Two applications of carbon dioxide (CO2) that help improve tap water quality are experiencing a surge in usage in Japan, according to The Gas Review.
First introduced in the early 1990’s, CO2 gas is often used to adjust the pH of alkaline water when it flows from rivers or dams to water treatment plants. Depending on the scale of the treatment plant, annual usage of CO2 can be as high as 300 tonnes.
Another application that is gaining popularity, is the use of CO2 to clean water pipes that carry tap water from treatment plants to homes. 500 kg/h of pressurised gas is used to quickly clear out contaminants.
Watertech develops, designs, installs, maintains, and manages water treatment facilities for tap water and sewage, including CO2 gas injectors for water treatment facilities. Tsugio Tachibanaki, General Technical Department Manager at Watertech, says the purpose of using CO2 gas is to “adjust the PH to neutralise alkaline water.”
The pH is a numeric value between 0 and 14 that gives the basic property of a water solution. A low pH indicates an acidic solution and a high pH indicates an alkaline solution. The PH of tap water in Japan is specified in Article 4 of the Water Works Law, and states that it must not be less than 5.8 or more than 8.6. Therefore, at water treatment plants that use raw water with a high pH, CO2, sulfuric acid, or another reagent is injected into the water to adjust the pH.
The advantage of using CO2 gas for pH adjustment, Tachibanaki mentioned, is its benefits in terms of safety. He said, “you can use CO2 gas safely because even if you inject too much gas, the pH will not drop below a certain value, like it can with sulfuric acid or other reagents.” At most facilities, the CO2 is supplied to the treatment plant by installing a storage tank. For example, at a treatment plant in Kyushu, 110,000 m3 of water is treated daily, so a 15 m3 storage tank was installed. To inject the CO2, a CO2 gas injector is controlled in proportion to the flow rate. The gas is injected at the well to adjust the pH to around seven. The receiving well is located after the water intake and settling basin, which remove algae, sand, and other debris, where the flow rate of raw water is measured and fluctuations in the water level stabilise.
In reference to recent trends in pH adjustment, Tachibanaki says, “Demand to lower high pH is increasing every year.” He goes on to say, “For example, eutrophication, or the excessive enrichment of water with nutrients, of river and dam water causes algae to flourish, which results in CO2 assimilation. This in turn causes the pH of the water to rise. Also, when murky water from floods flows into a treatment plant, it increases the risks in adjusting the pH.” He predicts, “Higher average temperatures in recent years have changed the properties of river and dam water and has made it easier for floods to occur.”
The conditions that cause high-pH raw water to flow into treatment plants is expected to continue into the future. It is therefore thought that the demand to adjust pH will continue to increase. However, according to the Japan Water Research Centre, there were 5,619 treatment plants that required pH treatment in 2014, but almost 90% of them already used sulfuric acid. Many of the pH treatment plants previously installed by Watertech inject sulfuric acid or a similar reagent. This was the result of the lower installation and running costs of chemical injection facilities in comparison with facilities for CO2.
However, for future facility renewals, it is noted that safety concerns will bring about an expanding need to switch to CO2. Keio Taketo, Head of the Technology and Development section of the General Technology Department at Watertech, expects much more from the future. He says, “If you inject too much sulfuric acid or other reagent, the pH drops below the standard range, so controllers are used to adjust injected amounts automatically. However, due to aging and other factors, the number of technicians that can adjust and maintain such facilities is currently decreasing, which has increased the demand for pH adjustment with CO2 gas to ensure safety.”
The Gas Review, issue no. 438