Anaerobic processes in sludge digestion

1. TREATMENT AND DISPOSAL OF
SLUDGES

2. There are two end products obtained from
various wastewater treatment plant:
(1)Effluent:The treated effluent is directly
discharged either in the receiving water or
on the land

3. (2)Sludge:The sludges are to be first
processed before their final disposal
Objectives of processing sludge are to extract
water from solids and dispose the dewatered
residue through a combination of physical
chemical and biological operations

4. SLUDGE TREATMENT
PROCESS
It includes following unit operation
1.Thickening or Concentration
2. Digestion
3. Conditioning
4. Dewatering
5. Drying
6. Incineration

5. 1.Thickening
The purpose of thickening is to reduce
moisture content of the sludge and
consequently to increase the solids
concentration
Three types of thickening are as follows
I. Gravity thickening
II. Air floatation
III. Centrifugation
IV Screw press

6. Gravity thickening
It is the most common practice for
concentration of sludges.
It is the simplest and least expensive.
It is useful for primary sludge or
combined primary and activated sludge
but is not useful for activated sludge
independently.
It is not effective when the activated
sludge exceeds 40% of the total sludge

7. Centrifugal thickening
It is used both to thicken and to dewater
sludges.
It involves the settling of sludge particles under
the influence of centrifugal forces.
The basic type of centrifuge used for sludge
thickening is the solid-bowl centrifuge.

8. Solid-bowl centrifuge
It consists of long bowl mounted
horizontally and tapered at one end.
Sludge introduce into the unit continuously,
and the solids concentrate on the periphery,
an internal helical scroll, spinning at a
slightly different speed and accumulated
sludge towards the tapered end where
additional solids concentration occurs and
the thickened sludge is discharged.

9. Compact screw press for thickening

10. Different
ial speed
gear box
Cover Feed ports Rotating bowl
Main
drive
sheave
Feed
solids
Thickened
solids dicharge
port
Thickened solids
Rotating conveyer
Centrate
Centrate
discharge
port
(Schematic diagram of a centrifuge used for sludge thickening)

11. 2.Digestion
Sludge digestion is a biochemical
phenomenon involving organisms,
enzymes, food and environment.
1. Anaerobic digestion
2. Aerobic digestion

12. ANAEROBIC
SLUDGE
DIGESTION
PRESENTED BY:
NITIN JAIN

13. INTRODUCTION
⬥Anaerobic digestion is a biological process
that uses bacteria that function in an oxygen
free environment. These bacteria convert
volatile solids into carbon dioxide, methane
and ammonia.
⬥ Reaction takes place in an enclosed tank that
may or may not be heated.

14. TYPES OF ASD
1) STANDARD RATE :
■ Standard-rate systems take place in a simple
storage tank with sludge added intermittently.
The only agitation that occurs comes from the
natural mixing caused by gases rising to the
surface. Standard-rate operations can be carried
out at ambient temperatures, although heat is
sometimes added to speed up the biological
activity. The required detention time vary from
30 to 60 days.

15. STANDARD RATE SLUDGE DIGESTER
To pipeline

16. 2) HIGH RATE:
■ High-rate systems use a combination of active
mixing and carefully controlled, elevated
temperatures to increase sludge stabilization.
These systems mostly use pre-thickened sewage
sludge introduced at a uniform rate to maintain
constant conditions in the reactor. They differ from
low rate digesters in solid loading rate which is
much higher in this case. Their limitation is of
sludge thickening and non-separation of
supernatant

17. HIGH RATE SLUDGE DIGESTER

18. TWO STAGE SLUDGE
DIGESTION PROCESS
⬥Two digestion tanks (primary & secondary) in
series
⬥Primary tank is used for digestion of sludge
through heating & mixing
⬥Secondary tank is used for storage &
concentration of digested sludge

19. TWO STAGE SLUDGE DIGESTION
PROCESS
SCUM

20. CONDITION OF ASD
⬥Sewage sludge must be treated in the digester
for a mean cell residence time of 15 days
between 35°C to 55°C (95°F to 131°F) and
60 days at 20°C (68°F). Temperatures greater
than 20°C(68°F) and less than 55°C(95°F)
can be interpolated, i.e. a temperature of
82°F must have a mean cell residence time of
at least 38 days.

