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Abstract The progressive exertion of BOD of freshly polluted waters normally breaks down into two stages : a first stage in which the carbonaceous material is largely oxidized and a second stage in which significant amount of nitrification takes place. Nitrifying bacteria are usually present in relatively small numbers in untreated domestic sewage, and their production rate at 20°C is such that their populations do not become sufficiently large to exert an appreciable demand for oxygen until a relatively long period have elapsed in the regular BOD test. This is attributed to the present of excess aoncentrations of organia matter which inhibits the ’growth and respiration of nitrifying organisms at the early stages of incubation period. As the time is elapsed, the organic matter concentration reduces gradually until its concentration allow for the respiration and multiplication of nitrifying organisms. Therefore, this study was undertaken to gain a complete understanding on the effect of carbonaceous organic matter concentration on the activity of nitrifying bacteria. Accordingly, several experiments were conducted on raw, |