by Rami E. Kremesti M.Sc., CSci, CEnv, CWEM

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
9. Soft water to avoid scaling otherwise dosing of appropriate anti-scalant

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

Options for pre-treatment technology are the following:

1. Coagulation-Flocculation and Filtration to remove colloidal suspended solids

2. Biofiltration in which a sand or AC filter is used as a medium to grow a population of bacteria that remove nutrients from the water that biofilm needs to grow

3. UV light can be used to kill bacteria or to reduce TOC

4. pH adjustment

5. CO2 degasification or removel by dosing NaOH

Some water chemistries require special designs. For example, water that is high in Silica, needs to be operated at a higher pH in which Silica is in its anionic form and does not form colloidal silica. High TOC waters are best operated in HERO mode, High Efficiency RO, which is RO at high pH where organics are anionic in form and stick less to the membrane surface.

Sea water is high in Boron and at acidic pH less Boron is rejected so it ends up in the permeate which is an issue in case the permeate is needed for potable water applications.

Some RO systems are designed to treated sewage effluent and employ techniques such as Direct Osmosis High Salinity to "backwash" the membranes from foulants every day.

A designer 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. The TDS of the water and the lowest winter temperature define the HP pump's rating. Ofcourse the design flow defines the number of membranes.

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
7. Temeparture

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. For example, RO permeate can be very soft and will damage steel based piping, for that reason, addition of Calcium and Carbonate alkalinity is a common post-treatment step. Sometimes Fluoride is added to potable water to protect the teeth.

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

The rest of the design process invovles selection of appropriate piping, a skid to carry all the equipment, instruments and valves as well as a control panel with a PLC. It is very important to have a non-return valve on the permeate site to prevent back-pressure which can delaminate and physically damage the RO membranes.

Advantages of RO:

1. No regeneration chemicals such as in Ion Exchange

2. Smaller footprint

3. Requires less energy that thermal desalination

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

Proper O&M practices are needed to maximise the design life of an RO system. For example, if the permeate flow drops by 15% or the feed pressure increases by 15% it means it is time for a CIP cleaning. The longer the time between drop in performance and CIP cleaning, the more the irreversible the fouling.