WEFTEC 2016
Mathematical Modeling of Biological Selenium Removal from Flue Gas Desulfurization (FGD) Wastewater Treatment Mehran Andalib1*, Sara Arabi1, Peter Dold2, Christopher Bye2 1
Environmental Operating Solutions, Inc. (EOSi), Bourne, MA, USA EnviroSim Associates Ltd., Hamilton, ON, Canada * Email:
[email protected] 2
ABSTRACT Due to increased enforcement of selenium regulations and understanding of selenium health and environmental effects, treatment of selenium contaminated waters has become a very important issue. The biological selenium reduction process is capable of transforming selenate into selenite and subsequently to nontoxic insoluble elemental selenium. Treatment of Flue Gas Desulfurization (FGD) wastewater is challenging due to high levels of metals such as selenium and total dissolved solids. Biological processes under anoxic/anaerobic conditions have been successfully applied to FGD wastewaters for removal of nitrate/nitrite and selenium. Unlike many other biological processes such as nitrification, denitrification and phosphorus removal, a mechanistic mathematical model as a design and analysis tool is presently lacking in the literature for biological selenium removal. In this work, a mechanistic mathematical model is proposed, added to the BioWin process simulator, and calibrated based on the operational data from a full-scale FGD Wastewater Treatment Plant (WWTP). This paper presents full-scale WWTP data on selenium concentration and speciation, and proposes a model that addresses the reductive competition between denitrifiers (nitrate and nitrite reduction) and Selenium Reducing Bacteria (SeRB). Four new state variables and five biological processes are proposed in this model to mechanistically simulate the two stage reductive pathways of selenate to elemental selenium. KEY WORDS Biological; Selenium Removal; Modeling, FGD Wastewater INTRODUCTION Effluent Limitation Guidelines (ELG) ELG are national regulatory standards established by the US Environmental Protection Agency (US EPA) for wastewater discharged to surface waters, municipal sewage treatment plants, and for industrial categories, based on the performance of treatment and control technologies. The regulations are implemented through the National Pollutant Discharge Elimination System (NPDES) program. The Steam Electric Power Generating ELGs apply to a subset of the electric power industry, namely those plants primarily engaged in the generation of electricity from a process utilizing fossil-type fuel or nuclear fuel in conjunction with a thermal cycle employing the steam water system as the thermodynamic medium (US EPA, 2009).The initial ELGs were
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established in 1974 with amendments in 1977, 1978, 1980, and 1982. The 1982 regulations for the industry were not updated after 1982. New ELGs were proposed in April 2013 and was finalized on September 30, 2015. The rule requires specific treatment of wastewater discharged from FGD units at coal-fired power plants. For FGD wastewater, the final rule contains limits for nitrate/nitrite, selenium, mercury, and arsenic (US EPA, 2015a) as presented in Table 1. To meet the ELGs for effluent nitrate, nitrite and selenium, FGD WWTPs will likely have to add a new treatment process to supplement existing solids and metals removal processes. US EPA estimates that about 12 percent of steam electric power plants will incur some costs. Power plants must comply between 2018 and 2023. Table 1 – Effluent Limitations for FGD Wastewaters (US EPA, 2015)
Pollutant
Unit
30 Day Average
Daily Maximum
ng/L
356
788
µg Se/L
12
23
µg/L
8
11
mg N/L
4.4
17
Total Mercury Total Selenium Total Arsenic NOx as N
ELG
FGD Wastewater Characteristics
Pollutant concentrations in FGD wastewater vary to some degree from plant to plant depending on the coal type, the sorbent used, the materials of construction in the FGD system, the FGD system operation, the level of recirculation in the scrubber, and the air pollution control systems operated upstream of the FGD system (US EPA, 2015b). FGD wastewater contains chloride, sulfate, Total Dissolved Solids (TDS), Total Suspended Solids (TSS), and bio-accumulative pollutants such as arsenic, mercury, and selenium. Additionally, inorganic nitrogen components such as ammonia, nitrate and nitrite (NOx) are present. Table 2 presents the average pollutant concentrations of the influent to FGD wastewater treatment systems (US EPA, 2015). As shown in Table 2, FGD wastewater contains significant concentrations of chloride, TDS, TSS, sulfates, and calcium. The majority of the chlorides present in the FGD wastewater originates from the fuel source. For sulfate, a plant burning a higher sulfur coal produces more sulfur dioxide (SO 2 ) in the combustion process, which in turn increases the amount of SO 2 removed in the FGD scrubber. Lime slurry [Ca (OH) 2 ] is utilized in the wet FGD scrubber to remove SO 2 . High calcium concentration in the FGD wastewater mainly originates from limestone.
