republished with permission from Ethanol Today, December 2010
In the agribusiness sector where acronyms abound, some corn producers and ethanol plant investors are learning yet another one: ZLD
It stands for “zero liquid discharge,” and it could be an important future trend for the ethanol industry as plants weigh the possible benefits of treating and re-using water that has already been through the plant, rather than discharging liquid into the environment.
“Over time I think you’ll see more and more states go for this,” said Rick Vondra, plant manager for Patriot Renewable Fuels, LLC of Annawan, Illinois, one plant that has already made the change. “State environmental agencies will be leading the charge.”
Charlie Kroeger of U.S. Water, who helped Patriot make the transition, said the factors making some plants consider zero liquid discharge are easy to pinpoint.
“The push behind ZLD is more stringent discharge requirements,” Kroeger said. “The regulations are tightening up in several states. As discharge regulations become more stringent, many plants are looking at this as the alternative.”
He adds that fuel ethanol biorefineries in the future not only must think about what they’re putting into the environment, but also must consider what they’re taking out. The simple fact is that many states are becoming more aware of the fact that water is a finite resource.
“Here’s what is going to happen,” Kroeger said. “Right now water is relatively cheap, but we are certainly heading for a water crisis. We are faced with a clean water shortage in the future. Depending on which expert you listen to, that could be sooner rather than later.”
In some areas, Kroeger added, ethanol companies are finding it almost a necessity to have a ZLD operation in mind in order to get a permit. Patriot was unique, he noted, in that they made the switch with no one from regulatory agencies leaning over them to require it. “They were proactive and they decided to do it in order to be a good neighbor and reduce water use,” Kroeger said.
Just how much can fuel ethanol biorefineries reduce water use? Vondra said Patriot Renewable Fuels’ plant in Annawan was using about 3 gallons of water for every gallon of ethanol produced when it was operating as a conventional plant. Now, under its first year as a ZLD facility, it appears that Patriot will use about 2.3 to 2.4 gallons of water for every gallon of ethanol – a savings of more than 80 million gallons of water annually. “That’s water that remains in the aquifer available for someone else,” Kroeger said.
LOCATION, LOCATION, LOCATION
At least in theory, there could be more siting options for building a zero liquid discharge ethanol plant because it will use less water than a conventional facility. But in practice, ethanol companies must still look for a location with abundant, good quality water as an early consideration when choosing where to build. In fact, water may be an even greater consideration for ethanol companies that are planning to treat and reuse it; a biorefinery that sees ZLD in its future must also calculate what it will cost to treat the water it will use and reuse in making ethanol. It’s a factor ethanol companies must weigh in the balance along with issues such as electrical supply, rural access, transportation issues, and natural gas.
Patriot President and CEO Gene Griffith said Patriot’s steering team recognized the need for a high quality source of water from the outset. They found it in Annawan, Illinois. “We chose to locate the plant in Annawan partially because an aquifer study indicated abundant water was available through the shallow wells. The water must be treated to remove iron, but it has a low chloride level that would not be difficult to treat or dispose of,” Griffith said. How to best use that water as a community resource was another consideration. “Important objectives in Patriot’s project plan included building a plant that will benefit out community economically and environmentally,” Griffith said. “We were aware through newspaper articles, that water consumption and discharge at some other plants had been a concern for their communities.”
Patriot addressed both of those concerns from the very start. Already during the plant design stage, those driving the project envisioned a model that could be adapted to make the operation a zero liquid discharge facility. They hired the firm that had experience working with ZLD design, U.S. Water in St. Michael, Minnesota. “We worked with U.S. Water and our contractors to design a zero liquid discharge system that would reduce our water usage and eliminate all discharges to assure that would be no negative impact,” Griffith said.
U.S. Water helped bring the first ZLD ethanol plant in the United States into operation in California in 2006 as the culmination of a year and a half of painstaking design work and construction. Since them, the company has worked with other clients who have been required to incorporate ZLD in their designs or who simply want equipment and piping options that allow for a transition to ZLD operations in the future. In several instances, Kroeger notes, U.S. Water has also been called upon to help correct ZLD designs by third parties that did not mesh well with the ethanol plants.
A HOLISTIC APPROACH
Making a ZLD operation work isn’t as simple as building a fuel ethanol biorefinery and building a water treatment plant to serve it. For want of a better word, Kroeger said, it’s the “holistic” approach that is missing in some of the operations where U.S. Water has been asked to troubleshoot ZLD programs. Whoever’s going to do our design, make sure they integrate your water plant design into the ethanol plant operation. These plants are unique,” Kroeger said.
In simplest terms, ZLD means using the right equipment and configuring equipment and plant operation to allow flexibility in reusing water streams internally. “The only water lost is what evaporates via the cooling tower,” Kroeger said. “The other water is treated and eventually incorporated into the final product.”
