Step-by-Step Installation and Maintenance of Reverse Osmosis Systems

Reverse osmosis (RO) is widely used in water treatment and drinking water plants to provide clear water. Reverse Osmosis (RO) is a technique in which water is passed through the semipermeable membrane by some driving force (Pressure, temperature, concentration gradient). The clear water that will pass the membrane will be called permeate or clear water whereas the impurities that are retained over the membranes will be called retentate.

Difference between Osmosis and Reverse Osmosis:

In Osmosis the fluid that passes from a semi-permeable membrane naturally from low solute concentration to high solute concentration until osmotic equilibrium is achieved.

Whereas in Reverse Osmosis (RO) the water will pass from high solute concentration to low solute concentration by some driving force which will be pressure, Temperature, or concentration gradient.

Membrane Processes Classification

• Based on Material (Natural Polymer, Synthetic Polymer, Inorganic)
• Based on Membrane structure and morphology (Symmetric Membranes, Asymmetric Membranes, Composite Membranes)
• Based on operational mode (Dead End, Cross Flow)
• Based on charges (Anion Exchange Membranes, Cation Exchange Membranes, Bipolar Membranes)
• Based on driving force (Pressure Membranes, Concentration Membranes, Potential Membranes)

Procedure for Installation Elements

Opening Pressure Vessel

1. Relieve Pressure

Shut off all sources of pressure and relieve pressure from the vessel.

b. Disconnect Permeate Port

 Disconnect and remove permeate piping from the permeate port of the vessel.

c. Removing Head Retaining Ring

It is best to follow the ring in a clockwise direction while removing it to avoid stretching the ring.

d. Remove Head

 Use the two tapped 5/16”-18 holes in the bearing plate to pull the head out. It is recommended that you contact the factory for head removal tools or instructions.

Element Install

1. Clean the vessel by passing fresh water to remove contaminants like debris and dust particles.
2. Check the membrane element surfaces carefully so that any imperfection that may damage to vessel bore.

Focus on the edges of the anti-telescope device (ATD / brine seal carrier). If any defects are found which cannot easily be corrected, contact the element manufacturer for corrective action.

• Using an approximate 50% mixture of glycerin in water, lubricate the inside of the vessel. This may best be accomplished using a suitably sized swab soaked in the mixture.
• Load the first element upstream and of the vessel. Leave a few inches of the element projection from the vessel to facilitate interconnection to the next element.
• Assemble the interconnector to the loaded element.
• Line up the next element to be loaded and assemble it to the interconnector already assembled on the first element.

CAUTION

Maintain element alignment carefully during the assembly process. Do not allow element weight to be supported by the interconnector.

 Misalignment can result in damage to the interconnector or permeate tubes or to the element’s outer surface.

1. Force both elements into the vessel.
2. When the final element is installed, push the element stack forward until the face of the first(downstream).

Closing Pressure Vessel

1. Inspect Vessel Shell

 Before installing the head, check the inside surface of the shell for any imperfections or foreign matter. Remove all foreign matter using a mild soap solution and rinse with clean water.

• Head Seal and shell Lubrication

 Ensure that the head seal is covered with a thin layer of glycerin and is free from any dirt or dust contamination.

Only a thin layer of lubricant is required. Silicone lubricants cannot be used.

• Install Head

Holding the head square to the axis of the shell, slowly push the head into the shell until the seal passes the retaining ring groove. As resistance is felt, tap the head into the vessel using a 2~3lb dead blow hammer until the head clears the retaining ring groove.

• Install Retaining Ring

Remove any moisture from the retaining groove before proceeding. As, the ring enters the groove, continue pushing as your hand slowly rotates counter-clockwise around the inside diameter of the shell.

• Reconnect Permeate Port

Reconnect the permeate manifold to the permeate port.

• Pressurize System

Before starting the system, double-check that each head has been correctly installed and that all piping connectors are in place. Check all necessary precautions are taken, and start the system.

• Check for Leaks

 After the system starts up, verify that all connections are leak-free. Fix any leaks at this time to prevent corrosion that may lead to component deterioration and possibly unsafe operation.

RO CHEMICAL INJECTION

The addition of chemicals is done to prevent scaling and salts adhering to the membrane pores resulting in choking of the membrane. Mostly following are chemicals that are added:

Sodium Meta Bi Sulphate :

Used for removing free chlorine by SMBS injection pumps during service of RO system continuous dosing is being done.

Scale inhibitor:

Used for preventing scaling added by SI injection pumps during service of RO system continuous dosing pumps.

Acid (HCL):

Added for maintaining pH for water to dissolve scale.

Checklist before start-up

Before starting up the RO system, it is important to make sure that the whole pretreatment process is working according to the specifications. If the chemical characteristics of the raw water are changed, then a full analysis of the water entering the RO unit should be done so that proper measurements can be made to put the variables under control.

Factors affecting the feed water quality and thus the system design are as follows.

• Flow, SDI (Silt Density Index), Turbidity, Temperature, pH, TDS, residual chlorine, and bacteria counts.

Also, the following mechanical inspections of the RO System are recommended for the initial start-up.

• Operational conditions of media filters and cartridge filters.
  • Chemicals addition lines and valves.The feed line is purged and flushed before pressure vessels are connected.Proper mixing of chemicals in the feed stream.Installation and calibration of such instrumentationPiping and securing pressure vessels for operation and cleaning mode.Valves for permeate line, feed flow, and reject flow control are in open position.
  • Initial feed flow is limited to less than 50% of the operating feed flow.

1. Start-up procedure
2. Before starting the initial operation sequence, the pretreatment section should be thoroughly rinsed to flush out debris and other contaminants without allowing the feed to enter the elements.
3. Make sure that all valve settings are correct.

• Use the feed water at a low flow rate to expel the air out of the elements and pressure vessels at a gauge pressure of 2 bar to 4 bar for more than 30 minutes. All permeate and concentrate flows should be directed to an approved waste collection neutralization pit during flushing.
• After the system has been flushed, close the feed pressure control valve, but make sure that the concentrate control valve is open.
• Open the feed pressure control valve little by so that feed pressure does not exceed 4.0bar and then start the high pressure pump.
• Increase opening of the feed pressure valve slowly to elevate the feed pressure and feed flow rate to the elements until the design concentrate flow is reached.

Then slowly close the concentrate control valve

until the ratio of permeate flow to concentrate flow approaches the designed recovery ratio.

Design recovery ratio is 80%(Permeate water – 27㎥/hr/Unit, concentrate water – 7㎥/hr/Unit)

• Repeat opening of the feed pressure control valve and closing of the concentrate control valve until the design permeate and concentrate flows are obtained, while checking the system pressure to ensure that it does not exceed the upper design limit.
• After adjusting the two valves, calculate the system recovery and compare it to the system design value.
• Check chemical additions of acid, scale inhibitor, and sodium meta-bisulfite etc.

Measure pH, conductivity, calcium hardness, and alkalinity levels to calculate the LSI or the S & DSI for a possibility of scale formation.

• Take the first reading of all operating parameters after allowing the system to run for 1(one) hour.

Read the permeate conductivity from each pressure vessel and identity a vessel with a malfunction.

• After 24 to 48 hour of operation, record all plant performance data such as feed pressure, differential pressure, temperature, flows, recovery ratio and conductivity reading. And also analyze constituents of feed water, concentrate, and permeate water samples. Compare system performance to design values.

Use the initial system performance information as a reference for evaluating future system performance.

Monitor the system performance regularly.