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Table 2 – Average Pollutants Concentrations in Untreated FGD Wastewater (US EPA, 2015b)
Pollutant
Unit
Total Kjeldahl Nitrogen (TKN)
mg/L
Average Total Concentration 34.9
Ammonia as N
mg/L
13.1
Nitrate – Nitrite as N
mg/L
91.4
Biochemical Oxygen Demand (BOD)
mg/L
8.2
Chemical Oxygen Demand (COD)
mg/L
345
Total Suspended Solids (TSS)
mg/L
14,500
Total Phosphorus (TP)
mg/L
4.02
Chloride
mg/L
7,180
Sulfate
mg/L
13,300
Total Dissolved Solids (TDS)
mg/L
33,300
Selenium
µg/L
3,130
Calcium
mg/L
3,290,000
The diverse chemistry of the contaminants and the very low concentrations specified in the ELG present a challenge to existing treatment technologies. Treatment technologies and management practices typically used for FGD wastewater treatment includes physical/chemical and biological treatment processes. In the past decade, removal of metals such as selenium and arsenic from FGD wastewater and other sources such as coal ash ponds has gained considerable attention and many research studies have been conducted to find or optimize a cost effective treatment technology for selenium. With the ever-present risk of wastewater spills to the environment (from ash ponds or other sources) and requirement to meet the ELG limits, it is imperative for the power industry to have a means for treating stored or discharged wastewaters for selenium. Available treatment technologies employed in the industry for selenium removal are presented in the next section.
Treatment Technologies for Selenium Removal from FGD Wastewater While developing rules for Steam Electric Power Generation, the US EPA identified 88 steam electric plants with FGD discharge after treatment (US EPA, 2015b). Most plants that discharge FGD wastewater use surface impoundments for treatment; however, the use of more advanced wastewater treatment systems is increasing due to more stringent effluent limit requirements
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(ELG) and some site-specific limits imposed by states. Figure 1 shows the distribution of FGD wastewater treatment technologies based on the Steam Electric Survey (US EPA, 2015 b).
Zero Discharge (51 plants, 37%) Other ( 11 plants, 8%) Biological (Anoxic/Anerobic (5 plants, 3%) Chemical Precipitation (33 plants, 24%) Surface Impoundments (39 plants, 28%)
Figure 1- Distribution of FGD Wastewater Treatment Systems (US EPA, 2015b) A list of the treatment technologies for FGD wastewater is included below. Chemical precipitation, biological treatment, and wetland systems are known to be effective technologies in removal of selenium from FGD wastewater. Surface Impoundments: Surface impoundments (e.g., settling ponds) remove particulates from wastewater by means of gravity. Impoundments are typically sized to reduce Total Suspended Solids (TSS) and are not intended to remove selenium. Chemical Precipitation and Physical Separation: In chemical precipitation systems, chemicals are added to enhance the removal of suspended solids and to remove dissolved solids, particularly metals. The precipitated solids are then removed from solution by coagulation/flocculation followed by clarification and/or filtration. This process has the obvious drawback of removing other, untargeted constituents, thereby resulting in large-volume process residuals. The dried sludge from the chemical precipitation process may be a hazardous waste, requiring either further treatment or resulting in high disposal costs. Coagulation / flocculation is an effective means of removing selenium. Since selenium is best removed in its reduced state, additional processing may be required to reduce the selenium to its elemental state. Evaporation: This type of system uses a falling-film evaporator (or brine concentrator) to produce a concentrated wastewater stream and a distillate stream to reduce wastewater by 80 to 90 percent (with a pretreatment step). Evaporation can be an effective means of removing selenium. While many power plants have evaporators as part of their wastewater management, these units are typically specifically sized for a final volume reduction stage prior to discharge to evaporation ponds or crystallizers (EPRI, 2004). Since evaporation processes are non-targeted processes, they typically are very high in both capital and operating costs.
