
Extending the Life of a WWTP Digester with a High-shear Tank Liner
Aquatera Utilities Inc. manages water and wastewater operations for multiple communities in northwest Alberta. Due to ongoing population growth in its service territory over the past decades, the company has created a water treatment plant capacity upgrade plan, to keep up with processing needs.
Recently, the utility realized the need for interim measures to maintain treatment capacity at its Grande Prairie Wastewater Treatment Plant (WWTP), until a master plan is complete and identifies a future upgrade path. Currently, a biological nutrient removal (BNR) treatment facility that processes roughly 21 million liters of sanitary waste per day, the plant uses an Autothermal Thermophilic Aerobic Digestion system (ATAD), to handle sludge removal.
Problem: Plant Capacity Limited by Deteriorating Infrastructure
Standing in the way of the plant’s ability to reliably maintain capacity was the fact that a key piece of infrastructure was no longer able to pull its processing weight, due to decreasing wall thickness.
The culprit was a digester tank, originally designed to help create Class A biosolids, but currently used as sludge storage prior to a centrifuge system.
“This usage created different conditions than the tank was designed for,” recalls Shane Kutin, P.Eng., Aquatera’s engineering manager. Shortly after the plant had removed the heating component circa 2015 and converted the ATAD to operate as a storage tank, the utility ended up having to replace the roof of the epoxy-coated carbon steel tank. “We started replacing the roof with stainless steel, but another eight years later, the tank walls were starting to show signs of localized corrosion at liquid levels,” Kutin says.
The tank was in fairly good shape below the liquid level, but above it, plant technicians began seeing a lot of serious pitting. Soon, they were actually starting to see holes forming all the way through the walls.
Kutin believes the conversion of the tank from its original function as digester to storage unit likely accelerated some of the localized corrosion issues, though it had previously been apparent that the tank wasn’t lasting as long as it had originally been specified for.
Management recognized that extending the life of this sludge tank — by repairing these voids in the inner and outer walls, then relining the interior — would buy them a few years of needed functionality to reach the point where the WWTP would get its own system-wide capacity upgrade plan. But they didn’t want to have to commit serious dollars to an asset which may or may not survive a near-future upgrade.
“We will be moving to, I believe, a Mesophilic Anaerobic Digestion (MAD) process,” says Kutin. “Hopefully we get a new system, but that’s to be decided. I think we could potentially incorporate (the tank) with our next system, at least in a short — five, maybe 10-year — term, then put in another. In essence, it could become strictly a holding tank for a new system.
“We need this tank. It really helps operationally, so we don’t have to centrifuge as we go. The sludge that comes out of our process goes through a centrifuge that dries it out, so we can haul it, then the rest of the liquids go back to be reprocessed. If we don’t have this tank (available), it’s just got to run 24/7; whereas right now, we just hold it for a couple hours, maybe a couple days, and just centrifuge as we need to.”
Solution: Apply High Shear Capacity Polymer Liner Technology
Plant management has performed other such asset lining, but for this badly deteriorated tank, would have had to replace panels, or done quite a bit of patchwork repair, due to the complete perforation of the walls. This would have meant a lot of prep to get the surface ready for welding, then re-prep before lining; quite a bit of work for a temporary fix.
They considered the option of foregoing repair work completely, by simply replacing the previous digester with a brand new tank, which would ultimately be designed into the new MAD treatment process. But cost estimates were five to seven times as much as even the most expensive lining solution.
“It wouldn’t be wasted money, since we’ll use it in the next-gen MAD system,” says Kutin, “but that cost probably doesn’t even include everything such as site prep for the berms, spill containment, things like that.
“We didn’t want to sink a whole bunch of money into tearing the tank apart, or new sections of tank, so we looked around for products that could help line it. There were a lot of lining products out there, but when I talked to infraStruct, they had this three-layer liner system that provided load-bearing capacity, as well as corrosion protection redundancy.
“That just made everybody feel a lot better. When you start having holes in your tank, to put a structural layer on, versus having to do spot repairs and some coating. So now we have not just a coating, but an extra bit of structure on this tank. It gives us peace of mind, that we had a lot of localized damage in one area, but now we have some extra support there.”
Creating an Interim Plan
InfraStruct approached this application using a methodology they created, called the “Diagnose, Advise and Treat (D.A.T.)” approach to solving complex problems. infraStruct project manager Rob Epp, explains:
“We assembled a group with diverse expertise to diagnose the problem, understanding the behavior we are trying to resolve. Advise: Do we repair, renovate or replace? Treat: Select the right technology to complete the project safely, on time, on budget, and resolve the problem.”
A solution of this magnitude requires an independent peer review process regulated by the Association of Professional Engineers and Geoscientists of Alberta (APEGA). It must be reviewed, stamped and sealed by a registered professional engineer in Alberta.
Through the D.A.T. process, infraStruct established that the OBIC Multi-Layer System addressed the behaviors needing resolution, to protect the tank from both corrosion and the hydraulic fluid loading force placed on the existing wall, currently and in the future.
The Team
InfraStruct retained Victoria, British Columbia-based Colquitz Engineering Ltd. to design the rehabilitation process. Chris Lee, P.Eng. led the design process and brought on Hillard, Ohio-based Rehabilitation Resources Solutions LLC (RRS), to provide design consultation for the rehabilitation design of the liner. RRS’s Ed Kampbell, P.E., is one of the most experienced rehabilitation engineers in North America in designing high shear capacity liners for structural applications, as well as providing national specifications for design and construction of these liners.
Result: Restored Tank Functionality
“Our treatment and ops team dealt with infraStruct, and I didn’t hear anything (during the liner installation), because it all went well,” recalls Kutin. “They loved working with them, said they were one of the most communicative contractors we’ve had in a while.
InfraStruct brought the project in on time and on budget, allowing Aquatera the breathing room they needed, without breaking the bank.
“We have a lot of big capital improvement projects coming up, so it’s nice that we don’t have to spend a lot more on this in the next few years; that we can hold off and get many more years out of this tank, and allocate budget to other critical items,” says Kutin.
Stormy Shafer is director of content development and public relations for Creative Raven.