Giardia cysts and Cryptosporidium oocysts at sewage treatment works in Scotland, UK

A three year study on Cryptosporidium oocysts and Giardia cysts in sewage was conducted with emphasis upon: (1) development and assessment of techniques for sampling and processing sewage for these parasites; (2) a field study in which six sewage treatment works (STW) were assessed for removal of these parasites from the liquid phase; and (3) assessment of effect of sewage treatment on survival/destruction of Cryptosporidium oocysts. In sampling, grab samples were superior to filtered samples. In concentration and clarification of samples, the most efficient technique was that in which sample manipulation was minimised. Although calculated parasite removal efficiency varied for both parasites and between and within STW, some consistent patterns were observed. First, Giardia cyst removal was significantly greater than Cryptosporidium oocyst removal. Second, most parasite removal occurred during secondary treatments, rather than during primary settlement. Third, no significant difference between efficacy of activated sludge treatment and trickling filters at parasite removal was detected. Also, in one STW which utilised tertiary sewage treatment, most removal from the liquid phase occurred during preceding treatment stages. Finally, one STW had significantly higher removal efficiencies of both parasites than the other five STW investigated. Low numbers of viable oocysts were detected at both influent and effluent. Survival was monitored by introducing oocysts, in containers bounded by semi-permeable membranes, into various environments within one STW. The results demonstrated that, over one week, none of the environments tested had a deleterious effect on oocyst viability, with the exception of the sludge-holding tank. Results are discussed in relation to other published work on Cryptosporidium and Giardia in sewage and recommendations for further areas of research on this subject are made.