Against the backdrop of worsening global water scarcity and water pollution, wastewater treatment has become a core link in safeguarding the ecological environment and ensuring the sustainable use of water resources. Whether it is the purification of urban domestic sewage or the discharge of industrial wastewater up to standards, an efficient wastewater treatment system can not only reduce the damage of pollutants to water bodies but also realize the recycling of water resources, providing support for the coordinated development of social economy and ecological environment. This article will elaborate on the basic concepts, core processes, mainstream technologies, and industry development trends of wastewater treatment, systematically explaining the key points of wastewater treatment to provide professional references for relevant practitioners, enterprises, and readers concerned about the environmental protection field.

I. Wastewater Treatment: Definition and Core Value

1.1 What is Wastewater Treatment?

Wastewater treatment refers to the process of removing pollutants such as suspended solids, organic matter, heavy metals, and pathogens from wastewater through physical, chemical, biological, and other technical means, and purifying it to meet national discharge standards or reuse standards. The sources of wastewater mainly include domestic sewage (such as domestic drainage from residents and commercial service drainage), industrial wastewater (such as drainage from chemical, textile, food processing, and other industries), and initial rainwater (rainwater that washes the ground and carries pollutants). Targeted treatment plans need to be designed for wastewater from different sources.

1.2 Core Value of Wastewater Treatment

• Protecting Water Resource Ecology: Direct discharge of untreated wastewater will cause pollution to rivers, lakes, and groundwater, damaging the aquatic ecosystem. Wastewater treatment can effectively intercept pollutants and reduce risks such as water eutrophication and excessive heavy metals.
• Realizing Water Resource Recycling: Purified wastewater (also known as "reclaimed water") can be used in scenarios such as irrigation, industrial cooling, and municipal greening, alleviating the contradiction between water supply and demand, especially applicable to water-scarce areas.
• Safeguarding Public Health: Pathogens (such as bacteria and viruses) in domestic sewage and toxic substances in industrial wastewater will threaten human health if they seep into drinking water sources. Wastewater treatment is a key barrier to blocking the spread of pollution.

II. Core Processes of Wastewater Treatment: From Pretreatment to Advanced Purification

A complete wastewater treatment system must follow the principle of "gradient purification" and gradually reduce the concentration of pollutants through multi-stage treatment. The core process can be divided into four stages: pretreatment, primary treatment, secondary treatment, and advanced treatment. Each stage complements each other in function to ensure that the effluent meets standards.

2.1 Pretreatment: Intercepting "Large Particle Impurities"

Pretreatment is the first line of defense in wastewater treatment, whose main purpose is to remove impurities with large volumes that are likely to cause blockage of subsequent equipment, reducing the burden on subsequent treatment links.

• Core Processes: Grilles (mechanical grilles / manual grilles), grit chambers.
• Specific Functions: Grilles intercept suspended substances such as branches, plastic bags, and fibers; grit chambers remove inorganic particles such as sand and gravel through gravity sedimentation to avoid abrasion of water pumps and blockage of pipelines.

2.2 Primary Treatment: Removing "Settlable Pollutants"

Primary treatment is a physical treatment stage, focusing on removing suspended solids and part of organic matter that can be settled by gravity from wastewater, reducing the turbidity and COD (Chemical Oxygen Demand) of wastewater.

• Core Processes: Horizontal flow sedimentation tanks, vertical flow sedimentation tanks.
• Treatment Effect: It can remove about 30%-50% of suspended solids and the COD removal rate is about 10%-20%. The effluent still contains a large amount of soluble organic matter and pathogens, which requires further treatment.

2.3 Secondary Treatment: Degrading "Soluble Organic Matter"

Secondary treatment is the core link of wastewater treatment, mainly based on biological treatment technology. It uses the metabolic effect of microorganisms (bacteria, fungi, etc.) to decompose the soluble organic matter that is difficult to settle in wastewater into harmless carbon dioxide and water, while removing part of nitrogen and phosphorus nutrients.

• Mainstream Processes:
  • Activated Sludge Process: Microorganisms form "activated sludge flocs" through aeration to adsorb and degrade organic matter, applicable to large-scale urban wastewater treatment plants.
  • Biofilm Process: Microorganisms attach to the surface of fillers (such as biofilters and biological rotating discs) to form a "biofilm". When wastewater flows through, the degradation of organic matter is completed. It has strong impact resistance and is applicable to small and medium-sized wastewater treatment projects.
• Treatment Effect: The COD removal rate can reach 80%-90%, and the BOD (Biochemical Oxygen Demand) removal rate exceeds 90%. The effluent basically meets the municipal discharge standards, but still requires advanced treatment to realize reuse.

