analysis of a pilot-scale anaerobic baffled reactor treating domestic ...

Nevertheless, it is concluded that the compartmentalised structure of the ABR had advantages over
UASB reactors and septic tanks in the treatment of domestic wastewater since:
• Solids retention in compartment 1 reduces the requirement for pre-settling and separate
digestion of wastewater solids
• Sludge beds in later compartments were protected from transient high organic loads through
solids retention in the first compartment
• The initial acidogenic zone was not contained exclusively to the first compartment when after
a long period of continuous operation, undigested particulate organics overflowed to the
second compartment, without any detrimental effects on the outflow stream (Section 6.8.1).
7.1.2.2 A-HRT as the critical design parameter
It was hypothesised that the critical parameter controlling effluent quality and sludge digestion rates in
an ABR treating sewage was the applied hydraulic retention time (A-HRT) and that low effluent COD
concentrations could be achieved at an A-HRT of 20 h.
It was found that the ability of the system to retain active biomass depended on the upflow velocity in
the reactor and that this controlled the quality of the effluent and the rate of sludge accumulation.
Although this was dependent on the feed flow rate, and therefore the A-HRT, the relationship between
A-HRT and upflow velocity is dependent on reactor geometry and therefore would be different in
another system. Therefore it was concluded that both A-HRT and upflow velocity should be regarded
as critical design parameters for design of an ABR treating domestic wastewater. (Sections 6.1.2,
6.1.4.1 and 6.6.2.1).
It was observed that at a 22 h A-HRT, a well-balanced and stable anaerobic biomass was not able to
establish in the pilot-scale ABR. However, it was concluded that this was due to the upflow velocity
being too high, and was not specifically due to the low A-HRT value. It was proposed that stable
digestion and acceptable effluent quality at A-HRT values of 20 h and lower could be achieved, but
only with reactor geometries that resulted in lower upflow velocities.
7.1.2.3 Performance of ABR treating domestic wastewater and mechanisms of treatment
The first objective of this study was to investigate the performance of a pilot-scale ABR in the
treatment of wastewater of domestic origin and understand the mechanisms of treatment therein.
Principle mechanisms of treatment in an ABR were shown to be through solids retention and
anaerobic digestion of retained solids. It was shown that the baffled design assisted in solids retention
and ensured good treatment of soluble components from the influent as a result of filtration through
and contact with sludge beds in the compartments.
It was observed that significant COD reduction could be achieved, and that the extent of treatment
depended on the upflow velocity and A-HRT of the system. Free and saline ammonia and alkalinity
concentrations were observed to increase as a result of treatment in the ABR.
It was also observed that although treatment in the ABR resulted in significant removal of pathogen
indicator organisms, especially of helminth eggs, the load of pathogens in the outflow stream was
nevertheless too high for the effluent to be discharged to a water course or to surface irrigation.
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