Pretreatment of Seawater for Desalination Plants Richard Dixon, ITT
Agenda Pretreatment Background Typical Contaminants Practical Examples Methods of Treatment and References 2
Typical Constituents in Water Salts/Minerals/Metals/Dissolved Ions Cations Calcium Magnesium Sodium Potassium Barium Strontium Anions Alkalinity Chlorides Sulfates Nitrates Phosphate Fluoride Other Silica Iron Manganese Aluminum 3
Typical Constituents in Water Micro-organisms Bacteria Protozoan Plankton Algae Organic Compounds and Chemicals Tannins/Lignins Oil and Grease Man Made Chemicals Particulate Large particulates, > 5 micron Silt, 0.5 to 5 micron Colloids, < 0.5 to 0.01 micron Gases CO 2 4
Occasionally Larger Contaminants 5
Ranges of Membrane Filtration Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis >0.1 µm 0.1-0.01µm 0.01-0.001µm < 0.001µm Suspended particles Viruses Low molecular weight compounds Ions Microbes Colloids, Turbidity Proteins Large Organics and oil emulsions Ions
Pretreatment System Objectives Turbidity< 0,5 NTU SDI < 3 No oxidant Low residual metal (Fe, Al) Low organic carbon content No algae cells Low particle counts 7
Pretreatment Components Screening Sand removal Dissolved Air Flotation (DAF) Multi-media filtration Microfiltration (MF) 0.1-1µm Ultrafiltration (UF) 0.01-0.1µm Chlorination and declorination Coagulation Flocculation ph adjustment Antiscalant chemicals Oxidation chemicals 8
Pretreatment Schemes Existing Membrane Plants 9
Pretreatment Schemes Existing Thermal Plants 10
Variability of Inlet Seawater Location Turbidity Temperature Mediterranean 1 to 10 25 South Pacific < 5 < 20 Persian Gulf 5 to 15 25 to 35 Australia < 5 20 to 30 11
Common Types of Intakes Open Surface (Channel) Open Submerged Beach Well Infiltration Gallery 12
Red Tide - Tocopilla 13
Red Tide found in many countries Non-toxic Dinoflagellate Noctiluca stretched 20 miles along southern CA, USA coast Non-toxic Noctiluca scintillans in New Zealand Cyanobacterial bloom contains the toxic species in the genus Microcystis A Noctiluca bloom in China 14
Intake and Red Tide 15
Intake and Red Tide Same dates at Escondida Coloso 16
Particle Sizes Relative nominal sizes of various particulates 1 micron = one millionth of a meter Dirt or sand Visible to the naked eye Silt and clay Protozoan cysts Bacteria and algae Colloids Viruses 40+ microns 40+ microns 0.5-20 microns 3-20 micron 0.5 5 microns 0.01 0.5 micron 0.01 0.25 micron Ions 0.00005 0.00025 microns 17
Performance of Filtration is Variable 18
Filtration requires Backwashing 19
Pressure filters 20
ITT Rapid Gravity Sand Filters (RGSF) for RO Membrane Pretreatment 21
Typical ITT Leopold Filter System 22
Leopold Filtration References 23
24
RGSF Typical Performance Feed Water Turbidity from 1 to 15 NTU Less than 1 NTU 2 mg/l FE +++ Over 1 NTU 4-5 mg/l Fe+++ First 10 hours filter run time 3.5 SDI then 38 hours of 2.7 2.9 SDI Avg SDI below 3, 95% of time 1000 mm sand (0.50 mm), 700 mm anthracite (1.10 mm) w/ 3 gpm/ft 2 filter rate 25
Media Filter Comparisons Leopold Filterworx Gravity Filter System ADV: lower operating cost, reduced backwash volume, less mudballs/channeling gives higher quality water DIS: not as economical for small systems Pressure Filters ADV: most economical for smaller systems, DIS: higher operating cost, increased backwashing volume, increased mudballs/channeling potential and hence lower quality effluent 26
Membrane Filtration MF/UF The ultimate in particulate and micro-organism removal and less prone to seasonal water quality variations Generally medium to low space requirements Low civil structure requirements Higher capital and operating cost Higher backwash volumes to process Lower overall system recovery / increases specific energy Many valves and process steps adding to control complexity and maintenance 150 micron backwashable pre-screens required 27
ITT Dissolved Air Flotation for RO Membrane Pretreatment 28
Typical DAF 29
Stoke s Law V = G (P1 P2) D 2 18 u V = Velocity -Time to Settle/Float G = Gravity P1 = Particle Density P2 = Liquid Density D = Diameter of the Particle u = Viscosity of the Liquid 30
Coagulant/Polymer Addition SIZE: Tens of Microns Buoyancy Flotation Particle Size Difference Gravity Settling SIZE: Hundreds of Microns Bubble Addition
How does a DAF Work? Flash Mix 5-20 mins floc time 4-20 gpm/ft 2 Loading Rate Floc 1 Floc 2 Skimming Devise Device Outlet RAW WATER + Coagulant Nozzles/ Valves Lateral Draw-off pipes Saturator Air Compressor 10 % Flow Recycle Pump Compressed Air dissolved in water at 80 psi 32
Small Systems 33
Large System Field Erected Concrete tank 34
DAF Applications Best for Contaminant Removal Algae Red Tide Organics Oil Colloidal Solids Best Operational Performance Lowest waste volume Highest Sludge Solids Low Chemical Usage Handles variable water quality and cold water 35
Conclusions Many factors to consider in pretreatment design: - Location - Water Variability - Intake Type - Desalination Uptime requirements 36
Recommendations Start EARLY: - Survey of Possible Locations - Bathymetry - Talk to the Specialists - Do Conceptual and Prefeasibility Engineering for more than one option - Involve Suppliers early - Involve all suppliers, not only Desalination OEMs, to understand options and consequences of designs 37
Thanks to: - Coloso - ElectroAndina - Veolia - Degremont AND THANK YOU! 38