Reverse osmosis feed pumps

250 psi over the pump suction pressure. If the suction pressure is 10 psig, the discharge pressure will be 260 psig). The curve also indicates that the net pressure suction head required (NPSHr) to prevent cavitation of the pump is about 3.45 psi (8.0 feet of water). The efficiency of the pump is about 68%, just about the maximum efficiency for this pump. This pump would be quite suitable for use in the specified RO application. However, in the case where the actual pump efficiency was far from the theoretical maximum, another pump would need to selected that would yield higher pump efficiency. Motor efficiencies run at about 90%. Each pump and motor combination has its own specific pump curve. 

Variable frequency drives (VFDs) are sometime used to adjust the operation of typical (US) standard of 480VAC, 3PH, 60hz operation of the motor. The functionality of a VFD is to convert frequency measured in Hertz (Hz) to motor speed. One Hz equals 1 cycle per second. When voltage is being received (input to the VFD), it is in the sinusoidal waveform. The sine wave is converted to a digital square wave that now controls the revolutions per minute (RPM) of the motor. 

An inverted duty motor is required for a VFD. Unless the pump in question has this type of motor, it cannot be retrofitted with a VFD. 

Pumps should be started slowly to prevent water hammer (a surge resulting from a sudden change in liquid velocity). Water hammer can cause cracks in the outer shell of the membrane modules as well as compaction of the membrane itself (compaction results in lower flux through the membrane at constant pressure). Also, water hammer causes the membrane modules to move in the vessel, which can cause wear to the O-rings used on standard interconnectors and lead to leaks of feed water into the permeate (see Chapter 4.3.3). An increase in pressure of no more than 10 psi per second is recommended.3 Some motors may be equipped with a soft start that regulates the speed with which they start up. Other considerations to minimize water hammer include  

0 Flow valves should be open when pumps are activated. 

Single open end with O-Ring with seal 

The VFD should receive an analog input signal from the permeate flow sensor. This is best practice for utilizing a VFD on an RO system. Some 

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After chemical addition, the feedwater is routed to cartridge filters which serve to mix the chemicals which have been added upstream and to insure that any particles that may have escaped the gravity filters, such as sand or other particulate matter is removed. In general, the cartridge filters do not improve the quality of the reverse osmosis feedwater to a large degree and they are not intended as continuous duty filters. The effluent from the cartridge filters is routed to the primary reverse osmosis feed pump wet well. 

Reverse osmosis feed pumps are sized using the required flow rate and operating pressure. Pump curves, as shown in Figure 6.4, are then consulted to determine the number of stages, impeller diameter, and horsepower (hp). 

The task of synthesizing an optimal RON can be stated as follows For a given feed flowrate, Qf. and a feed concentration, Cp. it is desired to synthesize a minimum cost system of reverse osmosis modules, booster pumps and energy-recovery turbines Chat can separate the feed into two streams an environmentally acceptable permeate and a retentate (reject) stream in which the undesired species is concentrated. The permeate stream must meet two requirements ... 

Figure 2. Schematic view of reverse osmosis test loop (I) hollow fiber membrane (2) pressure vessel (3) feed water (4) filter (5) pressure pump (6) relief valve.

Membrane distillation offers a number of advantages over alternative pressure-driven processes such as reverse osmosis. Because the process is driven by temperature gradients, low-grade waste heat can be used and expensive high-pressure pumps are not required. Membrane fluxes are comparable to reverse osmosis fluxes, so membrane areas are not excessive. Finally, the process is still effective with slightly reduced fluxes even for very concentrated solutions. This is an advantage over reverse osmosis, in which the feed solution osmotic pressure places a practical limit on the concentration of a salt in the feed solution to be processed. 

Some vendors place both passes on a single skid, thereby eliminating the RO feed pump to the second pass RO. The backpressure from the first pass is sufficient to provide the applied pressure required of the second pass. Care must be taken so that permeate backpressure does not exceed the applied influent pressure to the first pass, or osmosis rather than reverse osmosis will occur. Additionally, high back pressure can lead to delamination of the membranes (see Chapter 12.1.2.1 and Figure 12.1)... 

