Whether you are preparing for the impacts of future U.S. regulations, need to minimize your water-related plant costs, or want to develop more sustainable practices, a water management plan can help you meet your goals. Here’s a blueprint for developing a relatively simple, multi-use plan.
Recent articles in POWER and elsewhere have noted that all types of generating technologies are more frequently feeling the pain of constrained water supplies. The limitations can come from increased regulatory requirements, water quality changes, or growing water supply costs or limitations. Given the accumulating water-related pressures on power plants, having a clear understanding of how a plant manages its water and wastewater internally is becoming more important for plant operators and engineers.
Proactive management of how water is used in a power plant can do more than reduce water consumption. It can also lower operational costs and help a plant plan for a future in which it may have to deal with the onslaught of new and updated U.S. regulations expected over the next couple of years.
Even plants that do not expect to be affected by pending U.S. regulation changes can benefit significantly from a policy of increased sustainability, water conservation, and environmental awareness. Improved site water management and identification of increased reuse and recycle opportunities can result in lower water supply and disposal costs, decreased wastewater treatment needs, and an improved sustainability rating or good neighbor image.
Go with the Flow
One of the best ways that a plant can manage its water resources more effectively is to develop a plantwide water management plan. A water management plan is a comprehensive site-wide strategy for maximizing water use efficiency, minimizing wastewater discharge, and encouraging a policy of sustainability and reuse/recycle to the maximum extent practical to control a plant’s water footprint.
The first step in developing a plan is to construct an overall plant water balance depicting the current plant water picture. The water balance should identify all water consumers and wastewater producers throughout the facility and indicate all the flow routes and distribution pathways. (Click here for a downloadable flow diagram depicting a water balance from a typical coal-fired power plant.)
The most difficult challenge in developing an accurate site water balance is collecting adequate data to model the plant water and wastewater streams precisely. Inclusion of seasonal and historical data and its impact on the plant water balance is desirable, such as periodic degradations in water supply qualities that reduce cooling tower operating cycles.
This water balance should include average daily usage rates as well as peak and minimum flows to create an accurate picture of the plant’s water footprint. Reduced load operations (if they are relevant for a unit), partial plant operations (for instance, single-unit operation in a two-unit plant), as well as other off-design operating scenarios should be evaluated, as appropriate, to determine how they will affect water consumption rates and wastewater production.
This may require that the water management team perform its monitoring activities at several different times over the course of a year to capture the various plant operating scenarios of interest and collect the desired data. Off-design operating conditions, especially at multi-unit sites with integrated water and wastewater systems, often can produce the most challenging water operating scenarios and can drive decisions related to equipment sizing.
Once all flows have been charted, the second step is to understand the mass balance of contaminants present in the plant water and wastewater streams. This will likely involve sampling and testing the internal streams that are not normally monitored in order to paint a true picture of:
■ Where the plant is adding contaminants to the water streams or concentrating existing contaminants.
■ What contaminants are being added or concentrated.
■ In what concentration the contaminants are present in each stream.
Portable flow monitors and sample collection devices can be employed to monitor inlet and outlet flows and collect water samples from locations that are not instrumented to capture this data currently (Figure 1).
|1. Get the full picture. Portable flow monitors and sample collection devices can be used to monitor inlet and outlet flows and collect water samples from locations that are not typically instrumented to capture this data. Courtesy: HDR Inc.
Tracking plant water and wastewater flows, and developing a mass balance, can be as simple as developing an Excel spreadsheet to log and analyze the data (see here for a downloadable sample file), although a number of engineering and consulting firms have developed specialized tools to aid in this process.
The flow rates and the water quality constituents for individual plant water and wastewater streams—such as cooling tower makeup, boiler and cooling tower blowdown, equipment drains and washdown, pretreatment system wastewaters, captured storm waters, strainer and filter backwash waters, dewatering equipment filtrate, and many other plant sources and users—are assessed and tabulated to enable tracking throughout the plant.
Once the plant water balance is developed and the mass balance of contaminants is tracked throughout the plant, staff can use this tool in conjunction with the water balance to assess how operational changes or stream redirection might allow the plant to reuse some internal wastewater streams and reduce its water footprint, or reduce the level of contaminants that are discharged or that need to be removed.
Obvious examples of internal recycle/reuse opportunities include recycling reverse osmosis reject as cooling tower makeup, collecting and reprocessing boiler or heat recovery steam generator blowdown water, and recycling cooling tower blowdown as flue gas desulfurization makeup water. However, a close review of the plant water and mass balances can reveal quite a few opportunities for water reuse or water conservation. Many of these internal recycle/reuse opportunities can be inexpensive and relatively simple to implement, involving only piping and valving modifications, though some may involve the addition of treatment equipment such as clarification or filtration to reprocess the wastewater streams to make them suitable for reuse. Determining the water quality of the wastewater stream contaminants, therefore, is a crucial early step in evaluating wastewaters for reuse opportunities and performing a cost-benefit analysis.
Development of water and mass balances will also prepare a plant to intelligently evaluate the impact of and reaction to potential modifications to the plant water balance that may occur due to regulations changes. The new proposed U.S. regulations may affect a plant’s cooling water systems, material-handling and conveyance systems, and wastewater discharges. Potential changes such as the addition of wastewater treatment systems or the rerouting of wastewater streams can be simulated to assess their effects on overall plant operations.
Impoundments and Storm Water
Operators and owners tend to focus primarily on the quality of water coming into the plant and the wastewater quality going out of the plant, as required to meet their plant discharge permits. In contrast, knowing exactly what is going on within the power plant boundaries in impoundments and process sumps is not always as clear, especially in older facilities, where upgrades and plant modifications have considerably altered the plant water picture over the years.
Also, in many coal-fired power plants, coal combustion waste ponds and impoundments are utilized for equalization and treatment of numerous plant wastewater streams, complicating the plant’s ability to easily assess the potential for reuse of each individual stream. However, with some of the proposed U.S. regulation changes, coal-fired power plants may be required to do just that. Segregation and separation of plant wastewaters can minimize the volumetric flow rate that requires additional treatment to meet the proposed regulations.
For instance, at one facility where HDR has been assisting with water and mass balance activities in preparation for expected National Pollutant Discharge Elimination System permit changes, the engineering team was able to reconfigure portions of the wastewater collection system. It segregated storm waters into contact and noncontact storm waters and separated mixed process water streams, allowing the plant to reuse a fair portion of its previously discharged wastewater. As a result, the facility significantly reduced the potential treatment costs associated with meeting new permit requirements and realized an overall cost savings.
Especially in light of the upcoming regulation changes and the expectation that additional levels of wastewater treatment will be necessary to meet some of the new discharge limits, any plant likely to be affected should explore the potential for internal recycling or reuse of plant wastewaters and minimizing water waste in order to keep wastewater treatment costs as low as possible.
Starting now, evaluating the current plant water and wastewater conditions, conducting sampling and flow measurement programs, and assessing current wastewater treatment systems are key to development of a baseline and plan of attack for dealing with the impending regulation changes. Knowing exactly where your plant’s starting point is and developing a solid data history is crucial to determining what you may need to do to meet changes in regulatory requirements. ■
—Colleen M. Layman, PE is a technical practice director with HDR Engineering Inc.