CHEE 370
Waste Treatment
Processes
Lecture #22
Anaerobic Digestion
1
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”
2
Sludge Thickening

Wasted sludge (both primary and wasteactivated) 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
3
Read textbook pages 1488 - 1489.
Metcalf and Eddy
Gravity Thickening
Metcalf and Eddy
5
Centrifugal Thickening
Metcalf and Eddy
6
Centifugal Thickening
7
Sludge
Primary sludge
~ 45,000 mg/L solids
Contains untreated
biodegradable material
Thickened wasted activated
sludge
~ 40,000 mg/L solids
Contains active biomass
Anaerobic Digestion

Involves methanogenic bacteria which grow
on very simple carbon sources

2 General Types of Methanogens:
1.
2.

H2-utilizing methanogens
4 H2 + CO2  CH4 + 2H20
Acetoclastic methanogens
CH3COOH  CH4 + CO2
Strict anaerobes (O2 is lethal)
9
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
10
Anaerobic Digestion
Objectives
Destruction of organic material
1.


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
Pathogen Destruction
2.

Anaerobic digestion at 35 °C and at a solids retention
time of 15 days will lead to a high level of pathogen
destruction
11
Anaerobic Digestion
Objectives
Increase “dewaterability” of the final
sludge
3.




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%
12
Anaerobic Digestion
•
Methanogenic degradation of complex
substrates requires a concerted effort of
many bacterial species
•
Three Stage Process:
1.
2.
3.
Hydrolysis and Fermentation
Acetogenesis and dehydrogenation
Methanogenesis
13
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
14
Step 2
Acetogenesis and Dehydrogenation
Organic acids and alcohols are further degraded
by bacteria to produce acetic acid and H2

Step 3
Methanogenesis
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
15
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
16
Heterotrophs vs Methanogens
Activated Sludge
(Heterotrophs)
Bacteria
Anaerobic Digestion
(Methanogens)
Archaea
Assume they require aerobic
growth conditions
Strict anaerobic growth
conditions
Typical yield range:
0.3 - 0.5 g VSS/g bCOD
max at 20 C:
3 - 13.2 d-1
Typical yield range:
0.05 - 0.10 g VSS/g COD
max at 35 C:
0.3 - 0.38 d-1
Tables 8-10 and 10-10 in the text provide ranges for other parameters.
17
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
18
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
19
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
kT  k20

(T 20)
Where:





kT = reaction-rate coefficient at temp. T (C)
k20 = reaction-rate coefficient at 20 C
 = temperature-activity coefficient
T = temperature (C)
20
Digester Design Factors
1.
2.
3.
4.
5.
6.
7.
8.
******Solids retention time******
Hydraulic retention time
Temperature
Alkalinity
pH
Presence of inhibitory substances
Nutrient availability
Methane production
21
Single-Stage Digestion
Uniform feeding is important
Metcalf and Eddy Figure 14-20
Total solids are reduced by ~ 50 %
22
May have fixed or floating covers (Methane + Oxygen = Trouble)
Two-Stage Digestion
Not common in current practice
First tank is for digestion
Second tank is primarily for storage
23
Metcalf and Eddy Figure 14-20
Basic Model for Anaerobic
Digestion (AD)
Want to know:

1.
2.
3.
Simple model: CSTR with no recycle



Quantity of solids leaving the digester
VSS and TSS destruction in the digester as a function of
the SRT
How much methane is produced
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
24
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
25
Minimum SRT Calculation
K vfa  Svfa,available
min 
Svfa,available  (max,m  kd ,m )  K vfa  kd ,m

Where:


umax,m = maximum specific growth rate for the
methanogens
Kd,m = decay rate for the methanogens
26
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
design  2.5  min

The ministry of the environment requires at least 15
days SRT at 35 C


Compare with your calculation and select the larger value
27
Heterotroph Mass Balance


Assume there is no growth - only decay
Perform a mass balance on the digester for the
heterotrophic bacteria:
X H ,o
XH 
1 kd , H   c

As the SRT increases, the amount of active
heterotrophic biomass in the effluent decreases

28
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 :
 k   
d,H
c
X D  X Do  f d  X H ,o 

1 kd , H   c 

Where fd = debris fraction of the degraded biomass
(fd ranges from ~ 0.08 - 0.20)
29
VFAs for Methanogens

Multiple Sources:



Soluble biodegradable COD (Ss)
Biodegradable particulate COD (Xs)
Decay of heterotrophic biomass
 k   
d,H
c
Svfa,available  Ss  X S  (1 f d )  X H ,o

1 kd , H  c 
30

Effluent VFA and Formation of
Methanogenic Bacteria

CSTR without recycle
K vfa  (1 c kd ,m )
Svfa 
c (max,m  kd ,m ) 1
(Svfa,available  Svfa )
X m  Ym 
1  c  k d ,m
31
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
32
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:
FCH 4 

mCH 4 RT
64 P
Where mCH4 is mass-COD of CH4 produced/time
33
Methane Gas Production



Conversion of COD to methane gas
Consider glucose (C6H12O6)
Show: 0.35 L CH4/g COD consumed at STP
34
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!
35

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
36

Previously:
K vfa  Svfa,available
min 
Svfa,available  (max,m  kd ,m )  K vfa  kd ,m

If Svfa,available >> Kvfa:
1
min 
(max,m  kd ,m )

And:

design  2.5  min
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
38
Anaerobic Treatment of Mixed
Composition Sludge
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:
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
Combined Sludge Flow
39
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
40
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]
% VSS Destruction
VSSin  VSSout
%VSSdestruction
100%
VSSin


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 %
42
% TSS Destruction
TSSin  TSSout
%TSSdestruction
100%
TSSin


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 %
43

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
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
45
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
46
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.
47
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
48
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
49
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!
50