Drinking water, corrosion

CER 141.82, National Primay Drinking Water Regulations-Description of Corrosion Control Treatment Requirements, rev. July 1, 1991. 

Other Uses. As a biocide, chlorine dioxide is more effective than chlorine over a wider pH range. Chlorine dioxide is also less corrosive and more compatible with some materials of constmction. Chlorine dioxide has a wide variety of small appHcations in drinking water, food processing (qv), cooling towers, and oil recovery. In these areas, chlorite is the preferred source of chlorine dioxide. 

The electrolytes are non-corrosive and the electrodes do not corrode with time. This feature is of special significance when compared with an ordinary liquid resistance starter used commonly for slip-ring motors. Electrolytes do not deteriorate and therefore do not require replacement. The evaporated liquid can be replenished with drinking water when the level of the electrolyte falls as a result of evaporation. In Europe such starters have been used for over 15-20 years. Electrolyte switching is a costlier proposition compared to direct on-line or star/delta switching due to additional shorting contactor and timer, and the cost of electrolyte, its tank and thermostatic control etc. The cost may. 

Electrochemical corrosion protection of the internal surfaces of reaction vessels, tanks, pipes and conveyor equipment in the chemical, power and petroleum industries is usually carried out in the presence of strongly corrosive media. The range stretches from drinking water through more or less contaminated river, brackish and seawater frequently used for cooling, to reactive solutions such as caustic soda, acids and salt solutions. 

The water supply authorities normally insist that (for uses other than drinking-water taps) their main should discharge into a break-pressure vessel, after which the water quality becomes the consumers responsibility. The water tank should be covered against tramp dirt and access by birds, etc., and it must be shielded from sunlight to avoid the growth of algae. Nevertheless, access must be maintained for easy inspection. The distribution pipework is preferably all plastic and lead must be avoided altogether. The use of copper is doubtful with some corrosive waters, and soldered joints in it can lead to unacceptable concentrations of lead in the water. 

For many cooling waters, including seawater and also drinking water, where corrosion rates are 70 to 100% of the limiting diffusion current, the use of dimensionless group analysis can then be applied. 

The significance of this directive , from the corrosion point of view, is that for the first time legally enforceable limits for the concentrations of toxic metals in drinking water have been defined. This has greatly increased the importance of contamination as a consequence of corrosion, as opposed to simple mechanical failure, and has required a reassessment of the suitability of various metals and alloys traditionally used in the supply of water for domestic purposes. 

There is an increasing tendency to treat drinking waters to remove organic material. This is to minimise the formation of haloforms, produced when the water is chlorinated, which have heeilth implications . Organics are known to affect certain corrosion processes, e.g. type I copper pitting and the formation of protective corrosion product layers. However, the outcome of this development is difficult to predict as not all the organic material present is removed. 

There has been an increasing level of nitrate contamination of borehole supplies in the east of England, because of the use of agricultural fertilisers since the Second World War . Nitrates are known to exacerbate certain corrosion processes e.g. at soldered joints however the maximum value allowed for this ion by the EC drinking water directive (50 mg NO3 r ) should limit its significance. 

Humans may also be indirectly affected through exposure to increased levels of toxic metals in drinking water and food. Increased levels of toxic metal are a consequence of direct deposition of pollutants into water sources, increased leaching of metal from soils and lake sediments, and increased corrosion of water pipes. 

The increased requirements of drinking water in large cities becomes necessary to use sources of very soft water and because of its low salinity and pH are very aggressive and can bring on corrosion phenomena in the pipes of the pipeline, with the appearance of colour and turbidity... 

Lead levels ranging between 10 and 30 pg/L can be found in drinking water from households, schools, and office buildings as a result of plumbing corrosion and subsequent leaching of lead. The combination of corrosive water and lead pipes or lead-soldered joints in either the distribution system or individual houses can create localized zones of high lead concentrations that exceed 500 pg/L (EPA 1989f). 

Quantitative data on the nationwide range of lead levels in drinking water drawn from the tap (which would include lead corrosion by-product) were insufficient to assign a national value at the time of the 1991 EPA publication. One set of data comprised of 782 samples taken in 58 cities in 47 states shows that the average lead level in tap water was 13 pg/L with 90% of the values below 33 pg/L (EPA 199 Id). 

Ohanian EV. 1986. Health effects of corrosion products in drinking water. Trace Subst Environ Health 20 122-138. 

Distribution Systems. A substantial amount of contamination of drinking water can occur while the water is in transit to the consumer after treatment. Pipes are made of copper, galvanized iron, asbestos-cement, lead, or plastic, and often polymeric or coal tar coatings are used. All of these are capable of contributing contaminants to the water, especially if the water is corrosive. Lead, copper, cadmium, and polynuclear aromatic hydrocarbons in finished water are primarily problems of water distribution and not source water contamination. Physical deterioration of the distribution system can also permit biological contamination to occur during transit. 

For the experimenter in the laboratory, not only do materials have to be chosen on the basis of their corrosion-resistance, but also for their effect on ozone decay. Some metals (e. g. silver) or metal seals enhance ozone decay considerably. This can be especially detrimental in drinking water and high purity water (semiconductor) ozone applications, causing contamination of the water as well as additional ozone consumption. Moreover, the latter will cause trouble with a precise balance on the ozone consumption, especially in experiments on micropollutant removal during drinking water ozonation. With view to system cleanliness in laboratory experiments, use of PVC is only advisable in waste water treatment, whereas quartz glass is very appropriate for most laboratory purposes. 

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