Anaerobic Waste Treatment
What is anaerobic treatment?
M&E: Biotreatment without oxygen. Bacteria that survive in anaerobic
conditions are called obligate anaerobes.
Typically:
Organic waste
carbon dioxide + methane
Advantages Over Aerobic Treatment
1. low production of waste biological sludge
- lower free energy release in organic waste conversion
- slow biological growth, low solids production
2. high degree of waste stabilization
- aerobic: 85-90% BOD removal because of high biosolids, only
50% stable waste
- anaerobic: 85-95% BOD removal, 90% stable waste (CH4 not very
soluble in water so waste leaves system)
3. lower nutrients required
- operate at higher BOD, N:P ratios: P&P wastes often lacking in
these nutrients
Advantages Over Aerobic Treatment, cont.
4. no O2 requirements
- lower power requirements
- reduced treatment costs
- not limited by O2 transfer
5. methane production
- may be burned to generate heat
or power
CO2 + H2O
Soluble BOD
1 kg
Aerobic process
0.5 kg
New biomass
0.5 kg
CH4 gas
Biodegradable COD
1 kg
Anaerobic process
> 0.9 kg
New biomass
< 0.1 kg
Disadvantages Over Aerobic Treatment
1. optimum waste stabilization requires warm temperatures. Methanogens
grow slowly (limiting factor)
- mesophyllic: 35º - 38º (95 - 100º)
- thermophyllic: 58º - 60º (135 - 140º)
2. process is limited by slow biological (methanogenic) growth rates
- requires longer startup period
- limits rate of process adjustment to changing waste loads, temp., etc.
3. environmental sensitivity to O2, toxics, pH
4. more favorable when applied to concentrated waste streams
- BOD>10,000 mg/L
Process Description
A. Conventional
B. High-Rate
(θ=30-60 days)
(no mixing)
(θ~15 days)
(complete mix)
Gas out
(CH4 & CO2)
Influent
Influent
scum layer
supernatant
active layer
stabilized solids
sludge
sludge and supernatant to clarifier
Process Description, cont.
C. Upflow anaerobic sludge blanket (UASB)
D. Anaerobic contact process used on soluble BOD waste
E. Fixed film anaerobic treatment process
- anaerobic filter (fixed bed)
- expanded or fluidized
Anaerobic Microbiology & Biochemistry
hydrolysis
Complex
organics
Short-chain
soluble
organic
monomers
acid
forming
bacteria
Organic acids:
(acetic, proprionic,
formic, etc.)
acetogenesis
(waste conversion)
methane
forming
bacteria
CO2
CH4
methanogenesis
(waste stabilization)
Hydrolysis: cellulosic bacteria act on organic fibers to produce soluble organic
compounds (cellulose, lignins)
Acetogenesis: acetogenic bacteria convert soluble organics to short-chain
fatty acids (primarily acetic acid)
Methanogenesis: Methanogenic bacteria convert organic acids to CH4 and
CO2, most CH4 leaves the system
Common Volatile Acid Intermediates
•
•
•
•
•
acetic acid: CH3COOH
proprionic acid: CH3CH2COOH
formic acid: HCOOH
butyric acid: CH3CH2CH2COOH
caprioc acid: CH3CH2CH2CH2CH2COOH
Methane “Bacteria”
Methane bacteria
- very sensitive to small concentrations of O2
- slow growth rate
- low rate of organic utilization
Mechanism for methanogenesis
- acetic acid cleavage: C*H3COOH
Reduction of CO2: CO2 + 8H
C*H4 + CO2
CH4 + 2H2O
Note: Microbiologists no longer consider methanogens to be in the bacterial domain. Recent RNA
sequencing places methanogens in the domain Archaea. In fact, humans are more closely
related to cucumbers than methanogens are related to bacteria.
11th Ed. Brook Biology of Microorganisms
Methods of Predicting Methane Production
Waste composition :
a b

n a b
n a b
Cn H a Ob +  n − −  H 2O →  − + CO2 +  + − CH 4
4 2

 2 8 4
2 8 4
Prediction from waste stabilization:
One pound UBOD or COD stabilized = 5.62 cubic feet CH4 (STP)
Waste Characteristics for Anaerobic Treatment
•
organic strength (BOD, COD) + composition
•
alkalinity
•
pH (methanogenic range 6.8 – 7.4)
•
inorganic nutrient content: BOD:N:P ratio
•
temperature
•
potentially toxic materials
– heavy metals
– ammonia
– common cations
Stimulatory/Inhibitory Effects
from: Perry, 1964
Stimulatory and Inhibitory Concentrations
(mg/L)
Stim.
Mod. Inhibit.
Strong Inhibit.
Sodium
100 - 200
3500 - 5500
8000
Calcium
100 - 200
2500 - 4500
8000
Magnesium
75 - 100
1000 - 1500
3000
Effect of Ammonia Nitrogen on Anaerobic Treatment
Ammonia
Nitrogen
Concentration
(mg/L)
Effect on
Anaerobic
Treatment
50 – 200
Beneficial
200 – 1000
No adverse effect
1500 – 3000
Inhibitory at
higher pH levels
Above 3000
Toxic
Unbalanced Systems
Indicators:
• volatile fatty acids (VFA) conc.: most important or utilized indicator
• pH (related to VFA, CO2
• CO2 in gas increasing
• total gas production decreasing
• waste treatment/stabilization decreasing
Factors:
• temperature
•organic loading (waste strength)
•nature of waste