Integrated Biosolids and Organics Management Strategy in the City of Windsor, Ontario, Canada

The City of Windsor (City) owns and operates two municipal wastewater treatment plants; the Lou Romano Water Reclamation Plant (LRWRP) and the Little River Pollution Control Plant (LRPCP). These facilities provide secondary level treatment for municipal and industrial wastewater from the City of Windsor, ON and the neighboring Municipalities of Tecumseh and LaSalle. The LRPCP and LRWRP produce approximately 2,400 and 8,500 dry tons per year of residual wastewater solids, respectively. Under the current management strategy, the residual solids at each facility are dewatered by centrifuge (to a dry solids content of ~ 28 %) and transferred to the City-owned Windsor Biosolids Processing Facility (WBPF). At the WBPF, the dewatered sludge cake is thermally dried and pelletized to produce a fertilizer product which is registered under the Canadian Federal Fertilizer Act and sold throughout Southwestern Ontario, Canada. The WBPF has multiple drawbacks including utilizing an energy intensive drying process, aging infrastructure and equipment, and is not expected to be capable of accommodating future wastewater residual projections. In recent years, there has been increasing attention paid to managing the organic fraction of municipal solid waste. In an effort to divert organic material from the landfill, many municipalities have integrated organics management programs, which involves co-processing wastewater residuals with the diverted municipal organic wastes. In addition to the environmental and cost benefits of diverting and combining these waste streams, this process maximizes the recovery of their remaining value in the form of electricity, thermal energy, and/or fuel. Environmental benefits of diverting organic materials from landfills include reduced methane emissions, decreased leachate discharges, and opportunity for energy recovery from waste. Similar to the California program, the Ontario Ministry of the Environment, Conservation and Parks requires 70 percent of organic waste to be diverted from Ontario landfills by 2025. The City of Windsor does not currently have an organic waste collection program in place; therefore, a program is in development. The source separated organic waste materials which may potentially be accepted through this program include municipal food and organic waste; institutional, commercial, and industrial food and organic waste; agricultural organic waste; and high strength waste (HSW) such as food processing waste, dairy waste, and fats, oils, and grease. To address future biosolids management needs and comply with regulatory requirements for organic waste diversion, the City of Windsor completed a comprehensive study to develop an Integrated Biosolids and Organics Management Strategy. This study presents the complete planning and evaluation process for the Integrated Management Plan including the identification of the opportunity and evaluation of alternatives as well as a review of biogas potential, energy savings, and GHG reductions through anaerobic digestion and biogas utilization. To select the recommended management strategy, the following three alternatives were compared: (1) improvements to the existing WBPF, (2) composting, and (3) anaerobic digestion. The evaluation included social and cultural environment criteria, natural environment criteria, technical / suitability criteria, and economic criteria. Anaerobic digestion was identified as the preferred alternative as it provides the ability to process wastewater residuals in a less energy intensive way for improved energy recovery from waste, biogas production, and reduced GHG emissions. Additional benefits of an anaerobic digestion facility includes: that it has smaller land area requirements; proven and reliable process operation; higher potential for federal and provincial grant programs; is consistent with the City of Windsor's Community Energy Plan and Climate Change Action Plan; and results in the production of a well stabilized finished product suitable for farmland application. Under this strategy, the wastewater residuals and organic waste would be processed at a centralized anaerobic digestion facility with biogas utilization via a combined heat and power (CHP) unit. Figure 1 outlines the proposed process schematic for this alternative. Table 1 shows the feedstock quantity, volatile solids (VS) loading, and biogas production for each feedstock material. Feedstock quantity for the LRPCP and LRWRP is presented based on historical mass of wet dewatered sludge cake measured from 2016 to 2020. It is estimated that an average of 10,300 wet tonnes of source separated organics (SSO) and 22.7 m3/day of HSW could be collected throughout the City of Windsor and processed at the proposed facility. The biogas production from digesting sludge is estimated to be 2,050 m3 biogas/day and 6,950 m3 biogas/day for LRPCP and LRWRP, respectively. Co-digestion would increase the total biogas production by approximately 50% with 1,350 m3 biogas/day from HSW and 3,600 m3 biogas/day from SSO. Table 2 shows the energy balance for the LRWRP and LRPCP with the projected energy production from anaerobic digestion and biogas utilization. The energy consumption presented incorporates the historic energy consumption at the LRWRP and LRPCP and projected energy consumption required for thickening, dewatering, and anaerobic digestion. The energy produced from anaerobic digestion of sludge would amount to 40% of the energy required to operate LRWRP and LRPCP. Co-digestion with HSW and SSO would increase this contribution to 62% of the total energy required to operate both plants. In addition, Table 2 shows the effect that anaerobic digestion and biogas utilization had on GHG emissions for LRWRP and LRPCP. Anaerobic digestion of sludge reduced GHG emissions by 1,400 tonnes CO2e/year and 400 tonnes CO2e/year at the LRWRP and LRPCP, respectively. Co-digestion would reduce GHG emissions further to 3,300 tonnes CO2e /year, which corresponds to approximately 36% reduction in GHG emissions. Based on this evaluation, significant energy savings and GHG reductions can be achieved through anaerobic co-digestion of wastewater residuals and organic waste. This solution would move the LRWRP and LRPCP towards a net-zero energy future, provide energy savings to the City of Windsor, and reduce GHG emissions via onsite energy generation and biogas utilization. This integrated management approach will allow the City to meet the regulatory organics diversion requirements and address biosolids management needs for the two wastewater treatment plants. The operation and maintenance cost for the facility would be approximately $1,400,000 CAD/year; however, this would be offset by fertilizer revenue (~ $1,400,000 CAD/year) and electricity savings ($3,000,000 CAD/year). The opinion of probable cost for such an anaerobic digestion facility is approximately $151,000,000 CAD (inclusive of contingency and engineering allowance; exclusive of taxes). There are several government rebate programs available which help to facilitate municipalities implementing waste diversion programs and adopting technologies that generate clean affordable energy.