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Reverse Osmosis Pre-Treatment Design Strategies

Kremesti Environmental Consulting Ltd

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Introduction

Reverse Osmosis is a best available technology for water desalination/water recycling world wide. With proper pre-treatment, the problems of fouling, scale and bio-fouling can be ideally avoided or at least minimised.

Proper Design for RO Pre-Treatment:

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

1. Low SDI (silt density index) and MFI (Modified Fouling Index) – means there should be no colloids in the water
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 an anti-scalant is needed to be dosed
10. Low in bacteria and/or N/P (Biostable)

Sometimes it is not possible to have ideal conditions for RO operation, so certain operational strategies have to be used to manage scale, fouling and biofouling.

Classical RO Pre-Treatment

Classical RO pre-treatment schemes use screening, disinfection, coagulation/flocculation, Multi-Media Filtration, SBS dosing, cartridge filters and the dosing of an anti-scalant for scaling waters.

 

 

Figure 1: Classical RO Pre-Treatment Schemes

The weak point in these schemes is that the disinfection process itself breaks down bacteria and organics into Assimilable Organic Carbon (AOC) and any bacteria that make it onto the membrane proliferate and form biofilm – especially in the absence of a rigorous monitoring program and Clean In Place procedures.

 

More Advanced RO Pre-Treatment Schemes

A more sophisticated pre-treatment scheme that I have seen on a power station in Bulgaria where the RO membranes lasted ten years, was a DensaDeg clarifier, followed by Sand Filtration, chlorination and Active Carbon filtration to remove the residual chlorine. Active Carbon filters double as Bio-Filters and remove N and P from the water resulting in a Biostable Water.

On a power station in Carrington in the UK I have seen UF membranes used as a pre-treatment in lieu of the coagulation-flocculation/filtration steps.

High pH feed water schemes are termed HERO (High Efficiency RO) and result in a reduction of organic fouling as well as a lower bio-fouling index as the high pH is unfavourable for biofilm.

Some companies like Lenntech use a technology called CCRO = Closed Circuit RO in which the feed water and reject are recircualted using a high flow, low pressure pump resulting in higher water recycling rates as well as a surface shear force that flushes the surface of the membrane making it difficult for bacteria and colloidal particles to stick.

Some companies use softeners as a pre-treatment for RO’s in areas where the feed water is hard. Whether this is cost effective as compared to dosing an anti-scalant or lowering the pH needs to be calculated.

Some companies offer RO skids that come with a vibrating mounting mechanism that makes the membranes vibrate. This is called VSEP – Vibratory Shear Enhanced Process which is a membrane filtration technology that uses oscillatory vibration to create high shear at the membrane surface, preventing fouling and increasing filtration rates. Produced Water Treatment schemes are supposed to benefit from this technology.

Some companies offer AFM^® Activated Filter Media as a direct replacement for sand, doubling the performance of sand filters without the need of additional investment.

Cutting Edge Pre-Treatment Schemes

IDE, an Israeli desalination company has developed strategies for combating fouling and scaling that are very intelligent and sustainable.

Direct Osmosis High Salinity (DOHS) is a method that injects Brine intermittently into the RO feed water which, owing to forward osmosis, results in a form of backwash of the RO membrane. The changing TDS environment also doubles as a way to prevent bacteria from acclimatising to the osmotic pressure surrounding them.

Pulse Flow RO (PFRO), another technology developped by IDE uses the strategy of running in the RO in Dead-End mode for intermittent periods of time then flushing out the feed water. This creates a synergistic effect:

1. The feed water is released before the induction time of scaling resulting in scale free operation and saving costs on expensive anti-scalants
2. The changing osmotic pressure on the surface of the membrane creates unstable and unfavourable conditions for Bio-film to form.

IDE has also developed sustainable processes for producing green chemicals from seawater and brine reject.

 

Finally, Biostable water is increasingly emerging as a very smart strategy to combat biofouling. This process uses either a sand filter or an active carbon filter in which aerobic bacteria are encouraged to grow which remove N and P from the water making the water nutrient-deficient for any biofilm formation. DuPont offer such a commercial filter called the Be-Free.

Technologies that dose chemicals that disrupt QUORUM SENSING (the process by which planktonic bacteria communicate and congregate into sessile bacterial biofilm) are also out there but seem to be more in the academic idea-sphere at the current stage.

Conclusion

RO-Pretreatment strategies to combat scale, fouling and biofouling have evolved in the past decade making RO operation more headache free. However, the best pre-treatment scheme cannot replace a good operations and maintenance program that monitors RO performance and intervenes when necessary.

 

More Information/Training

Kremesti Environmental Consulting Ltd offers consulting and training services for designers and operators of RO systems.

 

About The Author:

 

Rami Elias Kremesti is a water treatment expert with a passion for RO Technology. He tutors an RO course with CIWEM in London, UK. He loves to cook, read and write. He is the proud father of two gorgeous teenage girls.