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Measuring Innovation: What's the most sustainable way of dealing with reverse osmosis and nanofiltration membrane concentrate, particularly in water-scarce Florida? Treat it and drink it. By Phillip Locke, Fred Greiner, and Ryan Popko U sing unconventional sources often requires the implemen- tation of new process applications to meet potable water treatment requirements and demands. Effectively treating waste streams can be an essential part of meeting alterna- tive water supply goals. Although membrane treatments — specifi- cally reverse osmosis (RO) and nanofiltration — feature significant benefits in treating water to a consistently high level, managing the treatment process byproduct, or concentrate, can be a challenge. Meeting The Challenge The City of Palm Coast, FL, had explored several methods to develop alternative water supplies, recover and reuse water, and reduce the util- ity's environmental impact. The city's consumptive use permit limited source water supplies, while regulatory constraints required an alterna- tive means of concentrate disposal. Because of these constraints, the city elected to treat and recover its RO concentrate as an alternative water supply, using a unique zero liquid discharge (ZLD) process at the water treatment plant (WTP) No. 2 membrane softening facility. The ZLD process is capable of treating 1.2 MGD of low-pressure RO concentrate using lime and soda ash softening, membrane filtra- tion, and disinfection. It allows the city to treat and recover nearly 100 percent of the WTP-produced concentrate that was previously discharged to a surface water body. The process treats the concentrate stream to a level that complies with all primary and secondary drink- ing water standards when blended with the plant's finished water and increases the facility's capacity by 1.2 MGD without additional source water supplies. Palm Coast's ZLD project evolved after the city's most recent National Pollutant Discharge Elimination System concentrate dis- charge permit renewal, as the U.S. EPA and Florida Department of Environmental Protection determined that the use of an extended mixing zone for water quality compliance was no longer feasible. The city evaluated 12 alternative disposal methods of concentrate based on treatability, feasibility, and economics. After bench-scale and pilot test- ing, the city selected ZLD for its WTP No. 2. The concentrate ZLD option provided concentrate recovery as an alternative water supply for potable use and for beneficial reuse of solids removed from the concen- trate stream. The local water management district's push for the use of alternative water supplies was also a factor in ZLD selection. An aerial view of WTP No. 2 shows many of the facilities built to treat and recover the plant's RO concentrate as an alternative water sup- ply using the ZLD process, including (clockwise from top left) a sludge dewatering facility, a lime silo, a post-treatment and ultrafiltration building, and solids contact clarifiers. Process Design The primary components of the ZLD process include lime and soda ash softening, clarification, ultrafiltration (UF) feed system, pressurized UF membrane system, UF backwash and cleaning system, disinfection system, process recycle and recovery system, solids handling system, sludge dewatering system, process and yard piping, electrical systems, and instrumentation and control systems. The table below presents the water quality parameters used to design the concentrate ZLD system. 14 wateronline.com n Water Innovations Zero Liquid Discharge Project Turns Treatment Byproduct Into Drinking Water An aerial view of WTP No. 2 shows many of the facilities built to treat and recover the plant's RO concentrate as an alternative water supply using the ZLD process, including (clockwise from top left) a sludge dewatering facility, a lime silo, a post- treatment and ultrafiltration building, and solids contact clarifiers. Parameter RO Permeate RO Concentrate (ZLD Feedwater) ZLD Water Goal 1 Actual Water Quality 2 Total Hardness (mg/L as CaCo 3 ) 4.1 1,650 <155 130 True Color (C.U.) 0 53 <65 41 Total Dissolved Solids (mg/L) 51 2,500 <2,320 966 Iron (mg/L) .02 1.7 <.17 .01 Chloride (mg/L) 11.1 600 <1,368 320 Turbidity (NTU) .10 8.8 <.41 .20