By Amelia Jordan
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 or “ZLD” systems have become more prevalent. ZLD is a term used to describe the 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.
A water treatment system built in California uses cold lime softening to achieve zero liquid discharge. The system was designed to adhere to increased environmental regulations for the region.
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 paper explores various traditional Zero Liquid Discharge options as well as some alternative approaches currently in use.
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 blow down, 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 blow down volumes can be greatly minimized with the use of high quality make up water. This can be achieved by treating the 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 the 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 the settled solids are collected in a filter press and disposed of. IEX backwash and regenerant waste can be recycled and reused.
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. The evaporated water (distillate) is recovered and recycled while the brine is continually concentrated to a higher solids concentration.
Concentrated brine is disposed of in a variety of ways, such as sending 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 land filled or land applied, depending upon the crystal characteristics.
In April 2009, US Water Services completed a complex water treatment system in Galva, IL, combining both high efficiency reverse osmosis with evaporation/crystallization technology for the first time ever in an ethanol facility in order to achieve Zero Liquid Discharge. Due to environmental restrictions, applications such as these were necessary in order 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.
US Water Services was the lead build firm on this project using reverse osmosis systems combined with an evaporator and crystallizer to achieve zero liquid discharge.
A high efficiency reverse osmosis 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 zero liquid discharge results can be achieved by evaporating down the waste stream volume by 80-90%. The remainder is then crystallized to a land fillable solid, in this case a salt cake, which is non-hazardous to the environment.
Cold Lime Softening
Some industrial water consumers have installed Cold Lime Softening (CLS) systems to pre-condition the water used for plant processes. Cold Lime Softening is a technology that has been around for decades. It is used to remove minerals, principally calcium, magnesium, iron and silica, from water fed to the cooling towers and reverse osmosis systems, subsequently increasing efficiency and reducing waste volume.
In the spring of 2006, a dry grind ethanol plant started in central California. One of the most unique features of this start up was that it was the first ethanol plant in the country to be designed and operated with no liquid discharge to the environment. The process that allows for this environmentally friendly system was developed by U.S. Water Services.
Listed as an environmentally sensitive area, California’s San Joaquin Valley does not allow any aqueous industrial discharge. In order 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 process using Cold Lime Softening 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 waste water management, Zero Liquid Discharge strategies are more often being evaluated for feasibility in industrial facilities. The ZLD approach selected greatly depends on the quality of water available for use. Precipitation, evaporation, crystallization, recycling and other creative approaches such as Cold Lime Softening are all viable approaches to this end.
Through the use of new treatment technologies, and by using old technologies in novel ways, a significant impact can be made on the amount and quality of water that a facility uses and discharges.
About the Author: Amelia Jordan, PE, has 10 years of experience in the water treatment industry. Areas of expertise include groundwater treatment, surface water treatment and pilot scale system design. She is a registered Professional Engineer in the state of Minnesota.