Sanitary_Note.pdf

Apex Educational Academy
Subject: Public Health Engineering
Sanitary Engineering
Note Collected and Compiled By:
Madhu Khanal
M.Sc Water Resource Engineering
[Ministry of Water Supply]
(madhukhanal72@gmail.com)
For
Central Level, Nepal Engineering Service, Civil Group, Gazetted Third Class
2019, December

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Prepared By: Madhu Khanal
1. Answering Framework:
Framework for answering generic questions like,
1) What are the sectorial challenges of wastewater management in the context of Nepal?
2) Why sanitation is important? Why wastewater management is an important aspect of sanitation? How
‘Total Sanitation’ can be achieved?
3) What are your opinions on present environment of wastewater sector of Nepal? & Similar questions..
Figure 1 Answering Framework
Framework for answering numerical or derivation based questions (Technical Questions)
Figure 2 Answering Framework (Numerical)
SECTOR POLICY
AND LEGISLATIVE
ENVIRONMENT
SECTORAL GOAL
SECTORAL
ACHIEVEMENT
GAP ANALYSIS
PRESENT
INSTITUTIONAL
FRAMEWORK
SECTOR ISSUES RECOMMENDATION
The Framework has been/will be
discussed in detail in the class.
ASSUMPTIONS
(Major Assumptions)
MAIN PART
(Numerical or
Derivation)
RESULT AND
CONCLUSIONS
LIMITATIONS (If
any)
The Framework has been/will be
discussed in detail in the class.

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2. Some Important Details of Wastewater Sector:
2.1 History
 From Malla Era works on Sewerage has been started.
 Establishment of “Pani Goswara Adda” in 1929 AD is considered as milestone in Water Supply and
sewerage System Development.
 Institution established on 1972, Department of Water Supply and Sewerage Management (DWSSM)
is making the important contribution in WSS sector.
 During the decade of 1970, in the three districts of Kathmandu valley, Centralized wastewater
treatment plants were established.
2.2 Guiding policy in Sewerage and Solid Waste Management Sector:
 Constitution of Nepal [Right to water supply and sanitation has been mentioned on article 35(4)]
 Solid Waste Management national policy, 2053
 Solid Waste Management Act, 2068 BS & Solid Waste Management Rule 2070 BS
 Sewerage Management policy, 2075
 Water Supply and Sanitation Sector Development Plan (SDP) – Drafted
2.3 Goal:
Figure 3 SDG Goal 6
Out of 17 goals of SDG, Goal 6 is dedicated to Water Supply and Sanitation.
BY 2030:
 (Goal 6.2 of SDG) Achieve access to adequate and equitable sanitation and hygiene for all and end
open defecation, paying special attention to the needs of women and girls and those in vulnerable
situations.
 (Goal 6.3 of SDG) Improve water quality by reducing pollution, eliminating dumping and minimizing
release of hazardous chemicals and materials, halving the proportion of untreated wastewater and
increasing recycling and safe reuse.
2.4 Achievement
 Basic Sanitation – Nepal is ODF declared (September 30,2019)
 Treated Waste Water – 0%
2.5 Gap:
 Challenge of maintaining ODF (Sustainability issue) and Movement towards Total Sanitation has been
just initiated.
 Wastewater to be treated by 2030 – 50%

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2.6 Organizational Setup/ Institution Involved
Figure 4 Sector Actors
2.7 Issues:
 Policy, Standard Gap
 Fund Gap ( Comparatively Sewerage System Demands a high cost)
 Rampant Urbanization, Land availability to construct Treatment Plant is being problematic.
 Lack of institutional ability and experience.
 Sewerage Management, not being a primary concern of government till date.
2.8 Recommendation
[E.g.: Sustainability Issue, Opportunity]
[These points can be incorporated in the recommendation section, if appropriate. Creative recommendation
will have positive impact on evaluation]
MoWS
DWSSM KUKL NWSC PID
FWSSMP
MoUD
DUDBC HPCIDBC
MoPID
WSSDO
PROVINCIAL
FEDERAL
LOCAL
M, RM
OTHER
NGOs,
INGOs,
DP,
WSUC
MoWS Ministry of Water Supply
MoUD Ministry of Urban Development
DWSSM Department of Water Supply and Sewerage
Management
KUKL Kathmandu Upatyaka Khanepani Limited
NWSC Nepal Water Supply Corporation
PID Project Implementation Directorate
FWSSMP Federal Water Supply and Sewerage Management
Project
DUDBC Department of Urban Development and Building
Construction
HPCIDBC High Power Committee for Integrated Development of
Bagmati Civilization
MoPID Ministry of Physical Infrastructure Development
WSSDO Water Supply and Sanitation Division Office
M, RM Municipality, Village Municipality
DP Development Partners
WSUC Water Supply and Sanitation Users Committee

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Table of Contents
1. Answering Framework:.............................................................................................................................. 1
2. Some Important Details of Wastewater Sector:........................................................................................ 2
2.1 History................................................................................................................................................ 2
2.2 Guiding policy in Sewerage and Solid Waste Management Sector:.................................................. 2
2.3 Goal:................................................................................................................................................... 2
2.4 Achievement...................................................................................................................................... 2
2.5 Gap:.................................................................................................................................................... 2
2.6 Organizational Setup/ Institution Involved........................................................................................ 3
2.7 Issues:................................................................................................................................................. 3
2.8 Recommendation............................................................................................................................... 3
5.2.1. Importance of waste water and solid waste management, Sanitation system, Types of sewerage
systems .............................................................................................................................................................. 8
1. Wastewater:........................................................................................................................................... 8
1.1 Definition: ...................................................................................................................................... 8
1.2 Classification: ................................................................................................................................. 8
1.3 Important Notes:............................................................................................................................ 8
1.4 Wastewater Management:............................................................................................................ 8
1.5 Solid Waste: ................................................................................................................................... 8
1.6 Sources of Solid Waste:.................................................................................................................. 8
1.7 Solid Waste Classification .............................................................................................................. 9
1.8 Why Waste [Solid Waste and Waste Water] Management is important?.................................... 9
2. Sanitation System: ................................................................................................................................. 9
2.1 What is Sanitation ? ....................................................................................................................... 9
2.2 Sanitation System: ....................................................................................................................... 10
2.3 Types of Sanitaiton System:......................................................................................................... 10
2.4 Difference in Conservancy & Water Carriage System:................................................................. 11
2.5 Type of Water Carriage System: .................................................................................................. 11
2.6 [Separate System]........................................................................................................................ 12
2.7 [Combined System]...................................................................................................................... 13
2.8 [Partially Seperate System].......................................................................................................... 13
5.2.2 Sources and nature of wastewater, effluent characteristics, Factors affecting sanitary sewage,
Determination of quantity of sanitary sewage, Determination of quantity of storm water........................... 14
1. Sources and Nature of Wastewater:.................................................................................................... 14
2. Sanitary Sewage................................................................................................................................... 14
3. Factors that affect Quantity of DWF.................................................................................................... 14

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4. Determination of quantity of Sanitary Sewage ................................................................................... 14
5. Determination of Quantity of Storm Water: ....................................................................................... 15
5.1 Time of Concentration: ................................................................................................................ 15
5.2 Runoff Coefficient:....................................................................................................................... 16
6. Rational Method:................................................................................................................................. 17
5.2.5 Typical design periods, flow velocity, self-cleaning velocity, flow diagrams, hydraulic formulae and
gradients, Estimation of quantity of sanitary sewage, collection systems, sewer design criteria, shape of
sewers, types of sewers, sewer materials: requirements, salt glazed stoneware, and plain or reinforced
cement concrete pipes, plastic, steel, brick, sanitary and storm water sewers for separate and combined
sewer systems, construction of sewer: excavation, laying, jointing of sewer, testing of sewer, water test and
air test.............................................................................................................................................................. 19
1. Design of Sewer: .................................................................................................................................. 19
1.1 Self-Cleansing Velocity (Minimum Velocity):............................................................................... 19
1.2 Non Scouring Velocity.................................................................................................................. 20
1.3 Discharge and Velocity Relationship............................................................................................ 20
1.4 Relationships: Full Flow VS Partial Flow....................................................................................... 21
1.5 Design:.......................................................................................................................................... 22
1.6 Shape of Sewer: ........................................................................................................................... 23
1.7 Material of Sewer:........................................................................................................................ 24
1.8 New Sewer Material [In Nepal].................................................................................................... 24
1.9 Laying and Testing:....................................................................................................................... 24
5.2.3 Characteristics and examination of sewage ........................................................................................... 28
5.2.4 Sampling of sewage, Physical, chemical and biological characteristics of sewage, Decomposition of
sewage, aerobic and anaerobic decomposition, Biochemical oxidation demand (BOD) and chemical
oxidation demand (COD), Test of solids, Dissolved oxygen (DO), pH-value, BOD, COD, chlorine demand .... 28
1. Characteristics of Sewage:................................................................................................................... 28
1.1 Physical Characteristics:............................................................................................................... 28
1.2 Chemical Characteristics:............................................................................................................. 29
1.3 Biological Characteristics: ............................................................................................................ 29
1.4 Decomposition:............................................................................................................................ 30
1.5 Sampling of Sewage:.................................................................................................................... 30
1.6 Bio Chemical Oxygen Demand (BOD): ......................................................................................... 30
1.7 BOD Test: ..................................................................................................................................... 31
1.8 BOD- Derivation: .......................................................................................................................... 32
1.9 DO Test:........................................................................................................................................ 33
1.10 PH Test: ........................................................................................................................................ 33
1.11 COD Test....................................................................................................................................... 33

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5.2.6 Sewage Treatment: Treatment methods, Secondary treatment processes and their types, BOD
removal, design criteria, activated sludge, oxidation ponds and ditches, aerated lagoons and lagoons,
Sewage filtration, intermittent sand filter, contact bed, trickling filters, bio- filters and design of trickling and
bio filters Wastewater Treatment ................................................................................................................... 35
1. Treatment Methods:............................................................................................................................ 35
1.1 Primary treatment ....................................................................................................................... 35
1.2 Secondary Treatment (Biological Treatment).............................................................................. 35
1.3 Intermittent Sand Filter (ISF) ....................................................................................................... 36
1.4 Contact Bed (CB):......................................................................................................................... 38
1.5 Trickling Filter (TF):....................................................................................................................... 40
1.6 Oxidation Pond: ........................................................................................................................... 44
1.7 Activated Sludge Process (ASP).................................................................................................... 45
5.2.7 Sewage disposal: Sewage disposal by dilution: essential conditions for dilution, self purification of
streams, factors affecting self –purification, the oxygen sag curve (streeter-phelps equation), Sewage
treatment by land treatment........................................................................................................................... 47
1. Sewage Disposal................................................................................................................................... 47
1.1 Dilution:........................................................................................................................................ 47
1.2 Essential Conditions for Dillution:................................................................................................ 47
1.3 Standard of Dilution..................................................................................................................... 47
1.4 Self-Purification:........................................................................................................................... 47
1.5 Action involved in Self Purification: ............................................................................................. 48
1.6 Oxygen Sag Curve:........................................................................................................................ 48
1.7 Land Treatment:........................................................................................................................... 51
5.2.8 Sludge treatment and disposal: Sources of sludge and necessity of treatment, Aerobic and anaerobic
digestion, Methods of sludge treatment: grinding and blending, thickening, stabilization, dewatering,
drying, composting and incineration, Methods of sludge disposal: spreading on land, lagooning, dumping
and land filling.................................................................................................................................................. 53
1. Sources................................................................................................................................................. 53
2. Necessity of Treatment........................................................................................................................ 53
3. Objective of Sludge Treatment:........................................................................................................... 53
4. Quantity and Characteristics................................................................................................................ 53
5. Sludge Volume ..................................................................................................................................... 54
6. Process:................................................................................................................................................ 54
7. Disposal................................................................................................................................................ 54
5.2.9 Community participation and introduce following under this heading Users committee, Village
maintenance workers, Pre construction/during construction/post construction trainings, Women
participation, Community mobilization/participation, Record keeping of WSP, Rehabilitation, Composting
toilets, eco-sanitation ...................................................................................................................................... 55

