Zero Liquid Discharge Considerations
EPRI Advanced Cooling Workshop
Dan CasiraroSalt River Project
Introduction
• Zero liquid discharge systems reclaim and reuse water that would otherwise be discharged.
• Although a total ZLD system includes all equipment involved with reclaiming all water streams, the brine concentrator and crystallizer are primary focus.
Introduction
• Zero discharge plants have water balance complexities and challenges
• Changes, excursions, and adjustments at one point in the water balance can have significant impacts at a separate point
Simplified NGS Water Balance
Typical BC Layout
NCG Vent
CrystallizerHeater
Feed
CrystallizerVapor Body
RecirculationPump
CompressionDevice
DewateringDevice
SlurryTank
Solids toDisposal
RecoveredWater
NCG Vent
CrystallizerHeater
Feed
CrystallizerVapor Body
RecirculationPump
CompressionDevice
DewateringDevice
SlurryTank
Solids toDisposal
RecoveredWater
Typical Crystallizer Layout
NGS Crystallizer
Crystallizers
Reasons to Install a ZLD System
• Water is in short supply• NPDES permitting obstacles• Discharge contamination issues• Increased CW blowdown flows due
to PM 2.5 rules
Design Considerations
• Characterize the water streams.• Determine mass loading.• Determine the final disposition of the
solids.• Optimize ZLD system capacity to the
plant situation.• Specify materials, instrumentation
and controls.
Solids In Equals Solids Out
Construction Considerations
• Participate during the performance guarantees.
• Review the proposed control package and upgrade if needed.
• Owner should be involved during new construction.
Operational Considerations• Surge storage between BC and
crystallizer• BC Blowdown Transfer Pipe / Trench• Process stability influences• Final solids transport• Operator training – Entry-level
position• Process monitoring• Contingency plans
Process Monitoring
Process Considerations
• High Boiling Point Rise• Sodium sulfate decahydrate• Sustained Limit Cycle Behavior• Seed recycle system• Condensate quality• High corrosion potential
Maintenance Considerations
• Operating with a restricted floodbox• Increased deposition potential
• Chloride concentration at saturation• Increased corrosion potential
• Process upsets vs. the maintenance schedule
• Planned maintenance versus emergency maintenance
• Chemical cleaning and routine maintenance
• Resource availability issues
Environmental Considerations
• ZLD system equipment is considered to be environmental equipment.
• Reclaiming miscellaneous wastes through a ZLD should be planned.
• A ZLD system cycles water to high concentrations possibly cycling an undesirable constituent too high.
Cost Considerations• Forced evaporation is energy intensive
and expensive.• Reclaim water outside the BC and
crystallizer if possible.• BC costs about $2 - $5 per 1000 gallons.• Crystallizer costs about 10X a BC.• Exotic materials are more expensive.• Complex water reclaim systems require
ponds and operator oversight.
Conclusions
• Although a ZLD system can be a challenge to design, construct, and operate, it is a viable solution to reclaiming water and eliminating water discharges.
• ZLD systems have unlimited configurations. Flexible configuration creates options.
• ZLD systems constantly evolve.
Top Related