Introducting wastewater/sanitation tariffs for the first time

For a project on a small island state of 160,000 people , where they are considering introducing sewerage tariffs for the first time, I am investigating the following:  - Generic (flat rate) tariff or tariff based on quantity- How to set up a tariff for septic tank discharges (pay per time or other option? )- What the timeline would be to set up and implement  – i.e how long does it take to implement this and;- How long does it take to start seeing a return after introducing tariffs? Has anyone here worked on something similar? Your feedback is much appreciated.   w