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Digestion stations

Disposing of food waste comes with all sorts of problems. Carolyn Cross looks at one of the latest options – aerobic digestion.

Aerobic digestion 1


9 December 2013

Whichever way you look at it, soggy, malodorous food waste is the antithesis of the perfect dining experience, but it’s also something that all catering enterprises, from Michelin-starred restaurants to prison canteens, have to deal with.


In the past, choices were straightforward but limited; food waste was simply pulped in a macerator and disposed of to drain, or bulked with other waste materials and trucked to landfill, out of sight, out of mind.

These days, however, a combination of more enlightened thinking, legislative changes and economic drivers such as Landfill Tax have driven caterers to work on strategies to reduce wastage, and to think of ways to dispose of unavoidable food waste more responsibly.

Composting and anaerobic digestion are well known alternatives to maceration, but an innovative on-site option – aerobic digestion – which does not require storage of food waste, vehicle movements for collection, or the building of large-scale industrial plants, is consuming the nation.


Small, but perfectly formed
Small enough to be sited in a standard commercial kitchen, the system employs bioremediation, a biological process. In layman’s terms, the equipment is inoculated with naturally occurring bacteria, which continue to multiply and digest the food waste, resulting only in a liquid digestate which is flushed straight to drain with no risk of causing blockages.
  
Waste 20 was designed and engineered by kitchen equipment specialist Mechline Developments. The machines are in use in restaurants, shopping centres, hospitals, hotels and even boats looking for a simple solution to the food waste dilemma. The units have also recently been installed at the Liverpool ONE retail centre (see p.22). Their appearance is uncannily similar to standard piece of kitchen equipment such as a dishwasher, and it slots into a space measuring fractionally more than one square metre, connected straight to drain.

Up to 180kg of food waste can be digested over a 24-hour period. Inside the machine, plastic poly chips provide an ideal living environment for bacteria, which make short work of food waste loaded straight into the machine from the kitchen preparation area.

Ian Cresswell, business development director at Mechline Developments, says: “We recommend that customers use special clear buckets to collect food waste prior to loading, which helps to monitor wastage. This simple visual reference means that operators can make real-time decisions to improve food usage and cut costs at the same time. Warwick Hospital, for example, reported savings of £8,500 per year after installing Waste 20.”


Industry approved
At present, Waste 2-0 is the only commercial food waste system to gain approval from the WRC (Water Research Centre) and also the WRAS (Water Regulations Advisory Scheme). Cresswell explains: “Approval has confirmed that we adhere to the water companies’ strict guidelines on what can be discharged into drains and that effluent quality, including acidity levels, falls within standard trade effluent levels. In fact, one water company – Yorkshire Water – has funded installation at a site where the food waste management system regularly caused blockages within the sewer system.”

Integral to the design of the machine is the fluid used. Cardiff-based Biological Preparations specialises in the development of products containing bespoke strains of bacteria and selected the group used in Mechline’s Waste 20 aerobic digestion system and its sister product GreasePak. The latter is a drain management system which offers an alternative to the traditional grease trap that is required by building regulations for commercial premises. 

Technical director Dr John Lear describes bioremediation as an ideal way to treat food waste and fats, oils and grease (FOGs): “It is a natural process, which breaks pollutants down completely rather than just transferring them elsewhere. I also think it is cost-effective as there can be less need to manually clean out pipework and grease traps.”

Costing in the region of £11,800, the machine compares favourably with collection and disposal contracts. Based on optimum usage, the current Landfill Tax rate of £72 per tonne equates to £4,730 per year even before factoring in the additional cost of collection and handling.



The science bit
Mechline has incorporated a number of safety features to minimise the potential for human error or misuse as well as energy-saving features to help reduce site operating costs even further. Once the kitchen staff has adapted to the idea of a new system, training required is minimal and it’s just a case of loading the machine and letting the bacteria work their magic.

The bacteria used in Waste 20 are supplied in the dormant spore form, becoming active once they come into contact with a suitable food source. Bacterial spores are extremely stable, with a very long shelf life, and can be easily incorporated into virtually any type of product. 

It is possible to source bacteria that do not form spores, sometimes referred to as “live cultures”, but the term is confusing and these bacteria may be less satisfactory. Lear explains: “Both spore-forming and non spore-forming bacteria are ‘live’, but when people refer to ‘live cultures’, they are usually talking about bacteria that do not form spores. These need refrigeration to ensure stability, and some types are not found in the lowest hazard rating set for bacteria, which means safety may be an issue.” 

Bacteria need to be provided with the right conditions to thrive, with many working best in a consortium. Lear adds: “They require a suitable temperature and an appropriate food source. Oxygen is also important for these strains, which is why the contents of the Waste 20 machine are agitated.”

When choosing the right blend for Waste 20, the first consideration was to find a range of bacteria capable of degrading all of the major food groups. Speed of degradation and ability to remove odours were also taken into account; however effective the system, it’s certain that no one would be impressed if the resulting reaction left diners holding their noses throughout the dessert course.

Bioremediation may seem like a novel approach, but in fact, bacteria may have been fine-tuning their approach to waste management since before Man took his first steps. Lear concludes: “Microbes have been nature’s binmen for many millions of years. Over that time they have evolved those processes and have become very efficient. Combining this process with the Waste 20 machine results in an effective on-site waste management solution.” 

 
Waste not, want not

When Bloxham School decided to reduce the environmental impact of its food waste, it chose aerobic digestion. The school quickly discovered that the on-site technology would also help to cut costs and maintenance.

Bloxham School in Banbury, Oxfordshire  is part of Woodward Schools, the largest group of Church of England schools in the UK, which educates 27,000 pupils a year. It caters for day and boarding pupils and produces around 180 litres of food waste per day. Historically, it had relied on macerators to dispose of uneaten food and waste from kitchen preparation areas, but in 2008 the school decided it was time to look for an alternative solution.

Alex Linnington, procurements and environmental manager, said that options such as anaerobic digestion had previously been considered, but that cost had proved prohibitive. Plus, Linnington added, it would have little effect on environmental impact due to the need to transport food waste.

Linnington said: “With the Waste 20 machine, our cost savings on labour are more than £6,000 per year, and as the Waste 20 system uses less water than a macerator, we’re saving there too. We also save around £400 per year on external drain maintenance – using a macerator meant that grease traps and drains needed cleaning regularly; since installing this machine we have cut that requirement out completely.”