Abstract
Project Overview UK Water Industry Research (UKWIR) are a leading institution for water research in the UK, creating a platform for innovation that helps to meet the challenges the water industry face. UKWIR commissioned a study of biochar production through advanced thermal conversion of sewage sludge. The project was steered through a committee of representatives from various water companies and led by Ken Shapland (UKWIR) and Dan Green (Wessex Water). Atkins and Cranfield University were commissioned to undertake the study. Advanced Thermal Conversion of Sewage Sludge to Biochar The UK and Ireland's current sewage sludge management practices heavily rely on treatment via anaerobic digestion (AD) or advanced digestion (AAD) and the recycling of biosolids to agriculture. Whilst considered the best practicable environmental option in most global circumstances, this remains susceptible to various concerns around sustainability and resulting regulatory changes, impacts to stakeholder confidence, practicality of operations and competing demand. This growing uncertainty means that the Water Industry may need to consider alternative or mixed approaches to manage these risks and to meet their 2030 Net Zero carbon challenges. An alternative approach could incorporate thermal strategies, readily deployed in the wider waste to energy sector but as yet not significantly utilized within wastewater. Atkins, supported by Cranfield University, carried out a study of advanced thermal conversion (ATC) technology for UKWIR. The study focused on producing biochar from sewage sludge and contains a literature review covering technologies, biochar products and markets, carbon sequestration potential, biochar quality and contaminants fate, environmental impact, regulations and incentives. The extensive study also includes an assessment of how biochar producing processes such as pyrolysis, gasification and hydrothermal carbonization could be integrated within existing wastewater treatment works. The comprehensive review explored case studies and literature from the UK and internationally plus outreach with operators and suppliers of ATC. The team built upon that knowledge to undertake a synthesis and appraisal to develop treatment flowsheets to gain an understanding of how the value of ATC could be realized in the Water Industry. Given remaining uncertainties and contextual differences, which prevent a single 'best choice' recommendation, hierarchies of options were produced, respective to several key driving factors Technology Review and Operational Outreach. An initial review of available ATC technologies was assessed. The range of technologies is shown in Figure 1. For each technology, process and operational conditions, dry solids feedstock requirements, outputs and technological readiness was assessed. A list of technologies is below. The key processes that produce biochar are emboldened and were the focus of the study: -Pyrolysis -Gasification -Combustion / Incineration -Hydrothermal Carbonization (HTC) -Hydrothermal Liquefaction (HTL) -Hydrothermal Gasification (HTG) -Wet Air Oxidation (WAO) -Super critical Wet Air Oxidation (SCWO) Pyrolysis, gasification and HTC were studied in more depth, including an assessment of biochar output yield and quality, global investigation of deployment for sewage sludge treatment, health and safety case studies and process energy requirements. In addition, several suppliers and operators were engaged with to understand key operational considerations for each technology. Regulations There is uncertainty in the UK as to how ATC of sewage sludge to biochar would be regulated, incentivized, permitted and planned. Our team's review of relevant policy and regulations highlighted the need for clarity particularly around applying biochar to land and ATC permitting routes. Biochar Factors Biochar from sewage sludge has the potential to be used and sold as a valuable product. This is due to biochar properties such as nutrient content, porosity, surface area, carbon content, stability of elements contained in biochar and quality in terms of emerging contaminant concentrations. Our team investigated the market potential of using biochar as a soil amendment, concrete additive, solid fuel, anaerobic digestion additive, pigment, catalyst and biocomposite. Each of these uses was ranked in terms of market potential and technological readiness. One of the key advantages of using ATC for sewage sludge treatment is the potential elimination of emerging contaminants. An extensive study of literature showed the effectivity of ATC processes in eliminating contaminants such as PFAS/PFOS, microplastics, anti-microbial resistance chemicals and others. The results indicated that ATC processes are favorable in terms of emerging contaminant removal from sludge and biosolids, however, there are some research gaps. In addition, we conducted a comparison of likely biochar quality with regular biosolids quality in terms of heavy metals and nutrient composition. Another key consideration for ATC is the stability of carbon within biochar. Studies suggest that carbon can be locked into the biochar structure for up to 1000 years, indicating that carbon sequestration is possible through biochar production. The study included a carbon assessment showing the potential carbon offsets for the UK water industry when biochar is produced from raw sludge or digestate. Figure 2 shows an example carbon balance for digestate, which suggests a significant amount of carbon could be sequestered into the biochar (approximately 16% of UK water industry emissions). Mass & Energy Balances Mass and energy balances were developed for several flowsheet scenarios to compare advanced thermal conversion with baseline anaerobic digestion and incineration in terms of energy demand / production, carbon fate, solid residuals to manage, pre-treatment requirements and process emissions. An example of one of the flow sheets it provided in Figure 3. This assessment highlighted several key considerations required when integrating ATC technologies into wastewater treatment works. These considerations include: -Mechanical Dewatering increasing the dry solids content from mechanical dewatering can significantly improve the energy balance across the flowsheet. -Drying drying efficiency is fundamental to the energy balance. Decreasing the energy required to evaporate water content can increase the excess energy production of these processes considerably. -Pre-treatment digested or thermally hydrolyzed sludge requires less energy for dewatering and drying The mass and energy balances will be presented in detail. Conclusions This extensive study highlights the complexity of ATC technology deployment in the water industry from process, output, market and regulatory point of view. Whilst ATC offers significant potential opportunities such as reducing land application risks and carbon sequestration, there are still gaps in knowledge that need filled and an overview of these research needs was presented in the report. This is an area of research that is growing rapidly, and industry should come together internationally to share best practise. A selection of the key recommendations for the UK water industry are provided below: -An end-to-end demonstration of ATC integration into UK wastewater treatment works is critical to establish best practice in design and operation of ATC for the UK water industry; and poses an opportunity for a collaborative UK trial. -A long-term trial to understand the ecological benefit of sewage sludge derived biochar application to land. Results could be accelerated in the shorter term through undertaking an 'over-compensation' of biochar application to provide a 'worst case' scenario. -Further assessment on the financial and carbon benefits of promoting different biochar outputs is required, including the impact that process type, processing conditions and feedstock properties have on additional benefits. It should also compare revenue streams based on available incentives, the requirements and costs of meeting these. As the world looks to harness the value of biosolids whilst mitigating against emerging risks, advanced thermal conversion of sewage sludge offers a potentially attractive solution. This study provides an up-to-date assessment of the art of the possible with regards to biochar from sewage sludge. It is hoped that global utilities will learn from this study, take inspiration from the potential opportunity it demonstrates and collaboratively develop advanced thermal conversion pilot plants and solutions to meet their regional needs and strategic aims.
This paper was presented at the WEF/IWA Residuals and Biosolids Conference, May 16-19, 2023.
Author(s)R. Wilson1, E. Piechozcek2, R. Lancaster3, S. Westlake4, Y. Bajon Fernandez5, E. McAdam6, K. Shapland7,
Author affiliation(s)Atkins1; Cranfield University2; UK Water Industry Research3
SourceProceedings of the Water Environment Federation
Document typeConference Paper
Print publication date May 2023
DOI10.2175/193864718825158864
Volume / Issue
Content sourceResiduals and Biosolids
Copyright2023
Word count9