ORKS DIRECTORATE ENGINEER-IN-CHIEF BRANCH MILITARY ENGINEER SERVICES MINISTRY OF DEFENCE IHQ (ARMY)

Transcription

1 TI NO. 03/2011 REVERSE OSMOSIS WORKS DIRECTORATE ENGINEER-IN-CHIEF BRANCH MILITARY ENGINEER SERVICES MINISTRY OF DEFENCE IHQ (ARMY)

2 TI 03/2011 TECHNICAL INSTRUCTIONS ON REVERSE OSMOSIS DIRECTORATE OF WORKS ENGINEER-IN-CHIEF BRANCH MILITARY ENGINEER SERVICES MINISTRY OF DEFENCE IHO (ARMY)

3 TI 03/2011

4 TI 03/2011

5 TI 03/2011 I N D E X TOPIC PAGE NO. 1. GENERAL 1 2.REVERSE OSMOSIS 2 3.GLOSSARY OF TERMS 3 4. DESALINATION PROCESS 4 5. LOG SHEET INFORMATION 6 6. MAINTENANCE 7 7. DO S AND DONT S 9 8. RO MEMBRANE ELEMENT CLEANING & FLUSHING 9 9. REMINERALISATION TROUBLESHOOTING 21

6 REVERSE OSMOSIS 1. GENERAL Even though water is a renewable resource, but its availability for the society is limited. There is a tremendous pressure on the available water resources due to increasing population and growing water consumption. The need for better management of available water resources is very much necessary to meet the basic necessities for ever-increasing population and industrial activities and to provide hazard-free water for the society. Sea water, though available in plenty, is unusable for all practical purposes without treatment. Areas that have either no or limited surface water or groundwater may choose to desalinate sea water or brackish water to obtain drinking water. Reverse osmosis is the most common method of desalination. Effective usage of Reverse Osmosis (RO) technology can make this abundant resource available as viable source of water in many coastal areas. TOTAL AVAILABLE WATER IN THE VARIOUS COMPONENTS OF THE HYDROLOGICAL CYCLE Components Volume of Water (TM30) Volume % (APP) Oceans % Ice Caps and Glacier % Groundwater and Soil Moisture % Freshwater Lakes % Saline Lakes % Rivers % Atmosphere % 1

7 2. REVERSE OSMOSIS Reverse Osmosis (RO) is primarily, a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely. Reverse osmosis is the reverse of the normal osmosis process, in which the solvent naturally moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The movement of a pure solvent to equalize solute concentrations on each side of a membrane generates a pressure and this is the "osmotic pressure." Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. In order to know the RO technology, the osmosis process is to be understood. Osmosis is defined as The tendency of a fluid to pass through a semi permeable membrane, like the wall of living cell, into a solution of higher concentration, so as to equalize concentrations on both sides of the membrane. When a Semi permeable membrane separates two solutions with different concentrations, the solution with the lower concentration travels through the membrane to higher concentration. This process will continue until the ionic concentration becomes equal. Osmosis Osmotic Pressure DILUTE SOLUTION OSMOTIC PRESSURE SEMI PERMEABLE MEMBRANE 2

8 Reverse Osmosis APPLIED PRESSURE Reverse Osmosis is defined as a process of a fluid to pass through a semi permeable membrane to lower concentration solution by applying external pressure. By applying external pressure, on the solution containing the higher concentration of dissolved ions, thus forcing the water through the Semi - permeable membrane, in the opposite direction leaving behind the dissolved ions and suspended solids. 3. GLOSSARY OF TERMS RECOVERY-The water, which passes through membrane as useful product is called, permeates. Remaining water, which does not pass through membranes and is discarded to drain with most of the slat from feed water is called reject. Recovery is defined as ratio of Permeate flow to the feed flow. SALT PASSAGE - Permeate TDS/ Feed TDS SALT REJECTION - (1-Salt passage) x 100 MEMBRANE FLUX- Amount of water flowing through unit membrane area in unit time. The unit of flux is gfd or lmh. ARRAY- RO plant has modular construction i.e. it is made by combining number of RO membranes, which are the basic building element of the plant. This arrangement of membrane is called array. 3

