Wastewater treatment is one of the most important and challenging environmental problems associated with coal-based power generation.
Using wet scrubbers to clean flue gas is becoming more popular worldwide in the electrical power industry. In the coming years, hundreds of wet scrubbers will be installed in the US alone. While wet scrubbers can greatly reduce air pollution, toxic metals in the resulting wastewater present a major environmental problem. The industry prepares to invest billions of dollars in the next decade to meet more the ever more stringent environmental regulations; unfortunately, a cost-effective and reliable technology capable of treating such complicated wastewater is still not available.
The compositions of FGD wastewaters vary greatly, depending not only on the types of coal and limestone used but also on the types of scrubber and processes used. Pretreatment method and management practices also affect wastewater characteristics. According to a recent survey by EPR1 (2006), untreated raw FGD wastewater could have TSS in ˜10,000 mg/L but after settlement, it falls to ˜10 mg/L; the pH typically falls in 5.8-7.3; sulfate is in the range of 1,000-6,000 mg/L; nitrate-N at level of 50 mg/L is not uncommon; chloride, alkalinity and acidity vary from hundreds to thousands ppm; selenium exists in various forms, ranging from dozens of ppb to over 5 ppm, among which, selenate could account for about half of total Se; arsenic ranges from a few ppb to hundreds of ppb; mercury ranges from below 1 ppb to dozens of ppb; and boron can be as high as hundreds of ppm.
Treatment of selanate-Se in wastewater is often considered to be one of the most difficult in toxic metal treatments. Selenium is a naturally occurring chemical element in rocks, soils and natural waters. Although Se is an essential micronutrient for plants and animals, it can be toxic at elevated levels and some of Se species may be carcinogenic. The hexavalent selenium is stable in oxic environments and exists as the selenate (SeO42−) anion, which is weakly sorbed by mineral materials and generally soluble. Tetravalent Se is the stable valence state under mildly reducing or anoxic condition (0.26 V<Eh<0.55 V at pH 7). It exists as the selenite (SeO32−) anion, which tends to be bound onto mineral surfaces (e.g., Fe and Mn oxides). Selenate and selenite are more toxic due to their high bioavailability than elemental selenium or metallic selenides.
A biological treatment system, ABMet, has been patented and is being marketed by GE Water.
However, there remains a need for a cost-effective and reliable treatment process for removing toxic pollutants from the wastewater generated by the wet scrubbers operated for flue gas desulfurization in coal-fired power plants.
The present inventor has developed a chemical treatment process that can cost-effectively treat all major pollutants in the flue gas desulfurization (FGD) wastewater in a single process.
The present inventor developed a fluidized reacting system using a hybrid reactive solid/secondary reagent reactor that can cost-effectively remove many toxic metals from wastewater. The system and process are effective to treat an aqueous suspension. The system uses a reactive solid and a secondary reagent as reactive agents to rapidly reduce selenate to become insoluble selenium species, which are then adsorbed or precipitated along with various of other toxic metals (such as As and Hg, if present) in wastewater onto the iron oxide sludge. The system is particularly effective for removing selenate-Se.
The present process is effective for removing almost all concern toxic metals in an aqueous suspension; in addition, it can remove oxyanion pollutants and metalloids. More particularly, contaminants removable by the present system and process are: most toxic metals such as arsenic, mercury, selenium, cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum, and the like; metalloid pollutants such as boron and the like; many oxyanion pollutants, such as nitrate, bromate, iodate, and periodate, and the like; and the like.
The present system and process use common, non-toxic, and inexpensive chemicals. The present chemical treatment system costs much less to construct and operate than biological treatment systems, which tend to be more complex.
