Media remedy range of contamination issues
An innovative media developed by UK industrial remediation specialist, Virotec, is ideal for applications requiring removal of metals and other pollutants from contaminated water and soil. Simon Tillotson, general manager at Virotec Europe, describes how the media permanently binds metals and phosphates to a high-capacity solid surface in a technique that can be used effectively to treat industrial and municipal wastewater and large volume contaminated water such as acid mine drainage.
It is a sustainable environmental contaminant treatment media created out of the treatment of another environmental problem. The media is very fine-grained (largely <10µm) and thus has high surface area-to-mass and surface charge-to-mass ratios creating a highly charged and reactive surface for attraction and binding of a wide range of metals and ionic compounds including phosphates.
Virotec media immobilises contaminants to its surface by a combination of physical (strong electrostatic adsorption) and chemical (precipitation, co-precipitation and recrystalisation) reactions; it has a high binding capacity (1500 meq/kg). The media also has an excellent acid neutralisation capacity (3.5-7.5 moles/kg), and is highly insoluble (<2% of the constituent minerals can be leached by rainwater). It has excellent settling and dispersive properties, well suited to applications where flocculation and formation of dense, easily dewaterable and stable sludge is required.
Contaminants are bound to the media in a non-leachable and non-bioavailable form, even under acid conditions. Better than 99% removal of metals from contaminated waters has been demonstrated and resultant dissolved contaminant concentrations in pore-water, leachate or effluent shown to be in the low ppb range.
Spent media, if not reused or recycled into, for example, building construction materials, will be classified as inert, or at worst non-hazardous, waste. Bound phosphates, whilst effectively retained, are not permanently bound; their slow re-release allows Virotec media used in phosphate removal applications to be sustainably reused as a soil treatment aid.
Here, not only is bound phosphate utilised as a nutrient source but the media's metal binding capabilities can be used for fixing leachable metals, and its acid neutralising capacity for additional soil improvement. Another example of its effective reuse is the application of media previously used for minewater treatment (and hence enriched in iron) for removal of phosphates from municipal wastewater. Here, the media's iron content can be increased by up to 15%, significantly enhancing its phosphate removal potential.
Toxicological studies show that Virotec products are safe. No adverse ecological impacts have been identified. Ecologically sensitive prawns thrive in pond waters treated for acidity using the media, and worms benefit from reduced metal contamination in vermiculture.
Virotec media is manufactured in a variety of formulations for versatile and effective deployment across a range of environmental situations. As a dry powder or slurry, media can be added directly and mixed with soils, sediments or wastewater.
Alternatively, pelletised media, as used in ViroFilter systems, are suited for passive flow-through applications such as electroplating wastewaters or acid mine drainage. The media also have the potential to be manufactured as porous blocks for use as permeable reactive barriers in groundwater treatment.
In flow-through applications, the highly porous nature of Virotec media is especially advantageous. An open skeleton and good interconnectivity between pores creates a permeable media with a high internal surface area maximising available reactive sites for effective treatment.
In direct addition applications, the media's fine-grained structure creates a large surface area of reactive sites for effective metals removal. Its non-dispersive nature and high density facilitates generation of dense and easily dewaterable sediments. Sediments of 70% dry solids are routinely produced with rapid settling characteristics. Sediments generated in this way have much lower volumes than, for example, equivalent lime sludges. Sediments are not leachable unlike lime sludges.
ViroMine technology, directly added to tailings, can successfully reduce mine leachate contamination. In a long-term independent USA Environmental Protection Agency (EPA) study at Gilt Edge Mine (South Dakota, USA), on-going since 2001, the media was applied at an optimised rate of 6-7% (w/w) to tailings. Tailing leachate concentrations for more than 20 metals, initially 100-1000's ppb, have typically been reduced to <10 ppb or undetectable levels. The leachate pH was rapidly improved, from 1.9 to 7.9 within one year, and incrementally to 8.6.
Acid mine discharge (AMD) has been successfully treated using ViroMine technology in the UK. Dissolved metal reduction and acidity neutralisation, and highly efficient removal of dissolved iron, is achieved by passage of the mine waters through the pelletised form of the media in ViroFilter treatment systems .
ViroSoil technology also has proven effectiveness for control of pH and fixing of metals in soils and sediments. Use of ViroSoil blends were shown in trials, at a former gold mine in Australia, to neutralise acidic soils and maintain pH far more effectively than the conventional liming alternative. Soil pH, initially 3-4, was raised to >7 when treated.
This level was maintained for the duration of the 15-month trial with a single application, compared with liming which could only achieve a pH of around 5. The ViroSoil addition also fixed metals within the soils.
Metal concentrations in treated soil run-off contained only trace to single figure ppb levels, despite exceptionally heavy contamination by iron (29000 ppm), copper (675 ppm), lead (1000 ppm) and zinc (1200 ppm). These results were not matched by the lime treatment.
The neutralisation of acid leachate from dredged marine and river sediments, generated during dewatering, is also an application for ViroSoil blends. Here the media tends to coat any sulphidic minerals present reducing exposure to oxygen, leading to generation of acid leachate. It also fixes any leaching metals and allows material to be safely disposed of, or with nutrient addition, to become a reusable soil.
ViroFlow offers solutions for a variety of industrial and municipal wastewater problems. Applications in metal finishing, metal smelting, metal refining, tanneries and car manufacture are currently being developed, with the main interest focused on ViroFilter.
ViroFilter systems are also particularly well suited for treatment of municipal wastewater. Excellent performance has been demonstrated both in phosphate removal trials, and effluent polishing of metals, COD and suspended solids.
Independent trials conducted by the UK Water Research Centre (WRc) confirm the effectiveness of the media for phosphate removal (>80%), demonstrate improved effluent polishing (< 1 mg/l P), and conclude that Virotec media has an exceptional phosphate removal capacity compared with conventional alternatives (Figure 1). ViroFilter systems can be readily fitted in-line with existing treatment systems, having a small plant footprint. These passive systems require minimal operator intervention and maintenance, and are ideally suited for deployment at smaller remote unmanned sites.