Chlorination is the most widely used method of drinking water disinfection throughout the world. However, studies have shown that chlorinated water can react with organic material and create undesirable disinfection by-products (DBPs) such as trihalomethanes (THMs). To minimize the environmental and health effects of THMs, regulations have been put into place on a global scale to reduce the formation of DBPs in drinking water applications.
In November 1988, the European Union published Directive 98/83/EC, which set quality standards for drinking water quality at the tap. This Directive led, in February 2003, to Spanish legislation in the form of Royal Decree 140/2003 that established a limit on THMs of 100 µg/l in drinking water applications, taking effect on January 1, 2009. During the transition period of January 2004 to December 2008, the THM limit was set at 150 µg/l. As a result of pending, more stringent regulations, drinking water facilities across Spain began to study, review and implement alternative, non-chlorine based, disinfection treatment alternatives such as chlorine dioxide.
The water and wastewater supply network of the Western Costa del Sol in Spain is owned and managed by ACOSOL, a public water utility that serves the needs of 11 municipalities: Benahavís, Benalmádena, Casares, Estepota, Fuengirola, Istán, Manilva, Marbella, Mijas, Ojén and Torremolinos. The Drinking Water Treatment Station of River Verde receives water from two sources: the La Concepción reservoir, which collects water from four rivers and has a maximum operating volume of 56.91 cubic hectometres, and a desalination plant with an annual production capacity of 20 cubic hectometres.
Over a one-year period, 2007-2008, ACOSOL monitored the concentration of total THMs in their supply network. At this time, ACOSOL was using gas chlorination as the primary form of disinfection treatment. Nearly 20 percent of the analyses indicated THM values close to 150 µg/l and 35 percent of the analyses indicated values above the proposed new limit of 100 µg/L. The average values of the analyses indicated a THM level of between 103 µg/L and 105.5 µg/L – levels that complied with the transition THM standard in 2008 but would fail to meet the new THM limit that would take effect January 1, 2009.
As a result, ACOSOL entered into a three-part study to identify and implement a disinfection treatment solution that would reduce THM concentrations to comply with the new 100-µg/L limit.
Chlorine dioxide as chlorine replacement
Prior to undertaking the study, three characteristic sampling points were determined:
- the treatment outlet: analysis of water taken from the regulating reservoir, where waters from the treatment station and from the desalination plant were mixed
- Forest Hill: a 315-m³ reservoir serving the utility's western region
- Toro III: a 25,000-m³ reservoir serving the utility's eastern region
THM analyses were carried out by external laboratories accredited according to UNE-EN-ISO/IEC 17025 for laboratory analysis by the national accreditation body ENAC and authorized for the Ministries of Health and Agriculture of the local government in Andalucía.
Phase one began in April 2008 and ended prior to December 2008, and it included the introduction of chlorine dioxide as an oxidant/disinfectant to the pre-treatment oxidation stage prior to settlement. Chlorine was still used as the primary disinfectant and dosed to maintain a free residual chlorine level of 1.2 ppm, ensuring adequate disinfection at the point of supply. The introduction of chlorine dioxide at this stage reduced the precursors of THMs by up to 30 percent. Results showed a decrease in the formation of THMs at the treatment outlet since chlorine was not injected during pre-treatment and the contact time of disinfectant chlorine in the treatment station was reduced compared to when chlorine was used as the sole disinfectant. However, in the supply network the chlorine precursors' contact time remained the same as under previous conditions, resulting in higher levels of THMs formed than at the treatment station outlet.
Phase two began in December 2008, ended in March 2009 and included the introduction of chlorine dioxide into the distribution network as chlorine was gradually withdrawn. There were periods of instability in the system as adjustments were made during the four months to achieve the proper dosing requirements of chlorine dioxide for disinfection and residual maintenance in the supply network. Chlorine dioxide doses were increased from 0.3 ppm to 0.7 ppm with an accompanying chlorine adjustment up to 1 ppm. Once the demand for chlorine dioxide in the distribution network had been exceeded, chlorine dioxide doses were reduced to 0.5 ppm. When the chlorine dioxide had been exceeded and stabilization had been achieved in the presence of chlorine dioxide in the network, a significant reduction in total THMs was observed compared to the use of chlorine only. The reduction was estimated at greater than 90 percent at the treatment station outlet and 80 percent in the distribution network. In all cases, the THM values were at a maximum of 30 µg/L. In addition to lowering THM values, the increased chlorine dioxide dosing enabled a pH reduction from a range of 8.1 to 8.3 to a value of 7.5.
Phase three began in April 2009 and ended in August 2009. It included the introduction of a constant and stable dosing of chlorine dioxide at 0.3 ppm accompanied by chlorine up to 1 ppm. The use of chlorine dioxide continued both in oxidation/pre-treatment and in disinfection so that a residual amount was obtained at all points of the supply network. THM formation from June until August reached a high of approximately 30 µg/L. It was also determined that the dosing of chlorine dioxide in disinfection facilitated a reduction in the concentration of disinfectant in the network. At the treatment station outlet, the combined concentration of chlorine dioxide and chlorine was 1 ppm, compared with a concentration of 1.2 ppm of chlorine that was previously necessary in order to ensure the existence of disinfectant at the end of the network.
The use of chlorine dioxide as a disinfectant resulted in chlorite and chlorate by-products at a maximum proportion of 60 percent and 30 percent, respectively. The residual chlorite value at any moment throughout the process was determined to be below the legislatively mandated value. Only when the dosage of chlorine dioxide exceeded 1.1 ppm did values of sodium chlorite approach the legislated value of 0.7 ppm.
Using chlorine dioxide as a disinfectant, ACOSOL complied with Spanish and European THM standards and could even comply with the standards of more demanding countries, where legislation for a maximum level of total THMs is 30 µg/L.
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