SEA WATER DESALINATION
- 1. JADAVPUR UNIVERSITY SCHOOL OF WATER RESOURCE & HYDRAULIC ENGINEERING
- 2. TOPICS OF PRESENTATION INTRODUCTION what is it? why is it? where is it? VARIOUS METHODS OF DESALINATION THERMAL BASED –Multi-Stage Flash distillation (MSF), Multi-Effect Distillation (MED), Vapour compression(VC) MEMBRANE BASED –Reverse Osmosis (RO), Electro-Dialysis (ED), Electro-dialysis Reversal (EDR) PREFERRRED TECHNOLOGIES IN SEA WATER DESALINATION MFD & RO why is it preferred STATE OF THE ART Reverse osmosis technology Pretreatment Post treatment Chemical cleaning Reject handling Energy recovery
- 3. INTRODUCTION WHAT IS DESALINATION? Desalination refers to the various processes that removes excess salt and other minerals from water in order to make it suitable for human consumption , irrigation and industrial application. desalination can be : Sea water desalination Brackish groundwater desalination WHY DESALINATION? Makes water suitable for human consumption and Irrigation Makes water suitable for industrial application Scarcity of fresh surface water in the world Hazardous contamination of groundwater Abundance of sea water
- 4. INTRODUCTION As per data from WORLD WATER BALANCE AND WATER RESOURCES OF THE EARTH, UNESCO 1975, following are some percentages of total water available on earth: 96.5% of earths water are oceans and seas which are saline in nature and are not fit for any usage in its natural form 0.93% of earths water are saline ground water which are unfit for usage without treatment 0.006% of total earth’s water are saline lake water and are not usable in its untreated form. Therefore nearly 97.5% of earth’s total water cannot be used directly into most of the applications and uses of humans which amount to a quantity of 1351 Million cubic kilometres. WHERE DESALINATION IS USED? Arid areas with scarcity of fresh water , contaminated ground water or brackish groundwater Sea water as source of raw water Power availability in abundance and cost of power is low renowned desalination projects of the world desalination plant table.pdf some major ro based desalination project in INDIA -nemmeli-chennai-100mld -minjur-chennai-100mld
- 5. METHODS OF SEA WATER DESALINATION THERMAL BASED SEA WATER DESALINATION Includes heat transfer Includes phase change of water to vapour heating evaporation condensation Includes operation at pressure lower than atmospheric pressure to reduce energy consumption for vaporizing water Different thermal processes are Multi-stage flash distillation Multi -effect distillation Vapour compression MEMBRANE BASED SEA WATER DESALINATION Includes use of very thin special plastics which acts as special barriers for selected particles Includes the reverse principle of osmosis Includes high pressure operations Includes the principle of electrolysis with ionic movement towards preferred electrodes. Different membrane based process: Electro dialysis & Electro dialysis reversal Reverse Osmosis
- 6. MULTISTAGE FLASH DISTILLATION MORE THAN HALF OF WORLDS DESALINATION PLANTS ARE BASED ON THIS PROCESS THE PROCESS INVOLVES Feeding of sea water into the system Preheating of sea water while passing through heat exchange tubes Heating of preheated sea water in a brine heater at normal pressure Passing of heated sea water through a series of closed vessel with progressively lower internal pressure called ‘stages’ Sudden boiling of heated sea water in the stages with progressively lower internal pressures called ‘flashing or exploding into steam’ Steam formed is passed over the heat exchange tubes carrying the incoming cold sea water causing it to condense Collection of the condensate as pure water on a tray under the tube Blowdown of stage chambers with concentrate. operating temperature varies from 90-120 degrees centigrade number of stages can be from 4- 40 stages power consumption figure varies from 23-27 kwh/cubic meter of distilled water production
- 8. MULTISTAGE FLASH DISTILLATION ADVANTAGES: Large production capacity (4000-30000 cum/day) Has flexibility in regards of quality of water More energy efficient with respect to other thermal processes Proven reliability Better quality of water is achievable than membrane processes Well developed construction and operation experience over other thermal processes Less pretreatment to feed water required DRAWBACKS: Operating cost high as it has both electromechanical equipments as well as requirement of fuel to generate heat Maintenance cost high due to high level of scale formations and corrosion due to high operating temperature
- 9. REVERSE OSMOSIS DEFINING RO Reverse osmosis is a filtration process which works by using pressure to force a solution through a semi permeable selective membrane, retaining the solute on one side and allowing the pure solvent to pass to the other side. This is the reverse of the normal osmotic process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied to balance the free energy available in the high concentration area. Osmotic pressure is driven by chemical potential which is a thermodynamic parameter. RO FEATURES AND MEMBRANES Involves high pressure operations (40-80 bars of pressure) Type of membranes can be cellulose acetate type or polyamide type. In sea water application polyamide membranes are used as these membranes can handle temperatures above 35 deg C. Spiral wound membrane is the most commonly used membrane construction in sea water reverse osmosis process Semi permeable membranes used has dense barrier layer in polymer matrix Spirally wound membrane picture MEMBRANE.jpg & spiral wound membrane.gif
