75 MLD Seawater Desalination Plant.

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Desalination: We will be grateful for suggestions to desalinate sea water with the following properties and to deliver 75 MLD of treated water:

Feed water pH at 25 deg C - 8.5, Conductivity at 32 deg C - 65000 µs/cm, temperature - 32 deg C, turbidity - 600 NTU, TDS - 42000 ppm, TSS - 2000 ppm, Ca - 500 ppm, Mg - 1500 ppm, Total hardness as CaCO3 - 7500, Na - 13000 ppm, K - 450 ppm, Cl - 24000 ppm, SO4 - 3500 ppm, M Alkalinity - 100 ppm, P Alkalinity - 15 ppm, F - 4 ppm, Oil & Grease - not detected, PO4 - 3 ppm, Fe - 1.6 ppm, TOC - 2 ppm, BOD @ 27 Deg C for 3 days - 10 ppm, COD (ASTM Method) - 50, B - 3 ppm, NO3 - 1 ppm, SiO2 - 25 ppm, NO2 - 0.5 ppm, Colloidal Silica - 1 ppm, Dissolved Oxygen - 6 ppm, H2S - 1 ppm, NH3 - 2 ppm, Sr - 7 ppm, Ba - 0.2 ppm, Total Al - 0.2 ppm, Dissolved Al - 0.01 ppm, Dissolved Mn - not detected, Total Mn - 0.01 ppm, Dissolved Fe - 0.15 ppm, Zn - 0.05 ppm, Cu - 0.1 ppm, Ni - 0.4 ppm, Cd - 0.2/106 ppm, Pb - 15/106 ppm, Hg - 0.05/106 ppm Desired treated water characteristics are TDS - 500 ppm, pH - 7.0 to 7.5, Langelier Index should be higher than 0.1. We are yet to decide between two options: PT-DMF-PSF-RO or PT-DMF-UF-RO. We need to give performance guarantees. Which one of the two options is better?

4 Answers

  2. Dear Mr. Conor Kenny, Mr. Anthony Matulick - thank you for your inputs. The feed parameters mentioned by me are correct, as they are substantiated with a lab test report. The sample was drawn from a point that is about 750 meters away from the shore in the sea, and at a depth of about 6 meters. The Bathymetry Study, The Hydrogeological Test Report, The High-tide-low-tide Demarcations, The Oceanographic Survey Report, The Soil Survey Report, The Seasonal Variation report, etc., in fact the entire Detailed Project Report and the Project Feasibility Report were completed in April 2014. TSS is indeed 2000 ppm.

  3. Hi CHAKRAPANI, I agree with Conor on the pre-treatment required. Also query if the TSS is realistic. If this is the number reported I suspect the sample was taken close to shore in the break. I suggest you re-sample at location relevant to where the intake will be situated to get accurate gauge of the TSS level. As Conor mentioned if the TSS is that high you will need additional treatment in your design to deal with this. Therefore it is important to get accurate information on this. Regarding the post treatment, the pH will possibly be lower than you require. Therefore I think you'll either need Caustic dosing to correct this. However to meet the LSI spec, some Lime/CO2+Calcite addition is needed to increase the Calcium - depending on this design you may not require Caustic. Hope this helps, Anthony

  4. Your BOD figure of 10 ppm for 3 day test is high as is ammonia of 2 mg/l and indicates sewage pollution . I assume the figure of 2000 mg/l TSS is an error as that is super huge. The rest looks like gulf seawater. A DMF solution before UF should ensure that and excellent feed to RO is maintained , However I would not use dual media , instead used mono media of a large grain biological active media (eg filtralite 2.5 mm to 4 mm) on which some biomass would grow over time and consume most of the readily biodegradable organics and ammonia. Let the UF do the job of fine particulate solids removal. Be very careful of the selection of UF , the key issue is often long term membrane integrity . Don't use any Ferric or polymer. A DMF and PSF solution correctly sized will also work provided that any dosing of chlorine is not allowed onto the DMF. But this will need lots of Coagulant and acid . If the TSS is really very high (> 50 mg/l) then a filter media solution is incorrect . You will need a gravity settlement step to drop out much of the solids probably with some coagulant use.