The present system and process are versatile and flexible. The present system and process are more robust and manageable than a biological process when exposed to toxic chemicals or any disturbances and changes in wastewater quality and quantity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustrating a single-stage fluidized bed reactor;
FIG. 2 is a flow chart illustrating a three-stage reaction system;
FIG. 3 is a schematic illustrating a single-stage fluidized bed ZVI/FeOx/Fe(II);
FIGS. 4A, 4B are pictures illustrating a bench scale single-stage reactor;
FIGS. 5A, 5B are pictures illustrating an alternative bench scale single-stage reactor;
FIGS. 6A. 6B, 6C are pictures illustrating a bench scale three-stage ZVI/FeOx/Fe(II) fluidized-bed reactor system; and
FIG. 7 shows three panels of pictures illustrating settling of a mixture of Fe0 and magnetite powder rich of surface bound Fe(II); pictures taken after settling for 1 min (left panel), 3 min (middle), and 6 min (right).
The present inventors have discovered a novel system for treating wastewater. Experiments have demonstrated the system operable for removal of selenium present as selenate.
According to some embodiments, a reactor system includes zero valent iron. According to some embodiments, ferrous iron is added to a reactor system. The present inventor believes that ferrous iron acts as a passivation reversal agent for zero valent iron. The mechanism is complex. While not wishing to be limited by theory, the present inventor believes that passivation is partially caused by corrosion of iron in a water environment. The present inventor believes that ferrous iron acts to cause conversion of iron corrosion product on the surface of the zero valent iron to magnetite. According to some embodiments, a sufficient amount of magnetite is produced so as to optimize removal of toxic materials by a reaction system including zero valent iron. According to some embodiments, the process produces removable solids. According to some embodiments, the removable solids contain toxic material encapsulated in magnetite. According to some embodiments, the encapsulated toxic material is solid.
Thus, according to some embodiments, the process uses a highly reactive mixture of zerovalent iron (Fe0), iron oxide minerals (FeOx), and ferrous iron (FeII) to react with, absorb, and precipitate various toxic metals and metalloids from wastewater, forming chemically inert and well crystallized magnetite (Fe3O4) particles that can be separated from water and disposed with encapsulated pollutants.
According to some embodiments, the reactive zone is maintained near neutral pH.
The present inventor believes that boron in the wastewater further contributes to passivation and that ferrous iron removes boron form the zero valent iron.
It will be understood that wastewater is illustrative of an aqueous suspension. For example, the present inventor contemplates treating oil refinery waste. Further, the present inventor contemplates treating wetlands.
It will be understood that selenium is illustrative of a toxic material. Other common toxic materials are contemplated. For example, the present inventor contemplates removing arsenic, mercury, cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum, and the like; metalloid pollutants such as boron and the like; many oxyanion pollutants, such as nitrate, bromate, iodate, and periodate, and the like; and the like.
It will be understood that ferrous iron is illustrative of a secondary reagent. The secondary reagent is desirable adapted to act as a passivation reversal agent. Passivation is generally the process of rendering an active material, for example zero valent zinc, inactive. Aluminum ion, Al3+, may substitute for (e.g. added as aluminum sulfate) for ferrous iron. It will be understood that iron is illustrative of a reactive solid. The present inventor believes that iron is particularly practical. However, the present inventor contemplates other treatment materials. For example, according to some embodiments, the treatment material is zinc. It will be understood that a reactive system may include the treatment material in zero valent form. According to some embodiments, the reactive system further includes a passivation reversal agent suitable for the zero valent form as may be advantageous.
According to some embodiments, a reactor includes an internal settling zone in communication with a reactive zone. The reactor is illustrated in schematic in FIG. 1. According to some embodiments, the internal settling zone uses gravitational forces to separate solids from liquids. According to some embodiments, mostly liquids remain in the settling zone. According to some embodiments, the internal settling zone is towards the top of the reactor (FIG. 1). According to some embodiments, communication with the reactive zone is via an inlet at the bottom of the internal settling zone. According to some embodiments, effluent is removed from the top region of the internal settling zone. According to some embodiments, the effluent is very clear. Magnetite is known to be black. Settling observed in an experiment over time is illustrated in FIG. 3 of the document “pilot test scale plan” appended hereto, which shows clearer separation of black material and clear fluid over time. The present inventor believes that settling for a separating method is particularly efficient. However, other suitable separating methods are contemplated.
