Chlorination

CHLORINATION of drinking and sewage waters. Chlorination of water is the treatment of it with gaseous chlorine or preparations of active chlorine (bleaching powder, calcium-sodium hypochlorite, chloramines, etc.) for the purpose of disinfecting it from pathogenic microorganisms. Chlorination, like other methods of water disinfection, does not free the water from bacteria 100%, but the single chlorine-resistant bacteria remaining after chlorination are of practically no significance, since in most cases they are non-pathogenic. The epidemiological significance of chlorination is very great: it sharply reduces the incidence of water-borne intestinal infections among the population and mortality from them. The application of chlorination on a wide scale began in America in 1908; to this same time belongs the beginning of the application of chlorination in Russia—in Kronstadt (1908) for the disinfection of the water supply network and in Nizhny Novgorod (1910) for the disinfection of water at the fair water supply system on the occasion of a cholera epidemic. Initially, chlorination of water in both Europe and America was used only in exceptional cases, usually during epidemics of water-borne intestinal infections, but over the past 15-20 years, water disinfection with chlorine in the practice of urban water supply in the USSR (Rostov-on-Don) and in other states of Europe and America has been used already as a permanent anti-epidemic measure. In the practice of field water supply for the Red Army and foreign armies, chlorination of water with bleaching powder or gaseous chlorine is also used for disinfection. The wide application of chlorination in the practice of stationary water supply (cities, state farms, collective farms, camps, etc.) and field water supply is due to the rapidity of action, good bactericidal effect, cheapness, and simplicity of using chlorine and its preparations in comparison with other means of water disinfection. Chlorination with gaseous chlorine has the following advantages compared with bleaching powder or other chlorine preparations: a) gaseous chlorine does not require bulky installations for preparing the solution (chlorine water); b) in view of the automatic nature of the devices metering gaseous chlorine (chlorinators), concerns about dosing accuracy are significantly eased; c) gaseous chlorine is a purer product, without side impurities that impart an unpleasant taste to chlorinated water; d) there is no loss of chlorine when storing it in steel cylinders. The storage of bleaching powder is usually fraught with significant losses of active chlorine. Chlorination of drinking and sewage waters with gaseous chlorine is carried out using special normal-type chlorinators (continuous action) and batch chlorinators, which measure out a specific amount of chlorine, mix the latter with water, and form so-called chlorine water, which is then fed into the water being purified for chlorination. The normal type of chlorinator (Engineer B. M. Remesnitsky), used in urban water supply systems and municipal stations in the USSR for the disinfection of sewage (Fig. 1), consists of parts 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 20, and 21, which serve for the uniform, continuous, and within certain limits adjustable supply of gaseous chlorine into the mixer 12, and parts 13, 14, 15, 16, 18, and 19, which serve to supply a certain amount of water (2-4 liters for each gram of chlorine) from the water supply system into the same mixer 12. In the mixer 12, mixing and dissolution in water of the incoming

Figure 1. Normal type of chlorinator: 1 - board; 2 - reduction valve; 3 - regulating valve; 4 - low-pressure pressure gauge with special flanges; 5 - high-pressure pressure gauge with special flanges and gas filter; 6 - chlorine quantity meter - differential pressure gauge; 7 - safety device for the meter with a regulating scale; 8 - tube connecting the regulating valve with the meter; 9 - drain valve with drain pipe; 10 - tube connecting the drain valve with the check valve; 11 - check valve; 12 - mixer; 13 - water reduction valve with water filter; 14 - tee; 15 - pressure gauge; 16 - connecting nipple; 17 - elbow; 18 - shut-off water valve; 19 - connecting nut with nipple; 20 - tube of the connecting shut-off valve with filter; 21 - shut-off valve; 22 - fastening pins.

gaseous chlorine takes place. A steel cylinder with chlorine is connected to the chlorinator. Normal-type chlorinators of the systems of Engineer Remesnitsky, Engineer Kulsky, Engineer Bromley, or their prototype Engineer Ornstein (see Waste waters,

Figure 2.

Figs. 3 and 4) are manufactured with varying capacities—ranging from 10 to 2,500 g or more of chlorine per hour. The field chlorinator of Engineer Kulsky of continuous and batch action serves for the chlorination of small quantities of water (Fig. 2). The Kulsky chlorinator consists of a piston pump (1), mixer (2), gas meter (3), and a cylinder of gaseous chlorine (4). The chlorinator is mounted in a box. The weight of the chlorinator with the box and spare parts is 12 kg. - The batch field chlorinator of Engineer Remesnitsky (Fig. 3) consists of the following parts: a) a metal cylinder with liquid chlorine with a capacity of 2.5 kg with a valve for releasing chlorine; b) a glass gas-measuring cylinder with a capacity of 1,500 mg of chlorine; the cylinder is graduated in weight doses from 100 to 1,500 mg of chlorine, has a valve tap at the top to regulate the inflow of chlorine from the cylinder and a tap on the left regulating the release of chlorine from the cylinder into the injector; the glass cylinder is connected at the bottom to a rubber reservoir, which during operation is filled with water; the latter is introduced into the cylinder before chlorine enters it; c) an injector for mixing chlorine gas with water entering through the pump; d) a manual piston pump for sucking water and passing it through the injector; e) two rubber hoses: lower left, suction, and upper right, connected to the injector, pressure hose, through which water mixed with chlorine goes. All indicated

