anaerobic treatment [ppt]- [part- i] 19-03-2013

8/11/2019 Anaerobic Treatment [Ppt]- [Part- I] 19-03-2013

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Process Biochemistry & microbiological

conditions required for Anaerobic process - I

Prof SK Maiti

Department of Environmental Sc. & EnggISM, Dhanbad


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INTRODUCTION

Anaerobic wastewater treatment is an oldest concept that dates back to 1964

(McCarty, 1964), but it is only after the extensive works of researchers likeLettinga (1979), Pette (1979), Jewell (1981) etc, anaerobic treatment

technology for treatment of domestic sewage and highly concentrated industrial

waste have been successfully used.

Anaerobic biological treatment processes offer several, advantagesover

aerobic system like

1) A high degree of waste stabilization is possible at high loading rate;

2) Significantly lower production of microbial sludge due to lower microbial cell yield

(approximately 0.1-g VSS/g COD vs. 0.5 to 1.0 g VSS/g COD in aerobic treatment),

3) Excess sludge has good dewatering characteristics,

4) Low nutrient requirement,5) No requirement of aeration;

6) Production of useful end products in terms of methane at a theoretical rate of 0.33 m3CH4/

kg COD stabilized;

7) Well developed sludge can be preserved for a period of one year or more without any

appreciable deterioration and

8) May be less sensitive to toxic compounds than aerobic process.


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Major disadvantages

1. Slow growth rate of methane bacteria, which means that low

substrate utilization rate, hence require relatively very long detention

time in the reactor.

2. Low growth rate resulting long start-up time (this is not only a

problem at the beginning of the operation but also after mechanical

disturbance and inhibition, which requires recovery).

3. The anaerobes are considered more efficient at temperature of 35oC

and higher, requiring some of the energy from methane to be used

for heating the content of the reactor and

4. Effluent from anaerobic treatment needs some post-treatment

before discharge because of lower oxygen content.


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ADVANTAGES Disadvantages

1. Less energy requiredAnaerobic processes may be net

energy producers instead of energy users.

2. Less biological sludge productionBecause the energetics of

Anaerobic processes results in lower biomass production by a

factor of about 6 to 8 times, sludge processing and disposalcosts are reduced greatly.

3. Fewer nutrients required- Many industrial wastewaters lack

sufficient nutrients to support aerobic growth. The cost of

nutrient addition is much less for anaerobic processes because

less biomass is produced.

4. Methaneproduction, a potential energy source

5. Smaller reactor volume required- Anaerobic processes

generally have higher volumetric organic loads than aerobic

process; Organic loading rates (OLR) of 3.2 to 32 kg

COD/m3.day may be used for anaerobic process, compared to

0.5 to 3.2 kg COD/m3.day for aerobic processes.

6. Elimenation of gas air pollution;

7. Rapid response to substrate addition after long periods without

feeding.

1. Longer start-up time to develop necessary

biomass inventory (months for anaerobic

treatment versus days for aerobic processes),

2. May require alkalinity additionThe most

significant negative factor that can affect theeconomics of anaerobic versus aerobic

treatment is the possible need to add alkalinity.

Alkalinity concentration of 2000 to 3000 mg/L

as CaCO3 may be needed in anaerobic

processes to maintain an acceptable pH.

3. May require further treatment with an aerobic

process to meet discharge requirements;

4. Biological nitrogen & phosphorus removal is

not possible;

5. Much more sensitive to the adverse effect of

lower temperatures on reaction rates;

6. May be susceptible to upsets due to toxic

substances;

7. Potential for production of odours and

corrosive gases.


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Biodegradation pathway


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Hydrolysis process

ProteasePeptidase

Lipase

FATS FATS

Fatty Acids + Glycerols

Cellulase

Carbohydrate

(starch, cellulose) PolysaccharidesMonosaccharides

(sugar)

Protein Polypeptides Amino acids


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ACIDOGENESIS (non-methanogenic)

The organisms are known as non-methanogenic, consists of facultative

and obligate anaerobic bacteria also known as Acitogens or Acid formers. Among the non-methanogenic bacteria that have been isolated from

anaerobic digester are Clostridium spp., Peptococcous anaerobus,

Bifidobacterium spp., Desulphovibrio spp., Cornybacterium spp.,

Lactobacillus, Actinomycetes and Escherichia coli.

Other physiological group present includes those producing proteolytic,lipolytic, ureolytic or cellulytic enzyme.

The sugar and amino acids are farther metabolized to acetic acid, H2or

higher volatile fatty acid (VFA) like- butyric acid, propionic acid, and

alcohol like methanol and ethanol.

The major routes of these metabolisms are via Pyruvic acid fermentation.Among these, only CO2, H2and acetic acid can be converted to CH4by

methane bacteria.

