Anaerobic Treatment

The use of microbes in the absence of oxygen for the

stabilization of organic material by conversion to methane,

CO2, new biomass and inorganic products.

Anaerobic processes

Anaerobic fermentation Anaerobic respiration

Anaerobic fermentation

• In anaerobic fermentation, there is no external electron acceptor The product generated during the process accepts the electrons released during the breakdown of organic matter. Thus, organic matter acts as both electron donor and acceptor. The process releases less energy and the major portion of the energy is still contained in the fermentative product such as ethanol.

Glucose Pyruvate

Energy

Electron

Ethanol

Anaerobic fermentation of glucose to ethanol

Anaerobic respiration

• Anaerobic respiration on the other hand requires external electron acceptor. The electron acceptors in this case could be SO42-, NO3- or CO2. The energy released under such a condition is higher than anaerobic fermentation.

Glucose Pyruvate

Energy

Electron

CO2 + H2O

SO42-

CO2

NO3-

H2SCH4

N2

Anaerobic respiration of glucose

Steps in Anaerobic Digestion

Step 1 - Hydrolysis

In the hydrolysis step large organic molecules such as proteins, poly-saccharides and fats are degraded into small and soluble components (sugars, amino-acids, fatty acids) by enzymes excreted by fermentative bacteria.

Step 2 – Acidogenesis

• ACIDOGENESIS In the Acidification step soluble

compounds are converted into a number of simple, low-molecular compounds: volatile fatty acids such as acetic acid, propionic acid, butyric acid, etc., alcohols, aldehydes, mercaptanes, CO2, H2, NH3. New biomass is also formed.

• Sugars, amino acids, and fatty acids converted to volatile fatty acids (76%), H2 (4%), and acetic acid (20%)

• Optimum growth rate occurs near pH 6

Step 3 - Acetogenesis

• ACETOGENESIS

Products of the acidification step can be converted into acetate, H2 and CO2 by acetogenic bacteria. New biomass is formed as well. Volatile fatty acids converted to acetic acid (68%) and H2 (32%)

Step 4 - Methanogenesis

METHANOGENESIS

• In the final phase of anaerobic decomposition, the products of the first three phases: acetic acid, H2 and CO2, formic acid and methanol are converted into methane and CO2 as well as new biomass. In this phase the actual COD-removal takes place.

• Limited pH range 6.7 to 7.4

Comparison between anaerobic and aerobic processes

Anaerobic AerobicOrganic loading rate:

High loading rates:10-40 kg COD/m3-dayLow loading rates:0.5-1.5 kg COD/m3-day(for high rate reactors, e.g. AF,UASB, E/FBR)(for activated sludge process)

Biomass yield:

Low biomass yield:0.05-0.15 kg VSS/kg CODHigh biomass yield:0.37-0.46 kg VSS/kg COD

(biomass yield is not constant but depends on types of substrates metabolized)

(biomass yield is fairly constant irrespective of types of substrates metabolized)

Specific substrate utilization rate:

High rate: 0.75-1.5 kg COD/kg VSS-dayLow rate: 0.15-0.75 kg COD/kg VSS-day

Start-up time:

Long start-up: 1-2 months for mesophilic : 2-3 months for thermophilic

Short start-up: 1-2 weeks

Anaerobic AerobicSRT:

Longer SRT is essential to retain the slowgrowing methanogens within the reactor.

Microbiology:

Anaerobic process is multi-step process and diverse group of microorganisms degrade the organic matter in a sequential order.

Aerobic process is mainly a one-species phenomenon.

Environmental factors:

The process is highly susceptible to changes in environmental conditions.

SRT of 4-10 days is enough in case of activated sludge process.

The process is less susceptible to changes in environmental conditions.

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Types of anaerobic reactors

Low rate anaerobic reactors High rate anaerobic reactors

Anaerobic pond

Septic tank

Standard rate anaerobic digester

Imhoff tank

Slurry type bioreactor, temperature, mixing, SRT or other environmental conditions are not regulated. Loading of 1-2 kg COD/m3-day.

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Anaerobic Sequencing BatchReactor (ASBR)

Anaerobic contact process

Anaerobic filter (AF)

Upflow anaerobic slugde Blanket (UASB)

Fluidized bed Reactor

Hybrid reactor: UASB/AF

Able to retain very high concentration ofactive biomass in the reactor. Thus extremely high SRT could be maintainedirrespective of HRT. Load 5-20 kg COD/m3-dCOD removal efficiency : 80-90%

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Anaerobic Sludge Granules (close up)

Comparison of the settling properties of granular, flocculent and disperse sludge after 5 minutes of settling time

Important considerations in UASB operation

Provide optimum pH of 6.6 to 7.6, and enough alkalinity i.e 1000 – 5000 mg/l CaCO3 .

