A new twist on a mature technology is allowing plants to minimize their water use and wastewater discharge.

The fast-growing renewable fuels industry is facing increasingly stringent state and federal water discharge restrictions. Some environmentally sensitive areas such as California’s San Joaquin Valley do not allow any aqueous industrial discharge.

When Pacific Ethanol decided to build a dry-grind ethanol plant in Madera County, Calif. - a prime agricultural location - in the spring of 2006, the plant had to find a way to reuse the discharge from the cooling tower, pretreatment equipment and process streams to avoid discharging any liquid into the environment. The plant contacted U.S. Water Services, an industrial water treatment company based in Cambridge, Minn.

In the cold lime softening tank, the mineral sludge sinks to the bottom while the clean water rises to the top and is siphoned off for reuse.

After carefully analyzing the local water quality as well as the plant process demands, U.S. Water Services designed a process using cold lime softening to achieve the plant’s goals. While not a new technology, cold lime softening is effective in precipitating many of the minerals from the water. The minerals, which are rich in calcium, then are added to the dried distillers grains and syrup (DDGS), supplementing the nutrient value of this valuable animal feed byproduct of ethanol production.

Startup of this first-of-its-kind process water recycling system came with a steep learning curve for plant operators. With no plant discharge, monitoring the water chemistry and controlling it to tight parameters are critical steps to a successful operation. The operations staff learned quickly that allowing chemical control to drift out of range resulted in fouled filters and reverse osmosis membranes and other troublesome issues.

However, once the system quirks were ironed out, the plant began reaping the benefits of the technology. Water quality to the ethanol process is excellent, water intake to the plant is low and, most importantly, no liquid is discharged to the environment. This facility is the first ethanol plant in the country to be designed and operated with no liquid discharge.

Other Benefits

While most plants do not have the restriction of zero discharge from their facilities, cold lime softening still can provide benefits. Depending on the specific permit regulations, the technology can be used to reduce liquid discharge volumes significantly without actually eliminating the discharge. In addition, many state permitting agencies see cold lime softening as a means to control pollutants, as it is one of the few available processes that removes minerals from the water. The extracted minerals form a sludge that can be compacted and reused as a fertilizer rather than simply discharged in a concentrated waste stream, as occurs with reverse osmosis and ion exchange systems. 

Another important feature of lime softening systems is that they allow much greater flexibility in the sources of water that can be used. For example, a plant in Minnesota made an agreement with the state Department of Natural Resources to monitor local wells. When plant operations caused the aquifers to drop low enough to impact local landowners, the plant installed a cold lime softening system. The plant has benefited in a number of ways. Water usage for cooling and process systems has dropped by more than 30 percent as a result of the improved water quality. More importantly, the water treatment system design allows the plant to use river water, well water and even water from a storm drainage pond, depending on which source makes the most sense at any given time. 

Water supply and discharge restrictions are becoming increasingly tight. By implementing new treatment technologies, and by using old technologies in novel ways, a significant impact can be made on the amount and quality of water that a facility uses and discharges. PC

Mike Mowbray is the marketing manager for U.S. Water Services, Cambridge, Minn. For more information, call (866) 663-7632 or visit  www.uswaterservices.com.

SIDEBAR: Cold Lime Softening Basics

Cold lime softening technology has been used for decades to remove minerals - principally calcium, magnesium, iron and silica - from drinking and process water. Raw water is fed into a tank supplied continually with a lime mixture, which “attracts” the minerals. As the minerals are removed, they form a sludge that sinks to the bottom. Clean water rises to the top and is siphoned off. In zero-discharge applications with stringent water quality requirements, the water is sent through a charcoal filter for additional purification before reentering the process.

Cold lime softening is a mature technology that slowly is being replaced in many instances by newer reverse osmosis and ion exchange technologies. However, benefits such as pollution control, water source flexibility and zero-discharge capabilities are allowing cold lime softening systems to find new uses in some unexpected applications.