anaerobic waste treatment
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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
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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
- anaerobic: 85-95% BOD removal, 90% stable waste (CH4 not verysoluble in water so waste leaves system)
3. lower nutrients required
- operate at higher BOD, N:P ratios: P&P wastes often lacking inthese nutrients
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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
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Soluble BOD
1 kg
Aerobic process
CO2 + H2O
0.5 kg
New biomass
0.5 kg
Biode radable CODAnaerobic process
CH4 gas
> 0.9 kg
1 kg
New biomass
< 0.1 kg
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Disadvantages Over Aerobic Treatment
1. optimum waste stabilization requires warm temperatures. Methanogens
grow slowly (limiting factor)- mesophyllic: 35 - 38 (95 - 100)
- thermophyllic: 58 - 60 (135 - 140)
2. process is limited by slow biological (methanogenic) growth rates- requires longer startup period
- m ts rate o process a ustment to c ang ng waste oa s, temp., etc.
3. environmental sensitivity to O2, toxics, pH
4. more favorable when applied to concentrated waste streams
- BOD>10,000 mg/L
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Process Description
A. Conventional B. High-Rate(=30-60 days) (~15 days)
InfluentInfluent
Gas out(CH4 & CO2)
(no mixing) (complete mix)
supernatant
active layer
stabilized solids
sludge sludge and supernatant to clarifier
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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
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Anaerobic Microbiology & Biochemistry
Complexorganics
Short-chainsolubleorganic
monomers
Organic acids:(acetic, proprionic,
formic, etc.)
CO2CH4
acidformingbacteria
methaneformingbacteriahydrolysis
acetogenesis
(waste conversion)
methanogenesis
(waste stabilization)
Hydrolysis: cellulosic bacteria act on organic fibers to produce soluble organiccompounds (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
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Common Volatile Acid Intermediates
acetic acid: CH3COOH
proprionic acid: CH3CH2COOH
formic acid: HCOOH
butyric acid: CH3CH2CH2COOH
caprioc acid: CH3CH2CH2CH2CH2COOH
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Methane Bacteria
Methane bacteria
- very sensitive to small concentrations of O2- slow growth rate- low rate of organic utilization
Mechanism for methanogenesis
* *- 3 4 2
Reduction of CO2: CO2 + 8H CH4 + 2H2O
Note: Microbiologists no longer consider methanogens to be in the bacterial domain. Recent RNAsequencing places methanogens in the domain Archaea. In fact, humans are more closelyrelated to cucumbers than methanogens are related to bacteria.
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11
th
Ed. Brook Biology of Microorganisms
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Methods of Predicting Methane Production
422
48248224
:
CHban
COban
OHba
nOHC
ncompositioWaste
ban
++
+
+
Prediction from waste stabilization:
One pound UBOD or COD stabilized = 5.62 cubic feet CH4 (STP)
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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
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Stimulatory/Inhibitory Effects
from: Perry, 1964
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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
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Effect of Ammonia Nitrogen on Anaerobic Treatment
Ammonia
NitrogenConcentration
(mg/L)
Effect on
AnaerobicTreatment
50 200 Beneficial
200 1000 No adverse effect
1500 3000 Inhibitory athigher pH levels
Above 3000 Toxic
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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