Ion Exchange Nutrient Recovery from Municipal Wastewater
Nitrogen and phosphorus discharge is regulated due to eutrophication. Typically ammonium-nitrogen is removed through nitrification which requires energy and phosphorus (P) is removed by metal salt addition to precipitate P. A more sustainable approach may be to implement nutrient recovery to form fertilizers. Most nitrogen fertilizer is formed through the energy intensive Haber-Bosch process. Phosphate rock, a limited resource, is mined for P fertilizer. Discharge regulations and the fertilizer production costs provide an economic driver for recovery. The current project used anionic and cationic ion exchange (IX) to concentrate the nutrients with recovery by struvite precipitation. Previous work lacks direct comparison of anion media, treatment of real wastewater with 2 IX columns in series, evaluation of regeneration characteristics and struvite precipitation from combined regenerants. During batch testing, Dow-Cu removed 76% of P while Dow-FeCu and LayneRT™ showed removal of >60%. These three media were tested in column mode using filtered municipal wastewater secondary effluent dosed with nutrients. Phosphate removal before breakthrough (PO4-P effluent>0.075 mg/l) for LayneRT™, Dow-FeCu, and Dow-Cu was 2.00, 1.49, and 0.46 g P/LBV, respectively. Desorption of >89% occurred for all 3 resins. Dow-FeCu provided the best conditions: effective removal, efficient regeneration at neutral pH and high (560 mg/l) regenerant P concentration. Natural zeolite was used for cation exchange. During ammonium exchange batch tests, NaCl solution with pH<13 was optimal for regeneration of zeolite. Column testing of zeolite was conducted using effluent from the>LayneRT™ column. Capacity during IX cycle 1 and 2 before breakthrough (NH4-N effluent > 1.5 mg/l) was 3.9 and 6.1 g N/LBV, respectively; performance variability is due to the difference in influent cation concentrations. Desorption was 74% and 68% in cycle 1 and 2, respectively. Struvite was precipitated from zeolite and anion regenerant with 71%, 13%, and 74% P removal for LayneRT™, Dow-Cu, and Dow-FeCu regenerants, respectively. Struvite precipitation was limited by Mg. Precipitates contained impurities such as Al3+, Ca2+, and Fe2+. The IX-recovery process removed greater than 98% P and 95% N and formed solids containing 13% P and 2% N.