Tackling plastics in digestate

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.

Nitrogen content

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 ?

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 dont 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.

AD plant

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

Consultations on biowaste treatment

Plastics in digestate is a key concern for the agricultural sector and the NFU believes more should be done to eliminate plastic contamination in materials spread to land and improve market confidence, says Philippa Arnold, NFU environment policy adviser.

She says the NFU has responded to the Environment Agencys ‘Standard rules consultation no 20: revision of standard rule sets for biowaste treatment issued last autumn and also the EAs review of the quality protocol frameworks for compost, anaerobic digestate, and poultry litter ash issued last December.

Summarising NFUs responses, Ms Arnold says: Contamination should be reduced at every stage, wherever possible and not just passed on throughout the chain to the end user; the food producer. This is of particular importance for sites which deal with local authority collections and supermarket waste food, which typically has a high plastic contamination percentage.

Support

The NFU would welcome support from Government both at a national and local level to assist local authorities in raising awareness to the householder.

She adds: We do not believe that limits imposed by current waste input contracts (around 5%) are sustainable and are therefore seeking a prompt and more stringent approach to inputs wherever possible, as we are already seeing a reduction in market confidence (even when materials are compliant with PAS100 and PAS110) solely due to plastic contamination.

The NFU says it fully supports investment in technologies which will help reduce the level of plastic contamination and feels there is more to be done in terms of informing and educating businesses and households who contribute to these waste streams in order to help reduce and eventually eliminate plastic contamination.

In the field: A Metcalf, N Yorks

JAdam Metcalf, farmer and contractor in North Yorkshire contract spreads 350,000 tonnes of digestate per annum which is nearly all from AD plants that take food waste.

He says: It would be a huge problem if plastic was going through. We spread a large proportion of the digestate on our own land. It is something I am very conscious of. We monitor it and have the digestate tested every month.

For the sites I look after, plastic is not an issue.

He believes this regular monitoring is key to getting high quality digestate.

In the contracts we have for removing digestate it clearly states we have the right to reject it if it does not meet the parameters in the contract. We ask for PAS110 standard. As long as those parameters are met, I feel it is acceptable.

PAS110 plants are inspected annually, including sampling, however, Mr Metcalf, who believes regular testing is very important, sends samples to independent laboratory NRM every month.

He says he has seen problems on other farms including in Scotland several years ago where there were lines of plastic in the field where the digestate had soaked in.

Soil and microplastics research

The Scottish Environmental Protection Agency (SEPA) has supported research to determine what effect microplastics have on soil.

Work at the University of Stirling looked at microplastic concentrations of between 0-2%.

Fiona Robinson says: This showed that at higher concentrations of microplastics in soil, worms lost more weight and were more likely to die than at lower concentrations. The 2% concentration group had the highest number of deaths at 12%.

SEPA also plans to publish a landbank report in autumn which will look at the barriers and restrictions to using organic matter in terms of climate, topography and soil, to help determine areas which should or should not receive organic materials such as digestate, says Ms Robinson.