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Making the most of the AD process

The by-product from AD is often seen as a burden but David Burrows finds out why the value of digestate is too great to ignore.


IT is no secret the Conservatives have been ripping up their commitment book on renewable technology.


Fiscal incentives have been slashed and environmental programmes rolled back or scrapped altogether.


Anaerobic digestion (AD) is not at the centre of the green policy bonfire but it has not been left untouched and investors are being warned to think more carefully about the plants.


Bill Elliott, owner of Best Organic Solutions, says: “I do not think subsidies will be here for much longer, so plants have to be viable on their own.”

Digestate, he says, is a ‘major burden’.


The fiscal drivers for AD are in waste management and power generation, with digestate a forgotten – and even costly – by-product.


Figures from the Waste and Resources Action Programme 2013 survey of organics recycling are based on ‘limited data’, says the organisation’s organics sector specialist Will McManus, but they do not paint a pretty picture.


For on-farm facilities, about 90 per cent of digestate is used on the producers’ own land, with the remainder given away to neighbours – usually free of charge – where neighbours bear the cost of collection and spreading.


For commercial and industrial users, about 50 per cent goes mostly to agricultural land but this is often at a cost of about £13/tonne to the operator.


Some 30 per cent is given away free of charge and about 20 per cent is sold for between £3-£5/t.


On average, digestate costs the operator £3.73 to dispose of, says Mr McManus. “There are a few leading lights who are doing a great job at providing a product which farmers value and are willing to pay for.”


One the one hand, it is a good deal for those taking AD digestate – they are paid for taking in a potentially valuable fertiliser. But few see it this way, instead viewing it as a risk.


Mr McManus says: “There are a range of factors but mostly [it comes down to] low fertiliser prices, relative cost of transportation and spreading, and of course the novelty of the product – many farmers have not had a chance to find out more about the value of using digestate as a renewable fertiliser.”


Tony Clutten, process sales manager at Huber Technology, agrees.


“While there is intrinsic NPK value in digestate and the fertiliser bill for local farmers can be substantially reduced by using digestate instead of proprietary fertilisers, there is a reluctance to pay for the material,” he says.

Digestate use valued at £55-£160 per hectare

The value of digestate has been shrouded in mystery, but data is beginning to emerge.


The Waste and Resources Action Programme, for one, has carried out a number of studies examining the financial benefits of nutrients provided in the compost.


In trials over five years at 22 sites, researchers found ‘digestates and composts can increase yields with no negative impacts on crop quality of safety’.


“Digestate is a good source of crop nitrogen,” according to the report Field experiments for quality digestate and compost in agriculture.


The increased nutrient supply from organic materials – phosphate, potash and sulphur – can also provide a ‘boost’ early in the season, particularly on shallow soils over chalk and limestone.


The benefit was valued at £55-£160/ha – figures ‘not to be sniffed at’, according to Anaerobic Digestion and Bioresources Association (ADBA) chief executive Charlotte Morton.


Those savings take into account the money saved on bagged fertiliser and the cost of spreading the organic materials, though they do not account for the cost of sourcing them.


This is the kind of information farmers need to hear, say experts.


At ADBA’s recent Research and Innovation Forum in York, sector heads outlined how the latest innovation could see AD plants ‘on a trajectory towards producing £250,000 worth of digestate each year by 2020’.


“As incentives continue to fall it will become increasingly important for AD operators to improve plant efficiency, maximise the value from outputs and drive up standards – digestate must therefore become a valuable additional revenue stream for the AD industry,” says Ms Morton.


Development of the anaerobic digestion market

Could the market for digestate also become a factor in the assessments made by banks when financing projects?


Not yet, says David Kinnersley, anaerobic digestion (AD) expert at Fisher German.


“The market is not sufficiently able to add value at the moment to make it a reliable or significant enough income stream to take this into account in financial forecasting – unless a market has already been contracted.”


In taking pasteurised food waste, digestate farmers face a number of potential headaches, including storage during wet weather and crop growing periods, controlling applications in nitrate restricted zones, possible contamination with plastics and transport charges.


De-watering the material prior to disposal overcomes some of the problems involved in managing liquid digestate, but this means the capital expenditure and operating expenditure of any project increases for the producer.


Lucy Hopwood, lead consultant at bioeconomy consultants NNFCC, says some companies supplying digestate driers are offering to pay installation costs and still own the dryer, buy the input material and sell-back the digestate as a means of ‘adding value to digestate’.


“It is a new business model and not one we have too much confidence in, but something the agents and suppliers are offering fairly bullishly at present,” she adds.


In April, NNFCC published its third report on AD in the UK, showing 130 new projects have come online as developers ‘rushed to get plants accepted onto incentive schemes’, including the Feed-in Tariffs and Renewable Heat Incentive, before Government changes them.


There are now 316 operational AD plants in the UK and 208 are farm-fed.


In all, 12 million tonnes of digestate are produced, NNFCC estimates.


The amount accredited by the PAS110 quality assurance scheme is rising all the time, increasing from almost 1mt from 19 sites in 2014 to 2.4mt from 42 last year.


Clutten at Huber is optimistic there will be a shift, with farmers overcoming their reluctance to embrace digestate from nearby AD plants.


“It will become increasingly important,” he says, ‘and once the full financial benefits of this material are known it will change from being a cost to a resource for the AD plant operator – as many AD plant operators working on their own farms will testify.’

In numbers:

  • 208 – Farm-fed plants currently operational in the UK
  • 108 – Waste-fed plants currently operational in the UK
  • 12m – Tonnes of digestate produced
  • 2.4m – Tonnes of PAS110 quality assured digestate produced
  • 97 per cent of digestate is used in agriculture
  • 90 per cent of on-farm produced digestate is used by the producer
  • £55-£160 per hectare (£22-65/acre) savings compared to bagged fertiliser


The market for digestate:


  • 50 per cent at a cost of approx. £13/tonne
  • 30 per cent given away free of charge
  • 20 per cent sold for £3-£5
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