Control of Struvite Deposition in Wastewater Treatment Plants

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1 Control of Struvite Deposition in Wastewater Treatment Plants Paul L. Bishop Associate Vice President for Research University of Cincinnati 11 th Annual Central States Water Environment Association Education Conference April 4, 2006

2 Incoming Wastewater Typical Municipal WWTP Flow Diagram Bar Screen Screenings Gravity Thickener Dissolved Air Flotation Tank Grit Chamber Grits Primary Clarifier Further Dewatering Anaerobic Sludge Digester Centrate/Filtrate Aeration Tanks Secondary Clarifier Return Activated Sludge Sludge Dewatering Facility Chlorine Contact Tank Sludge to Incinerator, Farmland or Landfill Plant Effluent

3 Problems Anaerobic sludge digestion releases ammonium, magnesium and phosphate, which can form struvite in digesters and downstream dewatering facilities Can result in scaling in pipelines and on walls of process equipment Centrate or filtrate from sludge dewatering is usually returned to the plant headworks where it can add to the wastewater burden

4 Struvite Magnesium ammonium phosphate MgNH 4 PO 4 6H2O Named after Russian diplomat, H.G. von Struve ( ) White, yellowish white, or brownish white in color FW = Specific density = 1.7 Very insoluble in water, pk so = at 25 o C

5 Struvite Chemistry NH 4 + NH 3 (aq) + H + pka=9.3 H 3 PO 4 H 2 PO H + pk a1 = 2.1 H 2 PO - 4 HPO H + pk a2 = 7.2 HPO 2-4 PO H + pk a3 = 12.3 MgOH + Mg 2+ + OH - pk=2.56 MgNH 4 PO 4.6H 2 O Mg 2+ + NH + 4 +PO H 2 O pk=12.6 Struvite formation occurs when the conditions are such that the concentration product exceeds the struvite conditional solubility product

6 Conditional Solubility of Struvite vs ph Log (Ps) Mg 2+ + NH PO 4 3- MgNH4PO4.6H2O Log (ionization fraction Kso Ps = CT, MgCT, NH CT, PO = 3 4 α α α γ γ γ ph Mg NH PO Mg NH PO Ps = conditional solubility product Kso = solubility product C T,Mg = total concentration of all soluble magnesium species C T,NH3 = total concentration of all soluble ammonia species C T,PO4 = total concentration of all soluble phosphate species " i = ionization fraction for component i γ i = activity coefficient for component i

7 Struvite Formation in Sludge Dewatering Process Anaerobically digested sludge, anaerobic supernatant (centrate/filtrate) Mixing & perturbations Carbon dioxide stripping ph elevation Phosphate equilibrium shifts towards PO 4 3- [Mg 2+ ] [NH 4+ ][PO 4 3- ] exceeds struvite solubility product (super-saturation) Nucleation and crystal growth Struvite precipitates

8 MgNH 4 PO 4. 6H 2 O Filtrate return line Ball check Struvite encrusted roller Productivity lost!! (Courtesy Schaner s Waste Water Products, Inc.)

9 Problems with Current Struvite Control Techniques Addition of iron chloride to form vivianite (Fe 3 (PO 4 ). 2 8H 2 O) " Chloride concentration increases " Ferric ion acts as an acid, lowering ph " Large volume inorganic sludge generation " Phosphate recovery from ferric phosphate salt(s) is nearly impossible Similar problems with ferric sulfate or alum

10 Objective Investigate the use of magnesium hydroxide to remove nutrients in a controlled fashion from digested sludge Can use waste flue gas desulfurization sludge as a source of Mg(OH) 2

11 Characterization of Mg(OH) 2 : Basic Properties that are Important to Wastewater Treatment Applications

12 Magnesium Hydroxide Dissolution Kinetics 10 ph Time (min)

13 Titration Curves of Several Neutralization Chemicals ph D C A B Titrant Added (eq.) A = calcium hydroxide; B = pure magnesium hydroxide; C = sodium carbonate; D = as-received magnesium hydroxide slurry

14 Relative Neutralization Capacity and Buffering Capacity of Several Neutralization Reagents (at ph = 8.5) Relative Neutralization Capacity Buffering Capacity(eq/pH) = pure magnesium hydroxide; 2 = sodium carbonate; 3 = calcium hydroxide; 4 = as-received magnesium hydroxide slurry.

15 Summary Mg(OH) 2 has unique features compared with other commonly used chemicals: slow dissolution process high neutralization capacity high buffering intensity

16 Sludge Digestion Enhancement Using Mg(OH) 2

17 NH 3 -N, PO P, Mg 2+, Ca 2+ and SO 4 2- Changes During Anaerobic Sludge Digestion

18 Biogas Production Profiles During Anaerobic Sludge Digestion 300 Biogas Volume (L Mg(OH)2 reactor Control reactor Digestion Time (hours)

19 Summary Applying magnesium hydroxide into an anaerobic sludge digester can: Result in greater destruction of COD and SS Enhance the production rate of biogas Increase overall treatment efficiency Reduce level of nutrients in the supernatant that must be returned to the plant s headworks Increase the nutrient content in the generated biosolids for agricultural use Improved sludge dewaterability, which will ease the operation of the down stream sludge dewatering facilities

