By Dr. David Kratochvil, BioteQ Environmental Technologies
Water conservation and reuse have become increasingly significant issues for industrial users. The rising cost of water and scarcity of supply in some regions, when combined with increasingly stringent regulatory demands for wastewater treatment and reuse, are driving the industry to consider alternative ways to remove contaminants from wastewater streams and increase conservation efforts.
Coal-fired power generation systems are among those that have come under significant scrutiny in recent months for their wastewater treatment practices. Numerous studies have been conducted to assess the total dissolved solids (TDS) that accumulate in process streams, including cooling and utility water, ash pond water and flue gas desulfurization (FGD) wastewater. While some successful measures have been put in place to remove contaminants from wastewater streams for reuse, these may fall short of the mark in terms of economic viability as the industry faces even stronger legislation in the future.
New water treatment technologies can achieve high rates of water recovery and clean water, at a lower life cycle cost compared to conventional alternatives. Photo courtesy BioteQ Environmental Technologies.
Ultra-filtration membrane and reverse osmosis systems, for example, are common solutions in use today for removing contaminants. However, these can be costly to manage, consume a considerable amount of energy and result in relatively low water recovery rates.
New technologies such as ion exchange can address this hurdle, offering more energy-efficient and cost-effective closed-loop alternatives that can help utility operations meet legislative requirements for water re-use (water recovery can be well into the 90+ percent range), while reducing operational costs. In addition, because these newer processes use low-cost reagents such as lime for resin regeneration, they are not only less expensive to run, they do not produce brine waste or require pre-treatmentand they can be applied to ash pond water, cooling water and zero liquid discharge (ZLD) systems.
The Water Dilemma
Water is becoming an increasingly precious commodity, especially in drought-prone areas such as the Western United States. Fossil fuel-fired power plants that operate in these areas especially are always mindful of the need to conserve as much water as possible.
For these types of power plant operations, cooling water accounts for the largest portion of overall consumption, followed by ash handling and FGD scrubber make-up water. Cooling water consumption is composed of evaporative losses, which are directly affected by a power plant’s overall thermal efficiency, and cooling tower blow-down, which can account for 35 percent of the total cooling water consumption. As a result, we see a significant discrepancy between overall water usage and actual consumption. Factors that can affect consumption levels include power plant thermal efficiency; make-up water quality; dry or wet flue and bottom ash handling and sulfur content of coal, gypsum, slurry dewatering; design of FGD scrubbers and steam line leakage.
Adding to the challenge is that, more often than not, the fresh water available in drier regions is often of poor quality and contains elevated levels of contaminants such as sulfate, calcium, chlorine, silica and TDS that limit reuse. In the U.S., the power plants most challenged in water management are those that rely on water sources with elevated TDS and hardness, such as water drawn from the Colorado River or the Ogallala aquifer covering South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico and Texas. These contaminants can lead to a myriad of problems that negatively affect water reuse, such as scaling, corrosion and emissions.
Sulfates in particular have become an increasing challenge, since they were not included under legislative mandates until recently. For some time, they were considered relatively harmless to humans. Recent research indicates however that crop yields and fertility rates in livestock have been negatively affected by the increasing concentration of sulfates in wastewater streams. Sulfate in process waters can also cause scaling in equipment, leading to reduced performance and premature equipment failure.
In addition, these contaminants present problems in terms of water discharge, since they can be toxic to aquatic life, livestock and humans. Power plant wastewater that cannot be reused or discharged therefore may be directed to zero liquid discharge systems in the form of evaporators.
Ion exchange technology, which has been shown to reduce sulfates in mining waste water, is now being considered as a viable option for power plant water management efforts. This process specifically handles the removal of TDS from hard waters with a high scaling potential and elevated sulfate levels.
Ion exchange technology operates on an entirely different principle from that of membrane systems for removing calcium and magnesium sulfates, among other contaminants, from wastewater. In simple terms, it is a two-stage process that uses cationic and anioic resins to remove calcium or magnesium and sulfate respectively, and then regenerates the resins. The ideal ion exchange solution for sulfate reduction is one that combines both anionic and cationic resins and works as follows:
- Feed water is passed through a series of contactors containing cation exchange resin to remove calcium and magnesium.
- The water is then passed through a second set of contactors containing anion exchange resins to remove sulfate.
- The clean water is then recycled or discharged safely to the environment.
Newer ion exchange processes use low-cost, off-the-shelf resins to remove calcium and sulfate ions from water in various concentrations. The process requires no pre-treatment and leaves no residual waste for special disposal. Besides clean water, the only other by-product of this process is gypsum, a saleable resource used in fertilizer production and building materials.
Depending on the ion exchange solution, it can also consume up to 90 percent less energy than a reverse osmosis system and can achieve 95 percent water recovery rates, thereby reducing the impact on local water supplies. This process can reduce operating and capital costs by at least half for certain resource and industrial applications. In some cases, the lower treatment costs and energy consumptioncombined with increased recovery ratescan translate into several million dollars of savings in annual power, fresh water and pre-treatment costs for power generation plants.
In addition to these savings, ZLD systems operating downstream of cooling towers using ion exchange technology can be smaller and require less salt throughput, thereby reducing the costs associated with constructing solar ponds and/or evaporator-crystallizer systems. And the gypsum produced from this process is not contaminated, making it a saleable product.
The Future of Water
The U.S. has been clear about its commitment to environmental stewardship. At the same time, the power generation industry is facing significant barriers in terms of its environmental impact on ground water aquifers if it does not find a way to reuse the resources it has. This is especially pressing in areas where water is a finite resource and where populations continue to grow, placing added strain on water allocation practices.
The challenge for the power generation sector will be to find solutions that not only remove contaminants, but also support environmental and fiscal sustainability without depleting precious water and energy resources in the process.
Author: David Kratochvil is the president and COO of BioteQ Environmental Technologies, a Vancouver-based water treatment company that applies technologies and operating expertise to solve water treatment problems.