Water & Wastewater Treatment

36 | JANUARY 2015 | WWT | www.wwtonline.co.uk In the know Technically speaking: resource recovery ed the full potential of the technology with near complete removal (below 0.1 mg/L) of not only phosphorus but also ammonium and nitrate (Whitton et al., 2014). In addition, similarly to BNR systems, algae have the added benefit of resource recovery with production of biogas from anaerobic digestion of the harvested biomass offering potential for energy neutral operation. Technology 3: Reed beds with reactive media These two novel technologies show great potential for integration in wastewater treatment trains; however, at this stage of development they require a level of infrastructure inappropriate for very small STWs where the low tech/low maintenance approach is prevalent. In this case other alternatives may be considered such as using reactive media in reed beds. Reed beds, known to be easy to implement and cheap to operate, are o†en used on small size STWs. These systems can then be optimised by replac- ing the bed media with reactive media, achieving phosphorus removal with minimal investment costs. For this, steel slag, a calcium rich waste by-product from the steel industry has been shown to be efficient at removing phosphorus. Indeed, the phosphates present in the wastewater to be treated react and precipitate with the calcium present on the surface of the media. Recent work at Cranfield University has demonstrated the potential of steel slag to achieve low phosphorus effluent concentrations be- low 0.5 mgP/L at contact times of 24-48 hours (Letshwenyo, 2014). A current limitation of the technology is the increased effluent pH observed. Size of the media used was found to have a significant impact on perfor- mance as a system with a smaller media size (6 mm) was shown to per- form even better with effluent phos- phorus concentrations below 0.2 mgP/L. The smaller media offer a larger surface area for reaction and better retain precipitates formed in the water phase, but may negatively impact the systems due to increased clogging potential. However, as reed beds are traditionally installed on small sites where consents are likely to be less strict, the performance of the larger media may be suitable. As part of the collaboration between Cranfield University, Atkins and Severn Trent Water, a 2-year demonstration trial of these technologies evaluated in parallel to other commercially available systems on the Packington STW will start this month. For further info go to wwtonline.co.uk Reed beds are a relatively simple and cheap option which o en make sense for smaller STWs "Current phosphorus removal approaches in conventional treatment involve either modifying activated sludge for biological nutrient removal (BNR), by precipitation with metal coagulants or a combination of both processes"

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