Chlorination of Water Part 13

[1] Nesfield. Public Health, 1903, =15=, 601.

[2] Darnall. J. Amer. Pub. Health a.s.soc., 1911, =1=, 713.

[3] Kienle. Proc. Amer. Waterworks a.s.soc., 1913, 274.

[4] West. J. Amer. Waterworks a.s.soc., 1914, =1=, 400-446.

[5] Jackson. Proc. Amer. Waterworks a.s.soc., 1913.

[6] Hale. Proc. N. J. San. a.s.soc., 1914.

[7] Pettenkofer and Lehmann. Munich Acad., 1887.

[8] Marshall. Conv. Amer. Elect. Chem. Soc., 1917. Eng. and Contr., 1918, =49=, 40.

[9] Faraday. Q. J. S., =15=, 71.

[10] Roozeboom. Rec. Trav. Chim., 1885, =3=, 59.

[11] Forcrand. Comp. rend., 1902, =134=, 991.

[12] Pedler. J. C. S., 1890, =83=, 613.

CHAPTER VIII

ELECTROLYTIC HYPOCHLORITES AND CHLORINE

Since 1889 when Webster first proposed the use of electrolysed sea-water as a disinfectant, various attempts have been made to introduce electrolytic hypochlorites for the bactericidal treatment of water and sewage. Two of these preparations were named Hermite fluid, and electrozone (c.f. page 5). Sodium hypochlorite, made by pa.s.sing chlorine into solutions of caustic soda, or by the decomposition of bleach by sodium carbonate, has also been used and preparations of this character have been sold under such names as Eau de Javelle, Labarraque solution, chloros, and chlorozone. These solutions contain mixtures of sodium hypochlorite and sodium chloride together with some free alkali.

Chlorozone was the name given by Count Dienheim-Brochoki to a number of preparations patented in 1876 and subsequently down to 1885. They were produced by pa.s.sing air and chlorine into solutions of caustic soda.

Lunge and Landolt[1] have shown that the air introduced is without effect and that the advantages claimed for chlorozone are illusory.

The earliest electrolytic installation on this continent was operated at Brewster, N. Y., in 1893 and since that date several plants have been erected where local conditions conduced to economical operation.

When a uni-directional current of electricity is pa.s.sed through a solution of sodium chloride, the salt is dissociated and the components liberated, NaCl = Na + Cl. If the elements are not separated, the chlorine combines with the sodium hydrate, formed by the action of the sodium on the water, to form sodium hypochlorite. The equations 2Na + 2H_{2}O = 2NaOH + H_{2}, and 2NaOH + Cl_{2} = NaOCl + NaCl + H_{2}O show that only one-half of the chlorine produced is found as hypochlorite; the other half reforming sodium chloride.

Several types of electrolysers have been used for the production of hypochlorites and chlorine but only two are suitable for water-works purposes: in one, the cathodic and anodic products recombine in the main body of the electrolyte; in the other, the diaphragm process, they are separated as removed and the final products are chlorine gas and a solution containing caustic soda and some undecomposed salt.

Until a few years ago the non-diaphragm process was the only one used for water treatment and it will consequently be discussed first.

_Non-diaphragm Process._ The theoretical voltage required for the decomposition of sodium chloride is 2.3 but when the products recombine in the electrolyte, side reactions occur which increase the minimum voltage to 3.54. On this basis one kilowatt hour gives 272 ampere hours and as one ampere hour is theoretically capable of producing 1.33 grams of chlorine, 1.21 kilowatt hours are necessary for the production of 1 pound of chlorine by the decomposition of 1.65 pounds of salt.

Charles Watt (1851) discovered this process and was the first to recognize the necessary conditions which are (1) insoluble electrodes, (2) low temperature of electrolyte, and (3) rapid circulation of electrolyte from the cathode to the anode. The control of the temperature is very important, for as it increases, side reactions occur with the formation of chlorates, and the efficiency is decreased.

