Transcript
Anaerobic Treatment of
Industrial Wastewater
BioE 202 Iowa State University
Anaerobic Waste Treatment : An Overview
Historical development
Mainly used for reducing mass of high solids wastes, e.g. human wast
(nightsoil), animal manure, agricultural waste and sludge.
Early applications of anaerobic waste treatment include:
• Mouras automatic scavenger - cited in French journal Cosmos in 18
• Septic tank- developed by Donald Cameron in 1895 (England)
• Imhoff tank: developed by Karl Imhoff in 1905 (Germany)
Popularity of anaerobic processes
No. of plants
Energy crisis in 70 and 80’s- a renewed interest in anaerobic process
Anaerobic treatment plants for industrial applications (Source:
Anaerobic treatment within wastewater
processing
Domestic wastewater (100)
Bar screen, Comminutor
Grit chamber etc.
Preliminary
treatment
(100)
Primary
sedimentation
Anaerobic digester
(60)
Primary sludge
(35)
(65)
e.g. Activated sludge,
Trickling filter, RBC
Aerobic treatment
Oxidized to CO2 (30)
Converted to sludge (35)
Secondary
sedimentation
Effluent (10)
Secondary sludge
(25)
Anaerobic treatment of solids
Anaerobic treatment of high solids such as animal manure, biologic
sludge, nightsoil, etc. is commonly known as “anaerobic digestion” a
is carried out in airtight container known as an anaerobic digester (A
AD is usually a continuous flow stirred tank
reactor (CFSTR) for which HRT ~ SRT
Design based on volatile solids (VS) loading rate
Anaerobic treatment of wastewaters requires a
long SRT to achieve better treatment efficiency
The ratio of SRT/HRT ~ 10-100
The high ratio allows the slow-growing methanogens to remain
in the reactor for a longer time
How do we achieve high SRT in anaerobic treatment syste
syst
Anaerobic Waste Treatment
Anaerobic treatment is a biological process carried out in the
absence of O2 for the stabilization of organic materials by
conversion to CH4 and inorganic end-products such as CO2
and NH3
Anaerobic microbes
Organic materials + Nutrients
Biomass
CH 4 + CO2 +NH3 +
Anaerobic processes
Anaerobic fermentation Anaerobic respiration
Anaerobic fermentation
In anaerobic fermentation, there is no external electron acce
The product generated during the process accepts the electr
released during the breakdown of organic matter. Thus, orga
matter acts as both electron donor and acceptor. The proce
releases less energy and the major portion of the energy is s
contained in the fermentative product such as ethanol.
Energy
Glucose
Pyruvate
Ethanol
Electron
Anaerobic fermentation of glucose to ethanol
Anaerobic respiration
Anaerobic respiration on the other hand requires external elect
acceptor. The electron acceptors in this case could be SO42-, NO
or CO2. The energy released under such a condition is higher
than anaerobic fermentation.
Energy
Glucose
SO42CO2
NO3-
Pyruvate
Electron
CO2 + H2O
H2S
CH4
N2
Anaerobic respiration of glucose, preference for electron accepto
O2 > NO3- > SO42- >
CO2
Advantage of anaerobic processes
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 p
2. Energy generation in the form of methane gas
1.16 kWh energy is produced for every 1 kg of COD fermented in anaerobic
3. Less biomass (sludge) generation
Anaerobic process produces only 20% of sludge compared with aerobic p
Soluble BOD
1 kg
Biodegradable
COD
1 kg
Aerobic process
CO2 + H2O
0.5 kg
New biomass
0.5 kg
Anaerobic process
CH4 gas
> 0.9 kg
New biomass
< 0.1 kg
…Advantages of anaerobic processes
4. Less nutrients (N & P) required
Lower biomass synthesis rate also implies less nutrients requirement : 20% of
5. Application of higher organic loading rate
Organic loading rates of 5-10 times higher than that of aerobic processes are
6. Space saving
Higher loading rates require smaller reactor volumes thereby
saving on disposal cost
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 a longer
start-up time to attain a biomass concentration
2. Long recovery time
If an anaerobic system is 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 conditions
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 conditi
Anaerobic microorganisms especially methanogens are prone to changes in
conditions such as temperature, pH, redox potential, etc.
