Problematik vid höga flöden - Gästrike Vatten AB

RESULTS AND DISCUSSION
Evaluation of the EBPR performance
Three different cases were identified during the study at the WWTP. These cases are
visualized in figure 3 and are linked to the operation with EBPR in relation to the hydraulic
load. At normal operating conditions the anaerobic phosphorus release is high and the overall
P-removal efficiency is around 98 % giving effluent concentrations of phosphorus below 0.3
mg/l. The anaerobic phosphorus mass release rate was 12.6 kg/h during normal conditions,
compared to 2.6 kg/h during high-flow conditions. During high-flow conditions, the overall
P-removal efficiency dropped to about 81 %. After a prolonged high-flow event the recovery
of the P-release was found to be almost instantaneous whereas the P-uptake in the following
aerobic phase demanded a much longer time for recovery. This caused the effluent phosphate
concentration to peak after such a high-flow event, leading to the need for chemical
precipitation. The main reason for this is probably the rapid increase of VFA in the influent
together with a rapid prolonged anaerobic retention time.
PO4-P (mg/l)
Normal conditions High-flow conditions After high-flow conditions
Figure 3. Three different observed cases of EBPR performance as linked to the hydraulic load.
Critical key factors:
The following crucial key-factors were identified as linked to high-flow conditions:
During high-flow conditions.
Lack of organic substrate (VFA) for the anaerobic process.
Occurrence of oxidative compounds (nitrate, oxygen) in the anaerobic reactor.
High levels of oxygen and an excessive aeration in the aerobic reactors.
Occurrence of chemical precipitants in the activated sludge.
After high-flow conditions.
PO4-P (mg/l)
Anaerobic Aerobic Anaerobic Aerobic Anaerobic Aerobic
Rapid increase of influent VFA together with a prolonged anaerobic retention time.
The main reason for the poor performance during high-flow conditions was identified as a
temporary lack of VFA. A correlation between the VFA-feed to the anaerobic reactor and the
phosphate release during different influx flows are plotted in figure 4. Here, a greater VFAsupply
and phosphate release is registrered during low-flow conditions compared to during
high-flow conditions. The anaerobic retention time was in this case shown to be of minor
importance since 100 % of the phosphorus release and 98 % of the TOC uptake occurred
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PO4-P (mg/l)