Struvite precipitation and COD reduction in a two‐step treatment of olive mill wastewater

BACKGROUND In the present work, a combination of several physicochemical methods has been applied aiming at recovering phosphorus from olive mill wastewater (OMW) in the form of struvite (NH 4 MgPO 4 ·6H 2 O, MAP) and at the same time achieving reduction of the chemical oxygen demand (COD) of the wastes. Phosphorus is a valuable raw material used for the production of fertilizers and numerous other products. RESULTS The experiments were conducted in a two‐stage process, over a pH range between 5 and 10. The first step consisted of a batch process, in which MgCl 2 or MgSO 4 were used as coagulants of suspended particles of the OMW. During the second step, a nitrogen source (NH 4 OH(aq)) was added to the supernatant and solid precipitation took place without delay. The measured COD values of the fluid phase of the OMW was reduced by up to 73% of the initial value, while the final crystalline product consisted of a mixture of struvite and dittmarite (NH 4 MgPO 4 ·H 2 O). CONCLUSION Over 90% dissolved phosphorus recovery from OMW samples in the form of struvite was achieved through the application of a two‐step process together with a significant reduction of the COD of initial raw wastewaters.