UASB reactors

UASB Reactors
(Upflow Anaerobic Sludge Blanket Reactors)
Dr. Akepati S. Reddy
School of Energy and Environment
Thapar University, Patiala
PUNJAB, INDIA

UASB Reactor
• Developed in 1970s by Prof. Gatze Lettinga, The Netherlands
• Suitable for high strength wastewater – still used for domestic
sewage in warm climates
• Differs from other anaerobic reactors in
– Existence of granular sludge (resistant to toxic shocks) with
• High mechanical strength & good settling properties (30-80 m/hr.)
• High methanogenic activity (0.5-2 COD/VSS.day)
– Internal 3-phase GSL (gas-solid-liquid) separator system
• Advantages
– Compact and require less land
– Low energy consumption, low operating costs and satisfactory COD
removal efficiencies (65-75%)
– Low sludge production, high levels of concentration and good
dewatering sludge
• Disadvantages:
– Bad odours, inability to tolerate toxic loads; start-ups requiring quite
long time; and wastewater needs further treatment

UASB Reactor
• A primary treatment unit – clarification sewage occurs
– Stabilization of the sludge also occurs
– To some extent even secondary treatment occurs
• UASB reactor includes two zones
– Reactor zone
• Sludge bed zone
• Sludge blanket zone
• Contain granular or flocculant sludge
– Settling zone
• 3-phase separator
• Inclusive of Gas system and Effluent system
• Includes
– Influent distribution system
• Flow division and distribution boxes
• Distribution tubes and necessary piping and fittings
– Sludge discharge system
– Provisions for sampling

Raw sewage inlet
Division box
Distribution
box
Distribution
box
Distribution
box
Feed box
UASB Reactor-1 UASB Reactor-2
Treated effluent

UASB Reactor
Sewage inlet Biogas outletBiogas outlet
Feed box
Distribution pipes
Effluent trough
Biogas collection tunnelBiogas collection tunnel
Diflector
Diflector pillar
Sludge bedSludge bed
Sludge blanket Sludge blanket
Sludge drains
Sampling ports
Diflector
Clarification
zone
Clarification
zone
Clarification
zone
Clarification
zone
Reactor zone Reactor zone

Basis for Design
There are no mathematical models and no clear guidelines for
the design – empirical equations are used in the design
• Two approaches, organic loading rate approach and upflow
velocity approach, are followed in the design
– Organic loading rate approach is followed when COD of
wastewater is > 5000-15000 mg/L
• Typical range of organic loading rate is 4-12 kg COD /m3.day
(average loading is 10 kg COD/m3.day)
– Up-flow velocity approach is followed when COD is <5000 mg/L
– followed for municipal sewage
• Typical up-flow velocity is 0.6-0.9 m/hr. (Volumetric hydraulic
loading rate is < 5 m3/m3.day and HRT is 4-12 hours)
• HRT, sludge concentration in the sludge blanket and in the
sludge bed, minimum SRT required and velocity through the
aperture are also considered in the design

Design of UASB for Sewage
• In case of domestic sewage, when temperature is >15°C, if
sufficient alkalinity is available,
– The organic loading rate can be >1.5 kg/m3.day (2.5 to 3.5
kg/m3.day)
– HRT can be >4-6 to 16 hours
• Organic loading rate for the domestic sewage and for the
wastewaters with COD <1000 mg/l, is 2.5 to 3.5 kg/m3.day
– higher organic loading rates result in excessive hydraulic loads,
and higher up-flow velocities
Sewage
temperature
Hydraulic detention time (HRT in hrs)
for average flow for peak flow
16-19 10-14 7-9
20-26 6-9 4-6
>26 >6 >4

Biological loading rates
• During start-up, the biological loading rate may be maintained
in the range of 0.05 to 0.15 kg COD per kg VSS.day
– Excessive loading rates can affect process stability (pH and VFA)
• Maximum biological loading rates depend on the
methanogenic activity of the sludge
– for domestic sewage 0.3 to 0.4 kg COD/kg VSS.day is the limit
Upflow velocity
• Maximum upflow velocities depend on the type of sludge
present and the organic loading rates applied
• For sewage, granular sludge is not formed and reactor is
designed for 0.5 m/hr. upflow velocity and 4 hour HRT
– For the flocculant sludge and for the organic loading rate of 5-6
kg COD/m3.day average upflow velocity is 0.5 to 0.7 m/hr.
– 0.9 - 1.1 m/hr. for peak flows (upto 10 m/hr for granular sludge)
– can be upto 1.5 m/hr for 2 – 4 hr. persistent maximum flow

