Cost of Construction. According to the replies received by the Committee on Water Supplies of the American Public Health Association[10] the total cost of equipment for disinfection varies widely and bears no apparent relation to the capacity of the equipment. This is due to the temporary nature of the plants erected in many cities and the necessity of erecting expensive structures in others. The cost of construction varies also in different localities. The cost of equipping hypochlorite plants with standard concrete tanks and dosage regulators would be more uniform and for capacities between 10 and 50 million gallons per day would approximate $15 to $50 per million gallons.
The operating cost of bleach plants shows similar wide variations. In some cases the labour required for mixing and supervision can be obtained without extra cost whilst in others the labour charge exceeds the cost of hypochlorite.
The price of bleach has shown violent fluctuations during the last three years (see [Diagram IX], [page 125]) but is now (1918) comparatively steady at $2.25 to $2.75 per 100 pounds. Assuming that 33.3 per cent of available chlorine can be extracted, each pound of chlorine costs 6.75-7.25 cents as compared with 15-25 cents for liquid chlorine. The fixed charges on the capital expenditures together with the labour and incidental charges almost invariably make the total cost of operation of a straight bleach plant higher than that of a liquid chlorine plant. The tendency during the last four years has been to substitute liquid chlorine for hypochlorite and the majority of the plants are now of the former type.
“Antichlors”
Substances used for the removal of excess chlorine are usually known as “antichlors” and those that have been most frequently employed are sodium bisulphite, NaHSO3, and sodium thiosulphate Na2S2O3. The reactions with chlorine are:
| (i) | NaHSO3 + Cl2 + H2O = NaHSO4 + 2HCl. |
| (ii) | Na2S2O3 + Cl2 = Na2S4O6 + 2NaCl. |
Sodium bisulphite is a very efficient “antichlor,” only 1.46 parts being required to remove 1 part of chlorine, but owing to its instability the action is uncertain. Sodium thiosulphate is a comparatively stable cheap salt, containing 5 molecules of water of crystallization, Na2S2O3·5H2O but 7 parts are necessary to remove 1 part by weight of chlorine.
“Antichlors” are used as aqueous solutions and the dosage controlled in the same manner as for bleach solutions. The action is an instantaneous one and it is consequently necessary that the germicidal action should be complete before the “antichlor” is added.
Filters, containing solid materials capable of absorbing free chlorine, have also been used for removing the excess of the germicidal reagent. Iron borings and aluminium were used experimentally by Thresh[11] but the process was not commercially developed. The “De Chlor” filter, in which carbon is the active substance, has been installed at several water works in England (Reading, Exeter, Aldershot) with apparently successful results. The Reading experimental installation, described by Walker,[12] consisted of a steel drum, 8 feet 3 inches in width, the top and bottom being domed. In the upper portion, 10 feet 9 inches in depth, provision was made for thorough admixture of the bleach solution and water and a subsequent storage of thirty minutes. The lower section of the filter was divided into three compartments, the first and last of which contained graded silica; the middle compartment was filled with a layer (20 inches deep) of specially prepared granulated charcoal or carbon.
The filter was operated under pressure and passed an average of 192,000 Imp. gallons per day, the rate being 32,000 Imp. gallons per square yard per day.