Oxygen Demand for the Treatment of Carrot Washing Water

摘要

During the production of vegetables large amounts of highly polluted washing water have to be treated in order to enable a reuse of the wastewater. Often the COD of the recycled effluent does not satisfy the German wastewater administration directive. Therefore, it should be shown for a carrot producing enterprise, how the removal of organic compounds can be enhanced in order to improve the treatment efficiency for the carrot washing water. During four months the water quality was studied at five stages of the treatment system consisting of a grit chamber, a combined settling/aeration tank and a constructed wetland. Organic pollutants were measured as COD, BOD5 and TOC in different fractions of the wastewater. The daily washing of carrots yields in about 300 m{sup}3 wastewater which is highly loaded with organic and inorganic solids. On average the efficiency of total COD reduction is 52%. Due to the fine grain size and the low density of suspended solids, there is a very slow and incomplete settling. 29 mg COD L{sup}(-1) in the effluent even derive from the fraction of particles which do not settle within 18 hours. The dissolved COD fraction attributes to 30 mg L{sup}(-1) and reflects the insufficient oxygenation of the wastewater. To optimize the treatment process a spatial separation of settling and aeration processes is recommended. Further the area of the constructed wetland must be increased to maintain its hydraulic conductivity and to obtain a high COD reduction.