Skill guide · Sanitation & Hygiene

Greywater Disposal Basics

The Sanitation & Hygiene chapter's addendum calls for a real greywater plan once a no-power, no-sewer sanitation setup is running, but the manual itself doesn't cover the technique. This guide covers which household water actually counts as greywater, the simple underground disposal method that keeps it from becoming a contamination or mosquito problem, and which soaps are actually safe to use with it.

Not all wastewater is the same

Greywater is the relatively lightly contaminated water from bathroom sinks, showers, tubs, and clothes washers. Toilet waste is blackwater and needs the dedicated treatment covered elsewhere in this chapter, not this guide. Less obviously, kitchen sink water is also effectively blackwater: it carries emulsified fats that clog soil pores and food particles that attract flies and rodents, the same vector problem this chapter's waste-management guidance exists to prevent. Route kitchen water to the same disposal system as toilet waste, not the lighter greywater system this guide describes.

Why greywater can't just run onto the surface

Greywater carries real pathogens, from skin, trace fecal contamination in shower and laundry water, and general household bacteria, even though the concentration is far lower than blackwater. Left standing on open ground, it also breeds mosquitoes and can carry contamination toward a well, garden bed, or neighboring property. The fix isn't complicated: get it below the surface and let soil do the filtering work, rather than letting it pool or run off visibly.

The mulch basin: the simplest system that actually works

A mulch basin is a shallow depression, dug below grade, filled with coarse woodchips or bark and connected to a greywater outlet by a short length of pipe. A single basin is commonly dug 2 to 4 feet across and 6 to 18 inches deep, and one basin of that size can handle roughly 40 to 80 gallons of greywater a week, in clay soil, figure on about 1 square foot of basin per gallon discharged per day; sandier, faster-draining soil needs less. The water disperses into the mulch layer and soaks down into the soil beneath, rather than pooling on top; the coarse organic material supports the aerobic bacteria that do the actual pathogen breakdown, and it also controls odor. Run the outlet pipe (1-inch inside diameter is standard for a washing machine line) into the basin and keep it covered by at least 2 inches of mulch, rock, or soil, an exposed outlet just creates a wet, visible pool right at the pipe's end instead of dispersing the water properly. Because most of that biological activity happens near the surface, a well-built mulch basin can process greywater more effectively than a much larger and more complex system.

Site the basin at least 10 feet from the house's foundation and any property line, and at least 50-100 feet from any well (check your specific local health department's setback, since these vary by jurisdiction and soil type), well downhill or away from any spring or garden bed growing food that gets eaten raw, and never let outflow reach standing water or a shared water source. Rotate between two basins if the household is generating meaningful volume day after day, giving each one a rest period to fully drain and re-aerate.

Warning

Never spray or pour greywater directly onto food crops eaten raw

Subsurface dispersal into a mulch basin or drip line is the safe method; watering vegetable leaves or root crops directly with greywater risks depositing pathogens onto food that won't be cooked before eating. Water ornamental plants, trees, or fully cooked-crop gardens this way instead, or route it well clear of any food garden entirely.

A sick household member's water carries a real, elevated pathogen load

Under normal aerobic conditions, pathogens in dispersed greywater degrade to safe levels within roughly three days, but that figure describes ordinary household greywater. Someone with active gastroenteritis (norovirus especially) sheds pathogens in very high numbers, and their bath and laundry water measurably carries more of that load than the rest of the household's. If a basin system has two basins in rotation, route that person's water to whichever basin is currently resting rather than the one actively receiving everyone else's water, and give it real extra rest time, closer to a week than three days, before rotating back. This is the same logic this chapter's cloth-wipe guidance already applies to a sick person's laundry: isolate it, don't just dilute it into the normal system and assume the usual timeline still holds.

Which soaps are actually safe for this

Not every soap or detergent is greywater-compatible. Look for biodegradable, phosphate-free, low-sodium, pH-neutral products; a simple liquid castile soap (olive oil, coconut oil, water, and potassium hydroxide) is a reliable, widely available option. Avoid anything containing bleach or chlorine, borax, petroleum distillates, or products marketed with "whitener," "softener," or "enzymatic" claims, since these either damage soil structure over repeated use or interfere with the bacterial breakdown the system depends on.

Pathogens don't linger forever in healthy soil

Under normal aerobic conditions, pathogens in dispersed greywater degrade to safe levels within roughly three days. This is exactly why the mulch basin's aerobic, well-drained design matters: a waterlogged or anaerobic basin doesn't break contamination down nearly as fast, another reason to keep the system draining freely rather than overloading a single basin.

Quick Reference

SourceTreat asNote
Bathroom sink, shower, tubGreywaterRoute to mulch basin or subsurface disposal
Clothes washerGreywaterUse a greywater-safe detergent (see above)
Kitchen sinkBlackwaterRoute with toilet waste; fats and food scraps attract vectors
ToiletBlackwaterCovered by this chapter's bucket-toilet/composting guidance

Full checklist:

Sources