Reactor height
• Depends on
– Type of the sludge present in the reactor
– Organic loading rates applied
– Volumetric hydraulic loading rates applied
• Includes height of the sludge layer and of the sedimentation
• Sludge layer height is 2-5 m for COD <3000 and 5-7 m for COD
>3000 mg/L and settling zone height is ≥1.2 m
• For the reactors treating domestic sewage sludge layer height is
2.5 to 3.5 m and settler is 1.5 to 2.0 m
• Up-flow velocity, reactor height and HRT are closely related
– Height is 3-6m for up-flow velocities <1.0 m/hr & HRTs 6-10 hrs
Reactor volume: HRT * Flow rate
– HRT is >4 hours (or hydraulic loading rate is <6 m3/m3.day)

Influent Distribution System
Feed water distribution should accomplish
– Optimal contact between the sludge and the sewage
– Avoid hydraulic short circuits and formation of dead zones
– Prevent channelling (high gas production rates minimizes)
Even distribution of the influent is more important in the lower
part of the reactor at lower temp. for low strength waste
water – low biogas production do not allow proper mixing
Short circuiting can also be caused by
– Short heights of the sludge bed
– Fewer number of influent distributors
– Concentrated sludge with high settling velocity
Distribution system includes division boxes, piping and fittings,
distribution boxes, and distribution tubes
Distribution boxes are installed over the reactor and distribution
tubes are issued from it
– A blocked tube can be easily detected

Distribution tubes
• Sewage velocity should be <0.2 m/sec. to avoid bubble dragging
– Maintaining anaerobic conditions becomes difficult
– Air bubbles can result in potentially explosive air-biogas mixtures in
the 3-phase separator
• Diameter of the tube should be 75-100mm to avoid frequent
blocking (effluent screening can help)
• Lower ends of the tubes should be installed at predetermined fixed
points
• Lower ends of the tubes should have nozzles to increase the tip
velocity to >0.4 m/sec. to allow good mixing and greater contact
with the sludge
– Tubes can have 40-50 mm diameter nozzle at the tip or, alternatively,
the tubes can have side apertures (windows) of 25x40 mm size
• The tubes should allow easy cleaning

Multiple cone bottom of the reactor can be helpful
Number of distribution tubes to be used depends on the basis of
– Area of influence of each of the tubes
– Total area of the reactor
Area of influence usually ranges between 1 and 4 m2 -
– For reactors treating domestic sewage it is 1.5 to 3 m2 (2-3 m2 is
recommended when COD is 400 to 600 mg/L)
Area of influence is a function of the type of sludge and the
organic loading rates applied
– More for flocculating sludge than for granular sludge
– Increases with increasing organic loading

Three Phase Separator
• Liquid entering the settling zone should be free from biogas
bubbles
– Deflectors overlapping with gas collection hoods ensure this
• Within hoods enough gas-liquid interface is needed to allow
release of gas bubbles breaking the scum layer
– Possible foaming should be taken care of within hoods
• Settling zone should have sloping bottom to allow sliding of
settled sludge into the reactor
– Depth of settling zone and upflow velocity in the settling zone
should ensure efficient clarification of the effluent
– Aparture between hoods should be big enough to allow settled
sludge return into the reactor zone
• For collecting the clarified effluent collection troughs with
enough weir length are needed
– Scum baffles should be provided in front of the overflow weirs

Three-phase Separator
Low upflow velocities, absence of gas bubbles, sufficient depth
of sedimentation compartment are important
• Depth of sedimentation zone should be 1.5 to 2 m
• Slopes of sedimentation surfaces should be 45-60
• HRT should be 1.5-2.0 hr. for average flow, >1.0 for 2-4 hour
persistent maximum flow and >0.6 hr for peak flows
• Aperture area between gas collection hoods should be 15-20% and
gas dome edge overlapping should be 200-300 mm
• Hydraulic surface loading rates should be
– 0.8 m/hr for average flow
– <1.2m/hr for 2-4 hour persistent maximum flow
– <1.6 m/hr for temporary peak flow
• Velocities in apertures are
– <2.0-2.3 m/hr for average flow
– <4.0-4.2 m/hr for 2-4 hr persistent max. flow
– <5.5-6.0 for temporary peak flows

Effluent collection
• Plates with V-notch weirs and submerged perforated tubes
are used for the effluent collection
• Scum baffle submerged 20 cm can be part of the launder with
V-notches
• Submerged perforated tubes
– Eliminates the risk of turbulence and release of gases and bad
odors and do not need scum baffles
– Solids can accumulate in holes and inside the perforated tubes
– For self-cleaning 1% slope is recommended