If the plant can operate so that no water is discharged to the environment, it is a zero liquid discharge facility. But making the move to ZLD means the plant designers, first, and then the plant operators, have to know exactly what they’re doing. “This brings additional challenges in a fuel ethanol biorefinery because the process water quality must be compatible with biological processes occurring in fermentation. The wrong decisions can be disastrous and costly, even leading to plant inoperability and shutdown,” Kroeger said.
At Patriot, the plant began operations as a conventional fuel ethanol biorefinery in August 2008, discharging water into the environment from its treatment processes as allowed under its permit with the state of Illinois. But not long after, during calendar year 2009, Patriot’s management team began thinking about ZLD, looking closely at how the plant was performing to evaluate how they could best make the transition. One very necessary step was to have someone on board licensed to operate a wastewater treatment facility – preferable someone who understood ethanol. Rick Vondra, as plant manager, took the test given by the state of Illinois to meet that requirement. “It’s more of a technicality than anything,” Vondra said. “We’re putting water that we’ve already used back into our plant, so instead of using well water, we’re treating wastewater so that we can use it again.
The good news for operators in other locations is that Illinois is the exception to the rule in that regard, Kroeger said. Most states don’t require operators of a ZLD ethanol plant to also have someone on-site with a license to operate a wastewater treatment operation. However, it’s likely that ZLD will add to labor costs somewhat because operating a ZLD plant is more complicated than operating a conventional plant. How much additional labor depends partly on the challenges associated with treating the water.
RETROFITTING VS BUILDING FOR ZLD
Of course the greater initial cost is from equipment, design and construction costs associated with adding the water treatment functions to complement the ethanol manufacturing. The cheapest way to go ZLD is to plan for it from the outset, as Patriot Renewable Fuels did. But Kroeger notes that with fewer new ethanol start-ups on the horizon, and with regulations tightening up in some states, going back in and retrofitting plants to make them ZLD operations may be in the cards for some ethanol companies. “Retrofitting is always more difficult. The cost of putting in the right plumbing and so on is always less if you do it at the outset,” Kroeger said. “Depending on how much equipment a plant has on site and how easily it can be reassigned within a ZLD operation, retrofitting could cost as little as several hundred thousand dollars to as much as a couple million.”
Vondra adds that the process of converting a plant to a ZLD operation will likely vary from location to location depending on factors such as mineral content in the water. “Water is not water is not water,” Vondra said. “Different water requires different treatment. You can treat almost any kind of water, but depending on what’s in there, it may require different steps. ”
In the case of Patriot Renewable Fuels, the first step in moving to a ZLD operation required eliminating a cold lime softening recycle loop that had been integrated into the water management software. The recycle loop was intended to maintain steady flow through the cold lime softening unit during periods of lower water use – a tactic that has a drawback in that treated water is re-treated even before use in the plant. That increases chemical usage. “In a ZLD environment, minimizing any chemical addition is critical to managing ion balance,” Kroeger said. “A cardinal rule of successful ZLD operations is to carefully assess every chemical addition and identify potential reductions or elimination. Chemical usage needs to be evaluated and minimized so that there is no unnecessary addition.”
Patriot requested that the software control be revised to eliminate the cold lime softening recycle. The plant now allows the finished water tank to rise and fall in a designated range to meet plant demand while maintaining adequate reserve storage. Next, Patriot replaced sulfuric acid with carbon dioxide generated by the plant for recarbonation, or pH adjustment, of the cold lime softening effluent. This step plats a key role in managing the water system ion balance. It has the added benefit in that it keeps carbon dioxide involved in the plant operation that would otherwise be vented to atmosphere.
Patriot then began recovering, in two phases, the waste streams that were previously released to the environment. First, using piping and valve manifolds included in the ZLD design, the operation began diverting cooling tower blowdown water into the same tank that holds well water. Cooling tower blowdown is an intentional water stream diverted from the cooling tower bulk water to manage dissolved solids levels in order to prevent scale and fouling. This stream becomes a significant portion of the total facility wastewater. This tank supplies the water for cold lime softening and includes a blend of well water, gravity filter backwash water and now, cooling tower blowdown.
After monitoring critical ions, such as sodium, sulfate, silica, chloride and hardness to assure that the water supply would still allow for healthy fermentation and ethanol production, Patriot introduced reverse osmosis concentrate into the well water tank. This step had also been planned as part of the ZLD design with piping allowing for easy diversion to the well water tank (now operating as a “blend” tank) feeding the cold lime softening. With this waste stream recovered, the plant stopped discharging any wastewater, effectively becoming a ZLD operation in February 2010.
If one of Patriot’s early estimates hold true, the Annawan plant could save upwards of 80 million gallons of water per year under a ZLD operation. But for the management team at Patriot Renewable Fuels, part of the payoff can’t be measured in gallons of water saved or gallons of ethanol produced. It’s in intangibles such as community goodwill.
“We live in this community and we want to be good neighbors,” Griffith said. “This system reduces water consumption and ensures there is no negative environmental impact. We’re proud to implement the ZLD design early in our existence. Others in the industry are talking about water usage and discharge, and we hope other plants will set these objectives as well.”