20 Nutrient Removal from Anaerobically Digested Sludge and Sludge Supernatant Using Mg(OH) 2

21 Nutrient Removal from Digested Sludge PO4 3- -P (mg/l) Phosphate---No mix control Phosphate---Mg(OH)2=100mg/L ph---no mix control ph---mg(oh)2=100 mg/l Time (min) Phosphate---Mixed control Phosphate---Mg(OH)2=250mg/L ph---mixed control ph---mg(oh)2=250 mg/l ph

22 Pilot Scale Experimental Results on Phosphate Removal from Centrate air stripping only PO4 3- -P (mg/l) aeration period settling period air stripping mg/l MgCl2 air stripping mg/l MgCl2 air stripping mg/l Mg(OH)2 air stripping mg/l Mg(OH)2 air stripping mg/l Mg(OH) Time (min)

23 Total phosphorus mass balance without metal phosphate precipitation from centrate/filtrate Influent Primary + secondary treatment systems effluent Sludge digester Filtrate/centrate Sludge dewatering 90 sludge cake Total phosphorus mass balance with metal phosphate precipitation from centrate/filtrate Influent 100 Treated filtrate/ centrate Metal phosphate precipitation reactor 7 Filtrate/centrate Primary + secondary treatment systems P-containing chemical sludge 61 Sludge digester 97 Sludge dewatering effluent sludge cake + chemical sludge

24 Summary Use of Mg(OH) 2 to remove nutrients from anaerobically digested sludge is effective only if the sludge is well digested. Removing phosphate from the side waste stream will: reduce the nutrient load to the headworks of the treatment plant (this is a current practice that adversely affects the overall treatment efficiency) lower the potential for struvite formation, which is a frequently occurring O&M problem in many municipal wastewater treatment plants generate a slow release fertilizer

25 Improving the Settleability and Dewaterability of Activated Sludge: Applications of Mg(OH) 2

26 SVI Effect of Mg(OH)2 on Activated Sludge Settleability Mg(OH) 2 Dosage (mg/l) SV(ml/L)

27 Surface Charge Density Changes vs Mg(OH)2 Dosage COO - ---Mg OOC Relative Surface Charge NH 3 NH Magnesium Hydroxide Addition (mg/l)

28 Mixed Liquor Sedimentation Curves under Different Mg(OH) 2 Dosage Conditions 180 Height of water/sludge interface (cm) Mg(OH)2: 0 mg/l Mg(OH)2: 100 mg/l Mg(OH)2: 300 mg/l Mg(OH)2: 500 mg/l 40 0:00:00 0:14:24 0:28:48 0:43:12 0:57:36 1:12:00 1:26:24 Time (hour:minute:second)

29 Sludge Dewaterability Changes with the Addition of Mg(OH) 2 CST (seconds) Mg(OH) 2 Dosage (mg/l)

30 Summary By charge neutralization, sweep flocculation and Mg 2+ bridging between the EPS matrices of the microorganisms, Mg(OH) 2 is effective in improving the settleability of activated sludge Besides enhancing the overall sludge digestion process efficiency, Mg(OH) 2 application to anaerobic sludge digester can also generate a digested sludge that is easier to dewater

31 Conclusions Mg(OH) 2 improved the biological phosphate uptake and release behavior of activated sludge Mg 2+ was found to stimulate the phosphate uptake during aeration periods The ph increase caused by Mg(OH) 2 addition enhanced phosphate release during the anaerobic sedimentation period Research results provide supporting evidence for the potential application of Mg(OH) 2 in EBPR processes

32 Conclusions Magnesium hydroxide can effectively improve the settleability of mixed liquor during sedimentation in secondary clarifier and the dewaterability of anaerobically digested sludge in sludge dewatering Magnesium hydroxide can enhance the overall process efficiency of anaerobic sludge digestion due to improved ph/alkalinity and the supplementation of Mg 2

33 Conclusions Magnesium hydroxide is effective in removing nutrients from anaerobic supernatant, thus reducing the nutrient load returned to the headworks of the plant It minimizes the risk of struvite formation and generates a good plant fertilizer Magnesium hydroxide is superior to other commonly used chemicals in this regard FeCl 3, alum and lime. Aeration (for mixing) plus magnesium chloride (Mg 2+ source) plus struvite seeding proves to be a good process for controlled struvite crystallization.

34 Potential Mg(OH) 2 Application Locations in Municipal WWTP Incoming Wastewater Bar Screen Screenings Gravity Thickener Dissolved Air Flotation Tank Grit Chamber Grits Primary Clarifier Further Dewatering Magnesium Hydroxide 2 Anaerobic Sludge Digester Centrate/Filtrate 1 Aeration Tanks Magnesium Hydroxide Secondary Clarifier Return Activated Sludge 3 Magnesium Hydroxide Sludge Dewatering Facility Magnesium Hydroxide 4 Chlorine Contact Tank Sludge to Incinerator, Farmland or Landfill Plant Effluent

Presented by Paul Krauth Utah DEQ. Salt Lake Countywide Watershed Symposium October 28-29, 2008

Basic Nutrient Removal from Water Beta Edition Presented by Paul Krauth Utah DEQ Salt Lake Countywide Watershed Symposium October 28-29, 2008 Presentation Outline Salt Lake County waters / 303(d) listings