…Limitations of anaerobic processes
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 to generate energy for their growth and maintenance is
much higher for anaerobic treatment systems. Anaerobic processes may
not be able to degrade organic matter to the level to meet the
discharge limits for ultimate disposal.
7. Treatment of high protein & nitrogen containing waste
The anaerobic degradation of proteins produces amines which are no
longer be degraded anaerobically. Similarly nitrogen remains
unchanged during anaerobic treatment. Recently, a process called
ANAMMOX ( ANaerobic AMMonium OXididation) has been developed to
anaerobically oxidize
NH4+- to N2 in presence of nitrite.
+
NH4
+ NO2
N2 + 2H2O
NH4+ + 1.32 NO2- + 0.066CO2 + 0.13H+ 1.02 N2 + 0.26NO3- + 0.066CH2O0.5N0.15
Comparison between anaerobic and aerobic proces
Anaerobic
Aerobic
Organic loading rate
High loading rates:10-40 kg COD/m3-day
Low loading rates:0.5-1.5 kg COD/m3-da
(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 COD
High biomass yield:0.35-0.45 kg VSS/kg C
(biomass yield is not constant but depends
(biomass yield is fairly constant irrespectiv
on types of substrates metabolized)
of types of substrates metabolized)
Specific substrate utilization rate
High rate: 0.75-1.5 kg COD/kg VSS-day Low rate: 0.15-0.75 kg COD/kg VSS-day
Start-up time
Long start-up: 1-2 months for mesophilic Short start-up: 1-2 weeks
: 2-3 months for
thermophilic
Comparison between anaerobic and aerobic proces
Anaerobic
Aerobic
SRT
Longer SRT is essential to retain the slowSRT of 4-10 days is enough for
growing methanogens within the reactorthe activated sludge process
Microbiology
Anaerobic processes involve multiAerobic process is mainly a onestep
species phenomenon, except for
chemical conversions and a diverse
nutrient-removal processes
group of microorganisms degrade
the organic matter in a sequential
order
Environmental factors
The process is highly susceptible to
changes in environmental conditions
The process is more robust to
changing environmental
conditions
How much methane gas can be generated through
complete anaerobic degradation of 1 kg COD at STP ?
Step 1: Calculation of COD equivalence of CH 4
CH4
16 g
+
2O2
CO2 + 2H2O
64g
16 g CH4 ~ 64 g O2 (COD)
1 g CH4 ~ 64/16 = 4 g COD ------------
(1)
Step 2: Conversion of CH4 mass to equivalent volume
Based on the ideal gas law, 1 mole of any gas at STP (Standard Temperature
and Pressure) occupies a volume of 22.4 L
1 Mole CH4
~
22.4 L CH4
16 g CH4
~
22.4 L CH4
1 g CH4
~
22.4/16 = 1.4 L CH4 ---------(2)
Step 3: CH4 generation rate per unit of COD removed
From eq. (1) and eq. (2), we have,
=> 1 g CH4
~
4 g COD
~
1.4 L CH4
=> 4 g COD
~
1.4 L CH4
=> 1 g COD
~
1.4/4 = 0.35 LCH4
or 1 Kg COD
~
0.35 m3 CH4
----------(3)
Complete anaerobic degradation of 1 kg COD
produces 0.35 m3 CH4 at STP
Organics Conversion in Anaerobic
Systems
hydrolysis
methanogenesisacetogenesis
acidogenesis
COMPLEX ORGANIC MATTER
Carbohydrates
Proteins
Lipids
Amino Acids,
Sugars
Fatty Acids,
Alcohols
INTERMEDIARY PRODUCTS
(C>2; Propionate, Butyrate
etc)
Acetate
Hydrogen, Carbon
dioxide
28
72
Methane
Carbon
dioxide
Process Microbiology
The anaerobic degradation of complex organic matter is
carried out by
a series of bacteria and archeae as indicated in the figure (with
numbers). There exists a coordinated interaction among these
microbes.
The process may fail if a certain of these organisms are inhibited.
Fermentative bacteria (1)
This group of bacteria is responsible for the first stage of
anaerobic digestion - hydrolysis and acidogenesis. These
bacteria are either facultative or strict anaerobes.
The anaerobic species belonging to the family of
Streptococcaceae and
Enterobacteriaceae and to the genera of Bacteroides,
Clostridium,
Butyrivibrio, Eubacterium, Bifidobacterium and Lactobacillus
are most common.
