The article presents a comparison of the share fraction of COD in raw wastewater in treatment plants which flow in a small amount of wastewater and the sewage treatment with high flow. Compared the constructed wetlands with an average capacity of 4 dm³/m,, the treatment plant with biological deposits with an average capacity of 8 dm³/m, and a sewage treatment plant with activated sludge in Bialystok with a capacity of about 70 000 dm³/m. The lowest percentages of dissolved fraction of soluble organic non-biodegradable substances SI was reported in raw sewage in small sewage treatment plants. Based on the available data wasn’t found significant correlation between the factions XI, SS, XS in raw sewage and the amount of wastewater.