Zero liquid discharge: the honest cost of the last litre
ZLD turns a discharge problem into an evaporation problem. It works — at an energy and capital price that should be paid knowingly, not discovered in operation.
Zero liquid discharge is a destination, not a technology: every litre of water entering the fence leaves as vapour or product, and every dissolved solid leaves as a solid. Regulators like the sound of it; boards like the headline. The engineering consists of concentrating brine against physics that resists harder with every gram per litre — and the cost curve is exponential, not linear.
The train
- Pre-concentration by membranes — conventional RO to its osmotic limit, then high-pressure or semi-batch RO toward 120–160 g/L. The cheapest water removed is removed here.
- Brine concentrator — falling-film evaporator, almost always mechanical vapour compression, taking brine to near saturation at 15–25 kWh per cubic metre of evaporated water.
- Crystalliser — forced-circulation MVC or steam-driven, finishing the job at 40–70 kWh/m³ and producing a wet salt cake.
- Solids handling — centrifuge or filter press, then landfill as the usual destiny; a saleable salt is the exception, not the plan.
| Stage | Energy, kWh/m³ removed | Relative capital per m³/d |
|---|---|---|
| Conventional RO | 1–3 | 1× |
| High-recovery RO | 3–6 | 2× |
| Brine concentrator (MVC) | 15–25 | 6–10× |
| Crystalliser | 40–70 | 10–15× |
Design judgements that decide the outcome
Silica, calcium sulphate and organics set how far membranes can go before thermal plant must take over — softening and pH strategy ahead of the high-recovery stage is where clever chemistry saves brutal kilowatt-hours. Materials are unforgiving: saturated hot chloride brine eats standard stainless; duplex, super-duplex and titanium appear on the datasheets and in the price. And the crystalliser is a chemical plant, not a water plant — sites that staffed it like a filter have the operating records to prove the mistake.
Minimal, not zero
The best ZLD decision I see repeatedly is stopping one step short — minimal liquid discharge at 90–97 per cent recovery, holding a small residual brine for ponds, injection or a shared outfall, and avoiding the crystalliser's capital and care entirely. True ZLD earns its cost where regulation forbids any discharge, where groundwater liability is existential, or where the salt has a buyer. Elsewhere, the honest comparison usually lands at MLD — and the site that knows this at concept stage buys its evaporators once, sized right.
- Tong, T. & Elimelech, M., The global rise of zero liquid discharge for wastewater management. Environmental Science & Technology 50.
- GWI, Water for the industrial world — brine management market data.
- Author's industrial brine and reuse study records, 2012–2026.
Kassem, M. S. (2026). ‘Zero liquid discharge: the honest cost of the last litre’. dr-kassem.com/writing/zero-liquid-discharge. Accessed 18 Sep 2026.