Hydrogen producing acetogenic bacteria (2)
This group of bacteria metabolizes propionate and
other organic acids (>C-2), alcohols and certain
aromatic compounds (i.e. benzoate) into acetate
and CO2
CH3CH2COO
CH3COO - + CO2 + H2
Syntrophic association of acetogenic organisms with
methanogenic H2- consuming bacteria helps to lower
the concentration of H2 below inhibitory level so that
propionate degrading bacteria are not suppressed by
excessive H2 level
H2 partial pressure 10-2 (100 ppm)
Homoacetogenes
(3)
Homoacetogenesis has gained much attention in recent
years in anaerobic processes due to its final product:
acetate, which is the important precursor to methane
generation.
The bacteria are, H2 and CO2 users. Clostridium
aceticum and Acetobacterium woodii are the two
homoacetogenic bacteria isolated from the sewage
sludge.
Homoacetogenic bacteria have a high thermodynamic
efficiency; as a result there is no accumulation H 2 and
CO2 during
growth
on multi-carbon
compounds.
CO2 +
H2
CH
3COOH +
2H2O
Methanogens (4 and 5)
Methanogens are unique domain of microbes classified as
Archeae, distinguished from Bacteria by a number of
characteristics, including the possession of membrane lipids,
absence of the basic cellular characteristics (e. g.
peptidoglycan) and distinctive ribosomal RNA. Methanogens
are obligate anaerobes and considered as a rate-limiting
species in anaerobic treatment of wastewater. Moreover,
methanogens co-exist or compete with sulfate-reducing
bacteria for the substrates in anaerobic treatment of sulfateladen wastewater.
Two classes of methanogens that metabolize acetate to methane
• Methanosaeta (old name Methanothrix): Rod shape, low Ks, high affi
• Methanosarcina (also known as M. mazei): Spherical shape, high Ks,
low affinity
Growth kinetics of Methanosaeta
andMethanosarcina
0.20
Specific growth rate,
, d
-1
Methanosaeta
Methanosarcina
0.16
0.12
0.08
0.04
0.00
0
50
100
150
200
Acetate concentration, mg COD/L
250
300
Essential conditions for efficient anaerobic
treatment
• Avoid excessive air/O2
exposure
• No toxic/inhibitory compounds present in the
influent
• Maintain pH between 6.8 –7.2
• Sufficient alkalinity present (mainly bicarbonates)
• Low volatile fatty acids (VFAs)
• Temperature around mesophilic range (30-38
o
C)
• Enough nutrients (N & P) and trace metals especially, Fe, Co,
Ni, etc.
COD:N:P = 350:7:1 (for highly loaded system) 1000:7:1
(lightly
• loaded
SRT/HRT
>>1 (use high rate anaerobic reactors)
system)
Best industrial wastewaters for anaerobic treatment
• Alcohol production
• Brewery and Winery
• Sugar processing
• Starch (barley, corn, potato, wheat, tapioca) and
desizing
waste from textile industry.
• Food
processing
• Bakery plant
• Pulp and paper
• Dairy
• Slaughterhouse
• Petrochemical waste
Environmental factors
The successful operation of anaerobic reactor depends on
maintaining the environmental factors close to the comfort of
the microorganisms
involved
in the process.
Temperature
Anaerobic processes like other biological processes
operate in certain temperature ranges
In anaerobic systems: three optimal temperature
ranges:
Psychrophilic (5 - 15oC)
Mesophilic
Thermophilic (50-55 oC)
(35 – 40 C)
Effect of temperature on anaerobic activity
Rule of thumb: Rate of a reaction doubles for every 10 oC
rise in temperature up to an optimum and then declines rapidly
pH
There exist two microbial domains in terms of pH optima namely
acidogens and methanogens. The best pH range for acidogens
is 5.5 – 6.5 and for methanogens is 7.8 – 8.2. The operating pH
for combined cultures is 6.8-7.4 with neutral pH being the
optimum. Since methano-genesis is considered as a rate
limiting step, it is necessary to maintain the reactor pH close to
neutral.
