Jeff Cooper explores the various food waste treatment options available to businesses and institutions to mitigate this particular waste stream’s detrimental impact on the environment.

Over the past few years, there has been a number of initiatives to divert food waste from landfill. This is mainly because of the cost of landfill, with landfill tax now at £64 per tonne and landfill fees amounting to over £100. There is also the requirement to reduce the amount of biodegradable waste going to landfill in order to fulfil the targets set by the EU Landfill Directive (1999/31/EC).

There have been longstanding initiatives by local authorities to encourage people to compost biodegradable wastes in their own gardens or to collect and compost certain food wastes, predominantly uncooked vegetable kitchen wastes. More recently, a number of institutions and organisations have decided that it would be useful to undertake the processing of food waste within the confines of their facilities.

Several food waste treatment options are available to institutions that think they have sufficient food waste to warrant treatment on site rather than utilise waste management services offered by the private sector, or in many cases take advantage of local authority provision for waste collection. The use of macerators for food waste has been a practice in some hospitals, but is now regarded as less suitable given both water and electrical services requirements and the issues of potential equipment breakdown and maintenance costs. There are, in addition, digesters of different types — indeed macerators are sometimes referred to as digesters — and compost units.

While this article focuses on composting, the critical point is to measure carefully the amounts of food waste being generated in order to determine the number and size of units to be installed. It is then a question of deciding the best option, whether that be a digester unit, a compost unit or a waste management company with a specialised food waste collection service.

Legislation

The anaerobic digestion (AD) and composting facilities treating food wastes in the UK and throughout the EU are subject to the EU Animal By-Products Regulations (ABPR), which set rigorous standards on the treatment of food wastes. However, there is an exemption from the UK’s environmental permitting regulations. Therefore, provided that the waste processed is generated on site and properly processed to eliminate any risk to the environment and public health, then a permit is not needed. This also has the additional benefit of avoiding the ABPR as well. Removing food waste, wet waste from other wastes will often mean that those dry wastes can be more easily separated and utilised.

AD versus composting at the larger scale

In the UK the preferred method of treating food waste is through anaerobic digestion (AD), mainly because it offers the opportunity to process organic wastes to produce a combination of one, two or three outputs: methane for energy recovery, liquid for fertiliser use or/and a solid residue that can be utilised as a soil amendment after further processing. The UK Government, after neglecting the potential of AD for many years, in 2005 completely changed its view and then actively encouraged the development of AD plants.

In July 2012, however, the Government announced, after an extended review of the incentives for renewable energy, that the renewable energy certificates (ROCs) for electrical energy from AD would be reduced from the current 2 to 1.9 in 2015/16 and to 1.8 in 2016/17. Also, from April 2013 those projects at or below 5MW output will not be eligible for ROCs, although this proposal is subject to further consultation. Industry sources suggest this will hit the overwhelming majority of AD projects.

For the supermarket sector, its preferred option for tackling the food wastes generated in store has been and continues to be AD, mainly because the technology can handle all types of food wastes and several types of packaging, such as paper, board and cellophane. There is, however, even for these food waste generators the options of diverting near end of food wastes to consumers at a discount or to charities distributing food to those in need, using certain food wastes for animal feed or even sending it to compost facilities.

The Vertal food waste treatment facility started operating in January 2010 in a factory which previously housed a dairy and distribution facility. The plant turns a wide range of food wastes into what Leon Mekitarian, Vertal Managing Director, described as “fertiliser” after only three days processing in the plant. Certainly the end product looks and smells like a high quality soil substitute and has a high nutrient content.

The food waste comes in from a wide variety of sources, much of it from other waste management companies such as Sita and Veolia; specialist food waste collection companies that collect food waste from restaurants, hotels, supermarkets and other food outlets; plus the newly instituted food waste collections from local waste collection authorities. The Vertal site has a newly installed weighbridge and 12 large waste processing tanks in the plant.

The processing system is based on a well-established sewage treatment process — ATAD (auto-thermal thermophilic aerobic digestion) — common in the Western half of the USA and Canada but not used so often in Europe. Therefore, water is added to the waste food that comes in from a variety of sources so that there is 80% water content when pumped into the first treatment tank. However, unlike the more common anaerobic processing system, the process is kept aerobic and completes its processing in 3 days compared to the 20–30 days for anaerobic facilities. The waste is then moved from one tank to another with the temperature increasing from 55oC in the first tank, 65oC in the second and 75oC in the third, in order to meet the pasteurisation requirements of the ABPR.

The use of composting facilities for selected food wastes has been undertaken in various parts of the country, sometimes kitchen waste separately, but more often with selected vegetable wastes added to green garden and other wastes. These are then treated in enclosed facilities and are required to conform to the ABPR.

