I live alone but pay the rate for multiple occupants. Is it possible to get a reduction?

It is possible to get a reduction for the water purification charge if you are a sole occupant

Note: this is not the same as single with children.

A reduction is possible if you are the sole occupant on January 1st of the tax year, but also if you become a sole occupant during the year. GBLT is automatically notified if a deregistration takes place at the municipal Registration of Persons (BRP). This happens for instance in case of death or divorce. We will send you a letter notifying you of the reduction approximately 8 weeks after this deregistration.

It is not possible to get a reduction for the water system charge for residents

This is a flat rate per property. The number of people living in that property does not affect this tax.