2.4 Advanced Treatment: Realizing "Water Quality Upgrading and Reuse"

Advanced treatment targets the effluent after secondary treatment, removing residual trace pollutants (such as heavy metals, refractory organic matter, and chromaticity) through physical, chemical, or advanced oxidation technologies, so that the water quality meets the reuse standards (such as industrial circulating water, landscape water) or more stringent discharge standards.

• Core Processes:
  • Filtration Technology: Quartz sand filters, activated carbon filters, or membrane filtration (MBR membranes, RO reverse osmosis membranes) are used to remove tiny suspended solids and organic matter.
  • Disinfection Treatment: Ultraviolet rays, chlorine dioxide, or ozone are used for disinfection to kill residual pathogens and ensure the safety of reuse.
  • Nitrogen and Phosphorus Removal: Chemical precipitation methods (such as adding PAC and PAM) or biological nitrogen and phosphorus removal processes are used to control the risk of water eutrophication.

III. Mainstream Wastewater Treatment Technologies: Classification and Application Scenarios

According to different treatment principles, wastewater treatment technologies can be divided into three categories: physical treatment methods, chemical treatment methods, and biological treatment methods. The applicable scenarios and treatment effects of various technologies are different. In practical applications, combined processes need to be selected according to the type of wastewater, pollutant concentration, and treatment goals.

3.1 Physical Treatment Methods: Low Cost and Easy Operation

Physical treatment methods separate pollutants in wastewater by physical effects without changing the chemical properties of pollutants. They have the characteristics of low cost, simple operation, and no secondary pollution.

• Common Technologies: Grilles, sedimentation, filtration, centrifugal separation, air flotation (removing light suspended solids such as oil).
• Applicable Scenarios: Pretreatment stage (grilles, grit removal), primary treatment (sedimentation), and advanced treatment (filtration), especially applicable to wastewater containing a large amount of suspended solids (such as papermaking wastewater, mining wastewater).

3.2 Chemical Treatment Methods: Targeted Removal of Refractory Pollutants

Chemical treatment methods add chemical agents to react with pollutants in wastewater, making them form easily separable precipitates or gases. They are applicable to the removal of pollutants that are difficult to treat by physical methods (such as heavy metals, refractory organic matter).

• Common Technologies:
  • Chemical Precipitation Method: Adding agents such as lime and aluminum sulfate to form precipitates with heavy metals (such as lead, cadmium) or phosphorus.
  • Advanced Oxidation Technologies (AOPs): Using ozone, hydrogen peroxide, ultraviolet rays, etc. to generate strong oxidizing free radicals to degrade refractory organic matter such as pesticides and antibiotics.
  • Neutralization Treatment:Neutralization Treatment**: Adding acids or alkalis to adjust the pH value of wastewater (for example, industrial wastewater is mostly acidic or alkaline and needs to be neutralized to neutral before entering the biological treatment link).
• Applicable Scenarios: Industrial wastewater treatment (such as heavy metal removal from electroplating wastewater, organic matter degradation from chemical wastewater), and phosphorus removal link in advanced treatment.

3.3 Biological Treatment Methods: Core Technologies with Environmental Protection and High Efficiency

Biological treatment methods use the metabolic activities of microorganisms to degrade organic matter, which have the advantages of high treatment efficiency, low cost, and strong environmental protection. They are currently the mainstream technologies for urban wastewater treatment and low-to-medium concentration industrial wastewater treatment.

• Classification and Application:
  • Aerobic Biological Treatment: Under aerobic conditions, aerobic microorganisms degrade organic matter, such as activated sludge process and biofilm process, applicable to wastewater with medium and high concentration organic matter (such as domestic sewage, food processing wastewater).
  • Anaerobic Biological Treatment: Under anaerobic conditions, anaerobic microorganisms decompose organic matter into methane (recyclable) and carbon dioxide, applicable to high-concentration organic wastewater (such as beer wastewater, livestock and poultry breeding wastewater), while realizing energy recovery.
• Technical Advantages: No large amount of chemical agents is needed, the operation cost is low, and the "harmless" conversion of organic matter can be realized, which is in line with the low-carbon environmental protection trend.

