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Making the most out of your digestate

How do you make the most out of your digestate?

Landbank, contamination and commercial potential are a few of the issues and challenges Panel 4 will be discussing at the ADBA National Conference, 11th December in London.

The industry is growing and so, too, does the need to find a viable use for digestate.

As we prepare to discuss this topic on 11th December, we pulled out our article from The Natural Fertiliser Files from our 2018 Winter edition of AD & Bioresources News, written by Jon Hughes:

Q: What single material can restore soil fertility and defend against soil erosion; reduce carbon emissions, air pollution and pollution of the waterways; and make a dent in the UK’s balance of payments deficit?

A: Natural fertiliser

‘What is natural fertiliser?’ I hear you ask. Naturalfertiliser is a product of anaerobic digestion (AD);the material that comes out the back end of the digester once the biogas has been captured from thedigested feedstock. Within the industry, however, this material is commonly referred to as digestate.

This is telling. Scientists generally add the suffix ‘-ate’ to indicate the consequence of an action; the by product of AD digestion is therefore digest-ate. The fact that this remains the go-to name for natural fertiliser indicates that operators have traditionally seen it as a secondary concern, behind the more pressing matters of feedstock, power and gas sales, and subsidy.

Nigel Lee of Amur says digestate is a legacy brand which is now outdated. “It does not pass the doeswhat- it-says-on-the-tin test. People don’t know what digestate is, or what advantages it offers. Rebranding is necessary and although individuals are having success in branding their own products an industry-wide solution would be beneficial.”

There have been efforts, with ‘bio-fertiliser’ being adopted by parts of the industry. It’s an improvement but, as a quick word association game will prove, the bio- prefix is one step away from being connected with ‘hazard’.

Natural fertiliser, which is emerging as a description in parts of the industry, is a vast improvement. It describes both how the material is derived – through the natural process of AD – and what it does: naturally restoring fertility to the land. More importantly, calling it natural fertiliser puts it in direct opposition to the chemically concocted mineral fertiliser, the crystal meth or crack cocaine to which modern farming is addicted.

The comparison with an addiction is fair; the more chemical fertiliser used to get high yields, the more you have to use to get high yields. In the process it leaches the earth of its organic matter and natural goodness, meaning you have to use more again. This face-off between natural and chemical fertilisers is required and is not one the AD industry should be afraid of.

Whereas natural fertiliser is produced by the natural biological process of AD, its man-made counterpart is principally produced by the chemical Haber Bosch process. Aside from its ruinous effect on the soil, chemical fertiliser is inherently environmentally unfriendly. According to multiple studies, the Haber Bosch process uses 3-5 percent of the world’s natural gas production – around 1-2 percent of the world’s annual energy supply – to produce around 450 million tonnes of nitrogen fertiliser a year. Natural fertiliser avoids all this.

Furthermore, one tonne of chemical fertiliser replaced by one tonne of natural fertiliser saves one tonne of oil, 108 tonnes of water and, depending on whose analysis you follow, between 5 (ADBA) and 9 (US Academy of Sciences) tonnes of CO2-equivalent.

There are 3.9 million hectares of arable land in the UK. In total, in 2015/16, according to the Agriculture Industries Confederation, 1.7 million tonnes of fertiliser was applied to farmland in Britain. Imported fertiliser cost the economy a little over £76 million in 2017, according to Trading Economics.

What natural fertiliser does?
A by-product of AD, containing water, nutrients and organic carbon for soils, natural fertiliser is derived from treating organic material such as food and farm wastes, manures and slurries, sewage, and purpose-grown crops.

It typically comes in three forms: whole, similar in appearance to livestock slurry, with typically less than 5 percent dry matter; liquor, where most of the dry matter has been separated;
and fibre, the separated dry matter. Whatever is fed into the digester, between 50-85 percent comes out as natural fertiliser. Like its chemical counterpart, natural fertiliser delivers nitrogen (N), phosphorous (P) and potassium (K) to the soil; relative levels of these nutrients vary according to the feedstock used.

