SWRO Desalination

It seems that  use of the design of Direct coupling of UF with RO  facilitates   the    intermediate feed tanks , pumps and cartridge filter are eliminated, simplifying the process and layout with reduced maintenance   on frequent replacement of cartridge filters and servicing of associated infrastructure.

   There are Desal plants using this Design especially   in Adelaide    (628mld)  /2002   and Perth(300mld) and recently commissioned in SIngapore Keppel Marina East     Desalination plant(137mld). MEMCOR CS Membranes of EVOQUA used for pretreatment and claims  to have achieved  the specific power consumption of 2.10 kWh/cu. m of water produced. There are modular containerised plants also. Really this is great  . In such case , why  this  is   not  widely adopted.?

    Besides,   it is   learnt that   many of the   marine micro algae  which  grow excessively when there is Algal Bloom can not withstand pressure more than 0.5 bars  and  hence algal  the cells would break down under pressure     or vacuum  and release    easily   biodegradable organic  compounds.  This would facilitate growth and formation of   Biofim  on the   SWRO membranes .This film would foul the membranes and thus resulting in reduction of plant production within weeks from the beginning of algal Bloom.

How this problem is addressed In the absence of Cartridge filters which would otherwise protect the  SWRO membranes from the damaged particles,

regards

Yuvaraaj

Freelance Consultant

(Retired . Chief Engineer,i/c;/Chennai Metropolitan Water Supply& Sewerage Board,Chennai,.India

Taxonomy

• Reverse Osmosis
• Sea Water Filtration
• Sea Water Desalinisation
• Sustainable Desalination
• Desalination
• Membrane distillation

8 Answers

1. Hi Yuvaraaj, 
   I agree with the need of the cartdrige filter as a guard filter: It is not advisable to remove them even when direct coupling of UF to RO. Ijn any case it is always very important to know the water source and quality in order to design the most suitable pretreatment, which is Akey factor on the performance of the downstream technologies.
   Check out this short video of our R&D department regarding the perfomance of self-cleaning disc filters to avoid the impact of harmful algal Bloom on desalination plants:
   https://www.youtube.com/watch?v=zgyExMnZf94

2. Isn’t salination of Sea Water a natural process? So is it mandatory that the desalination process has to be only an artificial synthetic process only? Or does it happen naturally too?
   
   Is there any natural process that’s only one way? Non-cyclic? Only linear?
   No way!
   
   Explore the secrets of natural desalination process in our latest article published by Smart Water Magazine, Spain, that’s called - Natural salination and desalination of Water
   Read the complete article on the following link :point_right:

3. Hi Yuvaraaj, 

   this is a very interesting question. Some of my thoughts. 

    

   1. The direct coupling of UF to RO  - although it has the advantages of eliminating several intermediate equipment which you have mentioned (Filtered water tank, pumps and even the cartridge filter), I do believe it also creates its own set of problems. 

   2. These problems include a need for more standby units - because it is likely the plant will be operating in a flow-constant mode and therefore, if some of the UF units are having outage for cleaning, the other UF units will have to increase the design flux instantaneously. Therefore, the UF system will be seeing a lot more flux fluctuation  – this creates pressure fluctuation which may have a potential detrimental impact on the UF fibre.

   3. The instrumentation would probably need standbys. in the situation of a break tank between the UF and the RO, the flowmeter serve as a check whether the pre-treatment units are producing enough filtered water for the downstream units. The operators largely rely on the level transmitters on the break tank to ensure continuous operation downstream. Without this break tank - the flowmeters now become a single source of failure in the plant. I see the need for multiple flowmeters to address this issue. Whenever there are multiple instruments reading the same parameters, the issues arises as to which value is the more accurate one.  Life becomes more complicated. Magnetic flowmeter which is more accurate will likely become mandatory.

   4. In terms of control, direct coupling is likely to require more automation. During a project, where timeline could be tight - more time is required to be spent on programming, debugging and making it more automated because no operator would be able to manually match the production of the UF system to the SWRO feed requirements on such a fast basis. in other words, the control system needs to be almost flawless because manual intervention is not quick enough. The project time saved from the intermediate equipment is just spent in other areas.

   5. with regards to the algae issue you posted, i view it as a separate matter altogether. Not all plant has algae issue so a direct coupling of UF to RO won't matter. For plants which sees active algae growth, there is a need to critically analyse the reason for this; could it be an issue of a poor intake design? Could it be presence of phosphorus or iron from illegal wastewater discharge causing this bloom? Is it seasonal? Perhaps using a chlorophyll analyser and shutting down the plant for a couple of day would suffice? Perhaps you could use a Dissolved Air flotation unit to reduce the severity of an algae bloom. 

