It's Something in the Water: Ensuring Profitable and Sustainable Operation

By Mike Mowbray and David Hume | June 05, 2007
Groundwater is one of ethanol production's key ingredients. With increased demand for the renewable fuel, groundwater quality and quantity issues are being brought to the forefront of the ethanol industry. Groundwater is as essential as it is a precious natural resource. Given the current rush to bring additional ethanol plants on line as soon as possible, developers run the risk of overlooking the importance of assessing the quality and quantity of their water sources before selecting a site. Such a lack of due diligence can turn out to be a costly mistake and ultimately jeopardize a project's economic viability.

As of January 2007, there was approximately 5.6 billion gallons per year of ethanol plant capacity in the United States. Most plants produce 40 MMgy to 50 MMgy using an ethanol production process generally requiring four to five gallons of water to produce one gallon of ethanol. This equates to approximately 1 million to 2 million gallons of water use per day. Groundwater is the primary water source for most plants mostly due to it being readily available, but also because it's generally of higher quality than surface water. Water quality and quantity play important roles in the operation of today's ethanol plants.

How Water Quality Affects A Plant
Water quality can vary drastically from different aquifers even within the same general location. Also, groundwater chemistry can change over time as aquifers become stressed due to continual pumping. Differences in water chemistry must be given serious consideration when designing and operating a plant.

When it comes to ethanol plant water quality requirements, the mash cook process is the least demanding since only a few water constituents, such as high concentrations of sulfates, chlorides or sodium, will have a negative effect on the process. However, a serious effort will be needed to identify another water source if the concentration of these salts in the water source is high enough to affect yeast, because high yeast levels lower yield and discharge issues will occur in heating and cooling systems.

Most operating ethanol plants and those under construction have heat recovery steam generators (HRSG). From a mechanical point of view, modern HRSGs are like a sports car. They pack a lot of performance, but they also require high-quality "fuel" and top-notch maintenance. In other words, it's critical that only the highest quality water be used to minimize scales and deposits on the tightly packed, small diameter tubes.

The scale that forms in the hot section of the HRSG can quickly plug and damage steam-generating tubes since they are very narrow and the heat flux very high. This can lead to a plant shutdown or derating while repairs are conducted. Furthermore, waterside tube scale will act as an insulator that blocks efficient heat transfer from the thermal oxidizer waste gas to the boiler water. As little as 1/64 of an inch of calcium carbonate or iron oxide scale can reduce heat transfer efficiency up to 14 percent.

Incoming water quality is also a major factor in cooling tower operation. Of all ethanol plant subsystems, the water cooling process requires the largest amount of water. A typical 40 MMgy ethanol plant has to remove 80 MMBtu to 100 MMBtu of heat per hour.

Low well-water mineral concentrations allow a plant to recirculate the water through the system several times before discharging it into the environment. Higher concentrations will either require an increased use of groundwater, which may be a scarce commodity, or a potentially costly water treatment process. While practically all water quality issues can be resolved, treatment costs can reach a point where they ultimately make plant operation prohibitive on a problematic site.

Mineral counts may still constitute a regulatory issue when it comes to discharging wastewater even in cases where the counts are sufficiently low enough not to hurt the ethanol production process. Unless a plant uses the very latest technology allowing mineral deposits to be discarded in the form of sludge or eliminated through a process of crystallization and evaporation, the discharge will most likely be into a local body of water. There may even be cases where a plant's original well water exceeds wastewater permit limits for a site. That's why it makes sense to bring in a water treatment professional as early as possible in the planning process.

How Water Availability Affects A Plant
Despite existing groundwater resources and well records suggesting that the necessary yield is available, it doesn't mean there will be sufficient groundwater available over the plant's operating life. If not assessed properly and managed effectively, pumping water from any aquifer means tapping into a limited resource. Aside from that, groundwater is also a shared resource, which is most likely already being used for domestic, agricultural, municipal and/or other industrial supplies.

Effectively operating an ethanol plant requires pumping a large volume of water 24 hours per day, seven days per week. If a plant pumps water at a aquifer's rate higher than what the aquifer can transmit to the well, or if the pumping rate exceeds the rate of recharge, the aquifer's water level will drop and could imperil the availability for others utilizing that source. Apart from promoting a good relationship with neighbors, being aware of how the production process affects groundwater levels will ensure the long-term operations and productivity of the plant.

In addition to potential well interference issues, lowering groundwater levels can also affect surface water if the two systems are directly connected. This can bring about public opposition, more regulatory involvement and potential legal issues. Groundwater quality can also suffer due to surface water impact, which means a plant may have to resort to additional water treatment.

To ensure a plant's economical operation and environmental sustainability, it is essential to have a thorough understanding of the geology of the groundwater source being tapped by the plant's well. A hydrogeological analysis of the plant site provides essential tools, such as a water supply well inventory, well interference evaluation, long-term aquifer sustainability evaluation and a groundwater chemistry assessment. It also provides clarification on whether the aquifer supplying the plant's groundwater will be able to sustain long-term pumping demands. It also determines if the required pump rate is sustainable without lowering water levels in the aquifer to an extent that will affect other wells or surface waters.

Moreover, hydrogeological consultants are able to assess groundwater quality and predict any changes that could occur due to pumping. They can also determine how many wells a plant will require and what studies are needed to obtain groundwater usage permits for a particular site.

Water quality and treatment have a significant impact on the profitability of an ethanol plant. Even if a particular state doesn't require working with a hydrogeological consultant, getting such a professional on board as early as possible in the planning process is highly recommended. Waiting until a couple of weeks before start-up to call in a water treatment chemical supplier and hoping he or she can solve water quality problems is simply asking for trouble.

Mike Mowbray is marketing manager with U.S. Water Services in Plymouth, Minn. Reach him at mmowbray@uswaterservices.com or (763) 553-0379. Professional Geologist David Hume is a senior associate in the St. Paul, Minn., office of Leggette Bashears & Graham Inc. Reach him at dave.hume@lbgmn.com or (651) 490-1405.

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