Severe bulking and foaming at the Himmerfjärden WWTP

Several years of trouble-free operation with pre-denitrification at the Himmerfjärden WWTP concerning bulking and foaming have been followed by an unstable period. Two bulking incidents with no foaming by Microthrix parvicella were found to be caused by ethanol addition. Other bulking and foaming occurrences by Microthrix could be correlated to the production of volatile fatty acids in some of the sludge treatment steps. It is possible that a more widespread use in society of washing and cleaning products based on vegetable oils has promoted the ability of Microthrix to compete with floc-forming organisms, especially when ethanol or volatile fatty acids are present.