1. CHEE 370 Waste Treatment Processes
Lecture #22
Anaerobic Digestion

2. Sludge Handling “50% of the cost, 90% of the headache”
Process Objectives:
Reduce the volume (Remove water)
Reduce the organic content
Reduce # of micro-organisms and pathogens
“Stabilize the Sludge”

3. Sludge Thickening
Wasted sludge (both primary and waste-activated) is “thickened” - the solids content is increased by removing some of the liquid
Makes handling the sludge more manageable
Reduces the required size for the anaerobic digesters and storage tanks
Minimizes the energy requirements for subsequent processes such as heat drying
Typically accomplished by physical means

4. Read textbook pages
Metcalf and Eddy

5. Gravity Thickening
Metcalf and Eddy

6. Centrifugal Thickening
Metcalf and Eddy

7. Centifugal Thickening

8. Sludge Primary sludge ~ 45,000 mg/L solids
Contains untreated biodegradable material
Thickened wasted activated sludge
~ 40,000 mg/L solids
Contains active biomass

9. Anaerobic Digestion
Involves methanogenic bacteria which grow on very simple carbon sources
2 General Types of Methanogens:
H2-utilizing methanogens
4 H2 + CO2 CH4 + 2H20
Acetoclastic methanogens
CH3COOH  CH4 + CO2
Strict anaerobes (O2 is lethal)

10. Anaerobic Digestion Anaerobic growth is a very slow process
Typically operate at higher temperatures (35 °C is optimal for methanogenic bacteria) to get high levels of conversion and minimize the reactor volume
Methanogens grow best at pH control the pH closely
By thickening the solids before digestion, the amount of water that needs to be heated is minimized
Reduces energy requirements and cost

11. Anaerobic Digestion Objectives
Destruction of organic material
Reduce the oxygen demand of the sludge - thereby making it more “stable” and suitable for release to the environment
Accomplished through oxidation
Methane is formed in the process
Methane can be used to run the heating system for the anaerobic digester
Pathogen Destruction
Anaerobic digestion at 35 °C and at a solids retention time of 15 days will lead to a high level of pathogen destruction

12. Anaerobic Digestion Objectives
Increase “dewaterability” of the final sludge
Wasted activated sludge cannot be thickened to more than 5% (50,000 mg/L)
Primary sludge can be thickened to as much as 6%
Sludge after anaerobic digestion can be thickened to as much as 130,000 mg/L
If dewatering is applied after digestion, the sludge can be thickened to as much as 25%

13. Anaerobic Digestion
Methanogenic degradation of complex substrates requires a concerted effort of many bacterial species
Three Stage Process:
Hydrolysis and Fermentation
Acetogenesis and dehydrogenation
Methanogenesis

14. Step 1 Hydrolysis and Fermentation
Performed by various facultative bacteria
Does not reduce the COD of the sludge
Carbohydrates  simple sugars
Proteins  amino acids
Sugars and amino acids  fatty acids and alcohols (fermentation)
Lipids  long chain fatty acids
Hydrolysis of lipids is the rate-limiting step
Important for the design of the digesters

15. Step 2 Acetogenesis and Dehydrogenation
Organic acids and alcohols are further degraded by bacteria to produce acetic acid and H2
4 H2 + CO2 CH4 + 2H20
CH3COOH  CH4 + CO2
Virtually all COD into the digester ends up as methane
Methanogenic bacteria have a very low yield - only a small amount of biomass is produced
Step 3
Methanogenesis

16. Anaerobic Digestion
Decomposition of organic and inorganic matter in the absence of oxygen
Main process used for the stabilization of sludge from municipal WW treatment
Advances in the design of digesters have made the process relatively economical
There are beneficial uses for the digested sludge
Methane produced in the digestion can be used as fuel

17. Heterotrophs vs Methanogens
Activated Sludge (Heterotrophs)
Anaerobic Digestion
(Methanogens)
Bacteria
Archaea
Assume they require aerobic growth conditions
Strict anaerobic growth conditions
Typical yield range:
g VSS/g bCOD
g VSS/g COD
max at 20 C:
d-1
max at 35 C:
d-1
Tables 8-10 and in the text provide ranges for other parameters.

