Evaluating water reuse practices and options for ethanol facilities
Every day in the U.S., more than 355 billion gal of water are withdrawn from surface and groundwater sources to serve industries and the public, according to a 2010 report from the U.S. Geological Survey. While nearly half of the water withdrawn serves thermoelectric power generating stations, ethanol production facilities also are under scrutiny to decrease their water usage. The production of ethanol takes on average 2.5 to 4.5 gal of water to produce 1 gal of ethanol. As the industry continues to grow, many existing ethanol facilities are implementing water recycling and reuse practices.
The primary drivers toward water recycling and reuse practices are environmental compliance and water availability. Every five years, a facility that discharges to a public body of water must apply for and renew its NPDES permit. During the renewal process, the local regulating authority may choose to impose tighter discharge restrictions on specific constituents. In states like Iowa, iron and sulfate are targeted, while in Minnesota and Wisconsin, phosphorus is under greater scrutiny. The ongoing permitting costs, costs to comply with new discharge restrictions and the threat of not being granted a permit are driving more facilities toward minimal or zero liquid discharge (ZLD) operations.
For those facilities that receive municipal water or discharge to a publically owned treatment works (POTW), water availability may become an issue. The costs to replace aging infrastructure approach hundreds of billions of dollars nationwide, according to a 2012 Washington Post article. Communities may not be able to supply facilities and may be forced to choose between supplying residents or supplying industry. Those plants that still are served may face double digit increases in water or sewer costs to maintain services. These costs have companies considering alternative water sources, installing their own intake systems or evaluating reuse options.
Water reuse and recycling projects can be some of the most difficult water treatment processes to design and implement. These treatment processes often require a combination of chemical and mechanical solutions. They also must be designed by someone familiar with ethanol plant water quality requirements, air and water permitting, and the nature of ethanol plant operational cycles. While it is easiest to design a greenfield plant to operate with a water reuse or ZLD system, any plant in operation today must look to retrofit existing equipment. The simplest and lowest-cost option often is to install treatment equipment for raw water intake to minimize wastewater generation at the back end of the plant (i.e., cooling tower blowdown).
Because there is no single design that works for all applications, it is important to find the right integrated solution for each individual plant. To create a successful water treatment system design for a plant, a thorough understanding of the chemistry, equipment aspects and plant conditions, such as plant design, operating conditions, available water quality and quantity, available personnel and training, capital and operating budgets, and environmental restrictions is necessary. Most water reuse and ZLD systems use one or more treatment technologies, including chemical feed systems, membrane filtration, reverse osmosis (RO), evaporation ponds (if climate allows), cold lime softening (CLS) and evaporation/crystallization. At first, evaporation/crystallization may seem like the simplest solution, but its initial capital investment and ongoing energy costs greatly exceed all other technologies combined.
Zero Liquid Discharge
ZLD is a treatment method that reduces water usage and focuses on eliminating water discharge completely. ZLD systems have become more prevalent due to tightening regulations and environmental compliance, heightening public perception of industrial manufacturing’s impact on the environment, and mounting concerns over the quality and quantity of water supply. These technologies can deliver valuable financial returns where water conservation and strict permitting regulations have increased the cost of industrial use of freshwater and, in some cases, continued operation.
Dry grind ethanol production facilities are particularly well-suited for ZLD systems as the ethanol process is a net water consumer. Water is required for fermentation that later is removed via the corn drying process. A carefully designed ZLD water treatment system at an ethanol plant replaces this process water makeup with waste streams from other processes. As long as the process water quality requirements are studied throughout the design and careful consideration is given to the operating conditions of the plant, a dry grind facility’s discharge stream can be eliminated.
The first ZLD water treatment system in a dry grind ethanol facility in the U.S. was commissioned at a California ethanol plant. After analyzing local water quality and discharge restrictions, a CLS-based system was selected. This decades-old technology—which is making a comeback—allows dissolved minerals, such as calcium, magnesium, silica, iron and others, to precipitate into a solid form using commodity, low-cost lime. The non-hazardous collected solids are dewatered and disposed of either in a landfill or applied as a soil amendment for local farmers.
The process is one of the only known technologies that removes dissolved minerals from water without creating a more concentrated brine solution. The facility’s water treatment system produces treated water quality low in hardness, silica and suspended solids. The CLS process also recycles waste streams for continual removal of certain ions, such as RO concentrate or cooling tower blowdown.
When a Minnesota ethanol plant voluntarily implemented a ZLD system to conserve water, its first step was a review of its water source and its variability to create water balance for ZLD operation. While the facility was already operating a CLS system, it was not originally designed to handle waste streams from other processes. The conversion to ZLD consisted of four key design changes.
- The replacement of sulfuric acid for cooling tower pH adjustment with fermentation generated carbon dioxide gas. This change removed additional sulfates from the plant wastewater and lowered annual tower chemical expenses.
- The recycling of cooling tower blowdown and RO reject to the front end of the water treatment CLS plant. The CLS process removes hardness, silica and other problem ions that limit cooling tower cycle concentration and RO recovery rate.
- Replacing the onsite regenerated boiler system water softeners with off-site regenerated ones. This change eliminated chlorides from the water balance and had a minimal increase in operating expense.
- Blending wastewater into the process water makeup stream. Sodium that impacts yeast production and sulfate that impacts distiller grain mineral content were considered so fermentation was unaffected by the process.
After implementation of system changes, water usage dropped from 2.9 gal of water per gallon of ethanol produced, to 2 gal of water per gallon of ethanol produced.
Alternative Water Sources
As groundwater sources become limited, facilities old and new are looking at alternative water sources to supply their plant. One of the most common sources today is municipally treated wastewater, called greywater.
This low-cost, or sometimes free, water source is abundant and does not strain local water sources. While attractive from a financial perspective, greywater carries many concerns when designing a water treatment system. Greywater is variable from hour to hour and day to day. Constituents like phosphorus and ammonia, which are common and abundant in greywater, can be costly to remove, and if left untreated can scale heat exchange surfaces or corrode equipment. The suspended solids content of greywater usually is much higher than most groundwater sources and requires its own method of treatment.
An Iowa ethanol facility implemented a greywater treatment system that eliminated its need for freshwater. While first designed to operate on potable water completely, during construction the facility learned the municipality could not supply the quantity of water required by the ethanol producer. Forced to find another water source, the municipality offered to supplement potable water with greywater to meet demands. The alternative water source changed the facility’s planned water treatment system to include microfiltration, a membrane-based filtration system effective at removing suspended solids and other organics common to greywater, and additional RO capacity.
The plant has been operating successfully since system integration approximately 10 years ago. This facility was one of the first of its kind to use the technologies of microfiltration and RO together for greywater reuse. Even with this robust treatment system, the greywater requires the treatment equipment to be cleaned and replaced more frequently and has higher associated chemical costs.
As water reuse and recycling projects continue to take center stage in long-term environmental plans, water-related projects will become even more common. Creative engineering and treatment technologies will continue to drive advancement and further water conservation.