The waters in question are pumped to a membrane bioreactor equipped with an air injection system, where part of the feed is recycled, making it move across a membrane ultrafiltration system, to prevent the presence of suspended microelements in the later phase of reverse osmosis. From the ultrafiltration process, two streams are obtained a concentrated stream of salts and microbial mass, which is recycled to the bioreactor, and a permeate stream that passes to the reverse osmosis plant. 

To meet the operating requirements of the reverse osmosis process, a pumping system Is commonly used to pressurize and circulate feed fluids at sufficient velocities to maintain the desired turbulence and to provide pressurized make-up fluid to replace withdrawn permeate and concentrate fluids. 

Figure 4.13 shows a flow diagram for a reverse osmosis unit with 75% recovery on a brackish feed. The pretreated feed is routed to the high pressure pump where the feed pressure is raised to between 250 and 400 psig as required for brackish water desalination. The pressurized feed is then pumped to the first pass pressure vessels where about 50% of the feed is recovered as product and 50% is reject. The reject from the first pass pressure vessels is then routed to the second pass pressure vessels where, again, about 50% of the first pass reject is recovered as product and 50% is reject which is sent to waste. Thus, the overall recovery of the unit is 75% as product. As can be seen, a normal array for a 75% recovery unit is two first pass pressure vessels feeding one second pass pressure vessel or a 2-1 array. If the system recovery were from 40 to 60%, all of the pressure vessels would be in parallel. However, if the system recovery were raised to between 85 and 90%, the pressure vessels would be arranged in a 4-2-1 array. 

Chlorine has been added to the feedwater upstream of reverse osmosis pretreatment. However, since chlorine will depolymerize the polyurea membrane barrier layer in the spiral wound element, with subsequent loss of desalination properties, the chlorine is removed in the pretreatment system dechlorination basin. This removal is chemically accomplished by the addition of sodium bisulfite. The chlorine level in the influent and effluent to the dechlorination basin is continuously monitored. The feedwater is then transferred from the dechlorination basin to the cartridge filter feed pumping station by gravity flow and it is then pumped to the cartridge filters. 

About 100 gallons/hour (GPH) are pumped from the first rinse tank through a cartridge filter and into a reverse osmosis unit. The reject stream contains 99% (59,400 mg/E) of the nickel in the feed stream with 1% (32 mg/E) remaining in the product stream. The reject stream is routed through an activated carbon column to the plating bath. The reverse osmosis product stream is combined with 5 GPH of tap water makeup, which is added to compensate for surface evaporation in the plating tank, and the combined stream is returned to second rinse tank. The waste stream (10 GPH) is sent to waste treatment which is a precipitation process. 

The detail of the permeation cell for reverse osmosis experiments and the flow system arc shown schematically in Figure 3.9. The permeation cell is made of stainless steel 310 and consists of two detachable parts. The upper part is a high-pressure chamber. A wet membrane is mounted on a stainless steel porous plate embedded in the lower part of the cell such that the active surface layer of the asymmeU-ic membrane faces the feed solution under high pressure. A wet Whatman filter paper is placed between the membrane and the porous plate to protect the membrane from abrasion. The feed solution is supplied to the feed chamber of the permeation cell by a pressure pump, while the permeate... 

The schematic diagram of the pervaporation apparatus is shown in Figure 3.12. The same permeation cell as the reverse osmosis static cell can be used for the experiment. The feed liquid mixture in the permeation cell (2) is either open to the atmosphere or under the pressure applied from a nitrogen cylinder (1). Vacuum is applied on the permeate side of the membrane by a vacuum pump (6). The permeate vapor is condensed and collected in a cold trap (5) cooled with liquid nitrogen. After a steady state is reached, the line is switched to the second cold trap. The permeation rate is determined by weighing the sample collected in the cold trap during a prescribed period. The sample is also subjected to analysis. 

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