1 chee 370 waste treatment processes lecture #22 anaerobic digestion

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1 CHEE 370 Waste Treatment Processes Lecture #22 Anaerobic Digestion

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Page 1: 1 CHEE 370 Waste Treatment Processes Lecture #22 Anaerobic Digestion

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CHEE 370Waste Treatment

Processes

Lecture #22

Anaerobic Digestion

Page 2: 1 CHEE 370 Waste Treatment Processes Lecture #22 Anaerobic Digestion

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Sludge Handling

“50% of the cost, 90% of the headache”

Process Objectives: Reduce the volume (Remove water) Reduce the organic content Reduce # of micro-organisms and pathogens

“Stabilize the Sludge”

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Sludge Thickening

Wasted sludge (both primary and waste-activated) is “thickened” - the solids content is increased by removing some of the liquid Makes handling the sludge more manageable Reduces the required size for the anaerobic

digesters and storage tanks Minimizes the energy requirements for

subsequent processes such as heat drying

Typically accomplished by physical means

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Metcalf and EddyRead textbook pages 1488 - 1489.

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Gravity Thickening

Metcalf and Eddy

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Centrifugal Thickening

Metcalf and Eddy

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Centifugal Thickening

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Primary sludge~ 45,000 mg/L solids

Contains untreated biodegradable material Thickened wasted activated

sludge~ 40,000 mg/L solidsContains active biomass

Sludge

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Anaerobic Digestion

Involves methanogenic bacteria which grow on very simple carbon sources

2 General Types of Methanogens:1. H2-utilizing methanogens

4 H2 + CO2 CH4 + 2H202. Acetoclastic methanogens

CH3COOH CH4 + CO2

Strict anaerobes (O2 is lethal)

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Anaerobic Digestion

Anaerobic growth is a very slow process Typically operate at higher temperatures (35 °C is optimal

for methanogenic bacteria) to get high levels of conversion and minimize the reactor volume

Methanogens grow best at pH 7.0 - control the pH closely

By thickening the solids before digestion, the amount of water that needs to be heated is minimized Reduces energy requirements and cost

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Anaerobic Digestion

Objectives

1. Destruction of organic material Reduce the oxygen demand of the sludge - thereby

making it more “stable” and suitable for release to the environment

Accomplished through oxidation Methane is formed in the process Methane can be used to run the heating system for the

anaerobic digester

2. Pathogen Destruction Anaerobic digestion at 35 °C and at a solids retention time

of 15 days will lead to a high level of pathogen destruction

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Anaerobic Digestion

Objectives

3. Increase “dewaterability” of the final sludge

Wasted activated sludge cannot be thickened to more than 5% (50,000 mg/L)

Primary sludge can be thickened to as much as 6%

Sludge after anaerobic digestion can be thickened to as much as 130,000 mg/L

If dewatering is applied after digestion, the sludge can be thickened to as much as 25%

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Anaerobic Digestion

• Methanogenic degradation of complex substrates requires a concerted effort of many bacterial species

• Three Stage Process:1. Hydrolysis and Fermentation2. Acetogenesis and dehydrogenation3. Methanogenesis

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Step 1

Hydrolysis and Fermentation

Performed by various facultative bacteria Does not reduce the COD of the sludge

Carbohydrates simple sugars Proteins amino acids Sugars and amino acids fatty acids and

alcohols (fermentation) Lipids long chain fatty acids

Hydrolysis of lipids is the rate-limiting step Important for the design of the digesters

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Step 2Acetogenesis and Dehydrogenation

Organic acids and alcohols are further degraded by bacteria to produce acetic acid and H2

4 H2 + CO2 CH4 + 2H20

CH3COOH CH4 + CO2

Virtually all COD into the digester ends up as methane Methanogenic bacteria have a very low yield - only a

small amount of biomass is produced

Step 3Methanogenesis

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Anaerobic Digestion

Decomposition of organic and inorganic matter in the absence of oxygen

Main process used for the stabilization of sludge from municipal WW treatment Advances in the design of digesters have made

the process relatively economical There are beneficial uses for the digested sludge Methane produced in the digestion can be used

as fuel

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Heterotrophs vs Methanogens

Activated Sludge (Heterotrophs)

