Ensuring Optimum Performance of RO Desalination Plant

Transcription

1 Ensuring Optimum Performance of RO Desalination Plant L.Y. Dudley PermaCare International, Nalco Limited Chapel House, Alma Road, Windsor, Berkshire SL4 3HD UK Tel:+44 (0) Fax:+44 (0) INTRODUCTION Reverse Osmosis (RO) desalination systems are being selected increasingly in the industrialised and developing world as an efficient, reliable and cost-effective technology. RO is now widely used for the production of municipal and industrial grade water treating seawater, brackish, or surface water sources. Membrane desalination is a pressurised operation which uses a semi-permeable membrane to reject salts and ionic solutes at its surface allowing water to permeate the membrane as potable quality product water. Typically these membranes are made from polysulphone with a polyamide rejecting layer. The salt rejection characteristics of reverse osmosis membranes typically range from 98%- 99.9%. Some of the earliest examples of municipal RO plant can be found in the arid regions of the Middle East. However, the utilization of membrane plant has spread throughout every region of the world as a viable economic alternative to traditional water treatment. Some of the largest capacity RO systems can be found in the USA, Saudi Arabia, Japan, Spain and Korea, but island communities in the Caribbean, Mediterranean, Canary Islands and the Far East have also seen the advantages of this process and have embraced the technology. Membrane systems are not new to the Caribbean, there are seawater RO systems which have been in operation for many years in Curacao, St Maarten, The Virgin Islands, Grand Cayman and Turks & Caicos producing between 1,000-10,000 m3/d municipal water supply. But in recent years there has been renewed interest and plans for a number of new sizeable systems to be built in the region notably in Trinidad, Jamaica and Barbados.

2 Figure 1: Examples of installed RO plant in the Caribbean- Wangnick Inventory (1) Country Capacity Water Source Year Use m3/d Commissioned Antigua 4,542 sea 1995 municipal Aruba, NA 4542 sea 1991 municipal Bahamas 10,000 sea 1997 municipal Curacao, NA 3,028 sea 1998 municipal Grand Cayman 2,271 sea 1990 municipal St Croix 946 brackish 1993 municipal St. Maarten, 9,800 sea 1997 municipal NA Tortola 1,862 sea 1995 municipal St Thomas 1,022 sea 1997 golf club Turks and Caicos 900 sea 1996 municipal Earier this year the largest brackish water desalination system in the Caribbean was commissioned in Barbados. This has a capacity of 30,000m3/d. Also in Barbados a 5,000m3/d seawater plant (expandable to 7,500 m3/d), is being built by Desalco Ltd (3). This is due to start up in 2001 at the exclusive Sandy Lane Hotel and Golf Club and will supply water for irrigation purposes. The current Point Lisas seawater project in Trinidad will become the largest RO plant in the Western Hemisphere when completed. 2.0 THE IMPORTANCE OF MONITORING AND PRE-TREATMENT It is the responsibility of RO equipment manufacturers (OEMs) to design systems with adequate pre-treatment and membrane configurations capable of producing optimum product quality and quantity. Poorly designed pre-treatment and inadequate plant monitoring and maintenance has in the past tarnished the reputation of RO technology. Today, good plant suppliers give consideration to all aspects of the pre-treatment design, monitoring systems, chemicals and control mechanisms to optimise their systems. Sophisticated design programs and other commercially available computer software aid them. The major plant performance characteristics are; energy consumption, percentage recovery, percentage salt rejection and product flowrate. These factors will dictate the product water quality and cost of production.

3 It is common for energy recovery turbines, low-pressure membrane technology and the provision for dosing of performance chemicals for scale and biogrowth control to be considered and incorporated into the initial plant design. Pre-treatment of all feed water sources is necessary to eliminate or reduce micropollutants in the feed water to the membranes minimising the risk of membrane fouling. This is achieved by removing particles typically larger than 5 microns, which may accumulate on, and 'blind' the membrane surface and its salt rejecting layer. Good pre-treatment with regular monitoring ensures consistently good production rates and water quality whilst optimising membrane lifetime. The degree of physical or chemical pre-treatment needed is dependent on the water source to be treated. This can range from a simple cartridge filter (5-10 micron) to a sophisticated system which may include ozonation, DAF, lime/soda softening, clarification, sand or mixed media filtration and cartridge filters prior to membrane filtration. Chemical pre-treatment may include coagulation, flocculation, acid dosing and antiscalant addition. It is usual to chlorinate the feed water to control microbiological growth, however when operating with polyamide membranes it is necessary to dechlorinate with sodium bisulphite to the feed-stream prior to the membranes to prevent damage by oxidation. In addition to obtaining detailed information about the feed water quality such as chemical composition and SDI, any significant seasonal changes in microbiological activity or ambient temperature should be determined. The Silt Density Index (SDI) measurement is an important on-site test to be performed regularly. An SDI <5 is recommended for spiral wound membranes and an SDI <3 for hollow fine fibre elements, such as the Du Pont membranes installed in some seawater systems in the region. Operators must be aware of the key parameters which influence apparent product flows and fouling potential. Plant monitoring requires daily logging of: System pressure drop Product flowrate Feed water temperature Feedwater analysis, SDI and ph Product water quality This information can be recorded manually or using computer software, which can be interrogated remotely. Some of this raw data requires normalisation to standard temperature and pressure conditions to allow accurate comparison of data easily highlighting marked changes in system performance during the lifetime of a plant. Figures