Gas system
• Includes provisions for biogas collection, conveyance, storage,
metering and disposal (either used or burnt)
– Collection includes a sealed compartment with hydraulic seal and
biogas purge
– Gas production rate and gas composition (CO2 and H2S) may need
monitoring
– Metering of the gas is essential for evaluating process efficiency
– If biogas is to be flared then gas reservoir can be replaced by a
security valve (flame trap!) and gas burner
– For avoiding drag of condensed liquids flow velocity in the piping is
maintained <3.5 m/sec.
• Liquid gas interface is maintained in the gas collection hoods for
facilitating easy release of gas bubbles
– Adjusting the overflow weir height
– Adjusting the pressure of biogas in the hoods
• Gas collection hood caps may have antifoam nozzles

Sludge Sampling and Discharge System
Sampling system includes a series of valves installed along the
height of the reactor compartment
• Helps to determine solids profile of the reactor and facilitates
establishment of sludge discharge strategies
• Helps in evaluating specific methanogenic activity and sludge
characteristics and knowing the ideal sludge discharge points
• Sludge sampling points can be 5 or 6, spaced at 50 cm distance - 1.5
to 2 inch dia. piping with ball valves can be used as sampling ports
• Monitoring and control of temperature and pH at different heights
may be needed
• VFA and alkalinity measurement may also be needed for the
process control
Sludge discharge system (meant for the removal of inert
material and excess sludge accumulating at the bottom)
• At least two sludge discharge points, one closer to the reactor
bottom and the other at 1-1.5 m height, to remove sludge from the
sludge bed zone and the blanket zone respectively - a third drain
can also be provided 0.5 m below the settling zone
• Sludge discharge piping can be of >100 mm diameter

Sludge Discharge System
• Determined by incoming TSS, TSS lost in effluent, TSS
hydrolyzed, sludge synthesized and TSS withdrawn as sludge
• Y (yield coefficient) is taken as 0.1 to 0.14 of COD removed
• Minimum SRT required is 3xTd
• Td is doubling time for methanogenic biomass
• SRT required depends on temperature
• It is 140 days for 15°C, 100 days for 20°C, 60 days for 25°C, 30
days for 30°C, 20 days for 35°C and 15 days for 40°C
• Sludge discharged in one batch should not be beyond 20-25%
of the total sludge present in the reactor
• Sludge is sufficiently stabilized, has good dewaterability , its
density is 1020-1040 kg/m3 and consistency is 3-5%
• Can be sent directly to the dewatering units (sludge drying
beds?)

Materials of Construction
Risk of corrosion is high
– Above liquid level by H2S (oxidized to SO4
- & cause corrosion)
– Below water level CO2 dissolves concrete at lower pH
Concrete and steel with an internal coating in an epoxy base, or
plastic fortified plywood can be used
Measures to minimize corrosion concrete structures
– Selection of appropriate cement
– Low water cement ratio
– Rigorous vibration of the concrete
– Adequate curing
– Use of special additives
– Acid resistant coatings/linings
– Painting with chlorinated rubber or bituminous epoxy
PVC, fiber glass and stainless steel for the solids and gas
separator (most exposed to corrosion)

Treatment Efficiencies
• Can remove COD by 70-80%, TSS by 70-80%, pathogens by 70-
90% and helminth eggs with 100% efficiency in case of
domestic sewage
• Not effective in nutrient removal
• Treatment efficiencies are very low at <10-15°C – hydrolysis
of particulate matter limits the process
• At 13-17°C for 14-17 hour HRT COD removal is 55-70%
• For 23-25°C at 4-6 hours HRT the removal is 80%
• Empirical formulae for COD and BOD removal efficiencies
Efficiencies are estimated by means of empirical relations
The above relations are applicable to domestic sewage for 20-27C
Efficiencies are substantially affected by HRT
The efficiencies are 40 to 70% for COD and 45 to 90% for BOD
 35.0
68.01100 
 CODE  5.0
70.01100 
 BODE

Treatment Efficiencies
TSS in the treated effluent is 40 and 140 mg/L and depends on
– Concentration and settling characteristics of the sludge
– Sludge wastage frequency and height of sludge bed and blanket
– Efficiency of the gas, solids and liquid separator
– Presence or absence of scum baffles
– Loading rates and HRTs in the reactor and sedimentation
compartments
• TSS in the treated effluent and HRT are related and often
shown by
TSS is total suspended solids in mg/L
‘t’ is HRT in hours in the sedimentation compartment ?
24.0
102 
 tTSS

