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 reed bed system was tested for the treatment of agricultural wastewater with average BODs of 1100 mg/l and NH4-N of 329.5 mg/l. At a mean flow rate of 2.0 m3/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 also achieved. NH4-N was reduced by 93.1%. Nitrification proved to be the major process for NH4-N reduction. Although a further polishing treatment is needed, the average BOD5 and NH4-N levels of the treated wastewater were close to those frequently required in UK discharge consents. The highest BOD5, COD and NH4-N removals and oxygen consumption rate were achieved in the tidal flow stage of the combined system; this suggests that the rhythmical air/water movement in the matrix of the tidal flow beds can benefit the treatment 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 this recycle. Therefore the recirculation of effluent around each treatment stage is a suitable technique for improving the performance of downflow reed beds.