Low pH reduces the activity of methanogens causing
accumulation of VFA and H2. At higher partial pressure of H2,
propionic acid degrading bacteria will be severely inhibited
thereby causing excessive accumulation of higher molecular
weight VFAs such as propionic and butyric acids and the pH
drops further. If the situation is left uncorrected, the process
may eventually
fail. This
condition
knownrates
as going
Remedial
measures:
Reduce
the is
loading
and “SOUR” or
STUCK”.
supplement
chemicals to adjust the pH: alkaline chemicals such as
NaHCO3, NaOH, Na2CO3, quick lime (CaO), slaked lime
[Ca(OH)2], limestone (or softening sludge) CaCO3, and NH3 can
pH dependence of methanogens
Relative activity of methanogens to pH
1.3
Activity
1.0
0.8
0.5
0.3
0.0
3
4
5
6
7
pH
8
9
10
11
Cont..
Natural buffering
Cont..
An anaerobic treatment system has its own buffering
capacity against
pH drop because of alkalinity produced during waste
treatment: e.g. the degradation of protein present in the
waste releases NH3, which
NH3 + H2O + CO2 NH4HCO3
reacts with CO2 forming ammonium carbonate as alkalinity.
The degradation of salt of fatty acids may produce some
alkalinity.
CH3COONa + H2O CH4 + NaHCO3
Sulfate and sulfite reduction also generate alkalinity.
CH3COO - + SO42- HS- + HCO3- +
3Hstarts
When pH
to drop due to VFA accumulation, the
2O
alkalinity present within the system neutralizes the acid and
prevents further drop in pH. If the alkalinity is not enough to
buffer the system pH, we need external additions.
Nutrients and trace metals
Cont..
All microbial processes including anaerobic require macro
(N, P and S) and micro (trace metals) nutrients in sufficient
concentration to support biomass synthesis. Anaerobic
micro-organisms, especially methanogens, have specific
requirements of trace metals such as Ni, Co, Fe, Mo, Se
etc. The nutrients and trace metals requirements for
anaerobic process are much lower as only 4 - 10% of the
COD removed is converted to biomass.
COD:N:P = 350:7:1 (for highly loaded system)
1000:7:1 (lightly loaded system)
Inhibition/Toxicity
The toxicity is caused by substances present in the influent
waste or byproducts of metabolic activities. Heavy metals,
halogenated compounds, and cyanide are examples of the
former type whereas sulfide and VFAs belong to latter .
Ammonia from either group
Types of anaerobic reactors
Low-rate anaerobic reactors
Anaerobic pond
High-rate anaerobic reactors
Anaerobic contact process
Anaerobic filter (AF)
Septic tank
Upflow anaerobic sludge
blanket (UASB)
Imhoff tank
Fluidized bed reactor
Standard rate
anaerobic digester
Hybrid reactor: UASB/AF
Anaerobic sequencing batc
reactor (ASBR)
Slurry type bioreactor, temperature, Able to retain very high concentration
mixing, SRT or other environmental of
conditions are not regulated.
active biomass in the reactor.
3
Loading of 1-2 kg COD/m -day
Thus extremely high SRT could be
maintained
Anaerobic contact process (ACP)
Anaerobic contact process is essentially an anaerobic
activated
sludge process. It consists of a completely mixed reactor
followed
by a settling tank. The settled biomass is recycled
back to the
reactor. Hence ACP is able to maintain high
.
concentration of
Biogasand thus
Biogas high SRT irrespective
biomass in the reactor
Settling tank
of HRT.
Degasifier allows the removal of biogas bubbles (CO 2,
Effluent
CH4) Influent
Degassifier
Completely mixed
attached to sludge
which may otherwise float to the
reactor
surface.
Recycled sludge
Waste sludge
…Anaerobic contact process (ACP)
Cont..
ACP was initially developed for the treatment of dilu
wastewater such as meat packing plant which had tenden
to form a settleable flocs. ACP is suitable for the treatme
of wastewater containing suspended solids which ren
the microorganisms to attach and form settleable flocs.
The biomass concentration in the reactor ranges from 4-6
with maximum concentration as high as 25-30 g/L depend
on settleability of sludge. The loading rate ranges from 0.