According to some embodiments, a reactive zone includes a central conduit. The central conduit improves mixing. For example, according to some embodiments, the central conduit promotes convective motion.
Thus, according to some embodiments, the reactor system operates as a fluidized bed that employs a motorized stirrer in conjunction with a central flow conduit to create a circular flow within the reactor and provide an adequate mixing between reactive solids and wastewater. An internal settling zone was created to allow solid-liquid separation and return of the solid into the fluidized zone.
FIG. 1 is a schematic illustrating an embodiment of the system and process. A single-stage fluidized-bed system includes a fluidized reactive zone, an internal solid/liquid separating zone, an aerating basin, a final settling basin, and an optional sand filtration bed.
Still referring to FIG. 1, the fluidized zone is the main reactive space where reactive solid, in the form of particles, is completely mixed with wastewater and secondary reagent and where various physical-chemical processes responsible for toxic metal removal occur.
Still referring to FIG. 1, the internal settling zone is to allow particles to separate from water and be retained in the fluidized zone. For high density particles, an internal settling zone with a short hydraulic retention time is sufficient for complete solid/liquid separation. This eliminates the need of a large external clarifier and a sludge recycling system.
Still referring to FIG. 1, the aeration basin has two purposes: (1) to eliminate residual secondary reagent in the effluent from fluidized zone; and (2) to increase dissolved oxygen level. For a single-stage reactor, effluent from fluidized reactive zone will always contain certain amount of secondary reagent. Oxidation of secondary reagent will consume alkalinity and therefore will lower the pH. To accelerate oxidation of secondary reagent, the aeration basin should maintain a pH of above 7.0. Chemicals such as Ca(OH)2, NaOH and Na2CO3 could be used for pH control.
Still referring to FIG. 1, the final settling tank is to remove flocculent formed in the aeration basin. The floc (fluffy) settled to the bottom can be returned to the fluidized zone and transformed by secondary reagent into dense particulate matter.
Still referring to FIG. 1, upon final settling, a sand filtration bed may be used to further polish the treated water before discharge.
Still referring to FIG. 1, the post-FBR (fluidized bed reactor) stages (aeration-settling-filtration) may not be needed under certain operation conditions.
Referring now to FIG. 2, several fluidized-bed reactors can be combined to form a multi-stage treatment system. It is recommended that each stage maintain its own reactive solid. That is, the solids are separated in each stage. In order to achieve a separate solid system, each stage may have its own internal solid-liquid separation structure.
Still referring to FIG. 2, depending on operating conditions in the FBRs, the wastewater characteristics, and discharge standards, the post FBR treatments (aeration+final clarifier+sand filtration) may not be needed.
Although a multi-stage system is more complex and may result in a higher initial construction cost, a multi-stage fluidized-bed reactor system has several major advantages.
A multi-stage system can achieve higher removal efficiency than a single-stage system under comparable conditions. Further, the FGD wastewater may contain certain chemicals (i.e., phosphate) that may be detrimental to the high reactivity of the reactive solids. A multi-stage system can intercept and transform these harmful chemicals in the first stage and thus reducing the exposure of the subsequent stages to the negative impact of these chemicals. As such, a multi-stage configuration is more stable and robust.
A multi-stage configuration facilitates the control of nitrate reduction, for example in an iron-based system. In a single stage system, because the presence of dissolved oxygen carried in raw wastewater, it tends to be difficult to operate the system in a rigorous anaerobic environment. In a multi-stage system, stage 1 can remove virtually all dissolved oxygen; as a result, the subsequent stages can be operated under rigorous anaerobic environment.
A multi-stage system allows flexible control of different chemical conditions in each individual reacting basin. The chemical conditions in each reactive basin can be controlled by adjusting the pumping rate of supplemental chemicals and turning aeration on or off. A multi-stage system can be operated in a mode of multiple feeding points. Each stage may be operated under different pH and dissolved oxygen condition.
