The aim of the study was to determine and compare the amount and biodegradability of organic compounds determined by the COD measure contained in raw sewage inflowing to small sewage treatment plants and municipal sewage wastewater treatment plants with high personal equivalent (P.E.). Raw sewage samples were collected in three chosen facilities. The research presented in the article were conducted with two small plants: constructed wetland plant with an average capacity of 4 m3/d and the treatment plant with rotary biological contactor (RBC) with an average capacity of 5 m3/d. Also big municipal wastewater treatment plant (WWTP) with activated sludge in Bialystok with a capacity of about 63 000 m3/d was checked. The lowest percentages of dissolved fraction of soluble organic non-biodegradable substances SI was founded in raw sewage in small wastewater treatment plants (CW and RBC).