Laboratory tests of the stability of disinfectants for different quality of water

¹ Department of Sanitary and Environmental Engineering, Faculty of Civil Engineering, Slovak University of Technology, Radlinského 11, 81005 Bratislava, Slovakia
² Water Research Institute, Nábr. arm. gen. L. Svobodu 5, 81249 Bratislava, Slovakia

^* Corresponding author: jan.ilavsky@stuba.sk

Abstract

The last water treatment process is disinfection and it has two purposes. The main purpose of disinfection is to kill or inactivate pathogens. A secondary purpose is to provide a residual disinfectant concentration in drinking water and to prevent microbial growth in water distribution systems. Although most pathogens (99 to 99.9%) are removed by coagulation, flocculation, sedimentation, and filtration, disinfection is a critical process for protecting the public from waterborne infections. Common disinfectants include chlorine gas, sodium hypochlorite, chloramines, chlorine dioxide, ozone and ultraviolet radiation. Each disinfection method has its advantages and disadvantages. Among the disadvantages of using sodium hypochlorite, in addition to the formation of disinfection by-products, the stability of the used agent is also important. Laboratory tests of the stability of disinfectants were carried out in drinking water of different quality depending on the pH value, water temperature and total water mineralization at different contact time in water (from 0 to 48 hours). To monitor the stability of disinfectants, chlorine-based disinfectants were selected, namely sodium hypochlorite prepared in the classic way and by electrolysis of brine. The stability of these disinfectants was verified in low-mineralized surface water from the Turček water reservoir, medium mineralized groundwater from the Sihot’ water source and groundwater from a private well in village of Ol’dza with highly mineralized water. Active chlorine doses in the range of 0.28-0.35 mg/L were used to verify the stability of sodium hypochlorite prepared in the classic way in selected drinking waters. When verifying the stability of sodium hypochlorite prepared by electrolysis of brine, doses of active chlorine in the range of 0.25-0.30 mg/L were applied. When verifying the stability of sodium hypochlorite produced by two different methods (chemical process, brine electrolysis), the dependence of the increasing stability of these disinfectants with a decrease in pH value and water temperature was demonstrated. The dependence of chlorine stability is significantly influenced by the water temperature, less so by the pH value of the water. Reaction kinetics of 0th, 1st and 2nd order was studied for better understanding of this process.