Water & Wastewater Treatment

18 | XXXXX 20XX | WWT | www.wwtonline.co.uk The Works: Sludge ● With the opening of the sludge market set to kick off with the implementation of a new price control by Ofwat in PR19, we run the rule over the various stages in the sludge treatment process and how innovation in the field might be boosted by reform The opening of a new market for bioresources may help accelerate the roll-out of innovative and cutting-edge technologies and new business models across the various function involved in dealing with sludge, including its treatment, its handling and transport, the generation of energy from biogas derived from sludge and other types of related resource recovery. Sludge thickening and dewatering is usually the first process in sludge treatment. Raw sludge at a wastewater treatment works will be in liquid form and will typically contain 1-10% dry solids (DS); in this form, it can be pumped but is not yet suitable for an anaerobic digestion process and would be expensive to transport. A‚er thickening and dewatering, sludge will be anything between 10-35% DS, the kind of consistency in which it could be handled by a shovel. The bioresources market implies more transport of sludge between sites, which will mean that dewatering processes will be a particular focus. Since Ofwat's new price control is likely to allocate money Biogas holders at Reading STW; a sludge transport silo at Harwich; and Wessex Water's Avonmouth WWTW according to the dry solids of sludge that emerges from each wastewater treatment site, it will become especially important to be able to measure the DS of your sludge and to produce it to a consistent DS when transferring it to another company. If the sludge is destined for anaerobic digestion then pre-treatments such as thermal hydrolysis are a useful step for making sure that biogas yield is maximised. This sees the sludge boiled under high pressure to temperatures of between 160 and 180 degrees Celsius, improving the biodegradability and viscosity of the sludge. This technology has now been implemented at around a dozen UK wastewater treatment works, the biggest of which is United Utilities' Davyhulme. Other pre-treatment methods to have been researched and developed in recent years include mechanical (e.g. the use of ultrasound), chemical (e.g. injecting ozone) and alkali treatments. This is followed by the anaerobic digestion process itself, which produces biogas which can be exploited for energy, most commonly through combined heat and power (CHP) engines, although some utilities have successfully injected biogas into the grid through gas-to-grid technologies (e.g. Severn Trent's Minworth) or even used them to power vehicles (e.g. Wessex Water at Avonmouth). Economies of scale are a distinct advantage when implementing all of these technologies, with those utilities that use them typically operating in hubs that import as much sludge as possible from smaller sites. The reforms will be good news for the companies operating these hubs as they will potentially be able to take sludge from an even wider catchment area. Resource recovery of phosphorus and other nutrients may also become more attractive – hence why Severn Trent's roll-out of a number of innovative P recovery technologies following trials at its Packington Sewage Treatment Works is especially timely. The recycling of sludge as fertiliser to land may also increase in popularity, especially if the multiple environmental regulations governing this are relaxed or simplified. An intriguing possibility is that water companies with plenty of agricultural land in their region (e.g. Anglian Water) could do deals with utilities with highly developed AD facilities (e.g. Severn Trent) for the transfer of sludge in both directions - raw sludge could cross the border from the agricultural area to the urban AD treatment works, while treated, dry sludge product could then return across the border to be used on the agricultural land as fertiliser. Finally, an alternative for this final stage of the sludge life cycle is pyrolysis, where sludge solids are turned into liquid and gas by being subjected to high temperatures with no oxygen. Thames Water has invested in research into this process, which is more sustainable than incineration and has the potential to produce more energy. A panel of experts who spoke on the subject of sludge at Utility Week Live recently named pyrolysis as the most exciting technology in the sludge arena at the moment and tipped it for wider application. • THE PROCESSES 18 | JULY 2017 | WWT | www.wwtonline.co.uk Sludge all set for reform

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