Long-Term Performance of Constructed Wetlands with Chemical Dosing for Phosphorus Removal

Abstract

Fourteen full-scale wastewater treatment plants (WwTPs) owned and operated by Severn Trent Water, a major UK water utility, were assessed to determine the long-term efficiency of using chemical dosing upstream of biofilm processes followed by horizontal flow wetlands to remove phosphorus and any residual metals. Results showed that the flowsheets used are an effective strategy to remove BOD, solids, ammonia, nitrate, and phosphorus, with all sites meeting their discharge consents for the past 10 years. Wetland removal rates of up to 6.7, 3.8, 0.24, 2.4, and 0.5 mg m⁻² d⁻¹ were found for solids, nitrate, ammonia, phosphorus, and iron, respectively. Under flooded conditions, settling mechanisms dominated as evidenced by average surface sludge accumulation rates of 2 cm year⁻¹. Subsurface flow resulted in lower or no sludge accumulation on top of the gravel. The average concentration of P and Fe in the sludge per site was in the range of 17–48 and 9–178 g kg⁻¹, respectively, much higher than sediment concentrations found in non-dosed wetlands and river sediments. Analysis of suspended, colloidal, and dissolved fractions showed a change in phase for P from particulate to dissolved after the wetland, suggesting release. Phosphorus release was observed under low P conditions in wastewater (<1 mg L⁻¹) and high sludge P levels, with release rates of 12–52 44 mg m⁻² d⁻¹ in the field and 44 mg m⁻² d⁻¹ under laboratory conditions. Iron release was observed under controlled experiments but not detected in the field, indicating additional transformation reactions take place in full-scale wetlands. Further research is required to provide adequate data for modifying the design and improving the operation and maintenance recommendations to ensure wetlands continue to deliver environmental benefit at WwTPs required to remove phosphorus.