9 LANGELIAR SATURATION INDEX (LSI) - An index that shows the tendency of a water solution to precipitate calcium carbonate. LSI is used primarily for brackish water RO applications. 4. DESALINATION PROCESS Desalination of water using Reverse osmosis process employing membranes, are most commonly used methods for treatment of Sea/brackish water since the early 1970s. As no heating or phase changes are needed, energy requirements are low. The typical single-pass system consists of the following components: Intake Pretreatment High pressure pump Membrane modules Remineralisation and ph adjustment Disinfection Alarm/Electric control panel TYPICAL FLOW DIAGRAM 4

10 A. PRETREATMENT - Purpose and role of pretreatment for RO installation. To remove turbidity/ suspended solids. To inhibit or control the formation of scales To inhibit or control the formation of sparingly soluble salts. To reduce the organic load. To avoid biological growth. To avoid heavy metal fouling Pretreatment is important when working with RO and nano filtration (NF) membranes due to the nature of their spiral wound design. Since accumulated material cannot be removed from the membrane surface systems, they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any RO or NF system. B. HIGH PRESSURE PUMP - The pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it. Typical pressures for brackish water range from 225 to 375 psi (15.5 to 26 bar, or 1.6 to 2.6 MPa). In the case of seawater, they range from 800 to 1,180 psi (55 to 81.5 bar or 6 to 8 MPa). C. RO PRESSURE VESSEL - Pressure vessels can be manufactured to house a single RO element or multiple elements. The only difference is the length of pressure vessel. The range is from 1-7 elements per pressure vessel. D. SELECTION OF MEMBRANE - The membrane assembly consists of a pressure vessel with a membrane that allows feedwater to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. RO membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and hollow-fiber. 5

11 E. RE-MINERALISATION AND ph ADJUSTMENT - A typical downside to RO as a method of purifying drinking water is the removal of healthy, naturally occurring minerals in water. The membrane of a reverse osmosis system is impermeable to natural trace minerals, which offer health advantages for the body. Water which has been stripped of these trace minerals at time requires mineralisation separately. Liming material is used to adjust ph between 6.8 and 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control. Detail method is given subsequently. F. DISINFECTION - Post treatment consists of stabilizing the water and preparing it for distribution. Desalination processes are very effective barriers to pathogenic organisms; however, disinfection is used to ensure a "safe" water supply. Disinfection (sometimes called germicidal or bactericidal) is employed to sterilise any bacteria, protozoa and viruses that have bypassed the desalination process into the product water. 5. LOG SHEET INFORMATION Flow Feed flow Product flow Reject flow Pressure Cartridge filter High pressure pump Feed pressure Inter-stage pressure Reject pressure TDS Feed Product Reject 6

12 Free Chlorine Content (if any) Dosing Chemical Details Number of Hours run Cleaning frequency Details 6. MAINTAINANCE A. SANITIZATION- In order to preserve the R. O Membrane, it is necessary to sanitize the system on a regular basis. The most efficient sanitization is achieved through injection of diluted Formaldehyde solution into the feed water line with an accessory injection pump. Flushing of solution is required before taking into service. B. REGULAR CHECKING -It is recommended to check the pressure drop across the membrane on regular basis and it should not increase more than 2 Kg/cm 2. C. PROLONGED SHUT DOWN - If the system is shut down for more than one week, it is necessary to fill the tube with Sodium Bisulphate solution of 10 PPM or 3% formaldehyde solution or fallow long term membrane storage procedure, however membranes must be flushed properly before taking into service.. D. PUMP INSPECTION-Once in a week, pump should be checked for any mechanical damage, current drawn. Necessary steps as given in manual should be carried out in case of any performance deterioration. E. CHECKUP FOR NORMAL STARTUP Pretreatment/ post treatment chemicals are prepared freshly. Electrical control panel: Manual auto switch should be in OFF position. Manual feed water isolation valve- Open RO unit concentrate valve should be open RO high pressure pump discharge throttling valve should be 25% open ph, Chlorine, Hardness Levels should be checked as per the limits specified. 7

