Because of the industry’s awareness of its environmental impact, focus has always been on increasing energy efficiency associated with the process. Water usage, on the other hand, has only been seriously addressed within the past few years. As the article “Running Dry?” from the October 2006 issue of EPM points out, the industry has made tremendous strides in technology to reduce process water discharge, in some cases eliminating it. However, process water demands only account for about one-third of the required water in the plant. The other two-thirds of the demand stems from utility systems, specifically the cooling systems that account for approximately 90 percent of the total utility water.
As the industry continues to expand beyond current geographies and feedstocks, water supply quantity, quality and discharge issues will continue to be a major hurdle for plants under construction. Meanwhile, the owner; the engineer, procurement and construction (EPC) contractor; and technology firms are left to solve these problems on their own. What reuse or recycle options exist? Is zero liquid discharge (ZLD) an option? What impact will these choices have on operations, the discharge permit and asset life? These questions would be answered through the implementation of a sound water management process. This process has key inputs, outputs, critical steps and resources—all operating under critical constraints—and can be applied regardless of the feedstock or site requirements.
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Outputs
Instead of starting with the solution, let’s begin with the end. What is the goal? Zero discharge? Reduced water use? Permit compliance? At its most basic level, an owner wants to have a water supply that will provide the ethanol plant with a sustainable quantity and quality of water. The water supply should allow operation of the plant without concern of damaging assets or adversely affecting production. The systems that produce the water supply and maintain its quality during use need to be easy-to-use, low maintenance, affordable and in environmental compliance. The EPC firms want to have a high level of assurance that the design they guarantee will perform. The technology firms want to be sure that choices made in handling water issues don’t adversely affect their methods of ethanol and distillers grains production. The key stakeholders in the process expect a specific outcome and look to a sound water management process to yield these answers to a high degree of certainty.
Inputs
A complete understanding of an ethanol plant’s water requirements is critical. Figure 1 provides a high-level overview of a general water balance in a typical corn-based ethanol plant.
The ethanol plant has two water demands: the utility systems and process. It is critical to understand the loading of the utility systems and the amount of process water that will be required. These requirements vary between technology providers. The plant’s water source options are the final key inputs to the process and have a substantial impact on the final solution. Water source choices include well water, surface water or municipal water. In some locations, the option to use gray water—the effluent flow from a municipal wastewater plant—is also available and could be an excellent source of water if managed properly. Many heavy industries—steel, chemical processing, petroleum and power—facing similar water issues have looked to gray water to assist in meeting their water shortage problems. Not to be forgotten is a fifth water source introduced through the corn and its approximate 14 percent moisture content.
Constraints
Most processes would be easy to implement if it weren’t for specific constraints, and water management is no different. The constraints within this process can be grouped as quality, quantity of water and expense limitations.
Quality of the inlet water dictates to what degree the water will have to be treated to minimize impact to assets, the process and the outfall. Moderate levels of minerals may be acceptable in the process but would be detrimental to heat-exchanger efficiency. A lower level of minerals in the system may be appropriate for the boiler system to avoid scaling, but could lead to high levels of corrosion in the cooling system and heat exchange equipment. Certain minerals that may not damage utility systems, like sodium, may have significant impact on production. Recycling of utility water streams to the process may impact animal feed regulatory issues. Having a service provider that understands all of the process constraints is important so that the resulting solution is credible.
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