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Design of an UASB Reactor
Guest article by Dr. Makarand M. Ghangrekar, IIT
UASB Process
Upflow
Anaerobic Sludge Blanket (UASB) reactor has been successfully used
to treat variety of industrial as well as domestic wastewaters.
The UASB reactor can be briefly described as a system in which
substrate passes first through an expanded sludge bed containing a
high concentration of biomass. The sludge in the reactor may
exist in granular or flocculent form, but the granular sludge offers
advantages over flocculent sludge. Most of the substrate
removal takes place in sludge bed. The remaining portion of
the substrate passes through a less dense biomass, called the sludge
blanket.
Provision of sufficient volume of
sludge blanket is necessary above the sludge bed, which will provide
further treatment to the wastewater bye-passed from the sludge bed
due to channeling, and will help in maintaining stable effluent
quality. Above sludge blanket the reactor has a three-phase
separator, Gas-Liquid-Solid (GLS), which separates the solid
particles from the liquid and gas, allowing liquid and gas to leave
the system.
Proper design of this device is
necessary to retain maximum sludge in the reactor to affirm high
Solid Retention Time (SRT), about 50 to 100 days or more, and to
facilitate treatment with short Hydraulic Retention Time (HRT).
In general, the sludge bed occupies 30 to 60% of the total reactor
volume, 20 to 30% of the total volume is provided for sludge blanket
and GLS separator occupies remaining 15 to 30% of the total volume.
The design of the UASB reactor is
carried out taking in to consideration the applicable loading rates,
such as, hydraulic loading rate considering limitations of
superficial liquid upflow velocity and Organic Loading Rate (OLR).
The reactor volume is worked out to suit desired range of loading,
and height and plan area are finalised considering upflow velocity.
With this approach in design, it may not be always possible to
accommodate the desired range of Sludge Loading Rate (SLR) due to
limitation of sludge bed volume. Also, it may not be always
possible to accommodate the GLS separator device, with dimensions
required for providing sufficient area for gas-water interface, for
settling, and for aperture at the bottom of GLS device to avoid
excessive liquid inlet velocities in the settler. The
schematic diagram of UASB reactor is presented in Figure 2.
Figure 2. Schematic diagram of
UASB reactor
Under certain situation e.g., for low
COD concentration of the wastewater (about 500 mg/L) and under very
high COD concentration of the wastewater (10,000 mg/L), the
dimensions required for the GLS separator may govern the overall
dimensions of the reactor. Hence, it is necessary to give due
consideration for the dimensions of the GLS separator required,
while designing the UASB reactor. With these objectives this
article aims to discuss range of parameters recommended for design
of the reactor and to illustrate the design procedure.
Design Procedure For UASB Reactor
The UASB reactor can be designed as
circular or rectangular. Modular design can be preferred when
the volume of reactor exceeds about 400 m3. It is necessary to
select proper range of operating parameters for design, such as, OLR,
SLR, superficial liquid upflow velocity (referred as liquid upflow
velocity), and HRT. The literature recommendations for all
these parameters and design procedure to account these
recommendations are given below.
Organic Concentration and Loading
For COD concentration in the range 2
to 5 g/L, the performance of the reactor depends upon the loading
rate and is independent of influent substrate concentration.
For COD concentration greater than 5 g/L, it is recommended to
dilute the wastewater to about 2 g COD/ L during primary start-up of
the reactor. Once, the primary start-up of the reactor is over
with granulation of sludge, loading rates can be increased in steps
to bring the actual COD concentration of the wastewater. The
loading above 1 - 2 kg COD/ m3.d is essential for proper functioning
of the reactor.
For primary start-up the optimal
loading rates for getting high COD removal efficiency (about 90%)
within short start-up time, coupled with generation of good quality
granular sludge, are OLR between 2.0 and 3.6 kg COD/ m3.d and SLR
between 0.15 and 0.25 kg COD/ kg VSS.d. The OLR to be used for
design of UASB reactor for different temperature is provided by
Lettinga and Hulshoff. In general, for temperature between 15
and 35 degrees C, the reactor can be designed for loading between 1.5 to 18
kg COD/ m3.d. Lower OLR should be preferred for low
temperature and higher OLR can be adopted for high temperature.
For sewage treatment, the design of
reactor at higher loading rate is not possible due to limitations of
upflow velocity, and maximum loading of about 2 to 3 kg COD/m3.d can
be adopted for design. Similarly, for high strength
wastewater, such as distillery, satisfying minimum velocity criteria
and maximum HRT limit is difficult. Therefore, categorization
of wastewater based on COD concentration was observed to be
necessary for generalizing the design procedure of UASB reactor to
meet the recommended operating conditions to the maximum extent.
