Water Efficiency as a Result of Sound Water Management
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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.
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.
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.
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.
Quality of the effluent is one of the most variable process constraints; it changes from site to site. Local, county, and state governments with different requirements govern effluent water. As previously mentioned, many of the process technologies are zero discharge, so process water generally doesn't have an impact on effluent. The discharge effluent comes primarily (if not exclusively) from cooling tower blowdown. With this system being the largest user of water, it is the primary—if not the only—contributor to the outfall. Therefore, the cooling system usually becomes the place where water is reused or recycled if the effluent permit has that flexibility. If the effluent permit has limitations, balancing the cooling system water requirements will require the most effort when modeling and engineering new options. Because of the concentration effects of the cooling tower, the blowdown from this system needs to be modeled and well-understood.
Quantity of water can be an issue in some locations and impacts the number of water sources that will be required. Multiple source water systems can lead to more complex pretreatment systems. Having multiple sources of water adds capital and operational costs to a project, and risks the plant operating without issue. If quantity is a primary constraint, again the cooling system will be looked to first to minimize makeup water requirements.
Capital and operational expense availability is the final major constraint. ZLD technology has been around for many years and has been put to use in many heavy industries, including chemical processing, steel, petroleum and power. It is expensive and adds millions of dollars to the total cost of the project. However, the substantial reduction in costs of membrane technologies over the years has contributed to improved water efficiency. It is important to understand the type of funding available, as solution providers today have a variety of options to make a water solution fit within a project's budget. See "New ethanol plants need a fresh look at water management" in the October 2005 issue of EPM for details on treated water outsourcing.
Process Steps, Resources to Execute
The goal is clear, inputs established and constraints understood. The process can be broken into five steps: analyze the water streams and design requirements; model the pretreatment equipment and chemical treatment to determine the impact on the process, environment, assets and production; recommend options for equipment and treatment in the form of required capital and expense outlays; have proper control mechanisms in place to maintain the water quality and water system designs; and have the ability to monitor the resulting systems and treatment in real time so as to minimize impact to the environment or production.
Analyzing the water sources being considered is important, as these are the inputs to all of the modeling. It is important that a water laboratory runs the analysis for a wide gamut of minerals, metals and conductivity, and that multiple samples are pulled if well or surface water is being considered. This is especially true if the well is shallow or if the surface water has wide variants due to seasonal changes. Having samples that misrepresent the actual water could lead to increased costs in wrong equipment, production issues or permit violations.
Modeling is the heart of process. The more variables that the model can incorporate, the more accurate the model is in representing what will actually happen once production begins. The model should include the ability to simulate pretreatment equipment, including clarification, filtration, reverse osmosis systems, cooling towers and boiler systems.
Models built on a database of actual results versus textbook equations are more accurate and can provide a broader range of options. The model should show impact to the assets through predicted corrosion rates, heat exchange efficiencies through predicted scaling potential, membrane fouling and recovery rates, and predicted effluent outfall contaminants. The modeling technique is iterative in nature, as one variable can cause the entire model to break down and violate constraints discussed previously. The number of dependent variables requires a robust model to ensure accuracy in scaling it to live operation. Owners, EPC contractors and technology firms should be vigilant in understanding the techniques being used in this part of the process because too much is at risk. There are many water treatment providers who say they do this step, but it is important to spend some time on understanding how it is done.
Recommendations should be presented as options versus a single solution, especially if there are multiple water sources. An example of the form of these recommendations can be seen in Table 1. Capital and operational expenses need to be considered when all options are examined and the selection of the treatment and equipment strategy is made. The rigor and complexity of the techniques used in the modeling step should allow the owner, EPC contractor and technology firms to have confidence that the final solution will result in what was modeled. With the peace of mind that the water systems have been accurately modeled, owners can quickly turn their attention to other critical project issues.
Controlling and monitoring the final design protects investments and allows the owner to maintain the balance between environmental and production impacts once the plant is operational. As was previously discussed, the cooling tower system uses the most water in the ethanol plant and therefore will be the place where most look to either reuse or recycle water. If these techniques are being utilized to increase water efficiency, proper control and monitoring of these systems is critical. These practices put incredible stress on the cooling systems and require the systems to be run within a tight control window to avoid asset damage and production slowdowns. The best control and monitoring systems should be able to immediately notify plant management and adapt automatically, as the quality and quantity of recycle and reuse streams vary during normal operations. Single upsets that last for hours generally are the cause of heat-exchange equipment failures or efficiency reductions, so knowing within minutes that the system is out of specification can save hundreds of thousands of dollars.
As the ethanol industry continues to expand, owners, EPC contractors and technology firms are being challenged to develop innovative solutions to the water issues associated with construction of new ethanol plants. The industry has taken substantial steps forward in recent years to continue to improve water efficiency for the process side of corn feedstocks. However, significant challenges still remain for both corn projects and many of the alternative feedstocks here in North America and around the world. These challenges are forcing owners to consider zero-discharge or advanced recycle, or reuse options, in order to make their projects successful. Regardless of feedstock or site selection, a sound water management process should be used to reach a balance between maintaining environmental sustainability and maximizing production, while minimizing the investment in the final solution.
Tony Stanich is Nalco Company's biofuels industry manager. Nalco is a leader in applying water management processes to a wide variety of industries, including the ethanol industry. Nalco has saved heavy industry over 700 million gallons of water a day through the application of advanced recycle technologies. Reach Stanich at firstname.lastname@example.org or (630) 305-2478.