Summary:

Two methods of assessment of the effectiveness of sewage treatment plant operationwere presented in the paper: first one – with the use of the reliability theory, and second one, with the use of the statistical quality control process by means of control charts. The analysis was carried out in a sewage treatment plant located in the village of Rokiciny Podhalańskie. The sewage treatment plant works in the A₂/O system. The evaluation of effectiveness of the sewage treatment was made for the following pollutants: BOD, ammonia nitrogen, total nitrogen and total phosphorus. The analysis showed the proper work of the sewage treatment plant for BOD and total phosphorus. The calculated reliability level were 93,3% and more than 99,9%. For total nitrogen the reliability level was lower: more than 46% for empirical distribution and more than 37% for normal distribution. This was caused by the character of inflow sewage. The analysis of control chart showed the stable work in reducting organic pollutants, and in spring and summer period unstable work for ammonia nitrogen and total phosphorus. Only once a disturb of process effectiveness of total nitrogen changes caused probably by inflow of storm water to the sewage system was observed.