13 F. Storage of Membranes SHORT TERM STORAGE days - flush with RO water every day for 1 hour Recirculation. LONG TERM STORAGE - Above 15 days - flush with 0.1 to 1% concentration of formaldehyde (or) 1% of sodium bisulfate solution. DRY STORAGE - Prior to installation, membrane should be protected from direct sun light and should be stored in a cool dry place of ambient temperature between deg. C G. MAINTAINANCE SCHEDULE Sr, No. Activity Frequency 1 Inspect and attend leaks Daily 2 Change Filter Cartridges Once in three month or when pressure drop >1.0 kg/cm2 whichever is earlier. 3 Lubrication of HPP Once in 6 months. 4 RO membrane cleaning When the normalized productivity rate is more than 15% below that for clean membrane, or When the pressure drop across the membrane increases nore than 2 kg/cm 2. 5 RO membrane replacement When after cleaning performance is not restored to the desired extent, even after replacing the O rings & Brine seals. 8

14 7. DO S AND DONT S DO S Do check the water level in feed tank, so that the pump suction is fully flooded. Check water temperature regularly and shut off the system immediately if feed water temperature is greater than 40 0 C Clean the pre filter when pressure drop across it exceeds 15 psi Do flush the system sufficiently for about 30 min at rated capacity before taking samples. DONT S Don t run the system dry or without sufficient water in the buffer. Don t run the system with feed water temperature more than 45 0 C. Don t run the system when pressure drop across pre filter exceeds psi Don t take sample without proper flushing of dead zones in the system. 8. RO MEMBRANE ELEMENT CLEANING AND FLUSHING The membranes elements in place in the pressure tubes are cleaned by re circulating the cleaning solutions across the high pressure side of the membrane at low pressure and relatively high flow. A cleaning unit is needed to do this. CLEANING SOLUTIONS The following chemical solutions ate recommended for cleaning the RO membrane elements. The appropriate solutions to use can be determined by chemical analysis of the fouling material. A detailed examination of the results of the analysis will provide additional clues as to the best method of cleaning. Keeping records of the methods used and results obtained will provide data useful in the methods and solutions that work best under the feed water conditions at hand. 9

15 Solution 1 is recommended for inorganic fouling. Solution 2 is recommended for Calcium Sulfate and organic. Solution 3 is recommended for high organic fouling. All solutions are to be used the highest available temperature up to 40 0 C or upto 60 minutes of cleaning. Prepare the solutions by proportioning the amount of chemicals to the amount of cleaning water to be used. Use chlorine free permeate to mix the solutions. Mix thoroughly. If additional information is needed, please contact the manufacturer. A general procedure for cleaning the RO membrane is as follows: Flush the pressure tubes by pumping clean, chlorine free product water from the cleaning tank (or equivalent source) through the pressure tubes to drain for several minutes. Mix a fresh batch of the selected cleaning solution on the cleaning tank, using clean product water. Circulate the cleaning solution through the pressure tubes for approximately one hour or the desired period of time, at a flow rate of 133 to 151 L/min per pressure tube for 8.0 and 8.5 inch pressure tubes, 57 to 76 L/min for 6.0 pressure tubes, 34 to 38 L/min for 100 mm pressure tubes. After completion of cleaning, drain and flush the cleaning tank; fill the cleaning tank with clean product water of the same ph as that of cleaning solution for rinsing. This is done to avoid precipitation of the matter which was dissolved during cleaning. Report above step with permeate at neutral ph. After the RO system is rinsed, operate it with the product dump valve open until the product water flows clean and is free of any foam or residues of cleaning agents (usually 15 to 30 minutes). 10

