By Mike Preston
The onslaught of new and updated regulations pertaining to the power industry is no longer big news. Utilities and power generation owners have been diligently studying and evaluating the impacts of these new and proposed regulations for some time now and trying to determine how they will respond. The sheer volume of regulations and associated ambitious implementation scheduled has led some to characterize the next 3 to 5 years as a regulatory “train wreck”.
While most of the regulatory emphasis has been on air quality, several proposed regulatory programs will directly impact power station water management plans. Once implemented, these regulatory programs will result in a greater emphasis on water conservation, water reuse, and wastewater treatment for power generators. This revised emphasis will provide an opportunity for power stations to re-evaluate their water management approach and consider some fundamental changes in water intensive processes in the plant. In the long run, this may prove beneficial for older power stations that were constructed when water resources may not have been as constrained or wastewater discharge as tightly regulated.
Air regulations dominate the regulatory discussion. Their impact on water management is not as direct as some of the other proposed regulations. However, that impact is fairly well understood now that the initial wave of flue gas desulfurization (FGD) systems has occurred. Generally, new wet FGD systems require a water supply for makeup and a wastewater treatment system for the scrubber bleed stream. FGD wastewater treatment requirements are not uniform at this time. Consequently, many different methods of dealing with FGD wastewater can be cited. However, this is likely to change with the issue of new power plant discharge regulations.
It is interesting to note that as the high cost of FGD bleed stream treatment has become better defined, costs are becoming a recognized factor in new FGD system planning, to the extent that new systems give significant consideration to wastewater treatment issues. One FGD supplier has even devised a combined wet FGD/spray drier approach that proposes to eliminate wet FGD wastewater.
Proposed regulatory changes with the most direct impact on water management are described in the following sections.
Clean Water Act Section 316(b)
Section 316(b) of the federal Clean Water Act (CWA) requires the location, design, construction and capacity of cooling water intake structures to reflect the best technology available (BTA) for minimizing adverse environmental impacts to aquatic species. EPA released a proposed Phase II rule in April 2011 to establish BTA for all existing facilities withdrawing more than 2 mgd for cooling purposes based on impingement mortality standards and case-by-case entrainment reviews. One approach to reducing mortality is limiting the intake screen velocity to 0.5 fps through-screen velocity.
The impact of this new rule may require many power stations using once through cooling to modify their intake structures to meet new requirements, or convert their systems to cooling towers to reduce surface water intake.
Effluent Limitation Guidelines (40CFR 423)
EPA entered into a consent agreement with various environmental organizations in which it agreed to propose revisions to power plant wastewater effluent limit guidelines by July 23, 2012, and finalize this rulemaking by January 31, 2014. Once the rule is finalized, plants would need to come into compliance with the new standard within 3 to 5 years of the final rule or as their NPDES wastewater discharge permits are renewed. The revised limits will likely include minimum best available technology (BAT) and/or water quality based limits for metals (arsenic, mercury, selenium, etc.) in power plant effluents.
The impact of this new rule will vary depending upon the final effluent standards and the water quality of the waterbody receiving the wastewater discharges. It is anticipated that additional treatment systems, likely physical/chemical and/or biological, will be required to meet new limits. Alternatively, water recycling/reuse or costly zero discharge systems may be required in some power generation facilities.
Coal Combustion Residual Management
EPA published a proposed rulemaking in June 2010 that would regulate coal combustion residues (CCRs) - i.e. fly ash, bottom ash, boiler slag and flue gas desulphurization materials generated from coal-fired power plants – as either “special wastes” subject to regulation under Subtitle C of the Resource Conservation and Recovery Act (RCRA), or solid waste under Subtitle D of RCRA. The practical result of either action will force coal-fired power plants to eventually convert their collection, handling, transport, and storage of CCR from wet to dry systems, and close or retrofit existing wet surface impoundments (ash ponds). EPA is expected to finalize this rulemaking in 2012, in which case coal-fired power plants will be forced to discontinue wet ash handling operations within 5 to 7 years depending upon how quickly the requirements are adopted and take effect in each state.
The impact of this new rule is uncertain at this time but could require nearly all coal-fired power generation facilities that currently use wet sluicing and/or ash ponds to convert to a form of dry handling and disposal system. Current impoundments may have to be closed or retrofitted and existing water and wastewater management systems reconfigured.
Water Management Review
Given the various requirements of new water and air regulations, this seems to be an opportune time for existing facilities to re-evaluate their water management approach. The overall goal of a water management review is to provide one piece of the information necessary to cost effectively meet the new regulation puzzle. While water management is only one of several considerations in this process, it should not be overlooked. It is the authors’ opinion that a holistic review of the water management plan will provide the best framework to consider all regulatory consequences and subsequent costs and benefits to the plant. In contrast to the approach of evaluating isolated cases in response to singular concerns, a holistic approach will permit facilities that are required to make modifications to consider the full range of solutions including their impact to the plant’s water use and wastewater production.
