Phosphorus Recovery and Nutrient Management with Magnesium Hydroxide: Struvite Recovery

Transcription

1 Phosphorus Recovery and Nutrient Management with Magnesium Hydroxide: Struvite Recovery Alan Bowers Nathaniel Barnes Dept. of Civil & Environmental Engrg. Vanderbilt University Nashville, TN Matthew Madolora Andrew Rupprecht Premier Magnesia,LLC Wayne, PA

2 Takeaways-Summary Struvite can be a profitable and recoverable resource in wastewater processes, instead of a maintenance expense. The issues with Struvite modeling can be handled statistically (Monte Carlo). Current Struvite modeling has a lot of uncertainty. Introducing a new index; SPI (similar to the Langelier Index for calcium carbonate). An SPI > 0 implies precipitation may occur, SPI < 0 implies precipitation is unlikely. Solution ph can be THE control in struvite formation. Magnesium hydroxide (Mg(OH)2) maintains a more even reactor ph (8.5 s.u.), conversely caustic (NaOH), may create regions of high ph (14 s.u.) and struvite scale. Struvite Scale can be managed where Mg(OH)2 is used for Alkalinity/pH control by adjusting ph and using scale mitigation strategies, such as the Thioguard Ωmega-S program investigated here. Struvite Recovery can be maximized by driving the reaction with elevated Mg and/or increased ph.

3 Magnesium Hydroxide, is useful for a variety of improvements in Wastewater Processes (increased ph and addition of Alkalinity): (Alkalinity, ph ) controlled 60% (Mg(OH) 2 liquid = 13 lbs Alkalinity/gallon) Mg OH - I. Collection System: 1. Odor Control (ph elevated > 7.0 reduces gas phase H 2 S) 2. Corrosion Control (Infrastructure Protection at elevated ph and Alkalinity) 3. Reduced Fats Oils and Grease (FOG ph or greater) mitigates obstructions in sewer lines II. In Plant Processes: 1. Reduced fats oils and grease (FOG ph or greater) mitigates plant flow obstructions 2. Improved ph and Alkalinity raises bioactivity and permits nitrification (7.14 mg Alkalinity consumed per mg N oxidized) (1 MGD 4.5 gal 60% (Mg(OH) 2 liquid consumed per mg N oxidized) 3. Addition to Digesters improves methane production and volatiles destruction, and improves Ca:Mg ratio (Mg ) for beneficial sludge reuse 4. Mg increases production of struvite reclaimed as a high-value fertilizer where recovery is practiced

4 What is Struvite? Struvite, MgNH 4 PO 4 (s), is a solid that precipitates in Wastewater Processes, where high concentrations of the individual components, i.e., Mg +2, NH 4+, and PO -3 4, are present at neutral or higher ph. Struvite may occur as a recoverable resource, or as a nuisance scale on pipes and equipment that requires frequent cleaning and maintenance. Struvite is mainly controlled by the concentrations of Magnesium, Ammonia, Phosphate and Solution ph. Increasing ph PO nh + H n PO 4 n-3 (n = 1,2,3) Increasing ph MgOH + OH - + Mg +2 Struvite Precipitate MgNH 4 PO 4 (s) Increasing ph NH 4 + NH 3 + H + At ph Values < 9.0, ph Is The Most Critical Factor, Small Increases in ph Result In An Exponential Increase In Struvite Precipitation (ph Struvite ). Struvite Scale example under strongly precipitating conditions. Struvite identified by x-ray Diffraction (100% Struvite)

5 Typical Processes where Struvite may occur include: Sludge Digesters (anaerobic/aerobic) where high nutrient concentrations are released Dewatering Equipment for Anaerobic Sludge (centrifuges, belt presses, etc) Treatment Processes for water removed by Dewatering Processes, such as Centrate Nitrification Processes Localized Regions of High ph (feed tubes, mixer blades, etc.) Processes specifically designed to recover Struvite as a valuable fertilizer, e.g., AirPrex, Ostara, STRUVIA, etc.

