Water is the source of life, and sewage treatment serves as the critical line of defense for protecting water resources. However, when it comes to sewage treatment, many people have a series of questions: What exactly is in sewage? Can treated water be drunk directly? Are the treatment methods the same for different types of sewage?
Today, we have compiled 100 of the most common questions in the field of sewage treatment. Covering basic concepts, core processes, equipment principles, and industrial applications, we will answer each question in plain language to help you fully understand the "secrets" of sewage treatment.
It refers to used water that has lost its original value. According to its source, it can be divided into domestic sewage, industrial wastewater, rainwater, etc.
Domestic sewage comes from households, schools, etc., containing organic matter, detergents, etc., with a relatively stable composition. Industrial wastewater originates from factories, containing heavy metals, chemical agents, etc., with a complex composition and toxicity varying by industry.
To remove pollutants from water (such as organic matter, suspended solids, nitrogen, phosphorus, etc.), so that the water quality meets discharge standards or reuse standards, avoiding environmental pollution and realizing water resource recycling.
China implements the "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plants" (GB18918-2002) and its amendment issued in 2023, which includes Grade 1A, Grade 1B, Grade 2, and Grade 3 standards. The specific standard applied is determined based on the discharge destination (such as rivers, oceans, municipal pipe networks).
The effluent from conventional sewage treatment plants mostly meets "discharge or reuse standards". It can only be used as drinking water (i.e., "direct potable reuse of reclaimed water") after advanced treatment (such as reverse osmosis, advanced oxidation, and strict disinfection) to meet the "Standards for Drinking Water Quality" (GB5749-2022). Currently, this application is relatively rare and requires strict approval.
COD (Chemical Oxygen Demand) refers to the amount of oxygen required to oxidize reducing substances (mainly organic matter) in water using chemical methods. It reflects the pollution degree of organic matter and reducing pollutants in water—higher values indicate more severe pollution.
BOD (Biochemical Oxygen Demand) usually refers to BOD₅ (5-day Biochemical Oxygen Demand), which is the amount of oxygen required by microorganisms to decompose organic matter within 5 days. It reflects the content of biodegradable organic matter. COD includes biodegradable and non-biodegradable organic matter as well as reducing inorganic substances. Generally, COD > BOD₅, and the ratio BOD₅/COD can be used to judge the biodegradability of sewage (a ratio > 0.3 indicates good biodegradability).
SS (Suspended Solids) refers to insoluble solid particles in water (including floating, suspended, and settleable particles). It can be removed through physical methods such as sedimentation, filtration, and dissolved air flotation (DAF).
Excessive nitrogen and phosphorus can cause eutrophication of water bodies, triggering the massive reproduction of algae and plankton. This consumes dissolved oxygen in water, leading to hypoxia, fish and shrimp deaths, and the formation of "water blooms" (in freshwater) and "red tides" (in seawater), which disrupt the ecological balance.
Also known as recycled water, it refers to non-drinking water that meets certain standards after sewage treatment. It can be used for green irrigation, road cleaning, industrial cooling, landscape water supplementation, etc. Some qualified reclaimed water can be used for agricultural irrigation (must comply with the "Standards for Irrigation Water Quality").
Yes, the main odorous substances include hydrogen sulfide (rotten egg smell), ammonia (irritating odor), and volatile organic compounds (VOCs). The treatment methods include source sealing and capping, negative pressure collection, followed by biological filter deodorization, activated carbon adsorption, chemical scrubbing, etc.
It is usually divided into four stages: pre-treatment, primary treatment (physical treatment), secondary treatment (biological treatment), and tertiary treatment (advanced treatment). Finally, the water must be disinfected to meet standards before discharge or reuse.
Pre-treatment removes large impurities, floating objects, and sand grains from sewage to protect subsequent equipment (such as water pumps and aerators) from clogging or wear. It includes processes such as grilles, grit chambers, and equalization tanks (for adjusting water volume and quality).
It mainly removes suspended solids (SS), some floating objects, and settleable organic matter. The SS removal rate is about 50%-70%, and the organic matter (COD) removal rate is about 20%-30%. The water quality is still poor and requires further treatment.
The core is biological treatment, which uses the metabolic activities of microorganisms (bacteria, fungi, protozoa, etc.) to decompose organic matter in water. The COD removal rate can reach 80%-95%, making it a key link in sewage treatment. The water quality after secondary treatment basically meets the discharge standards.
To further remove nitrogen, phosphorus, trace organic matter, heavy metals, color, odor, etc., that are not removed by secondary treatment, so that the water quality meets reuse standards (such as industrial reuse, landscape water) or more stringent discharge standards (such as Grade 1A standard).
Sludge is the solid waste precipitated or separated during sewage treatment. It mainly comes from: screenings intercepted by grilles, sand grains from grit chambers, primary sludge from primary sedimentation tanks, excess sludge from biological treatment systems, chemical sludge from advanced treatment, etc.
Yes, otherwise, it will cause secondary pollution. The treatment process usually includes: thickening (reducing moisture content and volume) → conditioning (improving dewatering performance) → dewatering (mechanical dewatering to reduce moisture content to below 80%) → stabilization (such as anaerobic digestion, aerobic composting to eliminate odors and pathogens) → final disposal (landfilling, incineration, land application, resource utilization as building materials).
It is a process in which anaerobic microorganisms decompose organic matter in an oxygen-free (or low-oxygen) environment. It is suitable for high-concentration organic sewage (such as food wastewater, brewery wastewater, slaughterhouse wastewater, with COD usually > 2000mg/L). It can produce biogas (energy recovery) with low energy consumption but a long treatment cycle.
It is a process in which aerobic microorganisms decompose organic matter in an oxygen-rich environment. It is suitable for medium and low-concentration sewage (COD < 2000mg/L) with fast treatment speed and high efficiency, but it requires oxygen consumption (high energy consumption). Anaerobic treatment has a slow speed, low energy consumption, and can produce biogas, but the effluent needs further treatment, and it is prone to producing odorous gases such as hydrogen sulfide.
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