Policies

Water Production : Chlorine Re-dispersion Facility

Date 2015-06-25 Category Water Supply (Arisu) Updater scaadmin
Writer
Seoul Metropolitan Government
Date
2015-06-25
Last Update
2016-12-19

Introduction

Reduction of disinfection odor in tap water
 
Tap water chlorination is a necessary step to ensure the hygiene and safety of tap water since it destroys harmful microorganisms; note, however, that it may also add an unpleasant odor to the drinking water. Before, most of the chlorine was usually added in purification stations; with the installation of hypochlorous acid producers, however, chlorine can now be added in multiple steps. In other words, only the minimum amount of chlorine was added at the purification plant, with a minimum amount added again at the water supply reservoir. This system, called “Chlorine Re-dispersion,” can significantly reduce unpleasant odor and related civil complaints. It also ensures that a sufficient amount of residual chlorine is maintained in areas located at the far end of the water pipe network. With the system, the same level of residual chlorine (0.1 ~0.3 mg/L) is maintained all throughout the water supply system while providing safe, odor-free water to our citizens.

Overview of the Policy

<< Installing a disinfection facility that forms hypochlorous acid at 18 reservoirs to allow multiple chlorine injection in the distribution system
<< Reducing the strong chlorine odor near purification plants to address civil complaints and consequently increase the tap water drinking rate
<< Ensuring safety from microorganisms in areas located at the end of the water pipe network by maintaining the proper level of chlorine

Background

In the past, the main objective was to ensure the safety of drinking water by adding a substantial amount of chlorine in purification plants to kill microorganisms and bacteria. The side effect, however, was the too strong odor of chlorine in water, especially near purification plants; thus making people hesitant to drink the water or prompting them to lodge civil complaints. On the other hand, households located at the end of the water pipe network were provided water that did not have enough residual chlorine; thus raising concerns of the disinfection and safety of the water they are drinking.

Process of Policy Implementation

To reduce chlorine odor near purification plants and to provide the proper chlorine level to distant places, the “Chlorine Re-dispersion System” was formulated in 2009 and pilot-run from 2010 to 2011 in 3 reservoirs. Based on the analysis of the pilot results, from 2012 to 2013, 14 reservoirs adopted the system, bringing the total to 17.

Details of the Policy

Installing a disinfection facility that forms hypochlorous acid at 18 reservoirs to allow multiple chlorine injections in the distribution system
Chlorine liquefied under high pressure is used; since disinfectants purification plants require safety devices and other protection facilities as per the High-pressure Gas Safety Control Act, which inevitably incurs higher costs such as hiring certified safety managers.
To reduce such high cost, we replaced high-pressure liquefied chlorine with equipment that generates hypochlorite through the electrolysis of NaCl. The system was first pilot-tested in 2009 at 1 site; after thorough analysis of the effectiveness, 17 reservoirs adopted this system by 2013. The system was complemented by electronic communication that enables remote controlling and monitoring system.

Know-how & Insights

Using chlorine at purification centers incurs additional cost of safety control and specialized labor. In contrast, a system in which hypochlorite is generated through the electrolysis of NaCl reduces safety-related costs and extra labor needs.
Even better, through the electronic communication control system, operation can be monitored and remotely controlled at each Waterworks branch.

Policy Outcome & Evaluation

We were able to reduce the chlorine concentration of water by 10.3% (from 0.58-→52mg/L) at purification plants; this translates into a 21.1% reduction of the chlorine concentration level (0.33→0.26mg/L) at faucets in homes near the purification centers. This substantially eliminated the disinfectant odor to the extent that people do not detect the unpleasant odor at all. For distant areas where the residual chlorine level was too low, hypochlorite was added to keep it at least above 0.1mg/L of chlorine concentration at the faucets.

Applicability of the Policy

This system is absolutely necessary for cities with long water pipe network.

Q&A

Why did Seoul set the proper residual disinfectant level at 0.1~0.3ml/L when the maximum chlorine concentration allowed in drinking water is 4.0 mg/L?
Though differing among individuals, various tests including blind tests and satisfaction survey find that people hardly detect disinfectant odor when the chlorine concentration level is below 0.3mg/L. Note that 0.1mg/L is the level required to protect water against bacteria and microorganisms.