6 Projections of commercially viable phosphate resources as an agricultural fertilizer suggest that these resources will be depleted and that agriculture will not be sustainable at the current scale: Estimated Commercially Viable Reserves: 50 to 100 years (current known reserves) 100s of years (if new reserves found) US Production Peaked in 1980: 54.4 Mt World Production: 250 Mt (2016) Top World Reserves: Morocco (70%), China (5%), Algeria (3%), Syria (3%), US (2%) Recovery and Reuse Appear Inevitable Struvite Recovery From Wastewater Processes

7 A Little Chemistry Struvite, MgNH 4 PO 4, forms in aqueous solution based on the concentrations of the chemical species involved, or, based on the solubility product: [Mg +2 ][NH 4+ ][PO 4-3 ] = K so (1) Where, raw solubility product values are known: log K so = to (literature range) and, the solubility product is highly ph dependent since all of the species involved are weak acids or bases, i.e., a ph-conditional solubility product, as shown in Figure 1. - ph-conditional Solubility Product Typical Wastewater Processes ph Decreasing Struvite Solubility Min. Solubility ph 10 to 10.5 Figure 1. Conditional Solubility Product, K so(ph-conditional), as a function of ph (after Ali, Schneider, and Hudson, 2005, Thermodynamics and Solution Chemistry of Struvite, J. Indian Inst. Sci., 85, ).

8 We can define the precipitation saturation ratio for struvite, Ω, as: Ω = [Mg +2 ][PO 4-3 ][NH 4+ ]/K so (2) where, Ω > 1.0 implies the solution is supersaturated, i.e., precipitating or scaling and, Ω < 1.0 implies the solution is undersaturated, i.e., not precipitating/not scaling Then, with the concentrations of each species (Mg +2, NH 4+, and PO 4-3 ) being ph dependent, this can be written in terms of the ph-conditional solubility product, K so(ph-conditional), and the total solution concentration of Mg, P and N, or: Ω = Mg T P T N T /K so(ph-conditional) (3)

9 The ph-conditional K so can be defined in terms of ph as follows: K so (ph-conditional) = K so /(f f f ) (4) Mg +2 NH 4 + PO 4-3 where, each f value represents the ph-dependent fraction of each species, for example: f = [PO 4-3 ]/P T and, P T = H 3 PO 4 + H 2 PO 4- + HPO PO 4-3 PO 4-3 Within the ph-conditional solubility product, there are 6 Equilibrium Constants that are known within some limits of solution composition and experimental error. Then, there are sources of uncertainty within these Equilibrium Constants that influence any predictive calculations to determine the struvite saturation ratio, Ω. The largest source of uncertainty being within the Solubility Product for Struvite (log K so = to -13.4, or variability within a factor of 10). Equilibrium Constants for struvite Solubility Product, literature values are highly uncertain: Literature values of pk so for struvite at 25 o C and 0 ionic strength (15 values) pkso

10 ph is generally used to optimize biological processes (ph = 6.5 to 8.5), and ph is the most practical parameter to control Struvite Precipitation (Mg, N, and P are process inputs with no control). Then, a Struvite Precipitation Index, SPI, can be constructed based on ph, where the SPI is linearly dependent upon ph, or: SPI = ph ph ss (5) where, and, ph ss = ph of Struvite Saturation, i.e., Ω = 1.0, for given Mg, N, and P ph = measured ph in solution SPI > 0 (struvite precipitates) SPI = 0 (struvite is at equilibrium) SPI < 0 (struvite does not precipitate, i.e., struvite dissolves) The SPI provides more insight into Struvite Precipitation and indicates how ph can be adjusted either to prevent Struvite Precipitation/Scaling or to manage/optimize recovery processes for Struvite as a beneficial product, i.e., ph is the Master Variable for struvite formation.

11 Based on this data: Consider an average set of solution conditions (hypothetical), as follows: Total Ammonia = 150 mg/l (as N) Total ortho-phosphate = 31 mg/l (as P) Total Magnesium = 24.3 mg/l (as Mg) ph = 8.20 (mean measured value) ph ss = 7.70 (Ω = 1.0, calculated from equilibrium data) Then, SPI = = and, based on these average solution concentrations and ph, we could lower the ph by 0.5 units to 7.70 (SPI = 0) and this would represent a 50% chance of success. To ensure success (about 90% chance), it is recommended that: or, SPI < (ph = 7.20, to guarantee no struvite forms in this case) SPI > (ph > 8.20 to guarantee recovery in this case) Then, ph could be used to manage scaling or to optimize recovery efforts

12 Within a Predictive Model for Struvite precipitation, the sources of uncertainty can be summarized as: 1. Equilibrium Constants literature values vary by up to a factor of Time dependent variability in parameters, i.e., Mg, P, N, and ph As an example, data collected on-site (several weeks) at a wastewater facility using conventional aerobic basins for nitrification of Anaerobically Digester Centrate was simulated using variability in Mg, N, P, and ph. These results are shown in Figure 2.

