Treatment of recycled carrot washing water

Large volumes of effluents and solid waste derive from industrial fresh packing and processing of carrots. Due to high organic loads and suspended solids and due to insufficient wastewater treatment, surface waters are polluted by effluents of carrot washing, as shown for one enterprise in Germany. Over a period of four months the water quality was studied at the stages of a treatment system consisting of a grit chamber, a settling tank combined with aeration devices and a constructed wetland. Organic pollutants were measured as COD and BOD5 in different fractions of the wastewater. The daily washing of carrots produces up to 300 m3 wastewater with a mean filterable solids content of 453 mg l(-1) and a mean COD concentration of 179 mg l(-1). On average the current efficiency of total COD reduction is 52%. Due to the fine grain size and the low density of suspended solids, settling is very slow and incomplete, primarily during the winter. As much as 29 mg COD l(-1) in the effluent derive from the fraction of particles which do not settle within 18 hours. The dissolved COD fraction accounts for 30 mg l(-1) and reflects the insufficient oxygenation of the wastewater. Strategies for enhancing the removal of organic compounds can be derived from the results of this study. Spatial separation of settling and aeration processes is recommended to optimise the treatment process. Furthermore, the area of the constructed wetland should be increased to maintain its hydraulic conductivity and to guarantee a high COD reduction.