My understanding as a landlord you are to provide a maintained system. BUT
The tenant must leave the property in the same condition as it was when they moved in. This means that if there was a clean and working trap then they must leave it in the same condition.
It would be the tenants cost if they abused the system. If they overloaded the trap, used products there were advised not to use, or put foreign objects into the system.
So if your maintenance program says it should only need to be emptied every 2 years but the tenant has been in there only 12 months and its has to be cleaned out and its not working as it should. Then yep tenants expense.