Evaluation of Alternate Process Chemistries for the Removal of Arsenic and Fluoride from Industrial Wastewater

Transcription

1 Evaluation of Alternate Process Chemistries for the Removal of Arsenic and Fluoride from Industrial Wastewater

2 Background Southeastern industrial client discharges process wastewater containing arsenic and fluoride On-site lagoons store, manage, and treat wastewater Project driver was to reduce operational treatment costs while minimizing capital costs WWT RAW WATER POND CLARIFICATION POND 2 Ryan Ames and Sam Jeppson, Dewberry November 2017

3 Project Driver Reduce Operating Costs Current process is labor and chemical intensive Process requires high lime dosage to raise ph over 11.8 before reducing ph with CO 2 to Excess lime causes clogging of piping Operators manually monitor lime slurry and outfall Site is remote and difficult to deliver bulk lime and CO 2 Client desired all liquid dosing from totes for simpler operation Current operating costs: ~$1,137/MG 3 Ryan Ames and Sam Jeppson, Dewberry November 2017

4 Current Treatment Process Current treatment process utilizes lime precipitation and CO 2 ph adjustment Arsenic in the arsenite form (at neutral ph and low ORP) is converted to arsenate (at high ph) Fluoride is treated by converting to CaF 2 which is highly insoluble in water The client s arsenic and fluoride discharge limits are 50 ug/l and 10 mg/l respectively. Avg. As & F removal rates are 93% and 25% respectively 4 Ryan Ames and Sam Jeppson, Dewberry November 2017

5 Current Process 5 Ryan Ames and Sam Jeppson, Dewberry November 2017

6 Background 6 Ryan Ames and Sam Jeppson, Dewberry November 2017

7 Background 7 Ryan Ames and Sam Jeppson, Dewberry November 2017

8 Background 8 Ryan Ames and Sam Jeppson, Dewberry November 2017

9 Treatability Approach Methods for As removal Chemical Precipitation Oxidation Adsorption Activated Alumina Ion Exchange Reverse Osmosis Methods for F - Removal Chemical precipitation Adsorption Activated Alumina Ion Exchange Reverse Osmosis Based on current infrastructure and long retention time in the clarification pond, Dewberry pursued chemical precipitation treatment of arsenic and fluoride 9 Ryan Ames and Sam Jeppson, Dewberry November 2017

10 Treatability Approach - Arsenic Proposed Treatment Raw Wastewater Current Treatment 10 Ryan Ames and Sam Jeppson, Dewberry November 2017

11 Treatability Approach - Fluoride Current treatment method uses the calcium in lime, Ca(OH) 2, to react to form CaF 2. Lime is not very soluble in water: 1.7 g/l Dewberry looked at multiple approaches for fluoride removal Improved process mixing Using an alternate calcium salt: calcium chloride Adding an adsorption aid: magnesium hydroxide Using an alternate coagulant: aluminum sulfate 11 Ryan Ames and Sam Jeppson, Dewberry November 2017

12 Laboratory Equipment ph/orp probe and meter Hach Fluoride TNT test kit DR 3900 Spectrophotometer Hach Arsenic test kit Phipps Bird Gang Stirrer 12 Ryan Ames and Sam Jeppson, Dewberry November 2017

13 Arsenic Indicator Test Raw water As concentration ~ ug/l Difficult to determine concentration within the ug/l range Arsenic kit was used as a qualitative test 13 Ryan Ames and Sam Jeppson, Dewberry November 2017

14 Bench Testing Setup 14 Ryan Ames and Sam Jeppson, Dewberry November 2017

15 Bench Testing Reactor Setup Order of chemical additions: Oxidant, calcium, magnesium, coagulant Mixing 1 min between chemicals 5 min of rapid mixing after chemicals added 15 min of slow mixing 30 min of settling Filtering Since client has a large treatment pond, decant was filtered to simulate discharge characteristics 15 Ryan Ames and Sam Jeppson, Dewberry November 2017

16 Trial Approach 1. Improve existing lime/ferric process a) Increase mixing energy & retention time b) Optimize lime dose 2. Evaluate alternate process chemistries a. Oxidant, calcium chloride, & ferric chloride b. Oxidant calcium chloride, aluminum sulfate c. Oxidant, Mg(OH)2, CaCl2, alum d. Oxidant, alum, ferric 16 Ryan Ames and Sam Jeppson, Dewberry November 2017

17 Results - Improved Mixing F- Concentration, mg/l Raw Fluoride Removal Pipe Mix Added Mix F- % Removal % F Removal As Concentration, ug/l Raw Arsenic Removal Pipe Mix As % Removal Added Mix % As Removal Conclusions 5% additional fluoride removal with additional mixing. 0% additional arsenic removal observed with additional mixing. 17 Ryan Ames and Sam Jeppson, Dewberry November 2017

18 Results - Optimized Lime Dose F Concentration, mg/l Fluoride Removal Raw Current Dose 0% 0% 5% 8% 21% F- ph ph As Concentration, mg/l Raw 0% Arsenic Removal Current Dose 60% 70% 80% 80% As ph ph Conclusions Increasing lime dose showed diminishing returns for As removal Additional removal of F required ph of 11 While increased lime dose increases removal, a lower dose can save on money with minimal impact 18 Ryan Ames and Sam Jeppson, Dewberry November 2017

