Tony Clutten, Process Sales Manager at AD specialists Huber Technology, discusses how process optimisation at AD plants can deliver significant operational and bottom line business benefits.

Huber: With anaerobic digestion set to play an increasingly important part in the UK's renewable energy mix, process optimisation is seen as key to delivering operational and bottom line business benefits to operators of AD plants.

An established technology in the UK water sector for many decades, the potential introduction of upstream competition in the market means the AD sector is poised for further growth.

Both established operators and the increasing number of new operators who now process food waste and other organic wastes are looking to optimise their operational performance in terms of reducing costs and maximising revenue streams.

They both face similar challenges, including grit removal and producing a clean digestate. Within supermarket and source segregated food waste for example, even a small percentage of grit and glass causes AD operators pipe blockages, abrasion, reduced digester capacity and costly cleaning routines to remove.

 At some AD food waste plants grit and glass is causing some plants to shut down at 12 hour intervals for routine maintenance, while others have major pump wear problems. Other plants have digesters which are substantially full of inorganic matter that is difficult and costly to remove, effecting both the retention time and subsequent gas yield.  

Two key areas where process improvements can deliver significant benefits are:

Grit removal

Huber was recently asked to look at an existing AD plant in Northern Ireland which takes in a mixture of food and green waste where grit in the system was causing blockages, sedimentation and wear issues.

Huber retrofitted a HUBER Longitudinal Grit Trap ROTAMAT® Ro6 Bio which allowed settlement of the heavy fraction and air circulation to reduce the organics that settle in the tank. The heavy fraction is then elevated and drained, followed by washing and dewatering in a HUBER Grit Washer RoSF4 G4E to wash out the organics which are then returned to the flow and provide a reduced volume for disposal.

Removal of materials including grit, glass and bone will dramatically improve the plant’s performance and reliability.

In a separate project, leading waste management company Shanks incorporated a longitudinal grit trap from Huber into their AD plant at Westcott Park in Buckinghamshire. For Shanks, grit was a key concern in the design of the Westcott Park AD facility – the firm was keen to ensure that as much as possible was removed before it could cause downstream problems.

Huber have developed a grit washer with some special features to suit the bio industry - air is used to aid organic separation from the grit and PLC control sequences the separation process and draining process.

An estimated 3% of the weighbridge feed was removed in the initial settlement channel and by washing and draining the heavy fraction the weight of the heavy fraction is reduced to less than 1% of the weighbridge feed weight. The other 2% is comprised of water and organics which are returned to the soup. The biomass-enriched returned water delivers a 1% increase in gas yield and also aids the water balance in the system, providing significant process and commercial benefits.

Photo:Trial grit washer

Reducing volume/tonnage to landfill and organics recovery to increase gas yield

Over a nine month period Huber carried out a trial at renewable energy firm Tamar Energy to assess the potential for reducing reject material and associated disposal costs which come straight off the bottom profit line. Typically, 7% to 12 % of the weighbridge food waste was being rejected by Tamar’s Tiger de-packaging unit.

Huber opted to use a HUBER Wash Press WAP® L modified for the application which has the key benefits of using very little water and accepting rejects continuously. The well-proven equipment has seen over 400 units installed in the UK alone. Tolerant to variations in feed materials, the technology delivers a 50% reduction in weight for disposal which rises even higher on supermarket waste.

In addition to the positive environmental and cost advantages of sending less organics to landfill, the organics can then be returned to the soup giving increased gas yield. The trial plant, which is easy to retrofit, delivered a payback period for the new equipment of less than 3 months.

Huber has been working with the AD sector both in the UK and overseas for decades – we understand the problems operators face and the type of solutions they need to get the best out of their AD plants.

Our ongoing development programme to further develop and test well proven equipment for anaerobic digestion plants is always focussed on meeting our customer’s requirements and providing them with the solutions they need - which optimise their process and ultimately deliver demonstrable bottom line benefits.

Huber has published a useful new Anaerobic Digestion Guide for existing operators of AD plants and companies interested in learning more about the AD process - email This e-mail address is being protected from spambots. You need JavaScript enabled to view it to receive a free copy.