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?
Wood-chip filter (Filtres a Broyat de Bois, FBB)
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
Land area for the filter bed TREATS GREYWATER ONLY. It needs the blackwater dealt with separately, which in practice means dry toilets: that is a whole-house decision, not a detail of this filter. Land area is modest, about 1 to 1.5 m2 per occupant.
A bed of wood chips that the used water from sinks and showers trickles through. Microbes living on the chips feed on the dirt and clean the water. It works with no power, where the ground slopes enough, and only needs the chips topped up as they break down.
Structure/basin: 20-30 years; the wood-chip media itself needs periodic renewal as it breaks down (INRAE guidance doesn't specify an exact interval)
Embodied carbon
Low
Space / material use
Medium
Output timing
Continuous process
Skill to repair
Basic
Regulation (FR example)
On-site sanitation (assainissement non collectif): the installation must be declared to and approved by the local SPANC before work starts, and is subject to periodic inspection. Arrete du 7 septembre 2009 modifie. Additionally, wood-chip filters treating greywater are authorised only under the EXPERIMENTAL regime of the arrete du 30 mars 2023, which runs five years from that date. That is a materially weaker footing than an agreed filiere and should be checked before relying on it.
Levels are relative to the other options for this need, not absolute. Fewer dots is lighter.
Wood-chip filter (Filtres a Broyat de Bois, FBB) (This page) 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.
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.
Greywater heat recovery (drain water heat exchanger) 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.
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.
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.