Application of chlorine dioxide as an alternative pre-oxidant in the treatment of eutrophic raw water

Abstract

The Vaalkop water treatment works (WTW) abstracts water from the Vaalkop Dam, which is situated in the Crocodile West/Marico Water Management Area of South Africa. The bulk of the inflow into the dam is through a canal fed from the Hartebeespoort Dam. The water quality of the Vaalkop Dam was of pristine quality during the time the dam was constructed but has since deteriorated gradually to highly eutrophic. The high nutrient levels have caused high concentrations of NOM, taste and odour problems, leaching of high concentrations of metals and operational problems such as reduced filter run times and high plant water losses. The currently available pre-treatment options have become inadequate to deal with the deteriorating raw water quality and this has prompted an investigation to explore the use of an alternative pre-oxidant in order to address these challenges in the raw water. An assessment of chlorine dioxide (ClO2) as an alternative pre-oxidant was undertaken. The aim was to investigate the effectiveness and economic viability of using ClO2 as a pre-oxidant as well as conditions under which ClO2 should be applied to obtain high quality water. The ClO2 was generated on site using the two chemical generation method whereby sodium chlorite is reacted with chlorine gas and the resulting ClO2 is directly injected into the raw water pipeline. A full scale plant trial was conducted in parallel with lab scale jar test experiments. The trial was conducted over a twelve month period. The operation of the generator was monitored by determining the generation efficiency and dosing adjustments were carried out based on the ClO2 demand. The water of the various treatment steps and the final water were sampled. Various parameters including NOM indicators, physicochemical and plant operating parameters as well as metal content, disinfection by-product (DBP), bacterial and algal concentrations were monitored. The two chemical generation method produced an excellent ClO2 yield of ≥96%, and the produced ClO2 was generally found to be a very effective pre-oxidant. This technology was used with very little operational interruptions and no safety related incidents were reported during the trial period. When compared with chlorine, the ClO2 pre-oxidant proved to be much more effective in the prevention of the formation of DBPs in the final water. Whereas a good algal removal rate of ≥97% was achieved during severe cyanobacterial blooms when ClO2 was used as a pre-oxidant, the algal removal rate dropped to 93% when the pre-oxidant was changed to chlorine. Compared to Cl2, a superior taste and odour removal efficiency was achieved when the ClO2 was used as a pre-oxidant. However, similar removal efficiency towards geosmin and 2-methyl-isoborneol (2-MIB) was recorded for the two pre-oxidants. Therefore, it was concluded that: (i) in addition to geosmin and 2-MIB, other unidentified taste- and odour-causing compounds were present in the raw water; and (ii) the ClO2 appears to selectively target these unidentified compounds much more effectively than chlorine. In addition, ClO2 was able to effectively remove the iron and manganese present in the raw water to below the South African National Standard (SANS) 241 limits in the presence of high levels of NOM and the unidentified taste- and odour-causing compounds. Other than leading to the formation of trihalomethanes (THMs), the application of Cl2 under such conditions has previously proven to be ineffective in the removal of iron and manganese as well as taste- and odour-causing compounds. Since ClO2 is much more expensive than Cl2, the chemical treatment cost increased by 6.8 c/kl at an average dosage of 0.8 ppm when ClO2 was used as a pre-oxidant. However, this increase seems to be offset by additional benefits such as reduction in coagulant demand and increase in treatment rates during times of severe algal blooms. To this end, an initial economic assessment points to ClO2 as a viable option for the treatment of raw water of poor quality for potable use. As evidenced by results obtained from the assessment of water quality and water treatment plant operational parameters, the application of ClO2 as an alternative pre-oxidant at the Vaalkop WTW was a success. However, Cl2 still remains the cheaper of the two pre-oxidant (Cl2 vs ClO2) and should be applied during periods of reduced organic loading when high rainfall and dam levels are experienced. Chlorine gas is also an efficient and cost effective treatment option to utilise when no taste and odour problems are experienced and low levels of iron and manganese are present in the raw water. Chlorine dioxide is definitely a pre-treatment step of choice during periods of high organic loading when reduced filter run times and high plant losses are experienced. It should also be applied during drought periods and/or low dam levels. During such periods, high algal concentrations coincide with anaerobic conditions, which is normally associated with high levels of iron and manganese contaminants in the raw water.