## 1 Introduction  
Methane is one of the main components of biogas in sewers, and is quite explosive when its concentration exceeds the lower explosive limit (about 5% by volume) [1]. Due to rapid development of the economy, urban populations continue to grow, and garbage and sewage are increasing by large quantities; this has led to an increase in the concentration of biogas in urban sewers, and sewer explosion accidents are commonly seen. In recent years, few gas explosions in sewers and septic tanks have been reported in developed countries, such as those in Europe and the United States [2]. Since the population densities of these countries are quite low and the urban facilities are complete, they produce less biogas and the operating pressure of the sewers is low. Therefore, portable gas sensors are commonly used for on-site sewer gas detection [3]. However, in China, as the urbanization process continues to accelerate and urban population density is increasing year by year, there is tremendous pressure on urban facilities. From 2004 to 2009, there were at least 26 sewage biogas explosions in the main area of the city of Chongqing, China [4]. If an on-site detection method were used to monitor the content of biogas, it would incur high costs in human and material resources; in addition, the content of sewage gas continuously changes over time, and such a method would not be able grasp the changes in the content of sewage biogas in real time. Once the concentration of hazardous gas (methane) reaches the lower explosive limit (LEL), it is not possible to take emergency measures to reduce the gas concentration to avoid accidents. Thus, developing a real-time sewer gas monitoring system is necessary for countries such as China. For sewer gas monitoring systems, manual sampling is usually followed by laboratory gas chromatography (GC) measurements. However, this method cannot capture the concentration of methane in the sewer in real time and requires a lot of manpower and resources. In response to this issue, [PERSON] proposed a device to monitor the dissolved methane concentration at the end of a rising main sewer network by pumping out sewage samples [5]; this device was able to capture dynamic changes in methane concentration in real time. Since the long-term monitoring of sewer gases requires a gas detection device with low power consumption and long-term stability, this system is not suitable for the networked monitoring of urban sewer gases. [PERSON] proposed the SewerSnort monitoring device, which used a float sensor to collect gas measurement values and the location information of sampling points [6]; gas measurement results could be obtained from the float sensor after its stroke was completed. The innovation of this technology lies in the use of SewerSnort for fully automated sewer gas monitoring, which is more efficient and cost-effective than previous methods. However, it is necessary to regularly allocate float sensors for sampling to understand how the level of gas in the sewer varies over time. To achieve low power consumption and high reliability in sewer gas monitoring, it is necessary to select suitable gas sensors. Commonly used methane sensors include catalytic sensors, semiconductor sensors, and photoacoustic sensors [7; 8; 9; 10]. Catalytic sensors are generally high in power, and their electrodes are affected by the toxic effects of hydrogen sulfide, so they are not suitable for sewer gas detection [11]. Semiconductor sensors use metal oxide semiconductor materials to cause changes in resistance or conductance after contact with the gas to be measured [12]. Since there may be other kinds of gases in a sewer that can be oxidized, such gases will react with the sensor, which will cause a large error in the measurement results. Photoacoustic sensors are usually expensive and are suitable only for experimental projects [13].