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Monitoring of waste degradation processes for sustainable MSW landfills - Conclusion



To address the shortcomings of “dry-tomb” landfills, the EPA and industry have experimented with bioreactor landfills. Advantages of bioreactor landfills include accelerated waste degradation and stabilization in a matter of years rather than decades to centuries in “dry-tombs”, increased generation of biogas, lower waste toxicity, reduced leachate disposal costs, an estimated 15 to 30 percent gain in landfill space due to increased waste density, and reduced post-closure care (EPA 2015). Despite these advantages, fewer than 2% of U.S. landfills are operated as bioreactors due to technological and scientific uncertainties (EPA 2015). Limited field data exist for landfills, though there is clear evidence that landfill gas collection systems are failing to efficiently capture gas resulting in methane release to the environment through cover soils that in turn lead to empiricism and inefficiencies in energy generation and collection (Pohland 1986). To optimize landfills and create the next-generation of sustainable landfills, processes must be actively and accurately monitorized to maximize energy collection, minimize environmental emissions, and use land as efficiently as possible to avoid encroachment on cities and communities.

The key components for monitoring landfills include leachate monitoring, gas monitoring, and in-situ monitoring. Leachate in conventional landfill slowly moves through the waste mass in an uncontrolled manner, where as in bioreactor landfills, it is recirculated as uniformly as possible to accelerate waste degradation. Key properties for leachate monitoring are chemical oxygen demand, biological oxygen demand, dissolved oxygen content, pH, temperature, redox potential, ammonia, heavy metals, and total dissolved solids.

Landfill biogas is generated during waste degradation, with the generation rate highly dependent on landfill operation and local climate. Gas collection and control systems (GCCS) are design to collect methane from biogas, in absence of GCCS, the methane is either flared or vented. Additionally, landfills are monitored for surface methane emissions to ensure that intermediate and final cover systems are adequate, and that the GCCS is operating as designed.

In-situ monitoring is necessary to understand the waste degradation processes, which typically occur over several years at significant depth. The three key parameters that distinguish conventional from bioreactor landfills for in-situ monitoring are moisture, temperature, and pressure (including both pore pressure and overburden pressure). Electrical resistance sensors is most commonly used for moisture content measures as it is low cost, reliable, and easily installed; however, its applications are limited to providing relative moisture contents in waste mass as opposed to absolute moisture contents. Temperature is measured by either thermistors or thermocouples, and must withstand extremely high temperatures up to 200°F, as research has demonstrated internal landfill temperatures of 170°F. Pressure is measured by pressure transducers buried in the landfill, with pore pressure representing both liquid pressure and gas pressure in surrounding pore space of waste and overburden pressure from the weight of waste lifts above the sensor.

Through monitoring of leachate, gas, and in-situ conditions, the waste degradation process can be better understood, and this knowledge applied to advancing the design and operation of MSW landfills. Field data collected by the sensors and instrumentation discussed can provide valuable information for incorporating the three physical interdependent processes that define waste degradation including a fluid model for leachate and gas, a mechanistic model of waste properties, and a biochemical model of the exothermic reactions during aerobic and anaerobic degradation. Through understanding and then control of degradation processes, MSW can be transformed from a hazard to be contained to sustainable energy source.



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