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Constructed Wetlands: Constructed wetlands are engineered systems that use natural biological processes involving wetland vegetation, soils, and microbial activity to reduce the concentrations of metals, nutrients, and TSS in wastewater. Constructed wetlands have been used to remove selenium from wastewaters. While viable and reasonably effective, this technology is not considered to be cost effective as a stand-alone process for selenium removal. If wetlands are used as an overall means of treating the wastewaters from a power plant, then this technology may be a reasonable choice for co-treatment (EPRI, 2004). Zero Liquid Discharge (ZLD): US EPA identified four design/operating practices available enabling power plants to eliminate the discharge of FGD wastewater: 1) several variations of complete recycle, 2) evaporation impoundments, 3) conditioning dry fly ash, and 4) underground injection (US EPA, 2015b). Other Technologies under Investigation: EPA identified several other technologies that have been evaluated for treatment of FGD wastewater; including iron cementation, reverse osmosis, absorption media, ion exchange, and electrocoagulation. Other technologies under laboratoryscale study include polymeric chelates, taconite tailings, and nano-scale iron reagents (US EPA, 2015b). Membrane separation and ion exchange, are also non-targeted processes but leave large volumes of undesirable residuals (EPRI, 2004). Selenium removal processes using membrane technology are complicated and the cost of membrane replacement and resin replacement should be considered. Recently, sorption processes have been successfully used for selenium removal. The operating cost for media and maintenance for sorption processes may be high and should be considered in the life cycle analysis. Biological Treatment: Two types of biological treatment systems currently are used in FGD wastewater treatment for selenium removal, including anoxic/anaerobic fixed-film bioreactors and anoxic/anaerobic suspended growth bioreactors such as sequencing batch reactors. Such biological processes are also effective in denitrification (removal of nitrate – nitrite). Figure 2 shows the process flow diagram for an anoxic/anaerobic biological system. Biological selenium reduction relies on the activity of a small group of bacteria that are able to reduce selenate to selenite, and subsequently to elemental selenium. The biological treatment system, in most cases, consists of a series of reaction vessels to accomplish the desired removals for nitrate/nitrite and selenium. For example, for wastewaters that contain nitrates, the first step is typically the removal of nitrates by denitrification microorganisms which reduce the nitrates to nitrogen gas which is stripped to the atmosphere. The denitrification step is followed by the selenium reduction process where the Selenium Reducing Bacteria (SeRB) microorganisms reduce the selenite and selenate to elemental selenium as evidenced by the formation of a red precipitate in the reactor. Although the ELG effluent NOx limit is 4.4 mgN/L, complete removal of NOx is desirable in the process since the biological selentate / selenite reduction process is most favourable under anaerobic conditions. Further details are provided below. There are many commercially marketed biological systems for biological selenium removal including iBIO™ (suspended growth reactor), ABMetTM which comprises of a packed-bed activated carbon system using proprietary SeRB, wetlands, fluidized and fixed bed reactors, Membrane Bioreactors (MBR) and Moving Bed Membrane Bioreactor (MBBR). For all the above biological processes, adequate carbon source, essential macronutrients (nitrogen and
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phosphorus) and micronutrients are required to ensure biological growth of bacteria capable of reducing NOx as well as selenate and selenite. The process may need to be followed by an aerobic reactor to remove process residuals like ammonia or degradable organics.
