Streamed sediment oxygen demand is calculated using a benthic respirometer. A pilot plant was erected to operate under controlled laboratory conditions using bottom sediments taken from different sites in the Merrimack River bed downstream of certain municipal sewer outfalls. The sediment studied in the pilot plant includes those solids which normally would be removed in a primary settling device receiving raw municipal sewage. The variability of the biochemical assimilation rate of the residues was considered. Findings were formulated in terms of the benthic equation of Camp. The effect of sediment depth on the aereal oxygen demand of these sediments as also studied.