In the production and application of water-based inks, a significant amount of wastewater is generated due to equipment cleaning. The ever-changing colors of these inks result in complex chemical compositions in the wastewater, which is characterized by high COD (Chemical Oxygen Demand), high color intensity, and poor biodegradability. If discharged into water bodies, this type of wastewater can cause severe environmental pollution. Therefore, the treatment process must be carefully designed according to the specific characteristics of the ink used. Currently, research and practical applications focus on combining pretreatment methods such as chemical coagulation, electrolysis, dissolved air flotation, and advanced oxidation with biological treatment systems.
Electrolysis has shown promising performance in pretreatment processes and has a solid foundation for domestic application. It offers several advantages: first, hydroxyl radicals (OH) produced during the process can directly react with pollutants, breaking them down into carbon dioxide, water, and simple organic compounds with minimal secondary pollution. Second, the electrolysis process also facilitates air flotation. Third, it operates efficiently at room temperature with low energy consumption. Lastly, it can function as a standalone treatment or in combination with other methods. For instance, when used as a pretreatment, it significantly improves the biodegradability of the wastewater, making subsequent biological treatments more effective. The successful application of electrolysis at Maoming Bantian Ink Co., Ltd. demonstrated that this method, when combined with biochemical treatment, can achieve standard discharge levels.
The decontamination mechanism of electrolysis involves using iron plates as anodes and aluminum plates as cathodes under a strong current. The main reactions are:
Anode: Fe → Fe²⺠+ 2e
Cathode: 2H⺠+ 2e → H₂
As the iron anode dissolves, it forms Fe(OH)â‚‚, which acts as a powerful coagulant. Simultaneously, hydrogen gas is released at the cathode, which helps in the flotation of suspended solids. The process includes oxidation, reduction, flocculation, and air flotation. Oxidation occurs through direct electron loss by pollutants or via active oxidants like [O] and Clâ‚‚. Reduction involves the transfer of electrons to pollutants, while flocculation leads to the formation of large flocs. Air flotation helps in removing both hydrophobic and hydrophilic contaminants.
The Upflow Anaerobic Sludge Blanket (UASB) process combines anaerobic filtration and activated sludge methods, converting organic pollutants into biogas. Developed in the 1970s, it uses a three-phase separator to separate sludge from wastewater, allowing for efficient treatment. The discovery of granular sludge improved its efficiency and led to the development of more advanced anaerobic reactors. UASB is widely used due to its simplicity, cost-effectiveness, and maturity.
Air flotation is another common method where tiny air bubbles attach to suspended particles, causing them to float to the surface. This technique is effective for removing oils and surfactants but may face challenges with high salt content and corrosion.
Coagulation involves adding chemicals to destabilize colloidal particles, allowing them to aggregate and settle. It is often used as a pretreatment step for oily or dyeing wastewater.
Biological contact oxidation uses biofilms on filler materials to degrade organic matter. It is efficient, requires no sludge return, and adapts well to fluctuating conditions. Membrane bioreactors (MBR) combine biological treatment with membrane filtration, offering high-quality effluent and compact design. They have been increasingly adopted in China for treating various types of wastewater, including industrial and high-concentration organic streams.
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