Anaerobic Digestion

(Methanogens)

Bacteria Archaea

Assume they require aerobic growth conditions

Strict anaerobic growth conditions

Typical yield range:

0.3 - 0.5 g VSS/g bCOD

Typical yield range:

0.05 - 0.10 g VSS/g COD

max at 20 C:

3 - 13.2 d-1

max at 35 C:

0.3 - 0.38 d-1

Tables 8-10 and 10-10 in the text provide ranges for other parameters.

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Effect of H2

Effective anaerobic digestion requires a diverse microbial community

Hydrogen gas partial pressure is the key to balancing the reactions and populations present in the reactor Formation of acetic acid and H2 by anaerobic oxidation is

inhibited by high ppH2

Methanogens cannot use organic acids other than acetic acid

H2-utilizing methanogens must remove H2 as fast as it is produced to allow anaerobic oxidation to proceed

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Effect of pH

Acid forming bacteria (pH 4.5 - 5) are much more tolerant of low pH than methanogens (pH 7.0)

Production of volatile fatty acids (VFAs) decreases the sludge pH Normally counterbalanced by buffering associated with

cellular CO2 production

Imbalances reduce the pH of the system, impairing methanogenesis “Stuck” or “Sour” digester Compounding problem - requires immediate attention

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Effect of Temperature Acid forming bacteria have a much higher maximum

specific growth rate (max) that changes more dramatically with temperature, as compared to methanogens

Where: kT = reaction-rate coefficient at temp. T (C) k20 = reaction-rate coefficient at 20 C = temperature-activity coefficient T = temperature (C)

kT k20(T 20)

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Digester Design Factors

1. ******Solids retention time******

2. Hydraulic retention time

3. Temperature

4. Alkalinity

5. pH

6. Presence of inhibitory substances

7. Nutrient availability

8. Methane production

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Single-Stage Digestion

Uniform feeding is importantTotal solids are reduced by ~ 50 %May have fixed or floating covers (Methane + Oxygen = Trouble)

Metcalf and Eddy Figure 14-20

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Two-Stage Digestion

Not common in current practiceFirst tank is for digestionSecond tank is primarily for storage

Metcalf and Eddy Figure 14-20

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Basic Model for Anaerobic Digestion (AD)

Want to know:1. Quantity of solids leaving the digester2. VSS and TSS destruction in the digester as a function of

the SRT3. How much methane is produced

Simple model: CSTR with no recycle HRT = SRT ( = c = V/Q)

No additional equations will be provided on the equation sheet - develop the relationships from basic mass balances and understanding of the process

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AD Model Assumptions Design based on the rate-limiting step - breakdown of

volatile fatty acids (VFAs)

Non-biodegradable fractions of COD remain unchanged by the digestion process

Heterotrophic bacteria only decays and the COD associated with decay will be accumulated as VFAs available to the methanogens

Complete hydrolysis and fermentation of biodegradable organic matter -> fully available to methanogens

Use the kinetics for the growth of the methanogens to determine the minimum SRT, then use this value with a safety factor to determine the operating conditions

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Minimum SRT Calculation

Where: umax,m = maximum specific growth rate for the methanogens Kd,m = decay rate for the methanogens

min Kvfa Svfa,available

Svfa,available (max,m kd ,m ) Kvfa kd ,m

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Factor of Safety for Growth

It is necessary to provide a factor of safety for methanogen growth (prevent “stuck” digester) and headspace

Use a factor of safety of at least 2.5

The ministry of the environment requires at least 15 days SRT at 35 C Compare with your calculation and select the larger value

design 2.5min

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Heterotroph Mass Balance

Assume there is no growth - only decay Perform a mass balance on the digester for the

heterotrophic bacteria:

