Greywater and Composting Toilets: The Sanitation Questions Everyone Asks

Photo by Magda Ehlers

A washing machine finishes its rinse on a Saturday morning, and the water in the drum is warm, faintly soapy, and headed for the sewer. In one state, sending that water under the mulch of a garden bed is legal with no permit at all, as long as thirteen written conditions are met. In another state, putting the same untreated water on the ground is simply not legal. Same house. Same washer. Same water. The difference is not the plumbing. It is the rulebook, and the rulebook changes at every state and provincial line.

That patchwork is why sanitation questions fill off-grid forums. People ask whether greywater can be stored (it cannot, not for long), whether a composting toilet replaces a septic system (in most places, it does not), and whether the toilet's output comes out as finished compost (a manufacturer says plainly that it does not). This article reads the code text, the makers' own manuals, and the failure reports, and answers the questions in that order: what the law says, how the systems work, and what goes wrong when the two are ignored.

What greywater is, and what it is not

Greywater is the used water from showers, tubs, bathroom sinks, and the clothes washer. What it is not matters as much. Toilet water is black water everywhere. Kitchen sink and dishwasher water sit in a contested middle: many codes exclude them from the greywater definition entirely, because that stream carries grease, food solids, and bacteria from raw meat. Austin's water utility classifies kitchen sink and dishwasher water as black water outright. Arizona's legal definition of greywater names the washer, tub, shower, and bathroom sink, and the kitchen sink is not in it.

How much water are we talking about? There is no single honest figure, and any guide that prints one without a source should be read with care. Washington State's Department of Health puts greywater at up to 40 US gallons per person per day and calls it the largest part of household wastewater. A measured Greek household study found about 26 gallons per person per day. A water-conserving household runs far lower; the worked example in Brad Lancaster's rainwater-harvesting material counts 6 gallons for a shower and 2 for a bathroom sink per person. The fixtures decide. For planning, the range of roughly 25 to 40 gallons per person per day in a typical North American home is the sourced version, with the conserving household well below it.

The two rulebooks worth reading first

Arizona runs the permissive model. Under its administrative code (R18-9-711, recodified as R18-9-D701), a private residence may reuse its own greywater at under 400 gallons a day with no permit, because the rule itself acts as a general permit. The conditions travel with the permission: the water stays on the property, serves household gardening or landscape irrigation, avoids human contact, and carries no hazardous chemicals and no water from diapers or infectious laundry. Surface application is by flood or drip only. Spray irrigation is prohibited. Surface-applied greywater does not go on food plants, with a narrow exception for citrus, nut trees, and other plants whose edible part never touches the water. And the moment a system blocks or backs up, the permission to use it stops until it is fixed.

California's laundry-to-landscape exemption is narrower and more exact. Plumbing Code Chapter 15 (§1502.1.1) waives the construction permit for a system fed by one domestic clothes washer in a one- or two-family home, and only while every condition holds. The heart of it is a labelled, easily reached three-way diverter valve, so the flow can be sent to the sewer instead of the garden. No pump may be added. The water stays on the site where it was generated. It cannot pond or run off. Where it reaches the surface, it must sit under at least 2 inches of mulch, rock, or soil, or under a solid shield. Diaper water and hazardous-chemical water go to the sewer, full stop. Builders in California call it "no permit, thirteen conditions," and the second half of that phrase is the half that matters.

At the far end of the patchwork sit places like Nebraska, where the state says flatly that discharging untreated greywater is not legal, and reuse means a treated, permitted, engineer-designed system. Pennsylvania's environmental department, as reported in a Permaculture Podcast account of its conversation with the permits office, does not distinguish grey water from black water at all. The article cannot give a reader a national answer, because there isn't one. The first step in any sanitation plan is reading your own jurisdiction's page, before a single pipe is bought.

