Abstract
Introduction/Context
Residual sludge produced at modern water resource recovery facilities (WRRFs) is both an asset - containing valuable resources- and a liability, as sludge still contains pollutants and requires significant management outside the facilities. Widely regarded as a potential recoverable resource, management of biosolids, the properly processed state of these residuals, still represents a significant challenge for water utilities. Land application is still the most common solution for biosolids management; however, there are growing concerns in our industry about the long term viability of this practice given rising costs, regulatory uncertainty, and public opposition. With growing awareness of climate change issues, use of renewable fuels for energy applications and contributing to the decarbonization of the 'used' water sector is also increasing in importance. Incineration of wastewater residuals is a well-known processing technology that offers an alternative to land application, but is most financially feasible in larger applications, faces significant opposition from some communities, and makes the recovery of some valuable resources like phosphorus more difficult.
Pyrolysis and gasification of treatment residuals are incineration alternatives that have received some interest within our industry over the years. However, until recently we haven't seen a significant increase in attention given to these thermal processing options to deal with both the challenges and opportunities presented by residual sludge management. These thermal technologies represent an environmentally friendly sludge processing pathway that produces eco-friendly products, particularly syngas and biochar, supporting the circular economy through greenhouse gas reductions, sustainable energy production, carbon sequestration, contaminant immobilization, and improvements in water retention and soil fertilization. This paper will present a series of case studies regarding the evaluation and implementation of full-scale facilities, including results from the startup and initial operation of a heat drying/pyrolysis facility. These case studies are summarized below. The City of Stamford Biosolids To Energy Project The City of Stamford, Connecticut is a leader in environmental stewardship, with such notable initiatives as nitrogen trading in the Long Island Sound and waste-to-energy linked to biosolids drying in the 1970s and 1980s. In keeping with this record, the city embarked on an innovative demonstration project (with the assistance of a U.S. Department of Energy grant) to show that dried and pelletized wastewater residuals can be used as a renewable energy source to generate electrical power. The project identified gasification as the most suitable technology to convert pellets of dried wastewater residuals into a renewable fuel which can in turn be used to generate power. The project consisted of the following components: •A literature review and assessment of biosolids gasification and other energy conversion technologies •Baseline and bench-scale characterization of biosolids and gasification byproducts •Bench-scale technology assessment •Preliminary design and cost estimates, including vendor pilot testing •Feasibility assessment report and decision to proceed to engineering/construction •On-site pilot testing (Figure 1) •Off-site vendor testing •Vendor preselection and preliminary design for design-build implementation Pyrolysis Technology Evaluation for Milwaukee Metropolitan Sewerage District In 2017, Jacobs evaluated pyrolysis and gasification technologies for the Milwaukee Metropolitan Sewerage District (MMSD) in Wisconsin. The project identified twelve vendors that have marketed and/or installed biosolids gasification and pyrolysis systems. Some of the vendors identified through the study included Environmental Power International, Biogreen® by Norris Thermal Technologies, Kore Infrastructure and Sulzle KOPF SynGas (which has two full-scale wastewater biosolids gasifier facilities in Germany). Conceptual level designs were developed for MMSD's two largest WRRFs (Figure 2). Edmonds Integrated Gasification Plant Design, Washington In 2019, Jacobs evaluated pyrolysis and gasification technologies for the Edmonds Wastewater Treatment Plant (WWTP) for City of Edmonds in Washington State. The project included procurement document development and design evaluation of three technologies that could fit on the tight treatment plant site. The evaluation led to selecting the Ecoremedy® Fluid Lift Gasification™ technology, an integrated drying, pyrolysis, and gasification process. (Figure 3) VCS Denmark Drying/Pyrolysis Full-Scale Demonstration Project While Danish water and wastewater utility VCS Denmark (VandCenter Syd/VCS) is already a net energy producer and has a viable biosolids management program, they are always exploring projects and innovations to expand their role as a sustainable utility. Their latest collaborative venture has changed one of their wastewater plants into a true WRRF, producing heat and biochar from sludge pyrolysis, a much cleaner fertilizer product than the biosolids that they produce today. For the past few years, VCS has collaborated with a small Danish startup (AquaGreen ApS) to evaluate a solution that combines steam drying of dewatered sludge with pyrolysis, aimed at producing biochar with reduced pollutants and increased plant-available phosphorus. A full-scale demonstration plant has been constructed at the VCS Søndersø WRRF (25,000 PE capacity). This demonstration facility (a 350kW unit with an estimated sludge processing capacity of 4,348 dry tonnes/year at 22-23 percent dewatered cake) uses the energy potential of the sludge's organic constituents to convert the sludge into biochar using steam drying and slow pyrolysis at low temperature (650 °C) in an oxygen-free atmosphere (Figures 4 and 5). The multi-year demonstration study seeks to answer questions about this technology such as its energy self-sufficiency, synthetic gas yield rates and quality, biochar production rates from biosolids, and the fate of several contaminants of emerging concern such as polycyclic aromatic hydrocarbons (PAH), di(2-ethylhexyl) phthalate (DEHP), linear alkylbenzene sulfonate (LAS), nonylphenol ethoxylates (NPE), microplastics, and per-and polyfluoroalkyl substances (PFAS).
Summary
This paper will offer valuable knowledge about technology selection, design, construction, startup, and operational results for sludge pyrolysis and gasification, promising technologies that produce recoverable resources of higher value while contributing to the decarbonization of 'used' water management. We anticipate that this information will be of great interest to other utilities, consultants, academics, and equipment providers of a new technology, offering the promise of a NextGen solution for resilient biosolids management and resource recovery.
There are growing concerns in our industry about the long-term viability of land application of biosolids given rising costs, regulatory uncertainty, and public opposition. Use of renewable fuels for energy applications and contributing to the decarbonization of the wastewater sector is also increasing in importance. Pyrolysis and gasification represent an environmentally friendly thermal sludge processing pathway that produces eco-friendly products supporting the circular economy.
Author(s)Julian Sandino2; Per Henrik Nielsen1; Todd Williams2; Niels Malmose Askjaer1
Author affiliation(s)VCS Denmark1; Jacobs2
SourceProceedings of the Water Environment Federation
Document typeConference Paper
Print publication date Oct 2022
DOI10.2175/193864718825158492
Volume / Issue
Content sourceWEFTEC
Copyright2022
Word count16