Greywater Systems

A household of four uses roughly 70–100 gallons (265–379 L) of water per day. Approximately 50–65% of that is greywater — water from sinks, showers, bathtubs, and laundry that has been used but not contaminated by toilet waste. Redirecting it to irrigation instead of the sewer extends a water supply dramatically during a shortage and reduces well draw or municipal consumption year-round.

Greywater systems range from a simple hose draped from a washing machine to an engineered branched-drain network serving an entire yard. This page focuses on systems a homeowner can install legally and maintain safely.

What Is Greywater — and What Isn't

Greywater includes: - Bathroom sink water (handwashing, toothbrushing) - Shower and bathtub water - Laundry water (clothes washing machine)

Greywater does NOT include: - Toilet waste (blackwater — requires septic or sewer treatment) - Kitchen sink water (fats, oils, food particles — too high in organics and pathogen risk for direct soil application) - Dishwasher water (high-temperature surfactants, food residues, sanitizers) - Water from washing diapers or cleaning wounds

The kitchen sink exclusion surprises many people. Kitchen greywater contains emulsified fats that clog soil pores and food particles that attract vectors (rats, flies). California's greywater code (Title 20, Ch 3) explicitly excludes kitchen water from permitted greywater systems — most other states follow the same logic.

Why Greywater Matters for Preparedness

During a grid-down event or extended water shortage, every gallon of greywater diverted to irrigation is a gallon of drinking water or stored water that does not need to be consumed on irrigation. A greywater system pre-installed and operating before a crisis:

  • Reduces irrigation demand by 20–40 gallons (76–151 L) per day for a household with a modest garden
  • Keeps fruit trees and perennials alive through an extended supply disruption
  • Requires no pumping — gravity-fed systems function without power

Even a temporary laundry-to-landscape redirect during a water restriction event reduces consumption meaningfully while maintaining garden productivity.

Greywater law is governed at the state level and varies widely. Always verify local ordinances — some cities impose stricter requirements than state code.

State Status Key Requirements
California Legal, relatively permissive Simple systems (laundry-to-landscape) require no permit; engineered systems need permit
Arizona Legal No permit required for subsurface systems under 400 gal/day (1,514 L/day)
Texas Legal with permit Must not create runoff or surface ponding
New Mexico Legal One of the earliest adopters; most system types allowed
Colorado Legal since 2013 Limited to toilet flushing and outdoor irrigation; indoor use prohibited
Oregon Legal Permit required; specific setback from water sources
Montana Legal DEQ approval required
Most other states Restricted or no specific code Default to prohibiting discharge outside septic/sewer

If your state does not have explicit greywater code, you are generally in a legal gray area — systems operating entirely underground with no surface ponding are rarely enforced against in practice, but permit-less installation carries risk.

Field Note

Before installing any permanent greywater system, call your county health department and ask specifically: "Do you have a greywater ordinance? What permits are required for a laundry-to-landscape system?" A five-minute call resolves the legal question. In many jurisdictions, the answer is "no permit required for small residential laundry systems" — and you can proceed without further bureaucratic engagement.

System Type 1: Laundry-to-Landscape (L2L)

The simplest legal greywater system. Water exits the washing machine via its normal drain hose but diverts outdoors to a mulch-filled basin around a tree or shrub rather than to the sewer.

Estimated cost: Inexpensive materials; $0 labor if DIY.

How It Works

The washing machine pump discharges through a 1.5-inch (3.8 cm) flex hose typically connected to a standpipe. You redirect this hose outdoors through a drilled hole in the foundation, sill, or through an existing utility penetration. The hose runs to one or more mulch basins.

Mulch basin design: Dig a 2-foot (61 cm) wide × 18-inch (46 cm) deep basin at the root zone of a tree or large shrub. Fill with wood chip mulch. The greywater drains into the mulch, aerates, and percolates into the soil before surfacing. A drain basin filled with mulch: - Prevents surface ponding (legally required in most codes) - Filters the water biologically as it passes through organic matter - Prevents direct human contact with greywater

Three-way valve: Install a 3-way diverter valve between the standpipe and outdoor line. This allows the washing machine to switch between sewer and landscape mode. During rainy season, system overload, or if someone washes heavily soiled items, route back to sewer. The diverter is an inexpensive standard plumbing fitting available at hardware stores.

Sizing a Laundry-to-Landscape System

A front-load washer uses 15–25 gallons (57–95 L) per load; top-load machines use 40–55 gallons (151–208 L). With 5–7 loads per week, a household generates 75–385 gallons (284–1,458 L) of laundry greywater weekly.

