4 hours. According to the official website of Zhongyi Testing, the hospital sewage treatment project must be designed and constructed in accordance with the relevant standards and specifications issued by the State Planning Commission and the State Council Environmental Protection Commission, and the pH value must be monitored no less than twice a day, every day. The interval is 4 hours, COD and SS are monitored once a week, and other pollutants are monitored at least once a quarter.