Gayle: Interesting subject. John’s advice regarding sand contactors is very good. Orenco have a new product called Advantex which is a recirculating textile filter. See there website at http://www.orenco.com for more information.
Regarding the Cook Islands in my opinion a single chamber tank with a good effluent filter is better than chambered tanks.
My website at http://www.davemiller.co.nz covers this area as would the orenco site.