Waste Containment System Monitoring Using Wireless Sensor Networks Weisong Shi§ and Carol J. Miller¶ §
Department of Computer Science Department of Civil and Environmental Engineering Wayne State University Email:
[email protected] and
[email protected] ¶
1. Introduction As new fabrication and integration technologies reduce the cost and size of wireless sensors, the observation and control of our physical world will expand dramatically using the temporally and spatially dense monitoring afforded by wireless sensor networks technology [Eea99, EMB03]. The commercialization of micro-sensors with wireless communication, such as Motes from Crossbow [Cro] and Intel, enables interesting applications where current wired sensor technology would fail, such as unattended environment monitoring, habitat monitoring, military surveillance, and others. In this position paper, we present our recent effort aiming to develop a waste containment system monitoring prototype using commodity-of-the-shelf wireless sensors at Wayne State University. This application represents a multidisciplinary collaborative effort to apply the wireless sensor network technology in a civil engineering application. The project team plans to investigate the impact of fractures, especially desiccation cracks, on the integrity of engineered geo-structures using wireless sensor networks. The proposed monitoring system will enable a comparison between the traditional compacted clay liner and a clay liner amended with fiber for structural reinforcement. Project results will provide significant insight into (1) feasibility of large-scale embedded wireless sensor network for containment system monitoring, (2) advantages of fiber-amended liner material [MR04], and (3) the nature and importance of crack progression in desiccating structures [Mea02]. The hydraulic integrity of containment systems can be assessed using a variety of commercially available monitoring systems. However, all of these use coaxial cables to transfer the response measurements from sensors to centralized data servers. The cables are problematic, as they may be a fault path for the formation of cracks, they are difficult to install and may interfere with other liner features, and the per-channel cost is high. This has limited the use of embedded monitoring systems, instead encouraging the continued use of external monitoring systems (i.e., leachate detection systems and monitoring wells) that have a much delayed response time in comparison with embedded systems. Wireless sensor network research has been continually evolving over the past five years. However, most of the published results are based on computer simulation, which requires extensive assumptions regarding operating conditions and others. The next step of wireless sensor network research should be application-driven. We envision that the collaboration of multidisciplinary team on wireless sensor network will benefit both sides. On the environmental engineering side, researchers will have opportunity to see the applications of this technology to research and practical problems that have been limited to the more traditional monitoring technology, on the sensor network side, computer scientists will benefit from the ability to test wireless sensor technology and optimized sensor communication protocols in real-world applications, that directly impact environmental quality. Therefore, this project is our effort to pursue the combination of the current “technology push” and “demand pull”. The objectives of this project include: (1) application of a wireless sensor network to evaluate the variation in hydraulic properties of compacted clay barriers during desiccation cycles typical of moderate and arid climates; (2) use of wireless sensor network in development of a prototype system to detect the occurrence and spread of desiccation cracking; and (3) developing a set of enabling techniques to apply wireless sensor network in an irregular field deployment, and (4) building a general infrastructure for application of technology to other waste containment systems. We are investigating the following four specific subprojects in this application: Accurate and timely fault zone detection, which is based on an evaluation of the extent of cracking.
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Impact of cracks on the hydraulic performance of barriers, which will be evaluated using the data measured by wireless sensors. We plan to evaluate this performance for both unamended clay liners, as well as fiberamended clay liners. Energy-efficient data dissemination and collection protocol, which takes both the application-specific information and general sensor lifetime optimization into consideration. A complete prototype for sensor-based waste containment system monitoring, including self-organized deployment and resilient data collection and routing. We expect that this prototype will be a general tool for the community for field deployment.
2. Problem Statement Figure 1 shows an abstracted view of a landfill cell where the sensors to be deployed. We are interested in both ambient surface environment and containment liner environment. The variables in ambient surface environment include precipitation, temperature, humidity, wind speed, and Sun index. The sensors in the cover liner will be placed using a grid of one sensor per 10m by 10m grid cell (1 per 100 m2) over a region of 100m by 60m area (6000 m2 area). This results in 60 sensors per grid layer, with 2 layers (elevations) of sensor placement, for a total of 120 sensors embedded in the liner. The vertical spacing of the sensors is 0.3m (center to center). The remainder of the 150 sensors budgeted for the field portion of the project will be used as back-ups and for the surface ambient environmental monitoring. Half of the test cell area will utilize a natural compacted clay barrier, while the other half will utilize a fiber-reinforced containment barrier. The sensors embedded in the containment liner environment must be capable of sensing multiple variables: soil moisture, temperature, pore water suction, and tensile stress. The recent progress in MEMS and manufacture engineering will make this a reality. For example, a multifunctional sensor will be made available by Crossbow in mid-2004 [Cro]. The lifetime of sensors is required to be 10 years minimum. In this application, we need to collect information regarding the environmental variables controlling crack formation in a clay liner that follows traditional construction techniques and a clay liner that has been modified with fiber to increase the tensile strength in an attempt to reduce the extent of cracking. waste area
wireless sensors
(b) plan view
(a) profile view
Figure 1: Proposed deployment for waste containment monitoring: (a) profile view, and (b) plan view.
