Phosphorus retention capacity of root bed media of sub-surface flow constructed wetlands

Phosphorus retention by sub-surface flow constructed wetlands is dependent upon the effluent quality, loading rate and type of root bed media. Three types of root bed media (Lockport dolomite, Queenston shale and Fonthill sand) at various stages of their use were sampled from a sub-surface flow wetland located in Sewage Waste Amendment Marsh Process Project (SWAMP), Niagara-on-the-Lake, Ontario, Canada, and their P sorption characteristics were investigated. Lower equilibrium P concentration (EPC(o)) and higher P sorption maxima (S(max)) and retention capacity (P(r)) of untreated Fonthill sand compared to untreated Lockport dolomite and Queenston shale indicated that Fonthill sand could be better root bed media for sub-surface flow constructed wetland systems to remove P from wastewater. In general, EPC(o) of the root bed media increased with the duration of their use. Untreated root bed media indicated that the greater the amount of poorly crystalline forms of aluminum (Al), and magnesium (Mg) contents, the greater their EPC(o). The P sorption maxima of root bed media increased with use by accumulation of amorphous and poorly crystalline forms of Al and iron (Fe). However, in alkaline root bed media, preferential P sorption occurred more on amorphous and poorly crystalline forms of Al than Fe. This study also indicated that increases in P sorption capacities of root bed media do not necessarily assure lower effluent P concentrations, since EPC(o) increased by several fold with use. (C) 2001 Elsevier Science B.V. All rights reserved.