The aim of the study was to determine the value and concentration indicators of BOD5, COD and general nitrogen in domestic wastewater after mechanical treatment, which should be considered in the design of the activated sludge reactor. Ninety-nine samples of raw wastewater and after mechanical treatment were then collected and physicochemical analysis was carried out. The quantity of pollutants in the raw wastewater was verified to determine if this was domestic wastewater. The next stage of the analysis included the definition of the characteristic quantity of the analysed indicators in wastewater after mechanical treatment, which should be taken into account while calibrating computer programs supporting the design of biological reactor chambers. A detailed analysis led to the conclusion that mean values for BOD5 - 300 mgO2·dm-3, for COD - 500 mgO2·dm-3and for total nitrogen 50 mgN·dm-3 should be applied. The work also determined the susceptibility of wastewater after mechanical treatment to the decomposition of organic and biogenic compounds.
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