How to predict RO membrane fouling potential?

Let us repeat a well known but frequently overlooked fact in Reverse Osmosis (RO) installations: The pre-treatment section is absolutely critical for the smooth operation of the RO unit and the long life of its membranes.

Fouling of membranes causes higher energy use, higher cleaning frequency and shorter life of the membranes.

What are the causes of fouling?

Sources of fouling can be divided into four main categories: scale, silt (very small, fine particles), bacteria (bio-fouling, microbiological slime) and organic fouling (from organic substances such as oil, grease).

The control of fouling requires proper design of the pre-treatment of the feed water in order to minimize fouling (multimedia filters, cartridge filters, chlorination-dechlorination) as well as regular cleaning to handle the fouling that still occurs.

Fouling by particulates (silt), bacteria and organics generally mostly affects the first modules in the RO unit. Scaling affects mostly the last modules in the RO unit, because they are exposed to the most concentrated water.

Silt is composed by suspended particulates of all types that accumulate on the membrane surface. Sources of silt are colloids, iron corrosion products, precipitated iron hydroxide, algae, and fine particular matter.

How to predict membrane fouling potential?

The standard way to predict fouling is by measuring the Silt Density Index (SDI) of the feed water. The method, which is based on experience, is described in ASTM test D4189-82. Silt Density Index testing is a widely accepted method for estimating the rate at which fouling will occur in membrane systems, especially using Reverse Osmosis (RO) or Nanofiltration (NF) membranes.
What is the Silt Density Index (SDI)?

SDI measures the time required to filter a fixed volume of water through a standard 0,45µm pore size microfiltration membrane with a constant given pressure of 30 psi (2,1 bar). The difference between the initial time and the time of the second measurement, after normally 15 minutes (after the built up of fouling), represents the SDI value.

When the SDI is high, one can conclude that the feed water contains a high amount of “membrane plugging materials”.

The SDI can effectively predict the fouling potential from all above mentioned sources except scale, as scaling mostly develops when the water is concentrated inside the RO unit.

 

SDI measurement Procedure & Calculation

Portable SDI kits are available. They use a standard 47mm diameter membrane filter of 0,45 μm pore size.

  • Adjust feed pressure to 30 psi (2,07 bar) and measure the initial time t0, necessary to filter 500ml of water. Keep feed pressure constant
  • Keep the filter in operation for 15 min
  • After 15 min, measure again the time t1 necessary to filter 500 ml of water
  • Calculate SDI = (1 – t0/t1) x 100/15

For example, if it takes 4 times longer to filter the second 500ml of water, after 15 min, compared to the first 500ml, then

  • SDI = (1 – 0,25) x 100/15 = 5

Useful Guidelines

The guideline is to maintain SDI at less than or equal to 5. A water sample that totally blocks the membrane has an SDI value of 6,7

The SDI values give the following “prediction” for reverse osmosis:

SDI < 1               Several years without colloidal fouling

SDI < 3               Several months between cleanings

SDI 3 – 5           Fouling is likely to be a problem, frequent cleaning will be required

SDI > 5               Unacceptable, additional pre-treatment is needed

 

The target SDI after filtration is normally 3 – 5 or less. Surface or seawater may have a very high SDI, requiring coagulation, flocculation, and deep-bed multimedia filtration before RO treatment.

 

Points to remember

  • Buy a portable SDI kit and measure SDI during service visits to your customers. Explain how SDI measurements are related to the protection of the customer investment
  • Check the specification sheet of the membranes used in your customer’s installation, they usually specify the maximum accepted SDI
  • If the feedwater is surface water, recommend to your customer to buy an SDI kit and to measure SDI daily
  • When considering the installation of a new RO unit make sure to check the SDI of the proposed feedwater and recommend a pre-treatment design based on the results of the measurement