Water treatment is an essential process that is not only vital for consumer health and safety but...
Water is the main human need for drinking, cooking, washing, irrigating rice fields, industrial raw materials, etc. But unfortunately water sources are limited and not all water sources can be directly used for human needs. For example, surface water (rivers and lakes) generally requires a filtering process and chemical injection first to remove impurities and kill bacteria to obtain clean water. These filtering and chemical injection processes are some examples of Water Treatment Plant (WTP) systems that aim to improve water quality as needed.
Selection of Water Treatment Technology
The technology in a water treatment plant varies greatly, depending on the water source and the desired product water quality. In general, to achieve the desired water quality cannot be directly obtained with one type of technology or equipment alone, but must go through several equipment in sequence. The thumb rule is that the sequence of equipment is based on the particle size of the impurity/contaminant to be removed from larger particles to smaller particles.
Thumb rule.
For example, to treat river water that is cloudy due to high Total Suspended Solids (TSS) and turbidity content and smells due to bacteria, it requires Bar screen - Chlorine Injection - Coagulation - Flocculation - Clarifier - Sand Filter - Carbon Filter - UV to get clean water. But if the product water will be used for boiler feed water with a higher purity level, namely Electric Conductivity <5 microsiemens/cm, there must be further equipment, which can be Reverse Osmosis or Ion Exchange Demineralization Plant.
For more details about the right water treatment system design, please contact and discuss with our Engineering Team. Don't forget to provide the following data:
- Where does the water come from (well/river/sea water)?
- Capacity of product water required (m3/hour, m3/day)?
- Analysis of water source quality?
- Desired product water quality parameters?
- Layout area provided (Length x Width x Height)
Reverse Osmosis (RO)
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 the 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 salt content in water. 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 salt, 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 reducing TDS by heating and evaporating water so that salt is left in the water and pure water vapor is obtained (very small TDS content). 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 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: 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 arrangement 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 TDS 500 - 20,000 mg/l
- Tap Water RO (TWRO) for clean water with TDS < 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. Hope this helps, and see you in the next article about Demin Plant.
