You are the operator of a 3,000-gallon health club spa with an average of 100 users per day. How often should the water in this spa be changed?

The correct choice for how often the water in the spa should be changed is based on the understanding of bather load and water capacity. In facilities such as health clubs, maintaining water quality is crucial due to the high volume of users, which can significantly affect the spa's water chemistry.

A commonly accepted guideline for changing water in spas is to consider the bather load. For a spa with frequent use, it’s advisable to change the water approximately every 5 to 10 days, depending on factors like usage, water treatment, and filtration efficiency.

In this scenario, the 3,000-gallon capacity with an average of 100 users per day suggests a higher demand on the water quality, making a water change every 10 days a reasonable frequency to ensure that the water remains clean and safe for users. This timeframe allows for effective maintenance of water hygiene and reduces the risk of bacteria or chemical imbalances arising from consistent heavy use.

Thus, selecting 10 days for the water change aligns with best practices concerning public health standards and the operational needs of a health club spa.