Every spring when the snow melts, there it is - rust. Unsightly, almost evil, rust plagues us. That small reddish-brown speck is suddenly a massive crater, slowly sucking the mechanical life out of your vehicle. Did a chunk of fender just fall off? We know this rust is usually caused by road salt or elements in the air reacting with a vehicle's metal surface, but where do other examples of rust originate? We also know that we could prevent or at least stall an automobile's corrosion by rust proofing our cars, washing off excess salt and dirt, and so on. Easy enough, right?
But what would you do with a Navy warship or an offshore drilling rig? Obviously more than bringing out the pail of soapy water on a Sunday afternoon. In tropical waters, street salt is certainly not causing corrosion and deterioration of ships' hulls, water-cooling systems or offshore structures. So what is? And how are these and other industries, the livelihood of which depends on being corrosion-free, solving the problem? Before a problem can be solved, it must first be defined.
Biofouling is simply the attachment of an organism or organisms to a surface in contact with water for a period of time. That explanation sounds fairly straightforward, but there are several organisms that cause biofouling, many different types of surfaces affected by it, and, due to the work of scientists, engineers and others, scores of solutions to the problem. Even this definition greatly simplifies what really occurs. This article examines the causes of biofouling and the numerous, innovative solutions being derived from our evolving knowledge of these causes.
Biofouling consists of biofilms, which include living and dead bacteria, their sheaths, stalks, secretions and other waste products embedded with metal hydroxide particles. These biofilms are natural and usually harmless, but can be a terrible nuisance when they cause build up in wells and pipes, or completely plug water filter systems. Biofouling is not as simple a process as it sounds. Organisms do not usually simply suck onto a substrate like a suction cup. The complex process often begins with the production of a biofilm. A biofilm is a film made of bacteria, such as Thiobacilli or other microorganisms, that forms on a material when conditions are right. Nutrient availability is an important factor; bacteria require dissolved organic carbon, humid substances and ironic acid for optimum biofilm growth. Biofilms do not have to contain living material; they may instead contain such once living material as dead bacteria and/or secretions. Bacteria are not the only organisms that can create this initial site of attachment (sometimes called the slime layer); diatoms, seaweed, and their secretions are also culprits. Coral reef diatoms' attachment depends on pH, and as in the Achnanthes and Stauronesis diatoms, the molecular structure of the organism.The study of the biology of the Achnanthes longipes (Bacillariophyceae) diatom can determine which temperatures produce maximum growth.
Almost any aquifer with an organic content will have some degree of bacteriological activity. The typical agents for microbiological fouling include iron, sulfur-reducing and slime producing organisms, although many others exist. An additional concern is that some of these organisms are opportunistic pathogens. Iron bacteria such as Gallionella are common in aerobic environments where iron and oxygen are present in the groundwater and where ferrous materials exist in the formation (e.g., steel or cast iron wells). These bacteria attach themselves to the steel and create differentially charged points on the surface, which in turn create catholic corrosion problems. The iron bacteria then metabolize the iron that is solublized in the process. Iron bacteria tend to be rust colored or cause rust colored colonies on the pipe surfaces. Sulfur reducing bacteria often are responsible for the hydrogen sulfide smell released when raw water is aerated. These bacteria are common where sulfur naturally exists in the formation, and will tend to form black colonies on pipe surfaces. While anaerobic, they will exist in environments where aerobic conditions that can lead to symbiotic relationships with aerobic organisms exist.
Slime producing bacteria are found in surface waters and in soil. Members of this genre often are used to protect farm crops from fungal growth, and as a result are to be expected in groundwater that has organics. However, these bacteria are highly adaptive. Research several years ago indicated that the bacteria would grow in any environment into which they were introduced. The Pseudomonas genera are facultative anaerobes that can persist in oxygen depleted environments by breaking down complex hydrocarbons for the oxygen. In some circumstances, they will use nitrogen in the absence of oxygen. Pseudomonas bacteria can permanently affix themselves to laser-polished 316L stainless steel in a matter of hours, so attaching to steel or lower grades of stainless steel is easily accomplished. Given that the Pseudomonas sp. are adhering bacteria, they are capable of producing a polysaccharide matrix (biofilm) that can act as a barrier protecting the bacteria incorporated in the films from harmful substances such as disinfectants and, in some cases, oxygen.
Biofilms also act to protect the bacteria from the shearing effect of turbulent flow, and can provide an environment for other species. Periodic sloughing occurs when the biofilm gets too thick. The microbiological accumulations/ biofilms pose several significant concerns. First, the accumulations on the metallic surfaces create anodes and, in conjunction with reactions caused by dissimilar metals, can lead to a steady catholic deterioration over time (with or without iron bacteria). Since the Pseudomonads are acid formers, ferrous materials are particularly vulnerable to deterioration, especially in the presence of iron bacteria.
The sloughing events pose a significant fouling concern for both the membrane softening and reverse osmosis membranes and could lead to some breaching of the membranes by the bacteria, whereby the bacteria could subsequently enter the distribution system. Because of the size of the openings in the membranes, it has been assumed that the membranes will filter out the bacteria, but the seals in the system may allow some leakage, allowing the permeate to be exposed to the raw water. The accumulation of bacteria in the concentrate causes concern from the standpoint of a point source discharge, as well as the potential for severe corrosion caused by concentration of inorganic salts and organic acids caused by the bacteria. The corrosion of the steel pipe at lime softening plants also could be partially attributed to the bacteria being brought in with the raw water.
Analysis of treatment processes indicates that lime softening does a relatively good job at removing the bacteria because of the mixing of lime and raw water that occurs and the "sticky" constituency of the bacteria. However, the proposed membrane softening process would not be as effective in the removal of the bacteria.
Plugging and biofouling problems in wells are prevalent throughout South Florida and other areas of the country. Unfortunately, despite the number of systems utilizing wells, the focus of operations personnel is more on the mechanical and electrical failures that routinely plague operators than on root causes of long-term deterioration such as colloidal, silt, sand, pump and well design and installation and biofouling. Conclusions from the case studies are that long-term microbiological problems may go unnoticed, undiagnosed or improperly diagnosed. Cathodic reactions from dissimilar metals pose significant risk to the long-term maintenance of the wellfield and may be enhanced and exacerbated by microbiological action. A complete investigation and proper analysis of the raw water supply including silt, sand and microbiological analyses are required prior to design of membrane processes.