The other two important products, VFA and alcohols must be converted to

acetic acid before conversion to CH4.


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ACETOGENSIS Process

The VFAs and alcohols are converted by acetogensto the H2,CO2,formate and acetate as follows:

(i) Propionate Acetate

CH3CH

2COOH + 2H

2O CH3COOH + CO2+ H2. [G

o= +76.1 kJ]

(ii) Butyrate Acetate

CH3CH

2CH

2.COOH +2H

2O2CH3COOH + 2H2 [G

o= +48.1 kJ]

(iii) Ethanol Acetate

CH3CH2OH + 2H2OCH3COOH + H2 [Go= +9.6 kJ]

(iv) Lactate Acetate

CH3CHOH.COOH + 2H2OCH3COOH+ CO2+ 2H2. [Go= - 4.2 kJ]


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METHANOGENESIS

These organisms are strict anaerobes, called as "methanogens" or"methane former".

The principle genera of microorganisms are rod shaped bacteria

(Methanobacterium.Methanobacillus), spherical shaped

(Methanosarcina, Methanoccocus).

These organisms convert Acetic acid, H2, and CO2into Methane.

The Acetoclastic methanogens convert acetic acid to methane as follows:

CH3COOH + H2OCH4+ CO2[Go= -104.6 kJ]

The Hydrogenclastic methanogens converted CO2and H2to methane as follows:

CO2+ 4H2CH4 + 3H2O [Go= -135.6 kJ]


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It is important to note that methane bacteria can only use alimited number of substrates for the formation of CH4.

Currently, it is known that, methanogens use following

substrate: CO2+ H2, Formate, Acetate, Methanol, Methalamine

and CO for methane processors.

(i) Formic acid Methane

4HCOOH CH4+ 3CO2+ 2H2O

(ii) Methanol Methane

4CH3OH3CH4+CO2+ 2H2O.

(iii) Methalamine Methane

4(CH3)3N + H2O9CH4+ 3CO2+ 6H2O + 4NH3

METHANOGENESIS


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In the anaerobic environment, 70% of methane is formed

from acetic acid and rest from CO2and H2 as shown below.

Complex

organics

Higher

organic

acids

H2

Acetic

acid

CH4

28%

72%

24%

52%76%

20%

4%

Carbon and hydrogen flow during CH4 production


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Hydrolysis

Acidogenesis

Complex

substrate

Simple soluble

substrate

Methanogenesis

Ace

tate Acetogenesis

H2, CO2,

Formate

Acetate

CO2, CH4

VFA

Alcohol

products

H2,

CO2,

Formate

SinglePhase digestion

Hydrolysis

Simple

soluble

substrate

Acidogenesis

H2, CO2

Methanogenesis

Acetogenesis

VFA

H2, CO2,

Formate

Acetate

Pha

se 1

Phase 2

CO2, CH4

TwoPhase digestion

Acetate,

Formate

Figure 1: Principle sequence of anaerobic biodegradation

Complex

substrate


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OPTIMUM ENVIRONMENTAL ONDITIONS

FOR ANAEROBIC TREATMENT

Biologically, it may stated that the optimum conditions for anaerobic treatment are

prevalent when the microorganisms are living in an environment allowing the highest

possible growth rateBiological optimum conditions for anaerobic treatment are

given below:

Parameters Optimum conditions

1. Wastewater Balanced in carbon source, macro and

micronutrients.

2. Composition Absence of oxidized compounds prone to

reduction (O2, NO3, H2O2, SO4); Absence of toxicmaterial

3. Temperature 55 to 60 0C

4. pH 6.8-8.0

Q4. Enumerate the optimal conditions required for CH4production.


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OPTIMUM REACTORS CONDITIONS FOR GROWTH OF (AF) AND (MF)

Parameters Acid Former (AF) Methane Former (MF)

TemperatureAcid formers (AF) favors the

growth in 30 0C .

For mesophilic methane former (MF),

the optimum temperature is 35-37 0C

pHAF, a more acidic pH range is

desirable. At pH 7.0 acid

formations is inhibited.

MF are very pH sensitive and reaction

should held in the pH range of 6.8-7.2.

The optimum pH MF reported is 6.6-7.6

(McCarty, 1964). At pH below 6.2, the

efficiency drops significantly.

Different Hydrogen

pressure:

AF developed under low partial

pressure of hydrogen. If suchcondition does not maintained,

they can not survive.

In case of MF, bacteria require

hydrogen as an electron donor and alsorelatively insensitive towards higher

hydrogen concentration.

Stirring velocity:Through mixing of the substrate -

bacterial mixture is a condition

of the optimal progress of the

hydrolysis and acid-forming

reactor.

Very slow internal mixing is require.

Different growth

condition:

AF are fast grower than MF MF are slow grower

Therefore, phase separation of AF and MF is envisaged for anaerobic degradation of waste.

