A Conversation about Residual Emissions
Speakers/Panellists
- Clive Humphreys, Senior Advisor, Environment Agency
- Dr Yadira Bajon-Fernandez, Senior Lecturer in Bioresources Science and Engineering, Cranfield University
- Paul Fountain, Senior Consultant Biosolids, Thames Water
- Steve Bungay, Senior Partner, AD Ingenuity LLP
Chair: Matthew Smyth, Director, Aqua Environment Solutions Ltd
Anaerobic digestion has transformed how the water industry manages wastewater sludges. It is widely considered that the recycling of anaerobic digestate to agricultural land is the best practicable environmental option, recycling nutrients such as nitrogen and phosphate, providing organic matter, and promoting sustainability through the production of renewable energy via biogas, with the production of biomethane being important for the UK meeting its Net Zero targets.
However, the treatment and storage of these wastewater sludges can be sources of emissions of methane, ammonia, and hydrogen sulphide into the atmosphere. Not only are all three environmental pollutants, which contribute to poor air quality, methane is a potent greenhouse gas. Therefore, emissions from AD plants can pose significant sustainability challenges, and there is a need for improved operational practices to foster sustainable growth of AD in the UK.
The Industrial Emissions Directive, which incorporates Environmental Permitting, Best Available Techniques (BAT), Appropriate Measures, and Improvement Conditions; in conjunction with the Clean Air Strategy, and the Methane Action Plan, require the reduction of emissions into the environment.
The Methane Action Plan incorporated an AD improvement programme, with requirements for leak detection and repair (LDAR) being added to environmental permits, and new and modified environmental permits are being issued to ensure BAT are adopted for around 120 AD plants used to treat wastewater sludge currently being operated by water companies.
Where there are open sludge tanks preceding AD, BAT is to be used contain and abate these tanks. There should be a primary anaerobic digestion vessel cover plan, and short retention post-digestion storage tanks or secondary digesters need to be enclosed and connected to the gas management structure, or in rare cases, to a suitable abatement system which treats all potentially polluting elements of the off gas. Gas utilisation and abatement should be in accordance with the guidance ‘Biological waste treatment: appropriate measures for permitted facilities’ and should include an updated Hazard and Operability Study and DSEAR risk assessment.
In addition to this the EA requires data on digester stability and the quality of the digestate being produced, to contribute to an industry wide benchmarking exercise to identify plants producing less stable outputs, and to provide a body of evidence to determine whether sludge cake storage is likely to result in emission in quantities sufficient to warrant abatement.
The Department of Energy Security and Net Zero (DESNZ) has just released a series of work packages investigating methane emissions from AD, looking at good practice guides for point source abatement, abatement strategies targeting post-AD area emissions, and developing a new residual emissions potential (REP) test.
The challenges facing residual emissions from AD are complex, and although, since the 1990’s, the UK total emissions of ammonia and methane have dropped by approximately 15% and 60% respectively, the emissions from AD have been steadily increasing.
An expert panel will discuss the challenges of managing these residual emissions, the difficulty with complying with the requirements of the IED and the Clean Air Strategy, and whether appropriate measures are indeed, appropriate.