- 10. REVERSE OSMOSIS MEMBRANE TYPES AND FEATURES Pleated Flat Sheet Membrane (MF) Spiral Wound Flat Sheet Membrane (RO & UF) Ceramic Monolith Element Membrane (MF & UF) Tubular Membrane (RO & UF)
- 11. REVERSE OSMOSIS
- 12. REVERSE OSMOSIS (SEA WATER)
- 13. REVERSE OSMOSIS ADVANTAGES: Suitable for desalinating both sea water as well as brackish ground water Flexible in regards of quantity and quality Requires less energy than thermal processes Flexible in regards of starting and stopping of pumps Easy operation with full automation Flexible in the matters of increasing the capacity of plant size by adding new membrane units Operating cost comparatively low with respect to thermal processes DRAWBACKS: More pretreatment required Water quality achievable is in the tune of 250-500 ppm Maintenance cost high
- 14. PRE-TREATMENT TO DESALINATION PRE-TREATMENT HIGHLIGHTS WHY PRETREATMENT OF RO IS REQUIRED No regular backwashing of the membranes are done due to the spiral wound characteristic of the membrane as effective cleaning of the membrane cannot be achieved Fouling in the membrane caused due to various factors results in loss of productivity of the system Membrane life gets hampered due to biological, organic and other deposits reducing the performance level For adjustment of Oxidation-reduction potential, PH and SDI of inlet feed water. For separating out the maximum of solid particles to make RO efficient. e.g.. cartridge filtration to remove 1-5 micrometer sized particles. and ultrafiltration to remove particles below <1 micron STAGES OF PRETREATMENT BEFORE RO PROCESS: Course and fine screen arrangement for physically separating larger sized wastes e.g. plastic, shoes, rags, sticks, sea weeds etc. in the sea water intake area. In presence of oil, oil removal systems like API, TPI and DAF are required for physical separation of oil by floatation and skimming technique Suspended solid separation by coagulation ,flocculation and settling process in clariflocculators, clarifiers Application of chlorine and sodium meta-bisulphate for prevention of biological fouling and necessary surface adsorption membranes for removing residual oxidizing agents. PH correction process for avoiding extreme PH conditions for prevention of RO membranes from corrosive atmosphere Various micro filtration processes viz. ultrafiltration, cartridge filtration, Nano filtration based on the type of RO membrane selected and final water quality requirement. primary purpose of pre filtration is to lower the particle size of the feed water and also take care of colloidal silica and reduction of SDI below 4.
- 15. PRE-TREATMENT TO DESALINATION DIFFERENT TYPES OF FOULING WITH NECESSARY PRETREATMENTS Fouling Cause Appropriate Pre-treatment Biological fouling Bacteria, microorganisms, viruses, protozoan Chlorination + Micro filtration Particle fouling sand, clay (turbidity, suspended solids) Flocculation & sedimentation + Filtration Colloidal fouling Organic and inorganic complexes, colloidal particles, micro-algae Chlorination +Flocculation & Sedimentation+ Micro Filtration Organic fouling Natural Organic Matter (NOM) : humic and fulvic acids, biopolymers Coagulation + Filtration + Activated carbon adsorption Coagulation+ Micro filtration Mineral fouling Calcium, Magnesium Barium or Strontium sulfates and carbonates Antiscalant dosing Acidification and Post PH correction Oxidant fouling Chlorine, Ozone, KMnO4 Oxidant scavenger dosing: Sodium (meta)bilsulfite Granulated Activated Carbon
- 16. PUMPING SYSTEM IN DESALINATION THE BASIC GROUP OF PUMPS AND TYPICAL TYPES IN A SWROP ARE: PUMPS TYPES Sea water intake pumps Vertical Turbine pumps, Depressed pit Horizontal centrifugal Pumps Pre-treatment Filter feed pumps Horizontal centrifugal Pumps Microfiltration Filter feed pumps Multi Stage Pumps RO high pressure feed pumps Multi Stage Pumps RO low pressure feed pumps Multi Stage Pumps Filter backwashing pumps (for backwashing filters of pretreatment system) Horizontal centrifugal Pumps Flushing pumps Positive Displacement pumps Energy recovery drive pumps (booster pumps) Horizontal centrifugal Pumps Product water pumps Horizontal centrifugal Pumps Chemical cleaning pumps Positive Displacement pumps, progressive cavity type or diaphragm type. Reject handling pumps Multi Stage Pumps
- 17. PUMPING SYSTEM IN DESALINATION The feed pumps supplies the pressure needed to push water through the membrane, while the membrane rejects the passage of salt through it. Typical pressures for sea water feed pumps ranges from 60 to 80 bars (high pressure operations) efficiency of pumps are high for lower power consumption Corrosion resistant material of construction to handle sea water Wide range of flow (rpm varies 700 to 3450) Centrifugal pumps for smaller and positive displacement pumps for comparatively larger systems Corrosion resistant material of construction to handle sea water are used. e.g. Duplex stainless steel, Titanium etc.
