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

COD [mgCOD/ℓ]
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Figure 4.6: Phase I: ABR inflow (♦) and outflow (□) total COD concentrations. Data for 60 h
(red), 32 h (green) and 20 h (blue) T-HRT (timer and PLC control) are shown.
The flow rate was increased again on day 205 resulting in a clear increase in outflow COD value to a
maximum measured value of 564 mgCOD/ℓ on day 206. By day 234, the COD value had decreased to
165 mgCOD/ℓ. For the remainder of the operating period at a T-HRT of 20 h, the average outflow
COD value was 272±40 mgCOD/ℓ (n = 20) (Table 4.3). This can be interpreted as an average COD
reduction of 66%.
The outflow COD concentration achieved in Phase I was not substantially better than values reported
for conventional septic tanks (Table 2.3), .i.e. between 200 and 300 mgCOD/ℓ. This result was
unexpected, given the improved performance of baffled reactors compared to septic tanks reported in
the literature (Section 2.5.1.3).
COD removal occurs as a combination of retention of particulate material and biodegradation of
biodegradable material. The absolute minimum COD concentration in the ABR outflow stream if all
biodegradable material was stabilised and all solid material was retained would be the concentration of
the soluble non-biodegradable components in the feed, and any soluble non-degradable components
generated by digestion. Additional COD in the effluent stream arises from entrained particulates and
undegraded biodegradable soluble material. Thus in order to draw conclusions about the effectiveness
of the system, especially in comparison to other systems, the measured outflow COD concentration
should be compared to the concentration of soluble non-biodegradable components in the inflow
stream. No reliable measure of this value was obtained although it is inferred from BOD5 data that the
total (particulate and soluble) non-biodegradable fraction the Umbilo WWTP wastewater COD may
have been as high as 65% in summer and 51% in winter (Section 4.2.1). It is also not known what
portion of influent particulate material was retained in the ABR by settling hence it is not known
whether the biodegradable COD removal was better or worse than could be expected of a
conventional septic tank.
4.3.4 Free and saline ammonia
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Figure 4.7 presents free and saline ammonia (NH3+NH4 + ) concentrations in the inflow and outflow of
the pilot-scale ABR during operation at Umbilo WWTP.
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60 h HRT 32 h HRT 20 h HRT
PLC control
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0.5