|By Michael G. Klidas, Research Engineer, Babcock & Wilcox Power Generation Group|
With the U.S. Environmental Protection Agency’s (EPA) April 2013 release of the effluent limitation guidelines (ELG) proposed rule regulating wastewater discharges from the steam electric industry, plant owners should investigate their options for compliance prior to the expected release of the final rule in May 2014.
The proposed rule will be implemented on a rolling schedule through National Pollutant Discharge Elimination System (NPDES) permits up for renewal and would set limits on the levels of pollutants in wastewater discharge from flue gas desulfurization (FGD) wastewater, discharges from fly ash and bottom ash systems, combustion residual leachate and gasification wastewater. Utilities, however, may want to pay particular attention to the selection of FGD wastewater treatment and the associated interactions with upstream equipment due to the large investment likely needed to comply with the FGD regulations.
Zero Liquid Discharge
One strong reason to consider going with a Zero Liquid Discharge (ZLD) option is to avoid potential issues with achieving and maintaining the specified numeric criteria and to avoid additional equipment installations for more stringent future regulations. However, implementation of a ZLD system comes with higher capital and operating costs than some other options. Some facilities may find that the trade-off between the higher capital expenditure and the uncertainty involved with other systems favors ZLD.
In the proposed rule, the EPA included a voluntary incentive program to encourage selection of ZLD systems. Plants electing ZLD for all of their wastewater streams, with the exception of cooling tower blowdown, could be granted up to five additional years for compliance. This could be an attractive way to postpone the large capital investment of a ZLD system.
ZLD system options include evaporation ponds, deep well injection of wastewater, closed loop FGD operation (in which chloride levels are increased and chlorides are purged through gypsum that is landfilled), spray dry evaporation in flue gas and evaporation via mechanical vapor compression (MVC). Since evaporation ponds are limited to deserts and deep well injection requires suitable geologic formations, few plants will be able to select these technologies as a ZLD solution. In addition, if the chlorine content of the coal being burned is high, the closed loop FGD operation may lead to unacceptable chloride concentrations and corrosion concerns . This leaves MVC and spray dry evaporation as the leading candidates for ZLD systems.
If a completely closed loop system is beyond the capability of the FGD system due to chloride levels, a smaller increase in chloride level can be achieved by decreasing, instead of removing, the blowdown stream. This offers cost savings for both an MVC system and a spray dryer due to the reduced flow of water to evaporate. This increase in chloride concentration may increase the scaling potential of the absorber tower and/or reduce SO2 removal.
Partial ZLD Solution
MVC systems also have the ability to go to a “partial” ZLD system. For a partial ZLD system, instead of completely crystallizing the dissolved solids in the wastewater, concentrated brine is created. This concentrated brine, around five percent of the initial blowdown flow, can be mixed with plant fly ash and landfilled as a solid waste still meeting zero “liquid” discharge. Partial ZLD avoids operational difficulties that may occur during the crystallization process. This is especially beneficial when softening is avoided leaving predominantly CaCl2 salts in the concentrated brine.
Biological Systems Solutions
For those who don’t go the ZLD route, there are other options. Due to the NPDES permitting process and the numeric criteria implemented at Merrimack Station, some believe the EPA is leaning towards regulations based on the performance of chemical precipitation followed by biological treatment.
Biological systems operate by creating a reducing atmosphere and converting nitrates to nitrogen gas, followed by conversion of selenium compounds to insoluble elemental selenium. Although these systems offer significant cost savings over ZLD systems, potential issues can occur as a result of fluctuating FGD wastewater chemistry. Close monitoring of wastewater will ensure minimal upsets to the biological system and enable suitable living conditions for the reducing bacteria.
Whatever solution a utility considers, understanding the interactions between the wastewater technology and the upstream process, using that information to plan viable options, and providing accurate design specifications will help ensure the best available option is selected for each plant.
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