Effective removal of ammonia nitrogen from waste seawater using crystal seed enhanced struvite precipitation technology with response surface methodology for process optimization

Traditional biological treatment was not effective for removing nitrogen from saline wastewater due to the inhibition of high salinity on biomass activity. In this context, a method of removing ammonia nitrogen from waste seawater was proposed by struvite precipitation which was enhanced by seeding technique. The abundant magnesium contained in waste seawater was used as the key component of struvite crystallization without additional magnesium. The effects of pH and P:N molar ratio on ammonia nitrogen removal efficiency were studied. The results showed that optimum pH value was in range of 8.5–10 and the P:N molar ratio should be controlled within 2:1–3:1. XRD and SEM-EDS analyses of the precipitates proved that Ca²⁺ and excess Mg²⁺ contained in waste seawater inhibited the struvite crystallization by competing PO₄ ³⁻ to form by-products. Then, seeding technique for enhancing the struvite crystallization was investigated, and the results indicated that using preformed struvite as crystal seed significantly improved the ammonia nitrogen removal efficiency, especially when initial ammonia nitrogen concentration was relatively low. Moreover, response surface optimization experiment following a Box-Behnken design was conducted. A response surface model was established, based on which optimum process conditions were determined and interactions between various factors were clarified. At last, economic evaluation demonstrated this proposed method was economic feasible.