Designing an RO System

by Rami E. Kremesti M.Sc.

The feed water ensured for the proper operation of an RO system must conform to the following guidelines:

1. Low SDI (silt densith index) and MFI (Modified Fouling Index)
2. No grease or oil
3. Disinfected (Low BFR = biofilm formation rate)
4. no heavy metals
5. low TOC (Total Organic Carbon)
6. No chlorine or strong oxidants
7. no extreme pH
8. No colloidal Sulfur

To achieve this, the designer needs to design a proper pre-treatment system.

Need inlet water quality and desired water quality to decide the number of stages needed and select the appropriate membranes (low TDS, Brackish or sea water). Also the system recovery is a defining paramter. The higher the system recovery, the more the stages needed.

A flux rate that is not too high needs to be selected. Too high a flux rate is tempting because it gives a higher permeate flow rate for a smaller number of membranes but this increases the fouling risk. Pilot studies are helpful at this stage or design data from systems operating with similar waters are used.

The parameters needed to monitor trouble free operation of an RO are:

1. Bacteria
2. ORP (oxidant levels)
3. pH
4. Water chemistry (cations, anions, ORP, and TOC)
5. Pressure and flow at inlet, permeate and reject
6. Conductivity

The minimum number of sampling points required is listed below:

1. Intake (surface) or well, before addition of any chemicals.
2. After a clarifier, settling pond, sludge contact unit, or similar sedimentation process.
3. After filtration units (sand, multimedia, activated carbon, or other).
4. Just before the membranes, after addition of chemicals (normally after cartridge filtration).
5. Concentrate stream.
6. Permeate stream.

An RO system needs a CIP (Clean in Place Station) and is best suited for continuous operation. A high speed demin water flush is highly recommended after shutdown.

An RO system might need an anti-scalant and/or pH adjustment.

An RO system might need post-treatment.

An RO system needs chemical preservation if shut down for extended periods of time.

Advantages of RO:

1. No regeneration chemicals such as in Ion Exchange

2. Smaller footprint

Limitations/Disadvantages of RO:

1. CO2 is not rejected

2. RO needs informed operations personnel

3. Expensive membranes compared to MSF Desalination

4. Removes minerals that can be useful