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Digestate Drying – Does It Make Sense Environmentally and Economically?

Digestate drying might seem like a great way to make a natural/ sustainable fertiliser product but think of the large heat demand to dry such a high water content material (at often 90% water)… It has to be asked: “Does It make sense environmentally and economically to evaporate it until dry”?

Digestate is a type of organic fertiliser created by the anaerobic digestion process that is mainly applied to land by spreading.

The nutritional composition of digestate varies depending on the feedstock mix and process design, but it generally includes adequate amounts of the important elements N, P, and K to be a useful fertiliser product.

Image text: "Digestate drying"

The Problem – Does Drying Digestate Really Pay?

Despite considerable efforts to stimulate markets for and enhance trust in digestate, its value potential remains low both for the liquid fraction and the solid/ fibre fraction.

That is due to its bulky form compared with synthetic alternatives, meaning that storage, transport, and spreading expenses can easily surpass the intrinsic value, making it a financial burden to the operator.

However, dry it and comply with the requirements of WRAP's PAS110 specification and you have a product to rival unsustainable synthetic (minerally extracted/ fossil fuel refined/ high travel miles) chemical NPK fertilizers.

Why Digestate is Under Valued

Make no mistake about it. In any reasonable market if the NPK fertilizer value of anaerobic digestion process digestate was set as a benchmark for the price of even raw unseparated digestate spread on land as a fertilizer it would have a considerable value.

In the UK, and we have no reason to suggest the situation is any different elsewhere, the market is influenced by the fact that farm business neighbours of AD plants know that the nearby digester operator has to dispose of digestate. It is a position in which the AD operator has no perceived room to negotiate a reasonable price, and hence seldom if ever gets one!

Usually, the recompense a local farm provides for spreading the AD operator's digestate amounts to little or nothing.

How to Sell Digestate as a Natural Fertilizer

To sell digestate as a Natural Fertilizer in the UK it has to comply with the WRAP PAS 110 specification.

Digestate must fulfil the criteria outlined in the Quality Protocol and BSI PAS110 specification (England & Wales) or the SEPA position statement in order to be sold as a natural biofertiliser (Scotland).

PAS110 defines minimum quality criteria for source-segregated biowaste whole digestate, separated fibre, and separated liquor. There is no necessity, however, for items to be specially dried or dewatered.

For bigger operators (commercial and industrial locations), digestate disposal is generally a cost to the business. But drying digestate to create a crystalized fertiliser can add significant value and make it economically viable.

Where digestate is sold offsite, WRAP surveys showed an average in 2016 price of approximately £3.73 per tonne, with similar studies ranging up to £5 per tonne, although data on this is scarce and often not published for being commercially sensitive.

The Digestate Drying Debate

Currently, digestate, both in raw form and as separated fibre and liquor, is a low-value product that is virtually often distributed on agricultural land or utilised in low-value horticultural applications. Spreading digestate to their own land or that of neighbouring farmers is the cheapest and simplest option for smaller farm-based plants digesting manures and crops, but the cost of transport and spreading often outweighs the nutrient value, making digestate at best a cost-neutral activity, and more often than not, a cash drain for many AD operations.

Improved transportability for the end product

The issue of digestate disposal is more of a concern for larger plants and those located in urban areas where access to suitable land banks for spreading is more limited or expensive. In these cases, dewatering digestate and drying digestate, as well as evaporating or further processing liquid fractions, can minimise storage needs and transport and distribution expenses. Water removal and clean-up that allows disposal to sewage or rivers under permission is a substantial advantage to large facilities that have disposal issues where distribution costs to land would otherwise be prohibitive.

Currently, very little digestate is marketed (only around 19% of digestate comes from the commercial sector (the primary source). This represents the current low market value for digestate, which is generally £3-5 per fresh tonne or less reported by plant operators, although digestate prices vary on a case-by-case basis depending on contractual agreements, responsibilities, land availability, and acknowledgement of the wider advantages.

Digestate Drying in Concept

Mechanical dewatering methods capable of producing up to 35% dry solids are already commonly utilised in the AD industry. Within limitations, these methods provide the most cost-effective technique of increasing the dry solid content of digestate.

