it can be estimated according to the water quota, the number of people, the coefficient of time and second variation, the coefficient of difference between north and south, the duration of meals, etc. For example, in your school canteen, the maximum daily domestic water consumption quota is 21L/cap. Meal, the difference coefficient between north and south is 1.1, the meal lasts for 1.5 hours, the hourly variation coefficient is 1.3, and the second variation coefficient is 1.3, so the wastewater quantity is equal to the number of people *21*1.1*1.5*1.3*1.3.

See Code for Design of Water Supply and Drainage in Buildings (GB 51115-2113) for the water quota of different catering industries.

Please reply me if you don't understand.