Reductions in densities of indicator organisms and pathogens were measured in maturation ponds receiving secondary effluents from trickling filter and activated sludge treatment works. Effluent detention times in the ponds were determined using dye tracing techniques and compared with nominal detention times calculated from pond volumes and effluent flow rates. Median detention times were substantially less than nominal times because of short circuiting due to pond design aspects and thermal stratification. Maturation ponds of 10 days median detention time were found to successfully disinfect a poor quality trickling filter effluent and were effective at removing parasite ova and reducing virus densities. Continued use of maturation ponds should be encouraged in developed and developing countries as they have low cost, operational, maintenance and skilled operator requirements and are an effective disinfection process. Pond designs should minimize short circuiting and thus the areal requirements of the ponds.

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