13 - No Scale Formation - Heavy Scale - Moderate Scale Figure 2. Projected SPI values using mean values for all components (Mg, N, P) and Equilibrium Constants compared to actual scaling (by metal coupons). Points shown in red = Heavy Struvite, orange = Moderate Struvite, blue = No Struvite (from aluminum coupons in treatment basins. Note, rates of scale formation and uncertainty (component concentrations and equilibrium constants) imply that that scale may not form until SPI > 0.50.

14 Comparison of Most Common Alkaline Agents in Wastewater Treatment I. Caustic, NaOH extremely rapid release of Alkalinity NaOH Na + + OH - (Instantaneous Reaction in water) a. Accidental Overfeeds result in high ph (unacceptable in many cases) b. Inlet Zones and Feed Tubes exhibit localized regions of high ph creating localized precipitation of struvite and/or calcium carbonate c. Safety Hazards to workers II. Lime, CaO or Ca(OH) 2 rapid release of Alkalinity Ca(OH) 2 Ca OH - (Fast Reaction in water) a. Accidental Overfeeds result in high ph (unacceptable in many cases) b. Inlet Zones and Feed Tubes exhibit localized regions of high ph c. Safety Hazards to workers d. Precipitates Ca 3 (PO 4 ) 2 or CaCO 3 Scale III. Magnesium Hydroxide* controlled release of Alkalinity (rate α particle size) Mg(OH) 2 Mg OH - (controlled Reaction in water) a. Accidental Overfeeds restricted in ph elevation (max. ph = 10.5) b. controlled Dissolution prevents localized regions of high ph c. No Safety Hazards d. Possible Struvite Precipitation/Scale *Supplied as Thioguard, a known highly reactive grade of Mg(OH) 2

15 Magnesia Where fast reacting forms of Alkalinity are applied, such as NaOH or Ca(OH) 2, regions of high ph, and thus high SPI, exist due to localized mixing conditions and/or accidental overfeeding

16 NaOH Injection into well-mixed reactor ph in Injection Plume > 8.2 (SPI > 0) Struvite Accumulation Around Caustic Feed Tube Struvite Forms Around Injection Tubes Bulk Solution ph < 7.50 (SPI < 0) Figure 3. Demonstration of localized high ph in vicinity of Injection Point (NaOH or Ca(OH)2) using phenolphthalein as an indicator.

17 Hot zones in aerated processes, created by manually-controlled, highly soluble hydroxides like caustic and lime can lead to biological upset, carbonate and struvite formation Hot Zones can be tempered by the use of less soluble, but still highly reactive magnesium hydroxide

18 Struvite is most sensitive to ph, with Ω (or SPI) being more responsive to elevated ph (exponential) than to increases in component (Mg, N, P) concentrations (linear), or: Favorable Conditions For Struvite Scale = Localized Regions of Elevated ph In wastewater systems, localized regions of high ph exist in several known locations: I. Caustic Feed Tubes (plumes of high ph surrounding inputs) II. Regions Supersaturated with CO 2 (evolution of CO 2 raises ph): 1. Inlet Zones; 2. Mixer Blades (lower pressure around blades evolves CO 2 ) Struvite Accumulation Around Caustic Feed Tube Struvite Accumulation On Spinning Mixer Blades No Struvite Accumulation On Static Mixer Blades

19 Lime Mg(OH) 2 Monthly Mean 71.7% to 77.8% Mg(OH) 2 Trial Lime Monthly Mean 64.3% to 69.9% Lime Lime results in significantly higher Inorganic solids (due to CaCO 3 precipitation, i.e., refer to Langelier Index as analogous to SPI) compared to Mg(OH) 2 (no Mg-based solids) Figure 4. Ratio of MLVSS to MLSS at a Conventional Aerobic WWTP. Note, Alkalinity added as Lime or Mg(OH) 2 to replace loss due to nitrification, solids in basins reported as as MLSS = MLVSS + MLNVSS, or, as MLVSS/MLSS, then MLNVSS or Inorganic solids decrease.

