A Reinvigorated Approach to Operational Safety in the Design of a New Digestion System for Spokane, Washington
The failure of Digester No. 3 at the Riverside Park Water Reclamation Facility (RPWRF) in Spokane, Washington triggered a host of changes to many aspects of project implementation, as well as daily operation. Planning, design, construction and startup of improvements now emphasize improved communication and collaboration between the design team and plant staff, as well as a heightened awareness of safety. Many components of the new digestion facility, developed by an integrated team comprised of City staff and the design team, are targeted at providing reasonable flexibility and control while eliminating unnecessary system complexity.
The changes to daily procedures are aimed to standardizing processes to ensure staff is armed with the knowledge and training to allow them to take appropriate and consistent action in operations and maintenance of the facility. Enhanced documentation is utilized to facilitate the implementation and practice of these procedures. Documentation also is a key component of the City’s active risk management process.
These enhancements can and should be considered as best management practices. Change is always difficult. It requires a commitment from all levels of the team. Management and policy makers must make sure adequate resources are in place, and promote a positive, proactive culture of communication and collaboration in support of staff. Designers must incorporate the needs and concerns of staff, while bringing the best design concepts to the table. Involvement and endorsement at the staff level is key to ensuring that implementation of new facilities, as well as new processes are successful.
In May 2004, while operating as an active digester, Digester No. 3 at the RPWRF in Spokane, Washington was damaged. The ‘fixed’ digester roof separated from the rest of the digester vessel, broke, and fell to the bottom of the digester vessel, claiming one life and seriously injuring two other persons. An estimated 200,000 gallons of digesting solids spilled from the digester, a portion of it into the adjacent Spokane River. The failure of Digester No. 3 significantly reduced the solids treatment capacity and eliminated liquid biosolids storage capacity at the RPWRF. The planning and design of new digestion facilities faced several unique challenges, including concern and resistance from the public and an increased sensitivity to safety and reliability from city staff.
The RPWRF provides treatment services for the City of Spokane and areas of Spokane County. The City’s sewer service area covers approximately 100,000 acres. The average day flows at the facility are currently 37 million gallons per day (mgd) and are expected to increase to 54 mgd by 2015. The design criteria for the new digestion facilities are based on 2015 design criteria. The City of Spokane has been implementing significant capital upgrades to their treatment facility to address the growth in the area as well as increasingly stringent regulatory requirements. CH2M HILL has provided program management services for the RPWRF since September, 1998.
PROCESS SELECTION AND VESSEL SHAPE
Although the digestion process was not slated for near term capital improvements, the failure of Digester No. 3 triggered a complete evaluation of several of the solids treatment processes. The RPWRF uses single-stage, high-rate, mesophilic anaerobic digestion for biosolids stabilization. In the planning stage, the evaluation of stabilization processes included thermophilic digestion, temperature phased anaerobic digestion and pasteurization prior to digestion. Continued use of the RPWRF’s mesophilic digesters was recommended with provisions for future prepasteurization facilities should drivers for Class A biosolids emerge.
Besides typical factors such as process performance and capital and annual costs, selection of vessel shape was heavily influenced by factors specific to the facility such as site constraints and constructability, grit, scum and foam management, odor control, and public acceptance. Eggshaped digesters were selected over conventional pancake, silo, or waffle-bottom styles for the following reasons:
- Steep bottom eliminates grit build-up, reducing if not eliminating the need to take a digester out of service for cleaning and allowing utilization of full digestion volume
- Best overall process stability and performance
- Lowest mixing/power requirement
- Significantly different shape perceived by public as more aesthetically appealing
- Perceived as safer and more ‘high tech’ than the conventional cast-in-place concrete digesters which failed
- Vessel construction allows for increased digester gas design pressures
- Increased ability to manage foam and reduced potential for scum formation due to concentrated vessel top
The two existing digesters are of the same design and construction as the digester that failed. Due to necessity, they will continue to be used as active digesters until the new digesters are brought on-line. Modifications to the overflow system and gas relief system have been made to allow continued operation during construction of the new egg-shaped digesters. At the end of construction, the existing digesters will be sequentially taken out of service, and undergo detailed structural and mechanical evaluations. Based on the results of those evaluations, the vessels will by modified to assure safer and more reliable operation unless replacement represents a more cost-effective solution. The vessels will then be used as dewatering feed tanks and liquid biosolids storage.