## 3 Composition of the Sewer Monitoring System  
The sewer gas monitoring system consisted of sensor detection nodes, gateways, and monitoring centers. We placed sensor detection nodes in the manhole covers of densely populated areas of cities (that is, areas prone to biogas explosion accidents) and set up  
Figure 1: Structure of the dual-channel acoustic tube measurement unit. Figure 2: Circuit composition of the ultrasonic sensor module. gateways in these areas. The detection information would be transmitted through the gateways to a monitoring center. A block diagram of the composition of the monitoring system is shown in Figure 3.  
The sensor detection node was the core part of the monitoring system, as it was responsible for monitoring the methane concentration in the sewer and sending detection information to the gateway. The detection node consisted of a methane detection unit, a carbon dioxide detection unit, a temperature and humidity detection unit, a central processing unit, and a wireless communication module. The methane detection unit was designed using the aforementioned dual-channel ultrasonic PD method. One reason for adopting this method was that other methane sensors generally have high power consumption, which cannot meet the needs of long-term use in a sewer. The ultrasonic measurement method has low power consumption, so it can ensure that the system can work continuously for several months. For the carbon dioxide detection unit, an infrared-absorption carbon dioxide sensor was selected for the system. There were two reasons for using this type of carbon dioxide sensor: one was that carbon dioxide has a high impact on the ultrasonic measurement of methane concentration, which needs to be measured and corrected, and the other is that a high carbon dioxide concentration can cause on-site workers to suffocate. Changes in temperature and humidity have certain impacts on the measurement results of ultrasonic methane sensors. In order to minimize these impacts, measuring the ambient temperature and humidity is necessary, and numerical corrections must then be made based on a theoretical analysis. The temperature and humidity detection unit used a single-bus integrated temperature and humidity sensor, which had the characteristics of low power consumption, ease of use, and good stability. The central processing unit of the detection node was responsible for collecting the detection data from the methane detection unit, carbon dioxide detection unit, and temperature and humidity detection unit, performing data processing on these data, and then determining whether the gas concentration was within a safe limit. The detection information was packaged and transmitted to the wireless communication module at given time intervals. The packaged information included the wellhead number, methane gas concentration value, carbon dioxide gas concentration value, safety status, etc. A Zigbee module with a low power consumption was used as a wireless data communication module. A communication network constructed with Zigbee modules meets the  
Figure 3: Block diagram of the composition of the sewer monitoring system. requirements of low-cost and low-power sensor networks, thus helping to extend battery life and improve the reliability of monitoring systems. Since the specific gravity of methane gas is relatively low, the methane concentration in the upper layer of a sewer is the highest. The sensor detection node was placed 0.5 m from the ground in the sewer. Each wellhead was numbered, reflecting location information for the corresponding sewer. The sensor detection node collected gas concentration information in real time and sequentially transmitted the wellhead number and concentration information to the nearby gateway via a wireless network. The gateway then transmitted the detection information to the monitoring center via serial communication.