20 Mitigation/Minimization should be practiced where Mg(OH) 2 is used and Struvite precipitation is predicted (SPI > 0). This can be done using a variety of techniques, including: 1. ph control (maintain SPI < 0) 2. Remove one or more key components, e.g., add Fe(III) to precipitate FePO 4 3. Prevent Struvite Formation, i.e., Thioguard Ωmega-S Program, or commercially available Struvite inhibitors Thioguard Ωmega-S is a proprietary program providing struvite management for Scale Minimization or Struvite Recovery that includes evaluation of feed points, smooth ph and Alkalinity control and reactor operation

21 Precipitation Region w/o Ωmega-S Case Study with Ωmega-S eliminates struvite scale under strongly precipitating conditions Figure 5. Case Study: Ωmega-S for Struvite minimization. Projected SPI values using mean values for all components (Mg, N, P) and Equilibrium Constants. Points shown in blue = No Struvite Scale (from aluminum coupons in treatment basins. Note,

22 Struvite Recovery as a Valuable Fertilizer Resource Struvite may be recovered as a high-value Fertilizer where higher concentrations of components (Mg, N, and P) exist: 1. Dewatering Waste Streams (pure struvite): 2. Digester Effluents (ppt out in the sludge for beneficial reuse)

23 Effluent Recycle Case Study: Ostara Process (Struvite Recovery from dewatering extracts) Return Back to Treatment Processes: Residual Mg, NH4 +, ortho-p Struvite Bed Solids Harvested for fertilizer Alkalinity Adjustment (ph control) Reactor Operating Conditions ph = 7.5 Ω 2.6 (SPI = 0.2) ΔMg = ΔN = Δortho-P (molar basis) Dewatering Waste: Mg, NH4 +, ortho-p

24 Ostara Operating Data (125 data points over one-year) Magnesium Ammonia (as N) ortho-p (as P) Mean = 117 mg/l Mean = 310 mg/l Mean = 88.1 mg/l SD = 35.4 SD = 129 SD = 25.5 Limiting 10.4% Limiting 3.2% Limiting 86.4% While ortho-p is the Limiting Reactant in the Struvite Recovery process most of the time (86.4%), it can not be added to the process without violating the initial intent (to remove N and P from the Treatment Processes). Therefore, to maintain Ω = 2.6 (SPI = 0.2, as the operational set point) and to maximize the production of Struvite, then, Mg is the Key ingredient (Mg can be increased) Note, Limiting Reactant refers to that component being below the 1:1:1 Stoichiometric Ratio to form Struvite, ortho-p is in deficit 86.4% of the time.

25 Table 1. Projected operational Struvite Recovery Based on actual reactor input concentrations of Mg, NH4 +, and ortho-p. Note, based on one-year operational data. Operating Conditions Mean Struvite Production, ton/d Stoichiometric a 1.09 SPI = 0.0 (Ω = 1.0) b 0.89 SPI = 0.20 (Ω = 2.6) c 0.72 Mg (+25%, SPI = 0.20) d 0.81 ph (+0.50, SPI = 0.20) e 0.94 a. Stoichiometric implies 100% limiting component consumed to produce struvite, i.e., absolute production limit b. Maximum recovery (operating at equilibrium ) at an operating ph of 7.5, i.e., theoretical limit c. Actual Operating Conditions (ph = 7.5), i.e., actual production projected d. Projected using operating conditions (ph 7.5, Ω = 2.6) e. Projected based on increasing ph from 7.5 to 8.0 and still operating at Ω = 2.6.

26 Conclusions Struvite can be a nuisance precipitate/scale or a valuable and recoverable resource in wastewater processes Struvite modeling has uncertainty due to poorly defined chemical equilibrium constants, limited reaction rates, and variability in solution concentrations/ph. These issues may be handled statistically Solution ph is a Critical Factor in Struvite Precipitation and a ph dependent Struvite Precipitation Index, or SPI (similar to the Langelier Index for calcium carbonate), can be determined for any set of solution conditions (SPI > 0 implies precipitation may occur, SPI < 0 implies precipitation is unlikely) Fast dissolving Alkalinity sources, such as NaOH, may create regions of high ph and consequently localized Struvite Precipitation/Scale, compared to Mg(OH) 2 that is controlled dissolving and maintains a more even reactor ph Nuisance Struvite Scale can be managed where Mg(OH) 2 is used for Alkalinity/pH control by adjusting ph and using scale mitigation strategies, such as the Thioguard Ωmega-S program Struvite Recovery can be maximized by driving the reaction with elevated Mg and/or increased ph