Biofouling

The buildup of biofilm on the membrane surface means an additional resistance to solvent flow as well as the possibility of enhancement of CP level by the biofilm, which is similar to the case of colloidal fouling [32,36], In general, the diffusivity is linked to the tortuosity factor of the biofilm [37]. Hence, it is likely that the backdiffusion of solutes in the biofilm on RO is hindered. The enhanced CP is important for two reasons. Firstly, the elevated concentration of solutes at the membrane wall means an increase in the osmotic pressure (CEOP) and hence a loss in the effective TMP. Secondly, the nutrient level is also enhanced and this will further accelerate the growth of the biofilm [32,36]. So, biofouling in RO becomes an interplay between C P and biofilm development. 

Sulfate-reducing bacteria (SRB) have been known to cause corrosion of copper and its alloys and various sulfides have been identified. Some of the sulfides are digenite Cu5S9, chalcocite Cu2S, covellite CuS, djuerleite Cu j 9fiS. 

This mode of failure is possible in all the copper alloys. The principal environment involved is ammonia. The evidence also exists for other media such as citrates, tartrates, nitrites, sulfur dioxide, carbonates, nitrogen oxides and phosphates to be conducive to a stress-corrosion cracking mode of failure of copper alloys. 

The most detailed studies of the phenomenon are confined to ammoniacal solutions. In the case of ammonia medium the familiar coordination complexes Cu(NH3) + and Cu(NH3)2 are formed, leading to selective removal of metal and causing the damage. In ammoniacal solutions traces of phosphorus (about 0.004%) increase the susceptibility to SCC. Other elements such as As and Sb have similar effects. These elements in trace amounts in the alloy produce precipitates at the grain boundaries and make the grain boundary region more anodic to the grain bodies. 

Some salient features of SCC of some selected alloys with respect to susceptibility to SCC are given below 60 

Aluminum bronzes Susceptible to SCC. Addition of 0.2-0.3% Sn or As provided immunity to intergranular cracking of a-Al bronze No effect on transgranular cracking in ammonia solutions 

PDMS surfaces modified with poly(ethylene glycol) have a near-perfect resistance to nonspecific protein adsorption, making them effective for 

Coatings of the block copolymers, PDMS-polyurethane and PDMS-acrylic-polyurethane, and PDMS-polyurea, are also effective antibio-fouling agents as are tethered quarternary ammonium salts in a cross-linked PDMS matrix.  

Microbial fouling of a surface is the net result of several physical, chemical, and microbial processes  

Control of microbial results in control of macrobial fouling. 

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An important and expensive problem in surface science occurs in the prevention of the attachment of marine animals such as barnacles to ship surfaces, a process known as biofouling. Baier and Meyer [159] have shown that the Zisman plot can be used to predict biofouling, thus avoiding costly field tests to find a successful coating to prevent biofouling. 

Corrosion associated with the action of micro-organisms present in the corrosion system. The biological action of organisms which is responsible for the enliancement of corrosion can be, for instance, to produce aggressive metabolites to render the environment corrosive, or they may be able to participate directly in the electrochemical reactions. In many cases microbial corrosion is closely associated with biofouling, which is caused by the activity of organisms that produce deposits on the metal surface. 

Hydrogen peroxide is also used as a treatment to clean wells of biological growth. One treatment is to pour a 0.5% solution into the well and allow it to sit for four hours after which pumping is resumed (31). Some treatments use solution concentrations as high as 10% to remove biofouling from wells (16). 

BloFIms. Microbiologists recognize two different populations of microorganisms. Free-floating (planktonic) populations are found in the bulk water. Attached (sessile) populations colonize surfaces. The same kinds of microorganisms can be found in either population, but the sessile population is responsible for biofouling. 

Corrosion. Copper and selected copper aHoys perform admirably in many hostile environments. Copper aHoys with the appropriate corrosion resistance characteristics are recommended for atmospheric exposure (architectural and builder s hardware), for use in fresh water supply (plumbing lines and fittings), in marine appHcations (desalination equipment and biofouling avoidance), for industrial and chemical plant equipment (heat exchangers and condensers), and for electrical/electronic appHcations (coimectors and semiconductor package lead-frames) (30) (see Packaging). 

Excellent resistance to saltwater corrosion and biofouling are notable attributes of copper and its dilute alloys. High resistance to atmospheric corrosion and stress corrosion cracking, combined with high conductivity, favor use in electrical/electronic appHcations. 

