WET News

14 WET NEWS DECEMBER 2015 Effective grit removal is often overlooked but without it process inefficiencies and increased electricity costs can occur. The challenge: Grit gets advanced THE VERDICT "The UK industry has accepted and grown to live with the burden of grit because of a 'one size fits all' industry standard. We believe the time has come to challenge accepted norms" Keith Hutchings, Hydro International leads to process inefficiencies and increases electricity costs. Grit: • Accumulates in channels, pipes, primary tanks, 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 • Reduces the quality of final effluent with a risk of exceeding pollutant limits As a result, 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 basins diff- users lead to wasted energy use and greater carbon footprint. INSIGHT SlUDgE TREaTMEnT & DiSpoSal Clogged aeration basins diffusers lead to wasted energy use and greater carbon footprint "We have calculated that 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," Hutchings continues. "For every 1% increase in grit downstream, there could be a 1% increase in wasted energy." When it comes to grit removal, many UK wastewater treatment plant operators may simply be unaware of how much grit they are missing. Current industry standards (WIMES standard no. 2.02 Grit Removal and Treatment Equipment 2008) make assumptions about the nature and form of grit. Similar to the current US standards (Water Environment Federation Met- hod of Practice 8 and Metcalf and Eddy) they make some key assumptions for 'convenience': • Grit is a clean silicate sand particle above 200 microns in size • The particle has a A abesp provides water to lthough grit is widely acknowledged to com- promise downstream processes through abrasion and accumu- lation, resulting in higher operating costs and poor energy efficiency, all too oœen, effective grit removal over- looked. The heavy burden in energy, materials, time and money is widely tolerated in UK water companies because it is accepted as unavoidable. "Most wastewater treatment operators in the UK have lived with a compromise," explains Keith Hutchings, Hydro International's group waste- water product manager. "Too oœen, the costs are borne as part of ongoing plant operation – almost without question. Whether a primary tank or digester needs cleaning out, a sludge pump stripping down, or aerators need unclogging, it is just something that has to be done." However, poor grit removal uniform, smooth spherical shape • The specific gravity is 2.65. The standards have estab- lished an all-purpose particle settling-out rate depending on flow rate, which defines conventional treatment and equipment design parameters. Conventional grit removal equipment is designed to remove up to 95% of this size and shape of grit. That is, if working at peak design efficiency. Says Hutchings: "Hydro is conducting sampling at a number of plants in the UK, which is consistently showing that this understanding of grit is wrong. We are finding that around 60% of grit is smaller than 200 microns and in some places an even greater proportion than that. The indication is that most grit removal systems may actually be removing as little as 20% to 30% of the incoming total grit load." NEED TO KNOW 1 grit shapes are seldom spherical and shape significantly affects the particle settling rate 2 The majority of grit particles can be in the 106 micron to 75 micron range 3 grit sampling has never been conducted in the UK 4 For every 1% increase in grit downstream, there could be a 1% increase in wasted energy THE CONCEPT • agM samples inlet waters to gain an accurate profile of the incoming grit • it recommends and tailors a combination of technologies to meet those conditions for an individual plant • HeadCell uses a large surface area and short settling distances to capture fine particles • HeadCell can be combined with the gritCup washer and classifier and SpiraSnail dewatering system

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