Zero Liquid Discharge Systems Offer Sustainability

Creating safe, sustainable and environmentally friendly water treatment systems is a priority for ethanol production plants. Zero liquid discharge systems are gaining ground as pragmatic solutions.
By Amelia Jordan | September 15, 2009
Regulations and environmental compliance parameters are being tightened, public perception of industrial manufacturing's impact on the environment is at an all-time high and concerns are mountining over the quality and quantity of our water supply. As a result, zero liquid discharge (ZLD) systems have become more prevalent. ZLD describes 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.

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 a critical step before selecting a treatment strategy.

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 the equipment generates.

Cooling tower blow down volumes can be greatly minimized with the use of high quality makeup 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 percent to 50 percent of the water it treats as waste; while a high efficiency system only has about 5 percent 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 sent for disposal. IEX backwash and regenerant waste can be recycled and reused.

A common ZLD approach is to concentrate (evaporate) the waste water 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. It can be sent to a publicly owned treatment works facility that uses evaporation ponds in areas with net positive evaporative climates (evaporation exceeds precipitation), or treated in a crystallizing system, such as a circulating-magma crystallizer or a spray dryer. Crystallized solids can be landfilled or land applied, depending upon the crystal characteristics.

In April of 2009, U.S. Water Services completed a complex water treatment system in Galva, Ill., combining both high efficiency RO with evaporation/crystallization technology for the first time ever in an ethanol facility in order to achieve zero 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. A high efficiency reverse osmosis unit was put in place as part of the third process, enabling water recoveries of 95 percent to 97 percent 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 percent to 90 percent. The remainder is then crystallized to a landfillable solid, in this case a salt cake, which is non-hazardous to the environment.

Some industrial water consumers have installed cold lime softening (CLS) systems to pre-condition the water used for plant processes. CLS technology 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 RO systems, subsequently increasing efficiency and reducing waste volume.

In the spring of 2006, the first dry grind ethanol plant designed and operated with no liquid discharge to the environment began production in central California. The production 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 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 with solubles 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, ZLD strategies are increasingly 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, CLS may still be a useful application. Depending on the specific permit regulations that a plant faces, it's often possible to significantly reduce liquid discharge volumes, without actually eliminating it. In addition, many state permitting agencies see CLS as a means to pollutant control, as it is 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 IEX systems do.

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. EP

Amelia Jordan, P.E., is design engineer for U.S. Water Services. Reach her at info@uswaterservices.com.