The influence of nitrogen (N)

phosphate (P) and N&P deficiency on sludge settleability and filamentous growth was investigated in four sequencing batch reactors. The results showed that P deficiency in influent would cause decrease of polyphosphate-accumulating organisms (PAOs) and inhibit the storage capability of floc-formers. Moreover

the proportion of protein in extracellular polymer substances (EPS) decreased and sludge volume index (SVI) of 200—250 ml·g-1 was observed. N deficiency in influent had less impact on the growth of PAOs and the storage capability of floc-formers. Protein in EPS was significantly improved

and SVI of less than 100 ml·g-1 was observed under this condition. The short-term deficiency of N&P in influent would cause deterioration of sludge settleability

but no further deterioration was observed after long-term domestication. No excessive growth of filaments but large numbers of spherical floc-formers existed in the N&P deficiency system

resulting in SVI of around 120 ml·g-1. The dominant filaments for N

P and N&P deficiency were N. limicola Ⅱ

Type 0092 and N. limicola I

respectively. All of these dominant filaments had limited effect on sludge settleability.