Microbiologically-related problems—including biofouling (slimes), deposition, and microbiologically influenced corrosion (MIC)—occur in the waste water (WW) industry in sewer pipes and treatment facilities. These problems often compromise the function and integrity of system components by interfering with fluid flow, plugging filter media and, in some instances, causing component failures due to blockages or leaks.
Why WW Facilities Experience Microbiologically-Related Problems
MIC occurs in WW facilities primarily because MIC-related microbes—including aerobes, slime formers, iron-related bacteria, anaerobes, organic acid-producing bacteria, sulfur-oxidizing bacteria (which produce sulfuric acid), and sulfate-reducing bacteria (which produce corrosive sulfides and consume hydrogen)—are usually present in such facilities in very large numbers. Nutrients suitable for the production of corrosive materials are also abundant and continually resupplied. Water and corrosive ions, such as chloride, are constantly present. Solids laden with MIC-related microbes are present in large amounts and can settle on metal surfaces forming biofilms and discrete deposits, under which pitting corrosion can occur very rapidly.
Microbiologically-Related Problems Experienced in WW Systems
WW systems may experience a variety of microbially-related problems. Many times, these problems occur simultaneously in an affected system and may not affect all systems uniformly, due to differences in local environmental, operational, chemical, and biological factors. These microbial problems include:
1. Biofouling of system waters due to biofilms.
- Biofilms (also called slimes), which are formed by microbial communities—in most cases composed of many different types of microbes (e.g., aerobic, slime-forming, anaerobic, acid-producing, iron- and manganese-depositing bacteria, molds, and sulfate-reducing bacteria)—attach to surfaces in contact with non-treated or inadequately treated waters. Biofilms are an early indicator of microbial activity.
2. Plugging of filtration media due to deposition, resulting in greatly reduced water “production rates.”
- Over time, biofilms grow in thickness and areal extent due to microbes depositing iron and manganese and accumulating corrosive materials such as chlorides. Biofilms mature by collecting more types of microbes (and, therefore, more biochemical reactions) and non-biological materials (e.g., sand, debris, organic and inorganic materials) and form deposits. Discrete deposits are a clear indication of microbial activity.
3. Loss of integrity in system components due to under-deposit pitting-type corrosion and exposure of components to corrosive microbial by-products.
- Once deposits are established, and given the right environmental conditions, microbial metabolic by-products—such as organic and inorganic acids, hydrogen, carbon dioxide, sulfides, and ammonia—accumulate within the deposits due to the activities of microbes and other chemical reactions. These metabolic by-products corrode the underlying metal and often lead to rapid under-deposit pitting-type corrosion beneath the deposits. This type of corrosion is a telltale sign of MIC. Microbes can both initiate corrosion and accelerate any existing corrosion.
4. Component failures due to under-deposit leaks.
- Once corrosion and MIC advance to the stage of under-deposit pitting corrosion, the metabolic by-products in deposits corrode the underlying metal until leaks develop under the deposits. Pinhole leaks are another telltale sign of MIC.
5. Damage to coatings and destruction of underlying metal due to microbial activities.
- Microbes enter holidays in protective coatings in pipes and storage facilities. Once established, microbial production and accumulation of corrosive materials under the coating (and next to the metal surface) causes further disbonding of the coating, allowing damage to the coating and underlying metal to spread and proceed at very rapid rates. The disbondment acts to trap and concentrate microbes and corrosive materials next to the metal surface, causing even faster corrosion to occur. Since this occurs under the coating, the site is not protected by cathodic protection and cannot be treated using corrosion inhibitors and biocides. These problems can occur on the inside or outside of equipment.
Most metals and alloys, except titanium, are rapidly attacked by MIC under some conditions in WW facilities. Several cases of MIC in WW facilities in the USA, Canada, and Jordan have been investigated by BTI Products’ sister company, BTI. An EPA-supported investigation, which involved BTI personnel, showed that concretes, rebar, steels, and other components of WW facilities across the USA have been extensively and severely affected by MIC.
Although still susceptible to MIC, fewer cases of MIC have been seen in cast iron and ductile iron.
The following WW facility systems are potentially susceptible to biofouling, deposition, and MIC:
- Steel pipes—even those coated on inside and outside with “protective coatings”—carrying sewage or circulating sewage in digesters and other applications.
- Concrete components in transportation and treatment facilities.
Whether this “MIC potential” progresses to rapid and severe MIC-type corrosion depends on local conditions—even in the same “system”—over time.
Locations in WW Facilities Where Microbiologically-Related Problems Are Likely to Occur
The following are typical locations in WW facilities where microbially-related problems often occur:
- Where bare carbon steel is exposed to the process fluids, rapid fouling and corrosion is almost certain to occur.
- Where coated steel develops imperfections (holidays), microbes can enter and rapidly cause corrosion.
- Where stainless steel is exposed to fluids, rapid MIC can occur, especially at field welds.
- Where cement lining is present and develops cracks or other imperfections, MIC can occur, often resulting in destruction of cement, rebar, and underlying steels.
- Where sulfides are present in the piping and escape and absorb to concrete and metals, the sulfides can be oxidized to sulfuric acid, causing rapid destruction of these components, especially at the crown.
Diagnostic Products for WW Industry Systems
BTI Products offers test kits designed specifically for diagnosis of microbial problems in water industry systems. Our test kits provide microbiological, chemical, and site-specific information important in diagnosis of microbial and other problems and in the design of treatment and prevention strategies. BTI Products and its many water industry clients use BTI Products test kits to investigate MIC and design effective treatment programs.
BTI Products kits can be used for all types of samples—including those containing water, particulates, corrosion products, hydrocarbons, and/or other materials. BTI Products test kits are designed to test for MIC-related microbes in samples from freezing to boiling temperatures and from a wide variety of salinities (from fresh water to sea water or production brines). They can be used to test for MIC-related microbes in almost all real-world samples and, most importantly, can be used by untrained industry personnel, on-site, to obtain accurate and pertinent data quickly and inexpensively.1
Recommended Diagnostic Test Kits
- MICkit® 4 to test locations using cathodic protection or coatings for chemistries important in corrosion diagnosis and treatment
- MICkit® 5 to test WW facilities for microbes involved in biofouling, deposition, and MIC
- MICkit® Comprehensive to test WW facilities for microbes and chemical parameters involved in biofouling, deposition, MIC, and other forms of corrosion
- MICkit® Pipe Inspection Kit to physically inspect pipes in systems that have tested positive for MIC-related bacteria and/or systems with a history of leaks/failures
- MICkit® Custom created specifically to suit your testing needs
Diagnostic & Mitigation Services for WW Industry Systems
BTI Products can also assist in evaluation of samples, test data, field-site investigations, and design and implementation of mitigation measures. BTI Products performs and supports research and testing programs related to causes, prevention, and treatment of microbial problems for individual clients.
Please contact us to discuss your specific needs.
- Scott, P.J.B. and M. Davies. 1992. Survey of field kits for sulfate reducing bacteria. Materials Performance, May, 64-67.