Automated closed-loop ROV cleaning for plug flow digester cleaning
Customer
1.2 million gallon plug flow digester (New York, USA)
Problem
The customer’s volatile solids reduction had dropped to less than 25%. They were unable to maintain the mesophilic temperature (95–101º) in the digester during colder months. The daily biogas production yield had dropped by 20%. The digester had not been cleaned out in over three years, and the customer wanted to delay the traditional cleanout until after winter to minimize the time required to reseed, reheat and commission the digester.
Potential Costs
The customer received proposals exceeding $650,000 to remove one concrete panel and bring in heavy machinery for the traditional cleanout. The cleanout was expected to take around 30 days, followed by an additional 30–60 days to recommission (increase methane-producing microorganisms) and achieve daily biogas production targets. Deferred revenue during this period was estimated at or above $75,000.
Solution
Bristola proposed using the 24 inch submersible ROV to remove and clean accumulated solids in and around the heat racking near the center wall of the digester. The Bristola technology would enable a more targeted cleaning approach, focusing on the most vital components, specifically the heat-radiant piping in the digester, to ensure the digester’s health and productivity throughout the winter.
Execution
The submersible ROV was deployed via the mechanical mixer locations on the perimeter walls of the plug flow digester. The customer removed the mixers, and these areas served as the deployment sites during the cleaning process. Bristola staged the dewatering trailer west of the digester near the adjacent concrete pad. This allowed for efficient removal and disposal of the solids from the digester. Bristola’s mapping abilities permitted the ROV to focus on the heat racking in Zones 1 and 2. Initially, the project’s scope was set for 10 days. The customer extended that to 15 days after seeing the amount of solids the ROV could remove daily. The SRCS system removed 1,375 yards of material in 15 days.
Project Costs
The project cost was $150,000 for operating the SRCS system on-site for 15 days.
Summation
Bristola’s SRCS system was a great solution for the customer as it was able to remove the accumulated solids without requiring them to incur the costs of downtime, taking the digester offline or performing a traditional cleanout in the winter. The customer now has a way to address solids accumulation within the plug flow digester moving forward. Bristola is working with them to implement a digester preventive maintenance schedule to address accumulated solids every 12–24 months. This will ensure peak biogas production and optimal VS reduction
targets are achieved.