10 kg COD/m3-day. The required SRT could be maintained
controlling the recycle rate similar to activated sludge pro
Anaerobic filter
• Developed by Young and McCarty in the late 1960s
to treat dilute soluble organic wastes
• The filter was filled with rocks similar to the trickling
filter
• Wastewater distributed across the bottom and the
flow was in the upward direction through a bed of
rocks
• Anaerobic microorganisms accumulate within voids of media
• Whole filter submerged completely
(rocks or other plastic media)
• The media retain or hold the active biomass within the filte
• The non-attached biomass within the interstices forms bigge
flocs of granular shape due to rising gas bubble/liquid
• Non-attached biomass contributes significantly to waste trea
• Attached biomass not be a major portion of total biomass
• 64% attached and 36% non-attached
Upflow Anaerobic Filter
Heate
r
Biogas
Effluent
Perforated
Al plate
Sampling
port
Water bath
Peristaltic pump
Media
Feeding
tank at 4oC
Constant
recirculation
line
temperature
Peristaltic pump
Sludge wastage
Anaerobic Filter Packing
Cont..
Originally, rocks were employed as packing medium
anaerobic filter. But due to very low void volume (40-50
serious clogging problems were witnessed. Now, many
synthetic packing media are made up of plastics; ceram
of different configuration have been used in anaerobic fil
The void volume in these media ranges from 85-95
Moreover, these media provide high specific surface a
typically 100 m2/m3, or above, which enhances biofilm gr
Anaerobic Filters
Cont..
Since anaerobic filters are able to retain high biomass, a
long SRT can be maintained. Typically HRT varies from 0.5
– 4 days and the loading rates vary from 5 - 15 kg
COD/m3-day. Biomass
wastage is generally not needed and hydrodynamic
conditions
Downflow
anaerobic
(DAF)
play
an important
role infilter
biomass
retention within the
void
space. anaerobic filters are similar to a trickling
Downflow
filter in operation. DAF is closer to fixed film reactor as
loosely held biomass/sludge within the void spaces is
potentially washed out of the reactor. The specific
surface area of media is more important in DAF than
UAF.
There is less of a clogging problem and wastewater
with some SS concentration can be treated using DAF.
Upflow Anaerobic Sludge Blanket (UASB)
UASB was developed in 1970s by Lettinga in the
Netherlands.
UASB is essentially a suspended growth system in
which proper hydraulic and organic loading rate is
maintained in order to facilitate the dense biomass
aggregation known as granulation. The size of granules
is about 1-3 mm diameter. Since granules are bigger in
size and heavier, they will settle down and be retained
within the reactor. The concentration of biomass in the
reactor
may become
as hydrolytic
high as 50bacteria,
g/L. Thus a very
The granules
consist of
high
SRT can be achieved even at a very low HRT of 4
acidogen/acetogens
hours.
and methanogens. Carbohydrate degrading granules
show
layered structure with a surface layer of
hydrolytic/fermentative
UASB Reactor
Effluent
biogas
Influent
Static granular bed reactor (SGBR)
• Developed at Iowa State University by Drs. Ellis and Kris
• Just opposite to UASB; flow is from top to bottom and the
is static
• No need of three-phase separator or flow distributor
• Simple in operation
with fewer
moving
parts
• Major issue: head loss
due to build-up of solids
Effluent
Effect of sulfate on methane production
When the waste contains sulfate, part of COD is
diverted to sulfate reduction and thus total COD
available for methane production would be reduced
greatly.
Sulfide will also impose toxicity on
methanogens at a
concentration of 50 to 250 mg/L as free
sulfide.
Stoichiometry of sulfate reduction
8e +8H+ + SO42-
8e +8H+ + 2O2
S2- + 4H2O
4H2O
2O2/ SO42- = 64/96 ~ 0.67
•
COD/SO42- ~ 0.67
Theoretically, 1 g of COD is needed to reduce 1.5 g of sulfate
Example 2
A UASB reactor has been employed to treat food processing
wastewater at 20oC. The flow rate is 2 m3/day with a mean
soluble COD of 7,000 mg/L. Calculate the maximum CH 4
generation rate in m3/day. What would be the biogas
generation rate at 85% COD removal efficiency and 10% of the
removed COD is utilized for biomass synthesis. The mean CH 4
content of biogas is 80%. If the wastewater contains 2.0 g/L
sulfate, theoretically how much CH4 could be generated?
Solution:
Maximum CH4 generation rate:
The complete degradation of organic matter in the waste could
only lead to maximum methane generation and is also regarded
as theoretical methane generation rate.