   1 Comment

   1. 1).Thanks  Mr.Junfeng for your reply.  What  seems to be interesting is  that this process claims to have achieved huge energy savings say 2.1 to 2.5 kWh/cu.mt against the average Specific power consumption of  about 3.5kWh/cu.m ,apart from capex savings due to elimination of some units ,foot print reduction and allied operational expenditure. There are plants working based on this process  for almost a decade. The recently commissioned one is in Keppel Singapore. When the  entire Desalination fraternity is striving for cost optimisation in terms of energy consumption , why this design is nor prevalent  despite this fact?

      2). Besides,  as  may be aware ,membrane pretreatment systems likely to be exposed to pressure surges and hence may be fibre breaks  releasing small particles. This would cause fouling/damage RO membranes, if  not captured by by CF.

      3) CF, apart from being protecting guard for RO membrane, is an indicator to assess the pretretment performance by monitoring the SDI upstream& d/s of CF.

      4) Added that, sampling ports to conduct SDI testing are installed u/s & d/s of CF before entering RO

      If  Opted for direct coupling, how the above  aspects could be managed?

       

       

      1 Comment reply

      1. Hi Yuvaraaj, some of my thoughts. 

         1. by eliminating the intermediate tank, one reduces the need for repumping to the SWRO. However, from my experience, the reduction in power is not expected to be in the values you have described. I would think it is over-stated. The main bulk of power consumed is still in the SWRO system, and that does not change with a direct UF couping. 

          

         2. I have offered some reasons why it is not prevalent in my initial post. The other reason I would think is that for large mega desalination tender, the technical specifications are prescribed. A bidder is not allowed to propose a direct UF coupling if this option is not available in the tender specifications. Unless consultants come on board with the idea, it is often not possible (or very difficult) to propose it from the bidder's point of view. 

          

         3. Having a direct coupling does not eliminate the need for quality monitoring d/s of the UF filtered water. Turbidity and SDI measurements are still the standards for determining this. 

         Without direct coupling, there are still reports and observations of UF fibre breakage and contaminant going down-stream, this does not change.  It remains to be seen if direct coupling result in more contaminants going downstream - but in my opinion, it certainty demands higher mechanical strength on the UF fibre due to the fluctuating pressures.

4. This is staggering in its ignorance. I'm shocked this misinformation was published.

   1 Comment

   1. Please explain what is wrong here to share you knowledge so that all learn. There are no "ignorant questions" someone asks because they do not know and look for insights. Much appreciated!

5. Thanks for your insights.  In case there is a rupture of  UF membranes and the bristles of  the  membrane fibre escape and damage the downstream  RO system, how this  could be the prevented?

6. ​MEMCOR CS ​Membranes (​EVOQUA/Dupont) is a submerged/immersed type membranes, generally installed in an open tank. As the water passes through the fibers, particles greater than 0.04 μm are rejected by the membrane and stays in the open tank only. With appropriate water wasting/recirculation of the open tank, as the same doesn't directly impact on the membranes. Hence, one can easily handle the high total suspended solid concentration loadings.

   The installed Disc Filters/Micro Strainers enable the removal of material larger than 0.1 mm, further ensure seawater filtration prior to Ulta Filtration.

   In the case of Algal Bloom, the same will be easily managed by the appropriate water wasting/recirculation of the open tank. This type of system has the further advantages of visual inspection of the Ulta Filtration Process.

7. Hi Yuvaraaj - I am in South Africa so I can't be of physical assistance to you there but please contact Merus Water Treatment in Australia or go to Merusonline and look at their website - I am the distributor for SA and this is an application that could be attended to. Regards, Bryan. 

8. Cartridge Filter is a guard filter.

   If the guard is not there, it doe's mean that anything can enter into the system.

   UF is a superior filter that takes care of algal bloom, bacteria, microorganisms, etc.  

   It is all an effective cost saving process design within technical acceptance.

   1 Comment

   1. I agree to what you say.  For UF in upstream, at initial plant proving, commissioning stage, it is advisable to have a guard in-line strainer.  Every time a plant re-starts after prolonged shutdown again the in line strainer elements can be re-used.  Plug and play approach.  Having said that, I would also like to emphasize on the issue of knowing your water source and its characteristics through out the the year, which would tell how elaborate the pretreatment system would be and how long it will used in all varying seasons.  This could play an important role in life cycle cost analysis.