parts are mounted in a box. The weight of the chlorinator is 8-10 kg. Losses of chlorine gas during operation with the chlorinator do not exceed 5-10%. Chlorination of water with chlorine preparations is carried out by introducing the latter in dissolved form into raw water using dosing devices of various types. In water supply practice, chlorination of drinking water is carried out by the following methods: a) Chlorination of clean water, i.e., water clarified by coagulation, settling, and filtration. In this method, chlorine is introduced into the filtered water at the beginning of the clear-water storage reservoirs or directly into the conduit leading the filtered water from the filters to the storage reservoirs. This method of chlorination, while yielding good results in the bactericidal action of chlorine at low doses of the latter (from 0.2 to 0.35 mg/L), at the same time requires a significant reservoir capacity to create a sufficiently prolonged contact of chlorine with clean water. A negative aspect of this method is also the danger of supplying bacterially substandard water to the consumer in the event of minor interruptions in chlorination, since during the filtration of water not treated with chlorine, the filtrate as a rule has a positive coli titer in relatively small volumes of water; b) Chlorination of settled water, i.e., introducing chlorine into the water before its filtration. This method has not found wide application in municipal water supply practice due to the significant consumption of chlorine and not always satisfactory results of the bactericidal action of the latter. The contact time of chlorine with water in this method is insufficient; c) Chlorination of raw untreated water—simultaneously with coagulation or prior to the latter. According to reports by water supply figures in the USA (Stritter, Enslow, Norman J. Howard), the USSR (Turchinovich, Zhitomirsky), this method, while requiring a large consumption of chlorine, gives very good bactericidal results. In field water supply practice, this method of water chlorination is one of the most portable in terms of the overall dimensions of water treatment plants and the duration of water treatment. Double chlorination. In recent years in America, Germany, and the USSR, chlorination has been used twice. In this method, the first dose of chlorine is added to raw untreated water or to coagulated and settled water, and the second dose is added to filtered water. Depending on the quality of the water, the chlorine dosage varies. Many authors recommend taking the first dose such that the water fed to the filter contains residual chlorine from 0.25 to 0.5 mg/L, and the second dose in the filtered water from 0.05 to 0.25 mg/L. Double chlorination provides a sharp improvement in the quality of purified water and has a great future in water supply practice. Hyperchlorination (superchlorination)—disinfection of water with large doses of chlorine followed by dechlorination. Chlorine doses in this method are used very large—three to five times or more exceeding the chlorine demand of a given water in the period set for chlorination. In this case, chlorine is added to raw untreated water, and its amount in the filtered water, i.e., after the filter, must be at least 1.0 mg/L. Hyperchlorination achieves the conversion of a number of foul-smelling chlorine compounds with phenol, cresol, and other substances into compounds that do not possess a bad odor. Subsequent dechlorination with carbon (activated) further improves the organoleptic qualities of the water, freeing it from excess residual chlorine. Dechlorination of water—treatment of chlorinated water containing an excess of active chlorine by chemical or physical means that destroy the odor and taste of active residual chlorine. When dechlorinating small quantities of water, chemical dechlorinating agents are used, such as hyposulfite (sodium thiosulfate), Na₂S₂O₃·5H₂O, or sodium sulfite, i.e., Na₂SO₃·7H₂O, while in the dechlorination of large quantities of water—in water supply practice—physical dechlorinating agents are also used, namely activated carbon. Filtering chlorinated water through activated carbon filters not only frees the water from the taste and odor of chlorine, but also generally improves its organoleptic qualities. The rate of water filtration through carbon filters is used from 5 to 200 liters per hour, depending on the water quality and the residual chlorine content. Activated carbon, after exhaustion, is regenerated (i.e., its absorptive properties are restored) by various methods. Chlorination of wastewater as a rule is carried out by introducing gaseous chlorine or a solution of bleaching powder into wastewater that has been preliminarily purified (by coagulation, settling, and filtration). Water chlorination gives completely reliable bactericidal results only when the following measures are carried out: a) correct establishment of the chlorine dose for the chlorination of a given water; b) correct establishment of the contact time of chlorine with water; c) ensuring thorough mixing of the entire mass of the chlorinated water with chlorine; d) chlorination must be controlled by chemical and bacteriological examination of the quality of the chlorinated water through systematic determination of residual chlorine and determination of the coli titer. The most essential and important measure ensuring positive results of chlorination is the determination of the necessary dose of active chlorine for each individual water and in each individual case of chlorination. The significance of this measure becomes clear if the following circumstances are taken into account: a) during water chlorination, active chlorine is consumed not only by the bacterial flora, but also by the oxidation of dead organic matter, as well as by interaction with inorganic substances; b) waters from different water sources with varying contents of organic and inorganic substances, including suspended solids, have different chlorine demands and require unequal amounts of chlorine for their oxidation, and the oxidation process itself proceeds differently in time and intensity; c) water of the same chemical and bacterial composition with the same content of suspended solids at different temperatures has a different chlorine demand, namely: the lower the temperature of the water, the less chlorine is consumed and vice versa. Hence it is clear that the chlorine dose necessary for the chlorination of water of different composition will be unequal; d) the amount of residual chlorine in the water after its chlorination during the time established for this must be at least 0.1 mg and not more. than 0.4 mg per liter; this amount ensures a reliable bactericidal action of chlorine with sufficient water contact with chlorine and does not spoil the taste of the water; e) larger doses of residual chlorine worsen the taste and odor of the latter; f) small, i.e., insufficient, doses of residual chlorine (less than 0.1 mg/L) do not provide a reliable bactericidal effect of water chlorination. Thus, the chlorine dose necessary for water chlorination is essentially the sum of the following components: a) the chlorine demand of the water in the period of time established for chlorination; b) residual chlorine in an amount of 0.1-0.4 mg of chlorine per 1 liter of water. Of the existing methods for determining the chlorine demand of water and the chlorine dose in the practice of stationary (municipal and field) water supply, the method of determination by experimental chlorination in vessels of various capacities (buckets, liter flasks, and beakers) has found application. Determining the chlorine demand of water and the chlorine dose by experimental chlorination in vessels of various capacities yields reliable results if it is carried out during the time that is established for the chlorination itself. To obtain a better disinfection effect during the chlorination of water with bleaching powder, in addition to determining the working dose of chlorine, it is necessary to: a) carefully prepare a solution of bleaching powder, which must not contain large suspended solids and must be as transparent as possible; introduce bleaching powder into raw water exclusively in the form of a solution and not in dry form; b) correctly establish the duration of chlorination, i.e., the time of contact of chlorine with water. The contact time of chlorine with water is a decisive moment for obtaining a good bactericidal effect and one of the main moments when choosing the chlorine dose. The shorter the contact time, the greater the chlorine dose must be, and conversely, the more significant the contact in time, the smaller the chlorine dose can be. At the same time, for each water there is its own minimum limit for the chlorine dose, below which, regardless of the duration of contact, chlorine does not exert a sterilizing effect on the water. Data from laboratory experiments given in the following table (Voytkevich) can serve as a characterization of the significance of contact time: Time | Chlorine content in mg: 0.5 | 1.0 | 2.0 | 4.0. Number of bacteria in 1 cm³: Before experiment: 232,000 | 232,000 | 232,000 | 232,000. After 30 min.: 65,000 | 2,300 | 50 | 77. After 1 h: 4,400 | - | 36 | 52. After 2 h: 20,500 | - | 23 | 47. After 4 h: 37,000 | - | 50 | 40. After 24 h: 75,000 | 232,000 | 9 | 5. Low water temperature (from 10° and below) retards the reaction of chlorine with water, due to which oxidation processes and consequently the bactericidal action are slowed down. Under these conditions, it is necessary to increase the contact time of chlorine with water. The contact time of chlorine with water is established for each water on the basis of bacteriological observation (determination of the E. coli titer) and counting the total number of colonies in 1 cm³ of chlorinated water.