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1. Water Users and Sanitation Committees WSUC ................................................................................. 55
2. Importance of WSUC............................................................................................................................ 55
3. Works of WSUC.................................................................................................................................... 55
4. Water Users and Sanitation Committees [Works and Responsibility] ................................................ 56
5. VMW, Village Maintenance Workers................................................................................................... 56
6. Some factors needed to be considered while selecting VMW............................................................ 56
7. VMW Skills ........................................................................................................................................... 56
8. SOP Format .......................................................................................................................................... 57
9. Preconstruction/ During Construction/Post Construction Trainings:.................................................. 57
10. Record Keeping of WSP.................................................................................................................... 57
11. Composting Toilets .......................................................................................................................... 57
12. Eco Sanitation .................................................................................................................................. 58
Undergoing Projects on Sewerage Management: ........................................................................................... 58
List of Figures
Figure 1 Answering Framework ........................................................................................................................ 1
Figure 2 Answering Framework (Numerical).................................................................................................... 1
Figure 3 SDG Goal 6 ......................................................................................................................................... 2
Figure 4 Sector Actors ....................................................................................................................................... 3
Figure 5 Solid waste Management Pyramid ...................................................................................................... 8
Figure 6 Waste Management ............................................................................................................................. 9
Figure 7 Barriers in Sanitation System .............................................................................................................. 9
Figure 8 Sanitation Value Chain...................................................................................................................... 10
Figure 9 Daily Variation of Sanitary Sewage .................................................................................................. 15
Figure 10 Time Area Graph of a Catchment.................................................................................................... 16
Figure 11 IDF Curve........................................................................................................................................ 18
Figure 12 Velocity Calculation Formulas........................................................................................................ 20
Figure 13 Partial Flow Condition..................................................................................................................... 21
Figure 14 Partial Flow Diagram....................................................................................................................... 22
Figure 15 Laying of Sewer .............................................................................................................................. 27
Figure 16 First and Second Stage BOD........................................................................................................... 31
Figure 17 Sewage Treatment Process .............................................................................................................. 35
Figure 18 Decomposition and Growth Process................................................................................................ 36
Figure 19 Intermittent Sand Filter.................................................................................................................... 37
Figure 20 Contact Beds.................................................................................................................................... 39
Figure 21 Trickling Filter................................................................................................................................. 41
Figure 22 Biofilm............................................................................................................................................. 41
Figure 23 Oxidation Pond................................................................................................................................. 44
Figure 24 Activated Sludge Process ................................................................................................................ 45
Figure 25 Sludge Sources ................................................................................................................................. 53
Figure 26 Eco sanitation .................................................................................................................................. 58

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5.2.1. Importance of waste water and solid waste management, Sanitation system, Types of
sewerage systems
1. Wastewater:
Recommendation: Google Search: Wastewater Management – A UN analytical brief. [In the first
suggestion of Google, you can find a good document related to Wastewater]
1.1 Definition:
Wastewater is defined as any water that has been negatively affected in quality by humans or by other means.
It is comprised of liquid and solid waste that is discharged from domestic residences, commercial properties,
industrial plants, and agriculture facilities or land. Wastewater contains a wide range of contaminants at various
concentrations.
1.2 Classification:
Sanitary Wastewater (Sanitary Sewage) – Sewage from residential or Industrial area.
Storm Sewage – It indicates Rainwater.
Total Sewage Volume = Sanitary Sewage + Storm Sewage
1.3 Important Notes:
Table 1 some important note related to Wastewater
 Untapped Resources.
 Sustainable Source of Water, Energy, Nutrients and other
recoverable by-products.
 Principle of 4R (Reduce, Reuse, Recycle, Recovery)
 Concept of Polluters Pay.
 Collection – Conveyance – Treatment – Disposal (Linear
System Vs Closing the loop Concept)
These topics will be/were
elaborated in the class.
[ If appropriate, incorporation of
these topics in your answer will
have a positive impact in
evaluation ]
1.4 Wastewater Management:
Wastewater management should consider the sustainable management of wastewater from source to re-entry
into the environment (‘reuse/disposal’ in the sanitation service chain) and not only concentrate on single or
selected areas or segments of the service provision process.
Many of today’s poorly thought-out and badly managed systems overload natural processes that purify water
and maintain soil structure. It is clearly important to design waste-water management systems that “work with
rather than against natural ecosystem processes” and, thus, understanding these processes before designing
infrastructure/artificial systems is fundamental for choosing a sustainable wastewater management approach.
Wastewater management should reflect the community and ecological needs of each downstream ecosystem
and user
1.5 Solid Waste:
Solid waste are any discarded, abandoned or waste like material. It can be solid, semi-solid or containerized
gaseous material.
1.6 Sources of Solid Waste:
 Household
 Construction and road sweeping.
Figure 5 Solid waste Management Pyramid

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 Institutional
 Horticulture
 Commercial
1.7 Solid Waste Classification
 Bio Degradable
 Non Bio Degradable
1.8 Why Waste [Solid Waste and Waste Water] Management is important?
 Because if Mismanaged...
 Affect Socio Economic Condition.
 Affect health.
 Affect Climate
 Affect Coastal & Marine Environment.
 Protect the future today.
 Profit Sector
 Ensure Sustainability.
2. Sanitation System:
2.1 What is Sanitation ?
WHO Definition:
Sanitation is the provision of facilities and services for the safe disposal of human urine and feces and
maintenance of hygienic conditions through services such as garbage collection and wastewater disposal.
Sanitation System:
Effective Sanitation System provide barriers between excreta and humans in such a way as to break the
disease transmission cycle. This aspect is visualized with the F diagram where all the major routes of fecal
oral disease transmission begin with the letter F
FEACE, FINGER, FLIES, FIELDS, FLUIDS, FOOD.
Figure 7 Barriers in Sanitation System
Figure 6 Waste Management

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2.2 Sanitation System:
Figure 8 Sanitation Value Chain
2.3 Types of Sanitaiton System:
1. Conservancy system
Sometimes the system is also called as dry system. This is out of date system but is prevailing in small towns
and villages. Various types of refuse and storm water are collected conveyed and disposed of separately.
Garbage is collected in dustbins placed along the roads from where it is conveyed by trucks ones or twice a
day to the point of disposal. all the non-combustible portion of garbage such as sand dust clay etc. are used for
filling the low level areas to reclaim land for the future development of the town. The combustible portion of
the garbage is burnt. The decaying matters are dried and disposed of by burning or the manufacture of manure.
Human excreta are collected separately in conservancy latrines. The liquid and semi liquid wastes are collected
separately after removal of night soil it is taken outside the town in trucks and buried in trenches. After 2-3
years the buried night soil is converted into excellent manure. In conservancy system sullage and storm water
are carried separately in closed drains to the point of disposal where they are allowed to mix with river water
without treatment.
2. Water Carriage System
With development and advancement of the cities urgent need was felt to replace conservancy system with
some more improved type of system in which human agency should not be used for the collection and
conveyance of sewage .After large number of experiments it was found that the water is the only cheapest
substance which can be easily used for the collection and conveyance of sewage. As in this system water is the
main substance therefore it is called as WATER CARRIAGE SYSTEM.
In this system the excremental matter is mixed up in large quantity of water their ars taken out from the city
through properly designed sewerage systems, where they are disposed of after necessary treatment in a
satisfactory manner.
The sewages so formed in water carriage system consist of 99.9% of water and .1% solids .All these solids
remain in suspension and do not changes the specific gravity of water therefore all the hydraulic formulae can
be directly used in the design of sewerage system and treatment plants.

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2.4 Difference in Conservancy & Water Carriage System:
Table 2 Difference: Water Carriage and Conservancy System
Item Conservancy Water Carriage
Initial Cost Low High
Foul Smell in latrines Yes No
Latrine Construction Away from Living Room (Cannot be
constructed as compact unit)
Can be constructed as Compact
unit.
Aesthetic Appearance Not Good Good
Area Large Area Less Area
Excreta Removal Takes time Immediate
System Human depended No human agency is involved in
transportation.
2.5 Type of Water Carriage System:
Separate System:
Combined System:
Partially Separate:
[Suitability condition of these system will be/were explained in the class]
Conditions favorable or unfavorable
It is decided based on following points
 Rainfall:
 Topography:
 Limitations of Available Funds:
 Pumping Requirement:
 Gradient of Sewers:
 Subsoil Condition:

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2.6 [Separate System]
Advantage
 The quantity of sewage to be treated being small the treatment works of smaller size would be needed
and the load on the treatment units will be less.
 The storm water (or rainwater) is not unnecessarily polluted and hence it can be discharged into natural
stream or river without any treatment.
 If pumping is required for lifting of sewage at the treatment works, the system will prove to be
economical both from the point of view of capital costs as well as from the point of view of running
costs.
 The sewers being of small size are economical. Further storm water (or rainwater) may be carried
through open or closed drains at or near the ground surface; consequently the cost of installation of the
system would be low.
 Sewers of smaller section can be easily ventilated as compared to those of larger section.
Disadvantage
 The sewers being of small size their cleaning is difficult.
 The sewers are likely to be choked.
 Unless laid at a steep gradient, self-cleansing velocity in the sewers cannot be assured. In addition,
flushing shall have to be done. This may prove unsatisfactory and expensive.
 The system requires two sets of sewers and hence it may prove to be costly.
 Maintenance costs of two sets of sewers are greater than that for one.
 The sewers or drains provided for carrying storm water (or rainwater) come in use only during the
rainy season. During other part of the year these may become the dumping places for garbage and may
thus be choked.
 Two sewers or drains in a street lead to greater obstruction to traffic while repairs of any one of them
are being carried out.
 In sewers of small size, there being lesser air contact foul smell may be produced due to the formation
of sewage gases.
 Double house plumbing would be required for making separate connections to two sets of sewers or
drains. Moreover, there is a likelihood of wrong connections being made because of which storm water
(or rainwater) may enter the sewer or drain meant for carrying sewage and thus cause overflow of
sewage.

