Simultaneous recovery of fresh water and ammonia from produced water by membrane contactor coupled with membrane distillation

Efficient treatment of shale gas produced water (SGPW) is a great challenge, while ammonia in SGPW has potential for recovery. In this study, a membrane contactor-membrane distillation (MC-MD) integrated process was proposed to recover fresh water and ammonia under different feed pH conditions from raw and reverse osmosis concentrate (ROC) of SGPW. The results showed that MC as pretreatment significantly enhanced MD performance, improving the normalized flux at 75% water recovery by 11.3%-16.2% and steadily reducing permeate salinity to below 15 mg/L. The ammonia flux and mass transfer coefficient became larger as the feed pH increased, reaching maximum ammonia flux of 2.47 g/(m²⸱h) at pH of 10.5 and maximum mass transfer coefficient of 60.5 m/h at pH of 11.5. However, the excessively high pH accelerated the diffusion of feed ammonia into the air, which increased the percentage of ammonia loss from the system. The addition of MC mitigated MD membrane fouling, increasing the contact angle of the membranes after use to 90.12-127.38°, which was 48.5-110.0% improvement compared to single MD (60.67°). The MC-MD system reduced the 81.2-86.0% cost compared to traditional methods, providing theoretical guideline for efficient SGPW treatment.