Addition of Ca2+ at 200 mg/L promotes granulation. Ca2+ conc. higher than 600 mg/L may form CaCO3 crystals which may allow methanogens to adhere to and then become washed out of the system.

Initial seeding of some well digested anaerobic sludge could be used. The seed occupies 30-50% of total reactor volume. Minimum seeding is 10% of the reactor volume.

Supplement nutrients and trace metals if needed. Provide N & P at a rate of COD: N:P of 400:7:1 (conservative estimate).

DESIGN CRITERIA FOR UASB REACTOR• LD in UASB Reactor 5 – 5.5 m

• Hydraulic retention Time 6 - 10 hrs

• Organic loading rate 1.5-2 Kg/m3/day

• Biogas yield 0.20 – 0.25 m3/kg of COD removed

UASB DESIGN

• In Calculation, Percent of COD removal is 75 - 85 %

• Organic loading rate ORL = Q (COD input – COD output) * 103

• Volume of tank W = C * Q / OLR = (kg COD/m3 * m3/h) / (kg COD/m3.h)

where C:  concentration of COD in wastewater Q:  flow rate of wastewater

• To determine the total liquid volume below the gas collectors, an effectiveness factor is used, which is the fraction occupied by the sludge blanket. It varies from 0.8 to 0.9

reqd. volume of reactor exclusive of gas storage area , VL = Vn/E

where VL = total liquid volume of reactor,m3

Vn = nominal liquid volume of reactor, m3

E = effectiveness factor

• Area of the reactor is given by: A=Q/v

where Q =flow rate of wastewater,m3/hr

v = design superficial velocity, m/hr

• Liquid height of reactor is determined using the following relationship: HL = VL/A

where HL = reactor height based on liquid volume, m

VL = total liquid reactor volume, m3

A = cross-sectional area, m2

• The gas collection volume is in addition to the reactor volume and adds an additional height of 2.5 to 3m.

Total ht. of reactor, HT = HL+ HG

where HG = reactor height to accommodate gas collection and storage

GLS separator Design• Slope of the separator bottom from 45 – 600

Free surface in the aperture between the gas collectors: 15 – 20% of reactor area.Height of separator from 1.5 – 2 m

• The baffles to be installed beneath the gas domes should overlap the edge of the domes over a distance from 10 – 20 cm.

Anaerobic Sequencing Batch Reactor

• Consists of four steps

– Feed

– React

– Settle

– Decant/effluent withdrawl

Settling time is about 30min.

Advantage of anaerobic process

1. Less energy requirement as no aeration is needed

0.5-0.75 kwh energy is needed for every 1 kg of COD removal by aerobic process

2. Energy generation in the form of methane gas1.16 kwh energy is produced for every 1 kg of COD removal by anaerobic process

3. Less biomass (sludge) generationAnaerobic process produces only 20% of sludge that of aerobic process

Soluble BOD1 kg

Aerobic process

CO2 + H2O0.5 kg

New biomass0.5 kg

Biodegradable COD1 kg

Anaerobic process

CH4 gas> 0.9 kg

New biomass< 0.1 kg

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4. Less nutrients (N & P) requirement

Lower biomass synthesis rate also implies less nutrients requirement : 50% of aerobic

5. Application of higher organic loading rate

Organic loading rates of 5-10 times higher than that of aerobic processes are possible

6. Space saving

Application of higher loading rate requires smaller reactor volume thereby saving the land requirement

7. Ability to transform several hazardous solvents including chloroform, trichloroethylene and trichloroethane to an easily degradable form

Limitations of anaerobic processes

1. Long start-up time

Because of lower biomass synthesis rate, it requires longer start-up time to attain a biomass concentration.

2. Long recovery time

If an anaerobic system subjected to disturbances either due to biomass wash-out, toxic substances or shock loading, it may take longer time for the system to return to normal operating condition.

3. Specific nutrients/trace metal requirements Anaerobic microorganisms especially methanogens have specific nutrients e.g. Fe, Ni, and Co requirement for optimum growth.

4. More susceptible to changes in environmental conditions

Anaerobic microorganisms especially methanogens are prone to changes in conditions such as temperature, pH, redox potential, etc.

5. Treatment of sulfate rich wastewater

The presence of sulfate not only reduces the methane yield due to substrate competition but also inhibits the methanogens due to sulfide production.

6. Effluent quality of treated wastewater

The minimum substrate concentration (Smin) from which microorganisms are able togenerate energy for their growth and maintenance is much higher for anaerobic treatment system. Owing to this fact, anaerobic processes may not able to degradehe organic matter to the level meeting the discharge limits for ultimate disposal.

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