Composting Toilets and Greywater Systems

Every off-grid property has to deal with two waste streams: toilet waste (blackwater) and the water from sinks, showers and laundry (greywater). Handled well, both can be managed safely with little or no electricity and without a large water supply. Handled badly, they contaminate wells, lakes and soil, spread disease and draw enforcement. This guide covers the main toilet options, how composting actually works, how long human waste must be stored before use, how to divert and use urine, how to size and build simple greywater systems, cold-climate problems and the regulations you need to check before you build.

Toilet options compared

Pit privy

A pit privy is a hole in the ground with a shelter over it. Waste stays in the pit and breaks down slowly, mostly without oxygen. Because the pit is in direct contact with soil, it is the option most likely to contaminate groundwater. The bottom of the pit must sit well above the seasonal high water table (a figure of at least 1.2 m (4 ft) is common, but your local rule governs), and never in gravel over a shallow aquifer, thin soil over bedrock, or a flood area. A vent pipe of 100–150 mm (4–6 in) diameter running from the pit to above the roofline, with an insect screen on top, cuts odour and flies considerably. Add a handful of sawdust, wood ash or dry leaves after each use. When the pit is full to within about 50 cm (20 in) of the surface, fill it with soil and move the shelter.

Self-contained composting toilets

These units hold waste in a drum or tray inside the toilet itself. A small fan vents moisture and odour outdoors. Capacity suits one or two people; overloaded, they produce a wet, smelly mass that has not composted. Most do not get hot enough to kill pathogens, so the contents must be finished in an outdoor bin before use. Look for certification to NSF/ANSI 41, which many health authorities require.

Central composting systems

A toilet on the main floor drops waste into a large chamber in a basement or crawlspace. The larger mass composts more completely and needs emptying once a year or less, but year-round use requires a heated space below the toilet.

Urine-diverting dry toilets

A divided bowl sends urine forward into a separate pipe or container and lets solids drop into a bucket or chamber. Keeping urine out of the solids is the most effective way to control odour and moisture in any dry toilet.

Incinerating toilets

These burn waste to a small amount of sterile ash. Electric models typically use in the order of 1–2 kWh per use, so a household of two using the toilet eight times a day would need 8–16 kWh daily. That is beyond most small off-grid solar systems. Propane models avoid the electrical load but have a fuel cost.

Septic systems

A conventional septic tank and dispersal field is the default where water is plentiful and soil conditions suit. It needs a pressurized water supply, a site assessment and, in most provinces, design and installation by an authorized professional. Tanks typically need pumping every 3–5 years.

How composting works

Composting organisms need carbon, nitrogen, moisture and oxygen in balance. Temperature is the result.

Carbon to nitrogen ratio

Microbes work best at a carbon-to-nitrogen (C:N) ratio of about 25:1 to 30:1. Human waste is very high in nitrogen, so it must be balanced with a high-carbon cover material. Too little carbon produces ammonia odour and a wet, anaerobic mass. Too much carbon slows decomposition but does not cause odour, so err on the side of more cover.

Cover material

After each use, cover deposits completely with dry carbon material. Fine, slightly damp sawdust from untreated softwood is the most common choice because it absorbs liquid and blocks odour well. Shredded dry leaves, chopped straw and wood shavings also work. Never use sawdust from treated, painted or manufactured wood.

Moisture and oxygen

Aim for 40–60% moisture, the feel of a wrung-out sponge. Too dry and microbes go dormant. Too wet and air spaces fill with water, oxygen runs out, and anaerobic bacteria take over, producing odour.

Temperature

A well-built outdoor compost pile with enough mass heats itself. The thermophilic (hot) stage runs at 55–65°C (131–149°F). Holding all of the material at 55°C (131°F) or higher for at least three days kills most pathogens. Above about 70°C (158°F) the beneficial organisms start to die off. Check the centre of the pile with a long-stem thermometer.

Most household composting toilets and small bins never reach these temperatures throughout. The outer layers stay cool and pathogens there survive. That is why long storage time, not heat alone, is the main safety barrier in home systems.

Handling and curing humanure

Human feces can carry bacteria (E. coli strains, Salmonella), viruses (hepatitis A, norovirus), protozoa (Giardia, Cryptosporidium) and parasitic worm eggs. Worm eggs, particularly roundworm, are the most resistant and are the benchmark for safe treatment.