A multi-stage system will lower chemical consumption. In a single-stage complete-mixed system, secondary reagent in the reactor are desirably maintained at a relatively high concentration in order to maintain high reactivity of reactive solids. As a result, the residual secondary reagent in the effluent will be high. This means that more secondary reagent will be wasted and more NaOH (or lime) consumption will be required just to neutralize and precipitate the residual secondary reagent in the effluent. As a result, more solid sludge will be produced and waste disposal cost will increase. In a multi-stage system, residual secondary reagent from stage 1 can still be used in stage 2. In this case, secondary reagent can be added in a way that conforms to its actual consumption rate in each stage. As a result, it is possible to control residual secondary reagent in the effluent in the final stage to be much lower than the one in a single stage system.
Referring to FIG. 3, according to some embodiments, in the system and process illustrated by FIG. 1, the reactive solid includes zero valent iron (ZVI) and iron oxide mineral (FeOx), and the secondary reagent is Fe2+. Thus, referring to FIG. 3, a single-stage fluidized-bed ZVI/FeOx/Fe(II) system includes a fluidized reactive zone, an internal solid/liquid separating zone, an aerating basin, a final settling basin, and an optional sand filtration bed.
Still referring to FIG. 3, the fluidized zone is the main reactive space where ZVI and FeOx reactive solids are completely mixed with wastewater and dissolved Fe2+ and where various physical-chemical processes responsible for toxic metal removal occur.
Still referring to FIG. 3, the internal settling zone is to allow ZVI and FeOx to separate from water and be retained in the fluidized zone. Because of high density of fully or partially crystallized FeOx particles, an internal settling zone with a short hydraulic retention time would be suffice for complete solid/liquid separation. This eliminates the need of a large external clarifier and a sludge recycling system.
Still referring to FIG. 3, the aeration basin has two purposes: (1) to eliminate residual dissolved Fe2+ in the effluent from fluidized zone; and (2) to increase dissolved oxygen level. For a single-stage reactor, effluent from fluidized reactive zone will always contain certain amount of dissolved Fe2+. Oxidation of Fe2+ will consume alkalinity and therefore will lower the pH. To accelerate oxidation of dissolved Fe2+, the aeration basin should maintain a pH of above 7.0. Chemicals such as Ca(OH)2, NaOH and Na2CO3 could be used for pH control.
Still referring to FIG. 3, the final settling tank is to remove iron oxide flocculent formed in the aeration basin. The ferric oxide floc (fluffy) settled to the bottom can be returned to the fluidized zone and transformed by Fe2+ into dense particulate matter.
Still referring to FIG. 3, upon final settling, a sand filtration bed may be used to further polish the treated water before discharge.
Still referring to FIG. 3, the reactive solid may initially be zero valent iron, with the iron oxide mineral formed in situ. The iron oxide mineral may coat the zero valent iron.
Still referring to FIG. 3, the system can be operated under various controlled conditions as needed.
According to some embodiments, an iron-based technique employs a mixture of zerovalent iron (ZVI or Fe0) and iron oxide minerals (FeOx), and Fe(II) species to react with, adsorb, precipitate, and remove various toxic metals, metalloids and other pollutants from the contaminated wastewater. According to some embodiments, an iron-based physical-chemical treatment process that employs a hybrid Zerovalent Iron/FeOx/Fe(II) Reactor to treat toxic metal-contaminated wastewater. For example, according to some embodiments, the present system and process involve a hybrid Zerovalent Iron/FeOx/Fe(II) reactor for removing toxic metals in wastewater. According to some embodiments, the process employs a fluidized bed system and use a reactive mixture of Fe0, Fe(II) and FeOx to absorb, precipitate, and react with various toxic metals, metalloids and other pollutants for wastewater decontamination. According to some embodiments, toxic metals are encapsulated within iron oxide crystalline (mainly magnetite powder) that are chemically inert and physically dense for easier solid-liquid separation and final disposal.
