By Thomas E Higgins, Dennis Fink, Brian Choi, Krystal Perez, and Jeff Tudini
Discharge limits for metals have become more stringent under the latest revision to the Steam Electric Effluent Limitation Guidelines (ELGs). Several key mechanical design and operational considerations are critical to ensuring that flue gas desulfurization (FGD) wastewater systems produce compliant effluent and are operable and maintainable.
A well-designed FGD treatment system includes specialized types of mechanical equipment that promote particle growth while minimizing shear to provide optimal conditions for metals removal. In addition, piping configurations and layout considerations must be well-thought-out to minimize solids plugging and also keep a suitable amount of flexibility and adaptability for potential changes in future plant operations.
|Several mechanical design and operational considerations are critical to ensuring wet flue gas desulfurization wastewater systems produce compliant effluent. Key design and operational considerations are critical to compliance and flexibility. Photo courtesy: CH2M|
Wastewater Treatment Process and Suggested Design Improvements
Complex chemistries associated with the wastewater generated from FGD systems usually require treatment systems with chemical and physical treatment processes for soluble metals and suspended solids removal followed by biological treatment processes for nitrate, nitrite, and selenium removal. Current metals removal is based on adding iron salts to remove anionic (negative ion) metalloids, such as arsenic and selenium, along with organosulfides to precipitate mercury and other cationic (positive ion) metals. To make these particles grow larger, which facilitates downstream settling, an anionic polymer is typically added last, which along with gentle stirring (flocculation), promotes larger particle formation just prior to clarification. Metal hydroxide precipitation is an acidic reaction, thus pH control is also required for metals precipitation. Chemical treatment is followed by coagulation of the resulting solids, and a clarifier for physical solids separation (Figure 1).
|Chemical and Physical Treatment System with Sludge Recirculation System|
Combined Chemical Treatment – The chemical and physical FGD treatment system typically involves an equalization tank followed by chemical treatment in multiple tanks to facilitate the precipitation/adsorption of metals. Our findings show that separate mix tanks are not needed for chemical treatment, and in fact counterproductive, as multiple mix tanks and long mixing times resulted in shearing of the precipitated mercury solids (Figure 2). Since iron salt addition is acidic and mercury removal is affected by pH, pH control was needed for all treatment steps other than polymer addition. Testing has demonstrated that use of a single mix tank was equal if not superior to multiple tanks, particularly if sludge recirculation is employed.
Desaturation and Scale Minimization – FGD wastewater tends to be slightly acidic and supersaturated with gypsum. It is desirable to adjust the pH to neutral or slightly alkaline to optimize metals removal. With the use of organosulfides, a pH of 6.5 to 8.5 is typically optimal, with mercury removal optimal at the lower pH. This pH control and reduction in calcium sulfate supersaturation is referred to as desaturation. Desaturation is typically accomplished by adding a slurry of lime and controlling pH to 8.5, depending on the optimum pH for metals removal.
However, reducing the formation of scale on downstream equipment is desirable. It has been our experience that for FGD wastewater that has significant magnesium content compared to calcium, use of lime actually increases downstream scaling. For those situations, using sodium hydroxide or sodium carbonate plus sludge recirculation were found to be better at reducing the supersaturation of calcium sulfate and minimizing scaling than adding lime.
Sludge Recirculation – CH2M has found that sludge recirculation provides several benefits, other than reducing calcium sulfate supersaturation. Sludge recirculation increases particle size, due to precipitation of new solids on older particles. The resulting solids are crystalline and compact, whereas solids that form in the absence of pre-existing solids in the water tend to be amorphous and trap water, making them less dense. The larger, denser solids created with recirculation settle and dewater better than when sludge is not recirculated. CH2M has done this recirculation in a closed sludge loop, using a single set of pumps for sludge wasting and sludge recycle. This is done by routing the recycle line first over the sludge storage tank(s), then on to the mix tank.
