Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | augusT 2014 | 29 Digging deeper Making energy bills easier to digest P ut simplistically, the cost of energy has two flavours in the water industry: usage reduction and energy generation. The former drives bottom line sav- ings as does the latter, however, on-site generation can also drive the top line if enough energy is produced to export to the grid. Couple this with the need to treat and dispose of sludge and anaerobic digestion is a clear win/win for the water sector, and many water companies are already making use of it for sludge treatment and energy recovery at their sewage treatment works. Sludge however, is not the only feedstock being exploited by the sector and food waste is increas- ingly attractive for use as a feedstock for onsite AD. It's not without its challenges though, as co-digestion, already being utilised in other parts of Europe such as Denmark, is still not happen- ing here in the UK. But what is preventing it from happening? And who is making the most from AD? The value of sludge Yorkshire Water is investing heavily in AD, includ- ing a £30m spend at its AD plant at Esholt (see 'In Action' box out) converting the system to advanced digestion with thermal hydrolysis, a brand new facility at Blackburn and refurbs at five more. The benefits are clear says Yorkshire Water investment and improvement manager Andrew Calvert from both a cost and a sustainable busi- ness perspective: "We use AD as our main sludge treatment process because our assessment of tech- nologies shows that AD is the lowest whole life cost treatment. It has the lowest carbon footprint, and creates bio-gas to produce renewable energy. It also produces a safe and environmentally benefi- cial material to recycle nutrients to agriculture." Welsh Water agrees, and the carbon reduc- tion potential of AD is a key driver for the com- pany, which is investing heavily as it pursues its long term strategy to reduce carbon emissions by 25% by 2015. Welsh Water currently has 11 sites with diges- tion and electricity generation from combined heat and power (CHP) engines. And already Yorkshire Water has spent £30m converting to advanced diges- tion with thermal hydrolysis at its Esholt plant. Pic: suave air Photos security of energy supply, reduced costs and built-in waste treatment it's no wonder aD is a no brainer for the uK water sector those sites process around 90% of the sewage sludge Welsh Water receives. The largest of these is at Cardiff wastewater treatment works (see 'In Action' box out)." And, says Welsh Water head of energy Mike Pedley, the plan is for all sludge to be processed via digestion by the end of AMP6: "During the coming investment period to 2020, we will move to 100% processing through AD with a further site in North West Wales and upgrading to our plant at Wrexham with thermo-hydrolysis and biomethane injection to the Grid." Scaling up Welsh Water isn't the only one to go big on AD. In fact, United Utilities has just gone 'biggest' at its Davyhulme Wastewater Treatment Works (WwTW), working with engineering, procure- ment and construction specialist Black & Veatch to open the largest advanced digestion plant of its type in the world – and makes the site energy positive, with the ability to export surplus energy to the grid.

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