Customer
1.2 million gallon EnviTec 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 four years. The customer wanted Bristola to provide an estimate for the cleanout, mixer replacement, roof removal and reinstallation of a new roof.
Potential Costs
The customer received proposals exceeding $300,000 for the roof and more than $300,000 for the cleanout. Bristola agreed to provide the customer with a cost to replace the roof, remove and reinstall the mixers, clean out the digester and, retrofit the entryway to allow for future live cleanings.
Solution
Bristola proposed using a crane, clamshell and skid steer to remove the roof and solids from the tank. At the start of the cleanout process the mixers would be removed and sent out to the customer’s repair shop, with the goal of reinstalling them once the new roof was ready. Bristola would also clean the walls and internal heat loops as part of the cleaning process. Two cranes would then be used to install the roof, and a smaller crane would be mobilized to install the mixers. The crane costs were significant, and swapping cranes would help to save the customer money on the project. Bristola also retrofitted the 4-by-4-foot entry door equipped the new door with a 24-inch pipe and isolation valve. This valve would allow for future live cleanings using the submersible remotely operated vehicle cleaning system.
Execution
The digester was drained to an adjacent holding lagoon. The roof was removed and the digester cleaned out in 12 days using the crane, clamshell and skid steer. During the cleaning process, it was observed that the gas zone of the inside concrete wall had heavy corrosion damage. Bristola worked with a nationally recognized vendor to supply concrete repair and corrosion-resistant epoxy products. The vendor and Bristola worked collaboratively on a plan and presented this to the customer. The customer agreed to address the problem, and the top 4 feet of the tank were cleaned, repaired and a corrosion-resistant epoxy applied. After the epoxy and concrete repair were finished, Bristola then installed the new roof and mixers. Lastly the retrofitted door was installed and the tank filled for recommissioning.
Project Costs
The total project cost was $411,000. This included the cleaning, roof removal and new roof installation, concrete repair and epoxy application, removal and replacement of five mixers, and installation of a retrofitted door and 24-inch isolation valve.
Summation
Bristola removed the roof and clean out the tank in a very short amount of time. The concrete corrosion inside the gas zone demanded a reevaluation of the plan and process. Bristola was able to assist the customer, find a reliable corrosion products company, and procure and apply the product for the customer. The roof install was a great success as the manufacturer also provided a support person to ensure the install was aligned with their processes and expectations. The manufacturer representative approved the install and verified the warranties for the customer. The mixers were not repaired by the customer’s service company in the allotted time, and Bristola came back at a later date to install the mixers before the tank was recommissioned.
