Fouling and Prevention

Fouling can be defined as irreversible deposition of materials onto or into the membrane, causing loss of flux and altered rejection (53-55). Membrane fouling can be caused by 

Colloidal fouling is caused by accumulation of particles and macromolecules on, in, and near a membrane. Materials accumulated at the membrane surface create an additional layer of resistance to permeation. Early work on colloidal fouling of RO membrane used to treat secondary effluents indicated that particles smaller than 5 im contribute more substantially to fouling than larger particles (56). It was postulated that as particle size increases, it is subjected to higher velocity and shear force at the membrane surface. Therefore, larger particles tend to be swept away in bulk flow rather than deposit on the 

Formation of a conditioning film composed of macromolecules, proteins, etc. 

A recent study has shown that some RO membranes are more prone to bioadhesion than others. The study involved a bioadhesion assay, which utilizes a model bacterium, SW 8, known to adhere to membranes. Examination of bacteria adhered to the membranes 

The ratio of carbon to nitrogen to phosphorus (C N P) has important effects on the rate of biofilm development. It has been reported that membranes which suffered severe biofouling were found to contain a high percentage (typically 60%) of organics. Laboratory characterization of membrane biofilms found that a typical biofilm contains  

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Chemical additives are often added to the circulating cooling water to reduce corrosion rates, reduce scaling and fouling, and prevent growth of bacteria and algae. Plants often contract with specialty service companies to provide these additives and supervise their use. 

Chlorinated by-products of ethylene oxychlorination typically include 1,1,2-trichloroethane chloral [75-87-6] (trichloroacetaldehyde) trichloroethylene [7901-6]-, 1,1-dichloroethane cis- and /n j -l,2-dichloroethylenes [156-59-2 and 156-60-5]-, 1,1-dichloroethylene [75-35-4] (vinyhdene chloride) 2-chloroethanol [107-07-3]-, ethyl chloride vinyl chloride mono-, di-, tri-, and tetrachloromethanes (methyl chloride [74-87-3], methylene chloride [75-09-2], chloroform, and carbon tetrachloride [56-23-5])-, and higher boiling compounds. The production of these compounds should be minimized to lower raw material costs, lessen the task of EDC purification, prevent fouling in the pyrolysis reactor, and minimize by-product handling and disposal. Of particular concern is chloral, because it polymerizes in the presence of strong acids. Chloral must be removed to prevent the formation of soflds which can foul and clog operating lines and controls (78). 

Reverse osmosis requires good pretreatment to prevent membrane fouling and loss of performance. Because it is seldom better than 60 to 70% efficient, there is a relatively high cost for pumping and discharging the additional supply water consumed. Nevertheless, it is good as a bulk water roughing process for purification. 

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

Analysis and prevention of fouling in tubes attached to micro mixers... 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

S. Sasso, R. Pierce, R. Walla, and A. Yacynych, Electropolymerized 1,2-diaminobenzene as a means to prevent interferences and fouling and to stabilize immobilized enzyme in electrochemical biosensors. Anal. Chem. 62, 1111-1117 (1990). 

Wet scrubbing requires that the water remain in the liquid phase, which requires that the product gas be cooled to below 100°C. This loss of sensible heat may be undesirable in some systems. Most gasification systems that currently use wet scrubbers do so primarily as a means to remove tars rather than particulates from the gas stream. Removing the particulates separately can prevent condensation of the sticky tars on the particulate surface, and that can prevent fouling and plugging of filter surfaces. 

The principal factor of complexity is obviously related to the origin of the wastewaters and to the presence of fouling and/or clogging materials (Table 6). Even if raw suspended matter seems to be at first responsible for preventing measurement, some other components such as soluble substances (grease, petroleum by-products,...) or living organisms (from bacteria to mussels) can be the main problem. 

A bottleneck in all membrane processes, applied in practice, is fouling and scaling of the membranes. These processes cause a decrease in water flux through the membrane and a decrease in retention. Much attention is paid, especially in case of nanofiltration and hyperfiltration, to prevent fouling of the membrane by an intensive pretreatment and the regular removal of fouling and scaling layers by means of mechanical, physical or chemical treatment. 

WHOI (Woods Hole Oceanographic Institution). Marine fouling and its prevention. U.S. Naval Institute. Annapolis. Ed. Iselin, C.O.D., 1952... 

The head is made of polypropylene and polyetheretherketone-based materials and is designed to run at an operating pressure of maximal 10 bar. All feed and eluent solutions are pumped directly into the column through tubes, to minimize dead zones and prevent fouling. 

Reverse osmosis can remove dissolved metals to very low levels. It can also remove a variety of pollutants such as cyanide and residual organics from refinery wastewater. However, because it is an expensive process, it would be competitive only if removal of total dissolved solids is also required. It also requires extensive pretreatment to prevent membrane fouling and deterioration [52]. The pretreatment processes may include filtration to remove suspended solids, pH adjustment, softening, and activated carbon treatment to remove organics and chlorine. A major drawback of the RO process is the handling and disposal of the reject stream, which can amount to 20-30% of the influent flow. 

Help prevent fouling and deposit formation on fuel injector nozzles... 

These deposits responsible for fouling can block out the reactants and prevent them from reaching the active sites, or even block the internal pores of the catalyst. Hydrocarbons and aromatics are usually the cause of coking. The chemical nature of the carbonaceous deposits relies on many parameters temperature, pressure, feed composition, nature of products, and catalyst age share the responsibility of the residue formation on catalysts. 

Many draw nozzles, especially those in the bottom of vessels, plug because of the presence of vortex breakers. Many designers routinely add complex vortex breakers to prevent cavitation in pumps. But vortex breakers are needed only in nozzles operating with high velocities and low liquid levels. Corrosion products, debris, and products of chemical degradation can more easily foul and restrict nozzles equipped with vortex breakers. 

This gave me an idea. Suppose we injected a liquid spray into the front end of the compressor (we eventually used a heavy aromatic naphtha, obtained from a local refinery). This could prevent the deposits from sticking to the spinning wheels. We tried it, and it worked. Rotor fouling and the consequent vibrations, and loss of capacity, became far less frequent. 

Where cooling systems require cleaning on-line, it is important to determine the basic reasons why fouling and deposition has happened in order to prevent a recurrence. The initial causes may include process contamination or an inadequate chemical treatment, but the situation may have been exacerbated due to a failure by staff or the service company to monitor the situation and take appropriate action at an early stage. 

Membrane fouling is the main cause of permeant flux decline and loss of product quality in reverse osmosis systems, so fouling control dominates reverse osmosis system design and operation. The cause and prevention of fouling depend greatly on the feed water being treated, and appropriate control procedures must be... 

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