Wastewater treatment plants process some of the most aggressive and corrosive liquids to travel through any piping system. Wastewater treatment piping, tanks, pumps, and related equipment are typically exposed to a high concentration of organic and inorganic compounds, sewage and industrial waste, corrosive chemicals, solids, microbiological organisms of all forms, as well as various gases.

     While the entire HVAC, manufacturing, and process plant industry strives to eliminate all biological presence and its corrosion threat, the fundamental basis of any wastewater treatment operation is to cultivate bacteria and certain other microbiological organisms at their maximum growth rate. A wastewater treatment plant therefore contains not only inherently corrosive water, but the highest potential for steel pipe and tank damage caused by microbiologically influenced corrosion (MIC) as well.

     An estimated $36 billion in corrosion damage is estimated to occur in water and wastewater handling systems each year - a sizeable percentage of which exists at wastewater treatment plants. Corrosion targets steel piping, tanks, transfer lines, and pumps - compromising not only their integrity, but the ability of the plant to to meet Federal and State established discharge permit limits. Corrosion also affects secondary piping such as required for compressed air delivery, chemical feeding, cooling waters, and digester produced methane gas lines.

     The wide range of different wastewater components, in itself, makes the planning to address any single corrosive threat an impossible task. Organic components of the raw sewage influent may consist of fats, greases, proteins, surfactants, oils, pesticides, and phenols, etc. The inorganic components of a raw sewage influent typically consists of heavy metals, nitrogen, phosphorus, sulfur, acids and alkalies, and toxic compounds of all possible origin, etc.

     Gasses such as hydrogen sulfide, methane, ammonia, oxygen, carbon dioxide and nitrogen are commonly found dissolved in wastewater, among other corrosives. Anaerobic decomposition of organic materials containing sulphur and nitrogen often result in generation of odorous compounds such as hydrogen sulphide, mercaptans, amines, and volatile fatty acids, etc. Chlorine and ozone, disinfecting agents in the final phase of treatment, add further corrosion threat.

     In addition, various chemicals in the form of coagulants, flocculents, metal precipitants, emulsifiers, antifoaming agents, neutralizers, and odor control agents are added along the waste treatment pathway. A wastewater stream therefore contains a wide range of chemical components, each with its own contribution to the corrosion of steel, concrete, and other components of the treatment plant.

     The interaction of the many primary components of a sewage influent will typically produce secondary chemicals and gasses having even greater toxic and / or corrosive properties. Micro organisms cultivated at different stages throughout the wastewater stream produce a multitude of chemical and gaseous by-products - hydrogen sulfide, or H₂S being a very common and particularly damaging by-product of MIC related bacteria.

     Sulfur reducing bacteria (SRB), for example, reduce sulfate to sulfite in an anaerobic environment to produce hydrogen sulfide - H₂S gas. Other aerobes, most commonly different strains of Thiobacillus, will oxidize the sulfur to sulfuric acid - producing pH values as low as 1.0, and attacking the concrete basins and most metals it comes in contact with.

     Corrosion is a significant concern at almost any steel surface within a wastewater treatment plant, and can extend to piping unrelated to the movement of wastewater itself. Due to the inherent threat which exists, many waste related treatment tanks and piping systems are coated to protect against such corrosive environments. Cathodic protection is also provided in some instances as another means to reduce corrosion, to extend service life, and to reduce maintenance costs. Even though holding, aeration, and sedimentation tanks are constructed of concrete, they inevitably suffer the damaging effects of corrosion and microbiological attack.

     New waste treatment plants extensively fabricated from stainless steel offer likely the only true solution to the corrosion problem, though at extremely high cost. Galvanized steel is widely utilized as another more common option. Yet, the effectiveness of all such preventative measures are never guaranteed, and require regular monitoring to ensure against unexpected problem.

     Due to the aggressive nature of wastewater, any uncoated steel pipe or tank surface in its contact becomes vulnerable to corrosion and therefore warrants some monitoring effort. Addressing this need, the installation of CorrView ^® at different locations provides an early indication should a corrosion problem exist. Corrosion activity may be directly related to the effects of the wastewater, or be related to the many other piping systems and operations which comprise a modern wastewater plant operation.

     Compressed air delivery to the aeration lagoons is one clear example where the piping is susceptible to corrosion on its inside due to the higher temperatures and the moisture produced in the process of compressing the air.

     Larger plants may also cogenerate power from the digester's waste methane - providing heating and cooling to the immediate plant, or co-generating power to sell back to the local power utility. Some may even generate power from the effluent outfall. Such secondary activity at a waste treatment plant then raises new corrosion concerns - such as the high corrosion activity typically found at waste methane lines, and in steam, steam condensate, chill and condenser water piping.

     While a growing amount of steel pipe and tank surfaces are now coated to ensure against corrosion, no reasonable method exists, once a plant is in operation, to monitor whether that coating is performing as planned. To this end, the addition of CorrView ^® monitors to any pipe or tank, prior to applying a protective coating, provides never before available advance notice of that coating's failure. Applying a protective coating over the wearable surface of CorrView ^® ensures almost indefinite life if it remains secure, intact, and fulfilling in its specification, or if it does not, a bright orange indication of an unexpected problem.

     Some key areas to consider monitoring at a wastewater treatment plant are:

• Any steel piping or transfer lines
• Any steel holding or transfer tanks
• Waste sludge lines
• Waste generate methane piping
• Overflow lines
• Oxidation and contact sterilization tanks
• Cooling tower or other process water
• Chill water or other closed loop systems
• Delivery piping for chemical additives, flocculents, coagulants, etc.
• Delivery piping for oxidants such as ozone, chlorine, and chlorine dioxide
• Steam condensate lines

     The monitoring and testing required to determine rates of corrosion and service life anticipation of a wastewater treatment plant component can be burdensome and expensive depending upon the area of concern. Most commonly, ultrasonic testing is employed in order to produce wall thickness measurement data, although too often only after a problem has developed.

     CorrView ^® simplifies the corrosion monitoring responsibility by operators, maintenance engineers and wastewater management personnel due to its simplicity. Since it is totally self-contained, has no power requirement, requires no follow-up testing, and is easy to install, CorrView ^® offers the dual benefit of providing a more accurate assessment of what corrosion is actually occurring at the pipe or tank metal itself, and by extending that monitoring capability to areas of the plant never before considered.

     We recommend the larger 1-1/2 in. CorrView ^® model due to its greater forward surface area available for corrosion to occur, and due to its greater viewing area. The option to extend the front further increases the available surface area available for corrosion to take place, and is recommended in any application where CorrView ^® is threaded directly into the pipe or tank.

     The following Technical Bulletins offer some excellent information based upon our own experience in wastewater treatment service piping over the past 10 years. In many cases we can provide hard statistical data in explanation of a specific corrosion problem. All Technical Bulletins are available for download in Adobe PDF format.