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SMALL REVERSE OSMOSIS SYSTEM DESIGN


There are several ways to design a reverse osmosis system to provide a specific water output. The cost of a system to provide a 1-gallon per minute flow can vary from approximately $3000.00 US to about 9,000.00 US and higher. Systems can be sold for less but they will have little flexibility.

In addition to the components listed below, all systems should have a line filter to provide 5-micron water feed and a carbon filter to remove chlorine from the incoming water. If the water hardness (calcium and magnesium) is excessive, antiscalant is injected to the water, the pH is adjusted, a softener is installed or the membranes can be changed out regularly.

The low cost system -

To design the lowest cost system, you use the lowest cost components. For a 1-gpm system, a PVC membrane housing rated at 200 psi would be used with a low-pressure membrane. The low-pressure membrane rejects about 95% of the hardness and about 85% of the Sodium and Chloride ions. This will result in the reduction of water dissolved solids by about 90%. So a 400 PPM (parts per million) water dissolved solids would end up being 40 PPM. Each membrane is rated at 1200 gpm at 75 psi but very few water systems deliver this much pressure so 2 membranes and housings would be required for 1 gpm (1440 gpd). Adding the extra membrane and housing is less expensive that adding and
maintaining a pump plus the efficiency is higher that with a single membrane system.

Efficiency -

All membranes require a rapid flow of water across the surface to keep them from plugging. If the flow rate falls below a certain level, surface fouling can result and the membrane cross flow is reduced. To maintain this flow in low-pressure membranes, about 70% of the water entering the system is sent to the drain. The 30% of the water not to drain is permeate and is good water. To get 1 gpm, the system would be fed at least 3.3 gpm (ideal flow to minimize plugging is 6 gpm).

The efficient system -

This system has higher quality components such as fiberglass membrane housings, more efficient membranes and a pump to operate the system at a higher pressure. The system would be operated at 125-235 psi. A 2.5 inch fiberglass membrane housing rated at 300+ psi and 3 membranes with a 98% reject rate would be used. An efficient high pressure pump would pressurize the system. Three membranes rated at 500 gpd each would be used and the reject rate of 98% would give an output of about 8 ppm dissolved solids.

Efficiency - This system will use less water that the first system, a triple membrane will allow permeate of about 40% of the influent water. To get 1 gpm, the system would be fed at least 2.5 gpm.

The high efficiency system -

In addition to the above, this system will have a feed tank with level controls. The reject is fed back to the feed tank and a bleed is run to drain. This allows a much lower flow to drain saving up to 95% of the water for use by the customer. In high hardness waters, a softener is placed in front of the system to prevent scaling (precipitation and plugging) of the membrane. Due to the recirculation of the reject water, the flow rate across the membrane can be kept in the ideal range to minimize plugging.