An increasing number of water and wastewater systems, structures, and components in the United States are being affected by corrosion and deterioration, which can shorten the life span of the system and increase costs for the consumer. To increase awareness of the corrosion problems encountered by municipal wastewater systems, members of NACE Task Group (TG) 466 recently published a report, “Corrosion Problems and Renewal Technologies in Wastewater Systems.”
The comprehensive report provides a roadmap that can guide decision-makers such as utility directors or operations managers in understanding the types of corrosion-control solutions available that can help them achieve system sustainability, says NACE International member Eric Dupré, business manager with Southern Trenchless Infrastructure Rehab Co. (Houston, Texas) and chair of TG 466. The report identifies materials of construction and the corrosion mechanisms that affect various components of a municipal sewer system, and explains repair, rehabilitation, and replacement methods for these components. Additionally, the report describes several current inspection technologies available for asset assessment.
The TG 466 report notes that ~190 million people in the United States are served by ~16,000 sewer systems comprising ~740,000 miles (1.2 million km) of public sewer mains—the publicly owned collection lines that gather the sanitary sewage from individual properties, convey it to a treatment plant, and then release it into a receiving body of water— plus 500,000 miles (800,000 km) of private lateral sewers, the portion of the collection system that connects a privately owned structure to the sewer main. These systems, however, are aging; 68% are more than 25 years old and 2% are more than 50 years old. Investment in upgrades and repairs is needed to maintain the nation’s wastewater infrastructure and prolong its service life; but the need comes at a time when municipal and state budgets are becoming more constrained and less able to maintain and sustain these deteriorating wastewater systems, and many asset management programs are doing more with less.
In its 2013 Report Card for America’s Infrastructure,1 the American Society of Civil Engineers (ASCE) says capital investment needs for U.S. wastewater and storm water systems are estimated to total $298 billion over the next 20 years, with 80 to 85% of capital investments addressing the country’s public sewer mains. The report gives the nation’s wastewater systems a “D” grade for infrastructure because they are “in poor to fair condition and mostly below standard, with many elements approaching the end of their service life.”
The costs to manage corrosion are also high. The 2002 cost of corrosion study,2 published by the U.S. Federal Highway Administration, reports the direct annual cost of corrosion in drinking water and sewer systems is $36 billion, which includes the cost of replacing aging infrastructure, lost water from leaks, corrosion inhibitors, internal mortar linings, external coatings, and cathodic protection. This figure comprises 75% of the total corrosion cost for all utilities—gas distribution, electricity, telecommunications, water, and waste water.
Wastewater pipe corrosion leads to untreated sewage releases into the environment that can cause soil and groundwater contamination. Pipe defects (such as holes, cracks, and failed pipe joints) in wastewater collection systems can cause blockages that lead to sewage overflow and backup into buildings. Pipe leaks/breaks can cause soil erosion and roadway damage, and disrupt service to customers.3 In the United States, there are up to 75,000 sanitary sewer overflows per year, resulting in the discharge of 3 to 10 billion gal (11.3 to 37.8 billion L) of untreated waste water.4
According to the TG 466 report, a variety of materials are used to construct wastewater pipelines and structures, including concrete, brick, iron, steel, various plastics, and composites. Portland cement-based unlined concrete is the most widely used material in existing wastewater systems in the United States, with ferric metals (iron and steel) coming in second. Corrosion of these materials in wastewater systems is caused mainly by hydrogen sulfide H2S corrosion and microbiologically influenced corrosion (MIC). Other corrosion deterioration mechanisms found in typical wastewater collection systems include hydraulic abrasion from turbulent flows and grit in the wastewater stream, and stress resulting from hydrostatic pressures.
The report describes the basic mechanisms of H2S corrosion, which can lead to rapid, extensive damage of concrete and metal sewer pipe and tanks, mechanical equipment used for the transport and treatment of wastewater, and electrical control and instrumentation systems. This type of corrosion occurs where biological activity of anaerobic bacteria results in the formation of sulfide. Under anaerobic (septic) wastewater conditions, sulfides cannot be oxidized. They combine with hydrogen to produce H2S gas, which has the characteristic “rotten egg” odor. When a sewer is operating partially full and exposed to air, the damp surface above the water line is exposed to aerobic bacteria that oxidize the H2S in the presence of moisture and produce sulfuric acid (H2SO4). The H2SO4 attacks exposed concrete and unprotected iron, steel, and copper surfaces. This results in corrosion of the collection system pipes, manholes, lift station wells, and other structures, with the majority of corrosion occurring above the water line in the headspace area of the structure. Systems that are particularly vulnerable to attack include electrical components, instrumentation systems, and ventilation units. Many variables, which are summarized in the report, directly or indirectly affect sulfide generation, H2S release, and H2SO4 corrosion.
