Membrane processes secondary flow

The performance of a membrane process is a function of the intrinsic properties of the membrane, the imposed operating and hydrodynamic conditions, and the namre of the feed. This chapter describes methods available to enhance performance by various techniques, mainly hydrodynamic but also chemical and physical. The focus is on the liquid-based membrane processes where performance is characterized by attainable flux, fouling control, and separation capabilities. The techniques discussed include secondary flows, flow channel spacers, pulsed flow, two-phase flow, high shear devices, electromagnetic effects, and ultrasound. 

The next important postulate of Mitchell s theory concerns the consumers of the energy produced by primary pumps and presupposes the presence in the organella membranes of secondary proton pumps which use the transmembrane proton flow for ATP synthesis and a number of other processes. Essential to this theory is the... 

A recent development of the secondary flow-enhanced membrane processes is the helical membrane module, which is characterized by a DNA helical-like spacer enveloped by polyester filter cloth with a pore size around 22 pm [31-33]. Figure 10.12 shows the schematics of the helical membrane spacers with different twisted helical angles. The main parameters of the helical membrane include the angles of twist of the spacer or membrane and the ratio of the membrane width to the length. 

Many treatment facilities at different locations were installed to produce water from wastewater for different uses. In some cases, MF membranes are used directly on strained wastewater to remove suspended particles that are too large for the gap between two membranes [30], Simple wastewater-treatment facilities in Europe exist along all large rivers. Secondary treated waters flowing into the rivers are again pumped at a distance of about 200 meters downstream, treated with active carbon and UF membranes, disinfected and then distributed to the system. This is wastewater treatment without an RO section due to the low salinity of the water. The process cannot handle dissolved medicines, hormones, drugs, and other contaminants that could be removed with RO or NF membranes. In some cases, NF membranes are used for better treatment of the water. Information on wastewater costing may be found in Adham et al. [31]. 

Many active-transport processes are not directly driven by the hydrolysis of ATP. Instead, the thermodynamically uphill flow of one species of ion or molecule is coupled to the downhill flow of a different species. Membrane proteins that pump ions or molecules uphill by this means are termed secondary transporters or cotransporters. These proteins can be classified as either antiporters or symporters. Antiporters couple the downhill flow of one species to the uphill flow of another in the opposite direction across the membrane symporters use the flow of one species to drive the flow of a different species in the same direction across the membrane (Figure 13.10). 

Chloroplast ferredoxin is a small water soluble protein M W 000) containing an Fe-S center [245]. Its midpoint potential ( — 0.42 V [246]) is suitable for acting as an electron acceptor from the PSI Fe-S secondary acceptors (Centers A and B) and as a donor for a variety of functions on the thylakoid membrane surface and in the stroma. Due to its hydrophylicity and its abundance in the stromal space, ferredoxin is generally considered as a diffusable reductant not only for photosynthetic non-cyclic and cyclic electron flow, but also for such processes as nitrite and sulphite reduction, fatty acid desaturation, N2 assimilation and regulation of the Calvin cycle enzyme through the thioredoxin system [245]. Its possible role in cyclic electron flow around PSI has already been discussed. The mobility of ferredoxin along the membrane plane could be an essential feature of this electron transfer process the actual electron acceptor for this function and the pathway of electron to plastoquinone is, however, still undefined. 

Such primary active transport systems are perfectly equivalent formally to the secondary active transport systems discussed in the previous section. Both systems require that true flows of substrate or progress of a chemical reaction must take place for coupUng to occur, for it is these processes that drive the carrier from one conformation, state or side of the membrane to the other. Both systems require absolutely that the reactions between carrier and substrate at one side of the membrane are shielded from those at the other, so that a true displacement of the carrier-binding sites from accessible at one side of the membrane to accessible at the other must take place. In the case of the primary active transports, it is the chemical reaction which brings about this vectorial movement of the carrier, since the two components of the chemical reaction, product and reactant, must combine with different forms of the carrier. In the case of secondary active transport, it is transport of the driving substrate which brings about reorientation of the carrier. 

A second aspect of the chemiosmotic coupling theory postulates that the proton-motive force (pmf) drives energy-consuming processes in the membrane by a reversed flow of protons [8] (Fig. 2). The energy of AjSji is thus either converted into ATP by a reversed action of the ATPase complex, or drives osmotic work such as the formation of solute gradients by secondary transport or drives mechanical work such as flagellar movements. 

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