The effect of purified sewage discharge from a sewage treatment plant on the physicochemical state of water in the receiver

摘要

The paper presents changes in the contents of physicochemical indices of the Sudó? stream water caused by a discharge of purified municipal sewage from a small mechanical-biological treatment plant with throughput of 300 m3·d?1 and a population equivalent (p.e.) – 1,250 people. The discharge of purified sewage caused a worsening of the stream water quality. Most of the studied indices values increased in water below the treatment plant. Almost a 100-fold increase in ammonium nitrogen, 17-fold increase in phosphate concentrations and 12-fold raise in BOD₅ concentrations were registered. Due to high values of these indices, the water physicochemical state was below good. Statistical analysis revealed a considerable effect of the purified sewage discharge on the stream water physicochemical state. A statistically significant increase in 10 indices values (BOD₅, COD-Mn, EC, TDS, Cl?, Na+, K+, PO₄3?, N-NH₄+ and N-NO₂) as well as significant decline in the degree of water saturation with oxygen were noted below the sewage treatment plant. On the other hand, no statistically significant differences between the water indices values were registered between the measurement points localised 150 and 1,000 m below the purified sewage discharge. It evidences a slow process of the stream water self-purification caused by an excessive loading with pollutants originating from the purified sewage discharge.