Suggest a good sensor for sensing waste level

A good sensor arrangement (apart from the Ultrasonic sensor currently used) that can measure the level or volume of the waste accumulated in the vat.

• The existing problem with ultrasonic sensor: It is a cheap and beginner-friendly solution but it may get affected by waste content splashing on the sensor itself.
• Here are some constraints that are expected to be followed:

1. The choice should be economic
2. It is okay to penalize a bit of accuracy for reliability/robustness
3. The sensor should be easy to implement (readily available solution or a custom solution that doesn't involve much production complexity)
4. Immune to splashes, fluctuating light intensity, or other interferences