I think you are taslking something like this.
http://www.polywest.ca/zoeller-fusio...atment-system/
We just had our 33 year old 650 gallon septic tank replaced. The property is on its 3rd septic field. We looked at options other than puttting in the "old-style" concrete tank, and just couldn't justify them.
1500 gallon septic tank = $9600 installed
1500 gallon fiberglass digester = $13,000 installed
2000 gallon fiberglass 3 or 4 stage digester/aerator producing virtually drinking water quality water = $20,000 installed.
We can highly recommend talking to the people who did our work and spec'ed our options. PM me if you want their contanct info.
Vic