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Battling the enemy

Battling the enemy

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    Crews at the Central Costa Sanitary District Wastewater Treatment Plant install deep-well anodes for a cathodic protection system designed to protect the secondary-treated-effluent welded steel pipe (shown with a white coating at the rear of the photo). The deep well was drilled to a depth of 320 feet and was about 12 inches in diameter. Photos: V&A Consulting Engineers Inc.

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    Left: Welders connect paired cathodic protection wires to below-grade wastewater pipes. Only a small fraction of U.S. wastewater systems are currently provided with cathodic protection. Right: Polyvinyl chloride sheeting—shown here in interior-wall form—will protect the new pump station from sulfide-induced corrosion.

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    Atmospheric conditions caused corrosion on this flexible coupling.

Concrete. Relatively immune to electrolytic corrosion, concrete is usually durable in underground service but should have a protective coating if the soil is acidic. If groundwater is high in sulfates, use Type V (sulfate-resistant) cement.

As long as it is submerged, concrete generally is safe from corrosive attack. But when submerged surfaces are exposed to air and humidity (as they are in a sewer pipeline during hours or seasons of reduced flow), trouble will follow, usually in the form of a biologically mediated sulfuric acid attack. Under the right conditions, concrete can be destroyed surprisingly quickly. Corrosion tends to be much faster and more extensive where concrete is exposed to combinations of fluids and air—such as sewer pipes, manholes, junction boxes, pumping stations, wetwells, grit chambers, clarifiers, aeration basins-in short, any place where the water level varies and biological slimes are present on the pipe or vessel walls.

The bottom line is that unprotected concrete should not be used anywhere high H2S concentrations occur. In particular, protective interior linings in large-diameter pipelines should extend well below the anticipated waterline. The most extensive sewer main rehabilitation or replacement projects arise out of failure to observe this principle.

Metals. All metallic components are vulnerable to corrosion, including steel pipe supports, wall anchors, slide gates, catwalks over clarifiers, roof vents, and clarifier mechanisms, where steel bolts can be a particular focus of corrosion. Aluminum's resistance to atmospheric sulfides and other pollutants makes it an attractive choice for numerous applications, but it will corrode quickly when used, as it often is, for digester odor-control covers without adequate ventilation. Aluminum also corrodes rapidly when in contact with high-alkaline substances such as concrete and therefore should not be used for handrails, door or access hatch frames, or windows unless isolated with an appropriate dielectric material. It should not be used for underground applications or in fluid exposures.

With appropriate precautions, copper and brass perform well in underground applications. In low-pH or high-chloride/sulfide soils, electrical isolation or cathodic protection may be needed. Copper and brass also resist atmospheric corrosion well under most circumstances but are attacked by the hydrogen sulfide prevalent in wastewater facilities. Their use is discouraged unless absolutely necessary—such as in electrical systems, fuses, and switches. Protective coatings, well-sealed stainless steel or nonmetallic junction boxes, tight seals on conduits and at conduit entrances, and vapor corrosion inhibitors can help retard corrosion of copper and copper alloys.

Steel and ductile iron need to be electrically isolated from dissimilar metals to prevent galvanic corrosion. Buried pipelines and structures should be coated or protected cathodically, or at least provided with a means of corrosion monitoring. Steel components that will be submerged (such as rake arms or clarifier mechanisms) also should receive cathodic protection and suitable coatings.

Stainless steel, particularly the 300 (austenitic or iron-chromium-nickel) series, is useful in the corrosive environment of a wastewater plant. In most facilities, stainless is replacing carbon steel and cast iron for slide gates and other water control apparatus. Generally, however, stainless is not suitable in buried exposures due to the risk of differential oxygen corrosion cells forming. Type 316L (a low-carbon molybdenum steel) generally performs best in flowing-water applications, though there is some risk of pitting if the wastewater is slow-moving or stagnant.

Coatings and Linings

As recently as 1950, bare concrete structures were the norm. The first tentative advancements emerged in the 1940s. Early products, alkyds or coal-tar epoxy resin coatings, were costly, often required several layers, adhered inconsistently, and were subject to pitting and corrosive gas penetration. Solvent-based coal-tar epoxies also shrank as they dried, leading to crazing, cracking, and separation.

Today's high-performance materials date back roughly to 1960, when intensive research on coatings began. The result is an extensive selection of polyvinyl chloride, polypropylene, polyurethanes, 100% solids epoxies, fiberglass-reinforced epoxies, and other specifically formulated products.

For example, elastomeric urethanes or high-solids epoxy coatings give good protection against chlorine; high-solids epoxies are suited for interior piping, equipment, and metalwork; aliphatic polyurethanes usually perform better on exterior surfaces. An array of reliable, affordable membrane linings also is available. Many of these materials have been in place and performing well for more than 30 years.