Each mulch basin handles approximately 40–80 gallons (151–303 L) per week. For a 3-basin branched system serving three fruit trees, the total weekly capacity is 120–240 gallons (454–908 L). Multiple outlets prevent overloading any single basin.

System Type 2: Branched Drain Gravity System

A more complete system that diverts greywater from multiple sources (shower, bathtub, bathroom sink) through a gravity-fed network of 2-inch (5 cm) drain pipes to landscape basins. No pump required — the natural fall of drain lines does the work.

Estimated cost: $200–$600 for materials; $500–$2,000 installed professionally.

Design Requirements

  • Minimum slope: 2% grade (1/4 inch drop per foot / 2 cm per 30 cm) to maintain flow without clogging
  • Pipe size: 2-inch (5 cm) diameter ABS or PVC drain pipe; larger for high-volume sources
  • Outlet spacing: Each outlet serves one mulch basin; a standard branched system has 3–6 outlets
  • Total head: The outlet must be at least 12 inches (30 cm) lower than the source fixture trap — verify this with a level before designing

The system splits flow between basins using flow splitters — Y-fittings oriented vertically that divide greywater flow to two or more branches. Each split reduces flow to each outlet by half, so an 8-outlet system requires the source fixture to generate sufficient volume.

Soap and Detergent Requirements

Conventional laundry detergents and many soaps contain components harmful to soil microbiomes and plants.

Avoid: - Phosphates: Cause algae blooms and disrupt soil chemistry. Most countries have phased phosphates out of detergents, but some still appear in commercial formulas. - Boron/Borax: Toxic to plants at low concentrations. Widely used in laundry boosters — check the ingredient list. - Sodium (high concentration): Sodium displaces calcium in soil, causing clay particles to disperse and form a crust that blocks water movement. Sodium-heavy detergents are identifiable by ingredients starting with "sodium lauryl..." - Chlorine bleach: Kills soil bacteria and plants directly. Do not use bleach in loads going to a greywater system. - Optical brighteners: Persist in soil; limited research on plant effects but generally excluded from greywater-safe products.

Use instead: - Plant-based soaps and biodegradable detergents (look for "greywater safe" on the label) - Oasis Biocompatible or similar certified greywater-safe laundry products (affordable per bottle) - For showering: castile soap, diluted liquid Castile products (like Dr. Bronner's), or unscented bar soaps free of synthetic fragrances

Pathogen Management and Application Rules

Greywater contains bacteria, viruses, and potentially parasites from skin, feces trace amounts, and washing cycles. It is not sterile. Managing application reduces health risk to near zero.

Three-day rule: Pathogens in greywater in soil degrade to safe levels within 3 days under aerobic conditions. This is the basis for most state rules requiring greywater to infiltrate soil rather than sit on the surface.

Application rules: 1. Subsurface only — outlets should empty below the mulch layer, not spray on leaves or soil surface 2. No edible portions — do not apply greywater anywhere it contacts food crops. Root vegetables and leafy greens are off-limits. Fruit trees, nut trees, ornamentals, and established shrubs are safe targets 3. No surface ponding — pooled greywater becomes a vector breeding site and violates most codes 4. 24-hour storage maximum — do not store greywater in tanks. Stored greywater rapidly turns anaerobic and pathogen-laden. If your system has any storage component, size it so it empties within 24 hours under normal irrigation load 5. Keep children away from outlet basins during and immediately after application

Combined System: Greywater + Rainwater

A greywater system combined with rainwater harvesting and bulk water storage creates a near-complete residential water cycle for irrigation:

  • Rainwater fills bulk tanks for garden irrigation during dry months
  • Greywater handles daily tree and shrub watering without tapping stored reserves
  • Stored clean water focuses entirely on drinking, cooking, and hygiene

In a prolonged grid-down scenario, this combination can sustain a productive garden indefinitely with zero municipal supply.

Prohibited Applications and When to Bypass

Always bypass to sewer when:

  • Someone in the household is ill with a gastrointestinal illness (norovirus, giardia, etc.)
  • You have used chlorine bleach in a laundry load
  • You have washed diapers, cloth wipes, or wound dressings
  • The system outlet basins are saturated (after heavy rain)
  • Temperatures are below freezing — greywater in above-ground pipes will freeze

The three-way diverter valve installed at setup makes bypassing a 5-second operation. Design for bypass from the start.

Cross-References