UDC 614.777:628.16.085

M. R. Petranovskaya, M. A. Semenova, G. L. Medrish, D. L. Basin

A New Direction in the Disinfection of Water with Ultraviolet Rays

Research Institute of Municipal Water Supply and Water Purification, K. D. Pamfilov Academy of Municipal Services, Moscow

The importance of treating and disinfecting drinking water and wastewater makes it necessary to seek new technological methods and improve existing ones.

The method of disinfection using liquid chlorine and chlorine-based reagents, which has been extensively tested and widely adopted in water supply and wastewater management, has recently come under serious criticism because water treatment produces organochlorine compounds of increased toxicity and carcinogenicity [7]. Numerous studies show that almost all chemical disinfectants produce intermediate compounds in water (trihalomethanes, ozonolysis products, chlorites, chlorates, and monochloramines) that affect the hematopoietic system, the liver, and the thyroid gland [5]. Accordingly, research is being conducted both in our country and abroad to modify chlorination regimes to reduce trihalomethane formation, replace chlorine with other reagents, and explore opportunities for wider application of physical disinfection methods [6, 12, 13, 23]. This explains the renewed interest in the practical use of water treatment with ultraviolet (UV) rays, a method free of the shortcomings mentioned above [8, 10, 15, 19, 21, 26].

The mechanism of action of UV rays has been known since 1878, when A. Downes and T. Beunt [9] studied their effects on bacteria and other protozoan organisms. Subsequent work in this field showed that UV rays, being absorbed at the surface of molecules, cause photochemical reactions that destroy cellular DNA [3]. These reactions proceed rapidly, and cell destruction occurs instantaneously, accounting for the strong disinfecting effect of UV rays [11].

The first experiments in disinfecting water with UV rays were carried out in 1910 by Baer Georg [3]. Although the potential applications of UV rays were investigated quite extensively throughout the world, technological development before the Second World War did not permit widespread use of the method. In subsequent years, powerful and economical radiation sources were developed, making it possible to create installations for effective and economically viable UV treatment of water [1, 14].

A comparison of three disinfection methods—UV irradiation, ozonation, and chlorination—in terms of suspended solids, temperature, pH, degree of toxicity and corrosive action, effects on the organoleptic properties of water, complexity of maintenance, and economic efficiency (Table 1) demonstrates the need for wider use of UV irradiation [2, 22].

A factor limiting the widespread use of UV rays was the more stringent requirements for the physicochemical and bacteriological characteristics of the water (turbidity no greater than 1.5 mg/L, color—

Table 1

Comparative assessment of disinfection methods

Table 2

Photochemical reactions and their results

…not exceeding 0.3 mg/L, coli index 1000), as well as the lack of a residual effect of the radiation, which could lead to secondary contamination of water in the distribution network. However, observations of water quality and the condition of water supply networks over 25 years in the United States [24, 27], together with operating experience from 2,000 installations in Europe, have shown this argument to be unfounded.

In recent years, a fundamentally new approach to UV water treatment has emerged. It consists in introducing small doses of oxidants (\(\mathrm{H_2O_2}\), \(\mathrm{O_3}\), \(\mathrm{Cl_2}\)) into the water before exposing it to UV radiation. It is known that UV radiation causes water molecules to dissociate, forming \(\mathrm{OH^\bullet}\) radicals with oxidizing power. An oxidant introduced into the water, such as hydrogen peroxide, dissociates under UV radiation to form two hydroxyl radicals whose oxidizing power is greater than that of the \(\mathrm{OH^\bullet}\) radicals formed from water [17]. The oxidation potentials of a number of substances, which determine their chemical activity, are, for example, 3.06 for fluorine, 2.80 for \(\mathrm{OH^\bullet}\) radicals, 2.07 for ozone, 1.77 for hydrogen peroxide, 1.67 for the permanganate ion, and 1.50 for chlorine dioxide, compared with only 1.36 for chlorine.

In the photochemical reactions induced by oxidants and UV radiation, \(\mathrm{OH^\bullet}\) radicals cause the rapid breakdown of organic molecules, releasing new radicals that sustain a chain reaction in which complex compounds are broken down into simpler ones. Examples of some photochemical reactions are presented in Table 2.

The presence of oxidants in the water being treated also promotes the conversion of iron and manganese ions into insoluble forms that readily precipitate. Similar reactions occur with sulfites and sulfides, nitrates and ammonia. Some organic dyes that are resistant to chlorine and other oxidants are readily destroyed by their combined action with UV radiation. A comparative assessment of the reduction in total organic carbon (TOC) concentration under the action of UV radiation and \(\mathrm{H_2O_2}\) or ozone showed that adding a 0.1–1.0% \(\mathrm{H_2O_2}\) solution followed by irradiation reduced TOC concentration by 84%, whereas the addition of \(\mathrm{O_3}\) reduced it by 100%; treatment with ozone alone (2 mg/L) for 8 h was less effective [16, 18].

