How do septic systems fit into LEED?
You pose an interesting question that gets to how products and technology are treated by the various LEED rating systems.
Firstly, as you may be aware, LEED and the U.S. Green Building Council do not certify products. Informed manufacturers use terms like “can help contribute toward earning up to five LEED credits” in their marketing materials.
LEED commercial rating systems, in general, provide points for overall performance improvement over a baseline. Therefore, your septic system, if it can be utilized in a commercial setting, could contribute toward achieving credits by cumulatively adding to overall landscape water savings (WE 1.1 & 1.2) and wastewater conveyance reductions (WE 2). If your system and local code also allowed a building owner to re-use the output water for non-potable uses inside the building, such as for toilets, urinals and mechanical systems, then it could contribute toward earning points for indoor Water Use Reduction (WE 3.1 & 3.2).
LEED for Homes differs from the commercial rating systems in that it actually does award points for utilizing specific technologies, most visibly in Environmentally Preferable Products credit (MR 2), but also in Graywater Reuse System (WE 1.2). The question is whether the system you describe would be interpreted as a graywater system and overcome the specific exclusion in the credit regarding septic systems. If the system you describe involves a holding tank such that the treated water is used by the irrigation system when irrigation is actually needed, then it could be argued that it meets the intent of the credit or even exceeds it. Traditional graywater systems involve isolating shower, lavatory sink and clothes washer water from blackwater (toilets and kitchen sinks), then treating it for re-use. Your system seems to be capturing and treating both, though you do not state the treatment quality of the output water.
The best way to find out if the system would earn points is to ask one of your customers who has registered for LEED for Homes certification to submit a Credit Interpretation Ruling through their LEED for Homes Provider. The Technical Advisory Subcommittee will then make a decision based upon the water-saving merits and technique versus the credit intent. They are the authority.
Another interesting note is that if the drip system fully serves at least 50% of the landscape planting beds of a home, that home would earn a point under High Efficiency Irrigation System (WE 2.1).
You also do not mention if the drip system is aboveground or subterranean. Most jurisdictions do not allow graywater to be used aboveground or applied on any food-producing plants (E. coli potential). It should never be sprayed. As the system you describe is actually treating blackwater, these principles take on much more importance. Since codes vary, it is always important to refer to the local code when researching graywater opportunities.