The use of chemicals such as aluminium sulphate and ferric chloride in biological wastewater treatment can achieve excellent phosphorus removals and is well established. Unfortunately employing such precipitants has disadvantages, influencing the anaerobic digestion of and altering metal distribution in the precipitated sludges.In order to study these effects, the two aforementioned precipitants were used at both the pre and co-precipitation stages to generate sludge which was concomitantly digested in laboratory scale apparatus. Two sequential extraction techniques, one based on particle size, the other on chemical selectivity were then applied to raw, activated and digested sludge to determine the complexation of the metals within the sludge matrix. The use of chemically precipitated sludge did influence the process of anaerobic digestion with detrimental effects observed when monitoring suspended solids, volume and methane composition of the gas produced and alkalinity.The proportion of solids contained within each particle size range was also affected.In raw sludge aluminium sulphate increased the concentration of small particles (2.5 ym), while ferric chloride precipitation produced larger particles (>100 ym).This trend was reversed for activated sludge. The anaerobic digestion of both precipitated sludges produced large particles (>100 ym). For the heavy metals Cd, Cu, Ni, Pb and Zn, chemical treatment appeared to shift the distribution throughout various chemical fractions in the sludge. More significantly, aluminium sulphate increased the solubility of both Cu and Pb in raw and digested sludge, while both precipitants appeared to reduce solubility of the other metals in all of the sludges.In terms of particle size, the raw, activated and digested sludges which were chemically treated contained metal complexes which were generally larger than the control sludges, although Cu and Ni appeared to be associated with smaller particles.In order to cut the cost of chemical treatment and reduce the environmental impact of precipitated sludges emphasis has been placed on the use of microorganisms to take up phosphorus in excess of normal metabolic requirements during activated sludge treatment.In an attempt to produce a working system two modified laboratory scale activated sludge units, one test and one control, were designed and constructed specifically for excess biological phosphorus removal. Once operational, several modifications were executed on the test unit to enhance the removal of phosphorus.These included improvement of anaerobic conditions; augmentation with acetate and Ac inetobacter/Aeromonas species; increases in sludge age and temperature.The major factor found to influence phosphorus removal was temperature: a 1°C increase resulted in a 2.6% increase in phosphorus removal in the 12-25°C temperature range. During periods of high phosphorus removal experiments were undertaken using metabolic inhibitors to identify the mechanism involved. It appeared that removal was dependent on some physio...
