EDT domain: d03_water_waste_circular
Definition
Reverse-osmosis (RO) desalination — make freshwater by forcing seawater (or brackish water) through a semi-permeable membrane at high pressure. In the NI water cascade (D22) it is the coastal local backstop: a community by the sea can produce its own water when rainwater, tank and borehole all fall short. It is local (does not import), but it carries the highest energy intensity of any backstop (~3–5 kWh/m³ for seawater), so it is the strongest water↔energy coupling term in the model. CR_032
Role — coastal local backstop, energy-heavy
- A local supply with effectively unlimited feedstock (the sea), so it never runs dry — but at a steep energy cost that lands directly on the community’s electricity demand. CR_032
- In NZ it is rare and feasibility-stage: island, emergency, or study-only (e.g. a Waiheke Island feasibility study); no large municipal seawater RO plant exists. So it is the backstop of last resort for a coastal community with no groundwater and insufficient roof catchment. CR_032
Cost & performance
- Capex (installed): ~USD 2,500 per (m³/day) of capacity ≈ NZD 4,200/m³/day (at NZD/USD ≈ 0.60). The industry/IDA benchmark is USD 1,000–2,500/m³/day; small community units sit at or above the top (USD 2,000–4,000) because RO capex is strongly scale-dependent. Low–medium confidence, international, scaled up for small size — no NZ community-scale installed cost was found. A 10 m³/day unit is order NZD 40,000+ plus an NZ install/freight premium. CR_032
- Energy intensity (the nexus term): seawater RO ~4 kWh/m³ at community scale (realistic band 3.5–5; state-of-the-art large plants reach ~2.8, a record plant 1.79 — not typical of small units); brackish-water RO far lower, ~1 kWh/m³ (0.7–1.5). High confidence on the bands (peer-reviewed), medium on the seawater point value. This is ~16× a shallow borehole (~0.25 kWh/m³). CR_032
- Opex (excl. energy): ~USD 0.50–1.00/m³ (chemicals, membrane replacement, labour, maintenance) — higher per-m³ at small scale; membranes replaced ~every 5–7 years. CR_032
Regulatory / NZ context
- Brine disposal + coastal consent: seawater RO recovers <50% of intake, so it discharges more than half the intake as concentrated brine. Discharging brine to the coastal marine area in NZ requires a regional coastal-permit / discharge consent under the RMA, and intake screening must manage marine-organism impingement. This is a material consenting constraint — often the binding one for a coastal NZ community. CR_032
Relevance to Neobiome (D22)
The coastal local backstop of last resort in the water cascade. Capex enters total_capex; the energy intensity (~4 kWh/m³ seawater) couples hardest of all backstops into the electricity model — desalinating to cover a water deficit can materially raise electricity demand and so depress electricity_ssi, a genuine cross-domain tension the engine must surface. Feasibility is gated by the brine coastal-discharge consent, not just cost. Brackish RO (~1 kWh/m³) is a much cheaper energy case where the feed is brackish groundwater rather than seawater. CR_032
Research targets
- An international community / island small-SWRO installed-cost case (the only path, since no NZ community plant exists) plus an NZ vendor quote (Bluemont / Xylem NZ island or emergency unit) to replace the scaled international capex figure (RT_262).
- NZ-specific brine-discharge consent precedent / cost, to quantify the feasibility gate (RT_262).
Connections
Links to
Sources (1): CR_032
EDT domains (1): D03: Water, Waste & Circular Systems
Referenced by
Sources (1): CR_032
EDT domains (1): D03: Water, Waste & Circular Systems