As the SRT increases, the amount of active heterotrophic biomass in the effluent decreases

XH XH ,o

1 kd ,H c

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Debris Mass Balance

Debris (XD) can enter the digester in the influent (XDo) stream and is also generated during biomass decay

Perform a debris mass balance on the digester :

Where fd = debris fraction of the degraded biomass (fd ranges from ~ 0.08 - 0.20)

XD XDo fd XH ,okd ,H c1 kd ,H c

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VFAs for Methanogens

Multiple Sources: Soluble biodegradable COD (Ss)

Biodegradable particulate COD (Xs) Decay of heterotrophic biomass

Svfa,available Ss XS (1 fd )XH ,okd ,H c1 kd ,H c

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Effluent VFA and Formation of Methanogenic Bacteria

CSTR without recycle

Svfa Kvfa (1ckd ,m )

c (max,m kd ,m ) 1

Xm Ym (Svfa ,available Svfa )1c kd ,m

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Methane Production

COD balance can be performed in order to determine the amount of methane produced

CODin = Q(SSo + XSo + XHo + XDo)

CODout = Q(Svfa + XH + Xm + XD)

CODin = CODout + CODmethane produced

CH4 + 2O2 CO2 + 2H2O

64 g COD/mol CH4

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Methane Production

Use the ideal gas law to calculate the volume produced per day (V=nRT/P)

Textbook example 7-9, p. 633 - Effect Of Temp! Volume of methane produced per day:

Where mCH4 is mass-COD of CH4 produced/time

FCH4 mCH4RT

64P

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Methane Gas Production

Conversion of COD to methane gas Consider glucose (C6H12O6)

Show: 0.35 L CH4/g COD consumed at STP

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Anaerobic Treatment of a Single Compound

Assume a single substrate is fed to an anaerobic digester at a flowrate of 1 L/day and at a concentration of 10,000 mg COD/L. The effluent COD concentration is to be less than 500 mg-COD/L and the transformation will be carried out by methanogens.

REMEMBER: ALWAYS DRAW A DIAGRAM!

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Determine: The SRT [50 days] The reactor volume [50 L] The recommended design volume [125 L] The volume of methane produced per day under standard

conditions in the designed reactor [3.3 L/d] Volume of methane produced at 35 ºC [3.72 L/d]

Given: μmax= 0.15 d-1

kd=0.03 d-1

Kvfa=1000 mg COD/L

Ym=0.03 mg VSS/mg COD

Page 37: 1 CHEE 370 Waste Treatment Processes Lecture #22 Anaerobic Digestion

Previously:

If Svfa,available >> Kvfa:

And:

min Kvfa Svfa,available

Svfa,available (max,m kd ,m ) Kvfa kd ,m

min 1

(max,m kd ,m )

design 2.5min

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Digester Volume

When designing the digester, it is important to include additional “head space” for the methane gas that is produced during the anaerobic digestion

Typically, an additional 25% volume is included in the design

This increase in volume does not influence the SRT

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Anaerobic Treatment of Mixed Composition Sludge

Combined Sludge Flow m3/day 210

TSS mg-TSS/L 21619

VSS mg-VSS/L 17073

Soluble biodegradable COD mg-COD/L 87

Soluble non biodegradable COD mg-COD/L 29

Particulate biodegradable COD mg-COD/L 18733

Active biomass concentration mg-COD/L 1256

Biomass debris mg-COD/L 989

Particulate non-biodegradable COD mg-COD/L 9413

Consider a waste treatment plant where the primary sludge and wasted activated sludge (WAS) are blended and sent to an anaerobic digester. The combined sludge is determined to have the following characteristics:

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Design of the Anaerobic Digester

The digester is to be operated at an SRT of 15 days, according to the Ministry of the Environment’s minimum regulations

