The Silt Density Index (SDI) is a crucial empirical measurement used in assessing the quality of feed water, particularly in the context of reverse osmosis (RO) water treatment systems. This index, standardized under ASTM Standard D-4189-82 (1987), serves as a predictor for potential silt fouling in RO systems.

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Silt in water comprises various suspended particulates that accumulate on the membrane surface of filtration systems. Common sources include organic colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particulate matter. The presence and concentration of these particulates significantly influence the performance and maintenance requirements of RO systems.

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The SDI is determined by measuring the time required to filter a fixed volume of water (typically 500 ml) through a standard 0.45-μm pore size microfiltration membrane. This process involves:

1. Initial Filtration: Measuring the time (Ti) to filter the initial 500 ml of feed water through a 0.45 micrometer Millipore filter (47 mm in diameter) at a pressure of 30 psig(2 bar).

2. Subsequent Measurements: Continuing the flow at 30 psig and recording the time (Tf) for another 500 ml to pass through the filter after intervals of 5, 10, and 15 minutes.

3. SDI Calculation: After completing the test, the SDI is calculated using the following formula:

   SDI=(1 - (Ti / Tf)) * 100) / T

   where Tt​ is the total elapsed test time (5, 10, or 15 minutes), Ti​ is the initial time in seconds for the first 500 ml sample, and Tf​ is the time in seconds for the second 500 ml sample after Tt​ (usually after 15 minutes).

Interpreting SDI Values

• SDI < 1: Indicates excellent water quality, allowing the RO system to operate for several years without significant colloidal fouling.

• SDI < 3: Suggests good water quality, with the system capable of running several months between cleanings.

• SDI 3–5: Implies a moderate risk of particulate fouling, necessitating frequent and regular cleaning.

• SDI > 5: Represents poor water quality, requiring additional pretreatment to make the feed water suitable for RO processing.

 

The tolerable SDI value varies with the design of the membrane module. Spiral-wound modules generally require an SDI of less than 5, while hollow fine fiber modules, more prone to fouling, need an SDI of less than 3.

 

To mitigate fouling by suspended solids, feed water filtration is essential. All RO units typically include a 0.45-μm cartridge filter before the high-pressure pump. In some cases, additional filtration methods like sand filters, supplemented by flocculating chemicals (e.g., alum or cationic polymers), are necessary. The target post-filtration SDI is usually below 3–5. Groundwater sources often have low SDI values, making cartridge filtration sufficient. However, surface or seawater can exhibit SDI values as high as 200, necessitating more intensive treatment processes like flocculation, coagulation, and deep-bed multimedia filtration before RO treatment.

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The following calculator is an adapted version from (Baker 233–234). The main difference resides in the naming of the terms. In all instances Tₜ is defined by the T₅, T₁₀, and T₁₅. 

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References

           Baker, Richard W. Membrane Technology and Applications. Third ed., Chichester John Wiley & Sons, 2012, pp. 233–234.

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Inputs Required

To effectively use the SDI Calculator, you need to input the following data, all measured in seconds:

1. Ti (Initial Time): This is the time taken for a specific volume of water to pass through a filter at the beginning of the test.

2. T5 (Time at 5 Minutes): The time taken for the same volume of water to pass through the filter after 5 minutes.

3. T10 (Time at 10 Minutes): The time taken for the water to pass through the filter after 10 minutes.

4. T15 (Time at 15 Minutes): The time taken for the water to pass through the filter after 15 minutes.

Note: You must enter the Ti value and at least one of the T5, T10, or T15 values.