To determine the degree to which tap water gets contaminated during its use and to assess the effectiveness of treatment of the sewage produced from this water, the mean values of some indicators and components of contamination were compared in raw sewage, in sewage leaving the mechanical-biological treatment plant, and in tap water. The work examined also the composition of sewage sludge and investigated the effect of the sewage on the receiving body and the environment. It was found that the ammonium nitrogen content of treated sewage was lower than that of drinking water. The results on total nitrogen showed that the amount of this component in the receiving body was three times higher than its amount in treated sewage, which, however, is practically impossible. Similar inconsistencies between the data on sewage and those on water in the river being its receiving body occurred for phosphorus and total suspended solids, which may indicate that the study methods employed in practice are inaccurate. Comparison of the results on the amount and quality of raw sewage, treated sewage, sludge, and water of the receiving body points to the conclusion that the existing method for determining the effectiveness of sewage treatment is inappropriate. It is suggested that the monitoring of the sewage treatment effectiveness of a treatment plant should be based on the balance of components in treated sewage and in sewage sludge.

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