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2.7 [Combined System]
Advantages
 In an area where rainfall is spread throughout a year, there is no need of flushing of sewers, as self
cleansing velocity will be developed due to more quantity because of addition of storm water.
 Only one set of pipe will be required for house plumbing.
 In congested areas it is easy to lay only one pipe rather than two pipes as required in other systems.
Disadvantages
 Not suitable for the area with small period of rainfall in a year, because dry weather flow will be small
due to which self cleansing velocity may not develop in sewers, resulting in silting
 Large flow is required to be treated at sewage treatment plant before disposal, hence resulting in higher
capital and operating cost of the treatment plant.
 When pumping is required this system is uneconomical.
 During rains overflowing of sewers will spoil public hygiene.
2.8 [Partially Seperate System]
Advantages
 Economical and reasonable size sewers are required.
 Work of house plumbing is reduced as rain water from roofs, sullage from bathrooms and kitchen, etc.
are combined with discharge from water closets.
 Flushing of sewers may not be required as small portion of storm water is allowed to enter in sanitary
sewage.
Disadvantages
 Increased cost of pumping as compared to separate system at treatment plants and intermediate
pumping station wherever required.
 In dry weather self-cleansing velocity may not develop in the sewers.
Note
The choice of the sewer system should be made with regard to the rain characteristics, the pollutant
concentration in the catchment and the sensitivity of the receiving water.
Generally, it can be said that the separate sewer system is cheaper if the rainwater is not treated. However,
many countries have implemented regulations, which enforce storm water treatment. Depending on the type
of treatment applied, the costs for separate sewer systems increase, so that the separate sewer system is then
generally more expensive than the combined sewer system

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5.2.2 Sources and nature of wastewater, effluent characteristics, Factors affecting sanitary
sewage, Determination of quantity of sanitary sewage, Determination of quantity of storm
water
1. Sources and Nature of Wastewater:
Sources Nature
Domestic effluent consisting of blackwater (excreta, urine and faecal
sludge) and greywater (kitchen and bathing wastewater)
High amount of organic
matter, greyish in color,
PH range of 7-8
Water from commercial establishments and institutions, including hospitals;
industrial effluent,
Might be toxic and
abnormal physical or
chemical characteristics
Storm water and other urban run-off – Storm Water Generally good in quality,
can be disposed with basic
or no treatment
Agricultural, horticultural and aquaculture effluent, either dissolved or as
suspended matter
Might contain different
chemicals.
Ground water Infiltration
2. Sanitary Sewage
Also known as Dry Weather Flow (DWF)
DWF
 Domestic, Industrial WW, WW from Public Facilities.
 Ground Water Infiltration.
3. Factors that affect Quantity of DWF
 GR of Population
 Rate of WS
 Type of Area Served
 Infiltration.
4. Determination of quantity of Sanitary Sewage
Design Period:
 Generally 30 Years, Pumping Plants 5 to 10 year only, Treatment Units 10 to 30 years
Population Forecasting Method:
(Future Population at the end of design period should be estimated)
 AI (Arithmetic Increase)
 GI (Geometric Increase)
 II (Incremental Increase)
 Decreased Rate of Growth
 Graphical
 Zoning
 Correlation.

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Water Consumption:
 Sewage flow curve is closely parallel to water consumption curve with obvious time lag.
 About 60-90% (Average 80%) of the consumed water is expected to return as sewage.
Peak Factor:
Due to Variation in sewage flow within a day (hourly variation), we need to adopt some peak factor to
design.
Babbit
Qmax =
5Qav
P0.2 , Population in thousands.
Harmon
Qmax = (1 +
14
4+P0.5 )Qav , Population in thousands. 1<P<1000
French Formula
PF = 1.5 +
2.5
√qm
Where qm is average flow in LPS
DWF = 0.8* Water Supply Rate* Design Population* PF
5. Determination of Quantity of Storm Water:
WWF- Wet Weather Flow (Storm Water Flow):
Portion of precipitation which flows over the ground.
Quantity depends upon:
 Surface area
 Slope and Shape of the Catchment.
 Intensity of Rainfall
 The condition of Surface.
 Initial Moisture Content of the soil.
 Atmospheric Temperature and Humidity.
 Obstruction in the flow of water.
Methods for Estimation of Quantity of Storm Water
1. Rational Method
2. Empirical formulae method
5.1 Time of Concentration:
The period after which the entire catchment area will start contributing to the runoff is called as the time of
concentration.
 The rainfall with duration lesser than the time of concentration will not produce maximum discharge.
 The runoff may not be maximum even when the duration of the rain is more than the time of
concentration. This is because in such cases the intensity of rain reduces with the increase in its
duration.
 The runoff will be maximum when the duration of rainfall is equal to the time of concentration and is
called as critical rainfall duration. The time of concentration is equal to sum of inlet time and time of
travel.
Figure 9 Daily Variation of Sanitary Sewage

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Figure 10 Time Area Graph of a Catchment
Inlet Time (Ti):
The time required for the rain in falling on the most remote point of the tributary area to flow across the ground
surface along the natural drains or gutters up to inlet of sewer is called inlet time. The inlet time ‘Ti’ can be
estimated using relationships similar to following. These coefficients will have different values for different
catchments.
Ti = [0.885 L3
/H]0.385
Where,
Ti = Time of inlet, minute
L = Length of overland flow in Kilometer from critical point to mouth of drain
H = Total fall of level from the critical point to mouth of drain, meter
Time of Travel:
The time required by the water to flow in the drain channel from the mouth to the point under consideration or
the point of concentration is called as time of travel.
Time of Travel (Tt) = Length of drain/ velocity in drain
5.2 Runoff Coefficient:
The total precipitation falling on any area is dispersed as percolation, evaporation, storage in ponds or
reservoir and surface runoff. The runoff coefficient can be defined as a fraction, which is multiplied with the
quantity of total rainfall to determine the quantity of rainwater, which will reach the sewers. It depends upon
the porosity of soil cover, wetness and ground cover.
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑅𝑢𝑛𝑜𝑓𝑓 𝐶𝑜𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑎 𝐶𝑎𝑡𝑐ℎ𝑚𝑒𝑛𝑡 (𝐶) =
∑ 𝐴𝑖 ∗ 𝐶𝑖
𝑛
𝑖=1
∑ 𝐴𝑖
𝑛
𝑖=1

17 | P a g e
6. Rational Method:
Assumption
 The peak rate of runoff is a function of average rainfall intensity during the time of concentration.
 The frequency of peak discharge is the same as the frequency of average rainfall intensity.
 The time of concentration is the time required for the runoff to become established & flow from the
most remote part (in time) of the drainage area to the point under design.
Storm Water Quantity (Q) =
𝐶𝐼𝐴
360
Where,
Q = Quantity of Storm Water in m3
/s
C = Average Runoff Coefficient of the Catchment.
I = Intensity of Rainfall for the duration equal to time of Concentration (mm/hr)
A = Area of Catchment (Ha)
This formula demands upon the information on C, I and A.
 Area of the catchment can be found easily with the survey or by using already prepared topo sheet or
with the help of DEM (Digital Elevation Method) by using GIS
 ‘C’ is calculated on the basis of land-use pattern of the catchment. Generally following values are
adopted according to type of land/cover,
Business areas 0.70 – 0.90
Apartment areas 0.50 – 0.70
Single family area 0.30 – 0.50
Parks, Playgrounds, Lawns 0.10 – 0.25
Paved Streets 0.80 – 0.90
Water tight roofs 0.70 – 0.90
 ‘I’ is calculated based on rainfall duration and frequency. As explained above rainfall duration is
taken equal to time of concentration (Tc)= Ti +Tf . The range of frequency (Return Period) often used
is as follows:
Residential Area 2-10 Year ( 5 Years most Common)
Commercial and high value districts 10-50 Year
Flood Protection 50 Year
 IDF (Intensity-Duration-Frequency) curve is generally used in order to adopt the value of ‘I’ based
on duration and frequency.

18 | P a g e
Figure 11 IDF Curve
 Some Empirical Relationship are available to compute “I”. In general the empirical relationship has
the following forms:
𝐼 =
𝑎
𝑏 + 𝑡
𝑂𝑅 𝐼 =
𝑏
𝑡𝑛
British Ministry of Health Formula:
For storm Duration 5 to 20 minute; 𝐼 =
760
10+𝑡
mm/hr
For storm Duration 20 to 100 minute; 𝐼 =
1020
10+𝑡
mm/hr
Limitation of Rational Formula:
 Finding exact value of ‘C’ is very difficult or never possible.
 Formula cannot be used for large catchment (A>500Ha)
 This formula does not consider slope of catchment and initial moisture content of soil.

19 | P a g e
5.2.5 Typical design periods, flow velocity, self-cleaning velocity, flow diagrams, hydraulic
formulae and gradients, Estimation of quantity of sanitary sewage, collection systems, sewer
design criteria, shape of sewers, types of sewers, sewer materials: requirements, salt glazed
stoneware, and plain or reinforced cement concrete pipes, plastic, steel, brick, sanitary and
storm water sewers for separate and combined sewer systems, construction of sewer:
excavation, laying, jointing of sewer, testing of sewer, water test and air test
1. Design of Sewer:
Design Criteria Follows:
1. Determination of Sewage and Storm Water
2. Selection of System (Combined or Separate)
3. Selection of Shape and type of Sewer (Circular Or Rectangular; Closed Or Open)
4. Calculation on Hydraulics of Sewer
5. Finalization of Sewer Size and Sewer Slope.
Design Period:
Generally 25 to 30 years is taken.
[Importance of Design period has been/will be explained in the class]
Specific Gravity:
As sewage contains 99.9% of water and 0.1% of solid. Its specific gravity is considered same as water. (1)
Velocity:
The velocity is the most important design parameter. It must lie in between ‘self-cleansing’ and ‘non-
scouring’ velocity.
1.1 Self-Cleansing Velocity (Minimum Velocity):
Solid particles remain in suspension at the velocity. To avoid deposition minimum velocity should be
maintained self-cleansing velocity.
Camp Shield Formula to determine self-cleansing velocity:
𝑣 = √(
8𝛽(𝑆 − 1)𝑔𝑑
𝑓
)
Here,
V Velocity (m/s)
𝛽 Dimensionless Constant ( 0.04-0.8)
S Specific Gravity
d Diameter of Solid Particles
g Acceleration due to gravity (9.81)