Assume the following parameters apply: max, m= 0.27 d-1,

kd,m= 0.03 d-1

Kvfa = 2000 mg COD/L

Ym= 0.03 mg COD/mg COD

kd,h = 0.22 d-1

fd = 0.2 VSS/TSS for particulate organic fraction = 0.90 mg VSS/mg TSS

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Calculate: Svfa,available [19590 mg-COD/L] Confirm that the ministry guideline for the SRT is

the appropriate choice for operating the system The recommended design volume for the

digester [3940 m3] Volume of methane produced per day in the

system at 35 C and 1 atm (Careful - biomass is already in COD units!) [1500 m3/d] Svfa [1115 mg-COD/L]

Xm [382 mg-COD/L]

XD [1182 mg-COD/L]

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% VSS Destruction

VSSin = 17073 mg/L (In this example)

VSSout = XH + XD + Xm + Xnon-biodeg particultes

Remember to convert from COD to VSS units % VSS destruction ~ 54 %

%VSSdestructionVSSin VSSout

VSSin100%

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% TSS Destruction

TSSin = 21619 mg/L (In this example)

TSSout = FSS + XH + XD + Xm + Xnon-biodeg particultes

Remember to convert appropriately to TSS units (given 0.9 mg VSS/mg TSS for the particulate organic fraction)

% TSS destruction ~ 47 %

%TSSdestructionTSSin TSSout

TSSin100%

Page 44: 1 CHEE 370 Waste Treatment Processes Lecture #22 Anaerobic Digestion

Methane is a usable product, and the amount of biomass produced through AD is low, so why don’t we use it for WW treatment in general?

Slow growth kinetics requires long SRT (large reactor volumes)

High temperatures required - COD present in WW will not generate sufficient methane to heat water to 35 °C

Effluent is not of sufficient quality Nitrifiers do not grow under anaerobic conditions Effluent from the digester usually contains high ammonia

concentrations Liquid stream from sludge processing is usually fed back

into the AS system

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Sludge Handling and Disposal

Heat Drying

Used to prepare the sludge for incineration or for sale as fertilizer

Sludge moisture content after drying is ~ 10% Dried sludge is termed “biosolids”

The high cost of drying and relatively low levels of nutrients in the biosolids have limited its use as a fertilizer

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Sludge Handling and Disposal

Incineration

Complete oxidation of the biosolids to produce CO2, H20, and ash

Advantages: Maximum volume reduction Destruction of persistent pathogens and toxins Potential to obtain energy

Limitations: Expensive Requires trained operators and constant monitoring Environmental impact Concerns with the disposal of the ashes

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Ultimate Disposal Ocean dumping of sludge is discouraged or prohibited Lagoons can be used for sludge disposal in remote locations

Excess liquid from the lagoon is returned back to the WW treatment plant

Landfills or land application are the most commonly used methods of disposal

Landfills are used for disposing sludge, grease, grit, and other solids Wastes are deposited in a designated area, compacted with a

tractor or roller, and covered with a 30 cm layer of clean solid or composted sludge to minimize odours and prevent attracting flies, rodents etc.

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Land Application Agricultural lands, forests, golf courses, parks … Concerns with public health risk through direct exposure or

consumption of contaminated crops and groundwater Controlling factors:

Utilization rate of nutrients by crops and vegetation Potential of plants to uptake toxic components (mainly metals)

from the sludge Accumulation of metal and salts in the soil Aesthetic

Standards and guidelines are developed based on toxicity studies and bioaccumulation within individual species and through food chains

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Review

Anaerobic Digestion

Decomposition of primary and wasted activated sludge (WAS) in the absence of oxygen Uses methanogenic bacteria Low biomass yields and methane gas production Digester modelled as a CSTR without recycle Design based on volatile fatty acids (from a

variety of sources) as the limiting substrate

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Review

Anaerobic Digester Design

Relevant Design Questions: How much methane is produced?

Solve using COD balance (64 g COD/mol CH4)

Quantity of solids leaving the digester? What is the % VSS and % TSS destruction across

the digester?

Important to always keep track of your units!