Analysis on the process flow of urban sewage treatment plants

From the perspective of technical feasibility and economic rationality, the removal of COD, BOD5, nitrogen, and phosphorus should be completed as much as possible within the secondary biological treatment process unit of sewage, especially for the removal of TN and NH3-N. Secondary water treatment should ensure sufficient removal rates of organic matter, suspended solids, and nitrogen and phosphorus nutrients. Generally, biological phosphorus removal and biological nitrification or denitrification methods are used. In some specific scenarios, chemical reactions may also be used to synergistically remove phosphorus. As for some special trace pollutants or toxic and harmful substances, they should be controlled at the source of industrial enterprises as much as possible, and if necessary, treatment units targeting water quality indicators should be set up in the deep treatment process.

In the advanced treatment after the secondary enhanced water treatment, the filtration process should be the core unit, and coagulation and sedimentation should be the strengthening means to efficiently remove suspended solids and colloidal substances, reduce the turbidity of the treated water, and eliminate some pathogens. If necessary, chemical phosphorus removal can be achieved by adjusting the dosage of coagulants such as Polyaluminum Chloride or polyferric sulphate. Advanced treatment is the selection unit for standard deep treatment, which uses physical, chemical, or biological methods to more fully remove certain specific components in water, such as denitrification filters to remove nitrate nitrogen, activated carbon adsorption and ozonation to remove male biodegradable organic matter, decolorization, and reverse osmosis to remove dissolved solids. Disinfection treatment is an essential unit for achieving standards and the final unit of advanced treatment processes. Its function is to use physical, chemical, or biological methods to remove and inactivate various pathogens in water.

Next, let's discuss the composition of the basic process flow for achieving Level A stability standards. The basic process flow is secondary strengthening treatment+chemical coagulation precipitation+media filtration+disinfection. The chemical coagulation precipitation filtration process includes the following three main combination methods. 1. Coagulation sedimentation filtration is divided into rapid mixing+flocculation+sedimentation+filtration. Rapid mixing is completed in a rapid mixing tank or inlet pipeline, and after flocculation reaction, it is settled in a clarification tank. The settled water enters the filter tank for filtration. In cases where the SS of the secondary treatment effluent is unstable or chemical phosphorus removal is required, this process combination needs to be adopted to ensure the comprehensive stability and compliance of the effluent. 2. Chemical flocculation filtration is divided into rapid mixing+flocculation+filtration. Rapid mixing is completed in the rapid mixing tank or inlet pipeline, and after a certain period of flocculation reaction, it enters the filter directly without sedimentation. Generally, polyaluminum chloride or aluminum sulfate are used as chemical phosphorus removal agents.

Finally, as a manufacturer of chemical coagulants, Lan Yao would like to emphasize some operational details of chemical coagulation treatment. The core purpose of setting up a chemical coagulant dosing system in the deep processing process is to improve the particle removal performance of subsequent filtration processes, in order to enhance the removal of suspended solids, colloidal substances, phosphates, and pathogens. Coagulants include polyaluminum chloride, aluminum sulfate, Polyferric Sulfate, polyaluminum ferric chloride, lime, and high molecular weight polymers. If the deep processing technology system can stably meet the TP removal requirements and 3 NTU turbidity without adding chemical agents, the coagulant dosing system is allowed to stop running, but the chemical agent dosing system must be kept running at least twice a month to ensure that the entire dosing system can be put into normal operation when needed. If the granular filter material filtration process is adopted after the coagulation process, the chemical coagulation process should at least meet the following points (here we take polyaluminum chloride as an example). The first point is to continuously monitor and record the turbidity value of the effluent from biological treatment, so that the subsequent equipment for adding polyaluminum chloride can automatically adjust the dosage of polyaluminum chloride according to changes in the incoming water quality. Secondly, in addition to micro flocculation filtration, chemical treatment facilities including rapid mixing and flocculation tanks should be provided in the design, and intermediate sedimentation facilities should be added if necessary to ensure that the subsequent filtration water quality requirements can be met under all operating conditions. The third point is that each processing unit of the deep processing facility should be equipped with at least two sets to ensure continuous regeneration treatment when one set of equipment is shut down for maintenance, upkeep, or backwashing. The fourth point is that while adding polyaluminum chloride, sufficient initial rapid mixing or equivalent measures should be provided to ensure the effective diffusion and efficient utilization of polyaluminum chloride in wastewater, and promote the efficient completion of subsequent flocculation reactions. The fifth point is that in the flocculation reaction tank, it is generally necessary to provide a mode that promotes the formation of floc particles. To control the turbulence or stirring intensity of the water flow through slow stirring, it is necessary to prevent both the sedimentation of floc particles and the fragmentation and disintegration of flocs. The sixth point is that insufficient coagulation time must not occur in process control to prevent further coagulation in the filtered water and affect the quality of the effluent.