Water & Wastewater Treatment

WWT May 2016

Water & Wastewater Treatment Magazine

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www.wwtonline.co.uk | WWT | MAY 2016 | 21 of sludge for generating energy - the disruption and cost that this finer grit can bring about in these downstream processes has added up. "Grit has a high wear rate on your ongoing sludge process," says Richard Murray, Head of Wastewater Operations at Northumbrian Water. "In our case we put our sludge through a thermal hydrolysis process, so there's wear on the pumps and all the stainless steel structures associated with that, and then ultimately it gets into our digesters. That's where the very fine grit accumulates - it just sits in the bottom of the digesters until you clean them out. In one case last year we removed 300 tonnes of grit out of just one digester. That's a real pain in terms of the costs associated with getting it out of there." Equipment that can remove grit to a finer level, by forcing the flow into a vortex pattern to assist separation, has come on to the market in recent years (one example is Hydro International's HeadCell - see box). While the WIMES standard remains at 200 microns, it is of course a bold decision to invest in equipment that goes beyond the industry standard; so is it time that the standard was reassessed? Andy Wall, Chief Technologist at United Utilities, thinks so. "We've been trying to get grit out to the current standard for perhaps 50 years or more, and my gut feeling is that the existing standard probably isn't good enough if we want to protect modern downstream equipment," he says. "When detritors were first designed, 50 or 60 years ago, downstream equipment on a wastewater treatment plant was pretty simple and heavy duty. These days, the equipment and process required to meet modern quality standards can be more susceptible to be damaged by grit. So I think the direction of travel should probably be to take more grit out and get down to a finer level, if we really do want to protect our processes and equipment downstream." The business case for finer grit removal relies on adding up the cost of the problems that the removed grit would have caused downstream: not a straightforward calculation, particularly as many of these costs are hidden. It is difficult to quantify how much a reduction in grit would extend a pump's life, for example, or lengthen the time that a tank or digester will operate without needing to be cleaned out. Furthermore, these longer-term savings are o™en not as clear as the immediate cost that is incurred in taking the grit away from site, adds Wall. "There is a cost of removing grit that's very obvious – because you are taking so many skiploads of grit away every week - but the cost of not removing the grit is o™en hidden, or masked for many years," he says. "It finds its way to many places in the treatment process, causing damage, filling vessels, channels and tanks, and nobody knows about it until it's too late." One design issue that comes with advanced grit removal is where in the inlet works it should be situated – in particular, whether it should be placed before or a™er fine screening. "A conundrum which has long presented itself to the water companies has been when to remove "My gut feeling is that the current standard probably isn't good enough to protect downstream processes." Andy Wall, United Utilities "In one case last year we removed 300 tonnes of grit from just one digester." Richard Murray, Northumbrian Water "As long as you are hitting the documented standard, then that's all people care about." Keith Hayward, Hydro International

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