Esholt trials AD pre-treatment technology

Although anaerobic digestion (AD) has played an important role in the sludge process at many wastewater treatment facilities, a lack of readily available data from new technologies means water companies and their contractors still face a steep learning curve.

Enzymic hydrolysis and high-pressure thermal hydrolysis are to make their mark at some of the larger UK sites, but one water company believes it may now have found a discreet pre-treatment technology for sludge digesters that is at the lower end of cost and the high end of simplicity.

At a time when AMP5 places great weight on the need to optimise existing AD facilities, Yorkshire Water is set to unveil the results from a process it took its first steps with in 2007, when a full-scale Cellruptor pilot plant was installed at Chesterfield’s Old Whittington Sewage Treatment Works (STW).

Despite lengthy interruptions when severe floods engulfed the area, completely overloading the works, results compared two identical digesters: one fed with thickened, mixed primary and secondary biosolids as normal; the other following Cellruptor’s pre-treatment that disrupts the particulate and cellular structure of the sludge.

The increase in gas generation during the periods of stable operation was sufficient for Yorkshire Water to invest in its first scale demonstration Cellruptor unit, which is now being put through its paces at Esholt STW near Bradford.

Consisting of a feed-pump, biogas compressor, three vessels (two pressurised) and a control valve, this technology from Eco-Solids International is available at something in the region of £0.5M, compared to approximately £5M to £10M for the far more complex thermal hydrolysis or enzymic hydrolysis systems.

Clive Rigden, managing director of Eco-Solids International, commented: “It’s been a longer journey than anticipated to get this far, but proving we can increase gas production by up to 40% on the SAS feed is well worth the wait – especially with the water industry now looking very, very closely at parasitic loads of some technologies that affect the real carbon cost and carbon performance of AD.”

Peter Barratt, Eco-Solids’ sales, marketing & product development manager, added: “Cellruptor’s technology not only produces significantly more gas but is set to show how it makes digesters easier to manage and less costly to operate. Operating at low pressures (10bar at most), Cellruptor may also reduce the amount of energy that goes into dewatering digestate, and decrease the volume of sludge for disposal.”

Jon Brigg, Yorkshire Water’s innovation delivery development manager, said: “Following our long R&D partnership with Eco-Solids International, we will shortly have the data to completely validate the Cellruptor process.

Knowledge gained from this installation (which is supported by the Carbon Trust) will go a long way in helping the industry maximise the generation of biogas and associated revenue from a sewage treatment works in a capital-efficient, low intervention manner.

“For smaller and medium-sized works we can see how Cellruptor offers a more capital efficient payback time than more complex technologies. This may translate to our larger, current and planned AD installations. Our work with Eco-Solids is one of Yorkshire Water’s key initiatives to increase our renewable energy generation and improve the energy efficiency of our assets.”

Eco-Solids International has also been working closely with some of the UK’s leading universities (including the University of Southampton) on R&D projects into sludge, food waste and manures, exploring ways to enhance renewable energy and improve digestate quality.

This work is leading to future product opportunities with other feedstocks, which could make it a valuable contributor to co-digestion. At Esholt STW, Cellruptor’s cost-effectiveness might also be enhanced because the process points to far less foaming from the extracellular polysaccharides present in Secondary Activated Sludge (SAS) – so far less need for expensive chemicals to treat this common problem.

Eco-Solids’ Clive Rigden concluded: “Some water companies still have to send treated sludge over 100 miles away for disposal. By improving volatile solids digestion to produce a far lower volume for disposal, Cellruptor ticks all the right boxes for those also trying to get to grips with balancing the complexities of the various energy units that need to be achieved.

“For the water industry, the wider roll-out of Cellruptor offers benefits to the bottom line of their businesses.”

With biosolids and waste now such big business, the challenges of finding appropriate and financially viable technologies make the evaluation of Cellruptor’s Esholt performance one of the most eagerly awaited landmarks in the industry’s bid to truly optimise its AD offering.

Eco-Solids is confident that Cellruptor will succeed, and while it is also developing its own Eco-Solids Process to convert sewage into an enhanced treated material without nuisance odours, it is also keen to push Cellruptor into the main wastewater treatment process where it can reduce the production of waste sludge. Possible test sites are already under discussion.