Ultrafiltration membrane operation detection

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Issue Time: Dec 16,2024

Summary

Detection during the operation of ultrafiltration membrane

In order to test the operation effect of the ultrafiltration device and the possibility of membrane fouling, it is necessary to monitor some key parameters during the operation of the ultrafiltration device.

Turbidity: refers to suspended substances such as mud, dust, fine organic matter, plankton and colloid substances in the water, which will cause the water quality to become turbid and present a certain degree of turbidity. Usually these suspended substances and colloid substances will also parasitize bacteria and viruses. For example, the turbidity of drinking water is generally required not to exceed 1 NTU, and the turbidity of the ultrafiltration membrane effluent is generally required not to exceed 0.1 NTU.

TSS (total suspended solids): refers to the solid matter that is retained on the filter membrane after the water sample passes through the filter membrane with a pore size of 0.45μm and is dried to constant weight at 103℃~105℃. Total suspended solids is one of the important indicators for measuring the degree of water pollution in water bodies. This parameter is generally more accurate than turbidity (turbidity usually cannot detect extremely fine particles).

SDI (siltation density index): It is one of the important water quality index parameters of reverse osmosis water treatment system. The SDI value represents the content of particles, colloids and other substances that can block various water purification equipment in the water. This parameter is usually used to judge the possibility of particles and colloids in the water blocking various water purification equipment (see the figure below).

The determination of SDI is to continuously add a certain pressure (30PSI, equivalent to 2.1kg/cm) to the measured water sample on a microporous filter membrane with a diameter of 47mm and a pore size of 0.45μm, record the time Ti (seconds) required to filter 500ml of water and the time Tf (seconds) required to filter 500ml of water again after continuous filtration for 15 minutes (T), and calculate the SDI value by formula; generally, the SDI value of the reverse osmosis inlet (i.e. ultrafiltration water) shall not exceed 5.

TOC (total organic carbon): This parameter is most commonly used to determine the organic content in water, which refers to the total amount of carbon contained in dissolved and suspended organic matter in water, including natural organic matter and synthetic organic matter. Total organic carbon is generally used to assess the possibility and trend of organic fouling and biological fouling of the membrane in the UF influent. When the TOC of the UF influent is greater than 2 mg/L, it indicates that the possibility of biological fouling on the UF membrane surface is very high.

DOC (dissolved organic carbon): The part of total organic carbon (TOC) that can be dissolved in water, generally refers to the organic carbon that can pass through the filter membrane with a pore size of 0.45 microns and is not evaporated and lost during the analysis process. Except for sewage, the proportion of dissolved organic carbon (DOC) in most natural water bodies is about 80~95% of the total organic carbon (TOC).

Iron and manganese: The oxidized forms of iron and manganese can be retained by the UF membrane system, but they can also cause membrane fouling. Iron ions generally exist naturally (such as groundwater, etc.), or are produced by corrosion of UF pretreatment pipelines or equipment, or are caused by the addition of flocculants in the coagulation and clarification equipment of UF pretreatment.

Calcium and magnesium: The hardness of water mainly comes from calcium ions and magnesium ions. According to the hardness, water can be divided into soft water (maximum 60mg/L in terms of CaCO3), hard water (maximum 180mg/L in terms of CaCO3) and very hard water (more than 180mg/L in terms of CaCO3). Hardness is not harmful to human health, but too high hardness in water can cause scaling on the surface of pipes, equipment or membranes during water treatment.

Conductivity: The conductivity of water is linearly related to total dissolved solids (TDS), indicating the conductivity of water.

pH value: used to indicate the pH value of water. pH values less than 7 are acidic, and pH values greater than 7 are alkaline. The pH value of pure water is 7, which is neutral. High pH values can make water taste bitter and easily cause scaling of water pipes and equipment. Water with low pH values can corrode or dissolve metals and other equipment.

Silica: It is divided into active silica (dissolved silica) or inactive silica (colloidal silica). In general, colloidal silica will accelerate the fouling of ultrafiltration membranes.