16 Summary of Recommended Cleaning Solutions Solution Ingredient Quantity per 100 Ltr 1. Citric Acid 2 kg RO Product (chlorine free) 2. Sodium Tripolyphosphate Tetra Sodium EDTA (Versen 220 or equal) RO Product (chlorine free) 3. Sodium Tripolyphosphate Sodium Dodecylbenzen RO Product (chlorine free) 100 liter 2 kg 1.85 kg 100 litre 2 kg 250 gm 100 liter ph Adjustment Adjust to 4.0 with Sodium hydroxide (NaOH) Adjust to 10.0 with Sulphuric Acid (H2SO4) Adjust to ph 10.0 with Sulphuric Acid (H2SO4) 11

17 RO Membrane Element Foulant Symptoms FOULANT GENERAL SYMPTOMS RESPONSE 1. Calcium Precipitates (Carbonate and Phosphates, generally found at the concentrate end of the system. A marked decrease in salt rejection and a moderate increase in pressure between feed and concentrate. Also a slight decrease in system production. Chemically clean the system with solution Hydrated Oxides (Iron, Nickel, Copper etc.) A rapid decrease in salt rejection and a rapid increase in pressure between feed and concentrate. Also a rapid decrease in system production. Chemically clean the system with solution Mixed colloids (Iron, Organic and Silicates) A slight decrease in salt rejection and a gradual increase in pressure between feed and concentrate. Also a gradual decrease over several weeks in system production. Chemically clean the system with solution Calcium Sulfates (generally found at the concentrate end of the system) A significant decrease in salt rejection and a moderate increase in pressure between feed and concentrate. Also a slight decrease in system production. Chemically clean the system with solution Organic deposits Possible decrease in salt rejection and a gradual increase in pressure between feed and concentrate. Also a Chemically clean the system with solution 2. For heavy 12

18 gradual decrease in system production. fouling, Solution 3. use 6. Bacterial Fouling Possible decrease in salt rejection and a gradual increase in pressure between feed and concentrate. Also a gradual decrease in system production. Chemically clean the system with either of the solutions, depending on possible Compound fouling. 9. REMINERALISATION Un-treated water produced by sea water reverse osmosis (SWRO) and thermal processes differ in product water salinity. The water produced by thermal distillers, is usually, characterized by very low salinity and high aggressiveness and corrosivity. Post treatment of seawater reverse osmosis permeate, with particular emphasis on carbon dioxide (CO 2 ), acid dosing and boron issues. A number of chemical factors influence the aggressiveness and corrosiveness of water produced by reverse osmosis and thermal distillation plants. Stabilisation of the distillate by recarbonation to a level that is nonaggressive, non-corrosive and palatable to consumers is therefore required. Langelier saturation index and Larson Indices are commonly used indicators of the aggressiveness and corrosiveness of potable water. The quality goals for remineralisation will depend on factors such as nature of distribution pipe work materials, palatability and client preference. An important factor influencing palatability, and which can strongly influence user preference, is the total dissolved solids content of the water. Blending remineralised desalinated water with treated brackish groundwater or treated seawater are frequently the cheapest options for increasing the dissolved solids content of desalinated water, if there is a requirement to provide a total dissolved mineral content of 150 mg/l or above in the final product water. The choice between the different processes is frequently project specific and depends on issues such as: installed capital and operating costs; volume of distillate to be treated; availability, quality and cost of locally available chemicals; issues associated with carbon dioxide such as ease and quantity recoverable from thermal desalination processes, feed water ph and resultant carbon dioxide present within SWRO permeate. Additionally boron has become an important issue associated with SWRO plants, feed water ph can influence boron removal efficiency and therefore the optimum and boron removal options associated with SWRO desalination. Additionally the simplicity of the competing 13

