Methods for advanced wastewater treatment

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To remove suspended solids from wastewater after secondary treatment, the treatment technique to be used depends on the state and particle size of these solids. For particles with a size of 1 μm or more, sand filtration is generally employed for removal； Particles with particle sizes ranging from a few hundred Å to several dozen μm are removed using microfiltration technology ； Particles with a particle size ranging from 1000 Å to a few Å can be removed using reverse osmosis. Particles in a colloidal state are removed using coagulation sedimentation. Industrial wastewater contains high levels of soluble inorganic salts, and effective desalination techniques include ion exchange, electrodialysis, and reverse osmosis. Ion exchange is primarily used to remove ionized substances from water. The COD value of wastewater treated by biochemical methods is relatively high, and most of the substances involved are non-ionic organic compounds. The organic matter in wastewater binds strongly to the fixed ions on the active groups of the resin; once this binding occurs, it is difficult to regenerate the resin, which severely affects its regeneration efficiency and exchange capacity ； Furthermore, resins have poor resistance to oxidation by oxidants such as Cl2 and O2, so they are not suitable for use. The electrodialysis treatment process is a type of membrane separation method; it features simple equipment and easy operation, does not require chemical reagents, but consumes electrical energy. Through primary electrodialysis, the desalination rate of wastewater can reach 20%–50%. To achieve a higher desalination rate, a multi-stage series system is required, which incurs high energy consumption and therefore is not suitable for use. Reverse osmosis is a membrane technology that has been developed over the past 20 years, and it is now widely used in applications such as water desalination and wastewater treatment. This method is specifically used to separate molecular and ionic dissolved substances in water. Its principle involves applying high pressure to an aqueous solution, causing the solvent water to pass through a reverse osmosis membrane and become fresh water, while the solutes remain trapped and form concentrated water. This approach achieves two objectives: one is to produce fresh water from saline water ； Second, it concentrates the dissolved pollutants in the wastewater; the treated wastewater can be either discharged directly or reused. A reverse osmosis system uses a molecular diffusion membrane as a medium, with hydrostatic pressure difference as the driving force to separate substances in aqueous solutions. Compared to electrodialysis, it offers significant economic advantages: it has higher electrical efficiency and lower energy consumption. Under the same feedwater conditions, the energy required to produce one ton of fresh water using reverse osmosis is 10% to 20% of that required by electrodialysis. Microfiltration also belongs to the category of pressure-driven membrane processes; in terms of the range of separation it provides, it fills the gap between reverse osmosis, nanofiltration, and conventional filtration. Microfiltration involves applying a certain pressure to the feed solution; polymeric substances, colloids, proteins, particles, and other such materials are retained by the semipermeable membrane, while solvents and low-molecular-weight substances pass through the membrane. The separation mechanism of microfiltration mainly includes the pore-size screening mechanism at the membrane surface, the blockage mechanism due to the clogging of membrane pores, and the primary adsorption mechanism of particles by the membrane surface and membrane pores. Generally, the transmembrane pressure difference in microfiltration operations is 0.2 to 0.7 MPa, which is much lower than that of membrane processes such as reverse osmosis. However, microfiltration devices do not remove salts, and thus cannot achieve the goal of advanced wastewater treatment