Imperial’s composting unit

The initial development of Imperial College London’s in-house composting facility was the installation of a Swedish-designed unit in February 2010. It was soon realised, however, that a larger unit was required and therefore a new IMC unit was installed in September 2011, which was housed in its own Portakabin together with a macerator, sinks, dishwasher (to thoroughly clean the collection caddies whenever necessary), weighing device and water and electrical services. The total cost amounted to £140,000. The unit also has a clean and dirty end so that, if necessary, the unit’s compost output can be approved for use beyond Imperial’s own grounds through approval by the Animal Health Inspectorate of Defra and permitting by the Environment Agency.

The operation of the unit has been assisted by Professor Sue Grimes and her colleagues at the Department of Civil & Environmental Engineering at ICL. Together they had investigated different options with the original Swedish unit in terms of its input characteristics and output parameters in order to optimise the second unit’s ideal operational parameters.

The throughput of the unit is some two tonnes per week of food waste sourced exclusively from all of ICL’s commercial food outlets but the amounts collected are variable, with the greatest variation and largest amounts generated in the vacation periods. This initially appears to be counter-intuitive but is due to the use of ICL’s residential premises by visitors outside term time. Their demand on catering facilities varies more than the student population. However, one of the important aspects of the work now is to work on the issue of food waste prevention using the guidance available through the Sustainable Restaurant Association. This is because the amount of food waste being collected for processing exceeds the unit’s capacity. As Nic Dent, Waste and Recycling Manager at ICL, admitted, they had found it very difficult to survey the food waste arising from food outlets in at the college in sufficient detail even prior to ordering the second unit.

The food waste is collected in caddies from each of the restaurant units, taken to the composting unit, macerated to achieve optimum particle size. Then, because the maceration operation adds water, the material is de-watered using an Archimedes screw with the water going to foul sewer. The material is weighed and placed in the composter and pellets of sawdust are added in order to achieve an optimum carbon/nitrogen balance, the mixture turned with air being added. The compost unit itself is turned three times a day for a couple of minutes each time to ensure proper aeration and mixing of the material. Thereafter, the compost emerges from the end of the unit after about 10 weeks with a further three months maturation in ICL’s existing garden unit that also has a green waste composting facility.

Initially, the compost from the unit was kept separate from the green waste compost at the site but the aim is to mix it in with the green waste compost. However, the compost from the unit will be tested to see whether it conforms to the standards set by PAS 100 (Publicly Available Standard 100). Because the first loads of compost have only just reached maturity, this has yet to be undertaken.

The benefits of in-house composting are not just the savings in landfill tax, currently £64 per tonne and rising to £80 in April 2014, and the minimal financial benefits of the value of the compost, but the greater utility value of the remaining waste, the increased flexibility of waste storage and transport and the cost advantages that brings. No longer does ICL use enclosed compactor units because the risk of rodent infestation has diminished to almost zero. Dry recyclable wastes can be easily separated at an MRF (materials recovery facility), for example, and the waste does not have to be moved so often. Indeed, in the case of ICL, the college is now able to access the local authority waste collection services and thereby save substantial amounts each year.

There are several other units now working in different institutions, such as hospitals and prisons, all of which can benefit from the exemption from permitting provided that the waste processed is generated on site and the product used on site.

Other options and facilities

In July 2012, the first WasteMET Asia Congress and Exhibition was held in Singapore. The exhibition had several exhibits focusing on the treatment and disposal of kitchen waste. One of the exhibitors had travelled from New Zealand where HOTROT had been originally developed to process lanolin from wool. The technique has now been adapted to undertake the treatment of kitchen waste.

There are several units of different size that are now operating in the UK. The most publicised HOTROT facility in the UK is the Twyford Cross Zoopoo treatment facility where a mix of animal manures and food wastes from the zoo’s catering facilities are added to the mix. The zoo has decided that on economic and environmental grounds it is preferable to process its food waste within the site. The compost will be used both for the flower beds and other decorative areas within the zoo’s premises but also in growing food for the animals housed within the zoo.

In contrast to HOTROT’s composting facilities, there were two further exhibitors showing how enzymes can be utilised to degrade and digest kitchen wastes so that the only end product is waste water which can be sent to sewer for treatment at a sewerage plant. Indeed the Marina Bay Sands Hotel has two one-tonne per day digester units in its basement to handle all of its restaurant waste. This is a more sophisticated form of maceration but undertaken in completely enclosed facilities so that that the only emission is dirty water to the sewerage system. The original equipment is expensive and the enzymes need renewing each year but it does provide easier handling of other wastes for recycling, for example.

Last reviewed 8 October 2012