Evaluation method of membrane performance in membrane distillation process for seawater desalination.

Membrane distillation (MD) is an emerging desalination technology as an energy-saving alternative to conventional distillation and reverse osmosis method. The selection of appropriate membrane is a prerequisite for the design of an optimized MD process. We proposed a simple approximation method to evaluate the performance of membranes for MD process. Three hollow fibre-type commercial membranes with different thicknesses and pore sizes were tested. Experimental results showed that one membrane was advantageous due to the highest flux, whereas another membrane was due to the lowest feed temperature drop. Regression analyses and multi-stage calculations were used to account for the trade-offeffects of flux and feed temperature drop. The most desirable membrane was selected from tested membranes in terms of the mean flux in a multi-stage process. This method would be useful for the selection of the membranes without complicated simulation techniques.

A large artificial dyke greatly alters partitioning of sulfate and iron reduction and resultant phosphorus dynamics in sediments of the Yeongsan River estuary, Yellow Sea.

We investigated sediment geochemistry, partitioning of organic carbon (Corg) oxidation by iron reduction (FeR) and sulfate reduction (SR), and benthic phosphorus (P) release, together with the P speciation in the sediments to elucidate the P dynamics in two contrasting sediments (i.e., estuarine vs. limnetic) separated by a large dyke in the Yeongsan River estuary of the Yellow Sea. In the sediments of the Yeongsan River estuary (St. YE), SR dominated the Corg oxidation pathway, accounting for 81.7% of total anaerobic Corg oxidation. Under the SR-dominated condition, H2S derived from SR reacts quickly with iron oxides to form iron sulfides, which ultimately release the P bound to Fe(III) into the pore water. The enhanced benthic P flux (0.24 mmol m-2 d-1) at the YE site accounted for 80% of the P required for primary production in the water column. In contrast, in the limnetic sediments of the Yeongsan Lake (St. YL), where high levels of CH4 accumulated, most P was bound to Fe and Al, which resulted in a low benthic P flux (0.03 mmol m-2 d-1). The results suggest that the frequent discharge of relatively P-depleted freshwater into the estuary via the artificial dyke may result in relatively P-limiting conditions in estuarine ecosystems. As a result, benthic P release from the SR-dominated estuarine sediment is a significant internal source of P in the coastal ecosystem. Our results indicate that the construction of a large dyke at a river mouth greatly alters Corg oxidation pathways and P dynamics in coastal ecosystems.

Effect of membrane type and material on performance of a submerged membrane bioreactor.

This study evaluated the impact of membrane type and material on filtration performance in a submerged membrane bioreactor (MBR) for municipal wastewater treatment. Three types of microfiltration membranes with similar pore size of 0.1 microm but different materials and types, phase-inversed polytetrafluoroethylene (PTFE), track-etched polycarbonate (PCTE) and track-etched polyester (PETE), were used. Changes in permeability with time for the PCTE and PTFE membranes appeared similarly, whereas the PETE membrane exhibited the most rapid flux decline. Lower TOC in the permeate compared to the supernatant was probably due to a combination of biodegradation by the biofilm (cake layer) developed on the membrane surface and further filtration by cake layer and narrowed pores. The faster permeability decline and higher TOC removal rate of the PETE membrane were attributed to an initial permeate flux higher than an average design flux, which led to a faster rate of fouling and thicker cake layer. Therefore, an MBR should not be operated at a flux higher than the average design flux for a specific type of membrane. A gradual increment of biomass concentration did not significantly affect membrane permeability of each membrane investigated. Dissolved organic carbon fractionation results showed that the composition of each fraction between the supernatant and permeates did not change significantly with time, suggesting that membrane hydrophobicity was not a dominant factor affecting MBR fouling in this study. The organic foulants desorbed from the PCTE membrane contained approximately 60% of hydrophobic fraction, which was probably attributable to the extracellular polymeric substances proteins released from the biomass attached to the membrane. While the total filtration resistance of the PTFE membrane was influenced by a higher surface roughness, those of the PETE and PCTE membranes, which had a similar and lower roughness, were affected by the initial operating flux.

Controlled generation of submicron emulsion droplets via highly stable tip-streaming mode in microfluidic devices.

Submicron emulsions could be produced via the tip-streaming process in a flow-focusing microfluidic device. In this article, the stability of the liquid cone and thread for tip-streaming mode could be significantly improved by employing a three-dimensional flow-focusing device, in which the hydraulic resistance was adjusted by modulating the channel heights in the flow focusing area, orifice, downstream and dispersed phase inlet channel. The pressure range for tip-streaming mode was enlarged significantly compared with two-dimensional flow-focusing devices. Therefore, monodisperse emulsions were produced under this tip-streaming mode for as long as 48 hours. Furthermore, we could control the size of emulsion drops by changing the pressure ratio in three-dimensional flow-focusing devices while the liquid cone was easily retracted during the adjustment of pressure ratio in two-dimensional flow-focusing devices. Furthermore, using the uniform submicron emulsion droplets as confining templates, polyethylene glycol (PEG) particles were produced with a narrow size distribution at the sub-micrometre scale. In addition, magnetic nanoparticles were added to the emulsion for magnetic PEG particles, which can respond to magnetic field and would be biocompatible.