PHOSPHORUS REMOVAL PROCESSES IN A WETLAND AFTER A DECADE OF RECEIVING A SEWAGE EFFLUENT

A study was made of the processes leading to phosphorus (P) removal in a wetland that received a high load of sewage-P for over a decade. The amounts of P accounted for by sediment deposition, sorption to detrital material, and uptake by above ground biomass were compared with the mass of P entering and leaving the wetland system. Phosphorus deposition was shown to be the most important sink for P with rates of up to 30 g P m-2 d-1 being recorded. The pronounced spatial distribution of deposition as well as laboratory experiments indicated that P deposition occurred predominantly by reaction between sewage-wetland waters (high in P, alkaline pH) and natural wetland waters (high in Fe and Al, acid pH) at or immediately below confluences joining their flow streams. Studies on P mass transport showed that the P removal rate (28-70%) was considerably greater than published values suggest could be predicted for this highly loaded (almost-equal-to 34 g P m-2 yr-1) system. It is suggested that the wetland configuration, whereby the flow from a relatively small area of sewage-impacted wetland, is joined by waters from much larger unimpacted wetlands, is the reason for the relatively large P removal in this system. The relative magnitudes of P sinks in this study are different enough from other studies to suggest that the present level of P removal in this wetland is sustainable.