Q5. How temperature, pH, stirring velocity and different hydrogen pressure effects the growth of acid formersand methane former during anaerobic degradation of sewage.


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PHASE SEPARATION1. In the two-stage process, the hydrolysis and fermentation/acidogenesis are contained in the first phase and the

acetogenesis/methanogenesis reactions in the second phase.

2. The two -phase processes isolate both potentially rate-limiting steps and thereby enhances each.3. The first phase can be enhanced by more contact between extra-cellular enzyme and complex substrate.

4. A smaller reactor volume containing primarily acidifying organisms can create higher concentration of enzymes.

5. In the second-phase, acetogenic and methanogenic reactions are promoted as well as biological reactions that

require symbiotic relationship.

6. The major advantages and disadvantages of phase separation is shown in table 1 (Fox and Pohland, 1994).

ADVANTAGES Isolates and optimize potential rate-limiting steps;

o Hydrolysis is encouraged during first phase, while methanogenesis is

encouraged during second phase.

Improves reaction kinetics and stability

o pH control in each phase;o improve reactor stability to shock load

o select for faster -growing microbe

DISADVAN-

TAGES

Disruption of symbiotic relationship

More difficult to implement, engineer and operate_

Lack of process experience and applicability to variety of waste

Uncertainty of linkage between substrate type and reactor configuration

Table 1: Major advantages and disadvantages of phase seperation


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FACTORS AFFECTS THE ANAEROBIC PROCESS

These are the following factors affect the performance of

anaerobic process:

1. Organic loading and the type of waste,

2. SRT and HRT,

3. Temperature,

4. pH and alkalinity,

5. Stirring velocity,

6. Nutrient content and

7. Inhibitors.


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V

SoQ.loadOrganic

Xe

Xr

XeQ

XrVSRT c

.

.

.)(

6.1 ORGANIC LOADINGOrganic loading is usually measured as COD or BOD, which is depends on the type of

organics. The Organic load (COD) is applied per volume of the reactor and expressed as Kg

COD/m3/day.

; Where. Q= Influent flow rate (m3/day); So = influent waste concentration (g/m3) and V =

reactor volume, m3.

If So is constant, load is proportional with the flow. Hence, by reducing the flow, the loadcan be reduced in the reactor. The organics i.e., sugar waste are degraded rapidly and

therefore, high organic loading can be used.

6.2 SRT and HRT

The potential parameter used for designing of anaerobic treatment process is the SRT. It isthe average time that the microorganisms remain in the reactor during the treatment. It is

de-fined as:

Xe

Xr

XeQ

XrVSRT c

.

.

.)(


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[3]. TEMPERATURE

Bacteria may be divided into the following groups based on their temperature growth

ranges;

Cryophilicable to grow rapidly below 200

C; Thermophilicspecies grow best at temperature above 55 0C;

Mesophilicare grow best at intermediate temperature range between cryophilic

and thermophilic (2255 0C).

Bacterial formation of methane has been demonstrate between 0 0C to 97 0C .It is well

known from the literature that, microorganisms participating in anaerobic treatment,divide faster at higher temperature.

Optimal temperature for biomethananation is at least 55600C.

Since SRT, is a measure of the amount of microorganism per amount of substrate, an

increase in SRT can be compensated for a lower temperature.

During treatment, constant temperature is desirable, and it should not fluctuate morethan 2 0C/day (Mosely, 1974).

The need for a constant temperature is explained by the different kinetic behavior of

acidogenic and methanogenic organisms.

Owing to their faster growth rates, the acidogens acclimatize more rapidly to changed

conditions, causing an accumulation of metabolic products, which result is an overallimbalance.


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[4]. pH and ALKALINITYSimply, the anaerobic process may be regarded as an acid producing step (hydrolysis

+acidification) and followed by acid consuming step (methanogenesis).

The methanogenic bacteria have stricter requirement for pH than acidogenic bacteria.

Optimal pH for methanogens is 6.8-8.0 , and if pH drops below 6.2 methane production rate

drops. In an anaerobic digestion system, pH is controlled by carbon dioxide- bicarbonate

equilibrium (MaCarty et.al, 1964):

3

32H

HCO

COHKt

Where, Kt= ionization constant for carbonic acid.

H2CO3= carbonic acid concentration, related to percentage of CO2in the reactor.

HCO3= bicarbonate ion concentration, forming of the total alkalinity.

The bicarbonate alkalinity is approximately equivalent to the total alkalinity for most waste,

when VFA concentration is low. When VFA begins to increase in concentration, the pH is

buffered by the bicarbonate alkalinity.

It is found that, CO2 gas and bicarbonate alkalinity are closely related. Hence, low alkalinity

values in an anaerobic reactor do not allow much safety factor for increase in volatile acid.

