Highly Efficient Recovery of Ammonium Nitrogen from Coking Wastewater by Coupling Struvite Precipitation and Microwave Radiation Technology

In this study, to reduce the removal cost of total ammonia nitrogen (TAN) from coking wastewater via struvite crystallization, a struvite recycling process combined with microwave radiation technology was investigated. The results indicated that the nitrogenous heterocyclic and phenolic compounds present in coking wastewater obviously influenced struvite crystallization. If the formed struvite was decomposed by microwave (MW) radiation at OH–:NH4+ of 1.15:1 for 300 s, about >97% of ammonium could be released from the struvite. In the decomposition process of MW radiation, it was found that the solid–liquid concentration of struvite in the solution system had a negligible effect on ammonium release. When the resulting decomposition product was recycled, to remove TAN from the coking wastewater at pH 8.5 and P:N molar ratio of 1:1 for 50 min, approximately 88% of TAN removal was achieved. In a seven-step process, the TAN-removal efficiency could be maintained at >80% with a little bittern supplementation. Cost analysis revealed that the proposed recycling process could reduce 54.4% of the struvite precipitation cost as compared to the nonrecycling process.