18. Effect of H2
Effective anaerobic digestion requires a diverse microbial community
Hydrogen gas partial pressure is the key to balancing the reactions and populations present in the reactor
Formation of acetic acid and H2 by anaerobic oxidation is inhibited by high ppH2
Methanogens cannot use organic acids other than acetic acid
H2-utilizing methanogens must remove H2 as fast as it is produced to allow anaerobic oxidation to proceed

19. Effect of pH
Acid forming bacteria (pH ) are much more tolerant of low pH than methanogens (pH 7.0)
Production of volatile fatty acids (VFAs) decreases the sludge pH
Normally counterbalanced by buffering associated with cellular CO2 production
Imbalances reduce the pH of the system, impairing methanogenesis
“Stuck” or “Sour” digester
Compounding problem - requires immediate attention

20. Effect of Temperature
Acid forming bacteria have a much higher maximum specific growth rate (max) that changes more dramatically with temperature, as compared to methanogens
Where:
kT = reaction-rate coefficient at temp. T (C)
k20 = reaction-rate coefficient at 20 C
 = temperature-activity coefficient
T = temperature (C)

21. Digester Design Factors
******Solids retention time******
Hydraulic retention time
Temperature
Alkalinity pH
Presence of inhibitory substances
Nutrient availability
Methane production

22. Single-Stage Digestion
Uniform feeding is important
Total solids are reduced by ~ 50 %
May have fixed or floating covers (Methane + Oxygen = Trouble)
Metcalf and Eddy Figure 14-20

23. Two-Stage Digestion Not common in current practice
First tank is for digestion
Second tank is primarily for storage
Metcalf and Eddy Figure 14-20

24. Basic Model for Anaerobic Digestion (AD)
Want to know:
Quantity of solids leaving the digester
VSS and TSS destruction in the digester as a function of the SRT
How much methane is produced
Simple model: CSTR with no recycle
HRT = SRT ( = c = V/Q)
No additional equations will be provided on the equation sheet - develop the relationships from basic mass balances and understanding of the process

25. AD Model Assumptions
Design based on the rate-limiting step - breakdown of volatile fatty acids (VFAs)
Non-biodegradable fractions of COD remain unchanged by the digestion process
Heterotrophic bacteria only decays and the COD associated with decay will be accumulated as VFAs available to the methanogens
Complete hydrolysis and fermentation of biodegradable organic matter -> fully available to methanogens
Use the kinetics for the growth of the methanogens to determine the minimum SRT, then use this value with a safety factor to determine the operating conditions

26. Minimum SRT Calculation
Where:
umax,m = maximum specific growth rate for the methanogens
Kd,m = decay rate for the methanogens

27. Factor of Safety for Growth
It is necessary to provide a factor of safety for methanogen growth (prevent “stuck” digester) and headspace
Use a factor of safety of at least 2.5
The ministry of the environment requires at least 15 days SRT at 35 C
Compare with your calculation and select the larger value

28. Heterotroph Mass Balance
Assume there is no growth - only decay
Perform a mass balance on the digester for the heterotrophic bacteria:
As the SRT increases, the amount of active heterotrophic biomass in the effluent decreases

29. Debris Mass Balance
Debris (XD) can enter the digester in the influent (XDo) stream and is also generated during biomass decay
Perform a debris mass balance on the digester :
Where fd = debris fraction of the degraded biomass (fd ranges from ~ )

30. VFAs for Methanogens Multiple Sources: Soluble biodegradable COD (Ss)
Biodegradable particulate COD (Xs)
Decay of heterotrophic biomass

31. Effluent VFA and Formation of Methanogenic Bacteria
CSTR without recycle

32. CODin = CODout + CODmethane produced
Methane Production
COD balance can be performed in order to determine the amount of methane produced
CODin = Q(SSo + XSo + XHo + XDo)
CODout = Q(Svfa + XH + Xm + XD)
CODin = CODout + CODmethane produced
CH4 + 2O2  CO2 + 2H2O
64 g COD/mol CH4

33. Methane Production
Use the ideal gas law to calculate the volume produced per day (V=nRT/P)
Textbook example 7-9, p Effect Of Temp!
Volume of methane produced per day:
Where mCH4 is mass-COD of CH4 produced/time

34. Methane Gas Production
Conversion of COD to methane gas
Consider glucose (C6H12O6)
Show: 0.35 L CH4/g COD consumed at STP

35. Anaerobic Treatment of a Single Compound
Assume a single substrate is fed to an anaerobic digester at a flowrate of 1 L/day and at a concentration of 10,000 mg COD/L. The effluent COD concentration is to be less than 500 mg-COD/L and the transformation will be carried out by methanogens.
REMEMBER: ALWAYS DRAW A DIAGRAM!

