My wife and I have a home on St.Croix that we keep rented. We have a septic system.
I have been reading with interest your wastewater problems. That is what I do for a living in Arizona. I run a state of the art wastewater treatment plant, and have been in the industry for many years.
Were I living in St. Croix, I would vote for the mechanical plant. Lagoons of the type that some are suggesting, do treat sewage naturally, but they require larger amounts of land that might have better uses. Also, lagoons are famous for not effectively removing fecal coliform which is hazardous to humans and animals. Add to this, the huge mosquito problem that lagoon type treatment causes, and you have sufficient reason to look towards a more controlled environment.
If possible, I would like to be put in touch with someone on the development committee. I would love to help and share my knowledge. I do plan on retiring to my home on island in 5-10 years.
Mechanical plants often use their treated effluent for reuse, such as land irrigation, or sprinkler systems. Our plant in Arizona uses no chemicals and is a biological nutrient removal plant of about 1.5 million gallons per day.