OCCURRENCE OF GRAM-NEGATIVE BACTERIA IN DRINKING-WATER UNDERGOING SOFTENING TREATMENT

A study was carried out on the presence of Gram-negative bacteria in t he municipal waters of Bologna (Italy) undergoing softening using dome stic ion exchangers with an automatic disinfection mechanism. The soft ening process was seen to cause a 15 fold increase in 22 degrees C and 36 degrees C heterotrophic plate counts. There was a 30 fold increase in Gram-negative bacteria and their number correlated directly with t emperature and inversely with active residual chlorine. Organic matter had no effect on bacterial growth. The most commonly found bacteria w ere various species of Pseudomonas (87.6%) (Ps. acidovorans, Ps. denit rificans, Ps. fluorescens and Ps. testosteroni) followed by Aeromonas hydrophila (5.6%) and Stenotrophomonas (Xantomonas) maltophilia (3.8% in outgoing water). Pseudomonas aeruginosa (present in 5.6% of incomin g water samples and 0.4% of outgoing water) and Yersinia enterocolitic a (present in 4.3% of incoming water samples and 1.1% of outgoing wate r) did not find favorable conditions for growth on the ion exchange re sins.