CONSIDERATIONS IN THE DESIGN

OF SEAWATER DESALINATION SYSTEMS

Gaining Water Production Efficiency Through Advanced Reverse Osmosis Technology

During the past decade, reverse osmosis equipment has steadily gained acceptance as a reliable, economical method of producing potable water from seawater. Reverse osmosis technology utilizes high pressure pumps and semi-permeable membranes to remove salt from seawater. The energy required to sufficiently desalt the seawater to meet potable water standards is proportional to the concentration of salt in the seawater and is directly influenced by system design, membrane productivity and cleanliness and pump and electric motor efficiency. Seemingly small differences in system design, features and operation can impact the actual cost of operation of a desalination plant significantly.

HydroMax, Inc. has taken tremendous strides in the production and operation of advanced seawater desalination plants. HydroMax Seawater Desalination Systems are now recognized to be the most efficient seawater conversion systems available. In order to gain this extreme efficiency, HydroMax spent many hours of research, experimentation and testing of various system configurations. We can offer systems that incorporate such advanced features as inline, water cooled, high pressure pumps, high productivity thin film composite reverse osmosis membranes, corrosion proof fiberglass membrane vessels, all 316 stainless steel pressure piping, back-washable raw water filters, ultraviolet sterilizers, the most advanced energy recovery turbine available today and a standard automatic control package that includes remote monitoring capabilities and electronic motor power consumption controls.

The following are explanations of areas that HydroMax, Inc. has identified as being important in the design of efficient, reliable desalination plants:

1. PRETREATMENT

   Proper preparation of the raw seawater for treatment is essential for cost effective desalination plant operation. Whenever possible, a sand well or bore hole should be the source of the seawater. As the seawater penetrates through the earth, a large portion of the biological and particulate material is removed by filtration. If it is not possible to gain access to seawater from a well, proper screening and filtration should be provided prior to the water entering the HydroMax system.

   Most frequently, the HydroMax system includes a self cleaning, permanent media, pressure sand filter as the initial barrier against waterborne debris. The HydroMax filters are easily back-washed of entrapped debris on a schedule according to the raw feed water conditions.

   An ultraviolet sterilizer should be installed following the sand filters for effective non-chemical reduction of biological contamination. Utilization of an ultraviolet unit helps to decrease the severity of biological fouling in the membranes and destroys bacteria that could be transported to the potable water. It is important that the ultraviolet unit is fabricated from corrosion proof materials and be sized for the feed water flow rate and clarity.

   The raw water should then flow through a 5 micron rated cartridge filter. HydroMax uses high-flow style pleated polyester filter cartridges. The durability and efficiency of the filter element is very important. Cheap filter cartridges are known to disintegrate during service and allow silt to flow through to the membranes. This fine silt can coat the membrane surface and decrease water production or even totally foul the membrane. Whenever membrane production decreases, it takes more time to produce a given amount of water. More operating time means more energy costs. It is considered wise to invest in a good cartridge filter system. With a back-washable sand filter upstream, even a very good quality cartridge filter system is economical to operate.

   It is usually recommended that the pH of the feed water be decreased prior to entering the reverse osmosis membranes. Sparingly soluble salts, such as calcium, are better kept in solution at a lower pH and are less likely to cause fouling of membrane surfaces when the pH of the feed water is controlled within a range of 6.0 — 7.0. Membranes can operate much longer, without frequent cleaning, when operated in this fashion. Reduced maintenance contributes to more economical system operation.

2. PRESSURE BOOSTING PUMP SYSTEM

   In order to produce potable water from salt or high salinity water, the pressure of the feed water must be elevated to between 600 and 1000 psi. This pressure increase is accomplished though the use of high pressure pumps. Typically, electricity is the most convenient method of driving these pumps. In most cases, HydroMax utilizes an advanced, in-line, water cooled pump for pressure boosting service. This type of pump has the advantages of extremely quiet operation, efficient transfer of power into flow and pressure, no external heat generation and the ability to operate in any ambient environment.

   The motor noise from the HydroMax in-line pump is muffled by the water flowing around it and by the pump casing. The smooth operation of the turbine impellers in the HydroMax pump are silent and do not generate high pulsations or clatter like piston type pumps that are frequently used in desalination plants. Unlike an open frame pump and motor, the HydroMax pumps can be used very close to noise sensitive areas and do not require air conditioning if used in enclosed spaces.

   The HydroMax in-line pump is permanently self lubricated and never requires any grease or oil. Since there are no friction wear parts in the HydroMax pump, there are significantly fewer parts to wear and replace. Since the motor is internal, and is submerged in water, there is no need for a potentially troublesome packing gland or mechanical shaft seal. The HydroMax pumps are constructed of passivated and electropolished stainless steel for corrosion protection. If a pump inspection or repair is required, the modular design of the HydroMax pump allows for quick, simple motor or impeller set disassembly and replacement.

