Water Well Journal
Issue link: http://read.dmtmag.com/i/692787
Unfortunately, the aspects of biofilm that make it advantageous for bacteria are extremely detrimental to well sys- tems. As the biofilm matrix grows, it can physically restrict flow paths and decreases production. Its excellent sur- face adhesion capabilities make biofilm an excellent catalyst for the accumula- tion of mineral scale, further complicat- ing well operation. Similarly, fine-grained sediments and debris, mobilized towards the well dur- ing operation, often become entrapped in biofilm, increasing the fouling potential. A phenomenon commonly observed in biofilm is stratification within the biofilm matrix—which not only in- creases the relative density and fouling potential of the biofilm, but also reduces oxygen levels in the deeper layers where anaerobic bacteria can thrive. This layer- ing effect can harbor more harmful or- ganisms, such as coliforms, and severely impede disinfection efforts. The in- creased stratification can also result in a more stubborn accumulation that challenges removal strategies. Several types of bacteria are also known to influence the basic principles which cause corrosion. Any mode of corrosion which incorporates microbes that react and cause corrosion or influ- ence other corrosion processes is called "microbiologically influenced corro- sion" (MIC). In order for MIC to occur, the proper environment and necessary nutrients must be available. MIC often occurs in the form of pitting but can also be seen in any number of other forms of corrosion. By growth and metabolism, microbes can introduce corrosion-assisting con- stituents such as acids and sulfides into a given system. The most recognized type of MIC has traditionally been asso- ciated with anaerobic organisms, which can create acidic conditions through release of compounds created during metabolic processes. A prominent ex- ample of this is the reduction of sulfate (SO 2 −4 ) to hydrogen sulfide (H 2 S) by sulfate-reducing bacteria (SRBs). Uneven colonization of microorgan- isms over a surface can also contribute to corrosion by influencing oxygen con- centration gradients. Aerobic bacteria lo- cated in biofilms near the water interface can create an oxygen gradient during oxygen consumption—leaving oxygen levels depleted in lower portions of the biofilm matrix near the surface. This results in the surface area under the biofilm becoming anodic to the area exposed to the bulk aqueous phase. Direct degradation of materials due to metabolic processes carried out by specific bacteria, such as iron-oxidizing bacteria, can also influence corrosion. One of the more recognized species of iron-oxidizing bacteria is Gallionella, which is a naturally occurring bacteria found in a variety of aquatic environ- ments, including aquifers. Gallionella are a stalked bacterium making use of iron as an energy source and secreting an iron-oxy-hydroxide byproduct. In its attachment to iron-bearing surfaces, Gallionella pits the metal in an effort to secure the iron necessary for energy. All iron-bearing structures, including stainless steel, are susceptible to this form of pitting. Degraded water quality is likewise a common result of bacterial fouling. Foul tastes and odors, resembling rotten eggs, can be created by the previously mentioned SRBs and other microorgan- isms. Increased turbidity can result from both living and dead bacterial cells in solution, as well as from the detachment or sluffing off of biofilm from surfaces. Discoloration, most commonly in the form of red water resulting from ele- vated iron levels, can also be a direct result of bacterial fouling. These un- pleasant conditions don't typically pres- ent a health threat, but they can become very costly to remedy during the treat- ment process. Outfitting Your Toolbox Because of the large number and di- versity of microorganisms which can be found in water supplies, their ability to impact well fouling is increased and any attempt to monitor their activities must be weighed against the effort required to do so. Albert Einstein once said, "Every- thing should be as simple as possible and no simpler." In that light, identify- ing the best approach to monitoring bacterial fouling is perhaps easier if considered as "the minimum effort required to obtain the highest return, and no less." A point of diminishing returns must be identified, and often that point is when the data, and insight derived from it, does not increase relative to the amount of time and expense employed to conduct the testing. If we use a basic understanding of bacteria and consider the types that have the greatest impact on our wells, com- bined with the knowledge of the advan- tages and limitations of the Bac-T (coliform) test, the selection of testing methods most suitable for monitoring bacteria then becomes more feasible. Coliform evaluation using the Simplate method. The test method confirms the presence of coliforms and allows for enumeration of the occurrence. TESTING from page 19 TESTING continues on page 22 waterwelljournal.com 20 July 2016 WWJ