Greywater System Designer

Reuse household water for irrigation. Daily yield, mulch-basin sizing, plant compatibility, soap chemistry — and what's legal where.

Plan a greywater system

Sources

Check sources you'll connect:

Household

People in household

Soil type Sand (fast drain) Loam (typical) Clay (slow drain)

Greywater rules of thumb

Rule	Why
Greywater on dirt, not plastic.	Soil + plant roots filter and treat the water; storage tanks turn anaerobic + smell within 24 hours
Use within 24 hours.	No storage; route directly to mulched basins
Below mulch, never on top.	Surface greywater = bug attractant + odor + skin contact risk. 2-4" of mulch over the discharge.
No edible roots.	Don't water carrots, beets, potatoes. Water tomato/pepper/squash bases (fruit above ground) is fine.
No spray heads.	Aerosolizing greywater can spread pathogens. Trickle / flood only.
Branched drain (no pumps if possible).	Gravity systems = no electricity, no maintenance, no failure modes
Soap compatibility matters.	Avoid sodium-rich soaps, boron, antibacterial agents — all damage soil/plants over time
Septic + greywater = compatible.	Diverting greywater from septic actually extends septic life by 30-50%

Soap & detergent compatibility

Avoid	OK in moderation	Best
Sodium-rich (most powdered detergents) — Na+ damages soil structure Boron-containing — toxic to many plants at low concentration Antibacterial soaps — kill soil microbes Bleach (chlorine) — kills plants Anything with EDTA chelators — bind plant micronutrients	Liquid detergents (sodium-light) Mild dish soap (Dr. Bronner's, Seventh Gen.) Most shampoo / conditioner (low salt)	Oasis Biocompatible Ecos (low-Na liquid) Castile soap (Bronner's pure) Plain water rinse cycles

Branched drain design

Standard greywater system: a 1.5" or 2" PVC line from the source drains downhill (1-2% slope), splitting at "double ells" (custom flow-divider fittings) into multiple outlets. Each outlet ends in a mulch basin around a plant.

Component	Specification
Main drain pipe	1.5" or 2" PVC; smooth (not corrugated); 2% slope minimum
Each split	"Double ell" fitting splits flow 50/50; never tee fitting (clogs)
Mulch basin (per outlet)	1-2 ft diameter × 12-18" deep, filled with wood chips or river rock
Outlet shield	4" hard plastic shield prevents critters; mulch above outlet
Cleanout	Sweep elbow + cleanout cap at every directional change
3-way valve	Diverter from house line: greywater system OR sewer/septic backup

Legal status

State category	Status
Lenient (CA, AZ, NM, TX, NV, OR)	Single-fixture systems often permit-exempt; Tier 1 systems (under 250 gpd) are simplified
Moderate	Permit required; technical review
Strict (most NE/MW states)	Treated as wastewater; full septic permit required for any greywater diversion
Local variations	HOA + local plumbing codes may add restrictions even where state allows

The simple "laundry-to-landscape" pattern (no plumbing modifications; just rerouting laundry hose to outdoor mulch basin) is permit-exempt in most states because no permanent plumbing change is involved.

Free under CC BY 4.0. Cite as "OAT Greywater System Designer (openagriculturetechnology.com)". Design principles from Art Ludwig's "Create an Oasis with Greywater" and state code summaries. Legal status varies — verify with your local jurisdiction.