3. Network-Related Research Challenges of This Application A great deal of efforts have been put on sensor network research, including energy-efficient routing protocols, query protocols, and aggregation schemes, however, there are still several open problems, especially related to the waste containment system monitoring: •
Many previous efforts in wireless sensor networks assume that each sensor has its position information available, either based on GPS or field surveyed determination. This assumption is generally true in coarsegrain fields with sensor location dictated by design or field constraints. However, positional information will
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not be available in the environmental containment application, with sensors embedded below ground surface. We have proposed a high accuracy landmark-free energy efficient localization algorithm called HALF to calculate logical positions of sensors while maintaining the neighborhood information [ZS04]. •
To our knowledge, topology discovery of sensors, i.e., determining the location of sensors without geographic information, and topology discovery in an irregular space, such as the clay cover of land cell shown in Figure 1, have not been addressed in any previous work, which focuses on regular space only. Based on the localization protocol, we are developing an efficient energy-efficient topology discovery algorithm for irregular space, which aims to make the sensor network deployment totally self-organized.
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Power consumption is an important problem regarding long-term field deployment of a wireless network system, and is especially important for this application with sensors embedded several feet below ground surface in a fragile environment. It is impossible to replace failed sensors after the sensors have been embedded inside the clay soil. Therefore, extending the lifespan of the whole network becomes extremely important in this application. We want to answer the following question: does the minimum energy path always result in the maximum lifetime of the whole sensor network system? The major objective of wireless sensor network research is to maximize the lifetime of the sensor network as a whole; however, many previous efforts focus on minimizing the energy consumption of individual pathways. Our initial results show that the answer to this question is no. Currently, we are working on an efficient load balanced query protocol to address this problem [SSS04].
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Current research in wireless sensor network treats all data in the same way, including both functional and temporal data. Therefore, the fidelity and consistency requirement of data, which is true for different kind of data, are totally neglected in previous research. By taking advantage of this observation, we are investigating an adaptive, parameter-specific data dissemination protocol.
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The data collected from sensors in this environmental application is highly dynamic, i.e., the data changes continuously, and periodic, i.e., the time between the updates and the value of the updates are not known apriori. For example, the collection rate of some parameters should be higher during raining day than that of sunny weather. Unfortunately, many previous efforts make the assumption that each sensor senses data at a fixed rate, and needs to report to base station periodically. Therefore, an adaptive protocol is necessary.
References [Cro] Crossbow Technology Inc., http://www.xbow.com [Eea99] D. Estrin and et al. Next century challenges: Scalable coordination in sensor networks. In Proceedings of the 5th Annual ACM/IEEE International Conference on Mobile Computing and Networking (MobiCom’99), August 1999. [EMB03] D. Estrin, W. Michener, and G. Bonito. Environmental cyberinfrastructure needs for distributed sensor networks, August 2003. A Report From a National Science Foundation Sponsored Workshop. [Mea02] C. J. Miller and et al. Impact of soil type and compaction conditions on soil water characteristic. Journal of Geotechnical and Geoenvironmental Engineering, 128(9):733–742, 2002. [MR04] C. J. Miller and S. Rifai. Fiber reinforcement for waste containment soil liners. to appear in ASCE Journal of Environmental Engineering Special Edition on Waste Containment Barrier Materials. [SSS04] K. Sha, S. Sellumuthu, and W. Shi, IQ: An Efficient Load Balancing Query Protocol in Wireless Sensor Networks, Technical Report MIST-TR-2004-006, Department of Computer Science, Wayne State University, February, 2004. [ZS04] Z. Zhu and W. Shi, HALF: A High Accurate Landmark-Free Localization Algorithm in Wireless Sensor Networks, Technical Report MIST-TR-2004-008, Department of Computer Science, Wayne State University, March 2004.
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