Impacts Of Powdered Activated Carbon On The Hybrid Powdered Activated Carbon/Ultrafiltration Process

Powdered activated carbon/ultrafiltration(PAC/UF) hybrid process, which incorporates the adsorption capabilities of PAC and the microorganism and particle removal ability of UF membrane, could substantially improve the safety of drinking water. The small-footprint process is flexible, and the dosage and age of PAC can be adapted to the variations in feed water quality. Therefore, the PAC/UF process is a favourable process to address the current challenges in drinking water treatment in China. However, PAC is in contact with UF membrane in the process, and thus PAC may affect the operation of UF. To date, managing the interaction between PAC and membrane remains a big challenge, and thus most of the full-scale adsorption-UF process avoid the contact of PAC with membrane. As a result, it is critical to understand the impact of PAC on the PAC/UF process.Firstly, the adsorption characteristics of UF membrane foulants onto PAC were investigated. Then, the combined fouling by PAC and organic matter and the deposition of PAC on the membrane surface were examined using a bench-scale flat sheet UF system. Finally, the removal of organic matter, ammonia and odor in the PAC/UF process was studied using a pilot-scale system.The adsorption of model foulants(Adrich humic acid, bovine serum albumin, and sodium alginate) and major foulants in natural organic matter(identified using parallel factor analysis of excitation-emission matrices) onto PAC were investigated. It was found that the adsorption of model foulants onto PAC followed the order of humic acid > humic acid > sodium alginate. The major foulants in the Songhua River water were protein-like fluorescent components, which could be effectively removed(54.5%) by the PAC adsorption. PAC adsorption pretreatment could mitigate membrane fouling by adsorbing a certain amount of membrane foulants.The combined fouling effects by HA and PAC in the presence and absence of 0.5 mmol/L calcium were investigated in terms of membrane fouling and HA transmission. The results showed that PAC and HA exhibited a significant synergistic effect when they formed fouling cake together regardless of the presence or absence of calcium. The synergistic fouling effect increased as the PAC dosage increased and as the PAC size decreased. In the absence of calcium, HA transmission increased in the presence of PAC due to the hindered back diffusion effect; and HA transmission increased as the PAC size decreased. In the presence of calcium, HA transmissions with and without PAC were almost identical. In the presence of calcium, the main mechanism of combined fouling was the steric effects; in the absence of calcium, in addition to steric effect, the hindered foulants back diffusion effect was also involved, as evidenced by the increase of HA transmission.The deposition of PAC on the membrane surface was investigated using model foulants. It was found that PAC itself rarely deposited on the membrane surface, but the mass of deposited PAC was substantially increased during the operation of the PAC/UF process. Organic matter and Ca2+ played a crucial role in the deposition of PAC. For HA, the mass of deposited PAC increased as the increase of the mass of deposited HA and as the decrease of PAC size. According to the theory of XDLOV, the attractive force between PAC and membrane decreased after HA adsorption, suggesting that the change of the interaction between PAC and membrane was not the reason of PAC deposition. The reason of PAC deposition was:(1) HA filled the space between PAC and membrane, and thus increase the contact area between PAC and membrane;(2) HA filled the space between PAC particles, and thus PAC was trapped by the deposited HA.The PAC/UF process could effectively remove turbidity(turbidity in the effluent was 0.041 NTU) and organic matter(removal was ~50%). When the PAC retention time was higher than 6 d, the process could also eliminate ~3 mg/L ammonia in the feed water. The PAC/UF process could be operated at an intermittent aeration mode, and the aeration interval could be determined according to the settling characteristic of PAC; the energy consumption of intermittent aeration was 0.085 k W·h/m3 less than that of continuous aeration. The volatile organic sulfides could not be effectively adsorbed by PAC, and could be partly removed by air stripping using the aeration system of the process. The PAC/UF process had the risks of membrane clogging and membrane breakage. In short, the PAC/UF an effective process for micro-polluted water treatment.