4 2,3 & 4 illustrate such data during 200 days of operation for one train of a brackish water RO plant located in South America. Figures 2, 3 & 4: Computer Generated Plant Performance Logs

5 3.0 SCALE AND FOULING CONTROL- PRACTICAL ISSUES Foulants most likely to affect the performance of RO membranes are: calcium carbonate, calcium sulphate and other inorganic scales organic matter iron oxide siilts/colloids biofilm silica natural humic and fulvic acids There are a number of effective measures using proprietary chemical treatments, which can be adopted in order to prevent this from occurring. 3.1 Scale Inhibition The primary fouling risk in untreated water treatment systems is that of scale formation which occurs when the solubility of any low solubility salt is exceeded in brine stream. The scaling potential varies with water quality, for example the scaling risk is greater for seawater than for brackish water sources due to the high ionic strength due to the predominance of sodium and chloride ions. Computer programs are available to assist plant designers with selection of appropriate scale inhibitors. Such software will also indicate the optimum plant recovery that can be chemically achieved. An example of this type of software is shown in Figure 5. Selection of an appropriate antiscalant will reduce the risk of scaling and often eliminate the use of hazardous acids, at the same time maintaining efficient plant operation with optimum percentage conversions. Antiscalant dosing can also be a cheaper alternative for membrane protection than ph control or ion exchange softening. There are a number of proprietary polymers and phosphonate based antiscalants available. These products are dosed into the feed water stream prior to entering the membranes. Threshold inhibitors, such as phosphonates, are able to stabilise a wide range of supersaturated salt solutions allowing engineers to design systems with maximum recovery rates

6 Figure 5: Example of an Antiscalant Prediction Program Figure 5 above illustrates the scaling potential of a treated and untreated brackish water brine stream, clearly indicating the advantages of dosing a proprietary scale inhibitor. In this example barium sulphate, calcium carbonate and iron presents the greatest anticipated fouling risk in the untreated brine. Figure 6: Table Showing Scale Inhibition Properties of a Phosphonate Antiscalant Foulant Index Maximum inhibition limits Typical dose rate in feed for Antiscalant A water calcium carbonate (a) LSI < to 4.0 mg/l calcium sulphate Ipb:Ksp < to 2.5 mg/l barium sulphate Ipb:Ksp < to 2.5 mg/l strontium sulphate Ipb:Ksp <12 N/A l calcium fluoride Ipb:Ksp <120 N/A Silica SCR(b) < to 4.0 mg/l Iron IFI <1.2 0 to 6.0 mg/ (a) Where the Stiff and Davis Saturation Index is calculated and Antiscalant A is used, the maximum S&DSI limit has never been reached.

7 (b) Saturation Concentration Ratio (SCR) at 25oC, ph 7.5 with 165 mg/l SiO2 in the concentrate. (c) Iron Fouling Index (IFI) developed by PermaCare International restricts the total iron level in the concentrate stream to 1.2 mg/l. It is important that any antiscalant formulation dosed on-line in a potable water plant has been approved by legislative bodies which authorise their use for municipal supply. Examples of such internationally recognised approvals are those issued by the UK Drinking Water Inspectorate (DWI), the Dutch approval body KIWA and NSF in the USA. Locally assessed approvals also apply in some countries. 3.2 Control of Biofouling The warm climate of the Caribbean region makes biofouling the greatest potential fouling risk assuming that some mechanism for scale inhibition is already in place. Specific pretreatment steps to control biogrowth throughout each treatment stage are essential. Particular attention is required to monitor evidence of high biological activity in the incoming feed water stream and at the sand or media filters and micron cartridge filters. This will be seen in the form of visible biofilm slimes and high microbiological counts in the feed. Regular inspection of every stage of the pre-treatment plant can act as an 'early warning' system before severe fouling of the membranes occurs. Attention should be given to areas of low flow in pipe work ('dead legs') where microbiological activity can accelerate and spread quickly throughout the plant. Bacterial cells (<2 microns) can pass easily through a standard micron pre-filter usually positioned as a final barrier prior to the membranes and if not removed will colonise the membrane surface causing the formation of a biofilm and hence reduced membrane performance. Typical indicators of biofilm development are; declining product flows, decreased salt rejection or increasing pressure differentials. However the threat of many of these fouling situations can be reduced or eliminated by improved system design, better plant maintenance procedures and a tailored biogrowth control program. Non-oxidising biocide formulations are recommended for sanitising membrane systems, as they will not present the risk of oxidative damage to polyamide membranes. Many nonoxidising biocides used by the water treatment industry are not approved for use in membrane systems, either for compatibility, carcinogenic or handling reasons. Membrane suppliers have approved several proprietary biocides for use in membrane systems. There are membrane compatible products, which can be easily deactivated for

