Influence of temperature and sludge loading on activated sludge settling, especially on Microthrix parvicella

During recent years modern full scale wastewater treatment plants with biological nitrification, denitrification and phosphorus removal have had increasing problems with foam formation on the surfaces of aerobic tanks and with bulking activated sludge. The results of a survey in 1995 (Kunst and Knoop, 1996) showed that most often the filamentous bacterium Microthrix parvicella is responsible for these problems. Up to today there is only little knowledge about its selection criteria in activated sludge. Therefore several experiments were done in full scale activated sludge plants and in laboratory systems under defined conditions to investigate the influence of low (< 0.1 kg/(kg.d)) and high (less than or equal to 0.2 kg/(kg.d)) BOD5-sludge loading rates on the growth and morphology of M. parvicella and the settlement of activated sludge. Furthermore the influence of temperatures of 5 degrees C, 12 degrees C and 20 degrees C on the growth of M. parvicella was investigated. It was shown that M. parvicella grows at low BOD5-sludge loading rate and low temperature and is the main causative organism of bulking and foaming sludge in nutrient removal plants. On the basis of this investigation it was concluded that the growth of M. parvicella and the settling problems of the activated sludge resulting from excessive growth of this filament will always appear in modern municipal wastewater treatment plants with BOD5-sludge loading rate less than or equal to 0.1 kg/(kg.d) especially under low temperature conditions. (C) 1998 IAWQ. Published by Elsevier Science Ltd.