Notice of RetractionPilot Study on Hybrid Biological Filter-Coagulation-Submerged Ultrafiltration Membrane for Drinking Water Treatment
Two parallel hybrid processes (Biological filter Coagulation/Ultrafiltration(BCUF) and Coagulation/Ultrafiltration(CUF)) for treating slightly polluted surface water were carried out in this experiment. In the process of BCUF, transverse flow water-dropping aerated biological filter was used as a primary pretreatment, and coagulation was used as the subsequent pretreatment of Ultrafiltration(UF). In the CUF process, coagulation was the unique pretreatment of UF. Turbidity was removed nearly completely in both two hybrid processes, and in the BCUF, effective ammonia nitrogen removal (87.1%) was also achieved mainly through the biological nitrification carried out in the biological filter, however, only 25.6% removal of ammonia nitrogen was achieved in the process of CUF. The removal rates of CODMn, TOC, DOC, UV254, AOC and BDOC were 55.2%, 87.8%, 71.0%, 61.1%, 91.4% and 89.3% respectively in the process of BCUF, which were much higher than that in the process of CUF. The biological pretreatment could improve the biological stability significantly and relieve the membrane fouling which made the transmembrane pressure (TMP) arising velocity lower. The pretreatment of biological filter and coagulation could remove the organic molecules in the range of 5000-500Da. Results showed the water-dropping aerated biological filter was fit for the pretreatment of UF.
Coagulation and ultrafiltration: Understanding of the key parameters of the hybrid process
Abstract A hybrid coagulation–ultrafiltration process has been investigated to understand membrane performance. Coagulation prior to ultrafiltration is suspected to reduce fouling by decreasing cake resistance, limiting pore blockage and increasing backwash efficiency. Coagulation followed by tangential ultrafiltration should gather the beneficial effects of particle growth and cross-flow velocity. Our study aims at determining the key parameters to improve membrane performance, by describing floc behaviour during the hollow fibre ultrafiltration process. Flocs encounter a wide range of shear stresses that are reproduced through the utilization of different coagulation reactors. Performing a Jar-test enables the formation of flocs under soft conditions, whereas Taylor-Couette reactors can create the same shear stresses occurring in the hollow fibres or in the pump. Synthetic raw water was made by adding bentonite into tap water. Five organic coagulants (cationic polyelectrolytes) and ferric chloride were selected. Floc growth was thoroughly monitored in the different reactors by laser granulometry. Coagulation–ultrafiltration experiments revealed different process performance. The effect on the permeate flux depended on the coagulant used: some coagulants have no influence on permeate flux, another enables a 20% increase in permeate flux whereas another coagulant leads to a decrease of 50%. Flocs formed with ferric chloride do not resist shear stress and consequently have no influence on permeate flux. These results show the necessity to create large flocs, but the size is not sufficient to explain membrane performance. Even if flocs show a good resistance to shear stress, a high compactness ( D f  = 3) will lead to a dramatic decrease of permeate flux by increasing the mass transfer resistance of the cake. On the contrary, flocs less resistant to shear stress, then smaller and also more open have no effect on permeate flux. An optimum was quantified for large flocs, resistant enough to shear stress facilitating flow between aggregates.
Preparation of hydrophilic poly(vinyl butyral) hollow fiber membrane via thermally induced phase separation
Abstract Hydrophilic microporous hollow fiber membranes were prepared via thermally induced phase separation (TIPS). Poly(vinyl butyral) (PVB) and three kinds of polyethylene glycols (PEG) with the molecular weight of 200, 400 and 600 were used as polymer and diluents. In all cases, liquid–liquid (L–L) phase separation occurred and asymmetric cellular structures with the smaller pores at the outer surface were obtained. The properties of membranes such as pore size, water permeability, solute rejection and intensity were investigated by changing molecular weight of PEG, concentration of PVB, composition of the bath and air gap distance. The smaller molecular weight of PEG, the more PEG in the bath and the shorter air gap distance resulted in the higher water permeability, the lower solute rejection property and the lower mechanical strength.