36. Determine: Given: The SRT [50 days] The reactor volume [50 L]
The recommended design volume [125 L]
The volume of methane produced per day under standard conditions in the designed reactor [3.3 L/d]
Volume of methane produced at 35 ºC [3.72 L/d]
Given:
μmax= 0.15 d-1
kd=0.03 d-1
Kvfa=1000 mg COD/L
Ym=0.03 mg VSS/mg COD

37. Previously:
If Svfa,available >> Kvfa:
And:

38. Digester Volume
When designing the digester, it is important to include additional “head space” for the methane gas that is produced during the anaerobic digestion
Typically, an additional 25% volume is included in the design
This increase in volume does not influence the SRT

39. Anaerobic Treatment of Mixed Composition Sludge
Consider a waste treatment plant where the primary sludge and wasted activated sludge (WAS) are blended and sent to an anaerobic digester. The combined sludge is determined to have the following characteristics:
Combined Sludge Flow
m3/day
210
TSS
mg-TSS/L
21619
VSS
mg-VSS/L
17073
Soluble biodegradable COD
mg-COD/L 87
Soluble non biodegradable COD 29
Particulate biodegradable COD
18733
Active biomass concentration
1256
Biomass debris
989
Particulate non-biodegradable COD
9413

40. Design of the Anaerobic Digester
The digester is to be operated at an SRT of 15 days, according to the Ministry of the Environment’s minimum regulations
Assume the following parameters apply:
max, m= 0.27 d-1,
kd,m= 0.03 d-1
Kvfa = 2000 mg COD/L
Ym= 0.03 mg COD/mg COD
kd,h = 0.22 d-1
fd = 0.2
VSS/TSS for particulate organic fraction = 0.90 mg VSS/mg TSS

41. Calculate: Svfa,available [19590 mg-COD/L]
Confirm that the ministry guideline for the SRT is the appropriate choice for operating the system
The recommended design volume for the digester [3940 m3]
Volume of methane produced per day in the system at 35 C and 1 atm (Careful - biomass is already in COD units!) [1500 m3/d]
Svfa [1115 mg-COD/L]
Xm [382 mg-COD/L]
XD [1182 mg-COD/L]

42. % VSS Destruction VSSin = 17073 mg/L (In this example)
VSSout = XH + XD + Xm + Xnon-biodeg particultes
Remember to convert from COD to VSS units
% VSS destruction ~ 54 %

43. % TSS Destruction TSSin = 21619 mg/L (In this example)
TSSout = FSS + XH + XD + Xm + Xnon-biodeg particultes
Remember to convert appropriately to TSS units (given 0.9 mg VSS/mg TSS for the particulate organic fraction)
% TSS destruction ~ 47 %

44. Methane is a usable product, and the amount of biomass produced through AD is low, so why don’t we use it for WW treatment in general?
Slow growth kinetics requires long SRT (large reactor volumes)
High temperatures required - COD present in WW will not generate sufficient methane to heat water to 35 °C
Effluent is not of sufficient quality
Nitrifiers do not grow under anaerobic conditions
Effluent from the digester usually contains high ammonia concentrations
Liquid stream from sludge processing is usually fed back into the AS system

45. Sludge Handling and Disposal Heat Drying
Used to prepare the sludge for incineration or for sale as fertilizer
Sludge moisture content after drying is ~ 10%
Dried sludge is termed “biosolids”
The high cost of drying and relatively low levels of nutrients in the biosolids have limited its use as a fertilizer

46. Sludge Handling and Disposal Incineration
Complete oxidation of the biosolids to produce CO2, H20, and ash
Advantages:
Maximum volume reduction
Destruction of persistent pathogens and toxins
Potential to obtain energy
Limitations:
Expensive
Requires trained operators and constant monitoring
Environmental impact
Concerns with the disposal of the ashes

47. Ultimate Disposal Ocean dumping of sludge is discouraged or prohibited
Lagoons can be used for sludge disposal in remote locations
Excess liquid from the lagoon is returned back to the WW treatment plant
Landfills or land application are the most commonly used methods of disposal
Landfills are used for disposing sludge, grease, grit, and other solids
Wastes are deposited in a designated area, compacted with a tractor or roller, and covered with a 30 cm layer of clean solid or composted sludge to minimize odours and prevent attracting flies, rodents etc.

48. Land Application Agricultural lands, forests, golf courses, parks …
Concerns with public health risk through direct exposure or consumption of contaminated crops and groundwater
Controlling factors:
Utilization rate of nutrients by crops and vegetation
Potential of plants to uptake toxic components (mainly metals) from the sludge
Accumulation of metal and salts in the soil
Aesthetic
Standards and guidelines are developed based on toxicity studies and bioaccumulation within individual species and through food chains

49. Review Anaerobic Digestion
Decomposition of primary and wasted activated sludge (WAS) in the absence of oxygen
Uses methanogenic bacteria
Low biomass yields and methane gas production
Digester modelled as a CSTR without recycle
Design based on volatile fatty acids (from a variety of sources) as the limiting substrate

50. Review Anaerobic Digester Design
Relevant Design Questions:
How much methane is produced?
Solve using COD balance (64 g COD/mol CH4)
Quantity of solids leaving the digester?
What is the % VSS and % TSS destruction across the digester?
Important to always keep track of your units!