The investigations were aimed to determine the probability of occurrence of sewage flows which are significant for the design, operation and modernisation of rural sewage discharge and treatment systems. The analysis included multiannual measuring data on daily sewage inflows to treatment plants from five rural sewerage systems located at Gdów, Koszyce, Kluszkowce, Łapanów and Stanisławice. The results showed that a sequence of daily sewage inflows to the treatment plants may be approximated by a theoretical log-normal distribution. The distribution parameters were estimated using a maximum likelihood method. The null hypothesis about the goodness of empirical fit of the daily sewage inflows to the theoretical distribution was verified using the non-parameter ?-Kołmogorow test. The parameters of theoretical distribution approximating the empirical distribution of daily sewage inflows from sewerage systems discharging sewage from 300 to 17 000 inhabitants are as follows: scale parameter -0.0999, shape parameter 0.4086. The calculated probability of exceeding the planned throughput of the sewage treatment plant to which sewage from the tested sewerage systems is directed revealed that the treatment plants at Kluszkowce and Stanisławice are overloaded in terms of hydraulics, and therefore require extension. At Kluszkowce 58.35% of all daily sewage inflows exceed the designed throughput and 26.75% – the critical throughput of the treatment plant, while at Stanisławice the respective values are 68.68 and 45.48%.

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