Uasb reactor

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Chapter 7
Anaerobic Reactor

Reactor Configurations
Slowly growing anaerobic bacteria require longer sludge
retention times (SRT) in anaerobic reactors.
Loading rates are therefore, primarily dictated by the
concentration of active biomass in anaerobic reactors.
Consequently, maintenance of a high SRT is the major
point of interest inpractical application of AD process.
High rate anaerobic treatment could be achieved by
employing efficient biomass retention methods.


Reactor Configurations
In anaerobic reactors to maintain higher biomass
densities, SRT has to be in excess of HRT (hydraulic
retention time).


High biomass densities also provide greater resistance to
any inhibitory substances in theinfluent.
To accomplish the higher treatment efficiency and
reliability associated with a long SRT, a number of novel
anaerobic reactor configurations have been developed.

History of anaerobic reactors
of anaerobic
1881 First conventional anaerobic digester was used to
liquidify the solid components of sewage
1891 First septic tank to retain solids in sewage
1905 Development of the ‘Imhoff’tank in Germany
1930s Digesters were started to be mixed and heated to
improve the digestion of solids in the sewage
1955 Anaerobic contact process was developed to treat
soluble organics and dilute wastewaters


Septic Tank System
Tank System

Imhoff Tank


Anaerobic lagoons

Anaerobic lagoons

Simple covered
anaerobic lagoon

ADI-BVF® lagoon

4 Conventional & High-rate ADs



Conventional & High-rate ADs
Conventional AD
Intermittant feeding and
Heated to 30-35oC
HRT based on liquid input
is 30-60 days
VS loading: 500-1600

High-rate AD
Homogeneous due to
Continuous or intermittant
feeding and withdrawal
Heated to 30-35oC
HRT based on liquid inputis 15 days or less
VS loading: 1600-800


High-rate anaerobic reactors
Completely mixed anaerobic digester
Anaerobic contact process
Anaerobic sequencing batch reactor (ASBR)
Anaerobic packed bed or anaerobic filter
Anaerobic fluidized and expanded bed reactors
Upflow anaerobic sludge blanket (UASB) reactor
Anaerobic baffled reactor (ABR)

Completely mixed ADCompletely mixed reactor with no solids recycle in which
the SRT equals the HRT.
Wastewater and anaerobic bacteria are mixed together
and allowed to react.
When the organic pollutant is reduced to desired level,
treated wastewater is then removed.
Can be operated in either batch or continuous mode and
depends on the continuous growth of new biomass to
replace that lost in effluent.

6 Completely mixed AD
At least 10 days of SRT and HRT are required because
of slow growing methanogens.
This necessitates a reactor with a very large volume.
Large volume requirements and wash-out of microorganisms in effluent pose serious problems and make
ADs unsuitable for use with most industrial wastewaters.
However, they can be used successfully for sludge
treatment and for wastewatersthat contain high solids
and organic matter content.

Completely mixed AD


Completely mixed AD

Egg-shaped Anaerobic Sludge Digester

Anaerobic contact process (ACP)
Link between high biomass concentration, greater
efficiency and smaller reactor size is the idea of ACP.
Settling of anaerobic sludge in a settling tank and its
return back to the reactor allows further contactbetween
biomass and raw waste.
In ACP, due to sludge recycling, the SRT is no longer
coupled to the HRT.
As a result, considerable improvements in treatment
efficiency can be achieved.


Anaerobic contact process (ACP)
Major drawback is poor sludge settlement arose from gas
formation by anaerobic bacteria in settling tank.
Gas formation problem can be minimized by;
employing vacuum...