Reverse Osmosis (RO)
As we discussed in the previous article, groundwater and surface water contain two kinds of solids:
- Total Suspended Solids (TSS), particle size 0.01 – 10 microns
- Total Dissolved Solids (TDS), particle size < 0.01 micron.
TSS generally floats in water because they are negatively charged and repel each other, making it difficult to combine and deposit. Therefore, to reduce the TSS content in general, a filtering step is carried out in the form of coagulation – flocculation – sedimentation – filtration.
TDS is closely related to salinity or dissolved salt content in water, and is usually expressed in units of mg/l or ppm (parts per million). As an illustration, the TDS of sea water in Indonesia is generally 30,000 – 40,000 mg/l, while the TDS of well and river water is in the range of 300 – 1000 mg/l. Permenkes No. 492 of 2010 requires a maximum TDS of 500 mg/l for drinking water. To reduce TDS or remove dissolved salts, a desalination process is carried out. There are three methods of desalination, namely Distillation, Reverse Osmosis (RO) and Ion Exchange Demineralization or Demin plant.
Distillation is the process of lowering TDS by heating and evaporating water so that the salt is left in the water and pure water vapor is obtained (the TDS content is very small). Distillation is not so popular today because it requires a very large amount of energy so it is relatively expensive.
Reverse Osmosis (RO) is the process of lowering TDS by means of reverse osmosis. Osmosis is the spontaneous movement of a liquid from a low concentration to a high concentration through the pores of a semipermeable membrane until equilibrium is reached. Reverse Osmosis means that on the contrary, water from the high TDS concentration side of the membrane moves to the low TDS concentration side of the membrane because it is forced by a high pressure pump that exceeds the system’s osmotic pressure.
There are 2 kinds of RO membrane materials: thin film composite (TFC) polyamide and cellulose acetate. TFC polyamide is more popular because it is more resistant to high temperatures and a wider pH range. With a pore size of 0.0001 microns, polyamide membranes produce TDS rejection rates ranging from 95-99%, depending on the inlet TDS, temperature and membrane structure design.
The choice of RO membrane also depends on the water source as follows:
- Seawater RO (SWRO) for seawater with TDS > 20,000 mg/l,
- Brackish Water RO (BWRO) for brackish water with a TDS of 500 – 10,000 mg/l
- Tap Water RO (TWRO) for clean water with TDS <500 mg/l.
In simple terms, the RO system usually consists of a feed pump – cartridge filter – HPP – RO membrane and housing, and is equipped with chemical dosing and Chemical in Place (CIP) module for chemical cleaning.
In designing the RO, it is necessary to pay attention to the limits of the permeate flux, the recovery rate and the pretreatment system. Permeate flux is the product flowrate per unit area of the membrane. Recovery rate is the percentage of permeate flowrate (product) divided by feed (inlet). By following the recommended flux and recovery rate limits and having an adequate pretreatment system to prevent fouling and membrane scaling, the membrane can be more durable and require minimal maintenance.
A good RO and pretreatment system is not only cheap to buy the system, but also has to consider operational and maintenance costs as well as resistance to fluctuations in water quality and temperature. Hopefully useful, and see you in the next article about the Demin Plant.