Greywater Loop
This pattern is shaped by
Problem
In a typical household, clean drinking water is used once — for a shower, a load of laundry, a sink full of dishes — and then sent to the sewer. Meanwhile, the garden is watered with more clean drinking water. The loop is open; the water passes through once and is gone.
Evidence and Discussion
Greywater — water from showers, sinks, and laundry (not toilets) — represents roughly 60% of household water use and is suitable for irrigation and toilet flushing with minimal treatment. Simple systems (gravity-fed, mulch-basin) are legal and practical in many jurisdictions.
Therefore
separate greywater (from showers, sinks, and laundry) from blackwater (from toilets) at the plumbing rough-in stage, even if the greywater system isn't installed immediately. Route greywater to subsurface irrigation for the garden or to toilet cisterns for flushing. In cold climates, route winter greywater to the sewer and switch to garden irrigation in growing season. The pre-plumbing costs almost nothing; the retrofit later is expensive.