In recent years, the integrated ultrafiltration (UF) membrane process has been widely used due to its high removal efficiency, slight membrane fouling, and small land use. However, a number of problems gradually occurred regarding the integrated UF process caused by the granular adsorbents used, such as powdered activated carbon, carbon nano-tube, nanoscale zerovalent iron, etc. Severe membrane surface damage was easily caused by these granular adsorbents after a long running time, and the cost of most adsorbents was very high. In this study, to effectively overcome these problems, cheap and loose aluminum hydrolyzed flocs were directly injected into the membrane tank in the presence of humic acid (HA), with the aim of investigating the removal efficiency of HA and the corresponding membrane behavior. The results showed that the removal efficiency of HA could be influenced by aeration mode, floc injection frequency, and floc dosage. Compared with intermittent aeration and one-time injection, a loose "protection membrane" layer was formed with continuous aeration and batch injections. Therefore, HA molecules were largely removed, leading to the dramatic alleviation of membrane fouling. The transmembrane pressure significantly increased to 74.8 kPa in the absence of flocs after running for 5 days, but that only increased by 6.3 kPa with continuous aeration and an injection frequency of once every 2 d (each addition consisted of 5.4 mmol·L-1 flocs) after running for 8 days. The removal efficiency of HA was 73.3% (8 d), which was much higher than in the absence of flocs (5 d, 32.1%). Additionally, only a few HA molecules were adsorbed onto the membrane pores with the batch injections, and a loose cake layer was the main fouling mechanism. With higher dosages of flocs injected each time, the average membrane pore diameter was larger after washing. Based on this excellent performance, this floc-integrated UF membrane technology indeed shows large application potential in water treatment.
Keywords: alum-based flocs; humic acid; integrated process; removal mechanism; ultrafiltration.