Water & Wastewater Treatment

WWT May 2016

Water & Wastewater Treatment Magazine

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Page 19 of 39

20 | MAY 2016 | WWT | www.wwtonline.co.uk Close-Up Insight report: wastewater grit G rit is a perennial problem for wastewater utilities, causing damage, inefficiency and maintenance costs throughout the network and treatment process. Whether it is emptying out tanks and digesters, replacing worn pumps or clearing blockages in channels and pipes, the costs associated with it can be considerable. Yet despite the current regulatory climate and continuing focus on cost, the methods employed for removing grit in the UK - and the industry standards applied for its removal - have changed little for several decades. So is it time for a fresh look at the issue? Small, loose particles of grit enter the wastewater stream from diffuse sources, typically in the form of surface run-off from roads. For this reason it has always been a fact of life Counting the cost of grit Are water companies ready to tackle the hidden costs of grit in sewage? in the UK's combined sewer network, and unlike ragging – which can be reduced to some extent by educating customers about what to put down their drains - there is very little that water companies can do to stop it at source. Grit causes a maintenance headache in the network, where it clogs up sewers and wet wells, and its abrasive nature means that it accelerates wear on pumping equipment. The problems multiply when grit reaches the treatment works, where it builds up in tanks, clogs basins and aeration lanes and compromises biological processes. For this reason the approach of utilities has been to attempt to remove the grit as soon as possible aƒer it reaches the treatment works. The most commonly-used equipment for this purpose, the detritor, works by slowing the flow down in a basin until the grit settles and can be mechanically raked out into skips. This method is effective in removing around 70% of grit, to the commonly-followed WIMES standard of 200 microns in size. However, even where this standard is achieved, grit that is finer than 200 microns still enters the works and causes problems. And as wastewater treatment and sludge treatment have become more sophisticated over the years – with the widespread use of anaerobic digestion and exploitation In figures 40,000 tonnes The amount of sewer grit removed every year by UK utilities (source: WRc) 200 microns The existing WIMES standard for the size of grit to be removed adipiscing elit. 30cm per second The flow rate needed to settle grit at 200 microns 90% The mineral content of sewer grit wwt And hydro international pRESEnT ThIS SponSoREd REpoRT

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