While not wishing to be limited by theory, the present inventor believes that the following are contributing mechanisms for the present iron based system and process: a) using the reducing power of Fe0 and Fe(II) to reduce various contaminants in oxidized forms to become insoluble or non-toxic species; b) using high adsorption capacity of iron oxide surface for metals to remove various dissolved toxic metal species from wastewater; and c) promoting mineralization of iron oxides and growth of certain iron oxide crystalline so that surface-adsorbed or precipitated toxic metals and other pollutants could be incorporated into iron oxide crystalline structure and remain encapsulated in a stabilized form for final disposal.
Experimental Results of Using a Hybrid ZVI/FeOx/Fe(II) Reactive System to Treat FGD Wastewater
The present system and process are a result of laboratory research conducted by the present inventor to develop a cost-effective method for removing toxic metals in the flue gas desulfurization wastewater generated from wet scrubbers of coal-fired steam electric power plants. Although developed specifically for treating the FGD wastewater with selenium as the main target contaminant, this chemical reactive system is suitable for general application of removing a wide spectrum of toxic metals in industrial wastewater, tail water of mining operations, and contaminated groundwater.
According to various experimental embodiments, as shown herein, a single stage may achieve 90% selenate removal within 4 hr reaction time. A three-stage system, in comparison, may achieve a 96% removal rate.
The present inventor believes that some exemplary novel aspects are:
1) Discovery of the role of externally-added Fe2+ in sustaining the reactivity of Fe0 with respect to selenate reduction. Externally-added Fe2+ may convert less reactive ferric oxide coating on Fe0 particles into a highly reactive mix-valent Fe3O4 oxide coating and therefore rejuvenate the passivated Fe0 surface.
2) Discovery that surface-bound Fe(II) on magnetite (Fe3O4) particles can rapidly reduce selenate to insoluble elemental Se and be removed from the liquid phase.
3) Discovery that the chemical conditions that promote the formation of magnetite (Fe3O4) as a reaction product from the oxidations of Fe0 and surface-bound Fe (coupled with reductions of dissolved oxygen, nitrate, and selenate in the water).
4) Development of a fluidized bed system with an internal settling zone and a central conduit that can (a) retain high concentration of Fe3O4 solid particles and therefore offer abundant reactive surface area that can host surface bound Fe(II)-selenate redox reaction; (b) offer an effective mixing condition so that Fe0, Fe3O4 and s.b.Fe(II) can achieve their respective roles in removing toxic metals; (c) avoid excess diffusion of oxygen from air into the reactive system so that less Fe0 and Fe(II) are wasted.
5) Development of a multiple-stage fluidized bed system that will (a) achieve better toxic metal removal efficiency; (b) control nitrate reduction efficiency to a level of desire; (c) reduce consumption of ferrous salt and Fe0; (d) reduce or completely eliminate residual dissolved Fe2+.
Bench Scale Tests
Single Stage Reactor
Three Bench-Scale Fluidized-Bed Reactors were Fabricated and Operated.
Referring to FIGS. 4A and 4B, Reactor#1 has an internal settling zone (the compartment on the left side) in which it allows reactive solid to separate from the water and be retained within the fluidized zone. Reactor#2 (not shown) is identical to Reactor#1. Reactor#1 and #2 both had an operating capacity of 7.2 L and had an internal settling zone (0.5 L) within the reactors (FIGS. 4A and 4B).
Referring to FIGS. 5A and 5B Reactor#3 is an integral system that has an internal settling zone (far left), an aeration basin (near left), and a second settling basin (right) within the reactor. Reactor#3 had an operating capacity of 10 L. It had a built-in aeration basin (0.6 L) and a built-in final settling basin (FIGS. 5A and 5B). Peristaltic pumps (Masterflex pumps, Cole-Parmer, Illinois) were used to pump in wastewater and the needed chemical reagents. A small aquarium air pump (purchased from Wal-Mart) as used to provide aeration. A motorized stirrer (max. 27 watt, adjustable rpm 100-2000, three-blade propeller stirrer) was used to provided mixing condition.
