Green Gas Support Scheme (GGSS): digestate management

Overview

Our April 2020 Future support for low carbon heat consultation sets out proposals for a new Green Gas Support Scheme (GGSS) to increase the proportion of green gas in the grid, through support for biomethane injection by the process of anaerobic digestion (AD).

In this consultation we propose 2 mitigations against ammonia emissions from digestate, a by-product of AD, for the GGSS scheme participants.

We welcome all views but due to the technical nature of the proposals, we expect responses mainly from those directly involved in the industry such as AD plant developers and relevant trade associations.

The government’s response to the April consultation will include decisions based on responses to this consultation. We expect to open the GGSS scheme Autumn 2021.

The proposals

Through the GGSS we aim to:

• encourage continued deployment of AD biomethane plants in order to increase the proportion of green gas in the gas grid
• ensure value for money
• minimise a market hiatus for the biomethane industry

Digestate is a nutrient rich by-product from AD that can be used as a fertiliser. Used like this it has many benefits but it also releases ammonia when stored or spread on land. It can also lead to water pollution if it is over-applied or used in the wrong place or at the wrong time. Ammonia is an air pollutant that combines with other chemicals in the atmosphere to form fine particulate matter which has negative impacts on human health.

Ammonia is toxic to some plant species even at low concentrations and sensitive species are currently being impacted by ammonia concentrations across large parts of the UK. Ammonia deposition also leads to biodiversity loss in sensitive habitats. Most of the UK’s protected habitats are currently receiving excessive nitrogen and any increase in ammonia emissions will worsen this situation. The UK government has legally binding targets to reduce ammonia emissions as well as a target to reduce nitrogen deposition in England. As stated in the Future support for low carbon heat Impact Assessment, ammonia from AD digestate currently accounts for around 5% of all UK ammonia emissions, and biomethane plants are a subset of all AD plants. Therefore, to maximise carbon savings on the scheme we need to consider all viable ways to mitigate ammonia produced.

In the April consultation we asked for views on digestate and ways to mitigate ammonia emissions throughout the AD process. In further engagement with Defra and the Environmental Agency (EA), we have identified some proposals to mitigate emissions on the scheme that would not impact on scheme deployment or present a significant risk to investment.

Proposal 1

Digestate needs to be stored in tanks or lagoons of sufficient size to comply with spreading restrictions to protect water and air quality and installing fixed covers on these new storage units will reduce ammonia emissions. Currently only food waste standard rules permits require these storage units to be covered, slurry and manures based digestate do not need to be covered. The EA is updating environmental standard rules permits to require all new storage tanks and lagoons to have covers. This requirement will be in force by the time the scheme opens. Existing sites will all be required to have fixed or floating covers.

We will make it clear in our government response to Future support for low carbon heat that all new participants using waste feedstock on the scheme will be required to obtain the requisite waste permit and ensure they have covered digestate storage. Please note that in the April consultation we proposed carrying over the minimum threshold from the RHI, set at 50%.

Proposal 2

Digestate can be spread on land as a fertiliser. There are a number of available techniques for spreading digestate with varying impact on ammonia emissions, from surface broadcast (splash plate) to low emission spreading equipment such as trailing hose, trailing shoe or injection. Current estimates suggest that 93% of the AD industry already use low emission spreading equipment. To capture the remaining 7%, we have identified 2 scenarios through which digestate is spread. We will include the following requirements in regulations to ensure low-emission spreading is used in all scenarios. Defra’s Clean Air Strategy sets out government plans to require digestate to be spread using low emission techniques by 2025 but we intend to require participants to adhere to either of the following from the start of the scheme:

• digestate is spread using low emission spreading of digestate as defined in the Code of Good Agricultural Practice for reducing ammonia emissions
• where the participant contracts another person to spread the digestate, that person complies with the National Association of Agricultural Contractors (NAAC) standards (or equivalent)

We have also investigated ammonia mitigation technologies that seek to reduce ammonia emissions from digestate. While these technologies such as acidification, pelletisation and drying are promising, few are advanced enough to provide the evidence necessary to develop robust policy for inclusion on the scheme. We intend instead to commission a study into the long-term feasibility and costs of all ammonia mitigation technologies to be published in time for a mid-scheme review.

Consultation questions

1. Do you support the measures proposed for low emissions spreading? Yes/no

2. Are there any further requirements that you think would be useful in this space, or would serve as better alternatives to mitigate ammonia pollution?

See the BEIS consultation privacy notice.

Please do not send responses by post to the department at the moment as we may not be able to access them.