Summary:

The evaluation of the influence of operation time the efficiency of on effectiveness sewage treatment process in a Lemna sewage treatment plant is presented in the paper. The basis for the evaluation included the study of the physical and chemical inflow of sewage to the treatment plant and outflow to the river, realized in years 1998–2000 and repeated in years 2004-2005. The research covered sewage temperature, pH, oxygen concentration, BOD₅, total suspended solids, ammonia nitrogen and phosphates. The analysis showed an increase of hydraulic load at sewage treatment plant from 75% in years 1998–2000 to projected 100% (150 m³∙d^-1) in years 2004–2005. Also, an increase was observed of concentrate pollutants in inflow sewage for following parameters: total suspended solids, ammonia nitrogen and phosphates. The increase of hydraulic load and pollutants load was caused by deterioration of the quality outflow sewage but for organic pollutants (BOD₅ – average 24,6 mgO₂∙dm^-3) and total suspended solids (average – 26,4 mg∙dm^-3), for which the discharge met the demands of water supply and sewage. Permission the increase of quantity of the sewage sludge on bottom ponds was caused by deterioration of quality outflow sewage in the analyzed sewage treatment plant. The anaerobic process in sewage sludge was caused by very high concentrations of biogenic compounds, main phosphates (average – 37,8 mg∙dm^-3).