Water is the wellspring of life. It is a universal solvent; many types of salts dissolve in water. It is the most important liquid in the world for maintaining the plant and animal life. It fills lakes, streams, the vast oceans, and flows under the ground. Water is a remarkable chemical, an excellent solvent capable of dissolving, to varying degrees, almost anything with which it comes in contact. Water picks up suspended matter as it runs across the ground and absorbs gases from the atmosphere. Impurities in the water come from various sources.
There are different sources of water namely
• Sea Water
• Surface Water
• Ground Water
• Waste Water [Sewage / Industrial]
Common impurities in water:
Total Dissolved Solids [TDS]: TDS as name signifies is total dissolved salts in water.
Hardness: Hardness is the part of TDS which forms hard scales, which composes of calcium Sulfate,
Barium Sulfate, Calcium Carbonate, Calcium Fluoride and others like
Metal Oxides etc.
With the ever increasing TDS content in raw water and treated effluent RO has been emerged as most economical technology. The building of deposits (scaling and fouling) on the membrane surfaces is usually unavoidable. However, it can be reduced to an acceptable level by using an adequate chemical treatment.
1. What is scaling?
Scaling refers to precipitation and deposition of sparingly soluble salts such as Calcium Sulfate, Barium Sulfate, Calcium Carbonate, Silica, Calcium Fluoride and any other super saturated salt on the immediate surface of the membrane. Typically scaling starts on the tail elements of the last stage (on the rejection side), since they are treating water with the highest concentrations of ions. Once a crystal of scale forms within the membrane element, it acts as a nucleation site for additional scale to form and the rate of scale formation increases exponentially.
2. What is fouling?
Fouling refers to entrapment of particulates, such as silt, clay, suspended solids, biological slime, algae, silica, iron flocs and other matter on the surface, or even worst, within the membrane pores. Typically fouling occurs in the lead elements of the first stage initially and then it works itself through the following elements.
2. Pretreatment of RO Membrane:
Both RO/NF technologies are manufactured, designed and built for “salt” and dissolved ion removal and not to particulate matter. Therefore, proper pretreatment plays a critical role in the performance, life expectancy and the overall operating costs of these systems.
For the most RO portable water system, cartridge filters (1-5 microns) should be a minimum pretreatment, even for the cleanest ground water sources.Inadequate pretreatment often necessitates frequent cleaning to restore product flux and salt rejection.
3. Antiscalants: As the name suggests that it is chemical compounds which prevents scaling occurrence in RO membrane and minimize the maintenance cost. There are three mechanisms by which AntiScalants work:
• Threshold Effect : Solubility enhancement by threshold effect, which reduces precipitation
of low solubility inorganic salts (CaCO3 in particular).
• Distortion Effect : Crystal modification, which deforms the growing inorganic salt crystal to
give small, irregular, readily fractured crystals that do not adhere well to surfaces and can be
easily removed during cleaning operations.
• Dispersion Effect: Dispersing activity, which prevents precipitated crystals or other
inorganic particles from agglomerating and depositing on surface. This also works for
preventing fouling to a good extent.
Advantage of using Antiscalants:
• Remarkable prevention of Scaling & Fouling
• Extended membrane life
• Effective running of RO System
• No pressure drop
• Reliable permeate water quality
• Good recovery & rejection rate