Curing time

• Hot composting: where the whole pile has reached 55°C (131°F) for at least three days, cure it for at least a further year after the last fresh material is added.
• Cool composting or storage only: World Health Organization guidance for dry storage at 2–20°C (36–68°F) is 1.5–2 years. In northern climates, where material sits frozen for months, use the full 2 years counted from the last addition.
• Frozen time does not count. Freezing preserves many pathogens rather than killing them. If a bin sits frozen from November to April, that winter has added little treatment.

Use a two- or three-bin rotation: fill one bin for a year, then close it and start the next. By the time the third bin is full, the first has cured for two years.

Safe handling

• Wear gloves and wash hands with soap and water after every handling task.
• Site outdoor bins away from wells, surface water and food gardens, and on ground where runoff will not reach water. Line the base with a thick layer of straw or wood chips to absorb leachate.
• Use finished compost on ornamental plants, shrubs and around fruit trees. Do not use it on root crops or leafy vegetables eaten raw.

Urine diversion and use

An adult produces roughly 1.5 L (0.4 US gal) of urine a day, about 550 L (145 US gal) a year. Urine carries most of the nitrogen excreted by the body (about 80%) and around half the phosphorus, but it is low in pathogens in a healthy person. Diverting it keeps solids drier, cuts odour and reduces the volume of material you must compost.

Storage

Stored urine turns alkaline (around pH 9) as urea converts to ammonia, and this kills most pathogens over time. WHO guidance is that urine from a single household can be used directly on that household's own crops, provided application stops at least one month before harvest. Urine collected from several households, or used on crops for sale, should be stored for at least six months, ideally at 20°C (68°F) or warmer. Store in a sealed, opaque container to keep ammonia in and odour down.

Application

• Apply to soil, never on leaves or the edible part of the plant.
• Dilute 1 part urine to 3–10 parts water for watering around growing plants, or apply undiluted to soil before planting and water it in.
• Rotate application sites. Urine contains salt, and repeated heavy use in one spot can build up sodium.

Greywater: sources and volumes

Greywater is wastewater from showers, bathtubs, bathroom sinks and washing machines. It contains soap, skin cells, hair, lint and small amounts of bacteria. Kitchen sink water carries grease and food particles, clogs systems quickly and breeds bacteria; many jurisdictions treat it as blackwater, and it should not go to a simple landscape system. Water from washing soiled diapers is blackwater.

Greywater must never be stored for more than 24 hours. Bacteria multiply quickly and stored greywater turns septic.

Simple greywater systems

The simplest systems use gravity, avoid storage tanks and pumps, and discharge below a layer of mulch so that no one contacts the water.

Branched drain

A gravity drain from the shower or sink runs to the yard and is split repeatedly using double-elbow fittings laid flat so that flow divides evenly. Each branch ends in a mulch basin. Requirements:

• A continuous downhill slope of at least 2% (1/4 in per ft) from fixture to outlet.
• Pipe sized to match the fixture drain, usually 38–50 mm (1.5–2 in).
• Outlets that discharge into a covered chamber or below mulch, never onto the surface.
• A three-way diverter valve near the house, so flow can be switched to the septic system or winter drain.

Mulch basins

A mulch basin is a shallow excavation, typically 30–45 cm (12–18 in) deep, filled with coarse wood chips and placed around or beside a tree or shrub. Greywater discharges into the mulch, which filters lint and hair and lets the water soak in. The mulch layer over the outlet should be at least 5 cm (2 in) thick. Top up the mulch as it decomposes. Water should soak in within minutes, with no pooling at the surface and no runoff.

Laundry to landscape

The washing machine's own pump pushes water through a 25 mm (1 in) line to a series of outlets at plants. No extra pump or filter is needed. Install a three-way valve right behind the machine so you can send water to the septic system or sewer when using bleach or washing heavily soiled loads. Keep the run short and the outlets lower than the machine.

Where to use greywater

• Trees, shrubs, fruit trees, berry canes and perennials.
• Not on root crops or leafy greens eaten raw, and never by spraying.
• Not on acid-loving plants such as blueberries and rhododendrons; greywater is usually alkaline.
• Not on saturated, steep or rocky ground where it can surface or run off.