Cont..
(7000 x 10-6)
Total COD removed = ----------------- x (2) kg/d
(10-3)
= 14 kg/d
From eq. (3) in example 1, we have :
1 Kg COD produces 0.35 m3 CH4 at STP
14 Kg COD produces ~ 0.35 x 14 = 4.9 m3 CH4/d at STP
At 20C, the CH4 gas generation = 4.9 x(293/273)
= 5.3 m3/d
The maximum CH generation rate = 5.3 m3/d
Cont..
Biogas generation rate
Not all COD (organic matter) is completely degraded. The fate of
COD during anaerobic treatment process can be viewed as :
Residual COD (in effluent)
COD converted to CH4 gas
COD diverted to biomass synthesis
COD utilized for sulfate reduction (if sulfate is present)
(7000 x 10-6)
Total COD removed = ------------- x (2) x 0.85 kg/d
(10-3)
= 11.9 kg/d
Cont..
As 10% of the removed COD has been utilized for biomass synthesis
remaining 90% of the removed COD has thus been converted to CH 4 gas.
COD utilized for CH4 generation
=
11.9 x 0.9 kg/d
=
10.71 kg/d
From eq. (3) in example 1, we have:
1 Kg COD produces 0.35 m3 CH4 at STP
10.71 Kg COD produces ~ 0.35 x 10.71 = 3.75 m3 CH4/d at STP
At 20C, the CH4 gas generation
= 3.75 x (293/273)
=
4.02 m3/d
The bio-gas generation rate is larger as it also contains CO 2 and H2S4C.02/0.80
= 5.03 m3/d
Cont..
Methane generation rate when sulfate is present
When the waste contains sulfate, part of COD is
diverted to sulfate reduction and thus total COD
available for methane production would be reduced
greatly.
Sulfate-reducing bacteria
Organic matter + Nutrients + SO42biomass
H2S + H2O + HCO3- + New
Theoretically, 1 g COD is required for reduction of 1.5 g sulfate
In this problem, there is 2 g/L sulfate.
Total COD consumed in sulfate reduction = 1.33g = 1330 mg
COD available for methane production
= (7000 –1330) mg/L
= 5670 mg/L
Cont..
(5666.67 x 10 )
Total COD available = ----------------- x (2) kg/d
for CH4 generation
(10-3)
= 11.33 kg/d
From eq. (3) in example 1, we have:
1 kg COD produces 0.35 m3 CH4 at STP
11.33 kg COD produces ~ 0.35 x 11.33 = 3.97m3 CH4/d at STP
At 20C, the CH4 gas generation = 3.97 x(293/273)
= 4.3 m3/d
The CH4 generation rate when sulfate is present = 4.3 m3/d
-6
Presence of sulfate reduces methane yield by about 19%
However, total biogas will now contain more H2S, adding volume
Expanded bed reactor (EBR)
• Expanded bed reactor is an attached
growth system with some suspended biomass.
•
The biomass gets attached on biocarriers such as sandman, pulverized
polyvinyl chloride, shredded tire beads.
• The bio-carriers are expanded by the upflow
velocity of influent wastewater and
recirculated effluent.
• In the expanded bed reactor, sufficient
upflow velocity is maintained to expand the
bed by 15-30%.
• The expanded bed reactor has less
clogging problems and better substrate
diffusion within the biofilm.
• The biocarriers are partly supported by fluid
flow and partly by contact with adjacent
biocarriers, which retain the same relative
position within the bed.
Fluidized bed reactor (FBR)
• FBR is similar to EBR in terms of
configuration. But FBR is truly fixed film
reactor as suspended biomass is washed–out
due to high upflow velocity.
• The bed expansion is 25-300% of the
settled bed volume, which requires
much higher upflow velocity (10-25
m/hr).
• The bio-carriers are supported
entirely by the upflow liquid velocity
and therefore able to move freely in the
bed.
• The fluidized bed reactor is free from
clogging problem short-circuiting and
better substrate diffusion within the
biofilm.
Hybrid system: UASB/AF
Hybrid system incorporates
both granular sludge blanket
(bottom) and anaerobic filter
(top). Such approach prevents
wash-out of biomass from the
reactor. Further additional
treatment at the top bed due to
the retention of sludge granules
that escaped from the bottom
sludge bed.
