Silting Density Index
Silting Density Index, referred to as SDI value, is one of the important parameters of water quality indicators.
The silting density index (SDI) is one of the important indicators for measuring the water inlet of the reverse osmosis system, and it is also the main means to check whether the effluent from the pretreatment system meets the reverse osmosis water inlet requirements. Its size is crucial to the operating life of the reverse osmosis system. It characterizes the content of particles, colloids and other objects in the water that can block various water purification equipment. By measuring the SDI value, the corresponding water purification technology or equipment can be selected.
Silting Density Index Description
SDI (Silt Density Index), also known as the sludge density index, is an important indicator that characterizes the water quality of reverse osmosis influent. Unlike turbidity, turbidity is the use of a spectrophotometer or visual turbidimetry to determine the content of particulate impurities in water, but cannot accurately measure some non photosensitive colloidal particles in water. The relationship between turbidity and SDI is not necessarily proportional: water with low turbidity may have high SDI values, because there are a considerable number of pollutants that can penetrate light and contaminate the membrane in the water, which are often undetectable when measuring the turbidity of raw water. The general requirement for reverse osmosis influent is SDI<5, with a maximum turbidity limit of 1NTU, and generally controlled between 0.3-0.5NTU.
Silting Density Index Disadvantages
Silt Density Index is very useful as an indicator for raw water pretreatment, but it cannot provide sufficient information for membrane design and control. The limitation of this test is that it cannot reflect the interaction between pollutants in the feed water and the membrane, which does exist in practical operations.
At the same time, the interception characteristics and flow mechanism of the test unit are different from the membrane components in the actual process. Therefore, experimental research and in-depth understanding of the mechanisms controlling material transfer and decreasing permeation flux are necessary for the treatment of a given feed water.
Standard Test Conditions for Silt Density Index
The standard test conditions of SDI are to first pass the raw water through a 47mm diameter membrane filter, and then conduct the test at 0.2MPa. The total test time t is 900s. Although the measurement of SDI is not absolute, it is the best method found to measure colloid content. A correlation between SDI and colloid deposition rate in permeators has been obtained.