20 | P a g e
Self-Cleansing Velocity:
Diameter (cm) Velocity (m/s)
15-25 1.0
30-60 0.7
>60 0.6
1.2 Non Scouring Velocity
Velocity above which scouring or erosion of inner surface will occur. Scouring takes place due to abrasive
action of harder materials like sand, grit, gravel etc.
Sewer Materials Limiting
velocities
Vetrified tiles 4.5 – 5.5
Cast Iron 3.5 – 4.5
Stoneware 3.0 – 4.0
Cement Concrete 2.5 – 3.0
Earthen Channel 0.6 – 1.2
Size:
Size is found at the multiple of 5 cm with minimum 15 cm and Maximum 3 m.
Slope:
Slope of the sewer generally follow the natural slope. Minimum of 1:100 and Maximum of 1:20 is
recommended.
1.3 Discharge and Velocity Relationship
Figure 12 Velocity Calculation Formulas
SEWER
Discharge
(Q) = AV
Velocity
(V)
Manning’s
𝑉 =
1
𝑁
𝑅
2
3𝑆
1
2
Chezy’s
𝑉 = 𝐶𝑅
1
2𝑆
1
2
Hazen Williams
𝑉
= 0.85 ∗ 𝐶𝑅0.63
𝑆0.54
Kutter’s
𝑉
=
23 +
0.00155
𝑆
+
1
𝑁
1 + (23 +
0.00155
𝑆
)
𝑁
√𝑅
√𝑅𝑆
Crimp and Burge
𝑉 =
1
𝑁
𝑅
2
3𝑆
1
2
𝑤𝑖𝑡ℎ 𝑁 = 0.012

21 | P a g e
1.4 Relationships: Full Flow VS Partial Flow
Notation:
Particulars Full Flow Partial Flow
Depth D d
Perimeter P p
Area A a
Velocity V v
Discharge Q q
Mannings Cofficient N n
Figure 13 Partial Flow Condition
Using Geometry and Manning’s Relationship for the velocity; following derivation can be made.
Table 3 Partial VS Full Flow Relationships
Name of Relationship Symbol Relationship Remarks
Depth Proportion 𝑑
𝐷
1
2
(1 − cos(
𝜃
2
)
Derivation
of
these
relationships
will
be/were
done
in
class
Perimeter Proportion 𝑝
𝑃
𝜃
360
Area Proportion 𝑎
𝐴
𝜃
360
−
sin(𝜃)
2𝜋
Hydraulic Mean Radius
Proportion
𝑟
𝑅 (1 −
360 sin(𝜃)
2𝜋𝜃
)
Velocity Proportion 𝑣
𝑉 (1 −
360 sin(𝜃)
2𝜋𝜃
)
2
3
(N=n Assumed)
Discharge Proportion 𝑞
𝑄
𝜃
360
∗ (1 −
360 sin(𝜃)
2𝜋𝜃
)
5
3
(N=n Assumed)

22 | P a g e
Based on above-mentioned Relationship, One diagram can be plotted, considering different Depth Proportion.
Thus, plotted diagram will be a useful tool in designing the sewers, it is known as partial flow diagram.
From the plot, it is evident that the velocities in partially filled circular sewer sections can exceed those in full
section and it is maximum at d/D of 0.81. Similarly, the discharge obtained is not maximum at flow full
condition, but it is maximum when the depth is about 0.95 times the full depth.
Figure 14 Partial Flow Diagram
1.5 Design:
Given Population, Water Supply Rate, Catchment Area and Characteristics, Manning’s Coefficient
STEP 1 Based on the given data calculate volume of Sanitary and Storm water Sewage. (Q)
(For this calculation we might need to forecast population, Assume some data like % of water
supply volume that goes to sewage, Peak factor etc.]
STEP 2 Select the System: Combined or Separate
STEP 3 Calculation on Hydraulics (Use the appropriate relationships as discussed above)
[Adopt suitable d/D ratio for the design]
STEP 4 Select Suitable diameter
[Computed diameter in STEP 3 might not be available in market, so select nearest market
available size]
STEP 5 Check Self Cleansing Velocity for selected diameter and low flow condition.
STEP 6 If everything is OK, Write your design results with a neat drawing. If not, Suggest an
appropriate way of redesign.
[Elaboration of these steps will be/were done in the class]
0
0.2
0.4
0.6
0.8
1
1.2
0 0.2 0.4 0.6 0.8 1 1.2
d/D
a/A, v/V, q/Q
Partial Flow Diagram
v/V q/Q a/A

23 | P a g e
1.6 Shape of Sewer:
Table 4 Shape of the Sewer
Shape Figure Properties
Circular The perimeter of circular sewer is the least with respect to the sewer of other shape.
The inner surface is smooth hence the flow of sewage is uniform and there is no chance of
deposition of suspended particles.
The circular sewers are easy to construct.
Standard
Egg
Generally used in combined sewers. These sewers can generate self-
cleansing velocity during dry weather flow.
New Egg
Horse Shoe The size is so large that the maintenance works within the sewer are very
easy.
Parabolic This type of sewer is suitable for carrying small discharges.
Semi
Elliptical
Rect. These are suitable for large sewers to carry heavy discharge of sewage. The
maintenance works are easy in this section
U shaped
Semi
Circular
Bucket
Handle
During dry season, the sewage flows through the lower portion and during
monsoon, the combined sewage flows through the full section.

24 | P a g e
1.7 Material of Sewer:
Table 5 Sewer Materials
Material Suitability Advantage Disadvantage Precaution
Brick Particularly
for large
diameter
Can be constructed to any
required shape and size
High Cost, Slow work, Large
Space
For chances of
Ground water
infiltration
Concrete Medium
diameter
Required strength Can be
achieved, Wide range of
pipe size, Rapid trench
backfill
Crown Corrosion, Outside
deterioration by sulphate from
soil water
Handling,
Laying,
Jointing etc.
Stoneware
Or
Vitrified
Clay
Resistant to most acids
and to erosion due to grit
and high velocities
Skilled labor required,
Normally available length is
just 90 cm
Handling,
Jointing
Asbestos
Cement
80 to 1000
mm in
diameter
freedom from electrolytic
erosion, good flow, light
weight, ease in cutting,
drilling, threading and
fitting with specials, ease
in handling, tight joints.
Subject to corrosion by acid,
highly septic sewage; Cannot
handle high superimposed load
Type of
sewage,
Superimposed
load
Cast Iron Pressure
Sewer
Variety of Joint available,
long laying length, tight
joints
Subject to corrosion by acid,
highly septic sewage
Type of
sewage
Student are advised to read about Steel,Ductile Iron Pipe & Non Metallic Pipe (PVC, HDPE etc)
1.8 New Sewer Material [In Nepal]
[CORR-FIT DWC Pipes]
Comparison with RCC Pipe
Parameter CORR-FIT DWC Pipes RCC Pipes
Length 6 m 2 to 2.5 m
Property Flexible Rigid
Jointing Socket and Spigot Collar
Number of Joint Low High
Pipe Weight Light Heavy
Hydraulics N=0.009 N=0.013
Handling Easy Difficult
Corrosion Resistance High Low
Life 70 to 100 year 25 to 30 years
1.9 Laying and Testing:
The laying of sewers is generally carried out by starting from the tail end or the outfall end, and proceeding
upwards. The advantage of starting the laying of sewers from the tail end is that the tail sewers may be
utilized even during the initial period of construction. On the other hand if the laying of sewers is started

25 | P a g e
from the head end the functioning of the sewerage scheme has to wait till the completion of the entire
scheme.
STEPS ACTIONS
Setting out
Sewer
Centre
Line
From the longitudinal section of the sewer line, the positions of manholes are located on the
ground because it is the general practice to lay sewer line between two manholes at a time.
The sewer centerline is marked on the ground by driving the pegs at an interval of 7.5 m or
15 m as per convenience. The sewer centerline should be properly maintained during the
construction.
(A) Offset Method
(B) Vertical Post Or Upright Method
Alignment
and
Gradient
of Sewers:
The sewers should be laid to the correct alignment and gradient by setting the positions and
levels of sewers so as to ensure a smooth gravity flow. This is done with the help of suitable
boning rods and sight rails, and a dumpy level. Modified levels of invert are first obtained
by adding a suitable vertical length to the invert levels mentioned on the longitudinal
section.
These modified levels of invert are marked on the sight rail. These levels are marked either
by fixing nails on sight rails or by adjusting the top of sight rails to the modified invert
levels of sewer line. Thus an imaginary line parallel to the proposed sewer line is obtained
on the ground.
In order to check the invert level of sewer boning rod or traveler is used. The boning rod is
a vertical wooden post fitted with a cross-head or tee at top and an iron shoe at bottom. The
boning rod is moved to and fro in the trench so as to obtain the invert-line of the sewer on
the prepared bed of the trench.
Excavation
of
Trenches,
Timbering
and
Dewatering
Excavation
The work of excavation is usually carried out in the form of open cut trenches but in certain
situations as indicated later tunneling is also adopted. The excavation is made so as to have
trenches of such lengths, widths and depths which would enable the sewers to be properly
constructed.
In busy streets and localities the length of the trench to be excavated in advance of the end
of the constructed sewer and left open at any time is usually not more than 18 m.
Timbering
The trenches may be excavated either with sloping sides or with vertical sides. Where
enough space is available, especially in undeveloped areas or open country, and when the
soil is such that vertical sides cannot be sustained, the excavation may be made with sloping
sides so that the sides are stable.
However, in many cases, it may be necessary to restrict the top width of the trench and
hence the excavation has to be made with vertical sides. When the depth of the trench
exceeds 1.5 to 2 m, and when the excavation has to be made with vertical sides, which
cannot be sustained, it becomes necessary to support the sides of the trench by sheeting and
bracing. This operation is known as timbering of trench. There are various methods adopted
for timbering of trenches out of which box sheeting is most commonly used.

26 | P a g e
Dewatering
Where the sub-soil water level is very near the ground surface, the trench becomes wet and
muddy because of water oozing in the trench from the sides and bottom. In such cases, the
construction of sewer becomes difficult. As such trenches for sewer construction needs to
be dewatered to facilitate the placement of concrete and laying of pipe sewer or
construction of concrete or brick sewer and kept dewatered until the concrete foundations,
pipe joints or brick work or concrete have cured.
Laying and
Jointing of
Pipe
Sewers:
Before laying the pipe sewer it should be ensured that the trench has been excavated up to
the level of the bottom of the bed of concrete or the bed of compacted granular material if
such a bed is to be provided, or up to the invert level of the pipe sewer if no such bed is to
be provided.
Along the trench sight rails are set at intervals of 30 m or so. After setting the sight rails
over the trench the centre line of the sewer is transferred to the bottom of the trench by
driving small pegs at an interval of 3 m or so. For laying the sewer at the desired gradient
invert-line of the sewer is set up. This is done by first adjusting the uprights.
After the invert-line of the sewer is set-up the sewer pipes are laid starting from the tail end
or the outfall end, and proceeding upwards. In the case of rocky or hard soil no concrete
bedding is provided and the sewer pipes are laid directly in the bed of the trench, but in the
case of soft soils the sewer pipes are laid on concrete bedding.
The pipes with socket and spigot ends are usually laid with sockets facing up the gradient.
In this way the spigot of each pipe can be easily inserted in the socket of the pipe already
laid. After truly bedding the first pipe, the second pipe is laid.
Testing Water Test:
Water test is carried out to find out the water tightness of the joints. This test is carried out
after giving sufficient time for the joints to set. In the case of concrete and stoneware pipes
with cement mortar joints, pipes are tested three days after the cement mortar joints have
been made. It is necessary that the pipelines are filled with water for about a week before
commencing the application of pressure to allow for the absorption by the pipe wall.
The test is carried out by plugging the lower end of the pipe-sewer by a rubber bag equipped
with a canvas cover and inflated by blowing air. The upper end is plugged with a provision
for an air outlet pipe with stop cock, and a connection to a hose ending in a funnel which can
be raised or lowered till the required pressure head is maintained for observation.
The water is filled in the pipe-sewer through the funnel and after the air has been expelled
through the air outlet, the stop cock is closed and the water level in the funnel is raised to 2.5
m above the invert at the upper end. Water level in the funnel is noted after 30 minutes and
the quantity of water required to restore the original water level in the funnel is determined.
The pipeline under pressure is then inspected while the funnel is still in position. There should
not be any leaks in the pipe or the joints (small sweating on the pipe surface is permitted).
Any sewer or part there of that does not meet the test shall be emptied and repaired or re-laid
as required and tested again.

