Environmental Water Treatment Knowledge: Pesticide Wastewater - Definition and Significance
Pesticide wastewater refers to the wastewater discharged by pesticide plants during pesticide production. Its quality and quantity are unstable. It is mainly divided into the following categories:
1. Benzene-containing wastewater: Producing 1 ton of BHC (hexachlorocyclohexane) discharges 3-4 tons of wastewater with a benzene content of 1,500-2,000 mg/L, which can be treated by distillation and adsorption on coal-gangue slag.
2. Organophosphorus-containing wastewater: COD above 10,000 mg/L, containing about 1,000 mg/L of organophosphorus. Dimethoate, methanol, dimethylamine, and other substances can first be recovered from the wastewater by extraction or distillation, and then the wastewater can be rendered harmless by biological treatment.
Combined treatment processes for pesticide wastewater
4. High-concentration phenol-containing wastewater: First recover phenol by extraction to reduce its content below 300 mg/L, then after appropriate pretreatment carry out biological or chemical oxidation treatment.
5. Mercury-containing wastewater: The wastewater is acidic and the mercury compounds are in a dissolved state, so it can be treated by the sulfide precipitation method. In recent years, reverse osmosis and the activated-carbon biofilm method have also been used to treat pesticide wastewater. Some countries have banned the production of organochlorine and organomercury pesticides such as BHC, and are actively researching microbial pesticides - the fundamental way to prevent pesticide pollution.
Pesticides come in a great variety and pesticide wastewater is chemically complex, with the following main characteristics:
1. High pollutant concentration - chemical oxygen demand (COD) can reach tens of thousands of milligrams per liter.
2. High toxicity - besides pesticides and intermediates, the wastewater contains toxic substances such as phenol, arsenic, and mercury, as well as many substances that are difficult for organisms to degrade.
3. Foul odor that irritates the human respiratory tract and mucous membranes.
4. Unstable quality and quantity. Consequently, pesticide wastewater causes very serious environmental pollution [1].
Anatase TiO2, under ultraviolet irradiation, can generate highly oxidizing hydroxyl radicals that oxidize and degrade organic matter into CO2, H2O, and inorganic substances, with fast degradation and no secondary pollution, offering a new approach to the degradation treatment of pesticide wastewater. Concerning the photocatalytic degradation of organic matter, the issues of concern are, on the one hand, the influencing factors and transformation pathways during degradation; the immobilization of nano-TiO2 and the integration of reaction and separation have become one of the challenging topics in the photocatalysis field, and on the other hand, the issue of improving the catalytic efficiency of the prepared catalyst.
Most photocatalytic systems use light sources such as high-pressure lamps, high-pressure xenon lamps, black-light lamps, and ultraviolet germicidal lamps, which consume substantial energy. If nano-TiO2 can be effectively and stably sensitized to broaden its absorption spectrum so that sunlight can be used directly as the light source, costs would be greatly reduced [2].
Ultrasound is sound waves with a frequency greater than 20 kHz. The principle by which ultrasound induces the degradation of organic matter is that, under ultrasonic action, the liquid undergoes cavitation - under the negative-pressure phase of the ultrasound, extreme conditions are produced that cause organic matter to undergo bond cleavage, aqueous-phase combustion, high-temperature decomposition, or free-radical reactions. Once the concentration increases beyond a certain value, the degradation rate changes little, and the solution temperature during ultrasonic degradation should be controlled at 15-60 C. Xie Bing et al. applied ultrasonic flotation pretreatment to wastewater from the production of monocrotophos and trimethyl phosphite, which reduced COD and toxicity and improved biodegradability; subsequent biological treatment dominated by photosynthetic bacteria brought the COD down to 200 mg/L. Research on ultrasonic-radiation degradation of organic pollutants has mostly been at the laboratory scale, lacking systematic studies and even more so pilot-scale data.
Domestically, most pesticide manufacturers have built biological treatment facilities, but almost none achieve ideal treatment results. Research on the biological treatment of such wastewater is therefore essential. A large body of studies has shown that fungi, bacteria, algae, and other microorganisms degrade pesticides well. The biofilm method fixes microbial cells on a packing medium, where the microorganisms attach, grow, and reproduce, forming a film-like biological sludge on it. Compared with conventional activated-sludge processes, the biofilm has the advantages of a high volumetric biomass concentration, long survival generations, and a rich variety of microorganisms, and is especially suitable for the application of specialized strains in wastewater systems.
The iron-carbon micro-electrolysis method is the combined effect of flocculation, adsorption, bridging, sweeping, co-precipitation, electrodeposition, and electrochemical reduction, and can effectively remove pollutants and improve the biodegradability of wastewater. The freshly formed iron surface and the large amount of nascent Fe2+ and atomic H produced during the reaction have high chemical activity and can change the structure and properties of many organic compounds in the wastewater, causing chain scission and ring opening; the electric-field effect around the micro-cell electrodes also causes charged ions and colloids in the solution to aggregate and deposit on the electrodes and be removed. In addition, the Fe2+, Fe3+, and their hydrates generated by the reaction have strong adsorption and flocculation activity, further improving the treatment effect.
Advanced oxidation technology generates highly reactive OH radicals through combinations of oxidants and is regarded as the best technology for treating refractory organic pollutants. Introducing ultraviolet light and hydrogen peroxide jointly, and controlling the pH of the reaction system, can further improve the efficiency of the ozone advanced-oxidation process. Chen Aiyin's research shows that UV-catalyzed ozonation is highly effective in degrading 2,4-dichlorophenoxyacetic acid (2,4-D) pesticide wastewater; among the four ozonation processes compared - ozonation alone, ozone/UV, ozone/hydrogen peroxide, and ozone/hydrogen peroxide/UV - ozone/UV advanced oxidation is the best ozonation treatment. For a 2,4-D water sample at 200 mg/L, the reaction for 30 min achieved complete 2,4-D degradation, and the mineralization rate exceeded 75% at 75 min. An alkaline environment favors the ozonation reaction. The introduction of hydrogen peroxide did not significantly promote 2,4-D degradation, because its decomposition consumes OH- and, in an unbuffered system, lowers the pH, limiting H2O2 decomposition and the OH radical chain reaction. It was shown that adding H2O2 somewhat improves the photolysis effect - at a dosage of 75 mg/L the COD removal rate of the sample rose from 20% with no dosage to 40%, but overdosing gave no further improvement. Aeration promotes the photolysis effect, especially for the UV/Fenton process; after 2 h of photolysis, the COD removal rate under aeration rose from 30% (no aeration) to 80%. Catalytic wet oxidation can achieve efficient degradation of organic pollutants while greatly reducing the reaction temperature and pressure, providing a highly efficient new technology for high-concentration, refractory biodegradable organic wastewater. The catalyst is the core of catalytic wet oxidation, and many scholars are devoted to developing new, highly efficient catalysts [2].
3. High-salinity wastewater: Producing 1 ton of dichlorvos generates 5-7 tons of wastewater with COD reaching tens of thousands of mg/L, containing 1,000 mg/L of organophosphorus and about 0.6% of the toxic dichlorvos, which is treated by concentration-incineration or wet oxidation.
When treating actual wastewater, the organic pollutants present are complex and diverse, so a single treatment process often fails to achieve the desired goal. In practice, the appropriate combination of processes can be selected by comprehensively considering the technical characteristics and the specific wastewater quality.
