BrianG, I am assuming that you are referring to a gravelless leach field alternative known as “a chamber system”. There are several manufacturers of this product. I have personal experience with the Advanced Drainage Systems, Inc. (ADS) product called “The Bio-Diffuser”. The ADS web site address is:
http://www.ads-pipe.com
You can read all the information regarding the chamber system on this site.
If you decide to use this product to expand your leach field, I recommend that you place a 4 foot wide layer of geotextile over the chambers to cover the louvers on the sides of the units. This will prevent siltation, or migration of the backfill soil into the void provided under the chambers. The geotextile also acts as a wick, allowing the water to “wick up” over the top of the chambers, and then into the soil. JWA