27 | P a g e
Air test
Air test becomes necessary, particularly in pipes of large diameter when the required quantity
of water is not available for testing. The air test is done by subjecting the stretch of pipe to
an air pressure of 100 mm of water by means of a hand pump. If the pressure is maintained
at 75 mm the joints may be assumed to be water tight.
In case the drop in pressure is more than 25 mm, the leaking joints should be traced and
suitably treated to ensure water tightness. The exact point of leakage can be detected by
applying soap solution to all the joints in the line and looking for air bubbles.
Figure 15 Laying of Sewer

28 | P a g e
5.2.3 Characteristics and examination of sewage
5.2.4 Sampling of sewage, Physical, chemical and biological characteristics of sewage,
Decomposition of sewage, aerobic and anaerobic decomposition, Biochemical oxidation demand
(BOD) and chemical oxidation demand (COD), Test of solids, Dissolved oxygen (DO), pH-value,
BOD, COD, chlorine demand
1. Characteristics of Sewage:
Composition
>99.0% Water
Solids
70% Organic
30% Inorganic
Sewerage characteristics can be divided into three broad categories:-
1. Physical
2. Chemical
3. Bacteriological
1.1 Physical Characteristics:
Temperature: The normal temperature of sewage is slightly higher then water temperature.
Temperatureabovenormalindicate inclusion of hot industrial wastewaters in sewage.
Color Fresh sewage is light grey in color. While the old sewage is dark grey in color. At a
temperature of above 20o
C, sewage will change from fresh to old in 2 ~ 6 hours.
Odor Fresh domestic sewage has a slightly soapy or oil odor. Stale sewage has a
pronounced odor of Hydrogen Sulphide (H2S).
Solids Solids in sewage may be suspended or in solution. Solids are a measure of the
strength of sewage.
Turbidity Highly turbid in nature as it contains suspended and dissolved solids.
SOLID
S
 Suspended Solid
 Fixed Solids
 Volatile Solids
 Fixed Solids

29 | P a g e
1.2 Chemical Characteristics:
Sewage contain both organic and inorganic chemicals. The tests representing these organic and inorganic
constituents come under the heading of chemical characteristics. Test like BOD, COD, NITORGON,
PHOSPHOURS, ALKALINITY etc. give the chemical characteristics of sewage.
PH Normally Alkaline. PH range (7 to 8). With time the PH range falls due to
production of acids by aerobic action and the sewage becomes acidic. After
oxidation, it again becomes alkaline. Abnormal PH values indicates that the
sewage from industrial sources.
BOD It is amount of oxygen required for bacteria to oxidize organic matter present in
the sewage.
COD Amount of Oxygen required for chemical oxidation of organic matters, readily
oxidizable carbonaceous and other matter.
DO Dissolved oxygen (DO) is a relative measure of the amount of oxygen (O2)
dissolved in sewage.
1.3 Biological Characteristics:
Aquatic Animal Bacteria and Viruses will be present in sewage.
Bacteria :
Aerobic: Cannot survive without presence of oxygen.
Anaerobic: Cannot Survive with presence of oxygen.
Facultative: Can survive in both environment.
Aquatic Plant Waterweeds and Algae
Note:
Evaporation is done in 103 to
105 degree Celsius for 24 hour
In Muffle oven temperature is
maintained 500+/-50 degree
Celsius for 30 minute

30 | P a g e
1.4 Decomposition:
 Aerobic (Occurs in presence of Oxygen)
 Anaerobic (Occurs in absence of Oxygen)
 Facultative (Combination of aerobic and anaerobic)
Table 6 Difference between Aerobic and Anaerobic Decomposition:
1.5 Sampling of Sewage:
Sampling is the process of collection of true representative sample of sewage to determine characteristics of
sewage. Generally, 24-hour sample collection of volume 100 to 150 cc is performed on hourly interval in a
clean vessel.
Sample Type Description
Grab Sample  Method :Manual
 Source: Beneath the surface where mixing of sewage takes place due to
turbulence.
 Suitable: For PH, DO tests.
Composite Sample  Grab sample are mixed as per weightage of sewage flow.
Sample should be label with information as follows:
Source Date Time Preservatives
Used
Collectors
Identity
Temperature Atmospheric
Pressure
1.6 Bio Chemical Oxygen Demand (BOD):
 BOD is a measure of oxygen used by microorganisms to decompose the organic matter present in
sewage.
 The amount of oxygen absorbed by a sample of sewage during specific period, generally 5 days at a
specific temperature, generally 20 Degree Celsius for the aerobic destruction of the organic matter by
living organisms.
 The organic matter present in the wastewater may belong to two groups, a) Carbonaceous Matter b)
Nitogenous Matter.
 The ultimate carbonaceous BOD of a waste is the amount of oxygen necessary for microorganisms in
the sample to decompose the biodegradable carbonaceous material. This is the first stage of oxidation
and corresponding BOD is called as first stage BOD.
 In second stage the nitrogenous matter is oxidized by autotrophic bacteria, and the corresponding BOD
or nitrification demand.

31 | P a g e
Figure 16 First and Second Stage BOD
 In fact, polluted water will continue to absorb oxygen for many months, and it is not practically feasible
to determine this ultimate oxygen demand.
 Hence 5 days period is generally chosen for the standard BOD test, during which oxidation is about
60 to 70% complete, while within 20 days period oxidation is about 95-99% complete. A constant
temperature of 20 Degree Celsius is maintained during incubation. The BOD value of 5 day incubation
period is commonly written as BOD5 or simply as BOD.
 Another reason for selecting 5 fays as standard duration is to avoid interference of nitrification bacteria.
Nitrification starts after 6th
or 7th
day. Sanitary engineers are generally interested in carbonaceous BOD
only, so by selecting 5 days we generally get only the carbonaceous BOD.
1.7 BOD Test:
The sample is first diluted with a known volume of specially prepared dilution water. Dilution water contains
salts and nutrients necessary for biological activity and phosphate buffer to maintain PH around 7 to 7.5.
Diluted water is fully aerated. The initial DO of diluted sample is measured. The diluted sample is then
incubated for 5 days at 20 Degree Celsius. The DO of the diluted sample after the incubation period is found
out. The difference between the Initial DO of the diluted sample after the incubation period is found out. [Initial
DO- Final DO]
𝐵𝑂𝐷55,200𝐶 = [𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐷𝑜 − 𝐹𝑖𝑛𝑎𝑙 𝐷𝑜] ∗
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐷𝑖𝑙𝑢𝑡𝑒𝑑 𝑆𝑎𝑚𝑝𝑙𝑒
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑢𝑛𝑑𝑖𝑙𝑢𝑡𝑒𝑑 𝑠𝑒𝑤𝑎𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒

32 | P a g e
1.8 BOD- Derivation:
𝐿0 BOD remaining at t=0, Ultimate BOD
t Time
𝐿𝑡 BOD remaining at ‘t’ time Or Oxygen equivalent of organic matter remained
after time ‘t’
𝑌𝑡 𝑜𝑟 𝐵𝑂𝐷𝑡 BOD exerted at time ‘t’
𝐾𝐷 BOD constant (Base e)
K BOD constant (Base 10)
𝜃 Temperature
The expression for the first-stage BOD is usually derived on the basis of the assumption that at a given
temperature, the rate at which BOD is satisfied at any time (i.e., rate of deoxygenation) is directly proportional
to the amount of organic matter present in sewage at that time.
𝑑𝐿𝑡
𝑑𝑡
∝ 𝐿𝑡
𝑑𝐿𝑡
𝑑𝑡
= −𝐾𝐷 𝐿𝑡
[ - Sign for decrease in rate with time ]
𝑑𝐿𝑡
𝐿𝑡
= −𝐾𝐷 𝑑𝑡
Log𝑒(𝐿𝑡) = −𝐾𝐷 𝑡 + 𝑐 [Integrating]
Log𝑒(𝐿0) = 𝑐 Imposing initial Condition, t= 0 , 𝐿𝑡 = 𝐿0
Log𝑒 (
𝐿𝑡
𝐿0
) = −𝐾𝐷 𝑡
2.30 ∗ Log10 (
𝐿𝑡
𝐿0
) = −𝐾𝐷 𝑡

33 | P a g e
Log10 (
𝐿𝑡
𝐿0
) = −
𝐾𝐷
2.30
𝑡
Log10 (
𝐿𝑡
𝐿0
) = −𝐾 𝑡 𝐾 =
𝐾𝐷
2.30
𝐿𝑡 = 𝐿010−𝐾𝑡 Taking Antilog
𝑌𝑡 = 𝐿0(1 − 10−𝐾𝑡
) Relation for BOD, depends upon time and temperature
Note:
L0 does not depend upon temperature.
K depends upon temperature.
𝐾𝜃 = 𝐾20 ∗ (1.047)𝜃−20
𝐵𝑂𝐷𝑡,𝜃 = 𝐿0(1 − 10−𝐾𝜃𝑡
)
1.9 DO Test:
Dissolved oxygen is the amount of oxygen in the dissolved state in the wastewater. Through the wastewater
generally does not have DO, its presence in untreated wastewater indicates that the waste water is fresh.
Similarly, its presence in treated wastewater effluent indicates that the considerable oxidation has been
accomplished during the treatment stages. While discharging the treated wastewater into receiving water, it is
essential to ensure that at least 4 PPM of DO is present in it. If DO is less, the aquatic animals like fish etc. are
likely to be killed near the vicinity of disposal. The presence of DO in wastewater is desirable because it
prevents the formation of obnoxious odor. DO determination also helps to find the efficiency of biological
treatment. Determination of DO is done by winklers method.
1.10 PH Test:
 The PH value of sewage indicates the logarithm of reciprocal of hydrogen ion concentration present
in the sewage. It is thus an indicator of the acidity or the alkalinity of sewage. If the PH value is less
than 7, the sewage is acidic and if PH value is more than 7, the sewage is alkaline.
 The fresh sewage is alkaline, with past of time PH tends to fall due to production of acid by bacterial
action in anaerobic or nitrification processes. However with treatment of sewage the PH tends to rise.
 Determination of PH is important because efficiency of certain treatment methods depends on it.
Especially the biological treatment, for better result the PH of sewage should be around 7.0 in
biological treatment as microorganisms can flourish in that PH range.
 PH meter (Potentiometer) is used to measure the PH value.
1.11 COD Test
https://www.slideshare.net/sherin23/chemical-oxygen-demand-analysis-using-apha-manual
 Water sample is refluxed in strong acidic solution with a known excess amount of potassium
dichromate.
 After digestion, the remaining unreduced K2Cr2O7 is titrated with Ferrous Ammonium Sulphate (FAS)
to determine K2Cr2O7 consumed.
 This gives us the oxidizable organic matter in terms of oxygen equivalent.

