Colloidal substances, together iron hydroxide and biofilm, are deposited in reverse osmosis (RO) or nanofiltration (NF) membranes, causing them to clog and reducing their lifetime.
The Silt Density Index (SDI) is a measure of the potential colloidal deposits that can be formed in membranes: high SDI values can cause irreversible blockage.
It is therefore essential to install a pre-treatment system capable of reducing colloidal deposits.
The SDI test determines how quickly colloidal substances and particulate fouling will clog a given membrane: for example, as shown in Table 1, SDI values >5 are too high, and pre-treatment with other, lower-cost filter media is therefore required.

SDI < 1 Functioning for many years without fouling
SDI < 3 Functioning for many months without fouling
3 < SDI < 5 Frequent fouling
SDI > 5 Not acceptable – pre-treatment required

Table 1

We at Zeocel Italia tested the natural zeolite ZEOWATER ZN using a standard method for measuring SDI.
Zeowater ZN reduced more than 85 % of fine particles with a size of 5 to 15 microns, and totally eliminated particles with a size larger than 30 microns!

Particulate (µm) SDI reduction
5,00 – 15,00 88,0 %
15,00 – 30,00 94,2%
30,00 – 50,00 100,0 %


Zeowater ZN is a natural zeolite product used for water filtration.
This zeolite reduces turbidity, suspended solids, and apparent colour, acting directly on the physical retention of suspended particles in water and effluents.

Thanks to our zeolite ZEOWATER ZN it is possible to:

  • – Maximise the membrane lifetime
  • – Reduce chemical costs
  • – Reduce operational costs

Find out more about Zeowater ZN

Contact us!