The rate of H2S corrosion is dependent on the construction materials used, the features of the wastewater stream and the collection system, and the type of transport and treatment processes used. Certain units and their processes are more susceptible to corrosion damage than others. The report describes the various components of a wastewater collection and treatment system that are most likely to promote the generation and release of H2S gas and reviews the basic steps typically involved in identifying existing or potential corrosion problems. It notes that system components experiencing H2S corrosion often require renewal—the application of a broad range of repair, rehabilitation, and replacement technologies to pipes, manholes, tanks, pump stations, and other mechanical equipment—to restore the functionality of the entire wastewater collection system.
Factors that commonly affect renewal planning include the ability to inspect and assess the condition and deterioration rate of each component, the extent of critical repair needs, and the availability of funding for rehabilitation work. When the pipe or structure is structurally sound and provides acceptable flow capacity, a repair technique is usually applied. A rehabilitation technique is employed when hydraulic conditions and structural strength need to be improved. When a pipe or structure is severely deteriorated, and/or f low capacity needs to be increased, a replacement technique is normally used.
The report focuses on trenchless technologies, which are methods that can be used to renew underground structures without full excavation so surface disruptions are minimal, and includes a description of typical repair, rehabilitation, and replacement methods for the primary components of a wastewater collection system—namely sewer mains, sewer laterals, manholes, force mains, and ancillary structures (pump/lift stations, valve or diversion structures, overflow structures, and drop shafts). H2S corrosion is often controlled by protecting structures with paint and other coatings and linings, as well as constructing structures of corrosion-resistant materials. The renewal technologies discussed in the report include sliplining, spiral-wound liners, cured-in-place pipe (CIPP) liners, close-fit liners, grout-in-place (GIP) liners, panel liner systems, sprayed coating and liner systems, and flood grouting.
The applicability of each technique is based on the condition of the existing asset, site circumstances, cost, track record, local availability of the technique, as well as its expected ability to meet performance requirements over an extended life cycle. The report contains several tables that provide a comprehensive overview of these sewer pipe renewal methods, including their features, work requirements, and applicable pipe parameters. According to the report, the performance of these technologies to date indicates that they do provide extended service life to infrastructure.
Other issues associated with managing wastewater system corrosion are also discussed in the report. These include design considerations, long-term performance and testing, and new materials, as well as condition assessment and inspection technologies. Condition assessment provides the critical information needed to evaluate the physical condition and functionality of a wastewater collection system and estimate its remaining service life and the value of its assets. The report lists a variety of inspection technologies used to assess wastewater collection systems, where they can be applied, and the type of defects they can detect. The technologies described include closed-circuit television (CCTV), acoustic technologies, electrical/electromagnetic current technologies, laser profiling, and other technologies currently under development.
This report is not intended to address all types of activities used to develop and implement a wastewater system renewal construction project; however, says Dupré, it does provide the reader with valuable information that will guide them when making corrosion management decisions that affect the future sustainability of their wastewater system.
Members of TG 466 include vice chair Frank Madero with MADERO Engineers & Architects; Erez Allouche with the Trenchless Technology Center at Louisiana Tech University; Alec B. Angus and Ramon Pelaez with Greenman-Pedersen, Inc.; Jason Iken with the City of Houston PWE Wastewater; Jeffrey Maier with the Metro Wastewater Reclamation District in Denver, Colorado; Dan J. Murray with the U.S. Environmental Protection Agency; Mohammad Najafi with the Center for Underground Infrastructure Research and Education at the University of Texas at Arlington; Jim Sepowksi with International Paint, LLC; and Cumaraswamy Vipulanandan with the Center for Innovation Grouting Materials and Technology at the University of Houston. Dupré acknowledges that a substantial amount time and effort was donated by TG 466 members to develop and publish this report.
Contact Eric Dupré, Southern Trenchless Infrastructure Rehab Co.—
e-mail: eric@southerntrenchless.com.
References
1 “2013 Report Card for America’s Infrastructure,” American Society of Civil Engineers, http://www.infrastructurereportcard.org (November 25, 2013).
2 G.H. Koch, M.P.H. Brongers, N.G. Thompson, Y.P. Virmani, J.H. Payer, “Corrosion Costs and Preventive Strategies in the United States,” FHWA-RD-01-156 (Washington, DC: FHWA, 2002).
3 “Aging Water Infrastructure Research: Science and Engineering for a Sustainable Future,” U.S. Environmental Protection Agency, Publication No. EPA/600/F-11/010, 2011.
4 “Aging Water Infrastructure (AWI) Research, System Rehabilitation,” U.S. Environmental Protection Agency, October 30, 2012, http:// www.epa.gov/awi/system-rehab.html (November 22, 2013).