Figure 2- Process Flow Diagram of an Anoxic/Anaerobic Biological Treatment System (US EPA, 2015b) Microbial (Biological) Selenium Removal Biological selenium removal has been the subject of many recent research studies. The first mention of an organism capable of using selenium (in the form of selenate) as an electron acceptor was a short note by Lipman and Waxman (1923). Over the past two decades, more details on the selenium biochemical cycle have been elucidated. The dissimilatory reduction of selenate via selenite to elemental selenium has been shown to be a significant and relatively rapid environmental process (Stolz et al., 2006). Several bacteria capable of catalyzing selenate reduction reactions have been isolated, and some of them have been extensively characterized (Maiers et al., 1988; Steinberg et al., 1992; Macy et al., 1989; Lortie et al., 1992; Oremland et al., 1989, 1994; Fujita et al., 1997). The use of selenium anions as alternative terminal electron acceptors is energetically favorable. The Gibbs free energies when coupled with H 2 oxidation are significant, with values of −15.53 kcal/ mol.e for selenite and −8.93 kcal/mol.e for selenate (Newman et al., 1998). Table 3 presents the Gibbs free energies for the chemical reactions of concern for FGD wastewater. In general, microorganisms which derive the most energy from a reaction will grow faster and outcompete microorganism that use lower energy pathways. Therefore, to achieve microbial reduction of selenite or Se (VI), it is necessary to remove oxygen, nitrate, and selenate or Se (VI).
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Table 3 – Free Energies of Various Electron Acceptors Coupled to H 2 Oxidation (Newman et al., 1998) ΔG' (Kcal/mol . e )
Reaction 1/4 O 2 (g) + 1/2 H 2
1/2 H 2 O
1/5 NO 3 - + 1/5 H+ + 1/2 H 2
-23.55
1/10 N 2 (g) + 3/5 H 2 O
-20.66
1/2 SeO 4 2- + 1/2 H+ + 1/2 H 2
1/2 HSeO 3 - + 1/2 H 2 O
-15.53
1/4 HSeO 3 - + 1/4 H+ + 1/2 H 2
1/4 Se + 3/4 H 2 O
-8.93
1/8 SO 4 -2 + 1/8 H+ + 1/2 H 2
1/8 HS- + 1/2 H 2 O
-0.10
Selenite is reduced to elemental selenium via SeRB, or converted to selenium sulfides via the abiotic reaction that is mediated by sulfate-reducing bacteria (SRB). However, simultaneous immobilization of selenite via the two pathways has not been studied (Tang et al., 2015). Factors affecting soluble selenium removal by a Selenate-Reducing Bacterium Bacillus sp. SF- 1 were studied by Kashiwa et al. (2000). The reduction rate of selenate to selenite was reported to be higher than that of selenite to elemental selenium, resulting in the transient accumulation of selenite during selenate reduction. In other words, the rate determining step of the removal or detoxification of soluble selenium in the SF-1 strain is always the reductive transformation of selenite into elemental selenium. Selenite concentration higher than 2 mM (158 mg Se/L of selenite) has been demonstrated to have an inhibitory effect on selenite reduction for this strain. This inhibitory threshold is significantly higher than the amount of selenate in a typical FGD discharge stream. Ike et al. (2000) conducted a research study with selenate-reducing bacteria, using aquatic samples with no significant selenium pollution. The authors concluded that there was a general tendency of the selenite reduction proceeding more rapidly and extensively than the selenate reduction. Strains FR-1, FK-2 and FK-121 which are capable of reductively transforming up to 5 mM selenate into selenite under anaerobic conditions were isolated from the enrichment cultures. During the selenate reduction, selenite accumulated almost stoichiometrically, and a small portion was further reduced into insoluble selenium. Their selenite-reducing activities were not efficient, and higher concentration of selenite (>2 mM) completely or mostly inhibited their selenite-reducing activity. The selenate-reducing activities of these strains were much lower than those of previously reported selenate-reducing bacteria which were isolated from Se-polluted environments. Maires et al. (1998) conducted a study with inocula from eight cultures with a solution containing 100 mg of selenate and 2 g of lactate per liter. Analyses revealed that selenite was present in concentrations ranging from 40 to 80 mg/L, indicating that 20 to 60 mg of selenium per liter had been reduced to insoluble selenium. Similar to other studies, there was a rapid initial reduction of selenate to selenite. When 90 to 95% of the selenate was reduced to selenite, the
