Wastewaters generated from large dwelling areas are collected and disposed of by means of long sewer trunk lines. The wastewater treatment potential of sewers is not a novel phenomenon, however, it has not been thoroughly investigated in the past. In this study, possible treatment efficiencies are estimated in the presence of sufficient oxygen. Suspended and attached growth kinetics are studied to explore the significance of aeration in sewer trunks. Although there are several existing models describing consumption mechanisms of soluble substrates and reaction rates, a new method is investigated in this paper for shortening the parameters used in biological rate equations. Soluble substrate consumptions of suspended growth microorganisms are taken into consideration by means of an experimental approach and they are estimated by using respiration rates. Relationships between respiration rate, substrate utilization rate and biodegradable organic matter concentrations are observed during the experimental program. An empirical relationship which provides the utilization rate in correspondence with diminishing substrate concentrations along a sewer line is developed based upon the experimental results. Expected treatment efficiencies are determined considering utilization rates of attached and suspended growth microorganisms.