21. Anaerobic digester
microbiology
1: Extracellular hydrolysis (e.g. cellulose)
2: Fermentation leading to organic acids (VFAs),
acetate, CO2 and H2
3: Fermentation leading to acetic acid (CH3COOH),
H2 and O2
4: Methanogenesis leading to CH4,CO2 and H2O

22. Complex organic material
(starch, protein fats)
Simple organic material
(sugar, amino acids)
Hydrolysis
Volatile fatty acids
(propionate, butyrate etc.)
Acidogenesis
Acetogenesis
H2 + CO2 Acetate
CH4 + CO2
Methanogenesis
Methanogenesis
Contd.

23. I.Single stage high rate
digestion
Heating, auxiliary mixing, uniform feeding
and thickening of the feed stream
characterise the single stage high rate
digestion process.
The sludge is mixed by gas recirrculation,
pumping and sludge is heated to achieve
optimum digestion rate.

24. Two stage digestion
In this system a high rate digester is
coupled in series with a second tank.
The first tank is used for digestion and is
heated and equipped with mixing facilities.
The second tank is usually unheated and
used principally for storage. Tanks may
have fixed roofs or floating covers.
This type of digestion is seldom used
because of expanse of building a large tank
that is not fully utilized and because the
second tank was of negligible benefit

25. 2.Aerobic digestion
It is used to treat
1. Waste activated sludge
2. Mixtures of waste activated sludge and
primary sludge
3. Waste sludge from extented aeration
plants.

26. Process microbiology
Con
td.

27. ⬥ Mixed culture of anaerobic micro-organisms
⬥ pH 6 to 9
⬥ Redox potential under - 300mV
⬥ Temperature :
- Psychrophilic (5 to 20 deg C.)
- Mesophilic (25 to 45 deg. C.)
-Thermophilic ( 45 to 70 deg. C.)
⬥ High growth rate of acidogenic bacteria
⬥ Low growth rate of methanogenic, homo- acetogenic
and synthrophic bacteria
CHARACTERISTICS OF METHANIZATION

28. Methane forming bacteria
Types:
⬥ Methanobacterium ruminantium
⬥ Methanobacterium M.o.H.
⬥ Methanobacterium strain AZ
⬥ Rods, cocobacillus appearing cells
Gram(+) Methanobacterium arbophilicum
⬥ Methanobacterium formicium
⬥ Methanobacterium M.o.H.
⬥ Gram (-) cells Methanospirillum hungatii

29. Non-methanogenic bacteria
Bacteria Genus
⬥ Aeromonus Aeromonus sp. Alkaligenes A. bookerii + 3 more
⬥ Bacteroides Bacteroides sp. Aerobacter A. aerogenes
⬥ Bacillus B cereus + 11 more Clostridium C. aminovalericum
⬥ Escherichia E. coli + 2 more Neisseria N. catarrhalis
⬥ Klebsiella Klebsiella sp. Leptospira L. biflexa + 1 more
⬥ Microcuccus M. candidus + 4 more Proteus P. vulgaris
⬥ Rhodopseudomonas R. palustris Paracolobactrum P. intermedium + 1
more
⬥ Streptomyces S. bikiniesis Pseudomonas P. aeruginosa + 8 more
⬥ Sarcina S. cooksonii + 1 more Serratia S. indians
⬥ Streptococcus S. diploidus
⬥ Gram (+) Sarcina Methanosarcina barkerii Long rods, Gram(+) cells

30. Important toxic compounds
⬥ Sodium Na+ 4000 - 6000
⬥ Ammonia NH4+, NH3 1500 -
2000
⬥ Potassium K+ 3000 - 5000
⬥ Calcium Ca2+ 3000 - 5000
⬥ Cyanide CN- 0,5 - 1
⬥ Sulphide (dissolved) H2S, HS-,
S2- 100 - 150
⬥ Methane analogues - Very low
levels
⬥ Alkyl benzene sulfonate - 500 -
750
⬥ Magnesium Mg2+ 3000
⬥ Nickel Ni2+ 100 - 1000
⬥ Zink Zn2+ 350 - 1000
⬥ Chromium Cr3+ 200 - 2000
⬥ Heavy metals Depends on
sulphide

31. Suitable wastes
⬥Sludges from waste water treatment
⬥Agricultural waste, especially manures
⬥Organic fraction of household waste
⬥Organic waste from the food industry
⬥Park- and garden waste

32. RESULTS OF A.S.D.
Characteristics of the rest product :
⬥The material is dark brown to black
⬥Neutral or soil aroma
⬥Low microbial activity (marginal gas production)
⬥Very low concentrations of VFA (Volatile Fatty
Acids)
⬥Elevated C/N ratio (often >10)

