Our paper shows how the quantity and quality of sewage from septic tanks affect the quantity and quality of mixed sewage undergoing a treatment process. The study was conducted in an exemplary small sewage system located in a rural commune. It lasted 24 months in 2013-14 and included an analysis of 24 samples of sewage collected both from septic tanks and the sewage system. The aim of the study was to determine an optimum amount of sewage supplied to the wastewater treatment plant (WTP) by vacuum trucks that would not cause significant fluctuations in the amount of organic waste expressed as BOD5 and COD in the mixed sewage undergoing the treatment process. Partial correlation analysis showed that the quality of mixed sewage is to a greater extent affected by the percentage of sewage from septic tanks than by their pollution degree. The analysis of the effects of two independent variables (i.e., percentage share and pollution degree of the sewage from septic tanks) on a dependent variable (i.e., organic pollution of the mixed sewage allowed for a preparation of nomograms useful for forecasting the quality of sewage undergoing treatment). A simulation carried out for the investigated wastewater treatment plant, whose average daily treatment capacity was about 230 m³, showed that the amount of septic tank sewage should be around 11.5 m³·d^-1, that is 5% of the WTP capacity.