ABSTRACT
The authors realize that this work is far from being an intensive study of the corrosive properties of Manila tap water; nevertheless the following facts have been determined: 1. The water of the city of Manila is undersaturated with calcium carbonate; it contains aggressive carbon dioxide; and its corrosive action on the distribution system cannot be removed by seration alone2. The amount of lime to be added to the city water to bring it to saturation is about 16 ppm expressed as CaCO3. If imported commercial hydrated lime is used (about 95% purity), this would be equivalent to about 11.7 ppm dose. Since the city is using about 80 million gallons of water daily, the amount of lime needed is about 7800 pounds, a little less than four tons, daily. At maximum cost of two centavos a pound, (lime, however, can be obtained locally at less than this) the daily cost would amount to about 160 pesosOur finding apparently shows the great amount of lime necessary. This dosage, however, is to be used only during the first few weeks of lime application until the system has "aged" - that is, a coating of calcium carbonate has been formed and the alkalinity at the plant is the same as that of the most distant point in the distribution system. After this condition is established, the dose may be reduced until a minimum is reached - that required to maintain the equilibrium so obtained. This minimum value is of course possible of determination only in the plant laboratory after the aging possible of determination only in the plant laboratory after the aging processAnother objection to the use of lime is the belief that the hardness of the water might be increased. Against this objection, however, is the fact that under certain limits the calcium causing hardness in drinking water may be diminished with addition of lime. Since the present hardness averages 45 ppm as CaCO3 the expected resulting hardness, when 16 ppm of lime as CaCO3 is added, would amount to 45 ppm minus 16 ppm, or 29 ppm as CaCO3. A water with this hardness is considered very soft for classification purposesOn the other hand, we should be warned that if proper corrective treatment is not instituted, serious red water troubles are likely to occur in the near future as has been observed already in Diliman, Quezon City, a place very near the water treatment plantA very convenient and practical method of controlling the prevention of corrosion by lime treatment is the use of the "Enslow Stability Indicator." In this apparatus, the water is passed through a solution oversaturated with pure calcium carbonate and then is made to come in contact with a bed of crushed marble. The water coming from this set-up will be saturated with the salt, and the pH of this water will be the saturation pH. By determining the actual pH of the water, the lime dose may be changed accordinglyCalgon, Inc., of Printing, Pa., has developed certain glassy phosphates which when added to waters cause the stabilization saturated solutions, thus preventing both the further deposition of the protective coating, and the eating up of the coating already formed. The use of sodium hexametaphosphate in the amount of 0.5 ppm is recommended. (Conclusions and Recommendations)