Treatment of agricultural wastewater in a combined tidal flow-downflow reed bed system

In this study, the efficacy of a full-scale combined tidal flow-downflow re ed bed system was tested for the treatment of agricultural wastewater with average BOD5 of 1100 mg/l and NH4-N of 329.5 mg/l. At a mean flow rate of 2 .0 m(3)/d, the BOD5 and COD of the influent were reduced across the system by 97.6% and 71.3%, respectively. Considerable SS and PO4-P removals were a lso achieved. NH4-N was reduced by 93.1%. Nitrification proved to be the ma jor process for NH4-N reduction. Although a further polishing treatment is needed, the average BOD5 and NH4-N levels of the treated wastewater were cl ose to those frequently required in UK discharge consents. The highest BOD5 , COD and NH4-N removals and oxygen consumption rate were achieved in the t idal flow stage of the combined system; this suggests that the rhythmical a ir/water movement in the matrix of the tidal flow beds can benefit the trea tment by providing higher oxygen flux and more efficient utilisation of the available bed volume. Better results for BOD5, COD and NH4-N removals were obtained in downflow beds with water recirculation than in those without t his recycle. Therefore the recirculation of effluent around each treatment stage is a suitable technique for improving the performance of downflow ree d beds. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserve d.