Silt density index

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 

Procedures to run an SDI test call for a 0.45-micron filter pad to be placed with the shiny side up in the filter holder. A small squirt of water from a water bottle is used to wet and completely seat the filter pad. (If the pad shows signs of air bubbles or erratic coverage of foulant of the pad upon removal from the filter holder, the filter was not seated properly and the SDI test is invalid). The isolation valve is opened and the pressure regulator is set to 30 psi. The time it takes to collect 500 ml of water through the 0.45-micron filter pad is then recorded. After this volume of water has been collected, water is allowed to run through the filter continuously for a total time of 15 minutes. If necessary, the pressure should be adjusted to remain at 30 psi. At the end of 15 minutes, the time it takes to collect another 

500 ml of water is recorded. Both recorded times are then entered into Equation 3.9 to calculate the SDI (note that SDI is unit-less)  

The test must be run at 30 psi. As mentioned above, if the pressure drops during the test, it should be adjusted back to 30 psi. 

The maximum SDI,. that can possibly be calculated from equation 3.9 occurs when the time to collect the final 500-ml of water, tn, is infinite. The maximum SDI15 then becomes (1/15) X 100 or 6.7. 

The lower the SDI, the lower the potential for fouling a membrane with suspended solids. Membrane manufacturers require that the SDI be less than 5 to meet their warranty conditions (Hydranautics warranty states an SDI of less than 4 is required). However, even though the SDI maybe less than 5, the water may still be capable of fouling the membranes. In practice, an SDIjj of less than 3 is usually necessary to reduce the potential for fouling to an acceptable level. 

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SDI the Silt Density Index, a measure of the rate at which the water blocks a 0.45 im filter. 

SEAWATER SILT DENSITY INDEX AS OBSERVED AT DIFFERENT LOCATIONS... 

MM pressure filtration has tended to become standard practice in many parts of the world as an RO pretreatment, and is designed to reduce the maximum feed-water turbidity below 1 NTU and the silt density index (SDI) below 5. 

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 5.21 The silt density index (SDI) test [49]. Reprinted with permission from Noyes Publications...

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.

ASTM Standard D4189-95 (2002). Standard test method for silt density index (SDI) of water. The American Standard for Testing Material. 

AFM atomic force microscopy SDI silt density index... 

Figure 3.6 Silt density index test apparatus and ancillary equipment.

Figure 3.7 Silt density index pads taken before and after a filter treating RO influent water.

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). 

Feed Water Source Silt Density Index Average Flux, gfd Conservative Flux, gfd ... 

SDI Silt density index tests should be conducted on a daily basis following start up, but once the system has reached equilibrium, weekly readings can be taken. Note that if the make-up water is a surface source, SDI may need to be taken daily to catch changes due to weather conditions (e.g., heavy rains), ship traffic, and so forth. 

There are two other types of technologies—reverse osmosis (RO) and electrodeionization (EDI)—that remove dissolved solids from a liquid stream. Both are widely used in the water purification industry and have potential for use in the treatment of CMP wastewater. RO is a method by which water is forced through a semipermeable membrane that does not allow ions to cross. EDI removes ions from a liquid stream by means of an applied voltage. Both the EDI or RO are very effective in removing anions and cations, but the tradeoff is that the feed to the EDI or RO must be preconditioned to prevent damage to the equipment. In particular, the feedwater to an RO should not have a silt density index (SDI) greater than 3.0, which may require additional filtration to ensure all the solids are removed from the liquid stream. Some EDI manufacturers recommend that the feedwater to the EDI be RO permeate or of better quality. 

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