The aim of this study was to assess the reliability of two on-site sewage treatment facilities (BCT S-12 and TJ EP-4) serving two school buildings located in rural communes in Małopolska region. The reliability was determined using Weibull reliability method. The research was conducted for 24 months and it involved the collection and analysis of 48 samples of raw and treated sewage from each facility. The analyses included organic and biogenic pollution, i.e. BOD5, COD, and total nitrogen. Reliability assessment revealed the following effectiveness for BCT S-12 facility: 77% for BOD5, 96% for COD and 25% for total nitrogen. For TJ EP-4 facility, the effectiveness was 81%, 88% and 72% for BOD5, COD and total nitrogen, respectively. Both facilities used the activated sludge process and experienced periodic disturbances in the treatment flow that resulted in reduced sewage treatment efficiency. Considering the above, the processes of nitrification and denitrification in the biological reactor should be streamlined to provide more effective removal of organic and nitrogen-based compounds in the investigated facilities.
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