Operation and control
Sensitive to the composition (concentration of various ions and
presence of toxicants like phenols) and strength of
wastewater , and to the temperature, pH, etc.
• Temperature effect is insignificant on hydrolysis and acidogenesis
– Temperature >5C
– Digestion rates are very low at <15C
• Optimal pH for methanogens is 6.8-7.2, but acid forming bacteria
favour acidic pH
Biomass washout can be a problem
COD:N:P ratio in the feed wastewater should be 350:5:1 (C:N:P
ratio of 200:5:1)
• Nutrient addition as NH4H2PO4 or (NH4)2CO3
Sulfur, potassium, calcium, magnesium, iron, nickel, cobalt, zinc,
manganese and copper may also be required
• Methanogens apparently have higher iron, nickel and cobalt
concentrations

Operation and control
Buffering capacity of the wastewater may be increased to
provide the alkalinity of 1000-5000 mg/L
– Sodium bicarbonate can supplement the alkalinity
TSS in feed should be <500 mg/L
• 50% of the COD in domestic sewage is contributed by TSS
• Insoluble matter can occupy volume, TSS can form scum layer,
and fats and lipids can cause foaming
Startup
• Startup time is 2-3 weeks for >20°C, otherwise 3-4 months
• Hydraulic loading during startup is <50%
• Startup needs sufficient amount of granular sludge
• During start-up, biological loading rate should be in the range of
0.05 to 0.15 kg COD per kg VSS.day
– Excessive loading can affect process stability (pH and VFA)
– Maximum limit for biological loading rates depend on the
methanogenic activity of the sludge
– for domestic sewage 0.3 to 0.4 kg COD/kg VSS.day is the limit

UASB Ponds
• These are modified and/or simplified UASB reactors
– The complicated and costly 3-phase separator is replaced (no
gas collection tunnels)
– Floating plastic collapsible membrane or fixed concrete slab is
used for the gas collection
– Deflectors are used underneath the treated effluent collection
trough to separate out the gas bubbles
• The plastic membrane can have the following three layers
– Top high tensile UV-resistant geomembrane
– Middle layer 12.5 mm thick polyfoam insulation and flotation
– Base layer of high density polyethylene welded to the base

UASB Ponds
• Other features and guidelines for the UASB ponds
– Clarification of the effluent is compromised with and
compensated by relatively lower upflow velocities
– Hydraulic short-circuiting is minimized through decreasing
spacing between distribution tubes towards the outer side
– For better performance the weir loading is reduced to half to
that for a secondary clarifier
– Aperture is maintained 15% and on this basis width of the UASB
pond cell is decided
– flow velocity through the aperture is maintained <<0.2 m/sec.
– The pond is left open over the deflector and used for having
• Overflow weirs and effluent collection trough
• Influent distribution boxes
– At regular intervals vertical sludge pipes are provided in the
deflector to facilitate sludge pump out.

Sewage inlet
Distribution box Effluent collection trough
Distribution pipe
Deflector
Treated effluent
outlet
Biogas outlet
UASB Pond – Top View
Sludge dredging drain

UASB Pond – (Section) Elevation
Sewage inlet Biogas outletBiogas outlet
Distribution pipesDiflector pillar
Sludge bedSludge bed
Sludge blanket Sludge blanket
Diflector
Effluent collection trough
Sludge dredging pipe
Distribution box
Biogas Biogas

Anaerobic Baffled Reactor
Anaerobic baffled reactor
• Baffles are used to direct wastewater flow in up-flow mode
through a series of anaerobic sludge blanket reactors
• Modifications to the basic process can include
– use of settler to capture and return solids
– use of packing in the upper portion to capture solids
• Long SRTs possible with low HRTs
• System is stable to shock loads
Anaerobic migrating blanket reactor
• Similar to anaerobic baffled reactor (ABR) but have an added
feature of mechanical mixing
• Influent feed point is changed periodically to the effluent side
• Organic loading rate 1 to 3 kg/m3.day and HRT 4-12 hours
• COD removal efficiency increases with temperature (60% at 15°C
and 80-95% at 20°C at lower organic loading)

Anaerobic baffled reactor
Anaerobic migrating blanket reactor
Influent Influent
reversed flow
Effluent
reversed flow Effluent
Influent
Effluent
Biogas
Biogas

Effluent zone
Biogas zone
Wastewater inlet
Capped vertical
section of inlet
Vertical
section of inlet
Effluent level
Raised discharge end
of the outlet
Hopper bottom
Extraction end
of the outlet
Outlet
Inlet to stage-2
Outlet
2-stage Anaerobic Baffled Reactor
Canopy

150
500
800
350
750
200
450
100
125
300
Overflow weir
Underflow baffle
Inlet pipe
Central upflow pipe
Partitioning wall of outlet box
Outflowing stream pipe
Flow distribution box

UASB reactors

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