Longterm experiences from four SBR plants operated with cold wastewater; comparisons and synthesis

An assessment of four different SBR plants in Scandinavia is presented with respect to treatment performance and especially nutrient removal. The plants have been in operation for 5 to 17 years, all under Nordic conditions of low water temperature during at least 4 months of the year. Thus, all the plants are operating at varying water temperatures, with typically low temperatures during the winter season, reaching 5 – 8 °C. Yet three of them perform quite satisfactorily, keeping to the consent values throughout the year. The fourth one suffers from a number of disturbances, resulting in poor annual performances. This paper gives not only performance figures from the plants, but describes the different operation modes that have to be exercised under the varying temperature and loading conditions. Special attention is given to the sludge quality, as these SBR plants all normally operate at low to very low SVI levels. Some comparisons with early findings and statements from other plants are shown with respect to operation modes and cycle lengths.