Case Study: Water Reclamation Plant, North Collier County

At the North Collier Country Water Reclamation Plant, two domes covering 100 foot diameter, 1.5 mg sewage surge tanks were specified to require plastic lining. Crom had completed PVC linings in the past, but had found them difficult to weld. They also found PVC welds to be unreliable once the work was done. Resicon's Corr-Tite Linear Low Density Polyethylene (LLDPE) was selected for this project because of its ease of installation and reliability, and Resicon's extensive experience in the design and construction of lined structures.

Most municipal wastewater tanks are coated above the operating water level with urethanes or epoxies to protect concrete from the aggressive attack of hydrogen sulfide gases generated by either raw sewage or the sewage treatment process. Many tanks are coated over the entire wall and dome surfaces.

There is a growing recognition in the engineering community that the reliability of coatings is unpredictable on concrete used under these operating conditions. For coatings to develop full adhesion to the concrete substrate, preparation of the concrete and installation conditions must be ideal. Achieving this level of installation conditions is very problematic, resulting in unreliable coating applications. Coatings that are applied under less than ideal conditions can develop pin holes which, over time, allow corrosive materials to get behind the coating, causing the concrete to deteriorate. Engineers are increasingly specifying Concrete Protective Liners (CPL) in areas where minimal or marginal protection can predictably result in concrete corrosion.

Concrete protective liners (CPL) are made from PVC, Polyethylenes or Polypropylenes, all of which effectively and reliably resist corrosive sewer gases. These membranes have integral locks on the sheet surface and come in thicknesses of up to 0.250 inches (5 millimeters). These membranes are laid on the formwork (or in the mold in the precast concrete process) before the concrete is poured, and lock into the cured concrete, forming an impervious barrier to any corrosive gas.

At North Collier, the walls of the prestressed concrete tank, the hallmark of Crom Corporation's 50+ years in the tank building business, were constructed up to the stage where the prestressing was done. The plywood dome form was then placed and readied for the Corr-Tite CPL.

The Corr-Tite had already been prefabricated and cut to the pie-shaped sections that would fit the dome, so it was simply a matter of placing the material on the dome and welding as much of the joint between the sections as was necessary to have the liner fit. Approximately 50 percent of the welding was done before the concrete was poured, and the remainder of the welding was completed after the dome was set and the formwork stripped.

One of the problems with polyethylene lining materials is that polyethylene expands and contracts in response to temperature changes. Resicon has extensive experience in designing liners for landfills and lagoons which are much larger structures. Expansion of the liner across these large structures is measured in feet rather than in inches. Resicon's experience in designing to accommodate material expansion and contraction was important to the project. Detailed calculations including the high and low temperatures of the region and the effect of the sun on the material were made. Panels were constructed and fastened to the dome form so that expansion and contraction was taken up in specific joints. These joints were left unwelded, so that all the movement in the liner would transfer to these "slip joints".

Two lining contractors experienced in the application of CPL membranes worked together to install and weld the Corr-Tite CPL. Premier Lining Services of Derry, New Hampshire, and Island Liners of Vancouver, British Columbia, worked over the weekends to prepare each dome so that Crom's work would not have to be delayed.

The interior of the finished dome was welded and completely tested. Butyl caulking was applied to the joint between the wall and the dome, completely sealing the dome from any gases generated by sewage contained in the tank.