Biological treatment uses microorganisms to absorb, decompose, and oxidize organic matter in sewage, and degrade unstable organic matter into stable and harmless substances, thereby purifying sewage. Modern biological treatment methods can be divided into two categories: aerobic oxidation and anaerobic reduction according to different microorganisms. The former is widely used to treat urban sewage and organic industrial wastewater. Aerobic oxidation is widely used and includes many types of technologies and structures.

Biofilm method (including biological filter tank, biological turntable), biological contact oxidation and other processes and structures. Activated sludge method and biofilm method are both artificial biological treatment methods. There are also natural biological treatments for farmland and ponds, namely irrigated fields and bioponds. Biological treatment is the most widely used wastewater treatment method because of its low cost.

It refers to the amount of solid matter in sewage that can be retained by the filter. Part of the suspended solids can settle under certain conditions. The determination of suspended solids is usually carried out using asbestos filter filtration. The main equipment is Gu's crucible. When laboratory equipment conditions are not available, filter paper can also be used as a filter, and the amount of suspended solids can be obtained from the difference between total solids and dissolved solids. When measuring suspended solids, large differences often occur due to different filters.

This indicator is one of the most basic data on sewage. Measuring the suspended solids in the incoming and outgoing water can be used to reflect the reduction of suspended solids after the sewage passes through the primary sedimentation tank and the secondary sedimentation tank. It is the main basis for reflecting the sedimentation efficiency of the structure.

Chemical oxygen demand (COD for short) refers to the amount of oxygen required by the oxidant to chemically oxidize organic matter in sewage. Potassium permanganate is used as the oxidant, and the measured result is customarily called oxygen consumption, expressed in OC. Potassium dichromate is used as the oxidant, and the measured result is called chemical oxygen demand and expressed as COD. The difference between the two lies in the choice of oxidant. Using potassium permanganate as an oxidant can only oxidize linear organic compounds in sewage, while using potassium dichromate as an oxidant has a stronger and more complete effect than the former. In addition to linear organic compounds, it can oxidize permanganic acid. Potassium cannot oxidize many structurally complex organic compounds. Therefore, the COD value of the same sewage is much greater than the OC value. Especially when a large amount of industrial wastewater enters the sewage plant, the chemical oxygen demand of the potassium dichromate method should generally be measured. The COD value of urban sewage plants is generally about 400~800mg/L.

The consumption value of the potassium permanganate method is often used in sewage plants as reference data to determine the dilution factor of the five-day biochemical oxygen demand.

The sludge index refers to the volume (in ml) occupied by 1g of dry sludge after the mixed liquid in the aeration tank has settled for 30 minutes.

The SVI value can better reflect the looseness and coagulation settling performance of activated sludge. The SVI of good activated sludge is usually between 50 and 300. If the SVI is too high, the SVI value measured at the same concentration is valuable. In addition, because the size of the measurement container has a certain impact on the number of measurements, the measurement containers must be unified.

The adsorbent should have the following properties: good adsorption selectivity, strong adsorption capacity, low adsorption equilibrium concentration, easy regeneration and reuse, good mechanical strength, stable chemical properties, wide source and low price. Before practical application, the adsorbent should also be made into porous fine particles so that the adsorbent has a larger surface area. The adsorbents currently used in wastewater treatment include: activated carbon, activated coal, clay, diatomaceous earth, activated alumina, coke, resin adsorbents, slag, wood chips, coal ash, humic acid, etc. Here we focus on activated carbon, which is widely used in wastewater treatment.

Activated carbon is made from carbon-based substances (such as wood, charcoal, coconut shells, coal, waste paper pulp, etc.) as raw materials. After being crushed and formed with a binder, it is heated, dehydrated, carbonized, and activated. Activated carbon is a non-polar adsorbent. Activated carbon has good adsorption performance and stable chemical properties, can withstand strong acids and alkalis, and can withstand flooding and high temperatures.

There are two types of activated carbon used in wastewater treatment: powder and granular. Powdered activated carbon has strong adsorption capacity, is easy to prepare, and has low cost, but it is difficult to regenerate and is not easy to reuse. The adsorption capacity of granular activated carbon is lower than that of powdered activated carbon, and the production cost is higher. However, it can be reused after regeneration, and the working conditions are good during use, and the operation and management are convenient. Therefore, granular activated carbon is mostly used in wastewater treatment.

Some organic substances can be biodegraded by oxidation (such as glucose and ethanol), some organic substances can only be partially biodegraded by oxidation (such as methanol), and some organic substances cannot be biodegraded by oxidation and are also toxic (such as ginkgo biloba). Phenol, ginkgolic acid, certain surfactants). Therefore, we can divide the organic matter in water into two parts, namely biodegradable organic matter and non-biochemically degradable organic matter.

It is generally believed that COD can basically represent all organic matter in water. BOD is biodegradable organic matter in water, so the difference between COD and BOD can represent the non-biodegradable organic matter in wastewater.