   The HydroMax in-line pump is water cooled. It can therefore easily operate in ambient temperature conditions of over 150 degrees F and is unscathed by dusty, sandy or corrosive external environments. Other pumps with external, air cooled motors can not come close to tolerating these commonly encountered desalination plant conditions.

   The primary HydroMax in-line pumps are typically installed in sets of two pump modules arranged in series. A third pump captures the energy gained from the energy recovery turbine and injects a high pressure stream of feed water into the system for more efficient operation. Pump impeller sets and motors are selected depending upon the salinity of the feed water. The HydroMax pumping arrangement allows for precise matching of pump output to the feed water conditions, Electronic motor power output controllers provide pumping flexibility and major energy savings in installations where the feed water quality may fluctuate.

3. MEMBRANES

   The reverse osmosis membranes used in the desalination plant determine the product water quality and output flow attainable. Membrane selection has a major impact upon the energy required to produce a given amount of water. If a membrane can produce water of good quality faster, or at lower pressure, the system pressure pumps will operate less frequently or with less horsepower to produce a given amount of water. Additionally, proper membrane selection can decrease the frequency of membrane cleaning or membrane replacement.

   HydroMax uses only top quality, field proven membranes in our desalination plants. High efficiency, advanced composite polyamide spiral wound seawater membranes provide the best performance possible and require the least amount of energy input.

4. ENERGY RECOVERY AND REDUCTION DEVICES

   HydroMax, Inc. systems utilize the most technologically advanced pressure exchanger energy recovery device available today. This proven, simple energy recovery device extracts energy from the desalination plant's brine discharge stream and transfers it directly back into the membrane feed water stream. The HydroMax design decreases energy consumption by over 60% when compared to a desalination plant operated without the energy recovery device. The energy savings is considerable. In areas where the electricity cost is only $ 0.12 USD per kWhr, the energy recovery equipment can pay for itself in about one year.

   The HydroMax energy recovery device also allows the desalination plant to be designed with the most efficient flow rates and recovery percentages in order to maximize membrane performance and longevity, rather than operating the reverse osmosis system improperly by attempting to reduce flow rates through the membranes or increase system recovery rates in order to minimize energy costs. The HydroMax energy recovery device also replaces flow restriction valves typically used in other desalination plants. These valves are costly to maintain, waste energy and allow the system to be operated incorrectly in respect to tangential membrane flow velocity and production rates.

   Proper pump and motor combination and energy recovery turbine size selection is important for proper and efficient system operation. Proper pump flow should be selected to minimize the fouling factor of the membranes and to provide sufficient flow for the desalination plant’s operating recovery rate. Pump output pressure effects the permeate production and flux rates. If an oversized pump is installed, membranes can be dangerously over-fluxed and energy is wasted. The HydroMax pumping system, selected for the specific source water chemistry, allows for precise pump and motor selection to minimize power consumption and maximize water production. The system flow and pressure can even be increased or decreased for optimization of the desalination process by either modifying the impeller module or through use of an electronic motor power output controller.

5. PIPING AND MEMBRANE PRESSURE VESSELS

   HydroMax, Inc. fabricates all the high pressure piping used in the desalination plant from corrosion resistant 316L grade stainless steel. All pipe joints are connected using a standard grooved pipe joint system with grooved pipe fittings. This reliable piping system allows low cost replacement of pipe sections or high wear fittings such as 90 degree elbows without having to cut and weld pipe at the plant. A worn elbow can be replaced within 30 minutes by any maintenance technician who knows how to use a wrench. HydroMax stainless steel piping and fittings are chemically passivated and then electronically polish hardened for further corrosion and abrasion resistance wherever possible. Low pressure piping in the desalination plant is constructed of heavy-duty, schedule 80 PVC.

   HydroMax membrane pressure vessels are fabricated of corrosion proof fiberglass reinforced plastic. The membrane vessels are designed and built to ASME standards for assurance of reliability and high pressure durability.

6. CONTROLS AND INSTRUMENTATION

   Controls and instrumentation are critical to operating a desalination plant. Proper automatic control sequences allow the plant to operate unattended and to shut down if an out of parameter condition exists to prevent equipment damage. Chemical feeds can be controlled to automatically maintain ideal feed water and permeate pH levels and for addition of chlorine. An operator should have immediate access to flow rate, pressure, and water quality readings. This information is used to check that proper water quality and production rates are being met and to help schedule system cleaning and maintenance before an irreparable condition or emergency develops. All controls in the HydroMax desalination plants are based upon durable, solid state and electromechanical devices. These components are easy to maintain and available throughout the world. No special programming, failure prone computer controls or unique printed circuit boards are used for actual system operation purposes.

7. REMOTE MONITORING

   HydroMax offers a PC based hardware and software package that enables an operator to view system performance from a remote location via modem. The monitoring system also allows an operator to shut the system down remotely if desired and can be programmed to call, fax, page or e-mail an operator when it is time for service. HydroMax, Inc. can monitor the performance of any system from our factory when it is equipped with the remote monitoring package.

8. FACTORY SERVICES