When adapting the pretreatment process of ultrafiltration pure water equipment to different raw water qualities, the first step is to conduct comprehensive testing and classification to identify the core contaminants and their characteristics—the foundation for subsequent process design. This requires determining whether the raw water is surface water (such as river and lake water, which often contains high levels of suspended solids, colloids, algae, and fluctuating organic matter), groundwater (such as well water, which may be rich in calcium, magnesium, iron, and manganese ions, with low microbial content but high hardness), or industrial wastewater (which may contain specific industrial pollutants such as heavy metals and high concentrations of organic matter). Furthermore, the concentration range, particle size, and chemical properties of the pollutants should be analyzed to avoid a "one-size-fits-all" pretreatment process due to a vague assessment of water quality, which could fail to specifically remove contaminants that damage ultrafiltration membranes.
For raw water with suspended solids and colloids as primary contaminants (such as surface water during the rainy season), the pretreatment process should focus on "physical retention and agglomeration removal," using a combination of coagulation, sedimentation, and filtration to reduce membrane clogging caused by these contaminants. First, a coagulant is added to the raw water. Its adsorption and bridging effects aggregate tiny colloidal particles and suspended solids into larger flocs. The flocs are then allowed to settle naturally in a sedimentation tank, initially reducing the water's turbidity. Quartz sand filtration or multi-media filtration is then used to further retain unsettled flocs and particles, preventing them from entering the ultrafiltration system with the water flow. If suspended solids and colloids adhere to the membrane fibers, they can quickly clog the pores, increase the transmembrane pressure, and cause physical damage to the membrane fibers over long periods of operation. Therefore, this type of pretreatment must ensure "agglomeration first, then retention" to reduce physical membrane fouling at the source.
When the raw water contains high concentrations of organic matter (such as eutrophic surface water and some industrial wastewater), the pretreatment process must prioritize addressing the risks of organic adsorption and biofouling on the membrane. Activated carbon adsorption can be used. The porous structure of activated carbon absorbs small organic molecules, pigments, and odorous substances in the water, reducing the adsorption and deposition of organic matter on the membrane surface. Prolonged attachment of organic matter to the membrane surface can form a difficult-to-clean organic fouling layer, which not only reduces membrane flux but also may breed microorganisms. If the organic matter concentration is too high, an oxidation pretreatment (such as potassium permanganate oxidation or ozone oxidation) can be added before activated carbon adsorption to break down large organic molecules into small molecules, improving subsequent adsorption efficiency. This also destroys the algae cell structure, preventing algae residues from entering the membrane system and causing clogging.
For high-hardness raw water (such as some groundwater) rich in calcium, magnesium, iron, and manganese ions, pretreatment processes should focus on anti-scaling and ion removal to prevent these substances from forming an inorganic scale layer on the membrane surface. If calcium and magnesium ion content is high, an ion exchange resin softening process can be used. By adsorbing and exchanging calcium and magnesium ions with the resin, the water hardness can be reduced, preventing the ions from precipitating on the membrane surface due to concentration during operation and forming inorganic scale such as calcium carbonate and magnesium sulfate. If the raw water contains iron and manganese ions, an aeration oxidation process is required to oxidize the divalent iron and manganese ions into trivalent iron hydroxide and manganese dioxide precipitation, which are then removed by filtration. This prevents the iron and manganese oxides from adhering to the membrane surface and forming a brown scale layer. This type of inorganic scale is not only difficult to remove through conventional backwashing, but can also scratch the membrane fibers, impairing the membrane's retention performance.
For raw water with high microbial content (such as untreated surface water and recycled sewage), a disinfection step should be included in the pretreatment process to prevent the formation of biofilms on the membrane surface. Sodium hypochlorite or ultraviolet disinfection can be used. Sodium hypochlorite kills bacteria and algae by disrupting microbial cell membranes and enzyme systems, while ultraviolet light kills bacteria by destroying microbial DNA. The choice of either method depends on the raw water conditions. If the raw water will subsequently pass through an ultrafiltration membrane sensitive to residual chlorine (such as certain PVDF membranes), a reducing agent should be added after disinfection to remove residual chlorine and prevent chlorine oxidation and damage to the membrane fibers. If microorganisms form biofilms on the membrane surface, they can clog the membrane pores and secrete corrosive substances, accelerating membrane degradation. Therefore, disinfection pretreatment must ensure that the microbial removal rate meets the influent requirements of the ultrafiltration membrane.
The sequence and parameter optimization of pretreatment processes must be dynamically adjusted based on the complexity of the raw water pollutants to avoid interference between processes and reduced treatment effectiveness. For example, when raw water contains suspended solids, organic matter, and microorganisms, the optimal process sequence should be "coagulation and sedimentation → multi-media filtration → activated carbon adsorption → disinfection → safety filtration." First, coagulation and sedimentation remove suspended solids, followed by filtration for further purification to prevent suspended solids from clogging the activated carbon pores and impacting adsorption efficiency. Disinfection after activated carbon adsorption prevents organic matter from reacting with the disinfectant to produce harmful byproducts. Finally, safety filtration (such as a 5μm filter element) intercepts particles that may have been dislodged during the previous process, providing a final layer of protection for the ultrafiltration membrane. Process parameters should also be adjusted based on water quality fluctuations. For example, when suspended solids concentrations increase during rainy seasons, the coagulant dosage can be increased and the filtration backwash cycle shortened to ensure stable pretreatment results.
For water quality requirements in different application scenarios (such as drinking water and industrial water), pretreatment processes must also take into account the operating characteristics of the subsequent ultrafiltration membrane to avoid over- or under-treatment. For example, pretreatment of drinking water using ultrafiltration pure water equipment requires extra attention to safety, such as selecting food-grade coagulants, ensuring disinfectant residues meet drinking water standards, and minimizing the impact of pretreatment chemicals on human health. Pretreatment of industrial water (such as that used in the electronics industry) emphasizes deep contaminant removal, such as by adding precision filtration or ion exchange steps to ensure that the levels of specific ions (such as silicon and boron) in the water do not affect subsequent production, while also protecting the ultrafiltration membrane from the highly corrosive effects of industrial pollutants. Regardless of the scenario, the ultimate goal of the pretreatment process is to ensure that the raw water entering the ultrafiltration system has "stable water quality and controllable contaminants," preventing the entry of substances that could clog, oxidize, or scratch the membranes, thereby ensuring the lifespan of the ultrafiltration membranes and the operational stability of the ultrafiltration pure water equipment.
