Water & Wastewater Treatment

30 | august 2014 | WWt | www.wwtonline.co.uk Digging deeper "the recycling of sewage sludge and of digested food wastes fall under two very different regulatory regimes without synergy" Mike Pedley Welsh Water head of energy Black & Veatch global director of water technol- ogy John Tattersall explains: "We recently com- missioned the £105m advanced sludge treatment facility at Davyhulme wastewater treatment works in Manchester. The process improves sludge's digestibility, increasing the biogas yield. The gas is used to generate up to 11.5 MW of electricity on site using CHP engines [enough to power 25,000 homes]. The WwTW is energy self-sufficient, and has the ability to export electricity to the grid. We have completed, or are executing, similar EPC pro- jects at Anglian Water's Cotton Valley, Whitling- ham, Colchester and Pyewipe WwTWs." Food waste challenges So, as a business critical issue, energy recovery during sludge treatment makes perfect sense for the water and wastewater sector, but with increas- ing AD expertise sitting with the water companies, it's not surprising that other feedstocks are being considered. Even Defra sees the water sector as the logical place to expand AD from food waste. However, this is not without its challenges. The UK regulatory system does not allow sludge and food waste to be digested together - Co-diges- tion. This is despite there being clear and proven benefits. Co-digestion makes some waste streams easier to digest and, in the case of sludge, adds to the calorific value thus increasing biogas output. Not to mention tying together the food/waste/en- ergy/water nexus in one incredibly neat model. Why then, with concepts such as the circular economy and closed loop processes so prevalent in the current resource debate, has it not been taken up on a meaningful scale in the UK? The an- swer involves a range of technical, demographic, economic, legal and policy barriers, the foremost of which is our regulatory framework. The privatised UK water sector has a unique regulatory landscape in which sludge and other waste, such as food waste, fall under different regimes. To satisfy them both creates high costs meaning co-digestion is financially unattractive. Welsh Water's Mike Pedley explains: "The lack of formal clas- sification for co-digested sludge means it falls outside the Sludge (use in Agriculture) Regulations and the Waste Permitting Regulations. The recycling of sewage sludge and of digested food wastes fall under Welsh Water currently has 11 sites with an- aerobic digestion, the largest of which is at its Cardiff wastewater treatment works two very different regulatory regimes without synergy." Andrew Calvert from Yorkshire Water agrees: "We have so far concentrated on getting our own sludge optimised, but we're aware that regulation can produce barriers to innovation and we need to understand this more." However, that doesn't mean that there aren't water companies who are making food waste work for them. Wessex Water, for example, was the first business to build a food waste treatment plant at a sewage works as part of its goal to become a zero waste business. However, co-digestion is still not viable and Wessex set up its own waste-to-energy company GENeco, outside of its regulated water business five years ago. The resulting plant in Avonmouth near Bristol has been operating for more than a year now and produced around 10GWhrs of elec- tricity in its first year - the equivalent power need for more than 3,000 homes. The site takes food waste from households, supermarkets and food manufacturers and is a good example of a water company exploiting additional feedstocks for AD. Severn Trent Water (STW) has also turned its attention to food waste, having already utilised AD for sludge treatment. It recently announced a £13M investment in a food waste AD plant at its sewage treatment works near Coleshill in Warwickshire. By the end of 2014 it should be converting up to 50,000 tonnes of food waste into energy and bio- fertiliser and plans to roll out food waste plants across the Severn Trent region. Project leader Mike Surrey said: "In our region there is 750 kilo-tonnes of available food waste every year. At the moment a lot of this goes to landfill, with no benefit to anyone. However, the new food waste anaerobic digester will be able to generate electricity to power our sites and sell the extra back to the grid, saving STW money and protecting the environment." So, while co-digestion may still be a long way off, AD from sludge and food waste will continue to provide a valuable means of disposing of waste • In Action Welsh Water ● The largest of Welsh Water's 11 sites with AD capability is at its Cardiff wastewater treatment works where it has invested £40m in advanced anaerobic digestion (AAD). The system benefits from pre-treatment through thermo-hydrolysis and generates roughly 20GWh per year of electricity. The AAD facility has reduced its own mains electricity consumption by 45% and mains gas consumption by 75%. It has also reduced carbon emissions by 25,000 tonnes per annum while it also pro- duces enough green energy to power 4,000 homes. Savings amount to around £5m annually which are passed back to Welsh Water's customers.

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