MVR Technology for Treating High-Salinity Wastewater

MVR technology is currently the mainstream core technology in the field of zero-discharge of high-salinity wastewater. It achieves extreme energy savings by recovering the latent heat of secondary steam and is suitable for treating various types of high-salinity wastewater with a TDS (Total Dissolved Solids) concentration of 1%-20%.

I. Core Working Principle

The MVR method is essentially a heat pump system. Electricity drives a compressor to compress and heat the low-temperature, low-pressure secondary steam (approximately 70-85℃) generated by evaporation to 95-110℃, increasing its enthalpy and reusing it as a heat source, forming a closed-loop heat energy cycle of "evaporation → compression → reheating".

Energy Conversion Formula: Electricity (driving the compressor) + Latent heat of secondary steam = Heating steam heat energy

Theoretically, 1 kWh of electricity can evaporate 5-7 kg of water, with total energy consumption only 20%-30% of that of a traditional triple-effect evaporator.

II. Complete Process Flow for High-Salinity Wastewater Treatment

Typical process chain: Pretreatment → MVR evaporation and concentration → Crystallization and salt separation → Resource recovery

1. Pretreatment Unit (Key Step)

pH Adjustment: Neutralize with alkali, control pH 6.5-7.5 to avoid equipment corrosion.

Hardness Removal: Add Na₂CO₃ and NaOH to precipitate Ca²⁺ and Mg²⁺, reducing hardness to <20mg/L.

Suspended Solids Filtration: Quartz sand + multi-media filtration, SS <10mg/L Special Treatment: For high-silica wastewater, flocculation and sedimentation are required (silica removal rate 90%).

2. MVR Evaporation and Concentration Unit Forced Circulation Process (Suitable for High-Salinity Wastewater):

2.1. Preheated material is fed into the system; the forced circulation pump starts after the liquid level reaches the target.

2.2. The material circulates at high speed in the tube side of the shell-and-tube heat exchanger (flow rate 2-3 m/s), exchanging heat with the compressed steam in the shell side to raise its temperature.

2.3. The heated material enters the negative pressure evaporation chamber (vacuum degree -0.07~-0.09 MPa).

2.4. Flash evaporation at 60-75℃

2.5. Secondary steam, after vapor-liquid separation, enters the compressor, and after compression, re-enters the shell-side circulation of the heat exchanger.

2.6. When the material reaches saturation, crystals begin to precipitate; it is discharged after reaching the discharge concentration (TDS 200,000-250,000 mg/L).

Optimization for high TDS wastewater:

Waste heat recovery: Preheating raw materials using condensate waste heat, saving 15% energy.

Segmented concentration: Falling film evaporation to 8%-10% in the first stage, forced circulation in the second stage. Concentrated to over 20%, balancing efficiency and scale prevention

3. Crystallization and Salt Separation Unit

3.1Separate Crystallization: NaCl and Na₂SO₄ are separated using a nanofiltration membrane (NF) and then crystallized separately.

3.2Product Purity: NaCl ≥ 98.5%, Na₂SO₄ ≥ 99%

3.3Mother Liquor Treatment: Mixed salt mother liquor (<5%) is treated by low-temperature drying or rotary kiln (1100℃).

4. Resource Recovery Unit

4.1Condensate: TDS < 500 mg/L, COD < 80 mg/L Recyclable in production

4.2Crystallized salt: Sold as an industrial raw material, with annual revenue reaching 2-3 million yuan (based on a treatment capacity of 10t/h)

III. Practical Application Cases

Case 1: High-salt wastewater from coal chemical industry

China Coal Mengda Project:

Water quality: TDS > 20%, silicon content > 100mg/L, high Ca²⁺ and Mg²⁺

Process: Lime softening + flocculation sedimentation (90% silicon removal) + DTRO concentration + MVR evaporation and salt separation

Result: Crystallized salt purity > 98%, System Recovery Rate 85%

Zhong'an Joint Project:

Scale: 80 m³/h

Process: HERO Concentration + Ozone Oxidation + TVR Evaporation + Freezing/Hot Melting Crystallization

Results: Annual wastewater reduction of 690,000 m³, recovery of 98,000 tons/year of industrial salt, and a 40% reduction in operating costs

Case 2: High-Salinity Wastewater from the Food Industry

Kimchi Production Enterprise:

Water Quality: Salted wastewater COD 32,000 mg/L, salt content 40,000 mg/L

Process: Gaseous... Flotation pretreatment + MVR evaporation crystallization + waste heat recovery

Results: Condensate COD < 800 mg/L, salinity < 500 mg/L, operating costs reduced by 40%

Pickled vegetable processing plant:

Scale: 200 tons/day, TDS 15,000-20,000 mg/L

Process: Coagulation flotation + salt-tolerant MBR + DTRO + MVR

Cost: Treatment fee reduced to 18 yuan/ton, 1.2 tons of crystallized salt produced daily, partially reused

IV. High TDS wastewater Economic Advantages

The economics of MVR significantly increase with increasing TDS concentration:

Low TDS (1%): Processing 101 tons of feed solution yields 1 ton of water; auxiliary equipment has high energy consumption, with a total power consumption of approximately 40-50 kWh/t.

High TDS (10%): Only 11.1 tons of feed solution need to be processed; auxiliary energy consumption is significantly reduced, with the total power consumption dropping to 30-35 kWh/t.

Conclusion: The higher the TDS, the smaller the MVR equipment size, the lower the unit energy consumption, and the more outstanding the economic advantages.

V. Technical Advantages and Limitations

Core Advantages

1. Extreme Energy Saving: Energy consumption is only 20-30% of traditional processes, saving 60-80% in energy.

2. Low-Temperature Operation: Evaporation temperature 60-75℃, suitable for heat-sensitive substances.

3. No Waste Heat Emission: Condensate heat recovery, cooling water consumption reduced by 90%.

4. High Degree of Automation: PLC/DCS control, reducing manual intervention.

5. High Resource Utilization Rate: Salt recovery rate >95%, impurity salt rate <5%.

Technical Limitations

1. High Material Requirements: When Cl⁻ concentration >30,000mg/L, titanium or duplex stainless steel must be used, increasing investment costs by 30-50%.

2. Sensitive to Water Quality: Hardness and silicon content need to be pretreated to <20mg/L, otherwise severe scaling will occur.

3. Organic Matter Interference: C… Wastewater with OD > 5000 mg/L requires additional oxidation pretreatment; otherwise, crystallization purity will be affected.

VI. Process Optimization Recommendations

1. Pretreatment is key: Enhanced softening and hardening removal using weak acid resin exchange to reduce total hardness to < 10 mg/L.

2. Membrane concentration coupling: DTRO concentrates TDS to above 12% before entering MVR, reducing evaporation load by 40-60%.

3. Anti-scaling technology: Forced circulation flow rate ≥ 2 m/s + online cleaning CIP + ultrasonic descaling (20-40 kHz).

4. Energy cascade utilization: Condensate preheating + waste heat recovery module, further reducing overall energy consumption by 15%.

MVR technology offers the best overall economic performance in the field of zero-discharge of high-salinity wastewater, especially in projects with TDS > 10% and treatment capacity > 10 m³/h.