Membrane scaling

Membrane scaling involves the deposition of saturated salts on the surface of the membrane, typically in the later stages of the RO system. Scale forms a layer on the surface of the membrane that becomes an additional barrier for water to have to flow through to the permeate side of the membrane, similar to that described above for foulants. 

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Electrodialysis can be applied to the continuous-flow type of operation needed in industry. Multi-membrane stacks can be built by alternately spacing anionic- and cationic-selective membranes. Among the technical problems associated with the electrodialysis process, concentration polarization is perhaps the most serious (discussed later). Other problems in practical applications include membrane scaling by inorganics in feed solutions as well as membrane fouling by organics. 

To overcome membrane scaling, the operating pH of the feed brine to the unit was lowered to a range between 4 and 7. A simple modification was made to the plant to control the pH of the plant feed brine by mixing acidic dechlorinated brine with alkaline dechlorinated brine. This modification has proven to be effective and no further membrane fouling has occurred over the last two years. 

Exhibits greater resistance to membrane scaling and fouling than other technologies. 

LoDICULE One of the delicate membranous scales borne on the torus outside of the stamens in the grass floret and probably representing a perianth part. 

Because of the relatively high concentrations of the acid and base as well as the salt solution the limiting current density is in general no problem and a bipolar membrane stack can generally be operated at very high current densities compared to an electrodialysis stack operated in desalination. However, membrane scaling due to precipitation of multivalent ions such as calcium or heavy-metal ions is a severe problem in the base-containing flow stream and must be removed from the feed stream prior to the electrodialysis process with a bipolar membrane. 

Table 3.5 Generally-accepted water quality guidelines to minimize RO membrane scaling.

Membrane scaling and degradation can lead to a loss in normalized salt rejection as can breaches O-rings and permeate tube. 

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

Membranes are typically not cleaned due to a drop in salt rejection. This is because in most instances, there is a mechanical explanation for the drop in salt rejection (see Chapter 12.2.2). However, in the case where membrane scaling is responsible for a drop in salt rejection, normalized permeate flow is generally the first indicator of this phenomenon (see Chapter 11.3.1.2)... 

Membrane Scaling Protection High Moderate to High+... 

Enoch et al. [90] used electrodialysis reversal (EDR) to prepare boiler makeup water for Dutch power stations from several types of surface waters. EDR uses automatic reversal of electrode polarity at regular time intervals to minimize membrane scaling. The EDR unit contained 200 anion and cation exchange membrane pairs, each with a surface area of 0.47 m. Polarity reversal occurred every 15, 20, or 25 min. Samples of surface water were desalted by 96% at an energy consumption of 1 kWh/m of product water and at a current density (8.3 A/cm ) that was 80% of the limiting current density (current density when the surface water cation concentration at the membrane surface drops to zero). 

Membranes scale to a hydrogen purification application based on a simple linear relationship if hydrogen throughput is increased by two times then, the required membrane area will increase by two times (all other factors held constant). This... 

Fig. 31 Double-immunofluorescent staining of 20-day rat cerebellar sections with antibodies to GD, gangli-oside and glial acidic fibrillary protein (GFAP). Purkinje cell dendrites are intensely GDj-immunoreactive (A) but do not extend to the pial surface, unlike the Bergmann glial fibers (B), which project brightly GFAP-immunoreactive end-feet onto the pial membrane. Scale bar is 35 /tm. Reynolds and Wilkin (1988).

Fig. 48. Electron micrograph of the synaptic distribution of immunoreactivity for the GluRl subunit of the AMPA receptor in rat cerebellum as detected by an antibody against the carboxy-terminal (intracellular) region of GluRl. A. A spine (s) emerging from a Purkinje cell dendrite (Pd) establishes an immunopositive type 1 synapse (solid arrows) with a parallel fiber terminal (pft). Intra-cellular immunoreactivity is present inside Bergmann glial cell processes along dendritic elements (e.g., open arrow). B. The peroxidase reaction end-product labels the postsynaptic density (psd) at the intracellular face of the postsynaptic membrane (pom) and not the synaptic cleft between the presyaptic (pem) and postsynaptic (pom) membranes. Scale bars in A = 0.5 /xm, in B = 0.1 jum. Baude et al. (1994).

This chapter reviews some fundamental science critical for the understanding, development and operation of many classes of dense composite inorganic membrane used for transport of hydrogen. A companion paper follows in this volume discussing some of the engineering issues of membrane scale-up. 

A scale-up hydrogen separation unit was designed with a membrane surface area of 63 cm. This unit, shown in Fig. 9.5, allows investigation into planar membrane scale-up variables such as supports, thermal expansion, and sealing. Outer diameter of the vessel is 15 cm. The unit was tested at 380°C, and, under full... 

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