7 Extended Application: Home Reverse Osmosis (RO) System

This application note discusses how to measure the TDS in water using a PIC16F17146 microcontroller. This application may be extended to implement a simple home RO system. Figure 7-1 shows a block diagram, including the components required to design and assemble a home RO system. A basic home RO system may or may not have the optional stages as shown in Figure 7-1, but these optional stages improve the drinking water taste and quality. Additionally, a home RO system may have an optional display/LED indication to provide system information.

Figure 7-1. Block Diagram of a Home RO System

A home RO system is made up of the following components:

  • Pressure Regulator and Solenoid Valve: A pressure regulator is used to detect pressure in the water inlet line that is more than 3 psi, the minimum required pressure for a home RO system. A Solenoid valve is used to cut off the inlet/outlet water flow when the RO system is not functioning.
  • Sediment Pre-Filter: The sediment filter cartridge removes sand, grit, precipitated mineral particles, insoluble iron oxide and other debris that can clog the reverse osmosis membrane surface or plug the drain flow restrictor, causing reduced water production.
  • Activated Carbon Filter: Most city water utilities require water leaving the plant to have a minimum chlorine level of 1.0 mg/L (1.0 ppm). The carbon filter removes chlorine and organic impurities in water while observing bad taste and odor. Additionally, it protects the membrane from choking.
  • Booster Pump: The booster pump works to maintain input water pressure to the RO membrane as the membrane requires a pressure approximately 75 psi to function well.
  • RO Membrane: The RO membrane does most of the work in the system. It removes over 96% of total dissolved solids (i.e., salts, minerals, metals), microorganisms and organic substances in the water. The membrane divides the water flow into two streams—filtered water from the membrane goes into the storage tank while rejected water goes into the drain.
  • Post Carbon Filter: This is a granular activated carbon polishing filter that improves water taste and quality.
  • Water Tank: A water tank is a RO system store for filtered water. It provides water when the drinking water faucet is turned on.

Optional RO components:

  • UV: This is required to inactivate any microorganisms present in water.
  • pH Balancing and Mineralizer Filter: A filter that remineralizes the RO water and raises its pH level. It also ensures taste and quality of drinking water.
  • TDS Meter: This meter measures the TDS levels in water and ensures that the RO system is working properly. It can also be useful for monitoring the cartridge’s life. Continuously monitoring the TDS levels in water helps to determine when a service call for the RO system is appropriate.

A home RO system has few electrically controlled components: viz. booster pump, solenoid valve, UV module, display or LEDs and TDS meters. All of the components require only ON/OFF functionality, which can be achieved using relays. In this application note, a TDS meter has been implemented. To control relays, simple GPIOs are needed, which can be achieved using a microcontroller device. This approach makes the PIC16F17146 microcontroller the best fit when implementing a home RO system.