The non-diaphragm cells used in Europe (Haas and Oettel, Kellner, Hermite, Vogelsand, and Mather and Platt) have been described by Kershaw.[2] In the Haas and Oettel electrolyser the electrodes are composed of carbon but in the other types at least one electrode is made from platinum or a platinum alloy. The Dayton electrolyser, which is the cell most familiar in North America, is shown in Fig. 9.

[Ill.u.s.tration: FIG. 9.--Dayton Electrolytic Cell.]

The outer cell is made of soapstone and is approximately 2-1/2 feet long and 2 feet wide. The main electrodes consist of four pieces of Atcheson graphite connected together by screws and metal strips to which is attached a clamp for connecting electrical terminals. Circulation of the brine is produced by gla.s.s baffle plates and secondary electrodes placed one inch apart between the main electrodes. The cell is intended to be used at 110-volts pressure but by wiring two cells in series a 220-volt circuit may be employed. An inlet and outlet are provided at each end of the tank to permit the direction of the flow to be periodically reversed for the purpose of removing the lime deposit from the graphite plates.

The salt solution is prepared in wooden tanks from coa.r.s.e clean salt (ground rock salt is unsuitable), containing as little iron as possible, in the proportion of 50 pounds to 100 gallons of water. After pa.s.sing through a gravel or other suitable filter the brine solution is carried by bra.s.s pipes to the electrolyser. The rate of flow is adjusted to the temperature of the hypochlorite solution leaving the cell but under normal conditions it is stated that the cell described will pa.s.s 40 gallons per hour with a consumption of 70 amperes and produce 2-1/2 pounds of chlorine per hour. This is equal to 8 pounds of salt and 3.08 kilowatt hours per pound of chlorine. After the cells have been operated for several months the efficiency usually falls and 10-11 pounds of salt and 3.5-3.7 kilowatt hours are required for the production of one pound of chlorine. The concentration of the hypochlorite solution is usually about 6 grams per litre.

Rickard[3] stated that by cooling the Dayton cell with ice 1 pound of chlorine could be produced from 2.65 kilowatt hours and 6.9 pounds of sodium chloride; without cooling the figures were 3.62 kilowatt hours and 7.2 pounds of salt. Based on the figures that have been obtained from mature cells, the efficiency of the Dayton cell as compared with those described by Kershaw is as follows:

------------------+----------------------+---------------------- | SALT. | POWER.

| PER POUND OF AVAILABLE CHLORINE.

Type of Cell. +----------+-----------+----------+----------- | Pounds. | Per Cent | Kilowatt | Efficiency | | Consumed. | Hours. | Per Cent.

------------------+----------+-----------+----------+----------- Haas and Oettel | 10.7 | 15.4 | 3.8 | 31.9 Kellner | 7.5 | 22.0 | 2.75 | 43.9 Hermite | 11.2 | 14.5 | 2.87 | 42.2 Mather and Platt | .... | .... | 2.75 | 43.9 Dayton | 10.0 | 16.5 | 3.6 | 33.6 Theoretical | 1.65 | 100.0 | 1.21 | 100.0 ------------------+----------+-----------+----------+-----------

The cost of production depends upon local conditions: if alternating current is available at $30 per horse-power per annum, and low-grade salt can be obtained for $5 per ton the cost of 1 pound of chlorine would be

-----------------+---------------------------- | COST (CENTS) PER POUND OF | AVAILABLE CHLORINE.

Type of Cell. +--------+----------+-------- | Salt. | Current. | Total.

-----------------+--------+----------+-------- Haas and Oettel | 2.67 | 1.97 | 4.64 Kellner | 1.87 | 1.43 | 3.30 Hermite | 2.80 | 1.49 | 4.29 Dayton | 2.50 | 1.92 | 4.42 -----------------+--------+----------+--------

The electrical and chemical efficiencies of the Haas and Oettel and Dayton cells, which contain carbon electrodes, are smaller than those containing platinum electrodes but for water sterilisation the carbon cells have been found to be more suitable. To prevent the action of magnesium salts on the platinum electrodes it is necessary to use a higher grade of salt or to provide means of purification. Because of the absence of a base and the presence of chlorides, electrolytic hypochlorite cannot be stored for more than a few hours without appreciable loss of t.i.tre. Unless used for the treatment of the effluent of a filter plant operated at a fairly constant rate a small storage tank is necessary to compensate for the irregular demand and to provide the head required by orifice feed boxes. Small variations can be made by regulating the flow through the cells but large ones are not compatible with efficiency, which is the highest under a constant load.

