Elucidating the formation pattern and mechanism of biofouling layer regulated by pre-deposition of powdered activated carbon in gravity-driven membrane systems

Powdered activated carbon (PAC) pre-deposition promotes pollutant removal in gravity driven membrane (GDM) but diversely affects flux. This study evaluated the effects of one-time deposition (GDM 1), batch deposition (GDM 2) and no deposition (GDM 3) of PAC on the biofouling layer formation mechanism and the GDM performance. The results showed that PAC particles effectively promoted the removal of dissolved organic carbon, ammonia nitrogen and fluorescent pollutants, while also significantly reducing the concentration of extracellular polymeric substances in the biofouling layer. The stable flux in GDM 1-3 were 6.58, 7.88 and 8.61 LMH respectively. One-time deposition of PAC compromised the structure of the biofouling layer and prevented its reverse relaxation. Pollutant floc particles continuously accumulated in the space between the PAC particles, forming a dense layer with low porosity and roughness. In GDM 2, the limited amount of PAC deposited each time helped to partially maintain the intact structure of the biofouling layer, which alleviated the decline in flux. The reverse relaxation of the intact biofouling layer in GDM 3 resulted in increased porosity, which positively influenced the flux. Additionally, the abundance of Bdelloidea significantly decreased in GDM 1 and GDM 2, and more predators with low agility showed limited modification on the biofouling layer, resulting in a dense biofouling layer. In conclusion, batch deposition of PAC is more recommended as the continuously renewed PAC promote pollutant removal, and compared with one-time deposition of PAC, the reduction in biofouling layer disturbance mitigates the flux decline.