Treatment of Agricultural Wastewater in a Pilot-Scale Tidal Flow Reed Bed System

Abstract

Tidal flow reed bed treatment is a process consisting of alternately filling and draining the bed matrix with wastewater. During the draining process, air is positively drawn from the atmosphere into the bed so the aeration of the system is significantly improved. A pilot-scale tidal flow reed bed system was studied for the treatment of agricultural wastewater, which was prepared by diluting pig slurry. At an average influent BOD₅ of 649 mg l⁻¹ and ammoniacal-nitrogen of 333 mg l⁻¹, the BOD₅, COD, SS and PO₄-P levels were reduced by 71.0, 58.3, 65.2 and 38.7%, respectively. The removals of BOD₅, COD, SS and PO₄-P increased with their influent strengths in straight line correlations. The NH₄-N was reduced by only 13.3% through the system; 69.0% of this removal resulted from nitrification into NO₂-N and NO₃-N, whereas 31% of the removal was due to other processes. The average oxygen consumption rate in this tidal flow system reached 203 gO₂ m⁻² d⁻¹, significantly higher than the rates obtainable in horizontal flow and downflow reed beds. The rhythmical air and water movement in the tidal flow system enhances the oxygenation ability of the beds and improves the efficacy of utilisation of the available bed volume. Accordingly, tidal flow reed beds appear to have much potential as initial treatment stages for small amounts of strong waste effluents where the bulk of the BOD₅ removal can take place.

Keywords:

• Reed beds
• tidal flow
• BOD5
• oxygenation
• agricultural wastewater