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reduction of the selenite and precipitation of the resulting selenium compound began. The extent of this reduction stage was variable, with 20 to 80% of the selenium being removed from solution. Further reduction of the remaining selenite appeared to be inhibited when >50 mg of selenium per liter had been reduced to below a IV oxidation state. This effect was assumed to be the result of end product inhibition or a limiting substrate other than carbon. Dissimilatory reduction of selenate (DSeR) to elemental selenium has been studied in more detail for soil/sediment samples. Biological rates of selenate reduction potential follow Michaelis-Menten kinetics in response to selenate concentration (Steinberg and Oremland, 1990). Measurement of selenate reduction potential, denitrification potential, and apparent K m were determined (K m is a kinetic parameter reflecting enzymatic affinities of resident microorganisms for selenate.).To date, no research has been published on reduction rate, growth rate, yield and other kinetic/stoichiometric parameters for SeRB in the context of wastewater treatment. Selenium Data Collection and Measurement - A Major Challenge The concentration of total selenium is determined using Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), which is capable of detecting selenium at concentrations as low as 0.5 μg/L. Chromatographic methods, as High Performance Liquid Chromatography (HPLC) and Gas Chromatography (GC), have been coupled to allow for the identification and determination of different selenium species at very low levels. The powerful separation technique (HPLC) has been coupled with the most sensitive and robust detector (ICP-MS) and the hyphenated technique (HPLC-ICP-MS) is the preferred method for selenium speciation in many biological samples. A separate selenium speciation analysis may be required for operation of selenium removal systems. Total selenium determination at low detection limits is no longer a problem (Santos et al., 2015). Speciation, however, has two common problems: the low concentration of the species (often below the limits of quantification) and the matrix interferences. Speciation into different inorganic and organic species is needed for some biological samples and can be a complex task (Santos et al., 2015). FGD streams are generally very high in TDS and halide ion concentration, which can interfere with analysis. Low concentration selenium measurements are required, which are often near the detection limit of analytical instrumentation and caution is required to avoid sample contamination from other sources, which can add to analysis error. Few laboratories can accurately perform wet FGD slurry analysis for these metals, though new equipment and methods can be utilized to increase capabilities. Furthermore, samples are often not at thermodynamic equilibrium when withdrawn from the scrubber, allowing for changes in chemistry downstream. Variance in coal and process chemistry over time has also led to significant variances in wet FGD effluent chemistry from day to day within the same unit, which makes the data analysis and prediction difficult (Brown et al., 2015). Although solution pH, density, and flue gas chemistry measurements have traditionally proven sufficient for unit characterization, more detailed information from analysis of grab samples in the bioreactors and equalization tanks are needed to better understand specific systems. Due to the specialized equipment required for such treatment and to the ever-changing variances in the effluent, the
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difficulty in real-time analysis has increased several-fold in recent years. Due to the relatively low number of full-scale FGD WWTPs employing biological treatment systems for selenium removal and the challenges with selenium species analysis described above, there is a very limited database to support mathematical modeling and kinetics studies for biological selenium removal. FULL-SCALE FGD WASTEWATER TREATMENT PLANT USED IN THIS STUDY This coal-fired power generating plant employs a wet FGD scrubber process and the scrubber blowdown (or purge) is directed to the Wastewater Treatment Plant (WWTP). The WWTP consists of multiple units for physical/chemical and biological treatment processes. Figure 3 presents the process flow diagram for this FGD WWTP: chemical addition tanks, sedimentation tank, pH adjustment and filtration, ion exchange, anoxic/anaerobic tanks, intermediate clarifier, SBR tanks, and final filtration. An ion exchange process is designed to remove high boron concentration in the influent FGD wastewater (average: 390 mg/L). The anoxic/anaerobic biological treatment system removes nitrate and selenium. Based on the Gibbs free energies presented in Table 3, when both nitrate and selenium are present in wastewater, nitrate will be preferentially reduced before selenium, and must be removed before effective selenium reduction will occur. Although a soluble COD concentration of 240 mg/L is measured in the anoxic tank influent, a small fraction of COD is readily biodegradable and this FGD wastewater does not have sufficient organic carbon to support denitrification and reduce selenium. Therefore, a carbohydrate-based carbon source, MicroC® 4100, is used as an external source. Urea and phosphoric acid are added to anoxic/ anaerobic and SBRs as nitrogen and phosphorus sources, respectively to support biological growth. Micro-nutrients are also added to promote microbiological growth. Hydraulic Retention Time (HRT) of the anoxic/anaerobic reactors is 24 hours based on influent design flow. The anoxic/anaerobic process is followed by aerobic SBRs to remove residual COD and ammonia before discharge.
