Reverse Osmosis (RO)

As we've discussed in previous articles, 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 of <0.01 microns.

TSS generally float in water because they are negatively charged and repel each other, making it difficult to coalesce and settle. Therefore, to reduce TSS content, in general, the filtering stage 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 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 ranges from 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, the desalination process is carried out. There are three ways 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 (very small TDS content) is obtained. Distillation is not very popular nowadays because it requires a lot of energy and is relatively expensive.

Reverse Osmosis (RO) is the process of reducing TDS by means of reverse osmosis. Osmosis is the spontaneous transfer of liquid from low concentration to high concentration through the pores of a semipermeable membrane until equilibrium is reached. Reverse Osmosis means the opposite, water from the side of the membrane with a high TDS concentration moves to the side of the membrane with a low TDS concentration because it is forced by a high pressure pump that exceeds the osmotic pressure of the system.

There are two kinds of RO membrane materials: polyamide thin film composite (TFC) and cellulose acetate. Polyamide TFC 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 design of the membrane arrangement.

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 TDS 500 - 10,000 mg/l
• Tap Water RO (TWRO) for clean water with a TDS of <500 mg/l.

A simple RO system usually consists of 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 RO, it is necessary to consider the limits of permeate flux, recovery rate and pretreatment system. Permeate flux is the product flowrate per unit membrane area. Recovery rate is the percentage of permeate (product) flowrate divided by feed (inlet). By following the recommended flux and recovery rates and having a pretreatment system that is sufficient to prevent membrane fouling and scaling, the membranes can last longer and require less maintenance.

Ro RO systems and their pretreatment systems can also be used to prevent membrane fouling and scaling.

A good RO system and its pretreatment is not only the cheapest system purchase price, but also must consider operational and maintenance costs and durability in the face of fluctuations in the quality and temperature of the water source. Hopefully useful, and see you in the next article about demin plants.