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.