In field conditions, chemical control of water chlorination is more accessible than bacteriological control, which, due to its complexity and duration (1-5 days), is not always possible to apply under such conditions. In carrying out chemical control of water chlorination, one is guided by the following: a) the chlorine dose is taken correctly if the residual chlorine in the water after chlorination for a set time is contained in an amount of no more than 0.4-0.3 mg and no less than 0.2-0.1 mg per 1 liter, with the absence of a clear odor and taste of chlorine; b) if the residual chlorine is greater than 0.4 mg/liter, then the water will have the odor and taste of chlorine, and such water must be dechlorinated. See also Sewage, wastewater disinfection. - Water chlorination in central water supply systems everywhere yields very positive results. Its widespread use is therefore currently regulated by a number of existing instructions and rules, which should guide sanitary-hygienic personnel and social workers. For the RSFSR there are: a) "Sanitary rules for the chlorination of drinking waters", approved by the People's Commissariat of Health on 15/V 1929; b) "Sanitary rules for the chlorination of wastewaters", People's Commissariat of Health 1930; c) "Regulations on permanent laboratory supervision over the chlorination of drinking and wastewaters", People's Commissariat of Health and the Supreme Council of the National Economy 3/XI 1930. In addition, the Military-Sanitary Directorate of the Workers' and Peasants' Red Army also issued its own rules: "Sanitary requirements for installations for the extraction, purification, and disinfection of water", Moscow, 1931.