Why ZLD Plants Cost Too Much to Run — and Seven Ways to Cut the OPEX

In a typical ZLD train — pre-treatment, biological ETP, RO, evaporator, crystalliser — the evaporator and crystalliser account for 50–70 % of operating cost even though they handle only 10–25 % of the flow. Multiple-effect evaporation consumes roughly 0.15–0.25 tonnes of steam per m³ of water evaporated; mechanical vapour recompression consumes 30–50 kWh/m³. Add cleaning, maintenance of exotic-metallurgy heat exchangers, and salt disposal at ₹4,000–8,000 per tonne to a TSDF, and the tail of the plant is the whole story.

By contrast, reverse osmosis concentrates the same water for 1–3 kWh/m³. Every cubic metre you can push through membranes instead of the evaporator saves an order of magnitude in energy. That single fact drives everything below.

1. Segregate streams at sourceHigh-TDS dye-bath, mother liquor or regeneration brine should never mix with low-TDS wash water. Segregation halves the volume the evaporator sees and often allows the low-TDS stream to be reused after simple RO.
2. Push RO recovery as far as scaling allowsStandard brackish RO stops at 70–75 % recovery. A second high-pressure stage, disc-tube RO or forward osmosis takes the brine to 90–95 %, shrinking the evaporator by two to four times. Softening and silica removal ahead of it is what makes this possible.
3. Choose MVR over MEE where steam is not freeIf your site has no waste steam, an electrically driven MVR evaporator at 30–50 kWh/m³ usually beats a steam MEE in life-cycle cost, and needs no boiler operator.
4. Recover a saleable saltTextile clusters recover sodium sulphate (glauber salt) for reuse in dyeing; chlor-alkali and tannery brine can be purified to NaCl. Selective crystallisation turns a disposal cost into a small revenue.
5. Remove COD before the membranes, not afterOrganic fouling is the main reason RO recovery collapses over time. A properly run biological stage, UF and, where needed, activated carbon protect the membranes and keep recovery high year after year.
6. Automate CIP and antiscalant dosingMembrane cleaning triggered by normalised permeate flow and differential pressure — not by calendar — extends membrane life and avoids the recovery losses of a fouled train.
7. Design the evaporator for the salt you actually haveForced-circulation for scaling salts, falling-film for clean brine, proper metallurgy (duplex or titanium where chloride is high). A wrongly specified evaporator scales, leaks and runs at half rated capacity — the most expensive failure in the whole plant.

*Illustrative for a 1,000 KLD textile effluent; actual figures depend on TDS, steam and power tariffs and salt market. The ranking, not the exact numbers, is the point.

• No segregation — the whole plant flow goes to ZLD-grade treatment.
• Spiral-wound RO on brine above 60,000 mg/L TDS — constant replacement.
• MEE bought on price with carbon-steel or 304 SS heat exchangers in chloride brine.
• Evaporator sized for design flow with no margin for the RO recovery falling to 60 % in year three.
• Salt stored in the open — dissolves in the monsoon and becomes effluent again.

We begin with a water-and-salt balance of the whole site, not just the ETP outlet. Segregation, reuse and recovery come before any evaporator is sized. Then two or three schemes with life-cycle cost over ten years, detailed engineering of the selected one, and supply, erection and commissioning. We also audit running ZLD plants whose OPEX has drifted out of control. See the zero liquid discharge page for the process description.