Soaps and products to avoid

Whatever goes down the drain goes into your soil. Some ingredients harm soil structure or plants:

• Sodium: many powdered laundry detergents use sodium salts as fillers. Sodium breaks down soil structure, so clay soils seal and stop draining. Liquid detergents are generally lower in sodium.
• Boron: borax and boron compounds are toxic to many plants at levels only slightly above what they need. Avoid any product containing them.
• Chlorine bleach: kills soil organisms and damages plants. Divert loads with bleach to the septic system.
• Softened water: ion-exchange water softeners add sodium. Do not route softened water to a greywater system.
• Antibacterial agents, solvents and drain cleaners: never send these to landscape.

Setbacks from wells and water

Separation distance is the main protection for drinking water. Exact figures vary by province and municipality, and by system type and soil, but the following are typical starting points:

• Drinking water wells: 30 m (100 ft) from a privy, compost bin, septic dispersal field or greywater discharge is the common standard. In BC this is the general separation between a well and a probable source of contamination.
• Lakes, streams, wetlands and springs: commonly 15–30 m (50–100 ft) from the high-water mark, sometimes more.
• Water table and bedrock: a minimum depth of unsaturated soil between the discharge point and the seasonal high water table or bedrock is required for privies and dispersal systems.

Shallow dug wells and springs are far more vulnerable than sealed drilled wells.

Cold-climate issues

• Composting stops in the cold. Microbial activity drops sharply below about 10°C (50°F) and stops near freezing. Unheated toilets and outdoor bins simply store waste through winter.
• Self-contained units need heat. They will not work in an unheated cabin in winter. Units with built-in heaters add a continuous electrical load that solar systems may not support in December.
• Urine lines and tanks freeze. Run urine pipes with a steady downhill slope and no low spots, keep them inside the heated envelope or below frost depth, and use a container large enough to tolerate a freeze. Stored urine freezes only a few degrees below 0°C.
• Greywater systems are usually seasonal. Frozen ground does not absorb water, and discharge onto frozen mulch builds ice and can surface. Use the diverter valve to send greywater to a septic system or approved winter dispersal from freeze-up to thaw.
• Septic fields need flow and snow cover. Do not plough, drive on or compact snow over the tank or field; snow is the main insulation.

Permits and regulations

Waste and greywater rules are set provincially and can be tightened by regional districts and municipalities. This section explains where to look, not what applies to your property.

• British Columbia: the Sewerage System Regulation under the Public Health Act covers systems that treat domestic sewage, and in BC greywater is domestic sewage. Sewerage systems, including those handling greywater, must generally be designed and filed by an Authorized Person (a Registered Onsite Wastewater Practitioner or a professional engineer or geoscientist) with the regional health authority. Installing a composting toilet does not remove the need for an approved method of handling greywater. Privies are addressed by health authority guidance and local bylaws.
• Ontario: Part 8 of the Building Code classifies on-site systems. Privies and composting toilets fall under Class 1, greywater leaching pits under Class 2, and septic systems under Class 4. Permits are issued by the municipality or a conservation authority acting for it.
• Other provinces and territories: each has its own private sewage standard.
• Northern US states: state health or environmental departments regulate on-site systems.

Before building, contact your health authority's environmental health office and your local building department. Ask what is permitted for your lot and whether a professional must design or file the system.

Key points

• Choose a toilet system based on water supply, winter heating, soil, water table and what local rules permit.
• Balance nitrogen-rich waste with carbon cover at about 25–30:1, and keep moisture at 40–60%.
• Most home systems do not get hot enough to kill pathogens; cure humanure for at least 1–2 years after the last addition, and do not count frozen months.
• Divert urine to control odour; use it diluted on soil and stop one month before harvest.
• Send greywater directly to mulched soil under plants; never store it longer than 24 hours.
• Avoid sodium, boron, bleach and softened water in any greywater system.
• Keep waste at least 30 m (100 ft) from wells, test well water yearly, and check provincial and local rules before you build.

This guide is general information. Codes, permits and safety requirements vary by province, state and municipality. Check local rules and use a qualified professional for electrical, gas, chimney and septic work where required.

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