anaerobic waste treatment

7/23/2019 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

anaerobic waste treatment

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