All Brass Monkey Ice baths utilise UV to help neutralise bacteria and viruses. The UV unit in your Brass Monkey has a bulb which will need replacing at regular periods to ensure optimum performance.

This is a replacement bulb is suitable for the Daro UV unit.

Please check you are ordering the right blub for your product type. You can check by clicking Here
This type of UV bulb should be replaced every 6 months.
Dispose of old bulbs at a local recycling centre. (edited)