Water Management Review Approach
A holistic water management review includes the production of an interactive water balance that depicts the current plant water management approach. This balance must be flexible so that it can be manipulated to reflect technology changes within the plant to meet new rules. A side benefit to this approach is producing an updated, detailed water mass balance for the plant along with an increased awareness of water and wastewater concerns.
The first, and most difficult, activity in producing this water balance is to collect and analyze sufficient data to accurately model the plant water balance. Examples of data needed include water quality and flow data measurements for raw water, various plant process waters, and wastewater. It is desirable to have historical data showing seasonal impacts on the water balance. However, some data may not be available and may have to be obtained during the development of the water balance model. Limited data is better than no data. As you might expect, obtaining this data can be challenging for the power station or their consultant, but generally with some diligence, it can be obtained.
Additionally, it is advisable to collect as much original equipment design data as is available so that equipment design constraints are understood and considered. Equipment design data needed would include water and wastewater treatment design data, cooling system data, steam cycle design data, ash handling data, FGD system data, pond/impoundment sizes, sump and sump pump sizes, wastewater collection system data, etc.
Once the water balance is modeled and calibrated against existing data, potential revisions to the water balance that may occur due to new regulatory requirements can be tested. As noted above, it appears the new regulations will potentially impact plant cooling systems, material handling systems (ash, gypsum, landfills, etc) and wastewater discharge. The following briefly addresses how revisions to these systems could impact plant water management.
Bottom Ash Handling Systems
Generally there are two types of bottom ash handling system common in power plants today. One that is typically associated with older plants uses water to sluice ash from the boiler. The sluiced material is either taken to dewatering vessels where the ash is dewatered and moved to storage or taken to a large ash pond where the ash resides permanently or temporarily prior to being moved to long term storage. The water in the sluicing system can be handled in several ways. Often the water is recovered and reused in the sluicing process with a relatively small amount of fresh makeup water required to cover losses. At some facilities, the pond makeup is wastewater and the pond is continuously blown down. In other facilities without ponds, the sluice water is used in a once through system.
The second type of bottom ash handling system uses a drag chain or scraper conveyor to move ash out from underneath the boiler. The conveyor is submerged in water to cool the ash and seal the system from outside air, thus it is often termed a submerged scraper conveyor (SSC). SSC cooling can be configured in two ways. Open cooling brings in fresh makeup water to the conveyor in sufficient quantity to cool the ash while an equal amount of warm water from the system is released. Closed cooling uses a water cooled heat exchanger to reject the heat from the SSC and return cool water to the conveyor trough. Ash that is conveyed from the boiler is dumped into dewatering bins or a dewatering pit and then removed to storage. Generally, the SSC is located directly under the boiler however; it can be remotely located to facilitate retrofitting ash sluice systems. In this application, ash would be sluiced to a remotely located SSC and the ash dewatered and conveyed to a storage location. This makes the SSC a primary option for replacing ash sluice systems.
A third option that is emerging is dry ash handling. Dry ash handling systems work very similarly to the SSC system except that air is the cooling medium. Thus, the ash is handled completely dry, with exception of small amounts of water for dust suppression. In the as yet unlikely event that ash is deemed a hazardous material, this would emerge as the preferred option because it eliminates contact with water and the associated issues of treatment and discharge of that water that would then follow.
Due to the proposed CCR regulations and the general liability associated with maintaining ash ponds, these types of systems are generally being considered for replacement. It appears almost certain that ash ponds will be eliminated in the future. This will require that sluice systems using ponds be retrofitted with dewatering bins or converted to SSC or dry handling. It would also be reasonable to anticipate that wastewater discharged from bottom ash systems will be collected and treated to meet new discharge limits proposed under 40CFR 423. A holistic water management review should be able to identify uses for ash water such as makeup to an FGD system or associated treatment costs for ash handling wastewater. These issues may factor in to the type of ash handling system modifications selected to meet new regulations.
The proposed Phase II CWA 316(b) regulations require that power plants withdrawing more than 2 mgd from surface water sources meet water intake criteria set forth in the regulation. Generally, any power station of significant size using once-through cooling will be subject to these new requirements. The proposed constraints in this new rule have many power stations considering converting once-through cooling systems to cooling towers.
Conversion of once-through cooling systems to cooling towers has many aspects for a plant to consider, not the least of which is water. While the cooling tower withdraws less raw water, it creates a more concentrated blowdown stream. In considering the switch to a cooling tower the owner may wish to consider how best to manage the makeup water and blowdown to match the particular plant operating profile and needs. The following considerations should be evaluated.
Should we consider another water source? For instance coastal plants forced to consider cooling towers may need to look for fresh water sources.
- Is cooling tower makeup treatment necessary or valuable? What type of treatment is necessary and to what extent should the makeup water be treated? Would additional pretreatment economically balance wastewater treatment concerns and costs?
- Can cooling tower blowdown be reused in other plant systems such as ash handling or FGD systems?
- If cooling tower blowdown cannot be partially or fully reused, can it be discharged without additional treatment?
