Commissioning a Biosolids Gasifier System at a Water Resource Recovery Facility

INTRODUCTION
The City of Edmonds' Carbon Recovery Project comprises a biosolids dryer, gasifier, and associated material handling equipment. This gasifier system is an emerging technology to produce Class A biosolids, biochar, or sand. The City of Edmonds (COE) is one of the first municipalities in North America to use gasification to process wastewater solids. The gasifier replaced a fluidized bed incinerator (FBI) that had reached the end of its service life at the Edmonds Wastewater Treatment Plant (WWTP). The Edmonds WWTP is in downtown Edmonds, WA on a tight site, adjacent to the Ferry Terminal Lanes. Figure 1 is an aerial picture of the Edmonds plant. Much of the facility is underground and primary sedimentation with conventional activated sludge is used to treat an average wastewater flow of 5.6 MGD (21 MLD). The main components of the gasifier system are the gasifier (Figure 2) and the rotary drum dryer (Figure 3). If space were available at the site, installing anaerobic digestion prior to drying and gasification would have been an option. This project was delivered by the Washington Department of Enterprise Systems (DES) through an Energy Services Company (ESCO). The ESCO was the overall project provider. A gasifier system provider (GSP) was the process designer, technology provider, and commissioning/training provider.

Several issues were encountered during construction and commissioning this new gasifier system and it took over two years longer than expected. Since the project began in 2020 (Parry 2020) and during this extended commissioning period, dewatered sludge was hauled to a landfill costing over $5 million. Construction during the COVID pandemic delayed the project well over a year. Incomplete installation issues delayed the startup by the GSP for months. Leadership changes at the COE, DES, and the ESCO also contributed to the delay. A change in operation and maintenance (O&M) management of the wastewater treatment plant delayed the commissioning required more training to accept the new gasification technology and integrate it into overall plant O&M. Only the leadership at the GSP remained the same through the project.

METHODOLOGY
DES hired an engineering consultant (EC) with expertise in designing and operating gasification systems (Parry 2012) to facilitate commissioning the project. The consultant led a 3-day project chartering session with the COE, DES, ESCO, and GSP that resulted in collaboratively working towards a common goal of commissioning and operating the gasifier system. After the chartering session, the gasifier system was successfully operated for four weeks, as witnessed by the EC, and DES issued the 'Limited Substantial Completion' for the project. Figure 4 is a picture of a COE operator controlling the system with guidance from the GSP operator. After the substantial completion of the project, the focus turned to operating the gasifier system. The city issued a six-month O&M contract with the GSP to assist the city with O&M of the gasifier system and provide more training to city O&M staff. Under this arrangement, the city operated the wastewater treatment plant and the gasifier system with the support of the GSP.

The EC prepared a mass and energy model of the overall liquid and solids processing to guide the operation of the gasifier system. The model started with the sludge inventory in the primaries and aeration basin. Management of the sludge inventory determines the amount of primary sludge (PS) and waste activated sludge (WAS) pumped to the blend tank. Blended sludge is dewatered by screw presses and dewatered cake is conveyed to the cake hopper. Dewatered cake is pumped from the cake hopper and mixed with dried biosolids to the gasifier and dryer.

RESULTS
The city is now operating the gasifier system as an integral part of the wastewater treatment plant and avoiding the high cost of hauling and disposing of dewatered cake. The city can produce a sustainable biochar product or a sand product for beneficial use as the biochar market develops. The city isn't dependent on developing a Class A biosolids market and dealing with PFAS issues. DISCUSSION The successful construction, commissioning, and operation of the gasifier system is progress toward resource recovery. It meets the original objectives of the Carbon Recovery project to provide the flexibility to produce biochar or sand. The gasifier system including the dryer fit in the limited space where the FBI was removed. With the limited space at the Edmonds WWTP, there wasn't room for anaerobic digesters. Anaerobic digestion would have provided a lower, consistent energy content that would have been easier to manage in operating the gasifier system. However, the gasifier system could handle the varying blend.

CONCLUSIONS
The Carbon Recovery Project was a success and provides the COE a sustainable solids processing system. The FBI system was replaced with an integrated gasifier and drying system to recover valuable resources of biochar and sand. The gasifier system is operating as an integral part of the overall wastewater treatment and solids processing system at the Edmonds WWTP. Management of other water resource recovery facilities can benefit from the lessons learned constructing, commissioning, and operating one of the first gasifier system in North America at a WRRF.