by Arable Farming July 2020
As plastics come under the spotlight environmentally, their presence in some digestates is of growing concern. Marianne Curtis finds out more.
Soil scientist David Tompkins says he is horrified by the prospect of spreading plastics on soils. However, this could be happening where digestate, produced from anaerobic digestion (AD) plants using food waste and sewage sludge as feedstocks, is being spread on agricultural land.
Dr Tompkins, of environmental consultancy Aqua Enviro, says the jury is still out on whether plastics harm soil biology.
“Research is underway but so far there is not enough evidence to say plastics are actually harming soil biology. Spreading plastic on soil does not seem right and should be stopped if harm is shown.”
PAS110, the specification to which the food waste digestate industry works, requires declaration of total glass, metal, plastic and any other non-stone man-made fragments greater than 2mm on a freshweight basis.
The permitted limits on the total of these contaminants vary on a sliding scale according to total nitrogen content. For example, with a total nitrogen content of 5kg/tonne of digestate, the specification allows up to 0.22kg/t of these physical contaminants.
“If all physical contaminants were plastics this would mean 220g of plastic for every tonne of digestate applied to land,” says Dr Tompkins.
Farmers should have a conversation with their digestate suppliers to determine what is in the digestate being supplied, he advises.
“If it was waste, could there be plastic residues? Does it comply with PAS110? Are you happy to take it or do you want a higher quality [lower plastic content]?”
In AD systems where purpose-grown crops are the feedstock, no plastics would be expected and with manures there may be the odd bit of silage wrap or other farm litter, says Dr Tompkins.
“Food packaged in plastics is a higher risk feedstock and these digestate suppliers should all be adhering to the PAS110 limits.”
In 2013, the latest figures available, more than 99% of digestate was destined for agriculture or field horticulture. However, use of land for spreading digestate may be under threat, adds Dr Tompkins.
“Changes, if they happen, are more likely to be in response to demand rather than policymakers, unless microplastics are shown to be causing harm. It is more likely the market will say I don’t like this any more and it will stop.”
In Scotland, reduced limits for plastics in digestate have been phased in since 2017. The example given earlier in this article would equate to 5kg total N = 17.6g plastics/t of digestate. These limits were originally developed by the Waste and Resources Action Programme and Zero Waste Scotland in conjunction with Quality Meat Scotland and NFU Scotland with the aim of delivering user confidence in digestate quality.
“Indications are that this has been the case, but more scientific evidence is needed to judge whether these limits are still too high – or even too low,” says Dr Tompkins.
He adds: “Digestate is a great fertiliser, but logistics are a headache, and ammonia emissions are a national issue. There are numerous threats to landbank – both real and perceived – with microplastics currently leading the field.”
Quality digestate does not start and end with the AD plant, says Fiona Robinson of the Scottish Environmental Protection Agency
“It depends what enters the AD plant and what people put in the bin.”
In Scotland, the emphasis is on preventing food waste, but where this cannot be achieved, undertaking high quality recycling either through composting or AD, which must meet PAS100 or PAS110 specifications, respectively, is the preferred option.
Ms Robinson says: “The Scottish Government wants to achieve a circular economy and has realised it needs to control the quality of what is arriving at the plant, so we looked at permits and have introduced permit conditions to make sure sites have pre-acceptance procedures for everything they want to bring in, to determine contamination rates.
“This gives them powers to reject loads exceeding contamination rates, depending on what depackaging or processing a site can do to meet PAS110.”
Biovale, which promotes Yorkshire and the Humber as a centre of innovation for the UK bioeconomy recently held a webinar: Preventing plastic pollution from AD digestate. For more information, visit biovale.org