The location and nature of accumulated phosphorus in activated sludge

Abstract

The removal of phosphorus from sewage works effluents is considered desirable in order to limit the eutrophication of lakes and impoundments. It has been observed that activated sludge systems are capable of producing effluents with extremely low phosphorus contents. Lack of understanding of the basic principles involved has been responsible for the inability to control the erratic behaviour of such systems. Because of the complexity of the activated sludge ecosystem, the elucidation of the mechanism involved in enhanced phosphorus-uptake has been slowly forthcoming with many controversial hypotheses being formulated. It was therefore decided to determine the location and nature of the accumulated phosphorus in activated sludge. The technique used was that of electron microscopy combined with the energy dispersive analysis of X-rays {EDX). To develop the techniques required for this investigation, use was made of a sludge which had exhibited enhanced phosphorus removal over an extended period of time. The location of the phosphorus accumulations were determined and analyses of these accumulations indicated that they were composed of polyphosphates. Seven sludges with varying phosphorus-uptake capabilities were then examined. It was evident that the phosphorus accumulations in all the sludges were located in one specific morphological bacterial type, the cells of which were characteristically grouped in clusters. It was also apparent that the more efficient sludges contained very large and more numerous clusters of these cells. Bacterial population analyses of four of the sludges indicated that Acinetobacter spp predominated. The phosphorus-rich inclusions in all the sludges exhibited similar elemental ratios, which indicated similarity in composition. Quantitative analyses of these phosphorusrich inclusions indicated that they contained phosphorus concentrations in excess of 30%. It was consequently concluded that these organisms can account for enhanced phosphorus removal by activated sludge. Accumulation of phosphorus-rich inclusions was induced in a pure culture of Acinetobacter and in a sludge which had poor phosphorus-uptake capacity. From these investigations it is suggested that enhanced phosphorus removal in activated sludge is dependent on the supply of suitable substrates to the organisms involved.