Minimize Salinity Variability – Keeping in mind that the complexity of the FGD wastewater chemistry typically requires a downstream biological treatment system for the reduction of nitrate, nitrite, and selenium, it is desirable to minimize the variability in salinity and other factors whose upset could affect the consistent performance of a downstream biological treatment system. Incorporating equalization, clarification, and internal recycle of sludge and filtrates from the dewatering system can limit rapid changes in salinity that improves the downstream reliability of the biological treatment system.
Selecting Mechanical Equipment to Minimize Shearing
Use of organosulfides has greatly reduced the solubility of metals, particularly mercury, however, CH2M has found that plants that have high-shear mixers and pumps have significant generation of colloidal-mercury-containing solids that are passing through their clarifiers and media filters, with mercury concentrations exceeding 1 ppb, and TSS exceeding 30 mg/L. In recent projects, CH2M added mix tanks with low-shear mixers for organosulfide coupled with iron and polymer addition to enhance an existing pond treatment and achieved mercury concentrations of less than 12 ng/L.
Mixers – CH2M tested a broad range of mixers to identify models with minimal shear for the removal of mercury and other metal-organosulfide precipitates. Hyperboloid mixers (Figure 3) equipped with variable-frequency drives provided a balance between providing adequate mixing and minimizing shear. Equipped with bottom supports, these mixers could also be operated at low water levels while maintaining solids suspension, making them ideal for equalization tanks.
|Low Shear Mixer|
Pumps – Based on the mechanical nature of pumping fluids, precipitated metal particles can be broken apart due to their fragile nature. Therefore, it is necessary to utilize pumps that minimize the shear when conveying the FGD wastewater or sludge from one-unit process to another. Power plant sludges can also be highly abrasive, causing rapid wear of conventional positive displacement pumps. Rotary lobe positive displacement pumps can be equipped with abrasion-resistant wear plates coated with tungsten carbide, limiting wear to resilient rotors that can be quickly replaced through end plates, without decoupling the pumps from their associated piping. This type of pump can also be reversed for back-flushing.
Downstream Filtration and Metals Removal – CH2M has found that metals removal is dependent on recirculation to generate large, dense solids; appropriate dosing with coagulants (ferric chloride) and flocculants (polymers); appropriate mixing intensity that limits shear; avoiding pumping where feasible; and using low-shear pumps for sludge recirculation. When this is done, solids settle well in conventional clarifiers, and filtration is not needed. However, when mixing results in solids shear, media filtration does not help mercury removal. A significant concentration of mercury can be present as particles smaller than 5 microns, which is the particle size effectively removed by media (sand) filtration. Many of our clients have found little additional mercury removal with sand filters over a well-operated clarifier.
Moreover, CH2M has found that sand media filters develop severe scaling problems with FGD wastewater because filters are typically fed low-solids water, with only the sand to provide solids for scale to form on. CH2M has tested membrane filters developed for high suspended solids applications (membrane biological treatment) and was able to limit scaling by maintaining a high concentration of solids on the upstream side of the membranes.
Piping Configurations Minimize Solids Accumulation and Plugging
Once the solids particles have been formed by chemical addition, the FGD wastewater is typically treated in a flocculating center well clarifier where the solids are gently mixed for particle growth (flocculation) then settled and removed. However, solids are prone to plugging. Well-designed piping configurations minimize the potential for settling of the solids within system piping. Additionally, ELGs do not allow for any dilution of the FGD wastewater to achieve the regulated limits, thus minimization of flush water for the movement of the solids and clearing of any blockages is desirable. Any flush water introduced into the system is also considered FGD wastewater.
