Conserving Water with Cold Lime
by Mike Mowbray

The fastest-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 US Water Services, an industrial water treatment company based in Cambridge, Minnesota.

After carefully analyzing the local water quality as well as the plant process demands, US 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.

Start-up 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 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.

While most plants do not have the restrictions of zero liquid 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 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 steam, as occurs with reverse osmosis and ion exchange systems.

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-liquid 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 iron exchange technologies. However, benefits such as pollution control, water source flexibility and zero-discharge capabilities are allowing cold lime softening to find new uses in some unexpected applications.