The combination of precipitative softening and ozonation as a pretreatment of ultrafiltration in flowback water treatment: performance and fouling analysis

Ultrafiltration (UF) technology is an efficient shale gas flowback water treatment method. However, severe membrane fouling is the primary restriction on the application of UF technology. Here, we studied the impact of three pretreatments: precipitative softening (PS), precipitative softening, followed by ozonation (PS-O) and ozonation, followed by precipitative softening (O-PS), on pollutants' removal efficiencies and membrane fouling. The results showed that (1) the hardness, bacteria, scaling trend and compatibility with formation water exceeded the requirements for water reuse; (2) pretreatments effectively increased water flux and prolonged ultrafiltration membrane life, and both of them followed the order of PS-O process > O-PS process > PS process; (3) the fouling mechanism was changed from the complete blocking model to the standard blocking model by the PS process and the addition of ozonation enhanced the correlation of standard blocking model; (4) the quality of fracturing liquid prepared by the effluent treated by the PS-O-UF process was the best and satisfied the requirements of slick water. This paper indicated that the PS-O-UF process was suitable for the treatment of Changning shale gas flowback water for reuse.