The aim of the research was to determine the COD fraction thereof in sewage and their changes in the effluent after further treatment processes. The study was conducted in a sewage treatment plant in Bialystok (RLM> 100000). In sewage the highest concentrations occurred in the suspension of the organic fractions slowly biodegradable X_S (303.7 mg O₂/l) and dissolved organic compounds readily biodegradable S_S (263 mg O₂/l). The lower amounts were irreducible fractions dissolved in sewage and suspended SI (56 mg O₂/l) and X_I (101.2 mg O₂/l). Almost 80% of the total COD fractions were biodegradable (S_S + X_S). In the treated wastewater soluble fraction S_I-biodegradable (56 mg O₂/l) occurred in the highest concentration. The flow of wastewater by components of sewage treatment plant resulted the complete removal of biologically degradable fraction of dissolved S_S. More than 94.5% of the total COD in waste water purified fractions were biologically decomposable (S_I + X_I). Moreover, based on the analysis of studies the following soil removal was found: BOD5 – 99.4%, COD – 92.9%, total nitrogen – 93.4%, total phosphorus – 92%. After waste water treatment, ammonia nitrogen was completely removed while the nitrate concentration increased to 4.6 mg N/dm³.