Figure 3 – Process Flow Diagram for the FGD Wastewater Treatment Plant
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Influent FGD wastewater, biological process influent, and effluent characteristics are presented in Table 4. Based on the data presented in Table 4, 96 percent NOx (Nitrate + Nitrite) removal efficiency was achieved. The majority of the influent concentration selenium was in the form of Se (IV). Based on total selenium concentration, 85 percent removal efficiency was achieved. Total selenium removal efficiency across the physical/chemical process and biological process was 62 and 63 percent, respectively. Higher soluble TKN concentration in the effluent compared with the influent is due to non-optimized urea addition to the anoxic/anaerobic and SBR systems. Table 4 – Influent, Biological Process Influent, and Effluent Characteristics
Unit
Influent FGD Wastewater
Biological Process Influent
Effluent
Soluble TKN
mgN/L
18 ± 5
12 ± 1.5
43 ± 3.5
Ammonia
mgN/L
2 ± 0.8
1.3 ± 0.4
5.3 ± 2.2
mg/L
241 ± 86
240 ± 38
240 ± 40
Nitrate
mgN/L
48 ± 16
28 ± 15
0.3 ± 0.25
Nitrite
mgN/L
2.7 ± 2.1
10.5 ± 3.8
1.6 ± 2.9
Selenium (IV)
µg/L
407 ± 84
120 ± 74
17 ± 10
Selenium (VI)
µg/L
31 ± 13
28 ± 5
10 ± 7.8
Soluble Selenium
µg/L
550 ± 125
234 ± 87
100 ± 63
Total Selenium
µg/L
623 ± 141
239 ± 69
88 ± 61
mgP/L
---
0.2 ± 0.3
0.7 ± 0.4
Parameter
Soluble COD
Ortho-Phosphates
In this work, a mechanistic process model for biological behavior of selenate and selenite reduction is proposed and data from this full-scale FGD WWTP is used to understand the kinetic parameters required for process modeling. BIOLOGICAL PROCESS MODELING Activated sludge mechanistic models developed for municipal wastewater treatment applications can also be applied to industrial wastewater treatment systems with appropriate wastewater characterization if oxygen and nitrate, and nitrite are the only electron acceptors in the system. For FGD wastewater, the stoichiometry and kinetics may differ from municipal treatment systems because of the biological selenium removal processes. In this case, it is beneficial to develop an industry-specific model for biological selenium removal which can be applied to FGD wastewaters as well as mining wastewaters containing high concentrations of selenium. Currently, a mechanistic model for biological selenium removal is not available. In this work, a mathematical model is proposed, added to the BioWin process simulator Activated SludgeDigestion Model (ASDM) (Jones and Takacs, 2004) and calibrated based on the operational data
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from the full-scale FGD WWTP described above. For this and similar treatment processes, a kinetic model for biological selenium removal will be a valuable aid for design, operation and control. Furthermore, the model will be a valuable research tool to improve understanding of the underlying fundamental processes and their interactions. Model Development BioWin’s “model builder” functionality was used as a framework to develop the model in this work. The main objective of this study is to propose a new mathematical model including four new state variables (selenite, selenate, selenium and SeRB) and five biological processes which are added to the current BioWin ASDM matrix (Jones and Takacs, 2004) and calibrated based on a concurrent biological nitrate and selenium removal in a single basin for a FGD wastewater treatment facility. This mathematical model addresses the reductive competition between denitrifiers (nitrate and nitrite reduction) and SeRB, and mechanistically simulates the two stage reductive pathways of selenate to elemental selenium. Model Processes and Components The proposed model is based on the following principles and processes, which are also summarized in pathways shown in Figure 4. The stoichiometric matrix is not included in this manuscript. Model components are described in Table 5. 