When is it necessary to install a dissolved air flotation (DAF) system in seawater reverse osmosis (SWRO) systems? From my point of view the following parameters are critical: TSS, Turbidity, SDI, DOC and TOC.
So the question would be which is the value of each parameter, that makes DAF system mandatory? My understanding is that with a Turbidity higher than 30 NTU, DAF is necessary.
Pretreatment of Seawater there are more steps for treatments like Fluctuating ,High turbidity High levels of organic Microbial and colloidal particles.The performance ofa SWRO plant can result from any of the following or acombination thereof.
1. Accelerated increase in net driving pressure.
2.Accelerated reduction in normalized permeate flow.
3. Accelerate increase in pressure drop across the vessel
4. Incresing RO cleaning frequency
5. Reduced RO membrane life resulting from increased RO cleaning
6. Reduced plant availabilty resulting from increased RO
The different types of membrane are used for Scaling,fouling, biofouling organiic fouling and colloidal fouling
I don't have any good rules-of-thumb, but certainly heavy diatoms/plankton, etc., turbid water and high organic molecules would be a good reason to attempt to remove. DAF would do a good job was anything that is buoyant-neutral or which tends to float.
Hi Patrick, this is my understanding also, but I have seen some recent plants being built without DAF. So I am trying to find out more about current developments and selection criteria for pretreatment systems. Have you seen new RO plants without DAF in the recent years? RegardsBenito
Dear Benito! DAF or another microflotation technology is the the only proper pretreatment for UF&RO if there is risk of algae blooms and if combined well with flocculation. In my experience, microalgae can be removed well given optimal conditions.
Media filters clog very fast during an algae bloom or let too much algae through. UF alone will not be capable to operate during a heavy algae bloom, but might be a good pretreatment for RO in regular conditions (I guess, that is what Nikolay referred to). DAF has a very high energy consumption though, especially in warm and saline waters. An alternative for DAF+UF is akvoFloat including a different energy and cost saving technology for flotation.
Regarding your initial question: I think it will be hard to give you numbers. In many cases, a DAF is installed to be operated during algae bloom only. So, the reason to install a DAF is a high probability of algae blooms (everywhere in the Persian Gulf e.g.) and at those events all given parameters increase dramatically during a couple of days or hours.
Hi Johanna, thanks for your feedback. Yes I guess it is not easy to set up parameters and mostly DAF is in operation during algae blooms. But suppliers needs to decide whether it is necessary or not, depending on the seawater analysis provided by the clients. My point is that I have seen some recent plants where DAF has not been selected. Only DMF or UF has been installed. I'm trying to find out if there are new developments in UF or DMF allow for removal of algae during red tide events without DAF system Regarding akvoFloat, is ti already under operation for seawater desalination plants?
The only condition when DAF could be beneficial is when the source seawater contians oil & grease/hydrocarbons. These compounds would foul the SWRO memrbanes irreversibly and DAF is very effective in remiving them to levels acceptable for processign through SWRO system. Often DAF is also condifered as a remedy for algal blooms. Expereicne to date on preactially all SWRO plants with DAFs is that they do not provide signficant algal and other suspendied solids removal when the content of these solids is lower than 5 NTU. The closer the desalination plant is to the equator, the smaller the algae are and the more dificult it is for them to be removed with conventional DAF technologies available on the market today even if the source seawater expreience significant algal blooms. Until the DAF technologies availalbe on the market are modified for the size of algae in the oceans and the density of seawater, DAF would not be suitable for removal of algae from seawater, espepcially in the Middle East, the Mediteranean and other parts of the world close to the equator where most algal blooms are dominated by pico and micro algae with sizes of 10 microns or less, which are too small to be effectively removed by fresh water DAF technologies availalbe on the market at present - most of these technologies are designed to remove fresh water algae with sizes of 30 microns and above.
Hi Nikolay, thanks for your detailed answer. You say there is no significant content removal when turbidity is lower than 5 NTU with DAF technology. So above that value, current DAF techonlogies are suitable and reduce turbidity at least to 5 NTU? Do I understand correctly your opinion? On the other hand , till DAF technologies improve, what is the option in current SWRO plants when you have peak turbidity values above 30 NTU? Double stage dual media filters or UF? I am not so sure If they can guarantee NTU values suitable for the RO membranes.
If a proper flocculation/coagulation is done, flotation can remove microalgae effectively. Of course flocculants mustn't be overdosed, so that no residuals reach the RO.
You are right when the Seawater containing high quantity of TSS, Turbidity, SDI, TOC, Micro-Organism which is necessarily be removed prior to reaching Filter, UF & RO.
Whatever pre-treatement is applied better DAF in most cases; the output from pretreatment to be
Hi Mansoor, thanks a lot. Yes I have the same understanding regarding Media Filter. So far I have taken 30 NTU as a reference value for DAF. But I am looking forward to get your feedback and see how is your experience with DAF. RegardsBenito
Dear Benito, DAF is still widely used in case of high TSS/Turbidity and Algal concentrations. Media Filters get clogged very quickly in case no pretreatment is applied. I will send you some parameters values, let me look into some case studies.Regards,Mansoor Ahmed
Dear Mansoor, thanks for your reply. What I am trying to find out is the value for each parameter. From which value is DAF recommended? I have seen recent plant being built without DAF in areas affected by red tide. So I wonder If there are new developments in Media Filter or UF, that makes DAF less necessary RegardsBenito