Recirculating aquaculture systems (RAS) have become an important development direction in modern aquaculture, providing advantages in water conservation, stable production, and reduction of environmental discharge. However, these systems produce wastewater containing high concentrations of suspended solids, nitrogen- and phosphorus-based pollutants, and large amounts of dissolved organic matter. If such wastewater enters a nanofiltration (NF) system without proper pretreatment, severe membrane fouling will occur, rapidly reducing flux and increasing operational costs. For this reason, selecting an effective pretreatment technology is critical for maintaining the stability of NF operation.
Recent research compared two major pretreatment technologies-flocculation and ultrafiltration-to analyze their pollutant removal performance, influence on NF flux decline, and membrane fouling characteristics. The findings provide valuable technical guidance for selecting suitable pretreatment processes in RAS wastewater treatment.
In flocculation pretreatment, sodium alginate showed the best performance among the three tested flocculants. At an optimal dosage of 45 mg/L and a stirring speed of 1200 r/min, it achieved a maximum suspended solids removal rate of 79.70%. However, the removal efficiency for ammonia nitrogen, nitrate, nitrite, and sulfides was generally below 40%, indicating that flocculation is insufficient for dissolved pollutants commonly found in aquaculture tailwater. The study also found that sodium alginate forms small, compact flocs that tend to deposit on the NF membrane surface, creating a dense cake layer. Due to its natural polysaccharide structure, sodium alginate also provides a substrate for microbial growth during the extended settling period, which significantly increases biological fouling on the NF membrane. As a result, the flux recovery after cleaning is relatively poor, suggesting that fouling caused by flocculation is more adhesive and difficult to remove.
In contrast, ultrafiltration (UF) membranes, with pore sizes of 0.01–0.1 μm, demonstrate excellent performance in removing suspended solids, achieving a removal rate as high as 98.54%. Although UF has limited ability to remove nitrogen and phosphorus pollutants, it effectively reduces the particulate burden entering the NF system. As a result, the NF membrane forms a looser cake layer during operation, reducing pore blockage and slowing the increase of transmembrane pressure. According to NF performance tests, wastewater pretreated with UF showed higher initial flux, slower flux decline, and significantly better flux recovery after repeated cleanings compared with flocculation-pretreated water. This indicates that UF can significantly delay irreversible fouling on the NF membrane.
Further analysis using excitation–emission matrix (EEM) fluorescence showed that the cleaning solution of the flocculation–NF system contained strong peaks representing soluble microbial byproducts, while these peaks were almost absent in the UF–NF system. This confirms that extended settling and the polysaccharide nature of sodium alginate promote microbial growth, thereby increasing membrane fouling in the NF stage.
Overall, the comparison clearly shows that ultrafiltration is more effective than flocculation as a pretreatment method for nanofiltration of recirculating aquaculture wastewater. Although flocculation provides a low-cost option for initial particle removal, its floc characteristics and microbial risks lead to more severe irreversible membrane fouling. Therefore, a UF + NF integrated membrane process is expected to become a key technology for advanced treatment and reuse of RAS wastewater.
Looking ahead, as aquaculture production scales up and environmental regulations become stricter, topics such as nanofiltration fouling control, ultrafiltration pretreatment optimization, and development of low-pressure high-efficiency NF membranes will continue to gain attention. Integrating UF and NF into more compact and energy-efficient membrane systems will promote the transformation of aquaculture wastewater treatment toward resource recovery, reduced discharge, and more sustainable water management.
