Report No DWI0273


Final Report to the Department of the Environment October 1991 to December 1992


Mar 1993


The possibility that sewage sludges and sewage effluents could represent sources of Cryptosporidium oocysts was noted in the report by the Group of Experts, under the chairmanship of Sir John Badenoch, to the Department of the Environment and Department of Health (Department of the Environment and Department of Health 1990). Since there was a paucity of information documenting their presence in these environments, there was reason for concern that sewage sludges, following disposal to land, could act as sources of Cryptosporidium infection. The present contract was initiated to assess the capability of the main sewage sludge treatment processes, described in the Department of the Environment's Code of Practice for Agricultural Use of Sewage Sludge (Department of the Environment 1989), to inactivate Cryptosporidium oocysts. The processes investigated were mesophilic anaerobic digestion, thermophilic aerobic digestion and sludge pasteurisation. Additional objectives of the contract were to assess the survival of oocysts in sludge-treated soil mesocosms and their removal during primary sewage sedimentation.

Initially it was necessary to develop a method to recover Cryptosporidium oocysts from sewage sludges of sufficient purity for subsequent viability testing by the fluorescent dye inclusion/exclusion technique. The adaptation of a method originally devised for the recovery of oocysts from faeces resulted in a procedure which yielded oocysts of a sufficient purity for viability assessment.

The capacity of primary sewage sedimentation to remove Cryptosporidium oocysts, studied using a laboratory simulation of the process, was found to be poor due to the low sedimentation rate of oocysts.

The laboratory evaluation of the main sewage sludge treatment processes described in the Department's Code of Practice for Agricultural Use of Sewage Sludge demonstrated that thermophilic aerobic digestion and sludge pasteurisation are effective treatments to render Cryptosporidium oocysts non-viable. Mesophilic (35ºC) anaerobic digestion for a period of four days followed by storage of the digested sludge resulted in complete inactivation of Cryptosporidium oocysts. However approximately 10% of the oocyst population, when assayed for viability immediately on recovery from the digester, were found to be viable after 18 days exposure to the actively digesting sludge.

The viability of Cryptosporidium oocysts decreased within the range 17% to 29% in sludge treated soil mesocosms over a period of 44 days. The survival data obtained implied that oocysts would survive beyond this period. However the treatment studies indicate that the levels of oocysts applied to land in sewage sludges should be extremely low.

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