Water & Wastewater Treatment Magazine
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26 | SEPTEMBER 2016 | WWT | www.wwtonline.co.uk real-time control is a key area for optimising the sludge treatment process to ensure a sufficient, but not excessive, amount of energy and resources are used. Hach is one company that has led the way in the real-time control of the dewatering process, ensuring that sludge entering the digesters has the optimal percentage dry solids (DS). Within anaerobic digesters themselves, a key challenge is ensuring digesters do not become clogged with grit or struvite, which reduces efficiency. Mixing is an important function to ensure smooth operation and efficiency can be promoted by new systems such as Landia's GasMix (overleaf). Meanwhile, the precipitation of struvite can be prevented by the use of a reactor such as the Ostara Pearl (marketed in the UK by ACWA Services) which also provides an opportunity for resource recovery of phosphorus. Finally, while the remaining digestate can be sold as fertiliser, pyrolysis (in which sludge solids are turned into liquid and gas when subjected to high temperatures with no oxygen) is an alternative which is gaining traction in urban areas where this is not an option. Thames Water has invested in research into this process, which is more sustainable than incineration and has the potential to produce more energy. A naerobic digestion is a simple process and one that has been widely deployed across the UK wastewater industry in the last few decades. However, almost every stage of the treatment and processing of sludge can potentially be improved upon, through innovations which improve the yield of valuable biogas, make more efficient use of this gas to produce energy, recover other resources or leave a higher quality digestate for onward use. The first area of improvement has been the development of pre- treatment processes that break down the sludge through heat, pressure or other methods prior to it entering the digesters; these can make a marked difference to digester performance. One relatively well-established such process is thermal hydrolysis, which uses pressure cooking of the sludge at high temperature followed by a rapid decompression. Cambi and Veolia are the two companies most associated with this technology, which has now been implemented at around a dozen UK wastewater treatment works, the biggest of which is UU's Davyhulme. However, 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. Whatever pre-treatment is used, Innovation Zone: Sludge and AD Powerful sludge ● Advances in anaerobic digestion (AD) mean that sludge is an increasingly valuable resource, a fact reflected by Ofwat's plans to open up a new market for bioresources. In WWT's latest Innovation Zone special, we run the rule over technologies that can help wastewater utilities get the most out of their sludge THE CHALLENGE Sludge treatment, anaerobic digestion and its related technologies are currently considered hot areas for innovation by wastewater utilities, not only because of the money to be made from exploiting the energy potential of sludge, but because of Ofwat's intent to open up a new market for sludge as part of its Water 2020 reform proposals. When these reforms come into effect, utilities that are leading the way on the efficient processing of sludge will be able to do deals to acquire sludge from less efficient neighbours, explore synergies with the wider waste sector and set up profitable subsidiaries and joint ventures concentrating on extracting maximum value from sludge. While in the past sewage sludge was seen as a waste product to be disposed of, commonly being dumped in the sea, the widespread use of anaerobic digestion (AD), a process which creates biogas which can be used as a fuel, changed all that. Over 80% of UK sewage sludge in 2014-5 went through an AD process, according to Ofwat figures. The biogas from the process can be converted into electricity and useful heat through CHP engines, or cleaned and injected into the national gas grid; meanwhile, the material le a er AD is finished, known as digestate, can be recycled as fertiliser to agriculture. Ofwat has estimated that opening up the market for sludge will result in a £780M boost to the industry, purely calculated through the extra energy that could be generated. With this in mind, water and sewerage companies with large-scale AD operations are keen to ensure that they work as efficiently as possible in order that they can compete strongly and gain a better slice of the pie from deregulation. Innovation, at every stage of the sludge treatment process, will be a crucial component of this. 13% The estimated proportion of UK sewage sludge that could be profitably transferred across water company boundaries (source: Ofwat) Real time control (Hach) P recovery: Ostara Pearl Thermal hydrolysis: Esholt