Corrosion at wastewater treatment facilities gets little attention from upper-echelon managers and policymakers but is all too familiar to those who actually operate and maintain the plants. Until this year, the 20-volume ASM Handbook did not even specifically address corrosion in wastewater plants. Unfortunately, it takes a series of unfortunate episodes to bring the problem to public awareness.
Recently, a number of large corrosion-related failures in San Diego, Phoenix, Miami, and elsewhere have been traced to corrosion and deterioration of concrete, and Los Angeles is contending with a failure expected to cost billions of unplanned rehab/replacement dollars. These incidents highlight corrosion as a major concern. The bright side is that plant designers, owners, and operators have an arsenal of tools for controlling corrosion. Properly selected and used, these tools can save hundreds of billions of dollars over the long term. The concept is both simple and innovative because it calls for a genuine revolution in how we look at public works infrastructure.
In the past, wastewater plants were expected to have a useful life of 25 to 50 years, depending on climate, soil, and other conditions. But the time has come to start thinking of an indefinite useful life.
At San Diego's Point Loma Wastewater Treatment Plant (peak capacity of 432 mgd), a recent rehabilitation of its 30- and 40-year-old sedimentation tanks brought the units back up to standard so effectively that the plant's senior civil engineer expects them to last another 30 years without a major renewal.
This will save taxpayers millions in rehabilitation and replacement costs, and it also enables the city to use the modified approach for its Governmental Accounting Standards Board (GASB) 34 report and claim an indefinite life for the $32 million asset. With advanced materials and methods, and a deeper understanding of their structure and behavior, agencies can extend the life of facilities indefinitely, making replacement or major rehabilitation rare.
Protecting a Plant
Corrosive environments within a wastewater facility are broadly classified as buried, fluid (submerged), or atmospheric. Every component is exposed to at least one of these and therefore vulnerable to several kinds of corrosive attack.
The problem begins with the potable water supply which may be non-corrosive, highly aggressive, or in between. Then, new constituents are picked up along the way, increasing the organic load and making the water increasingly aggressive. Testing wastewater—for pH, chloride-ion content, resistivity, chemical and biochemical oxygen demand, levels of suspended solids, and Langelier Index (a measure indicating the tendency for calcium carbonate buildup)—is an essential first step. Tests will reveal water acidity, expected rate of mineral deposition, pipeline corrosion, and potential bacterial generation of hydrogen sulfide (H2S).
Anaerobic sulfate-reducing bacteria such as Desulfovibrio desulfuricans in the gelatinous slime layer on interior walls produce H2S when they metabolize sulfur from organic matter in the wastewater. Other bacterial populations convert H2S to sulfuric acid, which attacks concrete, metallic, and other reactive materials. Climate (warmer regions are at greater risk), water temperature, abundance or paucity of dissolved oxygen, slope in pipelines, retention time, and wetted surface area all influence H2S generation.
Soil and air also should be tested prior to siting and before design and construction of a facility. Soil issues include soil resistivity and chemistry, and proximity to other structures, including cathodically protected structures and utilities. Inspection of existing structures can uncover important information. Ambient air should be tested for damaging substances including airborne salts, industrial contaminants, and traffic-generated sulfur dioxide.
Wastewater facilities are made mostly of two things: concrete and metal. Each of these has its own corrosion issues.