Vapor Compression Distillation (VC or MVC)

Vapor Compression Distillation (VC or MVC) produces high-purity distillate from concentrate streams in municipal treatment plants by evaporating water under vacuum and mechanically compressing the vapor to provide heating energy for continuous operation. Systems typically achieve 95-98% water recovery from RO concentrate or other high-TDS streams, with distillate quality consistently below 10 mg/L TDS. The primary limitation is high energy consumption (25-40 kWh per 1,000 gallons) and substantial capital costs, making it economically viable only for concentrate minimization or specialized high-purity applications rather than primary treatment.

• Landfill Leachate Concentration: VC systems treat high-strength leachate (20,000-50,000 mg/L TDS) at 5,000-25,000 GPD, producing 95%+ volume reduction. Selected for reliable organics removal and minimal chemical usage. Upstream: leachate collection tanks. Downstream: concentrate disposal, distillate to headworks.
• Septage/FOG Processing: Municipal facilities use VC at 2,000-10,000 GPD to process concentrated septage waste, achieving 90-95% volume reduction while meeting discharge limits. Chosen for odor control and pathogen destruction through thermal treatment. Connects between receiving tanks and residuals handling.
• Reject Water Treatment: At plants with advanced dewatering, VC treats high-strength centrate (5,000-15,000 mg/L ammonia) at 10,000-50,000 GPD. Selected when biological treatment is impractical due to toxicity or temperature. Upstream: dewatering equipment. Downstream: plant headworks or direct discharge.

Daily Operations: Operators monitor feed flow rates (typically 75-95% design capacity), steam temperatures (200-220°F), and vacuum levels (-25 to -28 inHg). Key adjustments include feed rate modulation based on TDS levels and anti-scalant dosing (2-8 ppm) to prevent scaling. Energy consumption tracking averages 18-22 kWh per 1000 gallons produced.

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Maintenance: Monthly tube cleaning using circulation cleaning (citric acid or HCl solutions) requires confined space procedures and acid-resistant PPE. Quarterly compressor maintenance includes bearing lubrication and vibration analysis. Annual tube bundle inspection requires crane operations and welding-qualified technicians. Typical tube replacement occurs every 3-5 years depending on feed quality.

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Troubleshooting: Scaling manifests as rising steam temperatures and declining production rates, addressed through immediate cleaning cycles. Compressor bearing failures show increased vibration 2-4 weeks before failure. Vacuum leaks cause production drops of 15-25% and require systematic leak detection. Well-maintained units achieve 12-15 year service life with major overhauls every 7-10 years.

• Evaporator Vessel: Vertical shell-and-tube heat exchanger, 4-12 feet diameter, 316L stainless steel construction. Houses heating tubes where feed water vaporizes. Sized for 50-200 GPH/ft² flux rates based on feed TDS and fouling potential.
• Mechanical Vapor Compressor: Centrifugal or roots-type blower, 25-500 HP, compresses vapor 2-8 psi to increase saturation temperature. Selection based on compression ratio requirements and energy efficiency targets (typically 15-25 kWh/1000 gallons).
• Preheater/Economizer: Shell-and-tube exchanger recovers heat from hot concentrate and distillate streams, achieving 80-90% heat recovery. 316L construction with removable tube bundles for cleaning. Sized for approach temperatures of 5-15°F.
• Feed Pretreatment System: Includes pH adjustment, anti-scalant dosing, and multimedia filtration to prevent fouling. Critical for maintaining 85-95% availability in municipal applications with variable feed quality.

