Generating "tide" in pilot-scale constructed wetlands to enhance agricultural wastewater treatment

The operation of tidal flow was studied using a pilot-scale system treating high strength piggery wastewater. Located on a farm in Staffordshire, UK, the system consisted of five wetland treatment stages vegetated with common reeds of Phragmites australis. Wastewater samples were collected from the inlet and outlet of each stage and analyzed for BOD5, COD, NH4-N, NO3-N, NO2-N, SSPO4-P and pH. Average hydraulic and organic loadings on the system were 0.12 m(3)/m(2) d and 240 g BOD/m(2) d, respectively, which is considerably higher than the typical loadings on conventional subsurface flow systems. On average, BOD5 and COD were reduced by 82 and 80 from initial concentrations of 2000 mg/L and 2750 mg/L, respectively, across the whole system. The first-order kinetics constant for BODS removal (K-BOD in m/d) in this tidal flow system is approximately 2.5 times the rate constant obtainable in a typical horizontal flow system, demonstrating a more efficient removal of organic matter in tidal flow wetlands. The overall efficiency of the system was found to increase with time before stabilizing towards the end of a start-up period. Straight-line correlations were established between the loading and removal of BODS and COD. Contributions by individual stages to the overall treatment were analyzed. SEM images of wetland media demonstrated the formation of biofilms and microbial activities inside the matrices of the wetland system, which accounted for the degradations of organic pollutants.