Bulking control with chlorination in a nutrient removal activated sludge system*

This paper deals with control of bulking by chlorination in a laboratory-scale (15l/ d) biological nutrient removal system. Bulking was caused by filamentous organisms characteristic of nutrient removal systems, i.e. Type 0092, Microthrix parvicella and Type 0914. Over a continuous dosing period of 19 d, at 8 mgCl2 /gMLSS.d the diluted sludge volume index (DSVI) decreased from 230 to 48 ml/g. Nitrificationdenitrification continued essentially unaffected. The biological P removal initially decreased from its normal 20 mgP/l (influent) to 14 rngP/l but recovered during chlorination to 19 mgP/l. Chlorination was terminated when overdosing became apparent and P removal declined precipitously to 12 mgP/l. After chlorination termination, biological P removal recovered to its normal 20 mgP/l in 5 d. Of the 3 filamentous organisms, Type 0914 was the most and M. parvicella the least susceptible to chlorination. Filamentous bulking in nutrient removal systems can be controlled by chlorination with a relatively minor loss of efficiency of biological N and P removal.