• Feed Water Quality Parameters: Total Dissolved Solids (TDS): 35,000-200,000 mg/L; Temperature: 40-180°F operating range; pH: 6.5-8.5 (pretreatment required outside range); Turbidity: <10 NTU (filtration typically required)
• Performance Parameters: Recovery Rate: 85-98% (typical 90-95% for municipal); Product Water Quality: <10 mg/L TDS, <1 mg/L chloride; Energy Consumption: 25-45 kWh/1000 gallons (varies with TDS); Gain Output Ratio (GOR): 8-15 (mechanical vapor compression)
• Sizing Parameters: Unit Capacity: 10,000-500,000 gpd per train; Heat Transfer Area: 100-2,000 ft²/unit; Compressor Power: 50-1,500 HP per unit; Operating Pressure: 5-25 psig (evaporator); Temperature Rise: 15-25°F across compressor
• Physical Constraints: Footprint: 400-1,200 ft² per 100,000 gpd; Height Requirements: 25-40 feet (including vapor space); Utilities: 480V/3-phase power, cooling water, compressed air

• What feed water TDS concentration and required recovery rate determine energy consumption? Municipal concentrate typically ranges 35,000-80,000 mg/L TDS. Recovery >95% increases energy consumption exponentially (40+ kWh/1000 gal vs. 25-30 kWh at 90%). Need detailed feed analysis including scaling potential (calcium, silica, sulfate) to determine pretreatment requirements and maximum practical recovery.
• Should the system use mechanical vapor compression (MVC) or thermal vapor compression (TVC)? MVC systems dominate municipal applications due to 60-70% lower energy consumption (25-35 kWh/1000 gal vs. 80-120 kWh for TVC). However, MVC requires higher capital cost and skilled maintenance. TVC may be justified where low-cost steam is available. Decision threshold typically occurs when steam costs <$8/1000 lb.
• What level of pretreatment is required to prevent scaling and fouling? Critical for reliable operation. Antiscalant dosing required for most municipal concentrates. Cartridge filtration (5-10 micron) standard. Softening needed when hardness exceeds 500 mg/L as CaCO₃. Wrong pretreatment selection results in 2-4 week cleaning cycles vs. 3-6 month intervals, significantly impacting operating costs and availability.
• How many parallel trains optimize reliability vs. capital cost? Municipal plants typically use 2-4 trains of 50,000-250,000 gpd each rather than single large units. Redundancy critical for continuous operation.

• Total Dissolved Solids (TDS): 35,000-200,000 mg/L
• Recovery Rate: 85-98% (typical 90-95% for municipal)
• Product Water Quality: <10 mg/L TDS, <1 mg/L chloride
• Energy Consumption: 25-45 kWh/1000 gallons
• Unit Capacity: 10,000-500,000 gpd per train
• Operating Pressure: 5-25 psig (evaporator)
• Heat Transfer Area: 100-2,000 ft²/unit
• Compressor Power: 50-1,500 HP per unit

• Material/Equipment Verification: Heat exchanger metallurgy certification (316L minimum for municipal brines); Compressor performance curves at design conditions; Electrical specifications matching site power availability
• Installation Requirements: Reinforced concrete pads for vibration isolation; Overhead crane access for maintenance; Steam/condensate piping with proper slope and trapping
• Field Challenges: Precise alignment of rotating equipment; Insulation installation around complex piping geometries; Integration with existing SCADA systems
• Coordination Issues: Early electrical coordination for VFD requirements; HVAC sizing for heat rejection loads
• Lead times typically 16-24 weeks for standard units.

• Aquatech International - HERO series (municipal concentrate management)
• Veolia Water Technologies - OPUS thermal systems (brine concentration applications)
• GEA Process Engineering - Varipond evaporators (municipal wastewater concentrate)
• Saltworks Technologies - SkyLoop systems (smaller municipal RO concentrate applications)
• All maintain North American manufacturing/service presence with municipal project experience.

• Thermal Evaporation - Lower operating costs but higher capital investment; preferred for >50 gpm concentrate flows. Roughly 20-30% higher capital cost.
• Deep Well Injection - Significantly lower cost where geologically feasible and permitted; regulatory approval increasingly difficult.
• Crystallization Systems - Higher capital cost (2-3x) but achieves zero liquid discharge; required where discharge limits are extremely stringent.

Establish manufacturer service agreements early - VC systems require specialized maintenance expertise not typically available in-house. Budget 15-20% of capital cost annually for service contracts. Negotiate performance guarantees based on actual feed water quality, not theoretical design parameters. Consider phased installation for larger systems to maintain plant operations during construction and commissioning phases.