Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | january 2015 | 35 In the know Technically speaking: resource recovery C oncerns over the environmental risk associated with the discharge of phosphorus from sewage treat- ment works (STW) effluent continues to intensify with stricter discharge consents likely to come into place as part of the Water Framework Direc- tive (2000/60/EC). Expected consents between 0.1 and 1 mgP/L open discus- sions about the future of phosphorus removal and the need for consideration around providing a balanced economi- cal and sustainable approach. Current phosphorus removal ap- proaches in conventional treatment involve either modifying activated sludge for biological nutrient removal (BNR), by precipitation with metal coagulants or a combination of both processes. Both are successful at meet- ing current consents, but become less attractive when meeting lower target New approaches for phosphorus removal Stricter phosphorus limits in the new WFD should encourage approaches that look beyond coagulation levels due to concerns over robustness of performance and excessive use of chemicals and energy. In this context, alternative approaches for phosphorus removal from wastewater effluents have to be explored. However, it is important to look beyond just treatment perfor- mance and consider other factors as part of a broader examination of the technologies to understand the true val- ue of emerging innovative approaches. Such thinking sits within the broad- er discussion concerning the future of wastewater management and the potential paradigm shiΠfrom treatment plants to resource recovery factories. To illustrate, currently available tertiary treatment technologies for phosphorus removal to very low levels are generally based on coagulation: either ballasted coagulation, or coagulation followed by filtration in a sand filter, membrane or cloth filter. Consequently, these technologies are heavily dependent on the use of chemicals, their price and the associated costs for sludge treatment and disposal which does not make them economically attractive. In addi- tion, these processes provide no easy route for resource recovery. However, innovative approaches currently undergoing development at Cranfield University offer alternative ways to meet the very low phosphorus discharge consents as well as opportu- nities for resource recovery providing a more sustainable alternative (Table 1). Technology 1: Iron nanoparticles The first technology is a physical pro- cess where phosphorus removal from wastewater is achieved by adsorption on iron nanoparticles embedded in an ion exchange resin (a resin currently used commercially for arsenic removal). The technology has been shown to remove phosphorus to extremely low levels (below 0.05 mgP/L) even at very short contact times (as low as 1 minute) allowing for small footprint units. As with all contact processes, the media requires regeneration once its capacity to remove phosphorus has been exhausted. Regeneration is achieved through simple pH swing to electrostatically dislodge the phospho- rus enabling selective recovery through generation of a phosphorus rich liquor. The regeneration liquor can be reused without impairment to the regeneration processes enabling highly concentrated phosphorus streams to be generated over multiple cycles. At preselected intervals the regeneration liquor is processed to remove the retained phosphorus through precipitation with calcium. This enables both recovery of the phosphorus and reuse of the regeneration liquid. The latter defines the overall total cost of the processes transforming the economic suitability of the approach and provides a route for nutrient recovery. Technology 2: Algae systems The second alternative is a biologi- cal process using algae for nutrients removal. The uptake of algae systems for nutrients removal from wastewater as either open ponds or photo bioreac- tors has been hindered by the costly downstream harvesting system and ex- tended retention times (days) required (Ometto et al., 2014). Immobilised algae in alginate beads offer a more viable ap- proach as harvesting is facilitated and high rate reactors with shorter contact times (6-20 hours) are used. Trials of the immobilised algae system demonstrat- Dr MArc PIDou acaDemic FelloW in reSource recoVery, cranFielD uniVerSiTy Table 1: Three emerging techniques for phosphorus removal

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