Same answer as other post what kind of treatment plant is it? Will it require a professional to operate it?
Ranking of cost to operate (lowest to highest)
Standard spetic tanks and drain fields
Modified septic tank (blower...) and drain field
Septic tank and Mound infiltration
Below systems usually require professional oversite. Varies greatly by state.
Septic tanks and sand filter.
Septic tanks and recirculating sand filter
Septic tanks and attached media (oreco or other)
Big driver of costs are lift stations and aeration.
Professional operator could be 50 to 60% of operating costs depending on system complexity.
You noticed I left out a straight lagoon for treatment and disposal of waste water. Im sure there are a few types of systems I have left out but you get the idea.