by vivek pant on June 11, 2019
There are several factors that affect the average lifespan of water purifier. In general, these factors can be divided into two groups: feed water characteristics and system configuration. This article focuses on the characteristics of the feed water. Different feed water can change the flow and rejection of an RO filter.
The main factors to consider are the temperature, the pressure and the concentration of salt in the feedwater.
Temperature: The RO permeate flow depends to a large extent on the operating temperature. The higher the water temperature, the greater the permeate flow. This occurs because water with a higher temperature has a lower viscosity and a higher diffusion rate, which facilitates the penetration of water into the RO membrane. For the elements of Dow Water and Process Solutions, a general rule is that for every 1 ° C increase in temperature, the permeate flow increases by 3%. Most membrane manufacturers provide temperature correction tables that allow more accurate conversions.
The temperature not only changes the amount of water that is produced but also affects the amount of salt that passes through the membrane. The warmer feed water increases the temperature of the RO membrane, allowing the salt to diffuse through it more quickly. The general rule is that the rate of salt increases by 6% for each 1°C increase in the temperature of the feed water, so that the salt flow increases approximately twice as much as the increase in water flow. Therefore, residential point-of-use (POU) systems that have large swings in feedwater temperature throughout the year will experience higher flows and worse rejections in the summer, and lower flows and better rejections in the winter.
Pressure: To understand the impact of changing the feed pressure in ro water purifier, the concept of net driving pressure (NDP) must first be considered. The NDP is the sum of all the forces acting on the membrane. These may include the pump or the feed pressure; back pressure of the restriction line and storage tank; and the osmotic pressure of the feed and the permeated water. NDP is the effective pressure that the membrane feels during the operation. If the net conduction pressure is doubled to one RO unit, the permeate flow will double.
As the feed water flows through the water purifier, clean water passes through the membrane. This increases the concentration of contaminants in the feedwater, which increases the TDS of the feedwater. Therefore, the input power TDS is always lower than the concentrated output TDS. In the same way, the friction of the water flow travelling through the filter and the container causes a drop in the supply pressure.
To calculate the average NDP on an element or vessel, the average feed osmotic pressure and the average feed pressure in the calculation should be used. The average osmotic pressure is achieved by taking the average of the osmotic pressure of the input feed and the osmotic pressure of the output concentrate. The average feed pressure should be determined from the average of the inlet feed pressure with the outlet pressure of the concentrate.
Salt concentration: The rate of passage of the salt through a membrane is determined by the salt concentration gradient, which is the difference between the TDS on the feed side of the membrane versus the permeate side of the membrane. The step rate of the salt is independent of the pressure. However, because more permeate water is produced as the feed pressure increases, the permeate salt concentration decreases. This occurs because the same amount of salt diffuses through the membrane, but dissolves in more permeated water.
As it is discussed earlier, higher salt concentrations (higher TDS) will decrease the permeate flow because of the osmotic pressure increases, decreasing the NDP. However, a higher concentration of feed salt will also increase the amount of salt that passes through the membrane. Therefore, the general water quality decreases for two reasons: first, less water is produced; and second, more salt diffuses and dissolves in less water.