Water & Wastewater Treatment Magazine
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www.wwtonline.co.uk | WWT | MARCH 2018 | 23 • WATER COMPANY SLUDGE DATA EXPLAINED • Water and sewerage companies were asked by Ofwat to publish information about the quantity, location, and quality of their sewage sludge by November 2017 as part of the regulator's efforts to open up a new market for bioresources. • The data is divided between larger treatment works serving a population equivalent of 2000 or more, and smaller treatment works with a P/E below this figure; significantly more information is being made available about the former than the latter. • Two key pieces of information included are the total volume of sludge produced by a works per year, and the percentage dry solids (DS) of the sludge, which gives a sense of its thickness. Other information published includes the treatment type (e.g. biological, or secondary activated sludge) and whether the works has screening and grit removal – these factors can each affect sludge quality • Since sludge is typically transported from a site via tanker, the data also reveals how many tanker collections there are per week and the size of tanker that is able to gain access to the site. • Publishing the data will allow other water companies to assess the possibility of bilateral deals for sludge that is within transportable distance; it will also encourage other commercial companies to get in touch with the utility with alternative proposals, for example, alternative waste management facilities where the sludge could be sent • The data also reveals the case for employing sludge thickening technology at many treatment works. If sludge is thickened to 6% dry solids for example, it will require half as many tanker trips to transport it than if the sludge is 3% dry solids. This transport benefit must be weighed against the cost of implementing a thickening solution. • BIORESOURCES INFORMATION - HIGHLIGHTS No of WWTWs over 2000 P/E Total raw sludge per year (tonnes) Average DS Largest treatment site (tonnes of sludge) Anglian Water 311 142,310 2.48% Great Billing (22,361) Dwr Cymru Welsh Water 107 13,392 3.82% Cardiff Bay (16,415) Northumbrian Water 66 69,921 3.65% Howdon (25,500) Severn Trent 260 205,604 4.68% Minworth (50,432) Southern Water 114 104,982 3.80% Budds Farm, Havant (9,924) South West Water 85 44,646 2.94% Brokenbury Quarry (5,085) Thames Water 153 382,500 3.12% Beckton (86,806) United Utilities 142 187,773 3.32% Davyhulme (24,460) Wessex Water 109 68,363 3.60% Avonmouth (21,079) Yorkshire Water 153 148,859 3.11% Knostrop (20,406) The water company bioresources data shows a wide range of sludge thicknesses being produced at wastewater treatment works, from 2% dry solids (DS) up to 6.5% or more. A sludge thickness of around 5-6% is ideal for onward transport: sludge that is thicker than this becomes more difficult to pump and manipulate and may choke equipment, while transport- ing more liquid sludge is inefficient as so much of it is water. Water companies are actively looking for low-cost ways of thickening sludge for better onward transport, but the type of sludge involved can have a big effect on what can be achieved: Surplus activated sludge (SAS) is more challenging to thicken than primary or co-settled sludge. One supply chain company active in this space, Plantwork Systems, says that it has seen a surge in interest for its non- mechanical Static Sludge Thickener (SST) as a result of the bioresource reforms. "The driver behind all of the compa- nies looking to this product is Sludge 2020 and the bioresources market being opened up," says Dominic Oliver, Associ- ate Director at Plantwork Systems. "All the teams contacting us for this particular piece of equipment are from the biore- sources departments of the water compa- nies. So it's a clear driver behind it." The released data has helped "massively" in identifying sites where thickening could make a difference, adds Oliver. Many smaller WWTW sites have no thickening technology in place, while larger 'hub' sites oŒen have belt or drum thickeners which could benefit from be- ing replaced with a solution that uses less energy. PWS's Static Sludge Thickener, which recently had a successful trial with Severn Trent (see next page) can thicken primary sludge to at least 6% and co-settled sludge to at least 5%, without requiring moving parts or polymer; it can currently achieve a minimum of 3% with SAS and tests are underway to prove it can reach a higher level than this. "Everyone in the industry knows that SAS is the big issue to thicken, and keep thickened without getting distortion and solid carryover," adds Oliver. There are similar drivers for grit removal and sludge screening at waste- water treatment sites, both of which can improve the quality of sludge, making it easier to extract energy from the product and reducing maintenance issues down- stream such as blocked digesters. "There is a need to invest across the entire Wastewater Treatment Works process chain, but that's easier said than done when resources are limited," says Kevin Mooney, Wastewater Sector Direc- tor at CDEnviro. "Ofwat's Water 2020 reforms mean that sludge quality should be top of the priority list, and one way to improve both sludge quality and system performance is to efficiently remove rag and grit." • SLUDGE THICKENING