The annual global production of milk is approximately 630,000 million litres and the volume of generated dairy wastewater accounts for 3.2 m3·m−3 product. Dairy wastewater is characterized by a high load of chemical oxygen demand (COD). In many wastewater plants dairy wastewater and municipal wastewater are co-treated. The effect of dairy wastewater contribution on COD fraction changes in municipal sewage which has been treated with a sequencing batch reactor (SBR) in three wastewater treatment plants in north-east Poland is presented. In these plants the real contribution of dairy wastewater was 10, 13 and 17%. In raw wastewater, SS fraction (readily biodegradable dissolved organic matter) was dominant and ranged from 38.3 to 62.6%. In the effluent, SS fraction was not noted, which is indicative of consumption by microorganisms. The presence of dairy wastewater in municipal sewage does not cause changes in the content of the XI fraction (insoluble fractions of non-biodegradable organic matter). SBR effluents were dominated by non-biodegradable dissolved organic matter SI, which from 57.7 to 61.7%. In raw wastewater SI ranged from 1.0 to 4.6%. Xs fraction (slowly biodegradable non-soluble organic matter) in raw wastewater ranged from 24.6 to 45.5% while in treated wastewater it ranged from 28.6 to 30.8%. In the control object (fourth wastewater plant) which does not process dairy wastewater, the SS, SI, Xs and XI fraction in inflow was 28.7, 2.4, 51.7 and 17.2% respectively. In the effluent the SS, SI, Xs and XI fraction was below 0.1, 33.6, 50.0 and 16.4% respectively.