Mechanical dewatering can be used to prepare digestate for composting or heat drying, for example, if it is not applied directly to the land, and used alone does ease storage and handling issues.

However using CHP (waste) heat from the anaerobic digestion process, which is mostly provided when cooling reciprocating engines used to generate electricity, to evaporate the water or “heat drying” has many advantages.

There are a number of markets where drying is an essential step, such as to:

  • extend storage periods,
  • protect or improve product quality,
  • produce higher-value products for more specialised markets (e.g. bagged fertiliser for horticulture or for combustion), or
  • where a dryer also facilitates a secondary process, such as pasteurisation. This efficiently uses ‘waste' heat, lowering costs and decreasing environmental impact elsewhere in the supply chain.

Possibilities for Using Dried Digestate

A major question is whether, in the absence of the RHI, the use of fossil fuels to dry digestate would be considered. Given its low market value, high equipment acquisition costs, and high energy prices, it is doubtful that anybody would pay for energy to dry digestate for lower value mainstream sectors such as agriculture and horticulture. Such systems rely on the use of unused heat from the AD system.

Even with such ‘free' heat as Assessment of digestate drying as an acceptable heat use under the Renewable Heat Incentive Page 26 of 28, payback within a reasonable timescale would need steady digestate prices substantially more than £5 per tonne in the absence of RHI support.

In cases where the heat is used efficiently and drying digestate is the only option for an AD site, they may look to use fossil fuels to supplement the surplus heat from the system, or if other heat uses are available, they may need to use fossil fuels to fulfil the drying process.

Only thermal drying may generate highly dry digestate (greater than 35% dry matter), but does this have any intrinsic value? It does have certain advantages:

  • a more straightforward handling
  • decreasing the amount of storage required
  • extended storage life
  • decreased transportation expenses and number of excursions to the field
  • access to a broader range of possibly more valuable end markets

The effects of digestate and fibre drying must also be distinguished from those of liquor treatment. The goals here can be to either a) clean up water for simpler on-site disposal (which also tends to create a concentrated fertiliser solution or fertiliser product as a by-product) or simply lower the amount of liquor, resulting in a more concentrated product for application to land. Different technologies are applicable for each of these goals, and only evaporation of liquor may be qualified for RHI payment.

Does Drying Digestate Really Pay?

Mechanical separation of digestate to 35% dry matter would allow operators to assign 25% of supply chain emissions to the digestate using the Renewable Energy Directive LCA approach. Thermal treatment (drying) of digestate to 90% dry matter improves the allocation to 38%, allowing feedstocks that would otherwise be judged unsustainable to meet the GHG requirements.

The RHI compensates in part for the market's failure to promote uptake of digestate drying because of low digestate pricing. In the absence of RHI assistance or other types of market intervention, digestate drying is unlikely to be considered.

It is also conceivable that without assistance, the use of heat for valid drying purposes, such as extending market reach to energy applications or higher value markets or assisting pasteurisation for safe spreading to field-grown crops, would not occur.

These instances, however, are frequently a significant driver in the overall AD project, and so the withdrawal of RHI assistance for all digestate drying would certainly prohibit some plants from proceeding and might have a detrimental influence on AD deployment rates in the UK.

Higher Returns, Lower Costs: Digestate drying

It would be potentially harmful to the industry to eliminate all support for digestate drying.

Nevertheless, legitimate causes and applications must be proven to prevent digestate from being dried for the sole purpose of increasing income by claiming RHI.

Digestate drying should not be prioritised above other legal heat uses, such as local heat customers, process heat demand, and so on.

Ths article is based upon the NNFCC, “Assessment of digestate drying as an eligible heat use in
the Renewable Heat Incentive”; Project Number: 16-015.5; March 2016. A report for DECC Heat Team.

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    • Ship Vessels
    • October 6, 2021

    Chemical fertiliser isn’t everything that’s wrong with the global agriculture economy. That doesn’t mean the issue isn’t serious; rather, it’s only a symptom of a much bigger problem. I’d want to see more people/channels take a deeper look at these subjects.

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