19 processes; client experiences, whether good or bad with the competing processes. Disinfection of desalinated water can be undertaken by utilising various forms of chlorine/hypochlorite, the cost of which will vary depending upon size of plant, client preference and availability of chemicals. Remineralisation can be distinguished, for the purpose of this TI, into four treatment processes: (a) Chemical addition excluding lime or limestone (b) Carbon dioxide addition followed by limestone or dolomite dissolution (c) Carbon dioxide addition followed by lime dosing (d) Blending with a water containing high mineral content. The methods available for recarbonation and remineralisation offer varying economic and technical merits and disadvantages. The actual detailed design or operation of the different processes can also control the quality of water produced, for example inclusion of a second pass within the SWRO or demister height above the brine surface at a particular brine temperature can strongly influence permeate/distillate salinity. The numerous issues associated with SWRO and thermal plant design are outside the scope of this paper; however SWRO pre-treatment and its influence on post treatment will be discussed further. The quality of the water produced from desalination processes and intended for human consumption, should meet all relevant standards and/ or guideline values i.e. World Health Organisation (WHO) Guidelines for Drinking Water Quality. Values for selected parameters are shown in below. Colour. Pt-Co Scale 15 Turbidity, NTU 5<I 2 Taste Acceptable Odour Acceptable ph <8 Total Dissolved Solids. mg/l <1000 Electrical Coductivity@25 o C, µg/cm Total Hardness,mg/l as caco Calcium, mg/l 200 Magnesium, mg/l 150 Sodium, mg/l <200 Chloride, mg/l <250 Aluminium, mg/l Iron, mg/l

20 Copper, mg/l 1.0 Zinc, mg/l 3.0 Manganese, mg/l 0.1 Residual Chlorine, mg/l In addition to meeting drinking water standards, the desalinated water should be non aggressive and non-corrosive. METHODS OF RECARBONATION AND REMINERALISATION FOR DESALINATED WATER Recarbonation for the purposes of this TI shall be defined as the introduction of bicarbonate and carbonate alkalinity and remineralisation as the increase of mineral content by means in addition to those which increase the bicarbonate or carbonate alkalinity of the desalinated water. Recarbonation will also re-mineralize the water in question. Provision of a total mineral content of approximately > 85 mg/l to the treated desalinated water by chemical recarbonation alone is more expensive than utilising a high salinity source for blending with the stabilized re-carbonated water. The value of 85 mg/l is an approximation associated with desalinated water produced from thermal as opposed to membrane based seawater desalination plants. There are various methods for increasing the mineral content of desalinated water; the primary requirements are to produce a slightly positive LSI and by increasing the bicarbonate alkalinity and ph value of water. Achievement of minimum TDS values is not usually an issue with desalinated water from SWRO plants, but it can be with distillate produced from thermal desalination units. This TI will not discuss the various issues relating to TDS increase associated with, for example, brine carry over within thermal desalination plants. With SWRO plants, a guarantee for the non-exceedance of maximum TDS and chloride is usually a more relevant issue. Recarbonation and remineralisation can be achieved by any of the following methods, but not all will be applicable for recarbonation and remineralisation of distillates from thermal and RO seawater desalination plants, due to cost, complexity of process, material handling issues etc. METHOD 1. APPLICATION OF CARBON DIOXIDE AND EXCESS HYDRATED LIME 2CO 2 + Ca(OH) 2 Ca(HCO 3 ) 2 This method is commonly used to add alkalinity to water to make it non-aggressive and/or non-corrosive. It is therefore widely used in the treatment of desalinated water. This process in relatively simple, the major complications being those associated with 15