34 | P a g e
Procedure:
 Wash 300 ml round bottom refluxing flask.
 In refluxing flask put one spatula (0.5 gm) of HgSO4 + 10 ml sample + 5 ml K2Cr2O7 + 15 ml
Concentrated H2SO4
 Add small amount of silver sulphate.
 Shake well and reflux for 2 hr.
 Cool and add little amount of distilled water to the flask through the condenser.
 Titrate the solution in the flask against FAS using Ferroin indicator.
 End point - green color to reddish brown.
Calculation:
The COD in PPM is determined by the formula,
𝐶𝑂𝐷 𝑚𝑔/𝑙 =
(𝐴 − 𝐵) ∗ 𝑀 ∗ 8000
𝑚𝑙 𝑠𝑎𝑚𝑝𝑙𝑒 𝑡𝑎𝑘𝑒𝑛
𝐴 = 𝑚𝑙 𝑜𝑓 𝐹𝐴𝑆 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑏𝑙𝑎𝑛𝑘
𝐵 = 𝑚𝑙 𝑜𝑓 𝐹𝐴𝑆 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑠𝑎𝑚𝑝𝑙𝑒
𝑀 = 𝑀𝑜𝑙𝑎𝑟𝑖𝑡𝑦 𝑜𝑓 𝐹𝐴𝑆
8000 = 𝑀𝑖𝑙𝑙𝑖𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛 ∗ 1000 𝑚𝑙/𝐿
Molarity of FAS solution (M)
𝑀 =
𝑉𝑜𝑙𝑢𝑚𝑒 0.04167 𝑀 𝐾2𝐶𝑟2𝑂7 𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑡𝑖𝑡𝑟𝑎𝑡𝑒𝑑, 𝑚𝐿
𝑉𝑜𝑙𝑢𝑚𝑒 𝐹𝐴𝑆 𝑢𝑠𝑒𝑑 𝑖𝑛 𝑡𝑖𝑡𝑟𝑎𝑡𝑖𝑜𝑛, 𝑚𝐿
∗ 0.25

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5.2.6 Sewage Treatment: Treatment methods, Secondary treatment processes and their types,
BOD removal, design criteria, activated sludge, oxidation ponds and ditches, aerated lagoons
and lagoons, Sewage filtration, intermittent sand filter, contact bed, trickling filters, bio- filters
and design of trickling and bio filters Wastewater Treatment
1. Treatment Methods:
Figure 17 Sewage Treatment Process
1.1 Primary treatment
Primary treatment removes material that will either float or readily settle out by gravity. It includes the physical
processes of screening, comminution, grit removal, and sedimentation. Screens are made of long, closely
spaced, narrow metal bars. They block floating debris such as wood, rags, and other bulky objects that could
clog pipes or pumps. In modern plants, the screens are cleaned mechanically, and the material is promptly
disposed of by burial on the plant grounds. A comminutor may be used to grind and shred debris that passes
through the screens. The shredded material is removed later by sedimentation or flotation processes.
1.2 Secondary Treatment (Biological Treatment)
Secondary treatment removes the soluble organic matter that escapes primary treatment. It also removes more
of the suspended solids. Removal is usually accomplished by biological processes in which microbes consume
the organic impurities as food, converting them into carbon dioxide, water, and energy for their own growth
and reproduction. The sewage treatment plant provides a suitable environment for this natural biological
process. Removal of soluble organic matter at the treatment plant helps to protect the dissolved oxygen balance
of a receiving stream, river, or lake.
Major objective of Biological Treatment Process is to convert volatile organic matter into stable inorganic
matters through aerobic/anaerobic/facultative process.

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Figure 18 Decomposition and Growth Process
1.3 Intermittent Sand Filter (ISF)
Visit: http://www.nesc.wvu.edu/pdf/WW/publications/eti/ISF_gen.pdf
In this process, the sewage has already undergone preliminary treatment.
Introduction:
 They are a viable alternative to conventional methods when soil conditions are not conducive for
proper treatment and disposal of wastewater through percolative beds/trenches.
 Sand filters can be used in sites that have shallow soil cover, inadequate permeability, high
groundwater and limited land area
 Treatment is accomplished through physical and chemical means but mainly microorganisms attached
to the filter media.
 The treated wastewater is collected in underdrains at the bottom of the sand filter and is then
transported to a line for further treatment or disposal.
Biological Treatment Process
1. Aerobic (Occurs in presence of Oxygen, Eg: TF,
ASP)
2. Anaerobic (Occurs in absence of Oxygen, Eg:
Anaerobic Lagoons & ponds)
3. Facultative (Combination of aerobic and
anaerobic, Eg: OP)
1. Suspended Growth (Microorganism maintained in
suspension, Eg: ASP)
2. Attached Growth (Microorganism are attached to some
inert material, Eg: TF)
3. Combined (Microorganism are maintained in suspension
as well as are attached to some inert material, Eg:TFAS,
ASTF
TF Trickling Filter
ASP Activated Sludge Process
OP Oxidation Pond
TFAS Trickling Filter Activated Sludge
ASTF Activated Sludge Trickling Filter

37 | P a g e
Figure 19 Intermittent Sand Filter
Construction:
 Rectangular tank with specially prepared bed of sand constructed below the ground surface without
lining.
 Depth 1 to 1.25m ; Area 1000-4000 Square Meter; L/B ratio 3 to 4; 0.15 to 0.3 m thick layer of filter
media (sand) with effective size 0.2-0.5 mm and Coefficient of Uniformity 2 to 5.
Process:
1. The sewage is applied through a dosing tank and siphon, which flows into troughs laid on the filter
bed.
2. These troughs have side openings, which allows the sewage to flow on the sand.
3. After 24 hours, the sewage is now applied over a second bed while the first bed rests.
Performance:
ISFs produce a high quality effluent by removing a very high percentage of the contaminants. The performance
of an ISF depends on how biodegradable the wastewater is the environmental factors within the filter, and the
design of the filter. The most important environmental factors that determine the effectiveness of treatment are
media reaeration and temperature. Reaeration makes oxygen available for the aerobic decomposition of the
wastewater. Temperature directly affects the rate of microbial growth, chemical reactions, and other factors
that contribute to the stabilization of wastewater within the ISF. Several process design parameters that affect
the performance of ISFs are the degree the wastewater was pre-treated prior to going through the sand filter,
media size and depth, the hydraulic loading rate, and dosing techniques and frequency. Although physical,
chemical, and biological processes are all at work to some degree in an ISF, the biological processes play the
most important role since bacteria are the primary workers in sand filters.

38 | P a g e
Advantages
 ISFs produce a high quality effluent that can be used for drip irrigation or can be surface discharged
after disinfection.
 Drain fields can be small and shallow.
 ISFs have low energy requirements.
 ISFs are easily accessible for monitoring and do not require skilled personnel to operate.
 No chemicals are required.
 If sand is not feasible, other suitable media could be substituted that may be found locally.
 Construction costs for ISFs are moderately low, and the labor is mostly manual.
 The treatment capacity can be expanded through modular design.
 ISFs can be installed to blend into the surrounding landscape.
 The soil cover prevents odors.
Disadvantages
 The land area required may be a limiting factor.
 Regular (but minimal) maintenance is required.
 Odor problems could result from open filter configurations and may require buffer zones from
inhabited areas.
 If appropriate filter media are not available locally, costs could be higher.
 Clogging of the filter media is possible.
 ISFs could be sensitive to extremely cold temperatures.
1.4 Contact Bed (CB):
Visit: http://www.engineeringenotes.com/waste-management/filters/filter-
types-6-main-types-of-sewage-filters-waste-management/39976
Contact beds also called contact filters, are in general similar to intermittent sand filters. A contact bed consists
of a watertight tank filled with filtering media.
Construction:
The tank is usually constructed below the ground surface by excavating the earth and it is provided with a
lining of cement concrete or watertight cement plaster on masonry, on sides as well as on bottom.
The filtering media in this case is gravel, ballast or broken stones. The size of the particles of filtering media
varies from 15 to 40 mm. The depth of the filter bed is kept between 1 and 1.8 m, the common value being 1.2
m.
The area of the filter bed is usually less than 2000 m2
. Usually 3 or 4 beds are provided adjacent to each other
so that they can work in rotation. A dosing tank with a siphon is provided to serve all the beds.

39 | P a g e
Figure 20 Contact Beds
Working:
Filling The outlet valve of the underdrain is closed and the tank is slowly filled with sewage
effluent from the primary settling tank through the dosing tank. The depth of sewage
applied may be 50 to 100 mm over the top of the bed. This filling may take about 1 to 2
hours.
Contact The dosing tank outlet is then closed, and the sewage admitted over the contact bed is
allowed to stand for about 2 hours. During this period the fine suspended, colloidal and
dissolved organic matter present in the sewage gets transferred to the filter media, and
comes in contact with the bacterial film covering the filter media.
Emptying The outlet valve of the underdrain is then opened and the sewage present in the contact
bed is withdrawn slowly without disturbing the organic film of the bed. This operation may
take about 1 to 2 hours.
Resting
For
Oxidation
The contact bed is then allowed to stand empty for about 4 to 6 hours. During this period
of rest, atmospheric air enters the void spaces of the contact media, thus supplying oxygen
to the aerobic bacteria, resulting in the oxidation of the organic matter present in the film.
Performance
The effluent received from contact bed is usually non-putresible, but it is turbid and high in bacterial content.
In general, contact bed filters remove 85 to 90% of suspended matter, 60 to 80% of organic matter and 50 to
75% of bacteria. Hence, the effluent from contact beds is passed through secondary settling tank.

40 | P a g e
Advantages
 Contact beds can be operated without exposing the sewage effluent to view.
 Contact beds can work under small heads.
 There is no nuisance of filter flies.
 The problem of undesirable odor is less as compared to that in the case of trickling filters.
Disadvantages
 Rate of loading is much less in comparison to trickling filters.
 Large areas of land is required for their installation.
 Operation of contact beds requires skilled supervision.
 Cost of contact beds is more as compared to that of trickling filters.
 Contact beds require long rest periods.
 There is relatively high incidence of clogging.
1.5 Trickling Filter (TF):
Visit:
https://www.slideshare.net/jshrikant/l-18-trickling-filter
http://files.dep.state.pa.us/Water/BSDW/OperatorCertification
/TrainingModules/ww20_trickling_filter_wb.pdf
 Also known as percolation filter.
 The attached growth process of biological treatment applies here.
 Sewage application is maintained through sprinkling.

