Urban Institute Ireland Research Collection

A gravel-based tidal flow reed bed system was operated with three different strategies in order to investigate its optimal performance for the treatment of high strength agricultural wastewaters. According to the three strategies, individual reed beds of the system were saturated and unsaturated with the wastewaters for different periods while steady hydraulic and organic loadings were maintained. Experiment results demonstrated that the system produced highest pollutant removal efficiencies with relatively short saturated period and long unsaturated period, highlighting the importance of O2 transfer into reed bed matrices during the treatment of high strength wastewaters. Significant removals of some major organic and inorganic pollutants were achieved with all the three operation strategies. Nitrification was not the major route of NH4-N removal when the system was under high organic loading. Due to the filtration of suspended solids and accumulation of biomass, gradual clogging of the reed bed matrices took place. The clogging caused concerns over the long-term efficiency of the current tidal flow reed bed system.

The purification capacity of a laboratory scale tidal flow reed bed system with final effluent recirculation at a ratio of 1:1 was investigated in this study. In particular, the four-stage reed bed system was heavily loaded with strong agricultural wastewater. Under the hydraulic and organic loading rates of 0.43 m3/m2.d and 1055 gCOD/m2.d, respectively, the average removal efficiencies obtained for COD, BOD5, SS, NH4-N and P were 77%, 78%, 66%, 62% and 38%, respectively. Even with the high loading rates, about 30% of NH4-N was converted into NO2-N and NO3-N from the mid-stage of the system where nitrification took place. The results suggest that the multi-stage reed bed system could be employed to treat strong wastewater under high loading, especially for the substantive mass removal of solids, organic matter and ammoniacal-nitrogen. Tidal flow combined with effluent recirculation is a favourable operation strategy to achieve this objective.