How to Protect Sewer Collection Assets and the Bottom Line
Many sewer collection assets are prone to degradation and corrosion from destructive hydrogen sulfide (H²S) gases and microbiological attack. Protective coatings are, therefore, required to enhance their service lives. But when it comes to specifying those coatings, there’s no shortage of opinions on the appropriate materials to use and how thick to apply them.
Material options range from reinforced cementitious mortars to high-build epoxies, polyurethanes and polyureas to more synthetic products such as polyethylene plastic products – and the list goes on. The recommended coating thickness will vary based on the coating solution, but some specifications call for film builds that are unnecessarily high, as they offer no additional protection beyond a certain thickness – and, therefore, add unnecessary cost with no added benefit.
So how do you determine which material is right, where it is needed and how much is required to ensure the best protection and asset life cycle? The devil is in the details, and it depends on the municipality’s expectations and available funding.
In a perfect world with unlimited financial resources, a municipality would achieve the longest life cycle by repairing all structural defects and degradation using a high-quality fiber-reinforced mortar and installing a resinous polymer lining in every manhole and other sewer collection asset. When properly installed, this system would provide protection for 40 to 50 years with regular inspection and maintenance. However, this is an expensive option, with a $250 to $750 cost per vertical foot, depending on the geographic location. Most municipalities don’t have the financial resources to take this approach. But they can adopt a multifaceted approach – one that uses this robust system, applied at a suitable but not excessive thickness, on assets facing high corrosive threats and other less expensive options on assets with less severe exposures. This approach can help a municipality optimize both its asset protection methods and budget.
Protection and Savings Combined
Optimizing protective solutions requires knowing where the biggest corrosive threats occur within systems. Typically, the highest levels of corrosive gases are found in areas with the highest turbulence, such as lift and pump stations, wet wells, force mains and the manholes immediately downstream of these assets. These areas require the most robust protection because the high flow and turbidity strip the H²S out of its aqueous form and turn it into gas that is then deposited on an asset’s surfaces, increasing corrosion potential (Lead image). A municipality will realize the best value by specifying a reinforced mortar at ¼- to ½-in. thickness (depending on the concrete’s condition) for these areas followed by 80 to 125 mils of resinous polymer material applied on top.
In other less turbulent areas with low H²S values, standalone cementitious mortars can be an excellent choice to line manholes. These systems made from calcium aluminate or microsilica can help the municipality save money overall – compared to lining all assets with a mortar/resinous liner system – or rehabilitate more structures within its allotted budget. Cementitious mortars will react similar to the original substrate to high H²S levels, but they can provide 15- to 20-year life cycles with an ongoing maintenance plan in low-H²S areas.
Given the above options, a complete rehabilitation plan does not have to be a one-product-fits-all scenario. A multifaceted approach can work as follows:
- High H²S levels > 50 parts per million (ppm): A resinous polymer liner should be installed for maximum protection. Apply a mortar resurfacer followed by a resinous lining (either a high-build, 100 percent solids epoxy, an elastomeric or rigid polyurethane, or a hybrid) in high-turbulence, high-H²S areas and on the first three to five manholes downstream of a lift station or force main (Figure 2).
- Moderate H²S levels from 25 to 50 ppm: Use a standalone calcium aluminate mortar liner (Figure 3) after the first three to five manholes downstream of the lift station or force main all the way to the next lift station.
- Low H²S levels < 25 ppm, or when the repair is simply due to structural issues: Apply a standalone microsilica mortar liner in these areas and also in areas where no H²S is present.
Following these guidelines, a municipality can protect its assets properly and rehabilitate more structures with the same allotted budget. Of course, this requires addressing all inflow and infiltration issues up front with the appropriate polyurethane injection grout before making any lining repairs. After all, one of the biggest challenges affecting municipal budgets and wastewater treatment capacities is having to unnecessarily treat ground water entering the system through cracks and voids.
Less Is More
Regarding how much resinous lining material is needed to protect against corrosion in high-H2S areas, specified dry film thickness (DFT) values have been rising for rehabilitation projects. For many years, the standard DFT for severe wastewater service has been 80 to 125 mils (Figure 4). However, today’s specifications are beginning to call for 250 to 300 mils of resinous liner applied in manhole rehabilitation projects, doubling the material cost.
Unfortunately, this added material is an unnecessary waste of a municipality’s budget dollars. At 80 to 125 mils, the top end is already sufficiently overengineered to capture worst case scenarios. There is no need to apply more than 125 mils in any wastewater structure unless the filler is a resinous epoxy aggregate-filled mortar, and the liner is applied in one application. If you’re rehabilitating the structure with cementitious mortar to restore the surface to its original plane and address all voids, bugholes and defects, topcoating with 80 to 125 mils of resinous liner is all that’s required for maximum protection.
When rehabilitating sewer collection assets, a municipality’s goal should be to achieve the best performance while maximizing value. Adopting a multifaceted approach that prioritizes robust protection where needed and more economical options where appropriate is the ideal way to meet this goal. Applying robust systems at an appropriate, and not excessive, thickness also helps to ensure a positive return on a municipality’s investments.