41 | P a g e
Figure 22 Biofilm
Figure 21 Trickling Filter
The trickling filter is filled with a high specific surface area material, such as rocks, gravel, shredded PVC
bottles, or special pre-formed plastic filter media. A high specific surface provides a large area for biofilm
formation. Organisms that grow in the thin biofilm over the surface of the media oxidize the organic load in
the wastewater to carbon dioxide and water, while generating new biomass.
Construction:
Tank and
Distribution
System
Type: Rectangular Or Circular. (Circular is common)
Materials: RCC or Masonry
Distributors: Rotary distributors having numbers of distributing arms.
Contact
Media/ Filter
Media
Media: Coarser materials of broken stone.
Effective Size: 2.5 to 7.5 cm
Quality: Hard and Durable
Depth: 1.8-2.4 m in NRTF & 1.2-1.8 m in HRTF
Under
Drainage
System
To collect filtered effluent.
Concrete blocks are generally used, which are placed at the slope of 1 in 50 to 1 in 200
Ventilation Generally, forced or artificial ventilation is not provided. However, if temperature
difference between atmosphere and filter media is more than 6 degree Celsius, force
ventilation is provided.

42 | P a g e
Working:
 As wastewater trickles through the media layer, a film is formed around the media, called slime layer
or biofilm.
 Depth of aerobic zone within slime layer, formed on media is 0.1 to 0.2 mm.
 The remaining part of film is anaerobic.
 As wastewater flows over the film, the organic matter is rapidly metabolized while colloidal organics
are adsorbed onto the surface.
 Since food concentration at the outer edge is more, therefore, microorganisms at outer level are in
rapid growth phase.
 As microorganisms grow thickness of biofilm increases.
 Diffused oxygen is consumed before it penetrates the slime layer.
 Therefore, anaerobic zone is established at the media surface.
 When microorganisms enter into death phase the loose their ability to hold the surface, and gets
detached from the surface.
 The phenomenon of scouring of slime layer is known as roughing, unloading of filter, or sloughing.
Recirculation:
Recirculation is the practice of recycling a portion of the trickling filter effluent back through the filter. The
recycled wastewater can be pumped from several different locations, such as a trickling filter effluent,
intermediate clarifier effluent, or secondary clarifier effluent. There are several primary reasons to recirculate
trickling filter effluent.
Reasons of Recirculation:
 Keeping the Filter Wet
 Diluting Toxic Influent Flow
 Improving Treatment Efficiency
 Controlling Excess Biomass
𝑅𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 (𝑟) =
𝑅𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝐹𝑙𝑜𝑤 (𝑅)
𝐹𝑙𝑜𝑤 𝑜𝑓 𝑅𝑎𝑤 𝑠𝑒𝑤𝑎𝑔𝑒 (𝐼)
𝑅𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟(𝐹) =
1+𝑟
(1+0.1𝑟)2

43 | P a g e
Types of TF:
1) Normal Rate of Slow Rate TF (No recirculation)
2) High Rate TF (Recirculation)
If two HRTF are connected in series, it is called bio-filter.
SN Parameters NRTF HRTF
1 Hydraulic Loading ( m3
/ha-day) 22000-45000 112000-335000
2 Organic Loading Rate (Kg BOD/Ha-m/day 925-2200 7400-18500
3 Depth of filter (m) 1.5 to 3 [1.8 to 2.4] 1 to 2 [1.2-1.8]
4 Recirculation Nil 1 – 2
5 Sloughing Intermittent Continuous
6 Cost of operation/Initial Cost Less/HIgh More/Less
7 Effluent quality Highly nitrified Nitrified up to nitrate
stage
8 Water Requirement Less More
9 Land Requirement More Less
10 Size of filter media 25-100 mm 30-60 mm
Design of TF
Some Relations used in design of TF:
1. 𝑅𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 (𝑟) =
𝑅𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝐹𝑙𝑜𝑤 (𝑅)
𝐹𝑙𝑜𝑤 𝑜𝑓 𝑅𝑎𝑤 𝑠𝑒𝑤𝑎𝑔𝑒 (𝐼)
2. 𝑅𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟(𝐹) =
1+𝑟
(1+0.1𝑟)2
3. 𝐻𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝐿𝑜𝑎𝑑𝑖𝑛𝑔 (𝐻) =
𝑅𝑎𝑡𝑒(𝑄)
𝑃𝑙𝑎𝑛 𝐴𝑟𝑒𝑎 (𝐴)
4. 𝑂𝑟𝑔𝑎𝑛𝑖𝑐 𝐿𝑜𝑎𝑑𝑖𝑛𝑔 (𝑈) =
𝑊
𝑉𝐹
=
𝑖𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝑜𝑟 𝑎𝑝𝑝𝑙𝑖𝑒𝑑 𝐵𝑂𝐷 𝑝𝑒𝑟 𝑑𝑎𝑦 (𝑄∗𝐶𝑖)
(𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑇𝐹)∗𝐹
Efficiency Formula (According to National Research Council Canada)
Single Stage Efficiency:
5. 𝐸1 =
𝐶𝑖−𝐶𝑒
𝐶𝑖
=
1
1+0.0044√𝑈1
Where U1 is Organic loading in Kg BOD5/Ha-m/Day
6. 𝐸2 =
𝐶𝑒−𝐶𝑒
′
𝐶𝑒
=
1
1+
0.0044√𝑈2
1−𝐸1

44 | P a g e
Design:
Given
1. Organic Loading (U)
2. Flow Rate (Q)
3. BOD level of influent (Ci)
4. Required BOD level of effluent (Ce or Ce
’
)
5. Recirculation Ratio (r)
Calculation:
I. 𝐼𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝑜𝑟 𝑎𝑝𝑝𝑙𝑖𝑒𝑑 𝐵𝑂𝐷 𝑝𝑒𝑟 𝑑𝑎𝑦 (𝑊) = 𝑄 ∗ 𝐶𝑖 Kg/day
II. 𝑅𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝐹𝑎𝑐𝑡𝑜𝑟(𝐹) =
1+𝑟
(1+0.1𝑟)2
III. 𝑂𝑟𝑔𝑎𝑛𝑖𝑐 𝐿𝑜𝑎𝑑𝑖𝑛𝑔 (𝑈) =
𝑊
𝑉𝐹
; Volume can be known.
IV. Assume Depth (1.2 to 1.8 m or 1.8 to 2.4 m)
V. Calculate Diameter form Volume and depth.
VI. Calculate efficiency from organic loading.
VII. Calculate effluent concentration from efficiency formula.
1.6 Oxidation Pond:
 It is also known as stabilization ponds.
 Stabilization ponds is biological treatment system in which stabilization of organic material is carried
out by bacterial oxidation and/or photosynthetic reduction of algae.
 Oxidation Ponds are used to treat sewage and bio-degradable industrial waste.
Types:
Aerobic Ponds; Anaerobic Ponds; Facultative Ponds
Figure 23 Oxidation Pond

45 | P a g e
Oxidation Pond Artificially constructed earthen basin or pond of controlled design
Theory Bacterial-Algal-Symbiosis
Function Sewage Treatment (Facultative Process)
Location At least 300 m from inhabitants
Should not be any tree or building around 50-60 m which can obstruct sunlight
Construction Earthen Dyke of top width 1 to 1.5 m
1:15 to 1:3 slope
Sewage is discharge at center of pond
An outlet chamber is made for combined outlet from different units
Commissioning a) Culture Method
Sewage is first filled to a depth of 15 cm.
Seeds of algae is grown and introduce to pond.
Everyday portion of decreased sewage is refilled, and pond should turn
completely green (within 1 week)
Now sewage is applied to operation level / Algae growth should be let to grow to
top.
a) Natural Method
Algae are made to grow naturally.
Sewage is kept to operation level (Inlet and Outlet Closed)
Reduced amount is added daily.
Operation and
Maitenance
Grass and fallen leaves should be removed daily.
Doesn’t require equipment or skilled manpower.
Special attention for mosquito breeding and flies
But chemicals should not be used as they can kill bacteria
Depth – 120 cm – Sludge removal once in 6 years.
Depth – 150 cm – Sludge removal once in 12 years.
Advantages Low initial Cost, No sophisticated technology, Low operation and Maintenance
cost, High efficiency (BOD >90%, SS>90%)
Disadvantage Larger land, Mosquito breeding, Not suitable in place where rainfall occurrence is
high
1.7 Activated Sludge Process (ASP)
Figure 24 Activated Sludge Process

46 | P a g e
Method Suspended Growth Method
Working Sewage from PST is mixed with 20-30% of own volume of returned AS.
Activated Sludge – Large concentration of highly active aerobic microorganism.
Time Mixture entered in aeration tank is kept for 4 to 8 hours
Principle Organism kept in moving state oxidizes the organic matter then the suspended and
colloidal matter tend to coagulate and form a ‘precipitate’, which settles down in
SST.
Mix Liquor Mixture of raw sewage and activated sludge
SS in ML MLSS
Action Physical Action : Small particles combined in aeration process and form bigger floc.
Biological Action: BOD reduction takes place, Bacteria decomposes organic matter to
inorganic matter
Efficiency 80-90% BOD removal
90-95% Bacteria removal
Aeration Diffused Aeration Tank (Ridge and Furrow & Spiral)
Mechanical Aeration ( Simplex, Link Belt and Kessner Brush)
Combined Type
(Students are advised to learn about aeration tank with figure)
Advantage Effluent obtained is of high quality (than TF), High Efficiency, Land required is less
(than TF), process is free from foul and insect nuisance, sludge has high fertilizing
value
Disadvantage High Quantity of sludge, Temperature sensitive, Closed supervision, High Operation
cost, At beginning plant requires seeding

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5.2.7 Sewage disposal: Sewage disposal by dilution: essential conditions for dilution, self
purification of streams, factors affecting self –purification, the oxygen sag curve (streeter-
phelps equation), Sewage treatment by land treatment
1. Sewage Disposal
Methods:
1. Dilution
2. Land treatment.
1.1 Dilution:
Disposal by dilution is a process in which the treated waste water from treatment plant is discharged into
large moving water body such as river or stream.
The effluent discharge and degree of treatment of wastewater depends upon the self-purification capacity of
the stream and its intended water use.
1.2 Essential Conditions for Dillution:
Wastewater River Water
Fresh
SS removed
Not Toxic
High water volume available
High DO available
Swift forward currents are available
Water is not used for drinking immediately after point of discharge
1.3 Standard of Dilution
Dilution Factor Standards of Purification Required
Above 500 No treatment requred. Raw sewage can be directly discharged into river
Between 300 to
500
Primary treatment such as PST is required so that SS concentration is less than
150 PPM
Between 150 to
300
Treatment such as screening sedimentation and chemical precipitation are
required so that SS concentration is less than 50 mg/lit
Less than 150 Thorough treatment is required, SS should be less than 50 mg/lit and BOD5
should be less than 20 mg/lit
[While making decision of sewage disposal, policy/standard of the nation should be reviewed]
1.4 Self-Purification:
Visit : https://ngojwg.org/study3-2-e.html
 When wastewater is discharged into the river or stream, the BOD of mix increases initially and DO
level starts falling.
 As river water travels further BOD gradually reduces and DO increases and reaches its saturation
level.
 Thus river gets purified on its own.
 This phenomena is known as Self Purification of Stream/River.

