Pond and wetland treatment

Wetlands are in use as adjuncts to wastewater treatment lagoons at many north temperate locations. Performance data for 21 systems show median removals of 67, 61, 61, 48 and 99.8% for TSS, BOD, NH₄-N, TP and fecal coliforms, respectively. Hydraulic loading rates range from 0.14 to 55 cm/d, areas from 0.02 to 200 ha, and latitudes from 30 to 54°N. Calibrations of first order models with temperature dependence show that rate constants vary from seasonal dependence at low loadings to temperature dependence at high loadings for ammonia. Phosphorus rate constants display seasonal, not temperature effects. BOD and TSS are not affected by season. Wetland rate constants are larger than those for lagoons for all constituents. The optimal winter operating strategy, if hydraulics allow, is partial storage during frozen months, coupled with winter use of the wetlands. The use of FWS wetlands for polishing lagoon effluents is cost effective when land availability is not drastically constrained. Many systems have been in operation long enough to demonstrate sustainable long-term performance. Infiltration beds are potentially a valuable addition to ponds and wetlands.