19 Results - CaCl 2 /Ferric F- Concentration, mg/l Raw Fluoride Removal F- Calcium Added Conclusions 17,000 mg/l CaCl 2 Added 0% 0% 0% 0% 0% 0% 5% % F Removal 80% Increasing CaCl 2 dose had no effect on F removal. Even at extreme doses 19 Ryan Ames and Sam Jeppson, Dewberry November 2017

20 Results - Mg(OH) 2 /CaCl 2 /Alum 18.0 Raw Fluoride Removal 6,000 F Concentration % 28% 55% 60% 57% 56% 60% F- Mg(OH)2 Alum CaCl2 Conclusions 5,000 4,000 3,000 2,000 1,000 0 Chemical Addition, mg/l Increasing CaCl 2 dose increased F removal until reaching 1,500 mg/l. Achieved 28% F removal with no calcium addition. 20 Ryan Ames and Sam Jeppson, Dewberry November 2017

21 Results - Mg(OH) 2 /CaCl 2 /Alum As Concentration, ug/l Raw No CaCl2 Arsenic Removal 0% 50% 75% 63% 50% As Ferric Oxid. Conclusions 0 Increasing CaCl2 conc. Increasing CaCl 2 dose had minimal effect on arsenic removal and no removal of F (not shown) Client liked economics of ferric and oxidant condition but wanted ~20% As removal Chemical Addition, mg/l 21 Ryan Ames and Sam Jeppson, Dewberry November 2017

22 Results Other Conclusions With additional trial data not shown: Greater than 60% As was removed at ORP levels over 400 mv Mg(OH) 2 did not influence F- removal Calcium did not reduce F- as much as alum Client liked economics of oxidant/ferric process needed to achieve a minimum F removal of 20% 22 Ryan Ames and Sam Jeppson, Dewberry November 2017

23 Results Alum Only F Concentration, mg/l Fluoride Removal Raw 0% 0% 4% 17% 33% 42% 54% F- Alum Chemical Dose, mg/l Conclusions Alum alone demonstrated significant F removals F removal was proportional to the amount of alum dosed. Alum alone did not remove As 23 Ryan Ames and Sam Jeppson, Dewberry November 2017

24 Results Multiple Parameters 20.0 Raw Fluoride Removal Raw Arsenic Removal F- Concnetration, mg/l % 20% 20% 24% Chemical Addition, mg/l As Concnetration, mg/l % 0% 60% 0% Chemical Addition, mg/l F- Oxid. Alum Ferric CaCl2 As Oxid. Alum Ferric CaCl2 Conclusions The presence of ferric chloride facilitated As removal in system Oxidant, alum, ferric combination reduced fluoride and arsenic by 20% and 60% respectively. 24 Ryan Ames and Sam Jeppson, Dewberry November 2017

25 Results Oxidant/Alum/Ferric F- Concnetration, mg/l Fluoride Removal Arsenic Removal Preoxidation No Preoxidation Raw Preoxidation No Preoxidation Chemical Addition, mg/l As Concnetration, mg/l % 25% 26% 26% 30% 27% 0.0 0% 14% 28% 37% 50% 49% F- Oxid. Alum Ferric Raw As Oxid. Alum Ferric Chemical Addition, mg/l Conclusions Achieved over 25% fluoride removal with ~ 80 mg/l alum As removal greatly improved with ferric. F removal improved slightly Pre-oxidation not required. Removal rates approximately the same. 25 Ryan Ames and Sam Jeppson, Dewberry November 2017

26 Treatment Costs Current Treatment Proposed Treatment % Fluoride Removal 32% 30% % Arsenic Removal 93% 50% WW Effluent ph Chemical Treatment Cost $933/MG $500/MG Labor Cost $204/MG $153/MG Total Treatment Cost $1,137/MG $653/MG 26 Ryan Ames and Sam Jeppson, Dewberry November 2017

27 Proposed WW Treatment 27 Ryan Ames and Sam Jeppson, Dewberry November 2017

28 Full Scale Trial Dewberry performed a full scale trial during the last week of July Intent of the trial was to: Determine if the modified process would work with the current mixing setup Collect enough sample for whole and acute toxicity testing Trial was cut short due to hurricanes and flooding 28 Ryan Ames and Sam Jeppson, Dewberry November 2017

29 Results Full Scale Trial Parameter Trial 5 Trial 6 NaOCl, mg/l Alum, mg/l Ferric, mg/l ph ORP Raw F, mg/l Raw As, ug/l Final F, mg/l Final As, ug/l F Removal 20% 30% As Removal 91% 71% 29 Ryan Ames and Sam Jeppson, Dewberry November 2017

30 Conclusions Oxidant + Ferric effective for As removal at lower ph Alum effective for F removal at near neutral ph Identified potential for cost savings (~43%) over current lime / CO 2 process. Improve ph control and dechlorination in pilot system to improve toxicity performance. 32 Ryan Ames and Sam Jeppson, Dewberry November 2017