Turning Pool Water into Safe Drinking Water | February 2022 | Industry-Academia-Government Collaborative Journal

First, I considered where to get the raw water to purify. Well water, spring water, river water, fire prevention water, etc. were suggested as candidates, assuming that the water supply is cut off due to an earthquake or flood. However, DUV-LED sterilizers can inactivate microorganisms that are biological hazards, but they can hardly remove harmful organic substances, compounds, and metals. Even if an activated carbon column is used for raw water pretreatment, there is a limit to the removal of contaminants. After consulting with Laurel Co., Ltd., a comprehensive water treatment manufacturer that is a member of this project, we decided to use the school's pool water as safe raw water. Schools are often designated as evacuation sites for local residents and are originally safe tap water. The collected water is contaminated with feces from animals and birds, and there is concern that pathogens may enter humans. DUV-LED is good at killing pathogens.

Therefore, Laurel was in charge of designing and manufacturing a prototype of a DUV-LED filtration device (a device that combines an activated carbon column and a DUV-LED sterilization device), and Kinki University was in charge of securing the pool water used for the purification test. . The Kinki University School of Applied Physics promotes industry-government-academia collaboration, and is also collaborating with Kinokawa City, Wakayama Prefecture, where the campus is located. When I consulted with Kinokawa City about pool water, it was decided that they would be able to provide pool water from two schools in the city that were closed and an elementary school pool water used by children. In the event of a disaster, chlorinated pool water that can be used for swimming may not necessarily be used as raw water. How many bacteria are there in pool water? How much does the pollution state of pool water change in spring, summer, autumn and winter? In order to purify pool water to a drinkable level, it is necessary to meet the water quality standards for tap water established by the Ministry of Health, Labor and Welfare. Standards of concern for pool water are the general viable count (100 CFU/mL or less, where CFU is the unit of viable count and the number of microorganisms that can form a colony on a standard agar medium) and E.coli (E.coli in 100mL). not detected). Therefore, while waiting for the completion of the prototype, I decided to investigate seasonal changes in pool water quality with my students.

The three schools surveyed are an active elementary school (School A), school T which has been closed since 2011, and school M which has been closed since 2005 and closed in 2019 (Fig. 1). School A is located in the city of Kinokawa City, School T is located on the mountainside surrounded by orchards and villages, and School M is located along a mountain stream. The pool water of School T and School M, which had not been replaced for a long time, was dark green and looked like a pond. Every month, Mr. Nishikawa from the Regional Revitalization Division, Planning Department, Kinokawa City, took me to three schools, sampled pool water, and examined the number of general viable bacteria and the presence or absence of coliform bacilli and E. coli. Coliform bacteria (aerobic or facultative anaerobic bacteria that degrade lactose with Gram-negative short bacilli; also includes E. coli) are used as food hygiene indicators. Some live in the natural environment, not just in the gut, and are not necessarily indicators of faecal contamination. Therefore, tap water quality standards require that E. coli be negative, not coliform bacteria. As a result of the survey from July 2019 to June 2020, although the number of general viable bacteria was kept low in the pool of School A, coliform bacteria and E. coli were occasionally detected. At the last water sampling in June 2020, the general viable bacteria count exceeded the tap water standard of 100 CFU/mL even in the School A pool. Normally, the pool is cleaned in June, tap water is filled and reset to a swimming state, but in 2020, the pool instruction was canceled due to the prevention of the new coronavirus infection, and it was neglected for a year. It was thought that the water quality of the school A pool had deteriorated. In many cases, the number of general viable bacteria in the pool water of School T and School M was at a high level exceeding the tap water standard throughout the year.

In the fall of 2019, the prototype of the DUV-LED filtering device shown in Fig. 2 was completed. In this device, raw water is first pretreated with an activated carbon column and then purified by passing through a DUV-LED water sterilizer. The water is designed to flow from the clarification tank to the raw tank so that it can pass through the sterilizer multiple times. Immediately, 100 liters of high-quality pool water from School A was pumped up in a plastic tank, transported to the university campus, and a purification test was conducted. As a result, by passing the raw water through the sterilizer once, the general viable bacteria count decreased from 21 CFU/mL to 0 CFU/mL, and we were able to obtain purified water in which general viable bacteria were completely killed. Next, the pool water from School T, which had poor water quality, was pumped up and a purification test was conducted. In the case of School T, passing the raw water through the sterilizer once reduced the general viable count from 164 CFU/mL to 10 CFU/mL, and circulating the water and passing it through the sterilizer twice resulted in 0 CFU/ Sterilized to mL. However, the positive E. coli turned negative after the first passage, so even if the water quality of the T-school pool was used as the raw water, the general viable count of E. coli was reduced after only one passage through the sterilizer. It was also found that purified water meeting the tap water standards can be obtained.