[Quantitative studies of the elimination of Salmonellae by biological wastewater treatment]

The wastewater pretreatment plant of Braunschweig works as a conventional, one step activated sludge plant without primary settling tanks. There were about 37,000 m3 per day of effluent during our investigations. During some months daily quantitative determinations of salmonellae were performed in the raw wastewater, in the activated sludge, and in the effluent. The mean concentrations of salmonellae were in the raw wastewater 2 X 10(5)/l and in the effluent after 11h of aeration and then settling 2 X 10(4) cells/l. The concentrations of salmonellae varied about the factor 10(3) (limits 10(2) to 10(5)). A small peak in the spring, a big peak in the late summer, and a week pattern were observed. The most salmonellae were adsorbed to unsoluble flocs of sludge, about 30-35% of the salmonellae only were found freely in the water and from that about 60% were concentrated round the flocs. Therefore, one step of settling removes about 90% of all salmonellae. In generally terms, the settling seems to be the most important step of removal of salmonellae from wastewater. Furthermore, the rate of removal is depending of the concentration. High concentrations of salmonellae cause a strong reduction and conversely. An immediate removal of salmonellae was observed in about 60%, however, there was a propagation of salmonellae in 30% of all samples, especially under conditions of low aeration, in an anaerobic environment, respectively.