Keep greywater and drinking water in separate pipe systems with no shared fitting, and let every drinking-water outlet discharge through an air gap above the highest water level of whatever it fills, because an air gap is the only backflow protection you can build without manufactured parts. A cross-connection lets contaminated water be drawn back into a drinking-water pipe whenever the pressure drops, which is how many mass water poisonings have happened.
Part of Greywater Recycling Systems
Plumbing separation is the design rule that drinking-water pipes, greywater drains and toilet waste never connect, so that dirty water cannot flow backward into the clean supply.
Three Systems, No Connections
- Drinking supply: pressurised pipework from the tank or spring to the taps (Gravity-Fed Water & Plumbing Systems).
- Greywater drain: open, gravity-fed, from sinks, basins, baths and laundry to the filter and garden.
- Toilet waste: carried by bucket or chamber to the compost; the parent page says the toilet has no connection to the greywater system.
The parent page also says to mark salvaged pipe by colour or label so the systems are not confused.
How Dirty Water Gets Into A Clean Pipe
A drinking pipe is clean only while the water in it is pushed outward. Two things reverse the flow:
- Back-siphonage: if pressure in the supply falls, for example a main breaks, the tank empties or a valve is closed while a low drain is open, the pipe above can go below atmospheric pressure and draw in anything its outlet is dipped in.
- Back-pressure: if a connected system is at a higher pressure than the supply, water flows backward through the join.
Worked Example: How Hard A Broken Pipe Can Pull
A water column exerts ρ × g × h of pressure: 998 × 9.81 = 9.79 kPa per metre, or 0.098 bar per metre.
- A tap is on the first floor, 3 m above a leak at the lowest point of the pipe, which is open to the air.
- The supply above has failed, for example the tank has run dry, but the pipe still holds water.
- A hose is fitted to the tap and its end lies in a bucket of greywater at the tap’s level.
- The bucket’s water surface is 3 m above the open leak. A pipe full of water joining the two is a siphon. The pressure driving it is 0.098 × 3 = 0.29 bar (29 kPa).
- Greywater is pushed through the hose and the pipe and runs out of the leak. If the leak is then repaired, or the supply returns, the pipe still holds greywater.
Nothing in the example is rare: pipes break, tanks run dry in droughts and hoses get left in buckets. An air gap, with the hose or spout well clear of any water surface, removes the siphon completely.
Rules That Work Without Parts
- No shared fittings, valves or pipes between the drinking supply and any other line. Not even a temporary hose.
- An air gap at every drinking-water outlet: the tap spout must end above the rim of the sink, bath or tank it fills. Plumbing codes set the gap at no less than about twice the diameter of the outlet, as I recall; I have not re-read it, so make it generous.
- No hose, pipe or spout that can lie below the water or soil surface. A hose attached to a tap goes dry-coiled and hung up.
- No top-up of a greywater tank or bed from the drinking supply except by an open pipe ending above the tank’s overflow level.
- Where pipes cross, drinking water goes above greywater, so a leak drips outward and downward, not inward.
- Do not trust a check valve or flap by itself. Grit or wear holds it open. An air gap cannot jam.
- Make the diverter valve choose between two open drains, for example garden or dry well, never between greywater and a pressurised line (parent page).
- Mark and map every pipe, and walk the system after any change looking for connections (Visual Inspection).
Network Design and Storage Tanks give the same warning about non-drinking connections.
Traps Are For Gas, Not For Backflow
A U-bend under a basin holds a water seal that stops drain gas rising into the room. It does not protect the supply. The sink’s drain should end in the greywater pipe through a free air space (a funnel or open end above the pipe), not a sealed join.
Common Mistakes
- Joining the greywater line to the toilet “to save piping”.
- A garden hose connected to the drinking tap and left in the greywater bed or a bucket.
- Using the same pipe colour for both systems.
- A “temporary” connection in a drought from the drinking supply to the irrigation line.
Safety
A cross-connection can poison every tap on the network with greywater or worse
Contamination enters when pressure drops, so it may not happen for months and then affect everyone at once. Never join a non-drinking line to the drinking supply, even through a valve. Use an air gap. If you suspect a connection, shut the line, flush and disinfect it, and treat the water as unsafe until it is found and removed (Water Purification).
What’s Next
- Learn next: Filtration Systems — treat the greywater you have kept apart
- Combine with: Indoor Plumbing Layout — apply the rules to a real house