Recognizing the risks around looming water scarcity and stress, industrial firms are increasingly turning to zero-discharge models for treatment of wastewater.
Gena Leathers, water issue leader for corporate water strategy at The Dow Chemical Co., explained to me in an interview that zero-liquid discharge at its core involves “bringing in technologies that allow you to capture, recycle, and reuse water.” She told me that “in some areas of the world zero-liquid discharge is a requirement.” This is the case, she said, increasingly in Europe and India, and “the concept is gaining more and more awareness in the U.S. as well.”
In the same interview, Neil Hawkins, Dow’s vice president for sustainability and global EH&S (environment, health, and safety), added that “in general, you’ll see that the trends are more toward zero discharge and ever-tightening requirements on quality of discharge.” In fact, he said, “in a lot of places the water we discharge is cleaner than the water we take in.” This is the case in Saudi Arabia, for example. In such markets, “you’re highly incentivized to keep that water and re-use it as much as you can. The more scarce the water, the more the economic incentive to go zero-discharge.”
Optimizing Water Resources, Minimizing Environmental Impact
As I reported in May, manufacturing and industrial firms rely heavily on water supplies for their operations but are increasingly competing for water with municipalities and agriculture. The United Nations’ Food and Agriculture Organization (FAO) estimates that industry uses 18.7 percent of available water worldwide, a figure that climbs to 43 percent in North America. The U.N.’s World Water Development Report says that “Water scarcity is viewed as an increasing business risk, with industrial water supply security dependent on sufficient resources.” This problem is compounded “by geographic and seasonal variations, as well as water allocations and competing water needs in a given region.”
Planning and permitting a new industrial plant is usually a long and arduous process. But “designing a plant for zero-wastewater discharge right from the start wins faster community acceptance and streamlines the permitting process,” write Joe Bostjancic and Rodi Ludlum of GE’s Water and Process Technologies division in a technical paper. Zero discharge can vastly improve the efficiency and economics of a plant, decreasing the amount of water that has to be purchased from local utilities and the treatment requirements for discharge. “Wastewater recycling also allows a greater freedom in selecting a site for an industrial plant because there are fewer concerns about adequate water supply,” they stress, and often “poor quality water can be used for make-up since it is upgraded in-house.”
Sellappan Eswaramoorthi, environmental scientist at Anna University in Chennai, India, writes in a paper on industrial wastewater treatment that the two essential processes in zero discharge are the recovery of water and other materials from wastewater and the minimization or elimination of pollution from the treatment facility into the environment.
Eswaramoorthi cautions that zero discharge theoretically means “no discharge of any kind of pollutants into the environment.” However, this is nearly impossible in an absolute sense, so “the term zero discharge is loosely used to define no liquid discharge into the environment.” Thus, while zero discharge is not technically the same as zero liquid discharge, the two terms are often used interchangeably. In practice, a zero liquid discharge process ultimately generates solid wastes that have to be disposed of in a conventional manner such as landfilling.
Eswaramoorthi says that a zero-discharge system adapts the three primary processes in conventional wastewater treatment (primary, secondary, and tertiary) so that the system eliminates discharge of pollutants by directing them into the solid phase, or sludge, which is then sequestered in a secure landfill. The system recovers reusable materials, especially water.
Since the requirements of an industrial zero-discharge system will vary greatly depending on the individual company and its waste stream, Eswaramoorthi says that the design of the system needs to be carefully thought out in advance, keeping in mind parameters such as:
- The quality of the wastewater being treated;
- The treatment system’s efficiency;
- The ability of the system to tolerate short-term shocks and long-term variability;
- Performance degradation of machinery over time;
- Operation and maintenance procedures such as backwash and cleaning;
- Mass-balance under various operating conditions, that is, the variability of volumes of materials at different stages and their input and output rates.
Strategic Re-Use of Water
According to Gary Howard, principal process engineer at engineering and construction firm Foster Wheeler, strategic reuse of recovered water is a key element of a zero-discharge model. “Across an industrial site there are many operations and processes that require water,” he writes in a conference paper, and “Good waste minimization techniques look at elimination or direct reuse of water as the first step.”
Foster Wheeler is a major builder of refineries. In such operations, Howard suggests, stripped sour water can be used in the desalters. Treated water “can be used in washing down dirty areas and for dust suppression in partially paved sites” or in cooling towers, if the level of dissolved solids is low enough. In some cases, treated water can even be used for irrigation, depending again on the mineral content and the particular agricultural requirements.
If “recovery to high-grade water is required,” Howard says, “the conventional treatment scheme is often enhanced to remove more contaminants.” Among the processes that might be employed, he lists nutrient removal, tertiary treatment such as sand filtration, membrane bioreaction (MBR), moving bed bioreaction (MBBR), and powdered activated carbon treatment (PACT). Dissolved solids can be removed to desired levels of purity by using reverse osmosis (RO) or multi-effect distillation (MED), followed further if needed by evaporation processes.
“Designing a plant for maximum water recycle and reuse is not the mystery it once was,” according to Bostjancic and Ludlum of GE. Employing new water treatment technologies, plants can now “recycle vast quantities of wastewater that once went to sewers, rivers, deep wells, spray fields, or percolation ponds.” Plants can now be “designed from the ground up with water conservation in mind.” In addition, they write, “difficult wastewaters” can be finished through such processes as evaporation, crystallization and spray-drying to reduce such waste streams to dry solids and “squeeze out the last bit of clean water for maximum recycle and reuse.”
A zero-discharge strategy has other environmental implications beyond optimizing the use of water resources and maintaining water purity. Ultimately, the solids and contaminants that get removed have to be dealt with, which could require special landfilling, as pointed out by Eswaramoorthi. Also, Howard says, “The extra energy expended in effectively distilling effluent to recover water increases the carbon footprint of the manufacturing process and increases the cost of the final product,” which has to enter into decisions around the design and costing of a facility.