Fouling microbial

Natural microbial fouling control strategies are environmentally sensible because they have been optimized by natural selection. A sensible innovation strategy then, is to observe natural control, try to understand it, attempt an imitation, and explain the copy1. The new chlorine alternative and its industrial water treatment applications were accordingly developed, as follows. 

Figure 1 The microbial fouling process on surfaces of certain macroalgae in aquatic environments is controlled by the selective oxidation of bromide with hydrogen peroxide and bromoperoxidase. Although chloride is many orders of magnitude more abundant in the sea, bromide is oxidized to hypobromous acid in situ.

These data show that bromine works better than chlorine in high pH waters such as the ocean. Similarly, most industrial water is quite alkaline and therefore, a practical form of bromine is also preferred. The technical attributes of bromine antimicrobials are of value in water treatment and are apparently also worth the cost to many aquatic plants. Further observations of natural microbial fouling control systems reveal that animals also preferentially manufacture, in situ, certain bromine-based antimicrobials. 

STABREX is safer to use because it is less toxic to aquatic wildlife, as shown in Table 7, and because less chemical is required to control microbial fouling. 

Disinfection by-products (e.g., adsorbable organic halides such as trihalomethanes) are more than 50% decreased compared to equivalent chlorine treatments in standardized AOX test with STABREX3. In practice, disinfection by-products are decreased even further in STABREX applications because less oxidant is required to control the microbial fouling process compared to bromine or chlorine applications. 

Some of the preferred tools used in natural microbial control programs are given in Table 8. The goal in nature is always to maintain control of a system, to avoid letting it foul until cleanup is the last resort. In nature, maintenance of a clean system is the only real hope for survival. Clean rarely, if ever, means sterile. The natural systems discussed in this paper, for example, are always comprised of vast microbial flora in close proximity to or actually a part of the protected portion of the system in which microbial fouling problems are actively managed by the animal or plant. 

The clear lesson from nature is that effective management of microbial fouling in complex dynamic systems requires a consistently administered program comprised of diagnostics, monitors, environmentally sensible remedies, and careful regulation of those remedies. 

A sensible approach to innovation in microbial fouling control technology can be simply stated Observe nature. Try to understand it. Try to imitate it. Explain the copy. There is much more to learn about natural microbial fouling control. Surely, there are many important clues still to be discovered. 

Microbial fouling is best dealt with before biofilm becomes mature. Biofilm protects the microorganisms from the action of shear forces and biocidal chemicals used to attack them. Microbes can be destroyed using chlorine, ozone, ultraviolet radiation, or some non-oxidizing biocides (see Chapters 8.2.1,8.2.2, 8.1.8, and 8.2.5, respectively). An effective method to control bacteria and biofilm growth usually involves a combination of these measures. Specifically, chlorination or ozonation of the pretreatment system, followed by dechlorination to protect the membranes, or UV distraction followed by periodic sanitation with a non-oxidizing biocide used directly on the membranes. 

Calcium carbonate scaling is perhaps the most common type of problem, with the possible exception of microbial fouling, that RO membranes experience. Fortunately, it is fairly easy to detect and handle. Basically, if the ion product (IP) of calcium carbonate in the RO reject is greater than the solubility constant (Ksp) under reject conditions, then calcium carbonate scale will form. If IP < Ksp/ scaling in unlikely. The ion product at any degree of saturation is defined as ... 

Carbon filters were once the standard method for removing chlorine from RO influent water. However, due to the microbial fouling... 

Membrane pretreatment includes microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Microfiltration and UF membrane processes can remove microbes and algae. However, the pores of MF and UF membranes are too large to remove the smaller, low-molecular weight organics that provide nutrients for microbes. As a result, MF and UF can remove microbes in the source water, but any microbes that are introduced downstream of these membranes will have nutrients to metabolize. Therefore, chlorination along with MF and UF is often recommended to minimize the potential for microbial fouling of RO membranes. The MF or UF membranes used should be chlorine resistant to tolerate chlorine treatment. It is suggested that chlorine be fed prior to the MF or UF membrane and then after the membrane (into the clearwell), with dechlorination just prior to the RO membranes. See Chapter 16.1 for additional discussion about MF and UF membranes for RO pretreatment. 

Non-oxidizing biocides are used on membranes to prevent microbial fouling. By definition, these products will not oxidize polyamide composite membranes and can be used directly on the membranes. There two most common, non-oxidizing biocides used with RO membranes sodium bisulfite and 2,2,dibromo-3-nitrilo-proprionamide or DBNPA. 

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

Cartridge filter replacement frequency, high replacement rates (every 2 weeks or less) could indicate a fouling problem. Low replacement rates (every 1 month or more) could lead to microbial fouling as microbes grow in the "old" cartridges. 

Microbial Fouling Finally, Figure 14.15 shows microbial residue on the surface of a membrane magnified 5000 times. 

As discussed in Chapter 8.1.9, MF and UF membranes can delay the onset of microbial fouling of RO membranes, but by themselves are not fully effective. Nutrients, in the form of low-molecular weight organics, can pass through these membranes such that any post introduction of microbes into the RO feed water will result in microbial fouling. Therefore, the use of chlorine is recommended in conjunction with these membrane processes to minimize the potential for microbial fouling of RO membranes. 

Hydantoin (Halane) used for microbial fouling— should be used infrequently... 

Peracetic acid used as a sanitizing agent to prevent microbial fouling... 

Sodium hydroxide used for microbial fouling and silica scale... 

Ridgway H.F., Kelly A., Justice C., and Olson B.H., Microbial fouling of reverse osmosis membranes used in advanced wastewater treatment technology Chemical bacteriological and ultrastiuctural analyses. Applied and Environmental Microbiology 46 1983 1066-1084. 

Okochi M, Lim TK, Nakamura N, and Matsunaga T. Electrochemical disinfection of drinking water using activated carbon reactor capable of monitoring its microbial fouling. Appl Environ Microb 1997 47 18-22. 

Figure 3.120 Microbial fouling of materials in ultrapure water—average cell count/cm at a fluid velocity of 91 cm/sec.>° i...

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