48 | P a g e
1.5 Action involved in Self Purification:
 Dilution
 Dispersion due to current
 Sedimentation
 Oxidation and Reduction
 Temperature
 Sunlight
If Cs & CR are any concentration (BOD, Temperature, DO etc.) in the sewage and river respectively.
Similarly, Qs & QR are discharge of sewage and river respectively.
Dilution:
𝐶𝑆 ∗ 𝑄𝑆 + 𝐶𝑅 ∗ 𝑄𝑅 = 𝐶 (𝑄𝑆 + 𝑄𝑅)
1.6 Oxygen Sag Curve:

49 | P a g e
DO Dissolved Oxygen
D0 Initial Deficit (Saturation DO – DO level after mixing sewage at t=0
DC Critical Deficit (Maximum Deficit, Minimum DO level)
tc Critical Time
L0 Ultimate first stage BOD at disposal point
Dt Deficit at any time ‘t’
K Deoxygenation Constant (Base 10) ; 𝐾𝜃 = 𝐾20 ∗ (1.047)𝜃−20
R Re-oxygenation Constant (Base 10) ; 𝑅𝜃 = 𝑅20 ∗ (1.016)𝜃−20
Deoxygenation
Curve
 In a polluted stream, the DO content goes on reducing due to decomposition of
volatile organic matter.
 The rate of deoxygenation depends upon the amount of organic matter remaining to
be oxidized at a given time as well as on the temperature of reaction, hence at given
temperature, the curve showing depletion of DO with time i.e. deoxygenation curve
is similar to the first stage BOD-curve.
Re-
Oxygenation
Curve
 In order to counter the balance of the consumption of DO due to deoxygenation,
atmosphere supplies O2 to water and the process is called re-oxygenation.
 The rate at which the oxygen is supplied by atmosphere to the polluted water depends
upon:-
 The depth of receiving water
 Condition of flow
 Oxygen deficit
 Temperature
Oxygen Sag
Curve
 In a running polluted stream exposed to the atmosphere, the deoxygenation as well
as re-oxygenation go hand in hand, if deoxygenation is more rapid than the re-
oxygenation an oxygen deficit results.
 The amount of resultant oxygen deficit can be obtained by algebraically adding the
deoxygenation and re-oxygenation curve.
 The resultant curve is known as oxygen sag.
In order to maintain clean conditions in a river stream, the oxygen deficit must be nil and this can be found
out by knowing the rates of deoxygenation and re-oxygenation.
Streeter Phelps Equation of Sag curve:
𝐷𝑡 =
𝐾𝐿0
𝑅 − 𝐾
(10−𝐾𝑡
− 10−𝑅𝑡) + 𝐷010−𝑅𝑡
At critical DO deficit:
𝑑𝐷𝑡
𝑑𝑡
= 0

𝑑
𝑑𝑡
(
𝐾𝐿0
𝑅 − 𝐾
(10−𝐾𝑡
− 10−𝑅𝑡) + 𝐷010−𝑅𝑡
) = 0
 (
𝐾𝐿0
𝑅 − 𝐾
[(−𝐾)10−𝐾𝑡
𝐿𝑛10 − (−𝑅)10−𝑅𝑡
𝐿𝑛10] + (−𝑅)𝐷010−𝑅𝑡
𝐿𝑛10) = 0 [𝐿𝑛10 ≠ 0]

𝐾𝐿0
𝑅 − 𝐾
[(−𝐾)10−𝐾𝑡
− (−𝑅)10−𝑅𝑡] + (−𝑅)𝐷010−𝑅𝑡
= 0

50 | P a g e
 −𝐾 ∗
𝐾𝐿0
𝑅 − 𝐾
∗ 10−𝐾𝑡
+ (
𝐾𝐿0
𝑅 − 𝐾
− 𝐷0)𝑅 ∗ 10−𝑅𝑡
= 0
 (
𝐾𝐿0
𝑅 − 𝐾
− 𝐷0) 𝑅 ∗ 10−𝑅𝑡
= 𝐾 ∗
𝐾𝐿0
𝑅 − 𝐾
∗ 10−𝐾𝑡
 (
𝐾𝐿0 − 𝑅𝐷0 + 𝐾𝐷0
𝑅 − 𝐾
) 𝑅 ∗ 10−𝑅𝑡
= 𝐾 ∗
𝐾𝐿0
𝑅 − 𝐾
∗ 10−𝐾𝑡
 𝐿𝑜𝑔10[(
𝑅 − 𝐾
)𝑅 ∗ 10−𝑅𝑡
] = 𝐿𝑜𝑔10[𝐾 ∗
𝐾𝐿0
𝑅 − 𝐾
∗ 10−𝐾𝑡
]
[Taking 𝐿𝑜𝑔10
on both side]

𝐿𝑜𝑔10 (
𝑅 − 𝐾
) + 𝐿𝑜𝑔10𝑅 + 𝐿𝑜𝑔1010−𝑅𝑡
= 𝐿𝑜𝑔10𝐾 + 𝐿𝑜𝑔10
𝐾𝐿0
𝑅 − 𝐾
+ 𝐿𝑜𝑔1010−𝐾𝑡
[𝐿𝑜𝑔1010−𝑅𝑡
= −𝑅𝑡
𝐿𝑜𝑔1010−𝐾𝑡
= −𝑘𝑡]

(𝑅 − 𝐾)𝑡 = 𝐿𝑜𝑔10 (
𝑅 − 𝐾
) − 𝐿𝑜𝑔10
𝐾𝐿0
𝑅 − 𝐾
+ 𝐿𝑜𝑔10𝑅
− 𝐿𝑜𝑔10𝐾
 (𝑅 − 𝐾)𝑡 = 𝐿𝑜𝑔10 (
𝐾𝐿0
) + 𝐿𝑜𝑔10
𝑅
𝐾
 𝑡 =
1
𝑅 − 𝐾
𝐿𝑜𝑔10 (
𝑅
𝐾
∗
𝐾𝐿0
)
 𝑡𝑐 =
1
𝑅 − 𝐾
𝐿𝑜𝑔10 (
𝑅
𝐾
∗
𝐾𝐿0
)
 𝑡𝑐 =
1
𝐾(
𝑅
𝐾 − 1)
𝐿𝑜𝑔10 (
𝑅
𝐾
∗ [1 − (
𝑅
𝐾
− 1)
𝐷0
𝐿0
])
R/K = f is called self-purification factor/constant.
 𝑡𝑐 =
1
𝐾(𝑓 − 1)
𝐿𝑜𝑔10 (𝑓 ∗ [1 − (𝑓 − 1)
𝐷0
𝐿0
])
Putting 𝑡 = 𝑡𝑐 =
1
𝑅−𝐾
𝐿𝑜𝑔10 (
𝑅
𝐾
∗
𝐾𝐿0−𝑅𝐷0+𝐾𝐷0
𝐾𝐿0
) in Streeter Phelps Equation, We can obtain
 𝐷𝑐 =
𝐾
𝑅
∗ 𝐿0 ∗ 10−𝐾𝑡𝑐 [R/K = f]

𝑓𝐷𝑐
𝐿0
= 10−𝐾𝑡𝑐
𝑡𝑐 =
1
𝐾(𝑓−1)
𝐿𝑜𝑔10 (𝑓 ∗ [1 − (𝑓 − 1)
𝐷0
𝐿0
) Gives
 𝑡𝑐𝐾(𝑓 − 1) = 𝐿𝑜𝑔10 (𝑓 ∗ [1 − (𝑓 − 1)
𝐷0
𝐿0
])
 10𝑡𝑐𝐾(𝑓−1)
= 𝑓 ∗ [1 − (𝑓 − 1)
𝐷0
𝐿0
]
Taking antilog
on both side

51 | P a g e
 (10𝐾𝑡𝑐)𝑓−1
= 𝑓 ∗ [1 − (𝑓 − 1)
𝐷0
𝐿0
]
 (
𝐿0
𝑓𝐷𝑐
)𝑓−1
= 𝑓 ∗ [1 − (𝑓 − 1)
𝐷0
𝐿0
]
𝑓𝐷𝑐
𝐿0
= 10−𝐾𝑡𝑐
1.7 Land Treatment:
Recommended Site : https://www.omicsonline.org/open-access/land-treatment-as-viable-
solution-for-waste-water-treatment-anddisposal-in-india-2157-7617-1000375.php?aid=82826
Land treatment is defined as the controlled application of waste water onto the land surface to achieve a
specified level of treatment through natural physical, chemical, and biological processes within the plant soil-
water matrix.
Advantages Disadvantages
 Water bodies will be prevented from
pollution.
 Wastewater can be utilized in irrigation
purpose.
 Operation and Maintenance cost is
relatively low.
 Requires Large area.
Ineffective in rainy seasons.
Not suitable in clayey soil.
 Supervision and special attention is
necessary
Methods Description Figure
Slow Rate
Irrigation
(SR)
Slow Rate (SR) systems are the predominant form of land
treatment for municipal and industrial wastewater. Such a
technology incorporates wastewater treatment, water reuse,
crop utilization of nutrients and wastewater disposal. It
involves the application of wastewater to vegetated land by
means of various techniques, including sprinkling methods or
surface techniques such as graded-border and furrow
irrigation. Water is usually applied intermittently (every 4 to
10 days) to maintain aerobic conditions in the soil profile.
Rapid
Infiltratio
n (RI)
Rapid infiltration (RI) is the most intensive of all land
treatment methods. In this method, usually high hydraulic and
organic loadings are applied intermittently to shallow
infiltration or spreading basins. The RI process uses the soil
matrix for physical, chemical, and biological treatment.
Physical straining and filtering occur at the soil surface and
within the soil matrix. Chemical precipitation, ion exchange
and adsorption occur as the water percolates through the soil.
Biological oxidation, assimilation and reduction occur within
the top few feet of the soil. Vegetation is not applied in
systems of this kind.

52 | P a g e
Overland
flow (OF)
Overland flow (OF) is a treatment process in which waste-
water is treated as it flows down through a system of vegetated
sloping terraces where waste-water is applied intermittently to
the top portion of each terrace and flows down the terrace to a
runoff collection channel at the bottom of the slope.
Application techniques include high-pressure sprinklers, low-
pressure sprays, or surface methods such as gated pipes used
with relatively impermeable surface soils in which infiltration
through the soil is limited in contrast to SR and RI systems

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