Thus, the COD concentration of the wastewater is suitably divided in
four categories.
It has been proposed to adopt loading
conditions as recommended in the Table 1, for design of UASB reactor
depending on the average COD concentration of the raw wastewater.
These loading rates recommended are suitable for temperature about
30 degrees C. For higher temperature, the loading rates can be
slightly increased and for low temperature these design loading
rates can be reduced.
Table 1.
Recommended loading range for design of UASB reactor
based on COD concentration at average flow
Reactor Volume
Based on the higher suitable value of
OLR, for given COD concentration, the volume of reactor required is
to be worked out as:
Volume = (Flow Rate x COD
concentration) / OLR
For the suitable SLR values for that
COD range (Table 1), the volume of sludge required can be worked out
considering the average concentration of VSS between 25 and 35 g/L
for medium and high strength wastewater, and 15 to 25 g/L for low
strength wastewater. This volume of sludge should be less than
50% of the reactor volume, worked out based on OLR, to avoid
overloading of the reactor with respect to SLR. If the volume
is not meeting the requirements, the OLR can be reduced to increase
the volume. The volume of the reactor is thus, finalized to
meet both the requirements. For this volume, the HRT should
not be allowed to be less than 6 h for any type of wastewater and
generally, it should be less than 18 h to reduce volume and hence,
cost of the reactor. For very high strength of the wastewater,
COD greater than 10,000 mg/L, it may not be possible to meet this
requirement, hence, under such situation the HRT may be allowed to
exceed even 24 h and as high as 200 h.
Superficial Liquid Upflow Velocity
Higher upflow velocities, favors
better selective process for the sludge and improve mixing in the
reactor. However, at very high upflow velocity, greater
than1.0 to 1.5 m/h, the inoculum may get washed out during start-up
or during normal operation granules may get disintegrated, and the
resulting fragments can easily wash out of the reactor. The
maximum liquid upflow velocity allowed in design should not exceed
1.2 – 1.5 m/h. Upflow velocities as 0.25 to 0.8 m/h are
favorable for granule growth and accumulation, during normal
operation of the reactor and maximum upflow velocity up to 1.5 m/h
at peak flow conditions for short duration can be used in design.
Reactor Height and Area
The reactor should be as high as
possible to reduce plan area and to reduce cost of land, GLS device,
and influent distribution arrangement. The height should be
sufficient to provide enough sludge bed height to avoid channeling
and to keep liquid upflow velocity within maximum permissible
limits. In order to minimize channeling the minimum height of
the sludge bed should be about 1.5 to 2.5 m. For this reason,
the minimum height of the reactor should be restricted to 4.0 m, to
conveniently accommodate sludge bed, sludge blanket and GLS
separator. The maximum height of the reactor can be about 8 m.
The height of the reactor adopted in practice is usually between 4.5
and 6 m.
While designing, initially suitable
height of the reactor (about 6m) can be chosen, and superficial
liquid upflow velocity is to be worked out as height/ HRT. It
is recommended to adopt upflow velocity of 0.7 m/h at average flow
and 1.0 m/h to 1.2m/h at peak flow. Accordingly, if the upflow
velocity exceeds the maximum limits height of the reactor can be
reduced in steps up to minimum of 4 to 4.5 m. If this is not
possible in the applicable range of height, HRT shall be modified
and fresh reactor volume and OLR shall be worked out. For low
strength wastewater, the maximum liquid upflow velocity becomes
limiting and for very high strength wastewater very low velocity
(less than 0.1 m/h) is required while designing the UASB reactor.
Under certain situations, the revised OLR may be less than the
initial OLR recommended. It is advisable to allow lowering of
OLR in such situations to control upflow velocity in the reactor for
proper performance of the reactor.
After these iterations for volume and
height, the plan area can be worked out and suitable dimensions of
the reactor can be adopted. Generally, the maximum diameter or
side length of single reactor should be kept less than 20 m.
Before finalizing the dimensions of the reactors, it is necessary to
consider the dimensions required for GLS separator, because to
accommodate the GLS separator meeting all requirements, it may be
necessary to alter height and plan area of the reactor.
Gas-Liquid-Solid (GLS) Separator
In order to achieve highest possible
sludge hold up under operational conditions, it is necessary to
equip the UASB reactor with a GLS separator device. The main
objective of this design is to facilitate the sludge return without
help of any external energy and control device. The guidelines
for shapes and design of GLS separator are given by Lettinga and
Hulshoff. The GLS should be designed to meet the requirements
such as, provision of enough gas-water interface inside the gas
dome, sufficient settling area out side the dome to control surface
overflow rate; and sufficient aperture opening at bottom to avoid
turbulence due to high inlet velocity of liquid in the settler, to
allow proper return of solid back to the reactor. Due
attention has to be paid to the geometry of the unit and its
hydraulics, to ensure proper working of the GLS separator.
