Water & Wastewater Treatment

In the know Technically speaking: sludge treatment 40 | APRIL 2015 | WWT | www.wwtonline.co.uk gas to the process as a single pulse of up to 20 minutes. This process has the advantage of promoting biogenic carbon sequestration, reducing the greenhouse gas (GHG) emissions, at the same time as enhancing biogas production. Other processes such as incineration and co-incineration, gasi• cation, pyrolysis, wet air oxidation, supercritical wet oxidation and hydrothermal treatment also look into recovering energy from sludge. Incineration and co-incineration are presently focused on energy recovery as heat or electricity and have been implemented successfully in many sewage treatment works (STWs) world-wide. More recently gasi• cation has been implemented in Europe with two plants in Balingen, Germany (2002) and Mannheim, Germany (2012) both supplied by Kopf Syngas, and one in the UK at Yorkshire Water's Esholt STW supplied by Intervate. Energy from gasifi cation In general, gasi• cation is completed by processing dried sludge into ash (also called biochar) and combustible gases (syngas) at high temperatures (>1000 °C) under reduced oxygen concentrations. The energy production by the gasi• cation is dependent on temperature, pressure and sludge characteristics (VS and TS content). The syngas produced can be used as gas turbine or a boiler fuel or, if the quality is high enough, can be used to replace natural gas. In same cases, dry feedstocks such as wood or green wastes might be added to the sludge to meet speci• c syngas quality production. The resulting biochar has been demonstrated to be useful for a number of applications on sewage treatment works such as soil conditioner, adsorbent for tertiary water treatment, construction materials, for phosphorus recovery etc. although full-scale use is still limited. Other emerging processes that have been implemented at full- scale are dedicated to phosphorus recovery. The main driver for the recovery/removal of phosphorus from sludge dewatering liquors and centrates is to avoid scaling. Formation of struvite and calcium phosphate occurs a" er changes in chemical and physical properties of the centrates such as temperature and pH, creating signi• cant scaling problems in pipes, centrifuges, heat exchangers, etc. that will need to be fully replaced to get back into service. On the other side, phosphorus has been named the "disappearing nutrient" due to the • nite rock reserves worldwide and there is also concern due to the unequal worldwide distribution of these reserves. Currently there are a number of full-scale struvite production plants world-wide, including one at Thames Water's Slough STW and another currently being commissioned with Paques Phospaq at Severn Trent Stoke Bardolph STW. A new option for struvite recovery is currently being researched at Cran• eld University. Laboratory-scale experiments have demonstrated the bene• ts of using speci• c bacteria that can produce biologically induced struvite as part of their normal metabolic pathways in sludge dewatering liquors opening a completely new route to recover phosphorus and ammonia as struvite from wastewater. Biological crystal formation of phosphorus compounds (e.g.: struvite; magnesium phosphate, etc.) has been demonstrated to be a by-product of the metabolism of speci• c bacteria that can be found frequently in the environment. Current research is focused on building and operating a pilot-scale bioreactor to demonstrate the bene• ts of the process and understanding the bio-struvite production yields and growth conditions of B. antiquum in mixed cultures. This bacteria is capable of forming bio-struvite crystals at pHs between 6-8 reaching 250 µm in size. Ammonia recovery can also take place through struvite production, but there are not many options available for ammonia recovery from sludge and centrates streams. Cran• eld University has been developing ion exchange processes to remove and recover nutrients from liquors/wastewater. More speci• cally the project considers 2 di£ erent ion exchange media: a hybrid ion exchange media (HAIX) for P removal and Al 3+ ion exchange media (MesoLite) for ammonia removal. Recovery of the ammonia and phosphate will be achieved through the regeneration cycles as ammonia sulphate and calcium phosphate, which are fertiliser precursors. These technologies are being demonstrated at pilot-scale with the support from Yorkshire Water through the long-term strategic partnership with Cran• eld University. Exciting technologies This article has aimed to highlight just a few possible options for resource recovery from sludge. Other exciting technologies that are being tested at pilot-scale such as recovery of heavy metals, production of bio-plastics, bio- fertilisers and bio-pesticides might reach full-scale implementation in the coming decade. Although relevant technologies for recovering resources from sewage sludge are being developed and demonstrated to be technically possible even at full-scale, the social and economical feasibility is still a major issue. The fact that products are being generated from sewage and sludge raises a number of regulation and social concerns that are di¨ cult to address. More successful cases need to be made as examples so others can follow. Liaising with regulators, the general public and the creation of viable supply chains are key steps to ensure the success of these technologies. Hear more on resource recovery when Cranfi eld's Dr Marc Pidou appears at Utility Week Live, at Bir- mingham's NEC, April 21 at 10.30am. Details: www.utilityweeklive.co.uk

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