21 the majority of lime systems throughout the world. Many of these complications can be solved if the correct design is used. For example: The installation of anti bridging devices to storage silos. There are many such systems, some more successful than others. Keeping the lime slurry to pump pipe work as short as possible and ensuring flow velocities of at least 1.5 m/s. The inclusion of access points in pipe work which may be prone to scaling. The use of alkalinity free make up and carrier water for lime slurry. Hydrated lime delivery lines (assuming pressurised off-loading from bulk delivery should be kept short and straight. Hydrated lime discharge and slurry preparation systems should be located indoors. Sizing of hydrated lime silos to minimise the possibility of carbon dioxide reaction with lime surface within the silo. Dry air or nitrogen aeration of a silo would also help to minimise calcium carbonate formation on the lime surface. Use of 3% lime slurry solution strength as optimum. Keeping lime slurry suction pipe work simple and short. If the hydrated lime supplied is not at least >96% Ca(OH) 2, production of limewater using saturators in addition to lime slurry preparation systems may well be required for stabilisation of thermal plant distillates, in order to produce a final water with turbidity less than 5 NTU. WHO Guidelines for drinking-water quality, third edition state The appearance of water with a turbidity of less than 5 NTU is usually acceptable to consumers, although this may vary with local circumstances. Use of lime saturators for treatment of SWRO permeate will be dependant on the quantity of lime required to produce a water which meets all of the required water quality requirements and is non-aggressive/corrosive to materials in contact with the water. The use of hydrated lime slurry will undoubtedly increase product water turbidity, and if hydrated lime below 96% Ca(OH) 2 is utilised without the use of saturators, a final water turbidity of greater than 5NTU will most likely result. This would be non compliant with, 16

22 for example, Schedule 2 of the Water Supply (Water Quality) Regulations 2000 in the United Kingdom. It is unlikely that the 5 NTU WHO guideline for turbidity would be exceeded if lime slurry dosing utilising hydrated lime of 98% Ca(OH) 2 were undertaken. However, process control problems for such de-salinated waters with low buffering capacity would be expected, i.e. the treatment of desalinated water with lime slurry alone will undoubtedly create problems in maintaining consistent product water ph, and consequently LSI requirements. For big desalination plants where relatively large quantities of hydrated lime are utilised, only bulk hydrated lime delivery and silo storage should be considered. That is, bag handling systems are not appropriate in this situation. METHOD 2: PASSAGE OF THE DESALINATED WATER DOSED WITH CARBON DIOXIDE THROUGH A BED OF LIMESTONE This process will produce water with a ph equal to the ph s. The reaction in the process is as : CO 2 + CaCO 3 + H 2 O Ca(HCO 3 ) 2 The method theoretically requires only 50% of the quantity of CO 2 required in Method 1, because of the contribution from the carbonate content of the limestone (CaCO 3 ). However, in practice, the CO 2 requirement of the limestone process compared to the lime process could typically be 65 85% of the required CO 2 quantity used within the lime process. Disadvantages of this process compared to method 1 include a greater degree of plant and process complexity, for example, the following plant items are required in addition to those necessary for method 1: Limestone absorption units (commonly referred to as filters, but not a strict definition of the process). CO 2 desorption tower(s). Re-pumping chamber(s). Lime, caustic soda or sodium carbonate dosing will be required to neutralise any excess CO 2. Although CO 2 stripping is commonly practised, one of these chemicals is usually required to remove the last traces of free CO 2 to attain the 17

