Water: Lifeblood of the Process

Successful water reduction and recycling require close attention to treatments and system impacts in ethanol production. This feature appears in the February print edition of Ethanol Producer Magazine.
By Susanne Retka Schill | January 24, 2017

Water is the lifeblood of an ethanol plant: Mixed with ground corn in the mash to be fermented; cascading down the sides of the cooling tower where its evaporative cooling powers are used to cool the fermentation broth and other processes; clearing emissions in the scrubber; washing tanks and pipes; circulating through the plant to be reclaimed, cleaned and used again.

As the industry has improved its efficiency and recycling, water use has dropped by more than half in the past two decades, to an industry average of 2.7 gallons of water used per gallon of ethanol produced, as reported in 2012 by the Energy Resources Center at the University of Illinois at Chicago, the most recent industry survey data. In 2016, the center’s Steffen Mueller was still citing that as the average, even as some in the industry were reporting water use levels at, or just below, 2 gallons.
Reducing water usage requires recycling water wherever possible, and in about a quarter of the nation’s ethanol facilities, recycling has advanced to the point where no waste water is being discharged.
 
California Keys
In Keyes, California, Aemetis Advanced Fuels, has maximized water recycling and, indeed, has no discharge permit. Phil Cherry, plant manager, explains the plant brings in 32 gallons per minute (gpm) of fresh water from its two wells that gets incorporated into the 800 gpm of water flowing through the plant. The poor-quality well water, high in silica content, is cleaned using a reverse osmosis system. Other systems used at the plant include a cold lime softener and chemical pH adjustments. The cooling tower is a big user of water, along with the CO2 scrubber. Water flows through the scrubber, to the vacuum pump and fusel oil decanter, making its way back to the front of the plant. Additionally, about 270 gpm of water is recovered as side stripper bottoms and evaporator condensate and reused at the front end of the plant.  “It’s continuous thing,” Cherry says. “We need very good water going into the boiler and cooling towers. As that water circulates, the used water goes back into the process.”  The cost for the water treatment systems in the Aemetis plant is high, not far behind the plant’s energy cost.

Such extreme water recycling would cause many ethanol producers to hesitate, and not because of the cost. Many in the ethanol industry would fear multiple shutdowns from build-ups of fusels or bacterial contaminants that inhibit fermentation or from scaling issues in pipes and heat exchangers. With a 364-days-per-year run rate, Aemetis shows it can be done.  Designed by a plant engineer with decades of experience in the dairy industry, such performance wouldn’t be possible without paying close attention to the water treatment regimen, although the plant’s high run rate is also due to unique features such as multiple redundancies and isolation valves that make it possible to do regular maintenance on the fly.

Total Water Recovery
The Poet network of provides another example of successful maximum recycling of water. Since 2010, when it filed for a patent on its Total Water Recovery system, Poet has implemented some form of the recovery system in all its biorefineries. “With 28 plants, including Project Liberty, the one thing I can definitively say is that no two plants are alike,” says Neil Anderson, vice president of operations. “We have plants that use municipal water, plants that use water from a gravel pit quarry, which is surface water. Plants use lake water or cooling pond water from a power plant, or from shallow wells, deep water wells and grey water from municipal water treatment.

“When we embarked on the total water recovery initiative, we set the goal to reduce water usage and also stop discharging when and where possible.  That means you have to swallow that water in the process,”  Anderson says. Due to the varying water sources for each plant, exactly how water is treated and where the recycled streams are used varies. “Beyond the total water recovery process, there are other water streams in the plant that use process water that is not filtered,” Anderson says. “We have engineers that are very good at identifying what the opportunities are. We go after that to nip away at it—where are the 5 gpm or 10 gpm where we can save water? When we do identify ways to do that, we share that across the group so all the plants can take advantage of it.”

Driving Factors
U.S. Water has worked with a number of dry-grind ethanol plants to achieve zero liquid discharge (ZLD). The company estimates 5 to 6 percent of the industry is considering ZLD, on top of the roughly 9 percent it has already helped convert. “In addition, we feel that at least 25 percent are in the process of evaluating water-reduction projects,” says Randy Meyer, chief sales officer. The drive to reduce water use is motivated by three pressures, he suggests: regulatory, economic and societal.  Water cost and, in some cases, scarcity, have plants looking at new water sources, plus the plants that discharge—still the vast majority of the industry—are finding that in each permitting cycle, the limits get tighter and the number of constituents to track increases.  “Regulations are targeting constituents such as sulfates, total dissolved solids, phosphate, selenium, chloride, nitrogen, ammonia and others.  In addition to the constituents being regulated, discharge limitations vary from state to state, and even from plant to plant, depending on the conditions of the stream or watershed receiving the discharge,” he says.

