In the future, increased demands for high-quality fresh water supplies will translate into voluntary and, ultimately, regulatory mandates. These constraints will require facilities to limit or to reduce withdrawal volumes from the local watershed. Plant personnel in southeast Texas have been living this scenario as a severe drought continues to compromise the quality and quantity of fresh water. However, implementing temporary solutions is not the same as crafting permanent solutions.
A reasonable approach.
Water-reuse projects are typically long-term efforts. They require a significant amount of analysis before making any capital project commitments. The initial efforts will require participation by a broad range of personnel: technical service, operations, water treatment suppliers and outside experts.
Optimize present utility/process water systems.
This step is considered a low-hanging fruit action item: reducing water usage that requires minimal analysis and operational changes with little or no capital investment. Your chemical supplier may be willing to assist in identifying opportunities to improve system reliability with innovative treatment strategies. Typical examples include excessively long times for the final rinse of softeners and demineralizers, low recovery rates for reverse osmosis and high blowdown rates in cooling towers.
Construct a water and wastewater balance.
Most plants have a validated steam balance, but few facilities have an analogous water and wastewater balance. A water balance starts with a process flow sheet that includes the dynamic variability of flowrates. Ideally, this balance would include specification limits for each stream: pressure, temperature, pH and contaminant concentrations.
Constructing and validating this water balance is not a trivial task. For a medium-sized refinery, it requires several hundred hours by a process engineer who is knowledgeable about the utility water system. Validating the data is especially difficult because water systems have few flowmeters or historical flowrate data.
Estimating the requirements for utility water can be difficult because flowrates may be highly variable due to seasonal demand. Plant managers should consider temporarily or permanently installing recording flowmeters to gather data for a significant period. The information will help to understand the seasonal and operational variability of water consumption.
Tabulate the costs of water and water treatment.
Accurate calculations of unit costs for withdraw, treatment and charge of water and wastewater will require crafting defensible economic analyses to justify water reuse projects. Plant and project personnel must agree on the components for the unit costs. What set of costs constitute total cost of ownership? Is it a combination of direct costs and allocated overhead costs or something else? Some examples include the cost of heat exchanger cleaning and electrical pumping costs. If you are forecasting costs for future years, is there a standard escalation factor included?
Map the water and wastewater streams.
Sometimes, the most cost-effective solution is reuse of a single wastewater stream within the plant. Consequently, the proximity of the wastewater stream and the reuse candidate is important. A map of the water and wastewater flows is essential to evaluate the feasibility of water reuse recommendations. Ideally, this map would include additional information about water and wastewater streams, including equipment (pumps, storage tanks, sample points) and operating characteristics (pressures, flowrates and temperatures).
Conduct scenario planning.
Consider all of the alternative water sources such as storm water and treated wastewater from your facility and/or neighboring facilities. Ensure that all contingencies are considered, including increasingly stringent regulations for effluent quality and restrictions on withdrawal volumes from the watershed.
Assess the risks to your utility water systems.
Water reuse projects inevitably involve trade-offs between the expense and complexity of re-treatment and the impact of modifying the water-quality specification for a proposed application. Therefore, it is important to understand the current vulnerabilities of your utility water units. Likewise, the reuse project should ensure that the expected changes in water quality do not inadvertently compromise system reliability or operability under all anticipated conditions. For example, a reduction in water quality for a boiler or a cooling tower may result in higher blowdown (reduction in water and energy efficiency). It may also reduce heat transfer efficiency due to greater scaling, thus increasing operating costs for the cooling tower.
Similar to the previous plant efforts to benchmark and to improve energy efficiency, water reuse will become a strategic initiative. Water reuse will be an integral part of sustaining and improving competitiveness in manufacturing. The quality and availability of water are declining, while the cost for fresh water is increasing. HP
|The author |
Loraine A. Huchler is president of MarTech Systems, Inc., a consulting firm that provides technical advisory services to manage risk and optimize energy and water-related systems including steam, cooling and wastewater in refineries and petrochemical plants. She holds a BS degree in chemical engineering, along with professional engineering licenses in New Jersey and Maryland, and is a certified management consultant. She can be reached at email@example.com.