Hi Stanley,
We've got plenty similar situations in Ireland too. What you call "Grandfathered" we probably call "Legacy Sites" - sites where there is an existing system that either has planning permission but doesn't work, or predates planning (and still doesn't work).
A cost effective solution that I've found works well on clayey soils with an existing problem like yours is to build a soil based constructed wetland between the septic tank and the drain. In Ireland the recommended size is 5m wide x 20 or 30m long, depending on how clean you want the final effluent. Just excavate the area so that you're down c.6-12" below the septic tank outlet pipe. Backfill with c.2-4" of clean, weed free topsoil, and plant with a selection of wetland plants such as Typha, Phragmites, Sparganium and Iris (or plants with similar physical characteristics that grow locally in your area). Ideally the overall wetland depth from ground surface down to the wetland base would be <1m for safety. Set the water depth using an outlet pipe on an elbow. Twist the elbow of the pipe to hold back water to a depth of c.1" until the plants are well established, and then 4" thereafter.
Legally you will probably need planning permission. We certainly do here. I know of some people who have just gone ahead and built the system without permission and although that's not legally permitted, it does have the advantage of getting the water cleaner quicker. In some cases,
local authorities have demanded that the unauthorised wetland be removed again and replaced with a mechanical system. But then again, the opposite has happened too, where conventional systems need to be replaced with a wetland...
I've another alternative that may or may not need permission. Take Travis' one piper; but instead of gravel and a perforated pipe, split a 1' couri pipe (twin walled courigated plastic pipe) down the middle to make 2 gutters. Dig 2 or 3 trenches between the tank and the polluted drain (a few inches deeper than the existing discharge pipe) and lay your gutters face down on the trench base. Cover with soil and plant willow cuttings over the top. The willows are voracious feeders and will mop up liquid, nitrogen and phosphorus. They may mop up
enough to eliminate any discharge at all, certainly in drier times of the year. The pipe set up will prevent willow
roots blocking your pipes (whereas perforated pipes would block in sort order).
Hope that helps a bit,
FĂ©idhlim