New Coating Replaces Failed Lining System on Digester Lid
Located on 44 acres in Pierce County, five miles south of Tacoma, Wash., the Chambers Creek WWTP treats residential and commercial wastewater in its service area and currently processes 21 mgd.
Located on 44 acres in Pierce County, five miles south of Tacoma, Wash., the Chambers Creek WWTP treats residential and commercial wastewater in its service area and currently processes 21 mgd. The plant contains three grit tanks in the head works, six primary clarifiers, three digester tanks, six secondary clarifiers and five aeration basins. Pierce County’s treated wastewater is discharged into Puget Sound, and its treated biosolids are trucked to farmers in Lewis County for use as fertilizer on pasture lands.
Aggressive gas permeation and the corrosive environment of the headspace caused the original liner to bubble, wrinkle and delaminate from the substrate.
In the midst of a June 2001 facility upgrade, owner Pierce Utilities requested bids to provide new 36-inch-thick concrete lids with protective coatings for two of the three digester tanks at Chambers Creek. The specification called for an interior polyurea liner over the entire 80 foot diameter of the lids and five feet down the walls. For the rest of the walls below the waterline, a cross-linked cycloaliphatic amine epoxy with strong corrosion and chemical resistance - Tnemec’s Tneme-Liner - was specified. Principally used for immersion service, this coating was originally applied when these tanks were built in the mid-1980s: it had stood up to the aggressive environment of these tanks and had never lost its performance characteristics.
After trowel-applying a high-performance epoxy mortar, the applicator team then spray-applied a thick film, 100 percent solids, abrasion-resistant lining.
In the fall of 2001, the tanks were cleaned, the original floating lids were replaced, the new stationary concrete lids were installed, and then the coatings contractor, Coatings Unlimited (CU), Kent, Wash., applied the specified polyurea system.
To restore the first digester lid at the Chambers plant, the old liner was removed and the applicator team abrasive blasted the headspace, removed the original surfacer filler and neutralized all surfaces.
A systematic check less than a year later found that the polyurea linings were beginning to lose their elasticity and adhesion to the substrate. The thick film was beginning to deform, blister and wrinkle, starting in the crest of each dome and moving outward to the edges. The owner’s warranty required the material to be replaced, so CU removed the delaminated lining from both lids and applied the polyurea over a different primer.
By the summer of 2003, the coating on both digester lids was failing once again above the waterline in the tanks’ head space. It was apparent to everyone that a new coatings solution was needed. The warranty work was no longer actionable, and the delays were costing a great deal of money. CU recommended a call to Tnemec’s Seattle-based coatings consultant.
The utility emptied one digester and brought in a bank of intense lighting equipment. Looking up at the lid, it was immediately evident to the Tnemec consultant that the failure was likely due to elevated gas concentrations that had permeated the coating in the head space.
The environment in the head space of an anaerobic digester can be aggressive, often filled with methane, carbon dioxide and hydrogen sulfide gases. Those gases can permeate a liner, altering the physical properties of the film and ultimately affecting the adhesion to the substrate.
The recommended solution for the problem was application of a modified polyamine epoxy from Tnemec that is specifically designed for wastewater immersion and fume environments. Called Perma-Shield, the thick film, 100 percent solids, abrasion-resistant lining exhibits low permeation to gases including H2S, as well as protects against Microbiologically Induced Corrosion (MIC) and provides chemical resistance to severe wastewater environments.
Digester Lid Restoration
To remedy the situation, the consultant instructed the CU crew to remove the failed coating on the first lid. This meant pulling it off in sheets by hand and with scrapers. For the surface preparation, the CU team abrasive blasted per SSPC-SP13/NACE 6 Surface Preparation of Concrete, removing the lining system, the surfacer filler and neutralizing all surfaces so they were clean, dry and free of contaminants.
This is one of three digester tanks at the Chambers Creek Wastewater Treatment Plant in Pierce County, five miles south of Tacoma, Wash.
Following the preparation, the team then spray-transferred and troweled smooth the entire surface with an epoxy modified mortar. Called MortarClad, this aggregate-reinforced material was developed especially for surfacing, patching and filling voids and bugholes in concrete substrates down to 1/16 inch. MortarClad was applied to make the concrete substrate perfectly smooth for a pinhole-free finish coat application.
The finished digester lids, after application of a modified polyamine epoxy coating system.
Next, the CU team spray-applied a single coat of Perma-Shield to a dry film thickness of 40-50 mils with plural component spray equipment. A different type of application, each component is pumped separately through the hoses to an inline mixer, then to the gun and sprayed. This process is more efficient, allowing the crew to maneuver the scaffolding used without causing material potlife concerns. Moreover, the single coat application was possible because the MortarClad resurfacer provides a contiguous concrete surface.
The Perma-Shield system has been in place in the first digester lid for over a year and the Chambers Creek management team gives the system high marks.
About the Author:
Vaughn O’Dea is Technical Sales Manager, Water/Wastewater, at Tnemec Company, Inc. He is a SSPC Protective Coating Specialist and NACE Certified Coating Inspector and Corrosion Technician. O’Dea also is an ICBO Certified Reinforced Concrete Special Inspector and a member of NACE, SSPC, ICRI, WEF and AWWA. Tnemec produces approximately 120 architectural and industrial coatings products used in a wide range of protective applications for steel, concrete and other substrates. For more information on Tnemec Co. or any of its products, visit www.tnemec.com.