Description
Rainwater harvesting collects rainfall from a roof (or other clean catchment), stores it in tanks, and supplies it for household and community use — for non-potable uses (toilets, laundry, garden, baths/showers) directly, or for potable use (drinking, food preparation) with adequate filtration and treatment. In New Zealand it is long-established where mains water is unavailable, and is increasingly used in town for resilience and to reduce mains/stormwater load. At community scale the common pattern is dual reticulation — roof water reticulated to houses for non-potable use, with mains (or a treated supply) for drinking. OT_039 Interview VI [INT_006]
How it works
- Catchment → conveyance → storage → (treatment) → use. Roof → spouting with leaf/debris strainer → first-flush diverter (discards the dirty initial flow) → calmed inlet to a storage tank → pump/gravity feed → end uses. For potable use, add a sediment cartridge filter + UV (or chlorine) disinfection. OT_039
- Yield: collectable volume ≈ local average rainfall × roof collection area × collection efficiency. BRANZ sizing assumes 10–20% loss (spillage/evaporation), i.e. ~0.80–0.90 efficiency; the NI model uses 0.8 (the conservative figure, matching CR_016’s 75–90%). OT_039 CR_016
- Coverage: roughly 65% of a household’s water needs are meetable from roof collection in central NZ; the rest (and dry-summer shortfalls) come from mains or a larger store. CR_016
Performance & sizing
- Demand met: the model sizes the
rainwatertech against NZ residential demand (median ~159 L/person/day winter, 231 summer — OT_036); roof yield typically covers a majority of non-potable demand but not peak-summer irrigation without a large store. - Pumping energy: delivering rainwater from tank to taps is not free. A ‘typical’ household rainwater pressure pump uses ~1.5 kWh/kL (range 0.9–4.9, driven by pump selection plus on/off cycling and line re-pressurisation), so the NI engine charges rainwater volume × 1.5 kWh/kL to electricity (the Water→Electricity nexus). Australian data (Sydney/Newcastle monitoring); no NZ measurement exists, so this is a transferable proxy. LIT_131
- Storage is the binding design variable. Tanks run dry in dry spells: the Earthsong community’s tanks “run dry several times each summer,” at which point a monitor opens the mains top-up — a grounded NZ counterpoint to design-figure “100% on-site water” claims. Tank volume, not catchment, is the real constraint (and cost driver). Interview VI [INT_006] CR_016
- Consent-free storage (a hard sizing constraint, BRANZ Bulletin 664 Table 1): ground-supported tanks up to 35,000 L need no building consent; the no-consent ceiling falls with elevation (16,000 L ≤0.25 m, 8,000 L ≤0.5 m, 4,000 L ≤1 m, 2,000 L ≤2 m, 1,000 L ≤3 m). Community storage above 35,000 L (or elevated) triggers a building consent. OT_039
Cost (NZ)
- HDPE round-tank storage ~NZD 110–135/m³ for ≥15,000 L (excl. GST); a 10,000 L tank ~
2,100–2,600. A full **non-potable** household system ~**3,500–6,000**; a sole-potable system with UV ~$8,000–15,000. Cost-per-m²-of-catchment is a weak derived metric — storage volume drives cost. CR_016 - Tank material is an embodied-carbon ↔ cost trade-off (international evidence). Marinoski & Ghisi (2018) — LCA of a Brazilian low-income house — found the storage tank dominates a rainwater system’s embodied energy and CO2: a glass-fibre-reinforced-plastic (GRP) tank was environmentally best, but a reinforced-concrete tank was cheapest with the only viable payback (14.5 yr vs 25.3 GRP / 32.7 HDPE). Two cautions for NI water design: (1) any RWHS system adds embodied energy/CO2 over a no-tank baseline, so the net environmental case rests on lifetime operational water savings; (2) payback is highly sensitive to the water tariff and the non-potable demand fraction. Directional (Brazil low-income), not NZ costs. LIT_051
Regulatory / NZ context
- Potable rainwater must use materials meeting AS/NZS 4020:2018 and is recommended to meet the NZ Drinking Water Standards 2005 (rev 2018); new potable installs need a building consent. Non-potable must be lilac-labelled (AS/NZS 3500.1:2021, NZS 5807:1980, G12/AS1), kept clear of food areas, with backflow prevention where mains-connected. OT_039
- Multi-household threshold: collecting/reticulating rainwater for more than one household can cross into “public supply,” triggering a heavier public-health treatment regime — the regulation-driven reason Earthsong keeps drinking water on mains and roof water non-potable. This is the key consent question for a Neobiome community rainwater design. Interview VI [INT_006]
- Fully on-site water (catchment + greywater recycling) is achievable by design at small scale (e.g. the OT_018 autonomous-housing concept), but is design-derived, not metered. OT_018
Research targets
- First-flush diverter sizing (L per m² roof), sediment-filter micron rating and UV dose for potable treatment — qualitative in Bulletin 664; needed to size and cost the treatment train.
- Whether the Tasman/Lower Moutere pilot’s collective reticulation crosses the public-supply threshold (the int_006 question, restated for the pilot).
Connections
Links to
Referenced by
Sources (3): CR_016 · LIT_051 · OT_039
EDT domains (1): D03: Water, Waste & Circular Systems
Technologies (1): Greywater Recycling