Major factors affecting the stable operation of the EDI system

EDI The system is a new type of water treatment technology. Its system characteristics and technical maintenance have always been the focus of research. So what are the main factors affecting EDI system operation?

Influence of Influent Conductivity

        Under the premise that other conditions remain unchanged, when the raw water conductivity rises beyond a certain range, the working range of the membrane module moves downward, and even the regeneration zone disappears, the working zone penetrates, and most of the filler resin in the membrane module is in a saturated failure state. At the same time, the concentration of ions in the water increases, and under the condition that the voltage remains unchanged, the current increases, thus weakening the process of water ionization, and the corresponding water ionization H+ ，OH- decreases.

Influence of Influent Flow Rate

EDI Under the condition that the water production capacity of the system is constant, the worse the influent water quality, the heavier the unit processing burden of the membrane module, and the smaller the influent flow rate should be adjusted. In the start-up stage of the membrane module, attention should be paid to the fact that excessive instantaneous flow rate may cause membrane perforation.

        Since the electron flow in the membrane module is mainly transmitted through the filler resin, the concentrate current, to a certain extent, has become the key factor affecting the electron flow migration in the membrane module. Reducing the concentrate flow rate can increase the current of the system and improve the water quality to a certain extent. However, the concentrate flow rate is not the smaller the better. When the concentrate flow rate is too small, it will lead to a larger concentration difference between the two sides of the membrane, forming a concentration difference diffusion, affecting the water quality. On the other hand, due to weak electrolytic ions Si and its ionic compounds have very low solubility, they are easy to saturate in low-flow concentrate, thus affecting the removal of weakly ionized ions. The concentrate flow rate is generally 5%-10% of the influent. The role of electrode water is mainly to cool the electrode and remove the gas generated on the electrode surface. Generally, the flow rate of electrode water is 1% of the influent. When the electrode water is too small, it cannot remove the gas on the electrode surface in time, which will affect the operation of the entire membrane module.

Influent pH, temperature and pressure influence

        Influent pH value indicates the H+ content in the influent, generally controlled between 5-9.5 . Usually pH value is low due to CO2 dissolution. CO2 is also one of the factors that lead to water quality deterioration. Therefore, before entering EDI membrane module system, a decarbonization device can generally be installed to make the CO2 controlled below 5mg/L . In water pH value and CO2 have a certain dissolution relationship. Theoretically, when pH>10 the removal efficiency is optimal. High pH value helps to remove weakly ionized ions, but it must be removed before entering EDI membrane module system Ca2+ ， Mg2+ and other ions.

        Temperature directly affects system pressure and product water resistance. Usually EDI the influent temperature of the system should be controlled between 5-35° The optimal temperature is around 25° . Lowering the temperature will reduce the activity of water, macroscopically manifested as increased viscosity of water and increased system pressure.

        Usually, the pressure of product water > concentrate pressure > electrode water pressure, so as to effectively prevent the phenomenon of concentrate diffusion polluting product water. Pressure changes are also an effective means to judge EDI whether the system membrane module is polluted. When the pressure difference between the concentrate inlet and outlet increases, it is often accompanied by blockage of the concentrate pipeline. At this time, it is necessary to clean the pipeline and use chemical cleaning or other methods to reduce the pressure difference. Therefore, EDI at the inlet of the system membrane module, the pollution index of the influent should be within the qualified range.

Influence of Pollutants

        The pollutants that have a greater impact on EDI include hardness (calcium, magnesium), organic matter, suspended solids, variable valence metal ions (iron, manganese), oxidants (chlorine, ozone) and carbon dioxide CO2 ) and bacteria. These pollutants should be removed in the pretreatment process to improve EDI performance. EDI Chlorine and ozone will oxidize ion exchange resin and ion exchange membrane, causing

      component function reduction. Oxidation will also cause EDI TOC content to increase significantly, polluting ion exchange resin and membrane, and reducing ion migration speed. In addition, oxidation causes resin breakage, and the pressure loss through the component will increase. Iron and other variable valence metal ions can catalyze the oxidation of resin, permanently reducing the performance of resin and membrane.

        Hardness can cause scaling in reverse osmosis and

        units. Scaling generally occurs on the surface of the membrane in the concentrate chamber, where EDI is higher. At this time, the pressure difference between the concentrate water inlet and outlet increases, and the current decreases. Reducing the influent hardness to the minimum will prolong the cleaning cycle and improve pH 值较高。此时，浓水入水和出水间的压力差增加，电流量降低，使入水硬度降到最小将会延长清洗周期并且提高 EDI System water utilization rate.

        Suspended solids and colloids can cause membrane and resin fouling and blockage, and blockage of the resin gap leads to EDI increased pressure drop of the components.

        Organic matter is attracted to the surface of the resin and membrane, causing pollution, which reduces the efficiency of ion migration of the polluted membrane and resin, and increases the membrane stack resistance.

      Carbon dioxide has two effects. First, CO32- and Ca2+ 、 Mg2+ form carbonate scale. The formation of this scale is related to the ion concentration of the feed water and pH Secondly, due to CO2 the charge is related to pH value, and it is RO and EDI removal depends on its charge, so its removal efficiency varies. Even lower CO can significantly reduce the resistivity of the product water.

        Bacteria cause algae growth and fouling, increasing the pressure drop of the membrane module and reducing water quality.