# Research Status and Development Trend of Wastewater Treatment Technology and Its Low Carbonization
## 2 Conventional Wastewater Treatment Technology
### Summary Discussion  
Activated sludge is the main pollutant-removal process. In the cell composition of activated sludge microorganisms, carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) account for about 90-97%, and the rest (3-10%) are inorganic nutrient elements, mainly phosphorus (P). The treatment of domestic sewage generally does not require additional nutrients, and some industrial wastewater treatment do require additional nutrients. In conventional aerobic treatment, N and P nutrients are added in accordance with BOD:N:\(p\) = 100:5:1. The removal rate of nutrients by the activated sludge method is more than 95%. A 5000 m\({}^{3}\)/d municipal sewage treatment plant will produce a residual sludge volume of 7-19 g/(L-d) when treating municipal wastewater by the activated sludge processes described in 2.1, 2.2, 2.5 and 2.6. The 2.3 adsorption-regeneration activated sludge method can make full use of the regenerating tank to activate and reuse the residual sludge and reduce the sludge discharge. The amount of residual sludge produced is about 1-8 g / (L-d) [41; 42; 43; 44]. The 2.4 organic load of the delayed aerated activated sludge method is low, the sludge is continuously in a state of endogenous metabolism, and the residual sludge is basically absent. The sludge produced by the activated sludge process needs to be transported to the dewatering plant and then treated. After the initial dewatering treatment of the sludge enrichment tank, it is further dehydrated through a plate-and-frame filter press and other measures. In this process, the main energy consumption is the electric energy of the plate-and-frame filter press, as well as the auxiliary energy consumption of the sludge transport conveyor belt. In the process of wastewater treatment, the main energy consumption in the biochemical reaction tank is air blast aeration and electricity consumption of some auxiliary equipment. An urban sewage treatment plant with a 15,000 m\({}^{3}\)/d treatment capacity requires more than 3 blowers and at least 1 plate-and-frame frame filter press. Therefore, based on the calculation of 3 blowers and 1 plate-and-frame filter press, the daily energy consumption produced by full load operation would be 2582.68 Kw\(\cdot\)h [45; 46]. However, the wastewater produced in this process can be reused for greening, flushing, and so on. The dehydrated sludge can be used as plant fertilizer, building materials, and so on. Figure 5: Process flow chart of sequential-batch activated sludge method. In the process of sewage treatment, which consumes much external energy, the form of pollution is actually changed from water pollution to air pollution and sludge pollution, which is obviously not in line with the concept of sustainable development. The COD in sewage can also be used for anaerobic digestion to produce CH4 and fermentation to produce hydrogen. The CH4 and H2 produced can be reused as part of the plant thermal raw materials. The generated surplus sludge can also be used, and the best utilization is incineration capacity. The concept of resource reuse is also implemented to achieve sustainable development, realize low-carbon treatment of sewage and reduce carbon footprint [47]. In addition, in terms of resource utilization, traditional sewage treatment mainly recycles treated water for greening irrigation, washing or industrial cooling, but ignores the problem that sewage itself contains rich carbon resources [48]. Sewage is actually a carrier of resources and energy. It is estimated that the potential chemical energy of urban sewage with a COD of 400-500 mg/L is 1.5-1.9 kW\(\cdot\)h/m\({}^{3}\)[49], and a metabolic heat of 0.14 \(\times\) 10\({}^{8}\) J can be generated per kg of COD [50]. The heat generated by an increase or decrease of 5 \({}^{\circ}\)C of sewage is almost equal to the annual power generation of 332 large power plants, and about 4 times the metabolic heat of organic matter [51].