Lowtech Graph lists low-tech options for every need of a home. Here you can compare them, find tutorials to build them, and link them together into a circular system. Why low-tech?
Greywater heat recovery (drain water heat exchanger)
Passive
One of 5 ways to meet this need. Cost, carbon, footprint and rankings are relative to other solutions for the same need, not absolute.
What your site needs
A vertical drain run accessible for the exchanger to wrap or replace a section of pipe NEEDS A VERTICAL DRAIN RUN, and this is the constraint that decides whether it is worth fitting: the exchanger works because draining water clings to the pipe wall in a thin film, which only happens in a vertical section. Horizontal installations recover substantially less. It also only sees water that drains WHILE hot water is running, so it suits showers and not baths.
A pipe wrapped around the shower or sink drain, carrying the cold water on its way to the water heater. The warm water going down the drain passes its heat to the cold water coming in. The water heater has less to do, with no pump and nothing to switch on.
Levels are relative to the other options for this need, not absolute. Fewer dots is lighter.
Greywater heat recovery (drain water heat exchanger) (This page) Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Medium · Skill to build: Intermediate · Upkeep: Light · Change to daily life: None · Works without the grid: Yes Choose it if you want to get back the heat of shower water, with nothing to switch on.
Constructed wetland / reed bed filtration Can I use it? Not in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Intermediate · Upkeep: Light · Change to daily life: None · Works without the grid: Yes Choose it if you have room and a slope, and want greywater cleaned by plants with no power.
Sand filter Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Basic · Upkeep: Regular · Change to daily life: None · Works without the grid: Yes Choose it if you want a compact filter for greywater, on its own or ahead of a reed bed.
Membrane bioreactor (MBR) greywater recycling package unit Can I use it? Works in a flat · No rebuilding needed What does it cost me? Upfront cost: High · Skill to build: Specialist · Upkeep: Heavy · Change to daily life: None · Works without the grid: No Choose it if you want greywater clean enough to flush toilets indoors.
Wood-chip filter (Filtres a Broyat de Bois, FBB) Can I use it? Not in a flat · No rebuilding needed What does it cost me? Upfront cost: Low · Skill to build: Basic · Upkeep: Regular · Change to daily life: None · Works without the grid: Yes Choose it if you want the simplest greywater filter to build, topped up with wood chips.
We don’t list sizing or quantities, as the yield of each technique depends on the quality of its build. The catalogue compares options but doesn’t dimension them. Regulatory notes are indicative, with a French example, and need to be confirmed locally.