Biofouling. Organisms present in the fluid stream are attracted to the warm heat-transfer surface where they attach, grow, and reproduce. The two subgroups are microbiofoulants such as shme and algae and macrobiofoulants such as snails and barnacles. 

Removing suspended solids, decreasing cycles of concentration, and clarification all may be beneficial in reducing deposits. Biodispersants and biocides should be used in biofouled systems. Simple pH adjustment may lessen precipitation of certain chemical species. The judicious use of chemical corrosion inhibitors has reduced virtually all forms of aqueous corrosion, including underdeposit corrosion. Of course, the cleaner the metal surface, the more effective most chemical inhibition will be. Process leaks must be identified and eliminated. 

Owing to the laborious task of obtaining corrosion rates from gravimetric measurements, data for the effect of exposure time on corrosion rates have been very limited. However, with the more recent use of polarisation resistance measurements it would appear that in the absence of macro-biofouling... 

Condenser design usually calls for thin-walled tubing with high thermal conductivity characteristics plus a high degree of resistance to biofouling and the many forms of corrosion that may occur. (These forms include crevice and pitting corrosion, biocorrosion, and erosion-corrosion). 

Clean-in-place (CIP) units for periodic membrane washing and removal of organic deposits, inorganic scales, and biofouling... 

The widespread use of many metals such as silver, cadmium, copper, mercury, nickel, lead, and zinc has resulted in their accumulation in the environment. Sediments are often the repositories of toxic metals (e.g.. Table 15-2). For example, copper is used as an anti-biofouling agent in marine paints and many harbor sediments contain markedly elevated levels of copper. 

The non-random distribution of bacteria in biofilms has important applications for industry (biofouling, corrosion) and in medical practice (use of apphances within the human body). 

Figure 4. Annular deposits on 316L stainless steel after 13 days of exposure to fresh water (Reprinted from W. Dickinson, F. Caccavo and Z. Lewandowski, The Enoblement of Stainless Steel by Maganic Oxide Biofouling, Corns. Sci. 38 (8) p. 1413, fig. 4, Copyright 1996, with permission from Elsevier Science).

Figure 5. Evolution of cathodic response to 10 pA cm" galvanostatic pulse as Ecorr increases during biofouling of 316L stainless steel in fresh river water. Data shown by solid lines and circles indicate points generated from curve lit to Eq. (6). (Reprinted from Ref. 12 with permission from NACE International.)...

An increase in reducible surface-bound material during ennoblement was demonstrated using galvanostatic reduction" techniques to monitor potential as a stainless steel coupon was cathodically polarized. Coulombs of reducible material were calculated from the duration of regions of polarization rate lag that indicated reduction of surface-bound material. Longer exposure times and thicker biofouling were not sufficient to increase the abundance of reducible surface-bound material. The increase seemed to be associated with increased... 

Dickinson et al. also used microelectrodes to measure dissolved oxygen (DO), H2O2, and local within biofouling deposits on stainless steel surfaces exposed to river water to further resolve the interfacial chemistry that resulted in ennoblement. Data were then compared with those from similar measurements as close as possible to the environment of a non-fouled substratum. 

Stainless steel microelectrodes were prepared by sheathing 100-pm diameter stainless steel wire in glass. Tips were polished on a precision diamond wheel. Electrode response to variation in cathodic depolarizer concentration was confirmed by exposure to solution containing up to 6 mM HjOj. The increase in for the electrode was comparable to the change observed for sample coupons exposed to the same H2O2 concentrations. The electrode was conditioned by exposure to the influent reactor solution for several hours before measuring E, within the biofouling deposits. 

Profiles of DO and HjOj concentrations and microelectrode Ecm were measured within biofouling deposits on coupons. The microelectrodes, mounted in a commercial three-axis micromanipulator, were positioned above a deposit by adjusting the X-Y micromanipulator controls... 

Anonymous. Microbiologically influenced corrosion and biofouling in oilfield equipment. Nace TPC Publication TPC 3, 1990. 

Fig. 5.1.10 (a) MR measured propagators and (b) corresponding calculated RTDs for flow in a model packed bed reactor composed of 241 -pm monodisperse beads in a 5-mm id circular column for a fixed observation time of 300 ms and as a function of biofilm fouling. As the porous media becomes biofouled, a high veloc-... 

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