Effect of operating conditions on the chemical phosphorus removal using ferric chloride by evaluating orthophosphate precipitation and sedimentation of formed precipitates in batch and continuous systems

Chemical phosphate removal by Fe(III) involves the precipitation of ferric phosphate and subsequent sedimentation of Fe–P particles. Physical–chemical factors that affect the phosphorus (P) precipitation are well known; however, the factors affecting the sedimentation of Fe–P precipitates have been poorly studied. The aims of this work were: (a) To study the effect of pH, presence of biomass (3.0 g TSS/L) and settling time on the orthophosphate precipitation by ferric chloride in a batch system. (b) To determine the effect of these factors on the sedimentation of the formed Fe–P particles. (c) To evaluate the performance of a laboratory-scale continuous activated sludge (AS) reactor operated with phosphorus simultaneous precipitation by ferric chloride under the optimum operating conditions determined from previous batch assays. Results from batch studies showed that presence of AS biomass and prolonged settling times favored the P removal. According to the proposed equation, in presence of biomass, the settling rate of the Fe–P precipitates was 3–7 times higher than that corresponding to phosphate buffer system. In the continuous system, Fe–P precipitates were gradually incorporated to the biomass, improving the reactor performance. However, a gradual shift of the size distribution from large to poorly settling small flocs was observed. The system operated at a sludge age of 21 days achieved more rapidly better performance than at 40 days. This last system showed bad flocculation exhibiting smaller flocs than the reactor with younger sludge. Simultaneous determination of the accumulation rate of fixed solids, and floc size distribution allows monitoring these systems.

► Operational factors affected the process of phosphorus removal by ferric chloride.
► Activated sludge biomass and prolonged settling times favored the process efficiency.
► Fe(III)-treated bioreactor at a sludge age of 21 days showed the best performance.
► Higher sludge age produced the smallest flocs deteriorating the reactor performance.
► Floc size measurement using image analysis allowed monitoring these systems.