Have you ever heard the term “Out of sight out of mind”? Sewer collection systems qualify as “just that” to the general public. After spending the last several years in this segment of the water and wastewater industry, I am amazed at the current state of most of our buried infrastructure in the United States.
It has been estimated that 25-30% of water infiltration enters through manholes. Treating this excess water adds significant cost to the treatment of wastewater, for any municipality. Couple the additional treatment cost with the cost of maintenance and repairs to these structures from microbial induced corrosion and your current budget has been exceeded.
Microbial Induced Corrosion acts to breakdown the sulfates in the waste stream, producing hydrogen sulfide gas (H2S) and carbon dioxide (CO2). These acidic gases reduce the pH of the concrete and Sulfur Oxidizing Bacteria (SOB’s) attach to the surface as sulfates are produced and these SOB’s (Thiobacillus Thioxidans) consume H2S and discharge sulfuric acid (H2SO4).
For years the industry has addressed this problem with the lowest cost option and repaired these structures with similar cementitious products to those used for the initial construction. While these repair products (microsilica modified repair mortars and calcium aluminates repair mortars) slow the corrosion rates from an average of 18% loss by weight (Portland Cement) over an 8-year period to as little as 3% loss by weight (Calcium Aluminates)over the same period, they do not stop the process.
Correcting the problems that exist in the structures should incorporate a system of products for repair and protection.
- Infiltration – Typically corrected with hydrophilic or hydrophobic polyurethane grouts installed through trenchless technology.
- Structural Rehabilitation – The rehabilitation of the structure is typically addressed with the use of cementitous repair products that have increased physical properties over that of the original substrate. The right selection of these materials can be left as a stand alone liner in a mild sewer environment.
- Corrosion Resistant Linings – Epoxies, urethanes, and polyureas offers the greatest improvement to the life cycle of these assets. The selection of the right products should incorporate some specific physical performance requirements and in most cases are not those that the industry has promoted for years.
Sewer structure rehabilitation can be a long-term solution if the specified products are designed to provide the maximum life cycle protection. Considering solutions of this nature can be cheaper in the long run than what the low bid offers. The best point of view that I have heard in a long time may have been that of Mike Hicks, Program Manager – Sewer Repairs & Replacement for the City of Fort Wayne, Indiana in a recent Trenchless Technology article. Hicks stated “I don’t want to leave my granchildren with problems that effect the quality of their lives.”