Design of GLS separator
The shape of the GLS device
considered in design is presented in Figure 3. The
gas-water interface inside the dome is considered at the depth Dh
from top of the dome. In the beginning, the height of GLS
separator can be considered as 25% of the total reactor height.
For estimating initially the number of domes required the angle of
dome with horizontal can be assumed as 45o, and base width of dome (Wb)
can be calculated as 2(h+Dh)/ tan q. The Dh is to be
calculated as (Wt/2) tan q, and initially the top width (Wt) can be
considered as 0.2 to 0.3 m. The number of domes required for
given diameter (or width for rectangular reactor) can be calculated
by dividing width or diameter by WB, and rounding this number.
Where, WB=Wb+Wa, and Wa can be considered as 0.2 m initially.
After deciding the number of domes, the flow rate shared by each
dome, is to be estimated in proportion to the base area of each
dome, including aperture width, to the total area of the reactor.
Aperture width at bottom of gas dome:
The area of aperture (Ap) required can be computed based on the
maximum inlet velocity of liquid to be allowed. This area can
be estimated as flow rate per dome for rectangular reactor (or
central dome in case of circular) divided by maximum velocity to be
allowed. The maximum inlet velocity of 3 m/h is safe for
medium and high strength wastewater and for low strength the inlet
velocity less than 2.0 m/h should be preferred. The width of
aperture (Wa) is to be calculated as aperture area divided by length
(or in case of circular reactor by diameter) of the reactor.
It is recommended to use minimum aperture width of 0.2 m and if the
width required is greater than 0.5 m, then increase the number of
dome by one and repeat earlier steps till it is less than 0.5 m.
Figure 3. Details of the
Gas-Liquid-Solid (GLS) Separator
Width at gas-water interface: The gas
production expected in the reactor can be estimated based on the OLR
selected for the design and expected COD removal efficiency in the
range 70 to 90 percent. The gas production can be estimated as
0.35 m3 /kg COD removed at ambient temperature. From this gas
production the biogas collection per dome is to be worked out in
proportion with percentage of area covered by the dome. The
biogas loading at gas-water interface can be calculated as gas
collection per dome divided by product of top width of gas collector
(Wt) and length of the reactor (diameter, in case of circular
reactor). The loading of biogas at gas-water interface should
be kept less than 80 m3 gas/ m2.d (about 3 m/h). Initially the
top width can be assumed as 0.3 m and for this width if the biogas
loading is less than 3.0 m/h then adopt 0.3 m as top width.
If the biogas loading is greater than
3.0 m/h, calculate the top width required. Generally, top
width of 0.3 to 0.7 m can be adopted in design with maximum of 1.0
m. When even with maximum top width, if biogas loading is
greater than 3.0 m/h reduce the height of GLS separation device to
20% and repeat the earlier steps of GLS separator design, with fresh
number of domes. Even with reduction in height of GLS
separator if these checks are not satisfying, provide additional
layer of gas collector dome. When two or more layer of gas
collectors are used the height of each layer can be 15 to 20% of the
overall reactor height, with minimum height of each layer as 1.2 m
and maximum up to 1.5 to 2.0 m. The fresh biogas collection
per dome is to be worked out and further steps are repeated until
all design conditions are satisfied.
Check for Surface overflow rate
The width of the water surface (Ws)
available for settling of solids for each gas dome, at top of the
reactor, can be calculated as difference of WB and Wt. The
corresponding surface overflow rate is calculated as hydraulic flow
rate per dome divided by product of length (or diameter) and Ws.
It is recommended that the surface
overflow rate for effective settling of solids back to the reactor
should be less than 20 m3/m2.d at average flow and should be less
than 36 m3/m2.d under peak flow conditions. If the calculated
surface overflow rate is meeting these criteria the design of the
GLS separator is final. When it is exceeding the limits
recommended, it is advisable to reduce the height of the reactor,
thus, for same volume of the reactor more plan area will be
available. When the height of the reactor is reduced all
earlier steps for design of GLS separator should be repeated to
satisfy all design criterion. The minimum height of the
reactor should be restricted to 4.0 m (preferably 4.5 m).
Once, all the design criteria are satisfied the angle of inclination
of the gas collector dome with horizontal (q) can be calculated as q
= tan-1[ 2h/ (Wb – Wt)].
