Evaluation of iron ochre from mine drainage treatment for removal of phosphorus from wastewater.

Treatment of polluting discharges from abandoned coal mines in the UK currently produces ca 30,000 t y(-1) of hydrous iron oxides ("ochre"), for which there is no major end-use, but which has previously been shown to have potential for removing P from wastewater and agricultural runoff. The efficiency of ochre for P removal from wastewater was investigated in experiments at two sites in the UK: Leitholm in Scotland and Windlestone in England. The three-year experiment at Leitholm involved diverting secondary-treated wastewater effluent through a trough which contained granular and pelletized ochre at different times. In the nine-month experiment at Windlestone, beds of ochre pellets in horizontal and vertical flow configurations were tested. The ochre treatment systems at Leitholm reduced influent concentrations of total P (TP) and TP mass by ca 80% and 50%, respectively, during optimal flow conditions, and achieved a removal rate of up to 65+/-48 mg TP kg(-1) ochre d(-1). There was no detectable release of potentially toxic metals from the ochre during the experiments. P removal rates by concentration were inversely related to flow and declined during the different phases of the experiments, probably due to clogging. At Windlestone, higher removal rates up to 195 mg TP kg(-1) ochre d(-1) were achieved for short periods of time following cleaning of the experimental system. Ochre has considerable potential to remove P from wastewater in a multi-stage treatment system and has a lifetime estimated to be 10 times longer than other substrates tested for P removal.

Wetland treatment at extremes of pH: a review.

Constructed wetlands are an established treatment technology for a diverse range of polluted effluents. There is a long history of using wetlands as a unit process in treating acid mine drainage, while recent research has highlighted the potential for wetlands to buffer highly alkaline (pH>12) drainage. This paper reviews recent evidence on this topic, looking at wetlands treating acidic mine drainage, and highly alkaline leachates associated with drainage from lime-rich industrial by-products or where such residues are used as filter media in constructed wetlands for wastewater treatment. The limiting factors to the success of wetlands treating highly acidic waters are discussed with regard to design practice for the emerging application of wetlands to treat highly alkaline industrial discharges. While empirically derived guidelines (with area-adjusted contaminant removal rates typically quoted at 10 g Fe m(2)/day for influent waters pH>5.5; and 3.5-7 g acidity/m(2)/day for pH>4 to <5.5) for informing sizing of mine drainage treatment wetlands have generally been proved robust (probably due to conservatism), such data exhibit large variability within and between sites. Key areas highlighted for future research efforts include: (1) wider collation of mine drainage wetland performance data in regionalised datasets to improve empirically-derived design guidelines and (2) obtaining an improved understanding of nature of the extremophile microbial communities, microbially-mediated pollutant attenuation and rhizospheral processes in wetlands at extremes of pH. An enhanced knowledge of these (through multi-scale laboratory and field studies), will inform engineering design of treatment wetlands and assist in the move from the empirically-derived conservative sizing estimates that currently prevail to process-based optimal design guidance that could reduce costs and enhance the performance and longevity of wetlands for treating acidic and highly alkaline drainage waters.