• Precipitation, evaporation, crystallization, recycling and other creative approaches such as cold lime softening are all viable approaches explored to minimize industrial water needs.
Cold lime softening (CLS) system at Pacific Ethanol Inc.’s Madera, CA, fuel ethanol facilities, the nation’s first such facility with complete recycle, zero liquid discharge.
Regulations and environmental compliance parameters are being tightened, public perception of industrial manufacturing’s impact on the environment is heightened and concerns are mounting over the quality and quantity of our water supply. As a result, zero liquid discharge (ZLD) systems have become more prevalent. ZLD is a term used to describe complete elimination of liquid discharge from a manufacturing process. More companies and industries have to treat or eliminate waste streams to a much higher standard than ever before. There are several approaches to solving this problem, some of which can be integrated into existing processes. Thorough evaluation of the impact of feasibility, cost (capital and operational), and complexity is very important before selecting a treatment strategy. This article explores various traditional zero discharge options as well as some alternative approaches currently in use.
Water, Waste Minimization
The goal of a well-designed ZLD system is to minimize the volume of liquid waste that requires treatment while also producing a clean stream suitable for use elsewhere in the plant processes. Typical sources for waste streams in an industrial setting include cooling tower blowdown, reverse osmosis (RO) concentrate, multimedia filter backwash and spent ion exchange (IEX) softener regenerant. The key to reducing overall wastewater flow is to select and/or optimize the equipment in order to optimize the flow stream quality generated by the equipment.
Cooling tower blowdown volumes can be greatly minimized with use of high quality makeup water. This can be achieved by treating makeup water for cycle-limiting ions such as hardness and silica. RO concentrate volumes can be minimized by integrating high efficiency systems to condition the water upstream of the RO units, such as softening, alkalinity removal and pH adjustments. A typical RO system rejects roughly 25-50% of water it treats as waste; while a high efficiency system only has about 5% water waste. Filter backwash waste can be minimized by integrating backwash methods incorporating air wash scouring or simultaneous air and water techniques. The collected backwash water can be captured, settled and recycled, while settled solids are collected in a filter press and disposed of. IEX backwash and regenerant waste can be recycled and reused.
Evaporation, Crystallization
A common ZLD approach is to concentrate (evaporate) the wastewater and then dispose of it as a liquid brine, or further crystallize the brine to a solid. A typical evaporator uses tube-style heat exchangers. Evaporated water (distillate) is recovered and recycled while brine is continually concentrated to a higher solids concentration. Concentrated brine is disposed of in various ways, such as sending it to a publicly owned treatment works (POTW), using evaporation ponds in areas with net positive evaporative climates (evaporation exceeds precipitation), or by treatment in a crystallizing system, such as a circulating-magma crystallizer or a spray dryer. Crystallized solids can be landfilled or land applied, depending on the crystal characteristics.
In April 2009, U.S. Water Services completed a complex water treatment system in Galva, IL, combining both high efficiency RO with evaporation/crystallization technology to achieve zero discharge. Due to environmental restrictions, applications such as these were necessary for the plant to operate. This particular facility integrated four major processes for the water to travel through. The first process is dual softening, consisting of a strong acid cation cycle and a weak acid cation. The second process is a decarbonator which greatly reduces carbon dioxide. A high efficiency RO unit was put in place as part of the third process, enabling water recoveries of 95-97% to be achieved, thus greatly reducing discharge volumes. The last process is evaporation and crystallization, where ZLD results can be achieved by evaporating down the waste stream volume by 80-90%. The remainder is then crystallized to a landfillable solid, in this case a salt cake, that’s non-hazardous to the environment.
Cold Lime Softening
Some industrial water consumers have installed cold lime softening (CLS) systems to pre-condition the water
The evaporator/crystallizer
In spring 2006, a plant started in central California. One of the most unique features of this start-up was that it was the country’s first ethanol plant designed and operated with no liquid discharge to the environment. Listed as an environmentally sensitive area, California’s San Joaquin Valley doesn’t allow any aqueous industrial discharge. To build the plant in this prime agricultural location, a process for re-using the discharge from the cooling tower, pretreatment equipment and process streams needed to be developed.
After carefully analyzing the local water quality, as well as the plant process demands, U.S. Water Services designed a method using CLS to precipitate many of the minerals from the water. The minerals, which are rich in calcium, are then added to the Dried Distillers Grains and Syrup (DDGS), supplementing the nutrient value of this valuable animal feed byproduct of ethanol production.
As environmental, political and public health entities place more focus on wastewater management, zero discharge strategies are more often being evaluated for feasibility in industrial facilities. The ZLD approach taken greatly depends on the quality of water available for use. Precipitation, evaporation, crystallization, recycling and other creative approaches such as CLS are all viable approaches to this end.
While most plants don’t have the restriction of zero discharge from their facilities, cold lime softening may still be a useful application. Depending on the specific permit regulations a plant is faced with, it’s often possible to significantly reduce liquid discharge volumes, without actually eliminating it. In addition, many state permitting agencies see lime softening as a means to pollutant control, as it’s one of the few available processes that actually removes minerals from the water rather than simply discharging them in a concentrated waste stream as RO and ion exchange systems do.
The high efficiency reverse osmosis units
The other important feature of lime softening systems is that they allow the plant management much greater flexibility in the sources of water that can be utilized. An ethanol plant in Minnesota needed to reduce the impact on the local aquifer, so it contracted with U.S. Water Services to design a CLS system that could treat river water. The plant has benefited in a number of ways from this project. Water usage for their cooling water and process systems dropped by over 30% (roughly 45 million gallons per year) as a result of the improved water quality. More importantly, the water treatment system design allows them to utilize river water, well water and even water from a storm drainage pond, depending on which makes the most sense at any given time.
Conclusion
Through the use of new treatment technologies, and by using old technologies in novel ways, a significant impact can be made via ZLD options on the amount and quality of water that a facility uses and discharges.
About the Author: A registered professional engineer, Amelia Jordan is a design engineer with U.S. Water Services Inc., of Plymouth, MN. Contact: 866-663-7632, [email protected] or www.uswaterservices.com