The 24-hour rule is written law

Ask experienced greywater designers what kills systems and the answer is storage. Washington's rule says it in code language, WAC 246-274-200(1)(g): "If the greywater is stored, it may not be stored for more than twenty-four hours." EPA Victoria's publication 812.2 states the same limit in nearly the same words, and Queensland's guidance repeats it. The mechanism is not mysterious. Greywater carries skin cells, soap, and food for bacteria. Held in a tank, the bacteria multiply, consume the oxygen, and the water turns anaerobic and septic. The rotten-egg smell owners report from a surge tank is not a nuisance layered on top of the problem. It is the problem, announcing itself.

This is why Art Ludwig's branched-drain design, the reference system in his book Create an Oasis with Greywater, has no tank at all. Washer water splits through a series of double ells and runs by gravity straight out to mulch basins, doughnut-shaped rings of soil with wood-chip mulch in the centre and the plant's root crown kept on its own dry island. The water is used the hour it is made. The safest storage, in that design philosophy, is none.

Under the mulch, never through a sprayer

The three rulebooks agree on delivery even where they agree on little else. Greywater goes under mulch, under soil, or out through drip lines at the root zone. It never goes through a sprayer. Arizona bans spray irrigation of greywater outright. California allows surface release only under its 2 inches of cover. Washington's whole programme is written around subsurface irrigation. The reasons stack: spray puts droplets in the air where people and pets can meet them, it wets leaves and paths instead of roots, and it spreads the water across surfaces that dry into contact zones. Under mulch, soil microbes do the treatment work, and nothing pools where a child can step in it.

A hose fanning a sheet of water over a vegetable bed is ordinary garden watering with clean water. As a picture of greywater delivery, it is exactly what the rules prohibit, and the distinction is worth keeping in front of every reader who sketches a system on a napkin. Greywater design is mostly the discipline of keeping used water low, covered, and at the roots.

The kitchen sink stays out

An earlier article in this series called kitchen water the problem child of greywater. The codes explain why. Kitchen wastewater carries grease and fats that coat soil and pipes, food particles that feed the fastest bacterial growth of any household stream, and bacteria from raw meat preparation. New Zealand's building guidance reports kitchen wastewater was found unsafe for reuse for exactly that bacterial reason. A widely used Australian summary warns that kitchen and dishwasher water can carry more pathogens and pollutants than raw sewage. That is why so many definitions quietly leave the kitchen out: the stream is not regulated as greywater because, legally, it is not greywater. A household that wants its shower and laundry water in the garden still sends the kitchen sink to the septic or sewer, and designs the plumbing split before the walls close.

Detergent is the whole ballgame on the plant side

Greywater law decides whether the water may leave the house. Detergent chemistry decides whether the garden survives it. Three ingredients do the damage, and University of California guidance names them plainly. Sodium builds up as salt in the soil and blocks plants from taking up water; powdered detergents carry more sodium filler, and water-softener backwash can make greywater unusable. Boron, sold as borax, borate, or sodium perborate, is toxic to plants in small amounts and accumulates in soil instead of washing through. Chlorine bleach kills the soil microbes the entire treatment system depends on; the standard practice is to divert bleach loads to the sewer and reach for oxygen bleach instead.

The label word that matters is "biocompatible," not "biodegradable." Plenty of products break down eventually and still salt the soil on the way. The sources name greywater-designed or suitable brands, Oasis, ECOS, Bio Pac, Vaska, and Dr. Bronner's castile soap among them, but the label check outlasts any brand list: no sodium build-up, no boron, no chlorine, and antibacterial soaps, solvents, dyes, and softeners left on the shelf.

Three toilet families, and they overlap

The outdoor toilet is older than every product in this section, and the modern units are best understood as three families arranged around one problem: separating what rots from what smells, without a flush. Self-contained units put the toilet and the treatment chamber in one bathroom fixture; Sun-Mar's Excel, with its rotating drum, is the classic example, and Nature's Head sells a self-contained unit that also diverts urine. Central or remote systems put a simple seat in the bathroom and a larger composter below the floor or in another room, Sun-Mar's Centrex and the Clivus Multrum line. Urine-diverting dry toilets split the stream at the bowl: urine runs forward to a bottle or pipe, solids drop dry behind it, often into a removable container or bag, as in Separett's Villa. The families overlap, and any chart that pretends otherwise is tidier than the products are.