33. Biological limitations
⬥ A suitable, balanced microflora has to be established
⬥ The degradation velocity and efficiency as function of the
activity of the microorganisms
⬥ The degradation efficiency depends on the composition and
size of the microflora
⬥ The substrate (influent) in anaerobic digestion should have a
chemical structure allowing biological degradation ( normally
organic in nature)
⬥ Environmental factors such as pH, temperature, absence of
oxygen and water content have an important influence on the
process

34. APPLICATIONS OF ANAEROBIC
DIGESTION
⬥Wastewater
⬥Cannery
⬥Distillery
⬥Dairy
⬥Brewery
⬥Jam factory
⬥Ice cream
factory
⬥Slaughterhouse
⬥Starch factory
⬥Chemical
factory
⬥Petrochemical
factory

35. SLUDGE
DIGESTERS

36. ACID
REACTOR

37. Benefits of Anaerobic Sludge
Digestion
⬥ Superior Sludge Stabilization
⬥ Highly Efficient Solids Reduction
⬥ Improved Dewaterability
⬥ Reduced Solids Mass and Sludge Volume
⬥ Reduced Odour Emissions (putrecibility)
⬥ Reduce BOD of wastewater
⬥ Pathogen Reduction
⬥ Generation of Valuable Gas and Energy
⬥ Reduction of greenhouse gas emmissions(CH4,CO2)
⬥ Extremely Low Operation Costs

38. Disadvantages
⬥ Anaerobic digestion is a biological process and can therefore not handle many
anthropogenic substances, that instead destroy the quality of the product
⬥ With unsatisfactory control of the process, there is a considerable risk
for operational disturbances that may take a long time to cure due to the very
slow growth rates of anaerobic microorganisms
⬥ Anaerobic digestion requires a more stringent process control and a
considerably more sophisticated equipment & large tanks, resulting in high
capital investment cost.
⬥ In small scale plants, where it is difficult to economically justify a stringent
process control, this has sometimes led to an unnecessary shut down due
to such operation problems
⬥ Supernatant from anaerobic digestion often have a high OD & high
concentration of N & suspended solids.

39. Advantages of aerobic
digestion as compared to
anaerobic digestion
1. Lower BOD Concentrations in
supernatant liquor.
2. Production of an odorless, biologically
stable end products.
3. Recovery of more of the basic fertilizer
values in the sludge.
4. Operation is relatively easy.
5. Lower capital cost.
6. Suitability for digesting nutrient rich
biosolids.

40. Conditioning
It improves the drainability of digested
sludge.It is more amenable to dewatering. It
can be achieved by various method such as
elutriation, chemical conditioning, heat
treatment,freezing etc.

41. 4. Dewatering
It is a physical unit operation used to
reduced the moisture content of sludge and
biosolids.
Various methods used in dewatering device
for removing moisture are
1. Mechanical dewatering devices
2. Physical devices

42. Mechanical dewatering devices are used to
dewater the sludge more quickly
The physical device include filtration
squeezing, capillary action, centrifugal
separation and compaction.
Because of the improved design and
performance of solid-bowl centrifuges,
imperforate basket centrifuge are seldom
used in new dewatering installation.

43. The selection of dewatering device
is determined by the type of sludge
to be dewatered, characteristic of
the dewatered product and the
space available.The dewatering
process that are commonly used
include centrifuges , belt filter
presses, drying beds and lagoons.

44. 5. Heat drying
It involves the application of heat to
evaporate water and to reduce the moisture
content of biosolids below that achievable
by conventional dewatering methods
advantage of this method is to reduce
product transportation costs, improved
storage capability, and marketabilty.
Heat drying methods
1. Convection
2. Conduction
3. Radiation

45. Radiation
In radiation drying systems, infrared lamps,
electric resistance elements supply radiant
energy that transfers to the wet sludge and
evaporates moistrues.
Radiation heat transfers is expressed as
follows
qrad=CsAσ (Tr
4-Ts
4)
Cs=radiation heat-transfer rate
A=Emmisity of drying surface
σ=Stefan-Boltzmann constant
Tr=absolute temp. of radiant source
Ts=Absolute temp. of the sludge drying surface

46. Rotary sludge dryer
Rotary
dryer
furnace Blender
cyclone
Hot gasses
Air
Fuel
Feed
sludge
Dried biosolids
Recycled product
Particulates
Dried
biosolids to
reuse
Exhaust
gas to
odor
control/of
f gas
destructio
n system
Schematic diagram

47. 6. Incineration
It involves the combustion of the sludge in
a reactor under high temperature along with
auxiliary fuels. The purpose of incineration
is to destroy the organic material, the
residual ash being generally useful as land
fill. During the process all the gasses
released from the sludge are burnt off an all
the organism are destroyed. Cyclone or
multiple hearth and flash type furnaces are
used with proper heating arrangement with
temp. control and drying mechanism.