It is therefore, desirable to have bicarbonate alkalinity value in the range of 20005000 mg/L,

in order to provide a buffer capacity to increase with a minimum decrease of pH.

It is recommended that VFA concentration should be less than 250 mg/L. The alkalinity in the

reactor can be controlled by- (i) by reducing the feed rate, which will allow to utilized the VFA

in the reactor and (ii) addition of alkaline materiel such as lime or sodium hydroxide.


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[5]. NUTRIENTS

i. For any biological system to operate, inorganic nutrients

required for bacteria growth must be supplied.

ii. The minimum requirement of C:N:P ratio is 100:6:1. However

several investigations reported in the range of 200:5:1 to

800:5:1 (Matta, 1985).

iii. When the loading rate is high, the ratio is usually 100:5:1 to

500:6.7:1.

iv. For complex waste, especially higher carbohydrate contents,

the ratio of 300:6.7:1 should be preferred (Frostell, 1985).v. Besides nitrogen and phosphorus, a large number of other

elements have been shown to be necessary for optimum

anaerobic growth like, Ni, Co, No, Mg, Fe, K etc.

[ ]


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[6.] OXIDANTSIt has been long recognized that absence of free oxygen is an obligatory prerequisite for

stable anaerobic treatment. The chemically bound oxygen interferes negatively, if available to

anaerobes. The interferes of compounds like, NO3and SO4/SO3in gaining wider importance

in anaerobic treatment.(i) Nitrate: If wastewater contains high concentration of NO3, by induction of pre-

denitrification can improve the performance.

(ii). Sulfate: Sulfides requires special attention in case of all anaerobic processes. Sulfide may

come from industrial process or may formed in the anaerobic unit as a result of reduction of

sulfate present in wastewater. Sulfur reducing bacteria utilized sulfate as an electron acceptorwith H2S being a end product. Elemental sulfur or organic sulfur compound may also appear

in product.

The role of sulfite and sulfate is to divert the electron from methanogenesis thus resulting in a lower

methane production.

Another influences is the production of H2S gas, which inhibits anaerobic treatment when present athigher concentration. The inhibition of acetoclastic methanogens by sulfide was studied & found that

more than 40% inhibition of methanogenesis at free H2S level of 50 mg/L, and increases to 100%

inhibition at 250 mg/L.

It has been showed that, when TOC level is less than 170 g/L (equivalent to 500 mg/L COD), sulfate

concentration up to 160 mg/L do not effect the TOC reduction. Therefore, when TOC/sulfate ratio = 1,there will be no problems, which is common in majority of sewage.

/


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[7]. Inhibitors/ toxic materials

A large number of toxicants are known to affects the anaerobic biological process; like-

1. Heavy metals;

2. Detergent and disinfectant used in food industry clean up3. Solvent from degreasing operation,

4. Inhibitors formed as secondary products (e.g, cyanide in coal coking operation).

5. Chemical inhibitors for food preservation.

The available strategies to decrease the impact of toxic material have been listed as follows:1. Remove toxic material from waste

2. Dilute the waste,

3. Allow a long adaptation period,

4. Use detoxification stage

5. Design for a longer SRT,

6. Form an insoluble complex or precipitate,7. Antagonize toxicity with another materials.

Removal of toxic materials or dilutions of the waste are rather obvious solutions, but

the possibility of NH3, SO2, H2Sstripping is mentioned by Takesshita et.al (1982).


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MONITORING AND CONTROL OF ANAEROBIC TREATMENT

PARAMETER FREQUENCY

(i) Influent (rate. concentration

composition, toxicity)

+++

(ii) SRT +++

(iii) Temperature +++(iv) pH +++

(v) Gas production and composition ++

(vi) Treatment efficiency ++

(vii) Volatile fatty acid and alkalinity +

+++= Should continuously monitored and controlled

++ = daily monitored; + =Intermittent monitoring.

Q8. List the important parameters to be monitored for the improvement of anaerobicprocess efficiency and their frequency of monitoring.


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8.0 Points to be remembered

1. Anaerobic treatment definitely has the potential to treat low strengthwastewater.

2. Major advantages are low energy requirement, low nutrient

requirement, low sludge production, however slow growth of

anaerobes has disadvantages that causes higher solid retention time

in the reactor.

3. Separate reactor conditions are needed for anaerobic process,

however partial phase separation could be achieved in UASB,

anaerobic filter, and expanded bed reactor.

4. Organic loading, pH and alkalinity, SRT and HRT, nutrients, oxidantand toxic substances affect the performance of the anaerobic process.

5. Frequency of monitoring is important for process improvement. A drop

in pH, increase in VFA/alkalinity ratio and increase of VFA in effluent

alarms the process failure.

anaerobic treatment [ppt]- [part- i] 19-03-2013

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