Hygienic Effects of Sludge Pasteurization Prior to Anaerobic Digestion (Pre-Pasteurization)

Summary

Pasteurization of sludge after anaerobic digestion resulted under practical conditions in many cases in recontamination of the stored sludge caused by using contaminated tubes for sludge transport or by infections with bird droppings or, possibly, by infected aerosols from the biological treatment processes. In Switzerland the pasteurization plants were closed down for a certain time to find out the reasons for these recontaminations.

Comparing experiments with salmonella-contaminated raw sludge, pasteurized before it was digested or stored without digestion, showed that recontamination can be reliably avoided. In none of these experiments salmonellas could be isolated. Salmonella-contaminated digested sludge was pasteurized after the digester and stored. Also in these cases recontamination was never found. There was a tendency towards multiplication of the total bacteria and the coliforms in pre-pasteurized sludge and in sludge pasteurized after digestion wheras salmonellas never could be isolated.

The results show that pre-pasteurization is a good means to avoid recontamination of digested sludge with salmonellas. But on the other hand it is also possible to avoid recontamination of digested sludge which is pasteurized and stored after the digestion process provided that exacting hygienic standards are applied. If these cannot be assured it is recommended to use the method of pre-pasteurization.