Water & Wastewater Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/670690
22 | MAY 2016 | WWT | www.wwtonline.co.uk Close-Up Insight report: wastewater grit the grit," says Dave Holthofer, Head of Wastewater Treatment at Dwr Cymru Welsh Water. "Ideally, you would remove it in the network prior to the treatment works or pumping stations. At the treatment works, it is best to remove the grit before the fine screening process. However, historically these systems have become fouled with rag, and are frequently found to be either out of service or are incurring high OPEX costs to keep them running." Grit removal works much better when installed aƒer fine screening, but you then have to take into account that your screens and screening handling equipment will themselves incur grit damage, adds Holthofer. Northumbrian's Murray says that the ragging issue means that his company always deploys grit removal aƒer screening, and that while some utilities have pursued different approaches he believes this is now the consensus. Since EA regulations mean that grit must be washed free of organics before it is disposed of in landfill, there is a real prospect that it can be profitably recycled. Severn Trent is one utility that has led the way in this regard, and has used washed grit as backfill for its own capital projects as well as for limited other applications. There are two obstacles in the way of this becoming routine: one is regulatory, with the EA currently placing restrictions on the applications that grit can be used for. The other is logistical: with a large number of wastewater treatment works spread over a large area, few sewerage companies are in the position of having enough grit in one place for it to be a reliable, economically viable income stream. With the many and varied problems that grit causes in water company operations, striving to remove more of it at the earliest stage possible is likely to make economic sense; and if the industry is able to reach the point where grit can be reused as a resource then this would certainly represent an achievement. "It can take a long time to accumulate, but the reality is that grit doesn't evaporate - it will be in the system somewhere and at some time you will have to remove it," UU's Andy Wall says. "And the cheapest way of removing it is right at the front end before it gets mixed in with other things," he concludes. EXPERT VIEW "The cost of not removing grit feels like a hidden cost" O ne company which is seek- ing to raise the bar in grit removal is Hydro International, which markets the HeadCell – a hydraulically-driven grit separation system which can remove 95% of grit down to the level of 75 microns. The technology has already been successfully installed at more than 200 plants in North America, and a pilot HeadCell unit has been trialled at several UK utilities. "Grit is a problem that has always been there, so there's always been a need to clear out channels, digesters, sludge lanes and so on," says Keith Hayward, Sales and Marketing Manager at Hydro International. "But the cost associated with it feels a bit like a hidden cost; there's been an accept- ance that if you have to drain down your digester every seven years and dig it out then that's just what you have to do." The WIMES standard of 200 microns has certainly contributed to this attitude of acceptance, adds Hayward. "It has meant that whenever you are siz- ing equipment for a project or quoting equipment to supply, you get equipment that gets you to that standard. At Hydro we would have been able to quote you equipment that takes you way beyond that standard for some years already, but the commercial side of the market wouldn't accept that. As long as you are hitting the documented standard, then that's all people are concerned about, and there is no incentive to go beyond that." The HeadCell uses a stacked tray sepa- ration technique to create a large surface area with short settling distances capable of removing more fine particles. It can be combined with Hydro's GritCup washer and classifier and SpiraSnail dewatering system which wash and dewater grit at the smaller sizes. Hydro has produced a 'cost of grit calculator' for utilities to tot up the costs that grit brings about in their plants. Hayward says that these figures show how a system like the HeadCell can pay for itself in around 3-4 years at a large 250,000 P/E plant; a smaller plant might have a payback period of around six years. These savings will ultimately be taken from the opex of the plant and could be passed back to the customer, he adds. "There is no doubt that grit is a high cost, whether it is just the high cost of cleaning digesters out or whether it's the cost of damage done elsewhere. We can remove grit to a much finer level than it is being removed at the moment, which will benefit everybody, including the consumer in the end. And that's really what we are aiming to do here."