But whereas chemical fertiliser weakens soil structure, natural fertiliser delivers essential organic matter back to the land. Organic matter within soil serves several crucial functions, says the UN’s Food and Agriculture Organisation: “From a practical agricultural standpoint, it is important for two main reasons: (i) as a ‘revolving nutrient fund’; and (ii) as an agent to improve soil structure, maintain tilth and minimise erosion.” It also improves the ability of soils to capture carbon, vital for mitigating carbon emissions.

WRAP (the Waste and Resources Action Programme) describes natural fertiliser as being “an excellent alternative to bagged fertilisers. Using it improves the sustainability of farming by reducing emissions of greenhouse gases associated with fertiliser manufacture, and by reconnecting nutrient cycles.”

As the nutritional content of natural fertiliser reflects the feedstock that went into the digester, use of fertiliser derived from food or animal waste is strictly regulated under both animal by-products regulation and BSI PAS 110 (British Standards Institute Publicly Available Specification), which require all such materials to be pasteurised to prevent the possibility of any pathogens entering the food chain.

There are further restrictions on when natural fertiliser can be applied to the land in accordance with legislative and good practice requirements for the use of all organic materials.

The benefits of natural fertiliser
Natural fertiliser provides the essential crop requirements of NPK and reduces emissions from rotting manure, farm wastes and slurries. By replacing petrochemical mineral fertilisers, natural fertilisers abate significant amounts of carbon.

Currently 90 million tonnes of manure is spread to land each year, which if sent to AD would abate 6.65 million tonnes of CO2 (MtCO2) annually. As the Committee on Climate Change noted earlier this year “no progress has been made in reducing agricultural emissions over the past six years”. Natural fertiliser is the readily available solution to this challenge.

Natural fertiliser is flexible and versatile.
It is perfectly suited for agricultural nutrient management by providing flexibility on application rate, application time, and application placement. Natural fertiliser can return NPK to the land quickly in liquid form or more slowly through the use of the solid fraction. This versatility also helps to address the ammonia issue. The government holds agriculture responsible for 88 percent of the UK’s ammonia emissions, of which AD itself directly contributes only three percent.

Most ammonia releases from agriculture occur as a result of poorly managed nutrient management programmes, which result in the spreading of more nitrogen than the crops need. The nitrogen
that remains unused either runs off, polluting waterways, or photo-oxidises and is released into the atmosphere as ammonia.

A survey of AD operators, undertaken by ADBA and the Renewable Energy Association of their respective members, found that 93 percent of respondents employing natural fertiliser derived from the AD process used low-emission spreading techniques; bandspreading with a trailing hose or shoe, or using a shallow injector, which allows the natural fertiliser to be delivered close to the plant roots.

This is in line with best practice and increases the amount of nitrogen available to the crop, and reduces the amount lost to the atmosphere as ammonia gas. In contrast, the broadcast spraying of all types of fertilisers wastes a lot of the nitrogen, with only a very small proportion becoming available to the crop.

Natural fertiliser is a natural source of phosphorous

Phosphorous is important for UK food production, which requires 3kg per person per year, and the UK is an importer, with 75 percent of the world’s phosphate rock located in Morocco, according to the Plant Research Institute and Stockholm Environment Institute. The Global Phosphorus Research Initiative, led by Swedish and Australian scientists, estimates that the world’s readily available phosphorus supplies will be inadequate to meet agricultural demand within 30 to 40 years.

By returning phosphorous to land via natural fertiliser, the UK will have greater food security as farming will be isolated from geopolitical instability and protected against volatile price spikes.

Natural fertiliser also increases the organic matter in soils, improving soil structure and fertility and reducing water demand, soil degradation and run-off, which can pollute waterways. Poor management practice and an over-reliance on chemical inputs have rendered 30 percent of the world’s arable land unprotected and less productive.

Join in the conversation with our panel of experts on 11th December about how we make the most out of this natural fertiliser: 

Amaya Arias-Garcia, Engineering Design Manager, Suez (Chair)
Dr Jane Gilbert, Carbon Clarity
James Astor, Chairman, Regen Holdings / Whites Recycling
Angie Bywater, BBSRC NIBB Network Manager, University of Southampton
Louise Hall, Project Manager and Dr. Claudio Fuentes Grünewald, Swansea University

Find out more and book your ticket here:

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