Greywater Recycling Potential Calculator

A typical US household (4 people) produces 80-100 gallons (300-380L) of greywater per day, which is 50-60% of total household wastewater. Breakdown by source: shower/bath (35-40 gallons), bathroom sinks (10-15 gallons), laundry (15-30 gallons). Kitchen sink water is not considered greywater in most codes due to food grease and solids. Dishwasher water is also excluded. A family of 4 could save 25,000-35,000 gallons of fresh water annually with a well-designed greywater system, reducing water bills by 30-40% and dramatically cutting the load on sewage treatment or septic systems.

Best uses: subsurface irrigation of ornamental plants, trees, shrubs, and lawns. Greywater is excellent for fruit trees and berry bushes (nutrient-rich from soap residues). Mulch basins around trees can handle direct greywater application. Avoid: spraying on edible parts of vegetables (root crops, leafy greens eaten raw), storing greywater more than 24 hours (bacterial growth), using on acid-loving plants if greywater is alkaline (from soaps), and surface irrigation where puddles form. Design tip: alternate greywater irrigation with fresh water to flush salts. Use low-sodium, biodegradable soaps to keep soil healthy. Divert to sewer during heavy rain or when using harsh cleaning chemicals.

Greywater is wastewater from bathrooms (sinks, showers, tubs) and laundry, containing soaps, shampoo, lint, hair, and small amounts of organic matter. Blackwater is from toilets containing human waste and pathogens — never to be reused without treatment. Kitchen sink water is sometimes classified as dark greywater or blackwater depending on local codes (due to food particles, grease, and higher pathogen load). Greywater systems must be designed to never mix with blackwater. Distinguishing feature: greywater can be safely reused for irrigation within 24 hours without treatment, while blackwater requires full sewage treatment at a wastewater plant or septic system.

US regulations vary by state. The 2021 IPC (International Plumbing Code) allows: (1) Clothes washer direct-connect systems (laundry-to-landscape) — simplest, no permit needed in many states. (2) Gravity-fed branched drain systems — no pump required, limited to 250 gallons/day. (3) Pumped systems with surge tank and filtration — requires permit and professional design. Required: subsurface discharge (12-24" deep), no ponding on surface, 100-foot setback from wells, no connection to drinking water system, use of approved biodegradable soaps. California, Arizona, New Mexico, Florida, and Texas have progressive greywater codes. Check local health department before installing. Many areas offer rebates for greywater system installation.