As an author and expert on healing clays, I am frequently asked  the question, "Will taking clay baths cause build up in my septic systems?" For the answer I went to an expert on the subject, Lawrence Luecking.  
Perry A~ 

The following article should not be viewed as anything other than general guidance.

 Should I Worry About Draining  Bath Clay Into My Septic System? 
" Lawrence Luecking, 2008 

As a former designer of residences, licensed builder and one who has constructed many residences on septic systems I will opine that: if your septic was designed adequately and is working properly the heavier clay particles will never get to the leach field.  They'll settle [by their own weight] to the bottom of the septic tank compartments just as a teaspoon of clay does when you put it into a glass to drink.

Contrary to popular belief and the desires of [most of] the Septic Tank Pumping Services to extricate as much of your $ as possible while extricating septic wastes, a septic system should endure for upwards of twenty years without ever needing to be pumped.  It's a living and continuing reaction like a 'breeder' in that the bacteria consume the solid wastes and thrive on them! It's a living entity made up of trillions of anaerobic bacteria [tiny micro organisms that live and function in the absence of oxygen and light] and  consume organic waste products.  Like a human body, a Septic system does not like acidic materials and if enough coffee grounds, drain cleaning products, household cleansers and the like are emptied into the tank, you'll eventually kill these tiny friendly bacteria and the system will fail and indeed need to be pumped.  I always made it a point to advise anyone for whom I built to avoid putting such materials into the house drains and toilets and I was never aware of any system I installed ever having a need to be pumped.  I am currently aware of one system that I installed in the late seventies which still functions and has never been pumped.  
 
Now, addressing the leach field proper, IF somehow clay did get to the leach field it would eventually seep into the rock bedding materials under the lines that distribute the effluents but, based upon the relative large size of any leach field it would take many years and an incredible amount of clay to establish a membrane that would arrest the permeability of the leach field.  Since the tank itself functions on a floatation process as the solid wastes are digested by the anaerobic bacteria, clay, heavy by nature at approximately 68 lbs/cubic foot, would seem to settle to the bottom of the tank [there is no significant turbulence in the tank] and not ever get to the leach field.  Since by design the digestion of wastes is accomplished before leaving the tank proper, what goes into the leach filed is, for the most part only water.

Bentonite can, by function of its [clay] particles, create an impervious material that does not allow for solutions to seep through, and is, as such, one of the materials used to line manmade lakes and ponds to prevent it from leaking/seeping into the ground!  Again, it would take a very long time and many hundreds of pounds of clay to clog a leach field.  Contrary to the tanks, leach fields can, by nature and the local soil constituency, and what is put into them, eventually build up deposits and sometimes need to be dug up and replaced. 

In synopsis, if you do the math related to tank volume and frequency of your baths and amounts of clay used, it would take years of clay baths before significant build up would dictate tank pumping.  If/when that might happen a thorough pumping of the tank, with a special focus on reaching the bottom of the tank, would remove the majority of any clay residue.  Look at it this way, how many 5 lb bags of clay would you have to put into your septic tank to establish a layer an inch thick?  That of course, depends upon how many baths and how many cups of clay per bath, but still, a very long time: while a one cup clay bath taken weekly could generate [approximately] 26 lb of clay residue in a year, it would, at this rate, require 2.6 years to deposit a cubic foot of clay in an average 1500 gallon tank.  Since a cubic foot would occupy only 1,728 square inches [at a one inch thickness] and there are approximately 7,200 square inches of tank space on a flat bottomed concrete tank, it would take 4.16 years, to accumulate a mere inch of deposit, not in my opinion an amount that would be deleterious to the tank's function.      

" Lawrence Luecking, 2008 

Following the release of this article in a yahoo chat group, it was validated by this testimony of Darla's experience with clay and a 50 year old septic tank:

I've been giving my daughter, who is recovering from Asperger's, SID and Pandas, an average of 3 baths a week for the last year, with 3-4 cups of clay in each one. I don't use a strainer; I wash every bit of it down the drain and I haven't had the first bit of trouble. In fact, it seems like all my drains work better now than they did before clay. I used to get the drains unplugged a couple times a year for other reasons, but haven't had a problem since I started using clay.  I'm on a septic system that is 50 years old and I've used several different kinds of clays. 

Darla S.

Lawrence (Luke) Luecking, a career builder and construction consultant, began his quest for better health and nutrition in his late teens.  He is a practicing nutritionist in Colorado with successful focuses on Nutritional approaches to healing disease including Cancer, balanced body pH, brain and memory clearing, and the relationship of environmental toxins to Parasite entry into the human body, and Bio-electric medicine parasite eradication.  To contact Luke at Nutricon email wmlua06@yahoo.com