L.D. Geohring, T.S. Steenhuis, N. Corrigan, M. Ries, M. Cohen, K. Cabral, R. Stas, R. De, J.H. Peverly 1
Loadings of phosphorus (P) in effluent discharges are detrimental to the quality of receiving surface water bodies. Phosphorus concentrations in excess of 0.03 - 0.05 ppm have been associated with algal blooms. Wetland systems, either natural or constructed, have not typically been very effective in reducing inorganic P. The wetland may provide a temporary sink for organic P removal, however, chemical and biological equilibrium of the soil strata and wetland plants are generally not sufficient for extensive inorganic P reduction. One means of reducing inorganic P from effluents would be to use specialized soil substrates in a subsurface flow constructed wetland. Soil substrates which are efficient 'adsorbers' of P could facilitate P removal. Four wetland substrates initially evaluated were garnet (ferrous metasilicate), wollastonite tailings (calcium metasilicate mixed with ferrous metasilicate), iron ore tailings, and paper mill waste. Except for garnet, these substrates are `waste' products of other industrial processes. Experiments to determine P adsorption isotherms for these substrate materials were performed. Wollastonite was the best adsorbent of P adsorbing 5 mg of P per gram of substrate from effluent containing a P concentration of 2.5 mg/l in a 5 to 1 solution to substrate mixture. Further evaluation of other materials included coal fly ash, sand with oxidized iron, and sand with aluminum oxide. Fine wollastonite was still found to be more effective than iron oxide additions to sand over a wider pH range. By increasing column length and retention time, fine wollastonite was able to remove 90% of the P over an extended time period and final effluent concentrations below 1 ppm were achieved.
