Using data from two pilot-scale studies and one-full-scale investigation, this article demonstrates that the removal rate in biological water treatment is proportional to the influent concentration to the bioreactor, i.e., a first-order process. This is true not only for measures of biodegradable organic matter, e.g., assimilable organic carbon, but also for precursors of chlorination byproducts such as trihalomethanes. The slope of the removal rate versus the influent concentration relationship is termed the average specific removal rate, rAS. When data were available from different studies, the rAS values tended to fall in the same range. With a first-order model, the performance of biological treatment in a given situation can be estimated from limited data. The model is suggested as an approach for the preliminary design of bioreactors for drinking water treatment. Includes 19 references, tables, figures.