Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | SEPTEMBER 2015 | 43 In the know Technically speaking: wastewater treatment G rit entering the process stream of a wastewater treatment works has always posed a threat by raising operating costs and impacting on energy efficiency. But the heavy burden in energy, materials, time and money is widely tolerated in UK water companies because it is accepted as unavoidable. Whether a primary tank or digester needs cleaning out, a sludge pump stripping down, or aerators need unclogging, it is accepted as something that has to be done. Grit - The Silent Enemy Grit is a perennial problem in wastewater treatment works, but advances in removal techniques mean that even smaller particles can now be extracted KEiTh huTchinGS GRouP PRoducT ManaGER HydRo InTERnaTIonal Until now, most wastewater treatment operators in the UK have lived with a compromise, a "one size fits all" approach to keeping their grit nuisance at bay at the inlet works. By accepting this norm, many simply do not know how much grit they are missing. Now, our attitudes to grit are changing and the realisation of the true cost of grit – especially finer grit- has started to undermine this stance. Utilities are considering the total cost of capital and operating expenditure (Totex) in their investment decisions, so asset managers, operations staff and maintenance engineers no longer have to put up with grit. Why grit matters Grit threatens effective treatment through abrasion and accumulation; poor grit removal leads to process inefficiencies and increases electricity costs. Grit accumulates in channels, pipes, primary tanks and digesters; wears out pumps, valves and other mechanical equipment; blinds filters and membranes; clogs aeration basins and diffusers; disrupts biological processes; reduces the effectiveness of energy recovery; and reduces the quality of final effluent with a risk of exceeding pollutant limits. As a consequence, maintenance budgets are burdened with excessive plant and personnel costs to periodically remove grit build-up in tanks and basins. In the meantime, clogged aeration basin diffusers lead to wasted energy use and greater carbon footprint. Draining down and cleaning out a primary tank can cost as much as £100,000 and may need to be repeated every five to seven years. It is calculated that for every 1% increase in grit downstream, there is a 1% increase in wasted energy. Plant operation So what is happening to challenge the traditional norms for grit removal? First is the current AMP6 emphasis on Totex asset investment and management. Every aspect of plant operation, including the effects of grit, is under scrutiny. Secondly, wastewater technology is looking towards greater automation and higher efficiency. That means plants with fewer personnel, a higher volume throughput in a smaller space and more biological processes to achieve better water quality and to recover energy. Thirdly, and crucially, there is a growing realisation that the standard model of grit we have taken for granted more or less since the 1940s is wrong. There has been a large elephant in the room – or more accurately some very small ones slipping quietly through grit removal systems and into our works. Revising the standards Our current industry standards (WIMES standard no. 2.02 Grit Removal and Treatment Equipment A grit-filled equalisation basin

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