High Pressure at RO membrane

[0995Francis]

Hello! Can anyone help me on how to solve our problem with regards to our RO?
We are using a PA membrane and a 4-2 pass configuration. The Primary pressure is still the same but suddenly the final pressure went high. Usually final press was at 65psi ti 75psi but after we have passed 30ppm hardness, the final pressure went to 110psi. We are dosing NaOH for pH.

Many thanks

 

[EMKWR]

Sounds like scaling. Any idea which pH was reached after addition of the NaOH ? It's quite tricky to reach an optimum situation for RO with water containing hardness.

Can you maybe tell a bit more. You can contact me by e-mail.

Best regards,

Edwin Edwin Muller
KEMA Power Generation & Sustainables
Arnhem, The Netherlands
E-mail e.f.muller@kema.nl
Internet:

http://www.kema-water.nl

 

[EMKWR]

Some additional information, the problem with RO and hardness is that there is always one side of the membrane which will is subjected to high concentrations of hardness, in your case the retentate. The fact that the problem is especially observed at the last stage confirms this.

Above that it is likely that also the distribution of the caustic over the sides of the membrane will different to that of the hardness (the OH and Na are significantly smaller than Ca and CO3).

Finally there is also the possibility of biofouling. This is however less likely because one should expect this to build up more gradually and in the whole system (not only in the first stage).

Best regards,

Edwin Muller Edwin Muller
KEMA Power Generation & Sustainables
Arnhem, The Netherlands
E-mail e.f.muller@kema.nl
Internet:

http://www.kema-water.nl

 

[VER]

Francis, I'm new to Engineering tips and read your post only today.

Anyway, you didn't mention the analysis of your feed water. Usually, with high Calcium and Magnesium hardness and with high HCO3, you do not adjust pH upward since Calcium Carbonate solubility decreases with increasing pH does causing precipitation in your concentrate side of the membrane. You add acid ( H2SO4 or HCl)to breakdown the HCO3 to CO2 (which passes through your membrane) thus preventing the formation of Calcium Carbonates in your concentrate side of the mebranes and allowing you to operate at higher recovery.

What is the recovery and capacity of your system? How many membranes per vessel? If you can provide me the feed water analysis including temperature, I can make you a computer projections to show the best operating feed pH and optimum system recovery. Looks like your mebranes are scaling based on your Delta P across the mebranes. You need to clean the membranes and get analysis of your feed water before attempting to use Caustic Soda again. Membrane guidelines for delta p is 10 psi per 40" membrane.

Feed Water Analysis required (as mg./L ions except pH and Temp.):
Ca
Mg
Na
HCO3
SO4
Cl
SiO2
pH
TDS
Temperature

If you have high silica in your feed water, then this is a different story, since silica solubility increases with increasing pH and temperature (the opposite of Calcium Carbonate). If silica is the problem, you have to remove the total hardness first by installing a softener upstream of your R.O. before adjusting pH upward so that you can operate at higher recovery without precipitating silica in your concentrate stream.

Hope this helps.

Ver...

 

[HFP13]

VER,

CAn you kindly provide me a computer simulation analysis in order for us to head into the right solution to our problem?
-We have 2-pass (RO#1/2 machines in parallel is 1 st pass and RO#3/4 on 2nd pass).
-We don't have water softener installed in the raw water.
-We have MMFs
-We have degasifier installed before 1 st pass RO tank but is now not functioning ( is there an implication on this as regards to permeate quality?)
-The recovery of RO#1 is 50%, RO#2 has high pressure diff. and is producing a high conductivity of 17uS.
-RO#3 has high recovery of up to 80%

The ffg is our raw water analysis:

QUANTITATIVE WATER ANALYSIS



Sample Description : PRE R. O.

Appearance : Clear, colorless liquid
pH : 7.40
Total Dissolved Hardness, ppm CaCO3: 176.00
Calcium Hardness, ppm CaCO3 : 76.00
Magnesium Hardness, ppm CaCO3 : 100.00
P Alkalinity, ppm CaCO3 : 0.00
M Alkalinity, ppm CaC03 : 198.00
O Alkalinity, ppm CaC03 : 0.00
Chloride, ppm NaCl : 8.00
Conductivity, ?mho/cm : 350.00
Silica, ppm SiO2 : 64.20
Total Iron, ppm Fe : 0.01
Sulfate, ppm SO4-2 : 1.00
Nitrate, ppm CaCO3 : 218.36
Manganese, ppm Mn periodate oxidation: 0.00
Sodium, ppm CaCO3 : 80.73
Total Dissolved Solids, ppm TDS : 210.00
Temperaure (deg C) : 25-30
Note: TDS is determined by using Nalcometer conversion.

Hope the above are in order.

Thanks,

Rommel

 

[MikeJefferies]

From the top ....

Francis is not suffering from high DP. The primary pressure has stayed the same, and the secondary pressure has increased. My guess is somebody has closed the concentrate valve. There is a good chance you have scaled the system though, if this is true.

HP13 / Rommel - High dP could be biological fouling, scaling or colloidal fouling. From the analysis, the only scales you are likely to see are calcium carbonate and silica. If you can, open up a few of the reject ends of the vessels and look inside - if you have scale you will see it there. If there is scale, take a sample and try and dissolve it in 0.1% HCl (or any other acid really). If it dissolves and gives off gas, then it's calcium carbonate. If it's silica, you have a problem. My guess is this is not a natural water ? Is the analysis constant, or does it vary ?

The degasser should remove CO2, and if this is not working then you will see higher conducitivities in the water, and in the permeate from RO 3&4 since the RO will not reject CO2.

 

[MikeJefferies]

I really should look at the dates of the posts before replying....

 

[HFP13]

Thanks, Mike! Our raw water is coming from the idustrial zone developer which they claim is already pre-treated. The water analysis varies slightly.

By the way, we do chemical injection to remove CO2 prior to entering RO#3 & 4. The question now will be the effect of the degasser w/c is off-line to the water quality.

Thanks very much!

 

[MikeJefferies]

Ok, I guess you are adding sodium hydroxide (caustic soda) to the water. This will convert CO2 to HCO3 (NaOH + CO2 =>NaHCO3). The HCO3 will be rejected by the membranes.

If you can use a degasser to remove the CO2, then you will need much less NaOH.

Your final permeate will be dependent on the quality of the feed water entering RO#3&4. Because you are dosing more NaOH than designed (due to the lack of degasser) the quality will be worse. In the scheme of things, however, I'd guess you are only talking 0.5ppm TDS extra in the final water.

My guess is you are suffering from biological fouling. Regardless of how well the water is pretreated, you will suffer some degree of fouling. The trouble with biological fouling is once it gets hold it tends to spread rapidly. If the differential pressure gets too high - typically above 60psi/4 bar - you start to risk telescoping the elements.

I suggest you schedule a clean on this system as soon as possible, and consider a biocide dosing regime to prevent it reoccuring.

Regards
Mike
mjefferies@avistatech.co.uk

http://www.avistatech.co.uk

 

[HFP13]

Mike,

You are saying that over dosing the feedwater w/ NaOH will have a worse effect on water quality. Is this the reason for biofouling?

Thanks

 

[MikeJefferies]

You will achieve a better quality dosing NaOH than you will by just letting the CO2 pass through the membrane. It's just the lack of degasser means you need to dose more NaOH. The extra NaHCO3 will therefore increase the conductivity of the final water, because the membrane will reject a given percentage of the ions being fed to it. If you could remove the CO2 with the degasser, there would be less dose of NaOH required, and therefore less ions present in the feed water.