Influence of solution and operating conditions on the treatment of aquaculture wastewater using direct contact membrane distillation: Ammonia rejection and membrane fouling

In the context of recirculating aquaculture, the timely and efficient treatment of ammonia nitrogen, which is a key factor influencing the survival of aquaculture organisms, has become a critical issue that must be addressed. This study aimed to comparatively investigate the interactive effects of bovine serum albumin (BSA) concentration, ammonia concentration, pH, temperature, and flow rate on the efficiency and membrane fouling during ammonia rejection by direct contact membrane distillation (DCMD) in the presence or absence of BSA. The results showed that the elevation of ammonia nitrogen concentration, regardless of the presence of BSA in the solution, led to an increase in concentration polarization, resulting in elevated rejection for ammonia nitrogen and aggravated membrane fouling. Moreover, the conditions of ammonia rejection that were found to be more favorable included lower temperature, pH, and flow rate. Among them, temperature and pH change primarily impacted ammonia solubility, which were the two factors with the significant influence on DCMD's ability to reject ammonia. In the presence of BSA, significant alterations were observed in the rejection of ammonia nitrogen and membrane fouling. The inclusion of 5 mg/L BSA resulted in a remarkable 97.5 % rejection of ammonia through hydration force. The elevation of pH led to a notable mitigation of membrane fouling owing to the augmentation of electrostatic force. Furthermore, a strong correlation was found between ammonia concentration, BSA concentration, and DCMD's rejection of ammonia. This study provides valuable insights for the future implementation of DCMD in treating actual ammonia wastewater and mitigating membrane fouling.