Dissolved Oxygen Concentration As An Indicator Of Water Quality

Since oxygen is essential for the survival of most macro-organisms, it is important to ensure that there are adequate levels of dissolved oxygen in rivers, lakes, reservoirs, etc., if they are to be managed satisfactorily. Ideally, the oxygen concentration should be at least 90% of the saturation concentration at the ambient temperature and salinity of the water. It is therefore important to know how effluents containing soluble and particulate organic matter can influence the dissolved oxygen concentration. One widely used method of assessment is the 'biochemical oxygen demand' (BOD), which is a measure of the quantity of oxygen required for the oxidation of organic matter in water, by micro-organisms present, in a given time interval at a given temperature. The oxygen concentration of the effluent, or a dilution of it, is determined before and after incubation in the dark at 20° for 5 days. The oxygen decrease can then be determined titrimetrically and the results presented as mg of oxygen consumed per dm3 of sample. Mineral nutrients and a suitable bacterial inoculum are usually added to the initial sample to ensure optimal growth conditions. This test is only an estimate of biodegradable material, hence recalcitrant or inhibitory compounds might be overlooked (SCA, 1989).

Because the BOD test takes 5 days it may be necessary to resort to the 'chemical oxygen demand' (COD), a chemical test which only takes a few hours to complete. The test is based on treating the sample with a known amount of boiling acidic potassium dichromate solution for 2.5 to 4 hours and then titrating the excess dichromate with ferrous sulphate or ferrous ammonium sulphate (HMSO, 1972). The oxidized organic matter is taken as being proportional to the potassium dichromate utilized. Most compounds are oxidized virtually to completion in this test, including those which are not biodegradable. In circumstances where substances are toxic to micro-organisms, the COD test may be the only suitable method available for assessing the degree of treatment required. The BOD:COD ratios for sewage are normally between 0.2:1 and 0.5:1. The ratio values for domestic sewage may be fairly steady. When industrial effluents of variable composition and loading are discharged, the ratio may fluctuate considerably. Very low BOD:COD ratios will indicate high concentrations of non biodegradable organic matter and consequently biological effluent treatment processes may be ineffective (Ballinger and Lishka, 1962; Davis, 1971). A number of alternative tests are available to indicate the 'oxygen demand' of a wastewater, including total organic carbon (TOC) and permanganate value (HMSO, 1972; American Public Health Association, 1992).

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