Inactivation of Ascaris suum eggs during storage in lime treated sewage sludge

Survival and development of Ascaris suum eggs in sewage sludge mixed with 10% w/w of quick lime (85% CaO) has been examined. Fresh collected A. suum eggs were mixed in sewage sludge during lime treatment. The development of the eggs was followed every second week during a 10 weeks period and again after 5 months, parallel with control eggs. Lime treatment of sewage sludge resulted in a pH > 12 and development of A. suum eggs was strongly reduced during the observation period. When the Ascaris eggs were isolated from the lime treated sewage sludge and suspended at neutral pH, the proportion that developed to larvated eggs decreased with time. Thus, the inactivation of A. suum eggs was reversible following short time storage while prolonged storage of sludge with pH > 12 completely destructed the ability of A. suum eggs to embryonate. A. suum eggs uniformly mixed into the sludge lost the ability to embryonate at a faster rate than eggs deposited in nylon bags. After 5 months no intact eggs could be identified in the sludge. Incubation of A. suum eggs at neutral pH imitate spreading on agricultural land, where the eggs could be a potential infective hazard. In conclusion, lime treatment and storage of sewage sludge at pH > 12 for at least 3 months can be recommended as a hygienic measure before application on agricultural land.