Ultrapure Water (RO + EDI) (UPW)

In plain terms

Dissolved salts, silica and metals in feed water foul membranes, poison catalysts and speed up stack degradation. Electrolyzers therefore run on ultrapure water, measured by its conductivity or resistivity.

A typical water treatment for electrolysis train has three stages (DESNZ, 2024):

1. Pretreatment: filtration and dosing to protect the membranes downstream.
2. Reverse osmosis (RO): pressure drives water through a membrane that rejects most dissolved salts. A brine stream leaves the plant.
3. Electrodeionization (EDI): an electric field and ion-exchange resins polish the RO permeate to final purity, without chemical regeneration.

At coastal sites, a seawater RO desalination stage sits in front, and the feed must be treated twice: seawater needs two RO passes (DESNZ, 2024).

Typical range

Electrolyzers need feed water of at least ASTM Type II quality. Making it takes more raw water than the electrolyzer consumes, because reverse osmosis and EDI reject part of the feed as brine. In a UK government study (DESNZ, 2024), 1 kg of ASTM Type II water needed about 1.67 kg of potable, surface or groundwater, and about 3.27 kg of seawater, which requires two RO passes. These multipliers come from the study's own mass balance, not from plant measurements. Treatment energy depends on the source water, and the study gives no figure for it; take it from the water-treatment vendor's design.

Why it moves LCOH

Treatment is a small share of LCOH on a site with good fresh water. With seawater, desalination adds equipment, electricity and brine disposal, and these grow with total water intensity, not just the 9 L/kg the reaction needs. Poor water quality also costs money indirectly, through shorter stack life.

Common mistake