Can we install an aeration unit in an older septic system?
The airated water treatment systems (AWTS) are a two tank system usually new ones have a inner and outer tank in the same unit. It is possible to buy a two tank seperate system and you could use your old septic tank as the first tank then run the outlet into the awts unit. http://econocycle.com.au/index.php?option=com_content&amp;view=category&amp;layout=blog&amp;id=7&amp;Itemid=13. an example of a two tank system is on this page.

Although all of this is possible if your dispersal field has failed it will still need replacing as this system only Improves the quality of effluent leaving the tank. The other option may be to connect to the council sewer if it is now available in your area?  you can run the sewerage straight to sewerif avalible or install a maceration pumpIin your septic and pump upto the sewer if it is nearby but you are unable to get fall.