Baffle of sufficient overlap (0.1 to
0.2 m) should be provided below the gas collector in order to avoid
entry of biogas in the settling compartment. The diameter of
the gas exhaust pipes should be sufficient to guarantee easy removal
of the biogas from the gas collection cap, particularly in case of
foaming. Generally, lower reactor height is required for UASB
reactor treating sewage. Under certain situation, particularly
for very low strength of wastewater, even with reduction of height
to the minimum may not meet all design requirements. In such
cases the OLR adopted for design can be reduced to provide greater
volume of the reactor and hence more plan area to meet the entire
design criterion.
Effluent Collection System
The effluent has to leave the UASB
reactor via number of launders distributed over entire area
discharging, to main launder provided at periphery of the reactor.
The effluent launders can be designed in such a way that the weir
loading (m3/m.d) should not exceed the design criteria of Secondary
Settling Tank (i.e.185 m3/m.d). The width of the launders may
be minimum 0.20 m to facilitate maintenance. The depth of the
launder can be worked out as open channel flow.
Additional depth of 0.10 to 0.15 m
shall be provided to facilitate free flow. On both sides of
the launders ‘V’ notches shall be used. When effluent launders
are provided with scum baffles, the ‘V’ notches will be protected
from clogging as the baffles retain the floating materials. A
scum layer may form at the top of reactor and sludge accumulation
can occur in the launder hence, periodical cleaning of launders and
removal of scum should be carried out.
Design of Feed Inlet System
It is important to establish optimum
contact between the sludge available inside the reactor and
wastewater admitted, and to avoid channeling of the wastewater
through sludge bed. Hence, proper design of inlet distribution
system is necessary. Depending on topography, pumping
arrangement, and likelihood blocking of inlet pipes, one could
provide either (i) gravity feed from top (preferred for wastewater
with high suspended fraction), or (ii) pumped feed from bottom
through manifold and laterals (preferred in case of soluble
industrial wastewaters).
The rough guidelines for the number
of feed inlet points required in UASB reactor is presented by
Lettinga and Hulshoff for different concentration of the sludge
inside the reactor and applicable loading rates. In general,
the area to be served by each feed inlet point should be between 1
and 3 m2. Lower area per inlet point (1 m2) is to be adopted
for reactor designed for OLR of about 1 kg COD/m3.d, and higher area
(2 to 3 m2) per inlet point can be provided to the reactor designed
for OLR greater than 2 kg COD/m3.d. Apart from the number of
feed inlet points, the minimum and maximum outflow velocity through
the nozzles should also be given due consideration while designing.
This outflow velocity through nozzles
can be kept between 0.5 and 4.0 m/s. The equation of
’condition for maximum power transfer through nozzle’ can be used
for working out nozzle or inlet pipes diameter. The clogging
of the nozzles may represent serious problem resulting in uneven
distribution of the wastewater over reactor bottom, particularly
when treating partially soluble wastewater. Hence,
arrangements should be made for cleaning or flushing the inlet
system.
Other Requirements
It is necessary to keep provision for
removal of excess sludge from the reactor. Although, the
excess sludge is wasted from about middle height of the reactor, it
is also necessary to make arrangement at bottom of the reactor.
In addition, 5 to 6 numbers of valves should be provided over
reactor height to facilitate sampling of the sludge. For
treating high strength wastewater it is recommended to apply
effluent recycle, in order to dilute COD concentration and to
improve contact between sludge and wastewater.
For treating wastewater with COD
concentration greater than 4 - 5 g/L, it is recommended to apply
dilution during start-up, for proper granulation of sludge inside
UASB reactor. Auxiliary equipment has to be installed for
addition of essential nutrients, and alkalinity for control of pH of
the influent. The other equipments to be provided are for
measurement of pH, temperature, influent flow rate, and gas
production rate.
Thus, this design of the reactor will
meet all design recommendation proposed in the literature. The
proper design of reactor along with GLS separator will help in
better retention of the sludge inside the reactor. This will
help in reducing start-up time, required for the reactor to achieve
steady state, and better performance of the reactor during steady
state.
In this design procedure first
priority is given for maintaining maximum possible COD conversion
efficiency of the reactor, by selecting proper loading conditions
for that COD concentrations, and then to try for minimizing cost of
the reactor. For given COD concentration of the wastewater
maximum possible loading rates are used in the beginning and reduced
in steps till the design satisfies all the requirements. Thus,
this design will provide minimum volume of the reactor satisfying
all design recommendations.
About our author
For more information contact:
Dr. Makarand M. Ghangrekar
Assistant Professor
Department of Civil Engineering
Indian Institute of Technology
Kharagpur-721 302, India
E-mail:
ghangrekar@civil.iitkgp.ernet.in or
m_ghangrekar@rediffmail.com
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