Limitations, improvements, alternatives for the silt density index

Reverse osmosis (RO) and nanofiltration (NF) membrane systems are widely used in the desalination of water. However, fouling phenomena in these systems remains a challenge. Four main different fouling types are identified: 1) Particulate fouling due to suspended and colloidal matter, 2) Biofouling due to adhesion and subsequent growth of bacteria, 3) Organic fouling due to organic compounds and 4) Scaling due to precipitation of sparingly soluble compounds. Silt Density Index (SDI) testing is a widely-accepted method for estimating the rate at which colloidal and particle fouling will occur in water purification systems when using RO or NF. During the SDI test the time required to filter a fixed volume of water through a standard microfiltration membrane at a constant given pressure is measured. The difference between the initial time and the time of a second measurement after 15 minutes (after silt built-up) results in the SDI value. The ASTM describes this test as a standard test for RO fouling potential due to particles. According to the standard, the applied pressure is 207±7kPa (30±1psi). The water temperature must remain constant (±1°C) throughout the test. In practice, the SDI is used most often and has been applied worldwide for decades. From a practical point of view, the SDI for fine hollow fiber RO feed water preferably must be lower than 3. A pretreatment method such as UF therefore has to guarantee an RO feed water with an SDI <3. An SDI test is one of the criteria in designing new desalination plants and has to be performed on the RO feed water. SDI is a useful tool to monitor the efficiency of the RO pretreatment in removing the particles presents in the raw water. The main advantage of the SDI test is that the test is simple to execute even by non-professionals. The SDI test is used to choose and design RO pretreatment processes. The SDI has an economical value since it is mentioned as a condition in the pretreatment process contract. Although the SDI test is widely used, there is growing doubt about the value of the SDI test as a predictive tool for RO membrane fouling. These doubts consist of two factors: 1) the relation between the SDI value and the performance of the RO unit, and 2) the reproducibility and accuracy of the SDI test.