Report No DWI0664


Final Report to Department of the Environment


Oct 1985


This final report is submitted in fulfilment of the Department of Environment's contract PECD 7/7/124, Parasites in Sewage Effluents, awarded over the period 1 July 1984 - 30 July 1985.

The first part of the report is a summary of the literature, which shows that eggs of Taenia saginata are incompletely removed by sewage treatment, because their settling velocities, with a median value of about 1 m/h, and the wide distribution of these velocities are too similar to the upflow velocities encountered in sedimentation and settling tanks at sewage works. The second part of the report describes the development by WRc of an improved flotation method for recovering eggs of Taenia from settled sewage solids and of a conical settling tank for concentrating eggs and solids from sewage and effluents.

The third part of the report describes the testing, by the Liverpool School of Tropical Medicine (LSTM), under field conditions of WRc's settling tank, in comparison with LSTM's cylinder procedure and their evaluation of the procedure adopted for use in the field. The final procedure was use of automatic sampling equipment to provide a 20 litre composite sample of treatment liquors, concentration of solids with eggs by settlement in cylinders and recovery of eggs by the WRc flotation methods. Extensive surveys were then carried out at Southport and Crewe sewage treatment works to determine the incidence of eggs of Taenia and other parasites in crude sewage, settled sewage and in activated sludge effluent. Because very few ova were encountered, probably related to the prevailing state of public health, the effect of the treatment could be determined for only one case, at Southport, where primary settlement removed 76% of Ascaris ova. Only 4 eggs of Taenia spp were recovered, 2 from 140 litres of crude sewage (Crewe) and 2 from 220 litres of settled sewage (Southport). Sewage sludge (Southport) contained 660 times the count of Ascaris eggs in the crude sewage. The results of the survey are compared with those obtained in a similar published survey at the sewage works of Braunschweig, FR Germany.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.