Prices, from the makers' own listings checked in November 2026: Nature's Head lists its models at $965 US. Separett lists the Villa at $989 US and the Villa Extend at $1,049. Sun-Mar lists its non-electric Excel NE at $1,825 US; its electric Excel's price was not captured from the maker's own site in this research, and retailer listings ran roughly $2,000 to $2,229, so treat that range as a retailer range, not a maker price. A secondary market summary from Angi puts self-contained units broadly at $600 to $2,500 and central systems at $2,500 to $5,000.

What "composting" actually requires

Real composting is a recipe: oxygen, moisture near 50 to 60 percent, a carbon-to-nitrogen ratio around 30 to 1, warmth, and time. Human waste on its own runs about 10 to 1, and urine is richer in nitrogen still, which is why carbon bulking material is not an accessory in a true composting unit. It is half the recipe. Sun-Mar's own manual gives the moisture test as "damp as a squeezed-out sponge, about 50%."

A household chamber rarely runs hot. The pathogen benchmark for composting, from the US Environmental Protection Agency, is 55 °C (131 °F) held for three days in the vessel, and a cabin toilet does not reach it. So what leaves a small unit is partly broken down, dried material that needs months of secondary composting somewhere else. Separett prints the honest sentence on its own product page: "the waste is not composted when you empty the bucket. Human waste should be composted for at least 6 months, preferably 12 months." Plan the second stage before buying the first. A household that has nowhere legal and safe to finish the composting has not solved the problem; it has moved it to a bucket.

The vent is the odour system, and the fan is nearly free

Every composting and dry toilet lives or dies on its vent pipe. Moving air carries moisture and odour out and keeps the flow at the seat going the right way. The surprise is how little power that takes. Nature's Head rates its 12-volt fan at 0.07 amps, about 0.84 watts, running continuously. Separett rates the Villa's fan at 0.06 kilowatt-hours per day. Sun-Mar's non-electric Excel NE has no fan at all; its 4-inch vent works as a chimney and must run 2 to 3 feet above the roof peak, and its optional 12-volt fan draws 1.4 watts. The outlier is the heated electric drum unit: retailer specifications put the electric Excel's fan-and-heater draw around 150 watts on average, which is a genuine load on a small solar system and belongs in any power budget that includes one.

Cold draws the family line again. Sun-Mar's manual puts the best composting band at 70 to 100 °F (21 to 38 °C) and warns that below about 55 °F (13 °C) decomposition slows sharply or stops; Cottage Life's troubleshooting puts working bacteria stopping below about 5 °C. In an unheated winter cabin, a composting unit becomes a holding tank, and the drum should not be turned on frozen contents. The urine-diverting design sidesteps the biology: Separett states the Villa has no room-temperature requirement, because diversion and drying do not depend on microbes. Its winter warning moves to the plumbing instead, with an outdoor urine pipe needing at least 1½ inches of internal diameter and insulation against mineral build-up and freezing. One Alaska builder, pricing a composting toilet for an intermittently heated cabin, reached the same conclusion from the wallet side and built a vented pit outhouse instead: heating a bathroom all winter so a toilet can compost is a strange bargain.

What NSF/ANSI 41 certifies, and the catch nobody prints

The certification jurisdictions look for is NSF/ANSI Standard 41, for non-liquid saturated treatment systems. NSF describes the testing plainly: a certified system is verified over extended testing, including occasional overload, to handle its stated capacity, produce no offensive odours, and produce output meeting required bacterial-content levels, with its labelling and advertising checked and field testing run alongside the lab work. Sun-Mar's self-contained units are NSF-listed. No NSF/ANSI 41 certification was found in this research for Nature's Head or for the Separett Villa, and their acceptance rests on local rules for urine-diverting and dry toilets instead. That is not a verdict on how the units work. It is a warning against assuming a certification a product page never claimed.

The catch sits one level up. In much of North America, a composting toilet is accepted as an accessory to an approved onsite wastewater system, not a replacement for one. Humboldt County's wording, quoted in legal explainers, says exactly that. Indiana's guidance states a composting toilet "is not a substitute for an on-site sewage system." Pennsylvania requires an onlot system for washwater alongside the toilet in a new residence. The logic is simple and it closes this article's loop: the toilet answers the toilet question only. The shower, sink, and washer water, the greywater this article began with, still needs its own approved destination. A sanitation plan that solves one stream and forgets the other is half a plan.