1- Selenate-Se and selenite-Se are state variables 1 and 2 in this proposed model 2- The end product of these processes is selenium (the third state variable). 3- Two different populations of microorganisms are capable of reducing selenate and selenite at different rates and kinetics. The first population is a fraction of Ordinary Heterotrophic Organisms (OHOs), f[OHOs], which is also capable of performing denitrification. The second population of microorganisms is a specialist group of bacteria called Selenium Reducing Bacteria (denoted as SeRB, the fourth state variable). SeRB is only capable of reducing selenate and selenite to selenium at different reaction rates. 4- This model assumes that when a non-acclimatized sludge is introduced to a wastewater containing selenium, a fraction of OHOs are able to utilize selenate and selenite as terminal electron acceptors. 5- As shown in Figure 4, a portion of f[OHOs] use electron donor for growth process (Y) and a portion of f[OHOs] are shifted to SeRBs. Therefore, there is a microbial shift in OHOs to reduce selenite. 6- Each process contains two-step reactions for biological selenium reduction of selenate to selenite and selenite to Se. 7- Reduction of selenite to elemental selenium is considered the rate limiting step.
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8- Since the level of selenite concentrations never reaches the inhibitory level of 158 mg Se/L, the inhibitory impact of selenite was not included in the process equations. This inhibitory level is based on the research work conducted by Kashiwa et al. (2000).
Figure 4 – Proposed Biological Processes for Selenium Removal
Table 5 - Components for Biological Selenium Removal Process Model Symbol
Definition
Units
SeO 4
Selenate concentration
mg Se/L
SeO3
Selenite concentration
mg Se/L
Se
Selenium concentration
mg Se/L
K SeO4
Half saturation coefficient for selenate
mg Se/L
K SeO3
Half saturation coefficient for selenite
mg Se/L
Z OHO
Active Ordinary Heterotrophic Organisms (OHO) biomass
mg COD/L
Z SeRB
Active SeRB biomass
mg COD/L
µ max SeO4
Max specific SeRB growth rate on selenate
d-1
µ max SeO3
Max specific SeRB growth rate on selenite
d-1
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Decay rate for SeRB
d-1
f
Fraction of active OHO capable of biological Se reduction and denitrification
----
Y
Yield of Heterotrophs for Se removal
g cell COD per g COD utilized
Yield of SeRB for Se removal
g cell COD per g COD utilized
d SeRB
Y SeRB
Model Calibration Data from the full-scale WWTP discussed above were used for model calibration and verification. Figure 5 presents the schematic of the WWTP for modeling and simulation. The unit process sizes and supplemental carbon, phosphorus, and nitrogen addition rates match the actual WWTP conditions. Dynamic simulation was conducted at a temperature of 25°C. Determination of model components such as yield and maximum growth rate parameters are described in this section.
