Some researchers have suggested that photovoltaic power generation that provides electricity for sewage biological treatment can reduce the energy consumption of sewage treatment plants [ 42 ].
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Sewage treatment plants have a high electric load with large energy consumption, and they are operated continuously for 24 h with a stable load [ 43 ]. The combination of photovoltaic power and wastewater treatment with the implementation of contract energy management can further reduce the cost of wastewater treatment.
At present, it also has some application cases for treating sewage using solar power as a power source. Hudnell et al. Han et al. The basic principle of solar photocatalysis is that by exciting the electrons in the semiconductor, the electrons are excited from the valence band to the conduction band to generate photogenerated electrons, thereby generating photogenerated holes in the valence band, and the electrons and holes are respectively diffused to the semiconductor surface [ 48 ].
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The surface reacts with different organic matters to degrade the contaminates. Some organic or inorganic pollutants in the sewage are detoxified by an oxidation-reduction reaction. Photocatalysis has its unique advantages of rapid reaction, complete oxidation, no secondary pollution, mild reaction conditions, and no need for excessive equipment. It is often used for advanced treatment or wastewater reuse treatment in sewage treatment plants [ 49 ]. Although the distribution of wind energy has certain limitations, it is a widely distributed energy source compared to other renewable energy such as mineral energy, water energy, and geothermal energy.
All regions can make rational use of renewable energy sources, like wind and solar conditions. According to local conditions, different wind turbines are used depending on different loads, so that wind energy can be converted into mechanical energy to the maximum extent [ 50 ]. The power grid is used as an auxiliary energy source, and it is suitable for the fields of wind energy and sewage treatment plants at various scales.
The construction of wind turbines will combine grid power and wind power, which is suitable for areas with a rich wind source [ 51 ]. Wind energy and solar energy can be applied without emitting pollutants or exhausting gas into the atmosphere [ 52 ]. They are environmentally friendly and pollution-free energy sources. Both wind and sunshine intensity constantly vary with weather and climate, which makes wind and solar energy difficult to be used [ 53 ]. However, with the development of modern science and technology, the utilization of wind energy and solar energy has made breakthroughs in technology [ 54 ].
In particular, the comprehensive utilization of wind energy and solar energy Figure 4 take full advantage of their complementarity, establishing a more stable, reliable, economical, rational energy system in many aspects [ 55 ]. The sewage source heat pump uses sewage as an energy source to exchange heat between the sewage and the heat pump, and the internal heat pump was derived by electric power to achieve cooling or heating. Urban sewage has the characteristics of large sewage volume, stable water quality, and relatively stable internal temperature [ 56 ].
Therefore, the working performance is relatively stable when using the sewage source heat pump system. The energy efficiency ratio of the sewage source heat pump is mainly affected by the water volume and water temperature on the inlet side and the user side. Compared with common air-source heat pumps, sewage-source heat pumps have a higher coefficient of performance [ 58 ]. In addition, fossil energy is not required in sewage-source heat pump systems, and does not generate secondary pollution. It is a technically feasible, economically affordable, and environmentally friendly method for comprehensive utilization of urban sewage.
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Baek et al. The secondary effluent of sewage treatment plants used in the sewage-source heat pump conditioning system has many significances [ 61 ]. The recovery and reuse of the effluent from a sewage plant can alleviate the current situation of water shortages in China to some extent [ 62 ]. Development and utilization of low-level energy sources in sewage through heat-pump technology can replace part of coal-fired and oil-fired boilers, which can properly alleviate environmental problems [ 63 ].
The influent of the sewage treatment plant is at the end-pipe network system, and it has relatively low elevation. Therefore, it is necessary to use a lifting pump to lift the sewage into the treatment system [ 64 ].
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This process could be energy intensive, the main reasons for the high energy consumption of the pumps in sewage treatment plants is the incorrect pump design or selection [ 65 ]. Energy consumption can be improved by designing proper pumps in a right position. The energy savings of the sewage lifting angle needs to be comprehensively analyzed from the sewage lifting system. First of all, in the design stage of the sewage treatment process, it is necessary to broadly investigate the existing pipe network system and the whole process facilities of sewage treatment to minimize the wastewater elevation that needs to be upgraded with the elevation of the processing facility and consideration of the flooding flow mode [ 66 ].
Secondly, it is necessary to select the appropriate pump according to the combination of sewage lifting amount and changing characteristics. According to the variation curve of the pipeline system, especially the sewage flow rate Figure 5 , appropriate pumps are needed to meet the high operating efficiency range and high water level conditions [ 67 ].