8 disposal. These formulations have been successfully used at municipal and industrial plant as off-line sanitising cleaners to control biogrowth. Typically the plant would be shut down for up to 12 hours for biocidal cleaning, but 'fast acting' alternatives are also available. 3.3 Preventative Cleaning Pre-treatment procedures should be in place to remove the potential foulants from RO systems by either chemical or physical treatment. However it is inevitable over a period of time that the membranes will experience a degree of fouling. It is advised that membrane systems undergo periodic maintenance cleaning at least annually. If this is not carried out there may be a significant reduction in output and deterioration of product quality, also the expected membrane lifetime will be severely reduced. Cleaning procedures should be selected depending on the fouling potential of the pretreated feed water. A typical maintenance clean would usually consist of an alkaline surfactant clean to remove organic matter and biofilm followed by and acidic wash to remove traces of inorganic scales. In cases where there has been biofilm accumulation a biocidal clean should also be incorporated. It is advised that all cleaning practices include a period of membrane soaking and that warm cleaning solutions of up to 30oC are used. Ideally the total cleaning solution volume for a standard 8" x 40" spiral element should be 40 litres per membrane, as recommended by the membrane manufacturers. However actual cleaning tank volumes in place may not accommodate this but in such cases a minimum of litres cleaning solution per membrane element (excluding pipe-work volume) is advised. Cleaning may take between 4-12 hours to perform depending on the severity of fouling and may require up to 24 hours downtime incorporating overnight soaking. 3.4 Diagnostic Services for the RO Industry There are a number of analytical procedures, which can be performed on membrane elements taken from a system if fouling or damage is suspected. Membrane autopsy involves the dissection of a membrane. A spiral wound design element is unrolled to reveal the membrane leaves and plastic spacer material, if a hollow fibre module is being autopsied the fine fibres are removed from the fibreglass casing for investigation. Foulant sample is then obtained from a known surface area of membrane or quantity of hollow fibres for chemical and microbiological analysis. Surface analysis techniques and membrane characterisation tests can also be performed as necessary.

9 4.0 BRACKISH WATER RO PLANT, BARBADOS The Ionics Freshwater Ltd plant(2) in Barbados has a capacity to produce 30,000-m3/d potable water (Figure 7). This is the largest brackish water system in the Caribbean and is a joint venture between an American OEM and a local partner. The project was conceived after the drought left the island's capital short of potable water supply. The plant is capable of supplying up to 20% of the island's 264,000 population. This project follows the current world trend towards Build Own and Operate (BOO) plant, with which the potable water supply is produced by the OEM and sold to the local municipality (The Barbados Water Authority). The Ionics Freshwater plant is located close to the capital and the brackish feedwater is taken from 10 wells of approximately ft in depth. Spiral wound membrane technology is used and Toray polyamide RO membranes are installed. Chemical pretreatment includes dosing of a phosphonate antiscalant called PermaTreat 191 to inhibit scale formation and enable optimum percentage plant conversion. Provision has been made for annual maintenance cleaning of the membranes. Figure 7: Ionics Freshwater Ltd., Barbados (30,000 m3/d) The plant has been producing drinking water for mains supply since February 2000.

10 5.0 CONCLUSIONS Reverse osmosis is now a viable economic alternative to traditional water treatment purification processes, for both municipal supply and industrial use. Technological advances in membrane materials, energy recovery systems and improved proprietary antiscalants and cleaners have helped to optimise plant performance and production costs. Good plant design, operator training and regular monitoring and control practices are critical to the continued success of this technology. We will see a growing demand for membrane desalination in the Caribbean over the next decade as potable quality water requirements increase. A trend towards replacement of softeners with RO to treat boiler feed water is also anticipated. 6.0 REFERENCES IDA Worldwide Desalting Plants Inventory Report No.15, Published by Wangnick Consulting GmbH. 2. Article in 'Water and Wastewater International' June 2000 (pages22-24). Ionics Freshwater Ltd. 3. Private correspondance with Mr Rin Barensden, Desalco (Barbados) Ltd. Presented at: CWWA Conference, Trinidad 4th October 2000