How to Expect the Unexpected with Digester Cleaning? We've Got Some Grit Ideas.

Presentation Description: Routine digester cleanings are essential to improve digestion performance and increase the lifespan of digesters. Cleanings also provide an opportunity to perform additional maintenance work on the digesters when they are out of service for an extended period. However, digester cleanings can result in unanticipated costs associated with material dewatering, material disposal, and interior structural/coating repairs. Without proper digester cleaning specifications, “simple” routine cleanings can result in costly change orders and schedule delays. This presentation covers an in-depth case study and provides a methodology that other utilities can adapt to mitigate budget and schedule risks during digester cleanings.

El Estero Water Resource Center (EEWRC) performs digester cleaning approximately every 10 years, and they contracted Brown and Caldwell (BC) to develop a bid package for their most recent digester cleaning. Even though this is not the first time EEWRC has performed a digester cleaning, the team experienced several unexpected issues: significant grit accumulation, digester gas purging challenges, deteriorated interior coating, concrete spalling, and cracks, and complicated digester access points. EEWRC and BC applied lessons learned from this project to identify a proactive digester cleaning strategy that accounts for unexpected situations and opportunity for utilities to benefit from digester cleaning, not just in terms of improved process performance and extended digester lifespan but also with minimizing capital expenditures so additional projects can be maintained under their capital improvements plan. This presentation dives into the challenges faced during EEWRC’s digester cleaning process and the solutions implemented by the team. Findings from EEWRC will be compared to experiences at other wastewater treatment facilities. The steps to develop a robust digester cleaning specification including appropriate estimation of material quantities, digester structural inspection specifications and work sequence optimization will be discussed. The methodology recommended in this presentation may help encourage other utilities to proceed with digester cleanings in a timely, confident, and risk-free way knowing that their budgets and assets will be better protected.