Klaas J. Appeldoorn

Biological phosphate removal by activated sludge under defined conditions.

Abstract A simple, one reactor vessel system, called a fill-and-draw system, was developed for the study of enhanced biological phosphate removal under defined conditions. Sludge was grown in a medium with acetate and glucose as sole energy and carbon sources. The sludge was exposed to cycles with three distinct, consecutive periods; first an anaerobic period, then an aerobic period and finally a settlement period. In the period of settlement one third of the liquid was replaced with fresh medium. Sludge grown under this regime became considerably enriched with polyphosphate-accumulating bacteria. The polyphosphate content reached up to 110 mg P/g dry weight. The amount of polyphosphate in the cells during steady state depended on the acetate:glucose ratio, the nitrate and phosphate concentration in the medium. Highest phosphate accumulation was obtained with an acetate:glucose ratio of 9:1. Intracellular polyphosphate was formed during the aerobic period and was anaerobically hydrolysed and released as phosphate into the medium. In the absence of oxygen and in the presence of 2g acetate-COD/l, 80–90% of phosphate was released by sludge containing 100 mg P/g dry weight. In the absence of acetate only 2–19% of the accumulated phosphate was excreted.

Ecological aspects of biological phosphorus removal in activated sludge systems

Inorganic polyphosphate (poly-P) is a linear polymer of many tens or hundreds of inorganic phosphate (Pi) residues linked by high-energy phosphoanhydride bonds (Fig. 1) and usually consists of mixtures of different molecular sizes. Thermodynamically the standard free energy of hydrolysis of the anhydride linkage yields about 38 kJ per phosphate bond at pH 5. The energy storage function of poly-P depends on the ability of the bond cleavage reaction to effect phosphorylation and thereby conserve the energy associated with the hydrolytic action (Dawes, 1990).