23 desired alkalinity and ph. Additionally, in practice stripping of CO 2 to very low concentrations is not used because of the potential of CaCO 3 scaling problems which may occur within the tower. Acid or scaling inhibitor dosing to the CO 2 stripping towers may be undertaken as an alternative or in combination with partial CO 2 stripping. There is a need for limestone of very high purity, as impurities could be added to the water. Hydrated lime requirement is normally only 70 80% of the mass equivalent limestone usage. Typically the density (not bulk density) of 95% limestone is kg/m 3 compared with that for lime of kg/m 3 METHOD 3: APPLICATION OF HYDRATED LIME AND SODIUM CARBONATE Ca(OH) 2 + Na 2 CO 3 CaCO 2 + 2NaOH The method tends to produce a non-adherent CaCO 3 deposit. This mode of treatment is more appropriate to natural water containing some alkalinity and free CO 2. This method is not used in the treatment of large desalinated water treatment plants primarily due to the high operating cost associated with sodium carbonate. METHOD 4: APPLICATION OF SODIUM BICARBONATE AND CALCIUM SULPHATE CaSO4 + 2NaHCO 3 Ca(HCO 3 ) 2 + Na2SO 4 This method is not generally practiced since calcium sulphate has a low solubility in water (0.23% by weight), and therefore the dosing entails the use of very large dissolving tanks for chemical makeup. Furthermore, sodium bicarbonate is very costly, also has a low solubility, and storage of both the chemicals under humid conditions is difficult as they tend to cake in the presence of moisture. In addition, further ph correction will be required. This method is not used in the treatment of water. METHOD 5: APPLICATION OF SODIUM BICARBONATE AND CALCIUM CHLORIDE CaCl 2 + 2NaHCO 3 + Ca(HCO 3 ) 2 + NaCl As in Method 4, further ph correction will be required. The method will increase the chloride concentration of the water which could, in turn, render the water corrosive. Both chemicals are expensive and their storage under humid conditions is difficult. Calcium chloride is deliquescent. Plants which require a minimum TDS of for example 200 mg/l 18

24 or above could benefit by the presence of increased concentrations of chloride. In reality this would be an impractical method of obtaining the minimum required product water TDS. The method is sometimes used in the treatment of industrial water, but not in the treatment of desalinated water for municipal use. METHOD 6: BLENDING WITH A TREATED SALINE WATER SOURCE Only partial stabilisation can be achieved by blending the desalinated water with mineral rich waters such as brackish ground waters or seawater. This can help to improve the organoleptic quality of the water. This option is generally undertaken only for distillates from thermal desalination facilities but it must be stressed that blending alone will not achieve all of the necessary required water quality requirements, i.e. CO 2 and either lime or limestone treatment will still be necessary. For the permeate TDS values obtained from SWRO and brackish water reverse osmosis (BWRO) treatment plants blending is not necessary to increase the product water TDS. If a blending water source is derived from a limestone or chalk geological formation, recarbonation and hence stabilisation may be achieved to a greater extent than from non CaCO 3 bearing geological formations. If blending from a CaCO 3 Ground water is possible, substantial savings may be made in the operating and capital cost of the post-treatment plant required. The size and cost of the post treatment plant will decrease if groundwater s derived from CaCO 3 bearing geological formations are available, the size and cost will decline as: Groundwater alkalinity increases. Chloride and sulphate to alkalinity ratio decreases. Other non desirable contaminants decline in concentration. Available yield from groundwater source increases. When treated seawater is used for blending in the treatment of thermal distillates the major ions added are sodium and chloride and the increase in alkalinity of the blended water is insignificant. Only a very small amount of seawater can be added without increasing the TDS beyond the particular plant requirement, too much seawater will affect the organoleptic properties and corrosiveness (Larson Skold index) of the water. Therefore, this method is only practical when supplemented with chemical treatment to achieve the required LSI and Larson s index. Product water from thermal distillation plants with a requirement of 100 mg/l TDS only marginally warrants the use of seawater 19

