Bacterial numbers and SRB activity in biofilm from VFA during late biocide treatment and nitrate treatment (a); number of SRB quantified by viable counts (MPN, black bars) and by fluorescent antibody technique (FA, white bars) (FA analysis was not performed after October 2002) (b); SRB activity, mean value of two measurements where error bars indicate standard deviation (c); viable counts of NRB targeting facultative anaerobic NRB (black bars) and obligate anaerobic NRB (grey bars) (a, c); total number of bacteria (filled circles)