Recovery and removal of ammonia–nitrogen and phosphate from swine wastewater by internal recycling of struvite chlorination product

The recovery of the total orthophosphate (PT) and removal of the total ammonia–nitrogen (TAN) from swine wastewater were investigated through a combined technology of using bittern as the magnesium source in struvite precipitation along with internal recycling of the chlorination product of the recovered struvite. Results revealed that the PT recovery efficiency and the struvite purity was mainly depended on the wastewater pH and the Mg:PT molar ratio. Co-precipitations of Mg3(PO4)2, MgKPO4, Ca3(PO4)2, and Mg(OH)2 (pH > 9) were confirmed to be responsible for the decrease in the purity of struvite. The decomposition of recovered struvite by sodium hypochlorite (NaClO) was feasible. The TAN concentration of the swine wastewater was decreased to 63 mg/L by internal recycling of the chlorination decomposition product for seven cycles. An economic evaluation showed that 37% of the treatment cost of the proposed process could be saved as compared with struvite precipitation using pure chemicals.