Clarifier Piping – For the clarifier operation, a closed-loop sludge wasting and recycling system design (Figure 1) allows for maintenance of a scour velocity in the sludge piping that minimizes plugging and eliminates the need for the introduction of any flush water except in the instances when a treatment train is shut down. Sludge is continuously recycled while a fixed amount of solids is wasted to a solids storage tank to feed the plant’s dewatering operation. The continuous recycle keeps the FGD solids moving throughout the piping system and minimizes the potential for plugging in the pipe due to solids settling. In addition, minimizing the sludge recirculating pipe lengths to limit the potential for solids plugging is recommended. Thus, it is desirable to locate the sludge pumps next to sludge sumps and recirculation lines as close to the mix tank and sludge storage tanks as possible. This can be achieved by constructing a tunnel underneath of the clarifier for housing of the sludge pumps. Situating the sludge pumps directly underneath the clarifier in this below grade tunnel allows for a short, straight run of pipe directly to the sludge pumps and minimizes piping for the sludge wasting and recirculation.
Sludge Wasting and Recirculation Rate – CH2M has found that solids recirculation ratio of 20 parts old solids to 1 part newly formed solids is optimal and can be accomplished by providing a time ratio of 20:1 of recirculation to wasting. Pinch valves are used with a timer to alternate wasting and recirculation. Pipes are located after the valves such that they drain into their respective tanks by gravity to minimize plugging. By providing a constant recirculation at a velocity of 5 ft/sec, deposition of solids in piping is minimized compared to a system where sludge wasting is intermittent or at continuous but variable flow.
Mix Tanks Piping – It is important to introduce the treatment chemicals into the mix tanks such that adequate mixing occurs to maximize the interaction with the solids particles for a given residence time, which allows for maximum potential for solids particle growth. Thus, injecting the chemical feed near where the FGD wastewater is introduced into the mix tank takes advantage of the turbulence from the inlet flow to assist with the chemical mixing. Since minimal mixing is used to prevent shear of particles, there can be a tendency of solids to settle and buildup in the mix tank becoming thicker over time. By providing a dip tube along with a hyperboloid mixer, the accumulation of solids in the tank can be minimized. The mixer can be tuned to promote bottom suspension of solids rather than turbulent mixing (avoiding shear on solids).
Dip Tubes – Dip tubes (Figure 4) act as a vacuum pulling these solids out of the tank, preventing buildup. Chemical treatment can also be introduced into the dip tube between the mix tank and clarifier. Situating the top of the dip tube slightly below the top elevation of the mix tank allows the tank contents to overflow into the next tank and prevents overfilling should plugging at the bottom of the dip tube occur.
Layout Considerations Simplify Operation & Maintenance
Since FGD solids are prone to plugging, careful consideration must be made when laying out the mix tanks, clarifiers, sludge storage tanks, and dewatering systems such that pipe lengths and any stagnant lines are minimized. When possible, it is advantageous to locate the chemical and physical treatment system and the biological selenium treatment system within close proximity to each other.
Common Dewatering – Both treatment processes (chemical/physical treatment and biological treatment) generate waste solids that must be dewatered. Having both treatment trains located in proximity to each other allows for the sharing of solids storage and dewatering equipment. The majority of the solids in the FGD wastewater treatment train is generated in the chemical and physical treatment system. The additional loading from the biological solids is relatively small and does not significantly affect the sizing capacity of the dewatering equipment required to dewater the solids generated from the chemical and physical treatment system.
Common Access – Clarifiers can also be situated in a way that allows for a common platform and access walkways to be at a common height for common access. Major process treatment equipment is accessible from a main level where pipe racks can also serve as walkways among the associated solids dewatering equipment.
Editor’s note: Originally presented at the International Water Conference®: November 6-10, 2016. Please visit www.eswp.com/water for more information about the conference or how to purchase the paper or proceedings.
All of the authors work for CH2M. Thomas E. Higgins, Ph.D., P.E. is Power Water and Process Senior Technology Fellow; Dennis Fink, P.E. is Senior Project Manager; Brian Choi is Senior Process Engineer. Krystal Perez, P.E. is Global Practice Lead for Power Water and Process; Jeff Tudini is Project Manager and Environmental Engineer.