Silt Density , fouling

Silt is formed by suspended particulates of all types that accumulate on the membrane surface. Typical sources of silt are organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particulate matter. A good predictor of the likelihood of a particular feed water to produce fouling by silt is the silt density index (SDI) of the feed water. The SDI, an empirical measurement (ASTM Standard D-4189-82,1987), is the time required to filter a fixed volume of... 

Figure 10.1. Experimental procedure for the determination of the colloidal index (Cl). The Cl or the silt density index (SDI) test is used to predict and prevent particulate fouling on the membrane surface. It measures the time required to filter a fixed volume of water through a standard 0.45- xm pore-size microfiltration membrane with a pressure of 2.07 bar. The difference between the initial time and the time of a second measurement after normally 15 minutes (after silt was built up) represents the Cl or SDI value.

Silt density index (SDI) is a test that measures the potential of an influent water to foul an RO membrane with suspended solids and colloids. The test involves timing flow of the influent water through a 0.45-micron filter pad.5... 

Silt density index measures suspended solids, particularly colloids, such as alumina- or iron silicates, clay, iron corrosion products, and microbes, that have a great potential for fouling RO membranes (see Chapter 3.8 for more details about SDI). The SDI should be as low as possible to minimize fouling of the membranes, but must be less than 5 to meet warranty requirements set by the membrane manufacturers (best practices call for SDI in RO feed water to be less than 3). Note that there is no direct correlation of turbidity to SDI, other than high turbidity usually means high SDI (the converse is not always true). 

Binovi R. (1983), A replacement for the silt density index permanganate demand to predict reverse osmosis membrane fouling, Dissertadon, University of Cinciimati. 

SILT DENSITY INDEX - A measure of the tendency of a water to foul a reverse osmosis membrane, based on time flow through a membrane filter at constant pressure. 

Attempts to quantify the fouling propensity of a feed water such as the Silt Density Index (SDI) and Modified Fouling Index (MFI) have met with limited success [128] due to the complex interactions between membrane and foulant. Fouling by natural organic matter depends critically on solution pH, ionic strength, and the presence of divalent cations due to changes in macromolecular structure [129]. Techniques for monitoring biofilm and scale formation are summarized in the literature [130]. 

Short flow path disc-tube (DT) RO membrane modules developed by Rochem for treating highly turbid waters and folding feeds are now used extensively to minimise concentration polarisation, seating and fording. Typically, for RO systems the Silt Density Index (SDI) of feed water must be less than 5 to prevent premature fouling. 

Silt Density Index (SDI) It is an empirical measurement ASTM Standard D-4189-82, 1987) used for measuring the colloidal fouling potential of RO/NF feed water. It is not a direct measure of the particle concentration, which is more properly measured by turbidity. The SDI value is based on the time required to filter a volume of feed water throu a 0.45 pm filter pad at a feed pressure of 2.1 bar g. The SDI value is calculated from the formula ... 

Silt density index (SDl) is a sensitive method for determining the ability of a filter to remove colloidal particles 14). SDl (in min ) is used extensively as a criterion in minimizing fouling of reverse osmosis membranes. The lower the SDl value, the cleaner is the stream. Manufacturers of reverse osmosis membranes recommend that the stream be prefiltered so that it has an SDl factor less than 3.0. Typically 1 pm absolnte 6) filters have an SDl of about 4-5. Manufacturers of hallow fiber membrane filters claim SDI s in the range of 1.75 to 2.25. SDl measurements of effluents from the nano alumina media range from 0.5-1.0. The SDl of 32% PAC media was measured and found to be about 1.0. Turbidity as well as SDl tests have confirmed that the extent of shedding of PAC particles into effluent streams is minimal. 

The designer of the RO system chooses the flux rate it is not a property of the membrane. In general, the flux that an RO system is designed for should be a function of the influent water quality. This is because higher flux results in more rapid fouling of the membranes. So, the lower the influent water quality, the lower the operating flux of the RO system should be. Table 3.3 shows the recommended flux as a function of influent water source (which is an indirect measure of the water quality) and silt density index (SDI), which is a measure of the tendency of water to foul a membrane (See Chapter 3.9). When in doubt, a default flux of 14 gfd is usually recommended. 

Silt Density Index The SDI is a commonly used test to predict a feed water s potential to foul a membrane by colloidal particles smaller than 0.45 xm. It is only a guide for pretreatment and is not an indication of adequate pretreatment. The SDI is a static measurement of resistance, which is determined by samples taken at the beginning and at the end of the test (EPA, 2002). 

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