IV. Development Trends of the Wastewater Treatment Industry: Intelligence, Resource Utilization, and Low-Carbonization

With the stricter environmental protection policies (such as the upgrading of the "Discharge Standards of Pollutants for Urban Wastewater Treatment Plants"), technological iteration, and the promotion of the "double carbon" goal, the wastewater treatment industry is developing in the direction of intelligence, resource utilization, and low-carbonization. The specific trends are as follows:

4.1 Intelligent Upgrading: Improving Treatment Efficiency and Operation and Maintenance Level

With the help of Internet of Things (IoT), big data, and artificial intelligence (AI) technologies, wastewater treatment plants can realize full-process automatic monitoring and intelligent regulation:

• Real-Time Monitoring: Collect influent water quality (COD, pH, flow) and equipment operation parameters (aeration intensity, water pump speed) through sensors, and upload the data to the central control system in real time.
• Intelligent Regulation: AI algorithms automatically adjust the dosage of chemicals and aeration time according to changes in water quality, avoiding over-treatment or sub-standard treatment, and reducing energy consumption and chemical consumption.
• Predictive Maintenance: Predict the risk of failures (such as water pump wear, filter blockage) through equipment operation data, reducing downtime and improving operation and maintenance efficiency.

4.2 Resource Utilization: From "Up-to-Standard Discharge" to "Resource Recovery"

Traditional wastewater treatment aims at "up-to-standard discharge". In the future, it will focus on "wastewater resource utilization" and "sludge resource utilization" to realize the reduction of pollutants and resource recycling:

• Reclaimed Water Reuse: The wastewater after advanced treatment is used for industrial cooling, municipal greening, and agricultural irrigation. Some regions have realized "reclaimed water supplementing river ecological water" (such as Beijing, Shenzhen).
• Sludge Resource Utilization: After dehydration, drying, and incineration, sludge can be used to make building materials (such as bricks, cement) or co-generate electricity, reducing the amount of landfill (traditional sludge landfill is likely to cause soil pollution).
• Energy Recovery: Methane (biogas) produced by anaerobic biological treatment can be used for power generation or as fuel, realizing "wastewater treatment + energy self-sufficiency" (for example, Shanghai Bailonggang Wastewater Treatment Plant, biogas power generation meets 1/3 of the plant's electricity demand).

4.3 Low-Carbon Development: Responding to the "Double Carbon" Goal

The wastewater treatment process (such as aeration, sludge treatment) consumes a lot of energy, accounting for 10%-15% of the energy consumption of urban public facilities. Low-carbonization has become an important direction of the industry:

• Promotion of Energy-Saving Processes: Adopt low-energy aeration equipment (such as membrane aerated biofilm reactor MBR) and high-efficiency sedimentation tanks to reduce operation energy consumption.
• Carbon Emission Reduction Technologies: Construct artificial wetlands by planting aquatic plants (such as reeds, calamus), using plants to absorb carbon dioxide, and at the same time assisting in wastewater purification; explore sludge carbonization technology to reduce carbon emissions during incineration.
• Green Treatment Mode: Promote "decentralized wastewater treatment stations" (such as in rural areas) to avoid energy loss caused by long-distance transportation of wastewater, and combine with ecological treatment technologies (such as oxidation ponds) to realize low-carbon operation.

V. Conclusion: Wastewater Treatment is a Key Support for Ecological Civilization Construction

Wastewater treatment is not only an "end-of-pipe treatment" method to solve water pollution problems but also a core link to realize the recycling of water resources, promote the "double carbon" goal, and build ecological civilization. With the continuous upgrading of technologies (such as MBR membrane technology, intelligent monitoring) and the transformation of industry trends (resource utilization, low-carbonization), wastewater treatment will shift from "passive treatment" to "active recycling", providing stronger guarantees for global water resource security and ecological protection.

For enterprises, choosing appropriate wastewater treatment processes (for example, industrial enterprises need to select a combined process of "pretreatment + biological treatment + advanced treatment" according to the type of wastewater) and promoting intelligent transformation can not only meet the requirements of environmental compliance but also reduce operating costs through resource recovery (such as reclaimed water reuse, biogas power generation). For society, strengthening the construction of wastewater treatment facilities (especially in rural areas and small and medium-sized towns) and improving public awareness of water conservation and environmental protection are important foundations for promoting the sustainable development of the wastewater treatment industry.