The aeration tank in a small scale wastewater treatment plant was converted to a sequencing batch reactor (SBR) with a maximum volume of approx. 27 m3. The main purpose of this study was to examine low temperature biological phosphorus removal (BPR). The wastewater temperature varied during the study between 3 and 8°C, with a water temperature at or below 5°C during 7 months of the year.

The SBR unit has been in operation from the end of 1989, the study period discussed here covered July 1991 - December 1992. SBR cycle time was varied between 6 and 12 hours, giving a total daily treatment capacity of between 18 and 36 m3.

The influent biological oxygen demand - 7 days (BOD7) levels varied between 88 and 165 mg/l. Corresponding phosphorus levels were between 3.10 and 9.55 mg/l The mean effluent level of phosphorus was 1.57 mg/l and the BOD7 value was 23 mg/l. This gives a mean total phosphorus reduction of 74% and a BOD7 reduction of 81 %.

During the study, mean supernatant suspended solids (SS) levels were quite high, at around 36 mg/l. This high SS level contributed a major part of both outlet phosphorus as well as BOD7 value. Effluent soluble values for phosphorus and BOD7 were 0.79 mg/l and 9 mg/l.

The supernatant SS component of BOD7 and phosphorus increased at lower temperatures. It was not possible to reduce or balance this increase by increased cycle time or increased settling time within the maximum cycle time available (12 hours). Stable low supernatant phosphorus and BOD7 levels are thus to a large degree controlled by the effluent SS level. A maximum of 20 mg/l supernatant SS is necessary to reach target supernatant values of less than 1 mg/l of phosphorus and 15 mg/l of BOD7.

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