UASB reactor facing a chronic
sludge wash-out problem can be
retrofitted
usingmay
this be
approach.
Hybrid system
any
combi-nation of two types of
Anaerobic baffled reactor
In anaerobic baffled reactor, the wastewater passes o
and under the baffles. The biomass accumulates
Between the baffles which may in fact form granules
time. The baffles present the horizontal movement
of biomass in the reactor. Hence a high concentration
biomass can be maintained within the reactor.
Biogas
Sludge
Anaerobic Sequential Bed Reactor
BIOGAS RECYCLE
BIOGAS
SUPERNATANT
DECANT
PORTS
SETTLED
BIOMASS
SETTLE
DECANT
FEED
EFFLUENT
REACT
FEED
Anaerobic process
design
Design based on volumetric organic
loading rate (VOLR)
So . Q
VOLR = --------V
VOLR : Volumetric organic loading rate
(kg COD/m3-day)
So
: Wastewater biodegradable COD (mg/L)
Q
: Wastewater flow rate (m3/day)
V
: Bioreactor volume (m3)
VOLR can be selected!
Efficiency, %
So and Q can be measured easily and are
known upfront
How do we select
VOLR?
Conducting a pilot scale studies
VOLR
Find out removal efficiency at different VOLRs
Select VOLR based on desired efficiency
Design based on hydraulic loading rate
V =
a . Q
a . Q
A =
-------H
Permissible superficial velocity
(Va)
Va = H/ For dilute wastewater with
COD < 1,000 mg/L
H
: Reactor height (m)
a
: Allowable hydraulic retention time (hr)
Q
: Wastewater flow rate (m3/h)
A
: Surface area of the reactor (m2)
Solids retention time (SRT)
An anaerobic digester is a completely mixed reactor for
which solid retention time(SRT) and hydraulic retention time
(HRT) is the same.
Influent flow rate
(Q), m3/day
V, m3
Volume
HRT, days =
=
Flow rate
V (m3)
day
Q (m3/day)
For a given SRT (HRT), the size of reactor can
be
easily determined since flow rate (Q) is known
Digester
volume, V (m3) = Flow rate (Q) x SRT ( C )
to us
Volatile solids loading rate
The size of an anaerobic digester can also be
estimated based on volatile solids loading rate
expressed as kg VS/m3-day.
Influent VS
kg/day
V, m3
Volatile solids
Influent VS (kg/day)
loading rate, =
(kg VS/m3- day)
Reactor volume
3
(m
) loading rate, the size of reacto
For a given volatile solids
be easily determined since influent VS (kg/day) is known
Digester volume, V (m3)=
Influent VS (kg/day)
Volatile solids loading rate,(kg VS/m3-
Green cow power
Methane for power generation
The $4.9 million facility
near West Amana produces
methane biogas that
powers four electric
generators.
The system produces about
2.6 MW of power or 15% of
Amana Service Co.’s base
load electricity in winter
and 10% in summer.
The digester uses feeder cattle manure from Amana
Farms and industrial and food processing waste from
such industries as Genencor International, Cargill and
International Paper’s Cedar River Mill in Cedar Rapids.
What happens to the leftovers?
Common misconceptions about anaerobic digesters include that
anaerobic digestion and the resulting biogas production will reduce the
quantity of manure and the amount of nutrients that remain for
utilization or disposal. An anaerobic digester DOES NOT MAKE MANURE
DISAPPEAR! Often the volume of material (effluent) handled from a
digester increases because of required dilution water for satisfactory
pumping or digester operation. On average, only 4% of the influent
manure is converted to biogas. None of the water! The remaining 96%
leaves the digester as an effluent that is stable, rich in nutrients, free of
weed seed, reduced or free of pathogens, and nearly odorless. This
means that a farm loading 1,000 gallons per day into a digester can
expect to have 960 gallons of material (effluent) to store and ultimately
utilize. Depending on digester design and operation, solids can also
settle out in the bottom of the digester and/or form a floating scum mat.
Both the scum mat and the solids will eventually need to be
mechanically removed from the digester to assure desired performance.
When evaluating the actual performance and operation of a digester, it
is important to determine and account for the amount and type of
material retained in the digester and the cost of lost digester volume
and ultimate cleaning.