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TWC 4.02 – Filtration – Reverse Osmosis

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Reverse osmosis (RO) systems produce two types of treated water and then allow the user to blend them in the final step in the treatment process. One product of the RO system is called ‘bypass water’. This water travels only through the prefilter and then past the activated charcoal. It is therefore free of sediment, organic matter, odour, and chlorine, but it retains the same dissolved mineral content it had before it entered the system.

The other water that comes from an RO system is known as the product water, or permeate. This water has passed through an RO membrane and as a result is ultrapure (at or near 10 ppm). An RO membrane is a filter; the pores in its media are submicron in size, small enough to literally strain the minerals out of the water.

Water molecules surround each ion in solution. The water molecules closest to the ion are tightly bonded to it. So, in order to pass through a pore, not just the ion but all those water molecules would have to pass through together. The pores in an RO membrane are big enough to let individual water molecules through, but they are not big enough to permit the transfer of ions with water attached. The water molecules that do pass through are ones not attached to any ion. (There are many such molecules. Remember, we measure our ions in parts per million, or ppm, and even 300 to 400 ppm of hardness is considered high by coffee standards.)


RO System Schematic 

1: incoming mains water
2: isolation valve
3: sediment filtration
4: carbon filtration
5: tee junction
6: electronic solenoid valve
7: booster pump
8: RO membrane
9: check valve/non-return
10: permeate water tube
11: high pressure switch
12: 24V DC power supply
13: tank valve
14: pressure storage tank
15: check valve/non-return
16: inline TDS meter
17: blend/needle valve
18: TDS probe in tee junction
19: post carbon filter
20: flow restrictor valve
21: waste water outlet


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