The failures owners actually report

The greywater failures are plumbing failures with a theme: nowhere for the load to go. A homeowner on the DoItYourself forum ran a garage washing machine into a drywell of drywell block; it worked about two years, then backed up and overflowed with thick black water that would not drain. The thread's diagnosis was lint. There was no filter, synthetic fibres do not digest, and the soil sealed. Greywater Action's cold-climate guidance reads the same way from the north: any pipe holding standing water can freeze and burst, pumped systems need drain-back, and many owners simply divert to the septic for the winter. InspectAPedia records the frozen version of the drywell story, a shallow drywell freezing solid and backing greywater up into the house line. The diverter valve California demands exists for precisely this moment.

The toilet failures start with use, not hardware. Cottage Life's troubleshooting, with EcoEthic's Rob Davis and Sun-Mar's Fraser Sneddon, opens with the verdict that composting-toilet problems tend to start with what the user puts in, not the hardware itself. The fixes are rebalancing toward that 30-to-1 carbon ratio, restoring moisture to the squeezed-sponge mark, and keeping out the cleaners that kill the microbes and the non-degradables owners admit to finding. Wrong bulking material is its own failure class: sawdust clogs systems built for shavings, treated timber can poison the microbes, and Separett reverses the rule entirely and instructs its owners to add no bulking material at all, because the fine dust can damage its fan. Flies have a maker-published remedy, 5 cups of natural diatomaceous earth stirred into the compost on Nature's Head's Australian troubleshooting page, which also separates the smells owners confuse: musty is normal; sewage means too wet, often because someone used the solids section as a urinal. And fan failure is odour failure, the troubleshooting guides agree, with one cause owners miss: a bathroom exhaust fan can compete with the toilet's vent and pull odour into the room.

One failure has no owner's story attached, and the article will not invent one. Separett ships a Bioblock sample with the Villa "to prevent blockage in the internal urine hose" and warns that minerals accumulate in urine pipes. That is a manufacturer acknowledging a failure mode in its own packaging, which is evidence of the mechanism, honestly labelled, and no more than that.

The expensive shortcut, briefly

Incinerating toilets burn waste at roughly 1,000 to 1,400 °F and leave about a tablespoon of sterile ash per use. Retailer listings checked in November 2026 put the Cinderella Comfort electric at $5,600 Canadian, drawing 0.8 to 2 kilowatt-hours per incineration, and the Cinderella Freedom propane model at $5,990 Canadian, using roughly 140 to 190 grams of propane per flush, with running costs estimated at $150 to $400 a year. Secondary round-ups put electric models broadly at $2,000 to $4,500 US and propane at $2,500 to $4,800. No plumbing, no compost to finish, and a per-flush energy bill beside which the composting fan's single watt looks like a rounding error.

Plants that polish, and the order of decisions

Planted beds of reeds and rushes appear in greywater designs as the polishing step, the place where roots and microbes take a final pass at water already delivered under mulch. They belong at the end of the system and the end of the decision list, not the start. The order that the codes, the manuals, and the failure stories all point to is this: read your jurisdiction's rules first, because they decide what is possible. Split the streams honestly, kitchen to septic, bathroom and laundry to the garden where the law allows. Deliver low and covered, never sprayed, never stored past a day. Choose the toilet family that fits the climate and the household's tolerance for the second composting stage. And hold the whole plan up against the accessory rule: every stream needs an approved destination, not just the one the brochure photographs well.

Sanitation is the least glamorous system in an off-grid home and the one with the least forgiveness in it. Water that is handled well disappears into a garden. Waste that is handled well becomes a chore measured in minutes a month. Handled on folklore, both come back through the floor, the soil, or the vent pipe, and they introduce themselves by smell. The rules and the manuals are dry reading. They are also the shortest version of this subject anyone has written, because every line in them was paid for by somebody's failure first.