The combined use of UV radiation and oxidants produces a strong bactericidal effect. According to C. Bayliss and W. Waites [4], treatment of water with UV radiation and \(\mathrm{H_2O_2}\) (0.15–0.45 M) killed 99.99% of spore forms. The greatest reduction in the concentration of inorganic and organic substances was observed under the combined action of UV radiation and ozone, and this effect was achieved with small ozone doses. At the same time, the introduction of ozone disrupts the cell’s enzyme system and facilitates disruption of the cell membrane [20]. This preliminary conditioning of the cell allows UV radiation to penetrate more deeply and produces a greater bactericidal and virucidal effect. Whereas chlorine treatment killed only 75% of viruses, the use of UV radiation with an oxidant killed 99.99% [4, 12].

An assessment of the combined use of UV radiation and oxidants showed that disinfection efficiency increases by 20–30%, while reagent consumption is reduced by 20–50%. The method is especially promising because it requires no contact tanks and achieves disinfection within 30–60 s [25]. The advantages of UV water treatment include preservation of the water’s mineral composition and taste, no risk of toxic compounds forming, and effective action against resistant forms of bacteria and viruses [17]. From a technical standpoint, UV installations are easy to maintain and automate; they also permit centralized monitoring and remote control.

An economic assessment showed that the cost of constructing an appropriate installation is recovered within 10 years [25]. The method’s “environmental harmlessness” and its other advantages outlined above indicate that water treatment with UV radiation and an oxidant is the most viable alternative to chlorination and other disinfection methods that use chemical reagents.

References

1. Sokolov V. F. Water Disinfection with Germicidal Rays. Moscow, 1954. P. 73–95.
2. Angehrn M. // Aqua. — 1984. — No. 2. — P. 109–115.
3. Baer G. // Publ. Works. — 1979. — Vol. 110. — P. 59–61.
4. Bayliss C. E., Waites W. M. // Microbiol. Lett. — 1979. — Vol. 5. — P. 98–101.
5. Bull R. J. // J. Amer. Water Works Ass. — 1982. — Vol. 74. — P. 642–648.
6. Craig V., Stig R. // Ibid. — 1981. — Vol. 73. — P. 40–44.
7. Dressman R. C., Stevens A. A. // Ibid. — 1983. — Vol. 75. — P. 481–484.
8. Gelzhäuser P. // Wasserwirtschaft. — 1984. — Bd 74. — S. 338–340.
9. Groocock N. H. // J. Inst. Water Eng. Sci. — 1984. — Vol. 38. — P. 163–172.
10. Hener D., Hofmann E. // GWF (Wasser-Abwasser) — 1979. — Bd 120. — S. 476–480.
11. Hoather R. C. // J. Inst. Water Eng. — 1955. — Vol. 9. — P. 191–193.
12. Hoff J., Geldreich E. // J. Amer. Water Works Ass. — 1981. — Vol. 73. — P. 40–44.
13. Huff C. B. // Publ. Hlth Rep. — 1965. — Vol. 80. — P. 695–700.
14. Johnston C. // Mod. Power Eng. — 1982. — Vol. 76. — No. 9. — P. 10–15.
15. Latzer B. // Pat. 622963 (Switzerland).
16. Legan R. W. // Chem. Eng. (N. Y.). — 1982. — Vol. 89. — P. 95–100.
17. Lipovicz M. // Ibid. — 1981. — Vol. 88. — No. 22. — P. 40–41.
18. Malaiyandi M., Husain Sadar M., Lee P., O’Grady R. // Water Res. — 1980. — Vol. 14. — P. 1131–1135.
19. Morgan F. // Water Eng. Management. — 1983. — Vol. 130. — No. 5. — P. 26–28.
20. Nall A. E. // Water Waste Eng. — 1980. — Vol. 17. — No. 2. — P. 43; 48; 52–53.
21. Pecala W., Percowski J., Rusiak J. // J. Gas, Woda Techn. sanit. — 1983. — Vol. 57. — P. 151–153.
22. Peytone G. R., Huang F. Y., Burleson J. L., Glaze W. H. // Environ. Sci. Technol. — 1982. — Vol. 16. — P. 454–458.
23. Singer P. C., Barry III J. J., Palen G. M., Scrivner A. E. // J. Amer. Water Works Ass. — 1981. — Vol. 73. — P. 392–401.
24. Tobin R. S., Smith D. K., Horton A., Armstrong V. C. // Ibid. — 1983. — Vol. 75. — P. 481–484.
25. UV Disinfection Chambers. Require Custom Designing. // Water Sewage Works. — 1980. — P. 56–61.
26. Washino M. // Radiat Phys. Chem. — 1981. — Vol. 18. — P. 51–52.
27. Yip R. W., Konasewich D. E. // Water Poll. Contr. — 1972. — Vol. 110. — P. 14–19.

Received January 23, 1986.