I should have said safe or considered sanitary. Here if your discharge from an aerobic system does not meet the standards for surface irrigation then your system can be condemned. While the number is not zero it does not pose a health risk. You'd have more contamination from the horse pooping in a limited area. There are restrictions where they can be placed. Same for where my brother lives and we are in two very different climates and soil types. I don't have the system but my brother does. He adds calcium hypocholite tabs for the last stage. He has to collect and test his water every year. Here if you have a well and septic (anaerobic) then they recommend testing of your well every ten years but don't require it. Sad thing is that because of our soils and no frost the systems in most areas are mounds that as soon as the inspection is done the homeowner knocks the mound down because they don't want their lawn disfigured.