- 18. POST TREATMENT IN DESALINATION Depending on the treated water application the reverse osmosis permeate shall be reconditioned before end usage Usual permeate Character has an acidic PH TDS varying from 100-350mg/lt 2-6 mg/lt of calcium and magnesium content Boron content 0.5 to 1.2mg/lt based on the raw water salinity and temperature End usage can be human consumption, irrigation or industrial process water. FACTORS DRINKING WATER IRRIGATION WATER PROCESS WATER SODIUM CHLORIDE <450MG/LT AS PER SAR & EC OPTIMIZATION MINIMUM CALCIUM AND MAGNESIUM 6-10MG/LT AS PER SAR & EC OPTIMIZATION MINIMUM PH 6.5-8.5 6.5-8.5 6.5-8.5 BORON <0.5 MG/LT CROP DEPENDANT - DISINFECTION REQUIRED NOT REQUIRED NOT REQUIRED
- 19. POST TREATMENT IN DESALINATION POST TREATMENT SUMMARY PROCESS DRINKING WATER IRRIGATION WATER PROCESS WATER TO REMOVE SODIUM CHLORIDE 2ND PASS RO - 2ND PASS RO TO REMOVE CALCIUM AND MAGNESIUM REMINERALISAT ION REMINERALISATION - TO NEUTRALIZE TO PH +/-7 NAOH/HCL INJECTION NAOH/HCL INJECTION NAOH/HCL INJECTION TO REMOVE BORON NAOH+2ND PASS RO SPECIFIC BORON REMOVAL PROCESS - DISINFECTION HYPOCHLORITE ADDITION NOT REQUIRED NOT REQUIRED
- 20. CHEMICAL CLEANING IN DESALINATION Seawater flows tangentially along the membrane, creating a boundary layer on the membrane surface. Membranes have to cleaned typically when: - Normalized permeate flow varies by 10-15% - Normalized feed pressure varies by 10-15% - Normalized permeate conductivity varies by 10-15% - Pressure drop between feed and concentrate varies by 10-15% Normalized values take into account temperature and salinity variations in feed water. In order to ease cleaning, systems can be equipped with cleaning in place (CIP) station, readily connected to the membranes rack The CIP station, depending on the plant size includes a chemical tank with mechanical or manual stirrer, a CIP pump and a fine filter to avoid debris to enter the membranes. the chemical tank depends on the number of membranes to be cleaned at the same time. Alkaline and acid cleaning solutions are recirculated around the membranes for at least 30 minutes.
- 21. REJECT HANDLING IN DESALINATION Reject of sea water desalination plant are discharged 2-3 kms deep inside the sea through marine pipelines The pressure available in the reject is utilized in the feed system with energy recovery devices for reducing power consumption Rejects can be also taken into another RO system at a higher pressure for maximum utilization Rejects can be put into incinerators to produce salts as the solid waste by evaporation of the liquid. it helps in attaining zero discharge concept ENERGY RECOVERY IN DESALINATION The power required to drive the high pressure pumps is the largest component of operating cost in SWRO membrane system. Most of energy imparted into the feed water flowing to the SWRO membranes leaves with the membranes in the brine reject water. A number of devices have been developed to recover pressure energy from the brine reject stream. Effectivity of an energy recovery device can be judged by quantifying the power consumption of a high pressure pumping system with the device and without the device. Energy recovery devices can be divided into two general categories. CENTRIFUGAL : Captures brine pressure with a turbine and transfer it to an impeller spinning in the sea water Francis turbines, Pelton wheels and turbochargers ISOBARIC : The device accomplish a direct transfer of pressure from the brine to the sea water in constant pressure chambers pressure exchangers where number of free pistons operate to balance and transmit pressure from the brine side to the sea water feed side.
- 22. THANK YOU