New solutions for water treatment are needed every year, Meyer says. “It would be typical for a plant to start out using just a small RO to treat boiler makeup. As discharge regulations change, they’ve had to consider added solutions such as a filtration system to remove the iron and reduce conductivity, or even install a cold lime softener or evaporative crystallizer, depending on the severity of the regulations. We constantly have to migrate the solution. Every plant is different based on operating parameters, the quality of water and source of water, the components in the discharge stream, as well as the permit.”

Plants face multiple choices when planning water treatment upgrades. In addition to considering water makeup components, it is important to pay attention to operational efficiencies, such as preventing scale and corrosion in the boilers, cooling towers or heat exchangers. Sodium and sulfate levels in water used for fermentation should be reviewed. Ion mitigation may be needed to remove components like phosphates or chlorides. Meyer says there are more than 20 system components that can be utilized for water treatment. Filtration systems can be installed, which includes reverse osmosis. Solids separation can be accomplished through precipitation or flotation, which then require systems for managing those solids. Some ethanol producers may have the resources to invest in capital-intensive upgrades, while other plants may opt for systems that are less expensive to install, though more costly to operate.

Meyer says the top-five systems to review for improvements and upgrades in ethanol water treatment programs include reverse osmosis units, softeners, pressure filters, programming and automation and cybersecurity. The company will work with a plant to phase-in upgrades, with the first stage put in place that facilitates the next upgrade when needed.  

Avoiding Compromises
Andrew Ledlie, marketing manager North America for Solenis, describes another focus area—making sure that in the effort to reuse water, system protections are not compromised.  “We’re starting to see customers who open up the piping in the cooling towers and while the plant may be 10 years old, the piping looks like it’s 25 years old.”  The ethanol industry is somewhat unique in recycling cooling tower blowdown, he says. “When you go into other industries—we participate in mining, hydrocarbon processing, chemicals and pulp and paper—you don’t have the limitations from a water treatment perspective of water that might end up in a food product. Even in the food and beverage industries, the water treatment is very separate from the food side. They don’t put cooling tower blowdown in the front of the plant.”

Solenis is applying a technology used in other industries to monitor the impact of the alternative water treatments being used in the ethanol industry to avoid issues in the distillers grains coproduct. The company’s performance monitoring equipment simulates real-time conditions in a heat exchanger, for example, to measure fouling and corrosion levels and monitor the effects of adjustments to water treatment. “Then you can see in real time whether you’re going in the right direction,” Ledlie says. Some plants may think all is well because heat exchangers or chillers are not getting scaled, but high corrosion and biofouling will limit the lifespan of the equipment and reduce energy efficiency and throughput.

There are multiple tradeoffs to consider, when evaluating water treatment, he says. The cooling tower, for instance, is the biggest direct user of water in an ethanol plant, with the majority of the water being evaporated, leaving a concentration of minerals and solids. RO units are often used to remove the minerals and solids in cooling tower makeup water to reduce the sludge left behind. “But running an RO system can use about 40 percent of the amount of electricity used to run the fans and the pumps in the cooling tower,” Ledlie says. And, ironically, RO systems reject a certain amount of water.  “It’s not an invalid strategy, but there is an energy penalty, and you’re not actually saving all that water.”

Future
If continuous improvement has been the story for water treatment the past two decades, what’s in store ahead? Meyer suggests that regulations pertaining to phosphates and sulfur compounds are going to continue to receive attention in the near term. Poet’s Anderson says more efficient cooling towers would be an exciting development. Further water recycling may be possible in the future, as well, he suggests. As plants develop ways to capture more energy, the benefits include more condensates that can become part of recycled water flows.

Indeed, perhaps one day, so much water will be recovered that not only will water discharge permits become a thing of the past, but the ethanol plant’s signature steam plume may disappear as even more water is wrung out and recycled.

Author: Susanne Retka Schill
Managing Editor, Ethanol Producer Magazine
sretkaschill@bbiinternational.com
701-738-4922