Optimisation of chlorine dioxide generators and investigation into an ultra-pure chlorine dioxide generation method for drinking water treatment

Abstract

Chlorine dioxide (ClO2) is an oxidant used across many different industry sectors for water treatment, including for the treatment of drinking water. Chlorine dioxide has been successfully applied to drinking water supplies for disinfection, oxidation, colour reduction and taste and odour control. Historically chlorine, as gaseous Cl2 or sodium hypochlorite solution, has been used as the major disinfectant in drinking water treatment plants (DWTP). Research linking the generation of trihalomethanes (THM) as by-products from the chlorination of water is causing a slow withdrawal from chlorine dosing within the drinking water industry, allowing alternative disinfectants to become more widely utilised. Alternative chemical disinfectants are also not trouble free. Chlorine dioxide produces chlorite (ClO2 - ) and chlorate (ClO3 - ) inorganic disinfection by-products (DBP). Chlorite and chlorate ions are highly undesirable, potentially toxic, by-products formed during the commercial generation of chlorine dioxide. Chlorate was relatively unspoken about within the water industry until recently as it was found to be a degradation product present in sodium hypochlorite solution, a chemical commonly used for chlorination. Increased awareness and upcoming changes in regulation concerning disinfection by-products are causing increased scrutiny to be placed on all chemical disinfection processes. Manufacturers of chlorine dioxide generators commonly base the performance of their equipment upon ClO2 output and conversion efficiency of sodium chlorite precursor to chlorine dioxide. Conversion efficiencies, expressed as ClO2 percentage yield, greater than 95% are the industry standard for commercial ClO2 generators. Conversion efficiency expressed in this way only details the ClO2 concentration generated, it gives no additional information on the concentration of DBPs produced. The presence of ClO2 - in the product solution is overlooked and the formation of chlorate is assumed to be negligible and therefore there was no great concern over chlorate transfer to the treated water. Since performance of generators are based on these simplifications, the purity of the ClO2 product is an important parameter that is often unknown and unspoken of. Scotmas Ltd have taken this opportunity to investigate the scale of DBP formation, particularly concerned with chlorate, in their ClO2 generators with subsequent optimisation of the process to reduced DBPs produced. This entailed an investigation into the purity of solutions produced by a 3.5 ghr-1 ClO2 reactor subject to varying conditions. The effects of increasing chemical retention time within the reactor and increasing acid concentrations were investigated. It was found that a retention time of greater than 17 minutes was required to produce the highest purity ClO2 solutions and that a reduction in acid dose, reduced by 60% in volume compared to sodium chlorite dose, continued to generate ClO2 solutions of high purity. To satisfy future regulation changes and customer demand a commercial generator able to efficiently produce an ultra-pure ClO2 product is required. Initial research and investigations were carried out to design an ultra-pure ClO2 generator. The design concept is to generate ClO2 inside a reactor which allows the ClO2 gas to subsequently be removed from the reactor and absorbed in water to form an ultra-pure ClO2 solution product. Techniques, such as ultrasound and aeration, were investigated on a small scale to evaluate their effectiveness at degassing ClO2 from solution. The feasibility of incorporating each technique into the reactor design was also considered. Ultrasound was found to increase the rate of ClO2 degassing from solution, although it was concluded that the technique was not effective enough to be incorporated into a commercial ClO2 generator. Aeration of ClO2 solutions dramatically increased the rate of ClO2 degassing, although the effects of adding large quantities of air to a process could be problematic in real installations. The short trial investigating aeration produced respectable results and has been suggested as the technique to investigate further for incorporation in to an ultra-pure ClO2 reactor.