Once a year, the small town of Windsor in Northern California hires an external service that specializes in dredging, dewatering, and the beneficial use and disposal of sludge. Over time, the costs of these services have risen. To promote sustainability and establish a net-zero program, the town has decided to explore alternative options.
“The District has set specific objectives for future solids management, such as eliminating dependence on external contractors for biosolids disposal, enhancing the beneficial use of biosolids, and reducing both costs and carbon emissions linked to sludge disposal,” said Windsor Public Works Director, Shannon Cotulla.
To meet these objectives, the Town carried out a Feasibility Study and Preliminary Design, which identified a feasible project option. They chose thickening and dewatering, followed by biodryers and pyrolysis, as the top preference solution.
The Town currently produces Class B biosolids. Sludge ponds store and stabilize Waste Activated Sludge and sludge generated by the Advanced Wastewater Treatment clarifiers. This process does not treat PFAS, removal of which most districts today are striving to incorporate into their future sustainable biosolids management plans.
“We are very fortunate here in Windsor,” Cotulla said. “Our commercial properties are not heavy industry, and our PFAS is very low.”
The fact is that very few technologies are currently available to remove PFAS from biosolids. However, after careful study, Windsor has decided to follow their Bay Area neighbors in Redwood City, where Silicon Valley Clean Water (SVCW) is making significant strides with its pyrolysis biosolids program.
Research on methods to treat PFAS-contaminated biosolids is still in its early phases. Several ongoing studies throughout the nation are investigating the application of pyrolysis technology, with SVCW being one of the lead agencies testing this innovative approach.
SVCW began implementing biosolids management strategies, including pyrolysis and solar drying, to reduce PFAS levels in 2020. Once the biosolids are dewatered, SVCW utilizes two drying methods: solar drying in dedicated beds and biodryers/pyrolysis technology. Solar drying takes place in summer, achieving 75% dryness of the biosolids. In contrast, dryers and pyrolysis operate year-round, decreasing the weight of biosolids by up to 90% and assisting in producing marketable biochar.
“We were able to visit Silicon Valley Clean Water and see firsthand how the process is done,” Cotulla said. “Hats off to them for being leaders in this program. They have done a lot of the leg work, including working with the Air Quality Control Board and showing how to get the permits needed to complete the project.”
Windsor’s feasibility study recommended collaborating with regional partners to explore the possibility of consolidating biosolids and constructing a single biosolids processing facility. This will greatly benefit nearby communities and, for Windsor, help fund the pyrolysis project.
Windsor, with a population of 28,000, is situated in Northern California’s Sonoma County, 63 miles north of San Francisco. Its economic growth and population surged during the 1980s when many housing developments were constructed. Before that, Windsor’s economy primarily depended on agriculture, particularly grape production for wine.
The Windsor Water District is responsible for collecting, conveying, and treating wastewater from both the Town of Windsor and its neighboring areas. Although the current permitted average dry weather flow (ADWF) stands at 1.9 MGD, the Windsor Water Reclamation Facility (WRF) is designed to handle a capacity of 2.25 MGD.
PFAS have been detected in both effluent and solid residual streams (sewage sludge) from wastewater treatment plants (WWTPs), raising significant concerns about the management of these materials. In California, the typical approach for managing WWTP solids is to convert them into biosolids for land application, dispose of them in lined landfills, or incinerate them in sewage sludge incinerators.
These solids are rich in nutrients. According to the U.S. EPAi, the most common method involves digesting them either aerobically or anaerobically to create a stabilized biosolid that can be used as fertilizer on land. This practice is advantageous because the nutrients in biosolids supply essential nitrogen, phosphorus, and trace metals that support crop and soil health.
Pyrolysis involves breaking down materials at moderate temperatures in an oxygen-free environment. What sets pyrolysis apart from incineration is the absence of oxygen. Pyrolysis can transform input materials like biosolids into biochar while producing a hydrogen-rich synthetic gas known as syngas .
One significant advantage of pyrolysis is that the end product results in a material volume reduction of over 90% compared to the input solids. This improves transportation and disposal efficiency while also decreasing environmental impacts, such as reduced PFAS loadings in landfill leachate compared to biosolids disposal.
Pyrolysis represents a significant financial investment compared with direct biosolid land application alternatives, and these technologies have a number of challenges and data gaps. However, if these issues can be overcome, these systems could provide an effective means of treating PFAS in WWTP solid residuals and PFAS-impacted biosolids.
The system at SVCW and Windsor is different from traditional pyrolysis in that the syngas produced by other pyrolysis processes is recirculated into the combustion chamber and fuels the process. This reduces the energy needed to fuel pyrolysis and the GHG emissions.
The pyrolysis system’s target PFAS removal efficiencies (REs) were estimated to range from over 81.3% to more than 99.9% (mean = over 97.4%), with the lowest REs associated with the lowest detected PFAS concentrations and the highest MDLs.
Pyrolysis units are smaller in size and capacity than sewer sludge incineration units (SSI). Partial mass vaporization results in relatively low air flows, which decreases the size and capital costs of air pollution control equipment. Compared to strict air pollution emissions regulations for SSI, the regulatory framework for processes like pyrolysis is less defined. Pyrolysis generates a hydrogen-rich synthesis gas (syngas) stream that can be combusted, recovering heat energy.
Windsor has initiated the design of a Class A biosolids handling facility that incorporates a pyrolysis treatment stage. However, there is still much work ahead before construction begins. At this stage, the Town is focused on obtaining funding.
“We’re starting from scratch and putting together more precise details,” Cotulla said. “This is an excellent solution for a facility of our size. Greenhouse gas emissions will be reduced by 90%, and we will have a marketable product in the biochar.”
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