Operation of Both Systems at a Sewage-Treatment Works Both systems were installed at the Muirhead/Birkhill sewage- treatment plant which is located on the outskirts of Dundee and receive sewage from a combined sewerage system. The units were seeded with 40 I of mixed liquor from a nearby completely- mixed activated-sludge plant. The study was carried out during summer months when the ambient temperature was 14-25°C (and the wastewater temperature was 15-22°C). The flowrate into the RBC system was set at about 0.96 I/ min, representing a sewage flow of approximately 1380 Vd. The storage capacity of the system was not sufficient to ensure a constant delivery of the required daily flow into the unit and, as a remedy, an additional storage tank was installed to 'trickle feed' the system. As a result of the continuous sewage inflow, the SBR unit operated on an 8-h cycle (or 3 cycles daily), each cycle consist- ing of three operating phases, as shown in Fig. 2. Performance evaluation of the two systems was as follows: (a) Continuous flow, with variable-strength sewage; this (b) Zero sewage flow for up to 52 h; (c) Hydraulic surge, by allowing two-thirds of the daily flow through the plants within a short period of time; and (d) Shock organic loading, by adding concentrated solutions of organic compounds to the crude sewage.