Hi Yuvaraaj, this is a ...
Published by Junfeng Kok, Principal Process Engineer at Hyflux
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).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?
Published by Yuvaraaj Dhandapany
1 Comment
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.
Published by Junfeng Kok, Principal Process Engineer at Hyflux