Claims have been made that electrolytic hypochlorite is more efficient as a germicide than bleach when compared on the basis of their available chlorine content but no evidence of it has been produced. Bleach contains an excess of base, which r.e.t.a.r.ds the germicidal action, and electrolytic hypochlorite contains an excess of sodium chloride, which accelerates it (Race[4]) but the ultimate effect is the same with both.

This is shown in Table XXIV.

TABLE XXIV.[A]--COMPARISON OF BLEACH WITH ELECTROLYTIC HYPOCHLORITE

----------------+-------------------++------------------- | BLEACH. || ELECTROLYTIC | || HYPOCHLORITE.

+-------------------++------------------- Contact Period. | Available Chlorine. Parts Per Million.

+---------+---------++---------+--------- | 0.4 | 0.6 || 0.4 | 0.6 ----------------+---------+---------++---------+--------- Nil | 182 | ... || ... | ...

10 minutes | 130 | 10 || 120 | 8 1 hour | 66 | 1 || 60 | 0 2 hours | 3 | 0 || 1 | 0 3-1/2 hours | 0 | 0 || 0 | 0 ----------------+---------+---------++---------+---------

[A] Results are B. coli per 10 c.cms.

Electrolytic hypochlorite has a greater germicidal velocity than bleach but the difference is so small as to be of no practical importance.

Rabs[5] experimented with various hypochlorites but was unable to find any appreciable differences in their germicidal action.

If electrical power can be obtained at a very low cost, or if the cost is merely nominal, as it is when there is an appreciable difference between the normal consumption and the peak load upon which the rate is based, the electrolytic hypochlorite method offers some advantages but in the great majority of plants it cannot economically compete with bleach. The instability of the liquor and cell troubles have also prevented the process being generally utilised. Baltimore and Cincinnati experimented with this method but did not adopt it. Winslow[6] has reported that, owing to the difficulty in obtaining bleach since the outbreak of war, Petrograd has used electrolytic hypochlorite made from salt.

_Diaphragm Process._ The various types of diaphragm cells that have been commercially operated are of two varieties: (1) cells with submerged diaphragms and (2) cells in which the electrolyte comes in contact with one face only of an unsubmerged diaphragm.

The Le Sueur, Gibbs, Crocker, Billiter-Siemens, Nelson, and Hargreaves-Bird cells are of the submerged diaphragm variety. The Nelson cell has been operated for some time at the filtration plant at Little Falls, N. J. The cells are fed with brine solution previously purified by the addition of soda ash and have given fairly successful results although the cost of maintenance is comparatively high. Tolman[7] has reported that several towns in West Virginia use a bleach solution prepared by absorbing chlorine, manufactured by the Hargreaves-Bird process, in lime water; the solution contains about 1.95 per cent of available chlorine.

The diaphragms in both the submerged and unsubmerged types are usually constructed either with asbestos paper or cloth, placed in such a manner as to divide the cells into two separate compartments: the anodic, into which the brine is fed and where the chlorine is produced; and the cathodic, where caustic soda is formed.

By maintaining the liquor in the anodic compartment at a higher elevation than in the cathodic one, the direction of flow is towards the latter, but owing to osmosis and diffusion the separation is not complete and a portion of the caustic soda pa.s.ses the diaphragm and produces hypochlorite with a consequent loss of efficiency and rapid deterioration of the anodes. With the exception of the Billiter-Siemens cell, the submerged diaphragm cells operate at not more than 85 per cent efficiency and the cost of maintenance is usually high.

In the non-submerged diaphragm types the invasion of the anodic compartment by caustic is much reduced and the efficiency and life increased.