Zerovalent iron powder used in the tests was obtained from Hepure Technology Inc., including I-1200+ and HCl5 (see Batch Test results for more details). Other reagents used in the operation include HCl, FeCl2, and NaOH.
Contrary to what many experts in ZVI technology believed, fresh ZVI does not tend to be effective for chemical reduction of selenate. Batch test results (Appendix B and Appendix C) confirmed that ZVI grains coated with magnetite could achieve a much higher reaction rate than ZVI grains of a relative fresh surface with little or very thin iron rusts. To improve performance of a ZVI system, a unique start-up process is employed to coat the ZVI powder surface with a more reactive and passivation-resistant, chemically-stable magnetite coating. When a reactor was started with using fresh ZVI powder, it took some time under carefully controlled chemical environment to coat ZVI with a magnetite layer.
Several factors are desirably considered in order to have a rapid and successful start-up for a treatment system. First, the physical chemical properties of iron, most important the size distribution of iron particles, are considered. Both reductions of selenate by ZVI and by surface bound Fe(II) (s.b.Fe(II)) on magnetite are surface-mediated heterogeneous reaction; therefore, increasing solid-liquid interfacial area would increase overall reaction rate. Fine ZVI powders could provide larger surface area and therefore achieve higher selenate reduction under comparable conditions. This was confirmed in batch tests. The continuous flow reactor tests were successfully started up five times. It appears that finer iron particles (dominant size: <45 μm in diameter) may be started up faster than larger particles (dominant size: 45-150 μm in diameter). The chemical purity of ZVI powder was found to not a major factor. In batch and continuous-flow tests, various purities and composition of ZVI powder were used. No major differences were observed among the different iron sources with respect to reaction mechanism and rate for selenate reduction. Overtime, the zerovalent iron grains may all be coated with a magnetite coating and in the present of dissolved Fe2+, they all achieve high reactivity for selenate reduction.
Generation of a magnetite coating on a ZVI particle is helpful to the success of the system. Appropriate aqueous chemical conditions must be maintained for the purpose. Iron corrosion could produce various iron oxides under different chemical conditions. Our batch and continuous flow reactor tests show that in order to generate magnetite from iron corrosion reaction, three conditions must be met: a pH of 6.5 to 7.5; adequate dissolved Fe2+ that can form s.b.Fe(II); and appropriate species and concentration of oxidants. Oxidants can be certain oxyanions such as selenate, nitrate, nitrite, iodate (IO3−) and periodate (IO4−) in the wastewater. Oxidation of ZVI by these oxidants tends to form ferric oxides (most likely lepidocrocite, γ-FeOOH). The small quantity of ferric oxides can be transformed to magnetite in the presence of surface-adsorbed Fe(II). Dissolved oxygen can also serve as an oxidant to generate magnetite (Huang et al. 2006). Low-intensity aeration in the early stage could accelerate the magnetite-coating process. High-intensity aeration should be avoided because it could form large quantity of ferric oxides even in the presence of dissolved Fe2+ and moreover, it will waste ZVI. Our experiences from live successful start-ups using simulated FGD wastewater indicates that in general the system will take about one to two weeks for the fresh ZVI to mature; over time, the system will gradually improve before reaching a state of high performance.
As an alternative (and recommended) start-up procedure, we used nitrate solution (add 30 mg/L nitrate-N in tap water, operating HRT=12 hr) instead of simulated FGD wastewater to feed the system. Nitrate would be completely reduced and in the presence of adequate dissolved Fe2+, a high quality (better crystallized and less amorphous, containing less ferric oxides or ferrous hydroxides) magnetite coating can be formed on ZVI particles. Start-up with nitrate solution would take only two days.
A general start up procedure and exemplary controlled parameters are:
1) Select ZVI sources. Finer iron powder (<50 μm) is preferred. Low iron purity and rusty surface in general are not a problem.
2) Add 80-100 g/L ZVI powder in the fluidized zone. Turn on mixing equipment.
3) Start-up with FGD wastewater