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

Soluble COD [mg/ℓ]
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0
Influent
Figure 5.23: Phase IV: COD concentrations in compartments of the pilot-scale ABR. Data
points indicate mean values for each compartment. The dashed lines show upper
and lower 95% confidence limits on the mean. n=18 except for compartment 1
(n=15) inflow (n=16) and compartments 3 and 4 (n=17).
5.6.3 pH values during souring incident
1
2
3
4
Compartment
During Phase II, a souring incident occurred where low pH values were observed in all compartments
and the outflow of the ABR. It is theorised that the initial cause of the souring was illegal dumping of
septic tank contents into the Kingsburgh incoming wastewater, a practice that is known to occur from
time to time in the middle of the night. Figure 5.24 shows a series of pH profiles on different days
during Phase II around the souring incident. The dumping would have caused a slug of low pH, high
COD wastewater to enter the ABR, causing organic overload, and inhibition of methanogenesis 1 .
The pH values during normal operation, souring and 3, 4, 5, 9 and 10 days after souring are shown in
Figure 5.24 Sour anaerobic conditions resulted in pH values around 4.5, the pKa value of acetic acid.
It is expected that souring occurred first in compartment 1, and was propagated to subsequent
compartments. Measurements on the day of souring (0) were taken at around 13h00. Illegal dumping
is reported to occur between 20h00 and 04h00 suggesting that between 9 and 17 h had passed between
souring and pH measurement, hence sour compartment liquors would have been washed down to later
compartments and replaced with lower strength partially treated liquors by the time the measurements
were made. Data for the day of souring (day 0) showed that pH values as low as 4.5 were only seen in
compartments 7 and 8, indicating that the first 6 compartments had already begun to recover.
Assuming the apparent HRT to be 16 h , the time for wastewater flow to reach the sample valve in
compartment 7 is 7/8 x 16 h = 14 h (Section 5.5.2.3). Consequently, it is supposed that a high COD
load was delivered to compartment 1 before 23h00 the previous evening, and by the time the reactor
was sampled, the first 6 compartments had already begun to recover. Three days after souring the
1 It is reported that security guards are bribed to open the gates to contractors who engage in septic tank
emptying. They then drive their vacuum tankers to the head of works and discharge the contents into the influent
channels before the coarse screens. The only evidence of these activities is sludge splash marks in the influent
channels. The Works manager did not appear to be concerned by the practice since it did not appear to have any
significant effect on the operation of the plant due to dilution in the mixed activated sludge units. When plant
workers were questioned retrospectively by one of the project team, they were uncertain but thought that the
sludge splash marks may have been present on the day that reactor souring was observed, indicating that illegal
dumping may have occurred the previous night.
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Effluent