Description
Greywater recycling captures the relatively clean wastewater from baths, showers, bathroom basins and (sometimes) laundry, and reuses it — untreated for subsurface garden irrigation, or treated for toilet flushing — instead of sending it to sewer/septic and drawing fresh water for those uses. Kitchen-sink and dishwasher wastewater is excluded (too much grease, food and chemicals — that is closer to blackwater). It is the demand-reduction companion to rainwater harvesting (rainwater_harvesting, which adds supply): together they cut potable demand from both ends. OT_040
How it works
- Diversion (untreated): laundry/bathroom greywater is filtered for solids (lint, hair) and diverted — often gravity-fed — to subsurface garden irrigation (mulch basins). It must be used immediately or within 24 hours; untreated greywater must not be stored longer (ponded greywater contaminates soil). OT_040
- Treatment (for storage / indoor reuse): filtration + physical or chemical disinfection brings the water to a standard where it can be stored and piped indoors to toilet cisterns. Surge tanks must be sealed and vermin-proof and not store untreated water; treatment devices must be marked (AS/NZS 3500.2 §7.4). OT_040
- Greywater must never be potable or used for washing, bathing or swimming. OT_040
Performance & sizing
- Volume: a typical NZ household produces 100–200 L of greywater per day — roughly 20–45% of total household water demand (cf. ~435–504 L/day household, OT_036). It is a large, reliable, year-round reuse stream (unlike rainwater, it does not depend on recent rain). OT_040
- Self-sufficiency contribution: in the Gullberg multi-domain study, greywater recycling alone supplied ~71% of community freshwater self-sufficiency across all density cases — the single largest contributor to closing the water loop. Treat as a structural benchmark (Swedish context). LIT_022
- A hybrid rainwater + greywater system achieved ~42.5% potable-water savings at household scale; greywater from showers alone covered ~19.6% (overseas LCA). LIT_008
NZ regulatory context
- Greywater systems require a building consent granted as an Alternative Solution — there is no Acceptable Solution for greywater reuse, so each install is assessed case-by-case; a resource consent may also be needed to discharge to land. Council policies vary widely (some promote, some restrict, some have none), and no single NZ law covers greywater end-to-end. Plumbing must meet AS/NZS 3500.2:2021 with non-potable labelling per AS/NZS 3500.1. OT_040
- This is a heavier consent pathway than rainwater, and on-site wastewater reuse has hit NZ regulatory walls in practice — Earthsong engineered on-site wastewater treatment but Watercare refused consent, leaving a future-conversion option only. A community greywater design must plan for the consent pathway early. Interview VI [INT_006]
Cost
First NZ cost data (industry installers, indicative). Equipment (WCTNZ, ex-GST, spot-confirmed): gravity diverter NZD 1,130; pumped diverter NZD 1,281; Nature Clear treatment unit NZD 1,913. Installed totals (estimated): ~NZD 3,000–5,000 (diversion — filter + subsurface irrigation) / 5,000–8,000+ (full treatment + disinfection, needs a certifying drainlayer). Diversion is cheaper than treatment, consistent with the design split above. The installed totals are synthesis estimates (equipment prices are the sourced anchor). CR_048
A second NZ synthesis corroborates the WCTNZ diverter figure (unit 1,281 excl GST) and adds a **full treated in-house reuse band of ~8,000–18,000 installed** (biofilter + UV; a discharge resource consent may also apply) — so the installed spread runs ~500–1,500 (manual diverter) → ~1,800–3,000 (pumped GWDD) → ~8,000–18,000 (treated full reuse). The engine's interim `greywater_capex_per_dwelling` 4,000 sits at the diverter-installed end, not the treated end. CR_050
Reuse fraction: greywater is ~68% of the gross residential stream (160 L/person/day, Kāpiti Coast DC via BRANZ Level), but the realistic year-round reusable fraction is ~25–35% — untreated diversion serves seasonal sub-surface irrigation only; only treated+disinfected greywater supports year-round toilet flushing. This band brackets the engine’s interim GREYWATER_FRAC = 0.30, but the 25–35% figure is a synthesis estimate, not a sourced NZ number. CR_048
Research targets
- RT_211 partial → CR_048: equipment prices now sourced; a sourced NZ installed-total (diversion vs treatment) and a measured year-round reuse fraction (to firm the ~25–35% / engine 0.30) stay open — BRANZ Bulletin BU665 (paywalled) is the authoritative spec.
Connections
Links to
Referenced by
Sources (3): CR_048 · CR_050 · OT_040
EDT domains (1): D03: Water, Waste & Circular Systems