Effectiveness of Selected Stages of Wastewater Treatment in Elimination of Eggs of Intestinal Parasites

The objective of the study was to determine the degree of municipal wastewater contamination with intestinal parasite eggs of the genera Ascaris, Toxocara, and Trichuris at individual stages of treatment, and indication of potentially weak points in the hygienisation of sewage sludge. The study was conducted in 17 municipal mechanical-biological wastewater treatment plants which, to a slight degree, differed in the technological process of wastewater treatment and the method of hygienisation of sewage sludge. The selected treatment plants, located in seven regions, included five classified as large agglomerations (population equivalent - PE >100 000), ten as medium-size (PE 15 000-100 000), and two as smaller size with PE 10 000 - 5000. The largest number of viable eggs of Ascaris spp., Toxocara spp., and Trichuris spp. was found in the sewage sludge collected from the primary settling tank. A slightly lower number of the eggs were found in the samples of excess sludge, which indicates that the sedimentation process in the primary settling tank is not sufficiently long to effectively separate parasites’ eggs from the sewage treated. The number of eggs of Ascaris spp. and Toxocara spp. in the fermented sludge was nearly 3 times lower than that in the raw sludge. The effectiveness of hygienisation of dehydrated sewage sludge by means of quicklime was confirmed in two wastewater treatment plants, with respect to Ascaris spp. eggs, in three plants with respect to Toxocara spp. eggs, and in one plant with respect to Trichuris spp. eggs. The mean reduction of the number of eggs was 65%, 61%, and 100%, respectively. In one wastewater treatment plant, a reduction in the number of viable eggs of Ascaris and Trichuris species was also noted as a result of composting sludge by 85% and 75%, respectively. In the remaining treatment plants, no effect of hygienisation of sewage sludge was observed on the contents of viable eggs of these nematodes.