Most anaerobic digestion (AD) plants in the UK use a wet process, designed to process pure food waste and a range of agricultural organic waste streams. But large quantities of food waste come from supermarkets in plastic retail packaging and a significant level of contamination is common from household food waste collections.
Most AD plants remove non-food materials with a de-packager at the start of the process. The de-packager offtake is a thick ‘soup’ of all packaging materials and food residues, which is generally only suitable for disposal at incineration or landfill.
Up to 15-30% of what arrives at a plant is removed by the de-packager, so its disposal represents a major cost for AD operators. Costs per tonne stand at £86 median for incineration and £107 for landfill, according to WRAP’s 2018 Gate Fees Report. By contrast, £35 is the median commercial gate fees for in-vessel composting.
Located near Leeds, The Maltings Organic Treatment has developed a solution to compost its AD plant’s de-packager offtake. The method removes most of the disposal costs normally associated with de-packagers, and can recover plastics and create compost to return vital nutrients to local fields.
The Maltings is dedicated to the full recovery, processing, reuse and treatment of organic liquids and solids. Its facility has a permitted capacity of 225,000 tonnes a year, split across a variety of treatment and recovery systems, including AD and composting.
The company developed the system because it is committed to composting as a cost-effective and sustainable technology to complement its AD processes, and sees huge potential for the AD sector to reduce disposal costs using the method. Currently the majority of its feedstock is from commercial customers.
The compostable materials available now are reliable and break down effectively within existing systems. Aside from compostable bin liners, the method allows The Maltings to process packaging manufacturer Vegware’s compostable disposables, of which it is seeing more from foodservice clients such as Let Loose in Hull. Last month, Forge Recycling started to bring in used Vegware and food waste from Leeds, York and Harrogate.
The processes have been designed and adapted over many years. It originally started with Leeds City Council’s food waste contract and its need to use compostable bags. The Maltings is supportive of all compostable packaging instead of plastic and, in its opinion, the recent move by councils to push for the use of plastic bags in food caddies is a step backwards.
Some AD operators prefer food waste in plastic bags over compostable liners, but it is impossible to remove it all. Microplastics will inevitably be present within the resultant digestate and then be spread on land. Ocean plastics are a known issue, but soils also desperately need protection.
Defra’s recent 25-year environment plan has recognised this and places a much higher value on soils now than at any time in the past, and this is welcomed by the organics recycling community.
The company is committed to producing quality products that meet the Environment Agency’s quality approval schemes, and its liquid digestate is now PAS110-approved. Later in 2020 it will be ready for PAS100 approval to demonstrate the quality of the compost.
Steven Carrie is a director at The Maltings Organic Treatment
The Maltings’ method step-by-step
Creating compost and recovering plastics
Input materials are assessed for their use in one of The Maltings’ five separate types of processing, which includes anaerobic digestion (AD) and composting. Here we focus on the method of composting the de-packager offtake to recover materials and keep them in the circular economy
Removing packaging from food waste for anaerobic digestion
Packaged materials including plastics and compostable packaging goes into a reception building where it is loaded into large twin-screw mixing hoppers – the de-packager. There are no metals or glass, as these are sent to separate in-house processing facilities in the manufacture of concrete blocks.
In the hopper, the material is broken open by screws and waste waters are added – this is often liquids from the de-packaging of fizzy drinks and so on. The material begins to resemble a thick, wet, paste-like material with packaging. At this stage any compostable packaging, including Vegware products, cardboard and paper packaging, begins to absorb water.
The material then passes into a system where the high pressure and naturally acidic liquids begin an initial hydrolysis process. This then passes into a second chamber where the liquid is forced out through 10mm screening heads. This size is necessary to comply with the requirements of the Animal By-Products Regulation (ABPR). The resultant liquid carries with it the organic and food materials which are pumped to a batching tank for further mixing.
The remaining packaging material is forced through a final cutting head and on to a conveyor belt. From here it is sent to a cold wash and over a final screen to remove more of the organic residues.
De-packager offtake spends 5-7 days in the in-vessel composter
The run-off is now mainly plastics and residual compostable packaging, with around 5% food residues. This fraction is sent to an in-vessel composter, which uses naturally occurring aerobic microbes to break down the waste with other organic feedstocks in large enclosed vessels.
The process naturally reaches more than 70°C, as required for complete pathogen destruction – a strict criteria for the UK’s ABPR. The retention time in the enclosed vessel is around 5-7 days for full sanitisation.
The material is ejected from the bottom of the vessel, where it is now much dryer and the organics have begun to break down and turn brown. At this point, most of the compostable fraction is breaking down, but the oversize plastic packaging is still whole.
Maturation in open windrow composting
The material is transferred into an open windrow composting area for 6-8 weeks of maturation under aerobic conditions. It is at weeks 3-4 that there is no longer any compostable Vegware packaging. After the full 6-8 weeks this stage is complete, and the materials is screened through a 20mm or 10mm system to remove the plastics and compost as separate products.
Recovering plastics, creating soil improver
The compost is run under deployments to local farms within a 15-mile radius of The Maltings’ site. It owns two businesses within the community, so has always given back to them where possible.
As part of this commitment, the company provides local farmers with up to 8,000 tonnes a year of compost free of charge. The farmers have used the material year-on-year, and have successfully reduced their reliance on chemical fertilisers with no detriment to crops.
Composting de-packager offtake reduces The Maltings’ disposal costs and ensures that its system is viable, allowing it to gift compost to the community.
The plastics are sent as either refuse-derived fuel or to the company’s plastic grading facility in Hull, which recovers and granulates it for the manufacturing industry.
The entire process combines the benefits of maximising the capture of food waste through the use of compostable bags, the collection of biogas from AD and ensures that the packaging materials are fully recycled using an aerobic process. The methodology is used extensively and successfully in Italy, where all packaging and carrier bags by law need to be compostable.
This minimises the opportunity for plastics to enter soils through the more conventional use of polyethylene-based material, which causes harm to the environment.