Because of the complexity of the fouling phenomenon, predictive correlations of flux decline with time are important in process design. Most of the studies made in this area were conducted with the assumption that mathematical modeling is nearly impossible due to the nature of the fouling process. Only two models, the silt density index (SDI) and permanganate demand (PD) have become widely accepted.
Silt is composed of suspended particulates of all types that accumulate on the membrane surface. Sources of silt are usually organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particulate matter (Table 3). The SDI has been very popular for the prediction of fouling caused by colloidal and particulate matter in most of the RO and nanofiltration (NF) membrane processes. It is based on the time required to filter a volume of feed solution through a membrane filter at a fixed hydrostatic pressure or the rate at which a membrane becomes plugged at
Table 3 Silt density index values for selected common feedwaters
the feed pressure. In the accepted method, the filtration apparatus consists of a 0.45 mm cellulose acetate membrane filter and a feed pressure of 30psig. Two variables are measured in the SDI procedure: the first is the length of time required to filter 500 mL of feed through a membrane filter. The filtration is continued for an additional 5—15 min, depending on feedwater quality. The time to filter a second 500 mL of the same solution is recorded.
The two times recorded are translated onto a plugging factor (PF):
where t1 is the time required to filter the first 500 mL of feed solution, i2 the time required to filter the second 500 mL of the same solution, and T the time of continuous filtration. The value of T is reduced, if [1 — (t1/t2)] is greater than 0.75.
SDI could be a good parameter to evaluate the fouling potential of water with low turbidity and suspended solids. However, it does not identify every kind of fouling potential as in the case of dissolved iron . There were also reports of RO membranes getting fouled by using seawater with an SDI of less than 1 that underwent chemical pretreatment .
The PD test was devised after the fouling dependence was studied on the concentration of aromatic hydroxyl species in membrane feed was studied. Permanganate lowers fouling potential through many reaction pathways. Among the reaction possibilities are electron abstraction, hydroxide ion removal, oxygen donation to organic compounds, and formation of manganous ion in acid solutions. Studies have shown that permanganate oxidizes many substances. The advantage of the PD test is versatility. Permanganate concentrations are readily detectable through
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