- If cooling tower blowdown requires treatment, what is the most economical treatment approach?
Implicit in these questions is at least a conceptual analysis of the treatment systems, estimated size and location within the plant, and an estimated cost to install and operate such systems. These considerations and options are best evaluated in the context of a holistic review of the plant’s water management approach.
New discharge limits for power stations under this rule are currently under consideration. Draft rules will not be issued until 2012. It is believed that new discharge limits will include limits for a number of heavy metals and that these limits will require that various discharge streams in the plant (such as ash transport water, coal pile runoff, CCR landfill runoff, metal cleaning wastewater, and FGD wastewater) be collected and treated for heavy metals. The most likely treatment approaches will be iron co-precipitation or metal sulfide precipitation. In some cases biological treatment may be required to reduce specific metals to very low levels.
In addition to the new regulations, new internal monitoring points may be established in new NPDES permits. As compliance approaches are developed to meet new discharge limits, the following questions should be considered.
- Can wastewater that is currently being discharged be reused? For instance, FGD systems, ash handling systems, dust suppression systems, and cooling towers may be options for reusing certain plant wastewater streams.
- Can existing plant wastewater systems be used in part, or in whole, to address new limits?
- How will plant modifications necessary to meet new air rules, cooling water rules, ash handling rules, etc change the composition of the wastewater?
- Does it make sense to reconfigure the wastewater collection system to segregate certain waste streams or combine certain waste streams?
- Where will new wastewater treatment systems be located?
- What are the treatment options for meeting the new discharge limits and what are the costs?
- Does it make sense to modify certain systems within the power station to reduce potential wastewater treatment impacts/costs?
In considering these questions, a holistic review of the power station water management plan will provide the proper context for making decisions on how best to comply.
FGD systems are being installed at almost every coal fired power station planning to remain in operation in response to new air regulations. The type of FGD system used at a particular power station is a complex decision and driven by many factors. Until recently, water supply and wastewater treatment associated with FGD systems was a non-factor in these decisions. However, that is beginning to change.
Wet FGD systems use a bleed stream to control the chemistry of the scrubber liquor in the absorber. This bleed stream is a wastewater that has virtually no reuse potential within the power station. It is also a wastewater that is extremely costly to treat. FGD bleed stream treatment is generally physical/chemical treatment to reduce total suspended solids (TSS) and heavy metals. This is generally accomplished in two-stage clarification followed by filtration. Biological treatment may follow the physical/chemical treatment to further reduce selected heavy metals.
In a relatively few cases evaporator technology has been used to eliminate the FGD bleed stream, but at significant cost. It is not currently anticipated that this approach will be mandated by new limits established under 40CFR 423.
The water management benefit provided by FGD systems is that they can accept relatively poor quality water as makeup water. Generally, the poorer quality makeup water is used as makeup for evaporative losses in the FGD. However, FGD systems do have makeup water quality limits, so it is best to have a good understanding of the water quality of various wastewater streams being considered as FGD makeup.
When considering wet FGD, the wastewater treatment costs should be considered in optimizing the FGD operating chemistry and material selection. Lower chloride content in the FGD absorber means lower cost FGD materials of construction and lower risk for FGD material corrosion concerns, but results in a greater quantity of FGD bleed wastewater.
Additionally, FGD wastewater treatment should be considered in the ultimate disposition of the gypsum by-product. Selling the gypsum results in greater quantities of FGD wastewater treatment because the gypsum must have lower moisture content and is generally washed in the dewatering process to reduce chloride content.
Spray drier absorbers (SDA), when applicable, eliminate the need for wastewater treatment because there is no bleed stream.
FGD systems and their impact to the power plant’s water management scheme are best considered in the context of a holistic water mass balance.
In summary, a holistic review of the power plant water management plan in the face of new regulations offers the following benefits:
- A chance to update the plant’s water balance and heighten awareness of water and wastewater concerns at the plant.
- Avoid surprise water and wastewater treatment issues at a later time.
- Consideration of wastewater treatment costs in the selection and optimization of power plant system modifications necessitated by regulatory requirements.
- Proper consideration of regulatory costs associated with necessary changes to water and wastewater systems as a result of new regulations.
- An opportunity to avoid costly treatment systems by identifying means of reducing and/or reusing wastewater within the power plant.
- An opportunity to economically justify and balance the cost of wastewater treatment resulting from new regulations.
- An opportunity to conduct a comprehensive review of changes to a facility’s water management approach based on multiple and simultaneous changes to environmental regulatory programs.
About the Author: Mike Preston is Chemical Engineering Section Leader and Project Engineer in Black & Veatch’s Energy Business. He graduated from the Missouri University of Science and Technology with a Chemical Engineering degree in 1988 and is a registered professional engineer. In his current role he primarily supports the water and wastewater concerns of Black & Veatch’s Energy Business clients including power generation, oil and gas, and other energy intensive businesses.
A version of this paper presented at the Industrial Water Conference, www.eswp.com/water.