25 blending from an economic perspective, assuming that there is a requirement to achieve non aggressive and noncorrosive product water. The % seawater to recarbonated distillate would be so small (typically %), but even so, the additional cost of the seawater treatment plant would not be offset by the increased operational cost associated with achieving 100 mg TDS by the addition of for example extra lime + CO 2 or limestone + CO 2. If there is not a requirement to achieve non corrosive water (Larson Skold index of <1) then use of seawater blending becomes an increasingly attractive option for thermal desalination plants. Use of a good quality brackish water or beach-well source will negate the requirement to treat the high TDS source water; this option will decrease the complexity, manning and cost of the blending option. Another issue of note should be the source or abstraction point of the seawater. For example in some locations the seawater could be subjected to concerns such as: Intense algal episodes associated with micro or macro algae and cyanobacteria could result in toxins being introduced to the water. Petroleum hydrocarbons. Some chemicals which are abundant in seawater, such as boron (up to about 6 mg/l as B), need special consideration with respect to SWRO treatment requirements. Although asmentioned previously, the blending of treated seawater with SWRO permeate is unnecessary. Provided that sufficient treatment is provided for the seawater, and given the relatively small proportion required to achieve the distillate TDS goal, no quality problems should be associated with blending with treated seawater. Alternatively, suitable hydro geological conditions in the vicinity of the plant may allow beach wells to be utilised, which would negate some of the quality concerns mentioned above. The use of brackish water sources may be feasible in certain locations; a higher proportion of brackish water to re-carbonated distillate is required. Close attention to the effects of utilising such brackish sources on other potential users of the brackish sources is required prior to their utilisation. Seawater or brackish water blending is sometimes utilised on thermal desalination plants, but never on SWRO plants. An important issue for new power water cogeneration schemes to consider is the use of thermal/swro hybrid plants. Such plants can provide valuable benefits in providing ideal blended water TDS values and more importantly providing valuable economies 20

26 from power generation. Lime, limestone and CO 2 issues are also relevant to the hybrid plants post-treatment method. 10. TROUBLESHOOTING Sl No Symptom Likely cause Consequence if not corrected Corrective action Change in quality of raw water Check raw water quality 1 Increase in CF outlet Coagulant/flocculant dosing malfunction Channeling in the sand filter. Inferior quality of chemical Fouling membranes leading reduction product flow. of to in Check coagulant/flocculant dosing system. Carry out fresh jar test if required. Check sand filter Use right quality of chemicals. Inadequate backwash of filter Give sufficient backwash to the filters. 2 High or low ph Acid level low / blockage in acid line Feed water quality / flow change Fouling membranes damaged the membranes of to Refill acid / check acid pump suction and discharge. Check feed water flow, adjust dosage as needed 3 Residual cartridge filter outlet Change in quality of raw water. Mal operation of NaOCl pump Change in raw water flow Damage to RO membranes Check raw water quality. Adjust dosing rate. Repair the pump Adjust raw water flow. 21

27 4 Low HPP suction Raw water pump malfunction Malfunction auto feed water valve / solenoid valve. HPP may get damaged due to cavitations. Check raw water pump. Give backwash to MGF, change cartridges, recalibrate the pressure gauge / pressure switch. Check solenoid valve / auto feed water valve. 5 Auto feed water valve does not open Solenoid malfunction valve Wrong installation of solenoid valve RO unit will not function Check solenoid valve. Check installation Increase in RO feed conductivity Lower product flow Check the raw water quality. 6 Increase in RO feed pressure Fouling / scaling of membranes Reject valve mal operation, pressure gauge mal operation Lower membrane life Membrane cleaning Adjust recovery RO flow and pressure imbalance Adjust the reject control valve. 7 Low flow reject HPP malfunction Incorrect reject flow indicator Scaling membranes of Check HPP Calibrate reject flow indicator 8 Increase in pressure drop Pressure malfunction Membrane scaling gauge Entry of a particulate material Decrease product flow Lower membrane life in Check pressure gauge Clean the membrane Check SDI and particulate content 9 Decrease in normalized Scaling membranes of Lower product flow. Increase Check raw water quality / SDI 22

28 permeate flow Malfunction permeate indicator of flow in RO feed pressure Clean the membranes and check the permeate flow indicator. Change in temperature of water Use suitable temperature correction factor 10 Decrease in normalized salt rejection O ring leakage. Membrane damage Product conductivity indicator malfunction Inferior product quality Probing to find O ring leakage / membrane damage Recalibrate conductivity indicator product 23