Evaporation is the workhorse of zero liquid discharge systems, concentrating RO reject brine from 5–10% TDS to near-saturation prior to crystallisation. Technology selection and scaling control determine system feasibility.
Feed liquid is boiled under vacuum; the vapour generated is mechanically compressed by a blower or compressor to a higher pressure, raising its condensation temperature above the boiling point of the feed. This allows the compressed vapour to act as the heating medium in the evaporator, recovering >95% of the latent heat. Specific energy: 6–15 kWh/m³ evaporated. Preferred for continuous operation >200 m³/day where electrical energy requirement is moderate.
Steam from one evaporator effect is used as the heating medium for the next effect at lower pressure. Three to seven effects are common; each additional effect reduces steam consumption by approximately 1 kg steam/kg evaporation. Specific steam consumption: 0.15–0.35 kg/kg (3–7 effects). Capital cost is higher than MVR but preferred where low-cost steam is available from an adjacent boiler or CHP plant.
A thermocompressor uses high-pressure motive steam to entrain and compress vapour from the evaporator, achieving steam economy between single-effect and MVR. Capital cost is lowest of the three options; widely used in food-industry evaporation. Not preferred for ZLD where electrical efficiency is the key operating requirement driver.
Operating at 30–70°C under vacuum, vacuum evaporators handle heat-sensitive wastewaters and reduce scaling risk. Ideal for pharmaceutical and food industry effluents where thermal degradation is a concern.
| Parameter | MVR | MEE (3-effect) | MEE (7-effect) |
|---|---|---|---|
| Specific energy (kWh/m³) | 6–15 | 45–60 (steam equiv.) | 15–25 (steam equiv.) |
| Steam requirement | None (electrical) | 0.35 kg/kg | 0.15 kg/kg |
| Capital cost (relative) | Medium | Low | High |
| Best capacity range | 50–5,000 m³/day | Any | >500 m³/day |
| Turndown capability | Excellent (VSD compressor) | Moderate (steam valve) | Moderate |
| Scaling sensitivity | Moderate | Moderate | Higher (multiple heat surfaces) |
| Preferred when | Electricity <, no steam | Waste steam available | Very large plants, cheap steam |
Precipitates as pH rises with CO&sub2; stripping. Controlled by pre-acidification to pH 4–5 before the evaporator, or by seeded precipitation in a softening reactor to remove calcium before the evaporator. Threshold inhibitors (antiscalants) provide limited protection at the high concentrations in ZLD evaporators.
Has inverse solubility: solubility decreases above 40°C. A major scaling risk in FGD blowdown and mining AMD streams. Controlled by operating below the gypsum saturation point or by seeded slurry recirculation (seeded slurry evaporation — SSE). Alternative: remove sulphate with barium chloride precipitation or ion exchange prior to evaporation.
Amorphous silica precipitates above saturation (~100–200 mg/L at typical evaporator temperatures). Once deposited, silica scale is extremely difficult to remove chemically. Controlled by pH adjustment (high pH increases solubility but risks other scaling), or by upstream softening and silica removal with magnesium hydroxide dosing.
Operating at 30–70°C under vacuum, vacuum evaporators handle heat-sensitive wastewaters and reduce scaling risk. Ideal for pharmaceutical and food industry effluents where thermal degradation is a concern.
Design principle: Pre-treatment before the evaporator is as important as evaporator design itself. A well-designed pre-treatment train — pH adjustment, antiscalant, softening, and organic removal — can double the operating time between cleans and reduce life-cycle cost by 25–40%. Contact our engineers to review your brine composition before specifying evaporator type.
Forced-circulation crystallisers for salt recovery from concentrated brine.
CrystallisationOur engineers are available to review your site conditions and recommend the most appropriate treatment solution.
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