Statistical process control of removal of nitrogen compounds in the wastewater treatment plant in Krosno

The aim of the paper was to determine the possibility of the use of Shewhart control charts to monitor changes in the forms of nitrogen, showing the quality of wastewater discharged from the wastewater treatment plant in Krosno in the years 2010-2015. The performed statistical analysis showed the highest number of cases of elevated nitrate nitrogen and/or ammonium nitrogen when the temperature of treated wastewater was below 8-9°C. This low temperature resulted in adverse effect on the activity of bacteria that were involved in biological removal of nitrogen. It was found that in the winter months, the second stage of nitrifying bacteria responsible for oxidation of nitrite to nitrate, exhibited higher activity than the denitrifying bacteria. Graphical presentation of total nitrogen content using control charts for the mean of the process revealed the exceedance of the upper specification line (USL = 10 mgN_tot·dm-3) in the months from December to April. It was observed that the total nitrogen removal process in the months from June to November was stable with a very high 90% reduction in biogenic activity. The obtained results confirm the efficacy of control charts as a tool which can easily be applied in the statistical process control of total nitrogen removal in municipal wastewater treatment plants.