| The security monitoring and early warning system for the municipal sewer and septic tank combustible gas has been well developed and used in Chongqing city, for preventing the explosion and poisoning accidents caused by the combustible gas accumulation in the urban municipal drainage pipeline, and improving the reliability and real-time performance of the urban environmental emergency monitoring.This paper, under the sponsorship of the project of Chongqing Municipal Administration Committee --- Application Research on the Security Monitoring and Early Warning System for the Municipal Sewer and Septic Tank Harmful Gas (CJ2006-17), attempts to design a sort of remote on-line monitoring system relevantly for combustible gas based on IR gas sensor technology and general packet radio service (GPRS). This system works as follows: in the monitoring region, after the sewer combustible gases concentration information was collected by the IR gas sensor sensitively and stably, it will be caught by the GPRS wireless terminal and through which, to be sent to the remote monitoring center, where under the aid of database and software, the real-time monitoring and emergency early warning to the combustible gases concentration will be realized. Having been practiced in the sewers and septic tanks of the main districts of Chongqing, this system contributes well to seek for the abnormal information and give an early alarm in time and in effect ensuring of a precise monitoring result, a stable running and a reliable data transmitting. Thus, a safe circulation of the municipal drainage pipeline can be guaranteed. The main research includes:①In terms of the safety investgation to Chongqing municipal sewers and septic tanks, the construction aim for this system is discussed and firstly the wireless sensor network technology is adopted in the sewer gas monitoring. According to the designing principle for the remote monitoring system, combined with its construction aim and actualization requirement, the whole framework of this system is described through four aspects: gas detection, equipment selection, monitoring areas range and transmission mode. Adopting the modularization design way, this system is divided into three subsystems: gas concentration detection system, data transmission system and monitoring center, all of whose composition and implementation scheme are presented in detail. ②This paper discusses the working principle, basic properties of the gas sensor and the detection realization mode of combustible gas. Moreover, according to the design requirements and application environment, the fixed IR combustible gas monitor is selected. Based on the warning mechanism the lower explosion limits (LEL) is selected as the detection index for combustible gas concentration and a three-level alarm response is confirmed. With the practical operation situation, the gas alarm limit under different warning grade is separately set between sewer and septic tank. By field test the installation position and principle of gas sensor and field equipment are confirmed. After 6-month-use, three technical indexes of gas sensor are verificated including detection error, response time, and alarm error according to national standard. The result shows that each index is satisfied with the national standard, hence, the working performance of gas sensor is stable and alarm result is correct.③The design of data transmission system is from three aspects: configuration of GPRS data transmission terminal, design of wireless communication network and establishment of transmission and communication protocol, consequently the remote wireless transmission of gas concentration data comes true. The GPRS-DTU equipment is selected as the wireless data transmission terminal. Using CMNET mode accesses the wireless network and dynamic IP address access mode achieve the connection way of center-to-multipoint, and then accomplish the data transmission between monitoring center and different monitoring terminals. By theoretical analysis and experimental test the UDP protocol is selected for the system, which optimized the economy and efficiency of the data transmission. The accuracy of communication data is impoved by means of data verification and communication procotol set, and make the data bidirectional transmission realized.④This paper discusses the function design and software implement of monitoring center. Using modualarization idea and multithread programming technique, it's convenient for maintenance management of the system. The system software design of monitoring center is achieved by creating database and monitoring software utilization. According to the principle of designing the database and data service software separately, the scalability of system is optimized. The configuration software is used to create database and data service software is to transfer the database, and then accomplish the alarm, query function by the data from database. Moreover, the monitoring software can achieve these operations including concentration display, historical data statistics, database query and parameter setting. ⑤To improve the data transmission success rate of this system, an overall demonstration for the mechanism of"packet loss retransmission"is proposed theoretically and experimentally, moreover, to a certain extent, by which the transmission stability and reliability of this system are strengthened. After over one-year test-run test and through the system performance analysis, the stability and reliability of this system are verified. The combustible gas concentration in different monitoring areas can be well reflected, varying from seasons, monitoring time and drainage conditions. Through the correlation analysis among the combustible gas concentration, the amount of wastewater discharge and pollutant concentration, the variation law between the combustible gas concentration and sewage discharging by people's production activity is studied. The application and implementation of the system is applied in Chongqing city indicate that adopting the wireless sensor network technology is feasible and valuable to the remote monitoring and early warning of the urban drainage pipeline facilities. The variation of the combustible gas concentration can be effectively characterized by the system, ensuring of the early warning and forcast and finally realizing its real monitoring.⑥Based on the system construction, practical runningt test and implementation aim, the security monitoring and early warning system guide (appendix) for the municipal sewer and septic tank combustible gas is proposed, giving a good criterion direction to promote the system operation and successfully accomplishing an optimized design, development, management and maintenance to this system.
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