48. Various methods for incineration
1. Multiple hearth incineration
2.Fluidized bed incineration
3.Coincineration with multiple solid base

49. Air Pollution control
Incineration method for waste water sludge
have the potential to be siginificant
contributors to air polltion. Air
contemenents associated with incineration
can be divided into to categories:
1. Odors
2. Combustion emissions

50. ANAEROBIC SEWAGE
TREATMENT

51. HISTORICAL PERSPECTS
⬥1850:Louis H Mourac, France
Automatic scavengers for settled sewage sludge
⬥1895:Donald Cameron
Built first septic tank, collected CH4 and used it for
street lighting
⬥1904:
Travic hydrolytic tank achieving dust function of
sedimentation and digestion installed at
HAMPTON

52. INTRODUCTION
⬥DECOMPOSITION OF ORGANIC AND
INORGANIC MATTER WITHOUT MOLECULAR
OXYGEN
⬥PROCESS DONE BY ANEROBIC BACTERIA
⬥THREE TYPES

53. PROCESSES INVOLVED
⬥HYDROLYSIS
⬥ACIDOGENESIS
⬥METHANOGENESIS

54. MICROBIOLOGY OF
PROCESS
⬥HYDROLYSIS:TRANSFORMATION OF
COMPLEX COMPOUNDS TO SIMPLER
COMPOUNDS.
⬥ACIDOGENESIS:ACID FORMATION TAKES
PLACE.
⬥METHANOGENESIS:METHANE FORMATION
TAKES PLACE.

55. STEPS IN ANAEROBIC DIGESTION
COMPLEX
ORGANICS
HIGHER
ORGANIC
ACIDS
ACETIC
ACID
H2
CH4

56. Complex organic material
(starch, protein fats)
Simple organic material
(sugar, amino acids)
Hydrolysis
Volatile fatty acids
(propionate, butyrate etc.)
Acidogenesis
Acetog
enesis
H2 +
CO2
Aceta
te
CH4 +
CO2
Methano
genesis
Methano
genesis

57. STANDARD RATE ASD
• THE CONTENTS OF DIGESTER ARE
USUALLY UNHEATED AND UNMIXED.
• DETENTION TIME VARY FROM 30 TO
60 DAYS.

58. MIXING OF FIRST STAGE
DIGESTION CAN BE ACHIEVED BY
FOLLOWING METHODS:
⬥Installing a draft tube mixer (commonly used
in India)

59. ⬥Gas recirculation (practiced in USA)

60. ⬥Sludge recirculation(practiced in UK, FRG)

61. ⬥From maintenance point of view option 3 is
ideal since a pump can be easily replaced
/repaired without affecting digester
performance
⬥From mixing consideration 1 and 2 are
comparable
⬥Option 2 needs an auxiliary storage of gas
for starting up a digester.

62. STANDARD RATE SLUDGE DIGESTER
To pipeline

63. HIGH RATE ASD
⬥HERE THE SLUDGE IS HEATED AND
MIXED.
⬥DETENTION TIME IS TYPICALLY 15
DAYS OR LESS.

64. HIGH RATE SLUDGE DIGESTER

65. TWO STAGE SLUDGE DIGESTION
PROCESS
SCUM

66. FACTORS AFFECTING
ASD
⬥ DISSOLVED OXEGEN
⬥pH (6.6 to 7.6)
⬥NUTRIENTS AVAILABLITY.
⬥OPTIMUM TEMPERATURE(85 TO 100 F)

67. ADVANTAGES OF ASD
⬥LOW CONCENTRATION OF BIO-MASS.
⬥METHANE IS VERY USEFUL END
PRODUCT.
⬥RESULTING ORGANIC MATTER IS
WELL STABILIZED.

68. LIMITATIONS OF ASD
⬥SLOW GROWTH RATE(LONG
DETENTION TIME).
⬥HIGH TEMPERATURE.