According to the sewage treatment volume, head, head loss, and pump power, the appropriate and efficient pump combination are selected. This includes setting the ratio and regulation between the variable frequency pump with the frequency converter and the fixed power pump without the frequency converter. The aim of setting is to reduce the pump shaft power and avoid frequent opening of the pump which will reduce the service life of the pumps [ 68 ]. Furthermore, they are more focused on the matching performance between the pump and the motor to enhance the efficiency of motor operation.
In addition, the pipeline should be well designed to ensure the compact and smooth system for reducing the length of bends and pipelines, and the resistance and energy consumption of pipeline transport systems [ 69 ]. Finally, it is necessary to pay attention to process operation management, equipment maintenance, dripping reduction, scaling, and mechanical wear of the operating system.
In addition, ensure that equipment and systems operate under high-efficiency conditions [ 70 ]. In the design and operation of different wastewater treatment plants, the improvement of the lift pump is achieved mainly by the application of variable frequency control technology.
spanguetortu.tk The removal of pollutants in sewage is mainly achieved through microbial biochemical metabolic processes. Wastewater treatment biochemical processes mainly include the Anaerobic-Anoxic-Oxic process, oxidation ditch process, and the SBR process [ 71 ].
The process for biochemical metabolism of microorganisms to remove pollutants requires the presence of electron acceptors which is mainly provided by aeration and oxygen supply [ 72 ]. Therefore, effective aeration is an important method for pollutant removal and effective sewage treatment. In addition, in the process of pollutant removal, such as denitrification in the A2O process, the mixture reflux is required to provide nitrate nitrogen as an electron acceptor [ 73 ]. Chemical agents are required to enhance chemical precipitation in the process of chemical phosphorus removal, which also cause certain energy consumption.
Aeration control is the key point for energy savings and consumption reduction in the biological sewage treatment process [ 74 ]. The differences in the aeration profiles between traditional control and optimized conditions. Aeration control includes optimization of the aeration device, aeration pipe arrangement, and optimization of aeration supply mode [ 75 ].
For aeration methods, the A2O and SBR processes generally use microporous aeration, while oxidation ditch generally uses rotary brush aeration or inverted umbrella aeration [ 76 ]. Microporous aeration mainly enhances oxygen transmission efficiency by generating microbubbles with a diameter of 1.
However, the microporous aeration process requires high energy due to the large aerodynamic resistance of the air flow through the micropores. Therefore, it is necessary to introduce a stirrer at the bottom of the chamber for microporous aeration; this design has also been applied to many oxidation ditch processes [ 77 ]. In the past, it was considered that unilateral aeration could reduce the air volume, but it has been proven that uniform small vortex and local mixing can be formed by comprehensive aeration, which strengthen the transmission of small bubbles and enhances oxygen transmission efficiency.
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In addition to aeration devices and aeration methods, the supply of aeration is a key research object for energy conservation and consumption reduction. If the amount of aeration is too small, the quality of the effluent from the sewage treatment would be affected; if the aeration is too large, it will cause waste of energy, change in sludge floc structure, and effect sedimentation of the activated sludge [ 78 ].
The core of aeration energy conservation is to provide the required electron acceptor which dissolves oxygen on demand in order to ensure the effective removal of pollutants in the biochemical treatment process, the quality of the effluent, the supply—demand balance, and the energy efficiency in the aeration process.
When the activated sludge biochemical treatment process has a high load of COD, the main objective of aeration is to remove COD and carry out nitrification. So, the calculation of oxygen supply mainly considers these two biochemical processes. Through the precise aeration control biochemical section, the dissolved oxygen signal is used in order to access the control cabinet and turn to the wind pressure value by programming.
Then the air pressure is used to control the aeration amount to achieve energy savings [ 80 ]. In addition, the unit energy consumption of the oxidation ditch process can also be reduced by using the brushing timing control. One of the key factors for blower control is to avoid aeration blower surge problems [ 81 ].
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Controlling the blower outlet pressure is a necessary condition to solve the surge phenomenon and achieve DO automatic control, and the low DO and effluent ammonia nitrogen concentration control can achieve efficient automatic control and energy savings of the system. Meanwhile, precise aeration can also be achieved through Oxidation-Reduction Potential and pH-controlled wastewater treatment processes [ 82 ].
With the in-depth study of the functional bacteria in the wastewater treatment process, new wastewater treatment processes that can realize energy savings from the technical point of view are gradually proposed. Those technologies mainly include wastewater denitrification processes based on short-cut nitrification, denitrifying, phosphorus removal processes, and anaerobic ammonium oxidation processes.
At the same time, denitrifying nitrite instead of nitrate can also reduce the demand for a carbon source for denitrification and strengthen wastewater nitrogen removal efficiency. For the anaerobic ammonium oxidation process, it is mainly applied to high ammonia nitrogen wastewater.
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