SDI filter pads test turbidity seawater

What is Silt Density Index SDI Definition

The Silt Density Index (SDI) is an analytical procedure which measures the fouling potential of a sample stream, it is basically the parameter most used to determine how much pretreatment is required in designing Reverse Osmosis Systems. In reality, it is a very rough indicator of fouling potential. SDI is based upon the principal that when a filter gets plugged, less water will pass through at the same pressure. Less water passes through because of the resistance to flow caused by particles plugging the spaces in the filtering medium.

silt density index SDI clean dirty filter fouling

Silt Density Index SDI for Clean vs Dirty/Fouled Filter

A sample stream of the raw water is filtered through a new 0.45 micron membrane filter for fifteen minutes. The amount of time required to pass 500 ml through the filter is measured initially, (when the membrane filter is clean), and again after fifteen minutes worth of suspended material have been filtered. The reduction in flow rate (longer time to pass 500 ml at the end of the 15 minute period) is a crude measurement of the fouling potential of the stream.

SDI filter pads test turbidity seawater

SDI filter pads after testing turbidity in seawater

Most spiral wound RO membrane element manufacturers require that an RO System feed water has a fifteen minute SDI value (SDI15) of less than 5 (one manufacturer requires less than 4). An SDI15 of 5 is obtained when it takes four times longer to pass 500 ml after fifteen minutes worth of suspended solids have been deposited on the membrane than it did during the initial 500 ml. Most hollow fiber element manufacturers require that the SDI15 be less than 3.

When a membrane element supplier requires that an RO System feed water SDI15 be less than 5, this means that the feed water SDI15 must be less than 5 as it enters the Reverse Osmosis System The problem is that the particles may grow. In the case of living particles, they grow and reproduce like any organism. They add mass by accumulating nutrients from the environment. They add numbers by reproduction. Nonliving particles may grow in an Reverse Osmosis System as well. They grow by physical/chemical means called coagulation or agglomeration.

The purpose of our pretreatment equipment is to remove the relatively large particulate matter from the feed stream so it does not plug the Reverse Osmosis Systen and to limit the number of living particles entering a unit. The purpose of dispersant addition is to then keep the smaller, especially nonliving particles from agglomerating (growing larger) and remaining in the RO System.

silt density index SDI reverse osmosis system

Silt Density Index SDI and Reverse Osmosis System

Since SDI is a measurement of both living, dead, and nonliving particulate matter greater than 0.45 micron, it is a useful tool to determine the fouling potential of a feed water and how much pretreatment equipment will be required. Some general guidelines on what it will take to maintain an RO unit’s feed water SDI at less than 4 – 5 are:

RAW WATER SDI PRETREATMENT REQUIRED TO ACHIEVE SDI < 5
Greater than 10 – 20 Clarification, MMF, CF, +/- Dispersant (plugs in less than 5 – 10 mns)
7 – 8 TO 10 – 20 MMF (with polymer addition), CF, +/- Dispersant
5 TO 7 – 8 MMF, CF, +/- Dispersant
Less than 5 CF, +/- Dispersant

Read more about Silt Density Index SDI Measurement

industrial brackish water reverse osmosis system bwro

What is a Reverse Osmosis System – RO System Definition

A Reverse Osmosis System is basically the application of the reverse of the Osmosis process. Where pure water is produced out of brackish or seawater by applying a pressure to the concentrated salt solution above the applied and osmotic back pressures. An Industrial Reverse Osmosis System works the same way as illustrated. Net driving pressure (NDP) forces water through the membrane. In an operating Reverse Osmosis system, feed water is pressurized by a high pressure pump. Due to Net Driving Pressure (NDP), a portion of the feed water is forced through the Reverse Osmosis semipermeable membrane.

The membrane is completely impermeable (won’t allow passage) to particles and only slightly permeable to dissolved substances. The water that passes through the membrane is called permeate. Permeate usually has very few particles in it. Unless there is a membrane defect (hole) or other problem, any particles found in the permeate were produced there (either from bacteria or equipment). Permeate is also low in dissolved substances (a small amount of dissolved solids does pass through the membrane). Permeate, therefore, is a relatively high purity water. Figure below illustrates a Reverse Osmosis System in the format we have been using so far.

Reverse Osmosis System Illustration

Reverse Osmosis System (RO System) Illustration

Reverse Osmosis System Operation and Flushing

We know that when we pressurize the feed water and water passes through the membrane, the feed water is concentrated. If the concentration in the feed water gets high enough, the osmotic back pressure will rise to eventually give us a Net Driving Pressure (NDP) of zero and the flow will stop. In a Reverse Osmosis System, then, we must flush away the dissolved substances from the membrane surface so that the osmotic back pressure won’t keep going up. This is different from the other filters that we usually work with. Most of the filters that we have dealt with in our lives have been “full flow”, “accumulative” types of filters. “Full flow” means that there is one stream in (feed water) and one stream out (filtrate). “Accumulative” means that the filtered “stuff” accumulate in or on the filter.

From coffee filters, to cartridge filters, to multimedia filters, this has been the case. The feed water goes in, the filter removes the “stuff” that we want to take. When the filter gets full, we backwash or replace the filter. Membrane systems can’t be full flow or accumulative. With an RO membrane, we are filtering out ions which have an osmotic pressure. What would happen if we continue to filter out dissolved substances which produce an osmotic back pressure? Answer: The process would stop.

A Reverse Osmosis System therefore, must have a flushing flow which carries away the dissolved and suspended substances. This waste stream is called concentrate. A Reverse Osmosis System (RO System), therefore, has one stream going into it (Feed Water), and two streams coming out (Permeate & Concentrate).

The following illustration also shows a Reverse Osmosis System with a 100 gpm (22.7 m3/hr) feed water flow. The Net Driving Pressure (NDP) supplied by a high pressure pump forces around 75% of the feed water through the membrane. The water, suspended particles, and dissolved substances which don’t go through the membrane are concentrated and exit the Reverse Osmosis System as concentrate.

reverse osmosis system operation example

Reverse Osmosis System Operation Example

Reverse Osmosis System Contaminants Removal

Most water constituents retains on the feed side of the Reverse Osmosis membrane depending on their size and electric charge. While the purified water (permeate) passes through the membrane. Figure below illustrates the sizes and types of solids removed by Reverse Osmosis membranes as compared to other commonly used filtration technologies. RO membranes can reject particulate and dissolved solids of practically any size. However, they do not reject well gases, because of their small molecular size. Usually Reverse Osmosis membranes remove over 90 percent of compounds of 200 daltons (Da) or more. One Da is equal to 1.666054 × 10−24 g. In terms of physical size, RO membranes can reject well solids larger than 1 (Angstrom) Å. This means that they can remove practically all suspended solids, protozoa (i.e., Giardia and Cryptosporidium), bacteria, viruses, and other human pathogens contained in the source water. Reverse Osmosis membranes are designed to primarily reject soluble compounds (mineral ions) while retaining both particulate and dissolved solids.