Microbiological aspects of phosphorus removal in activated sludge systems

Abstract

Once the underlying reasons for the appearance of algal/cyanobacterial blooms were understood, and wastewater treatment plants were recognized as one of the major point sources for phosphate in waterways, effort was directed at designing activated sludge systems which could effectively remove phosphate from treatment plant effluent (see Chapter 2). Typical wastewater influent to treatment plants contains up to 11-15 mg/1 phosphate, although this value will vary, and in the US appears to be decreasing with the expanding use of low-phosphate detergents (Jenkins and Hermanowicz, 1991). Often the effluent needs to contain phosphorus (P) at less than 1 mg/1 (approximately 3 mg/1 phosphate). One method is to remove phosphate by chemical precipitation with compounds like Ca(OH)_Z, alum, ferric sulphate, ferric chloride or pickle liquor, and the principles of such removal systems have been discussed by Meganck and Faup (1988) and Jenkins and Hermanowicz (1991). These chemical removal processes, however, have several disadvantages. For example, they generate more sludge which often has to be dewatered before disposal. The chemicals are expensive in such large amounts, and there is a risk that their use will increase the salinity level in the receiving body of water (Jenkins and Hermanowicz, 1991; Kortstee et al., 1994). Despite these problems however, this is often the method of choice if very low phosphate levels (<1-2 mg/1) are demanded in the effluent (Roske and Schönborn, 1994).