Figure 5 – Schematic of the Full-Scale WWTP for Modeling Growth Stoichiometry and Kinetic Parameters In the absence of literature data for yield and growth rate parameters for biological selenium removal, biomass yield and maximum specific growth rate parameters are calculated from thermodynamics of biological growth (Heijnen and van Dijken, 1992) based on catabolic and anabolic stoichiometric reactions. The half reaction equations for biological selenium reduction are as follows: SeO 4 -2 + H 2 + H+ (1)
HSeO 3 - + H 2 O
ΔG' = - 15.53 kcal/mole.e
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Equation
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HSeO 4 - + 2 H 2 + H+ (2)
Se ↓+ 3 H 2 O
ΔG' = - 8.93 kcal/mole.e
Equation
The overall reaction stoichiometry for glucose for selenium reduction is presented in Equation 3: C 6 H 12 O 6 + 2 SeO 4 -2 (3)
2 CH 3 COO- + 2 HCO 3 - + SeO 3 -2 + Se + H+ + 6 H 2 O
Equation
ΔG'= - 758 kcal/mole Glucose (Blum et al., 2001) For this study, MicroC® 4100, a proprietary carbohydrate based carbon source, is used. Therefore, stoichiometry and kinetic rates for glucose is used for model calibration. In addition, model parameters are calculated for other electron donors typically used for biological selenium reduction including glycerol, acetic acid, and lactic acid. Table 6 presents the Gibbs free energies for the above mentioned electron donors used for model parameters calculations for biological selenium reduction. Table 6 - Gibbs Free Energies for Electron Donors for Biological Selenium Reduction ΔG' (Kcal/mol)
Reference
Glucose
-758
Blum et al., 2001
Glycerol
-533
Blum et al., 2001
Acetic Acid
-1,104
Nancharaiah and Lens, 2015
Lactic Acid
-507
Nancharaiah and Lens, 2015
Electron donor
Yield and maximum specific growth rates are calculated based on thermodynamics of biological growth (Heijnen and van Dijken, 1992) based catabolic and anabolic stoichiometric reactions. The following assumptions are made: standard temperature of 25 °C; ammonia used as nitrogen source for biomass growth; typical biomass formula of C 1 H 1.8 O 0.5 N 0.2 . True cellular yield and maximum specific growth rates are presented in Table 7. Calculated parameters for glucose were used for model calibration. Based on the calculated parameters in Table 7, acetate has the highest specific growth rate, with similar values observed for glycerol, glucose, and lactate. Comparing the yield values, glucose has the highest calculated value with somewhat similar values for glycerol, acetate, and lactate. Detailed comparison of electron donors is outside the scope of this paper. Table 7 – Calculated Growth Parameters for Typical Electron Donors Y SeRB (mg COD/mg COD)
µ max (h-1)
Glucose
0.560
0.382
Glycerol
0.296
0.332
Electron Donor
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Acetate
0.301
0.980
Lactate
0.272
0.370
The amount of COD for selenium removal is calculated based on stoichiometry of reaction 4 for a one-step reduction process, described in detail in Appendix A, as presented in Table 8. The amount of COD utilized for the consumption of one unit of selenate is calculated at 0.608/ (1 – Y SeRB ). 1 6
4 3
𝑒𝑒 ′ + 𝑆𝑆𝑆𝑆𝑂𝑂42− + 𝐻𝐻 + =
1 𝑆𝑆𝑆𝑆 6
2 3
Equation (4)
+ 𝐻𝐻2 𝑂𝑂
Table 8 – Half Reactions for Oxygen and Selenium
Redox Couple
Reaction 1/2 H 2 O = 1/4 O 2(g) + H+ + e-
O (-2)/O(0)
e- + 1/6 SeO 4 -2 + 4/3 H+ = 1/6 Se + 2/3 H 2 O
Se (0)/Se (VI)
For the inhibition coefficient for NOx concentration (K d,NOx ), the average NOx concentration in the effluent from the full- scale FGD WWTP was used. Model Validation and Results The model described above was implemented in BioWin® 5.0 (EnviroSim Associates, Ltd, Hamilton, Ontario, Canada). Comparison of predicted and measured results for 20 days for the full-scale FGD WWTP was conducted. Table 9 presents the comparison of observed values and model predictions for NO x . Figure 6 presents the simulated nitrate and nitrite results. Table 9 – Comparison of Observed Values and Model Predictions for NOx Parameter Flow
Min 25% percentile Average 75% percentile Max
(GPD) 281,000 370,000 418,684 486,000 499,000
Observed Conc. Modeled Conc. (composite) NO 3 -N NO 2 -N NO 3 -N NO 2 -N (mg N/L) 0 0
