At present, most domestic chemical enterprises are small in scale with backward technology development and application. Their products are mainly pesticides, pharmaceuticals, chemical fertilizers and so on, featuring extensive production mode and low technical level. The production wastewater is generally characterized by high contents of organic and toxic components, high ammonia nitrogen level, strong acidity and deep chroma, which causes severe water pollution and makes wastewater treatment a highly challenging task.
1. Elaboration of Characteristics
Mixed chemical wastewater generally refers to waste substances generated during industrial production, containing a large number of components that cause severe pollution to the natural environment. These pollutants have the following characteristics:
First, the wastewater is mainly a mixture of industrial production wastewater and domestic sewage, with the discharge volume of wastewater from chemical enterprises being much larger than that of domestic sewage. Second, the water volume and quality of the chemical wastewater to be treated change significantly with a wide fluctuation range. Third, even if the wastewater has undergone preliminary basic treatment before being sent to the treatment plant, it still has a complex composition with a wide variety of substances, high content of toxic components, resulting in great difficulty in treatment and low value for recycling. Fourth, although indicators such as COD meet the required standards after preliminary basic treatment, problems such as high salinity, high ammonia nitrogen content and high wastewater concentration remain major challenges for wastewater treatment.
2. Analysis of Treatment Processes
2.1 Physicochemical Treatment Processes
2.1.1 Regulation of Wastewater Quality and Quantity
The quality and quantity of mixed chemical wastewater treated at the sewage treatment station vary significantly and fluctuate greatly, making it difficult to stabilize. This leads to unpredictable impacts on the quantity and quality of wastewater during the treatment process, which hinders the normal operation of technical equipment and results in poor stability. In some cases, it may even cause equipment malfunctions and damage, thereby affecting the quality and efficiency of wastewater treatment.
To address this issue, measures need to be taken to regulate the wastewater quality and quantity. A common practice is to discharge the wastewater collected by the treatment plant into an equalization tank, which is designed for homogenizing water quality components and adjusting water volume. Only after such conditioning treatment can the subsequent wastewater treatment processes be carried out.
2.1.2 Isolation Treatment of Oil-like Organic Substances
Mixed chemical wastewater contains a large amount of oil-like pollutants with extremely poor water solubility. These pollutants have high adsorptivity, and can adsorb the active organisms on the surface of wastewater biofilms and oily sludge particles, thereby isolating them from oxygen. As a result, the biological activity of these organisms is reduced or even eliminated, which impairs the efficiency of wastewater treatment. Therefore, scientific treatment of such pollutants is essential to improve the wastewater treatment effect.
For the treatment of the above-mentioned pollutants, the chemical wastewater containing oil-like organic substances can be centrally discharged into an oil separation tank for organic matter sedimentation. This process separates the wastewater from the oil-like substances and reduces the content of precipitated substances in the wastewater. Subsequently, further treatment with chemical agents is applied to decrease the content of oil-like organic substances in the wastewater and enhance the treatment effect.
2.1.3 Air Flotation Treatment
This treatment process mainly relies on equipment capable of generating air bubbles. In wastewater treatment, a large number of highly dispersed bubbles are used to adsorb suspended particles in the wastewater; as the bubbles float upward, the substances are separated from the wastewater. Its adsorption targets are mainly hydrophobic solid suspended particles and oil-like suspended substances. Common air flotation methods adopted in China include pressure dissolved air flotation, rotary-cut air flotation, and cavitation air flotation.
2.1.4 Coagulation Treatment
The principle of this technology is to add a coagulant into the wastewater to be treated and mix it with the wastewater. Through a series of physical and chemical reactions, the suspended pollutants and poorly sedimentable substances in the wastewater are coagulated together to form larger particles, which then become separated from the wastewater. The coagulation treatment method often needs to be combined with technologies such as sedimentation and air flotation to ensure the quality of wastewater separation. Mixed coagulants are generally preferred, while the use of single-type coagulants is not recommended.
2.2 Biochemical Treatment Processes
2.2.1 Hydrolysis and Acidification
2.2.2 Anaerobic-Oxic Process (A/O Process)
2.2.3 PACT Wastewater Treatment Process
This process was developed by DuPont in 1972. It is an activated sludge-powdered activated carbon technology with a wide application range, and has been extensively adopted in the treatment of mixed chemical wastewater such as petrochemical wastewater, coking wastewater and papermaking wastewater. In addition, it has a significant nitrification effect. Due to its excellent performance in removing ammonia nitrogen and reducing chroma, it is also frequently applied in urban wastewater treatment during the process of urban environmental management. This process involves adding an appropriate amount of powdered activated carbon to wastewater, which reduces the organic load of the wastewater and enhances the efficiency of organic matter removal in the biochemical treatment system.
