The International Information Center for Geotechnical Engineers

Monitoring of waste degradation processes for sustainable MSW landfills - In-situ monitoring

In-situ monitoring

To understand the complex physical and biochemical processes of waste degradation, moisture, temperature, and pressure must be measured in-situ at various depths. These parameters provide an indication of the activity phase of the waste degradation process. Measuring these parameters during waste degradation provides the necessary data to create models, and to work towards optimizing landfill operation. Since the 1970s, bioreactor landfills with leachate recirculation have demonstrated accelerated waste degradation accompanied with more rapid generation of methane.  However, the processes for this accelerated waste degradation are poorly understood. In-situ monitoring enables operators and researchers to understand and optimize these processes to create more sustainable landfills. 

Moisture content

Due to low moisture levels, MSW in sanitary landfills degrades at a very slow rate. Conversely, bioreactor landfills increase moisture levels to accelerate degradation. However, monitoring moisture at landfills presents a significant challenge due to preferential channeling of liquids in the waste. Studies have shown that liquids preferentially flow through a relatively small part of the waste, resulting in non-uniform waste degradation (Oonk et al. 2013). To be successful, bioreactor landfills must be designed and operated to have more uniform moisture levels through adequate recirculation of liquids. Moisture content sensors, at the current practice, are usually more indicative of relative moisture levels rather than of absolute moisture contents.

 fig7

Figure 8: Preferential channeling of liquids in a MSW Landfill (Oonk et al. 2013)

Several methods have been proposed to measure moisture levels at landfills including borehole devices, buried instruments, and most recently surface techniques. Borehole devices include neutron probes which are lowered into boreholes and estimate the moisture content at varying depths of the surrounding waste. Neutron probes emit neutrons into the surrounding waste, and water results in the neutrons slowing down and a cloud building up around the probe. This neutron cloud is measured and calibrated, providing an estimate of moisture content (Townsend et al. 2015).

Ground instruments include time domain reflectrometry sensors (TDR), time domain transmissivity (TDT), and electrical resistance technology. TDR is conceptually similar to radar – the device emits an electromagnetic signal and then analyzes the reflected signal to measure physical characteristics of interest in the medium The reflected signal measured by TDR depends on the moisture content of the medium (typically soil), with moisture content based on the relative permittivity or dielectric constant of the material. TDT is similar, but the signal is analyzed but the emitted signal rather than reflected signal is measured. Both TDR and TDT  are relatively expensive at approximately $500 per unit, electrical resistance devices on the other hand are approximately $25 per unit (Reinhart et al. 2002). Low cost moisture content sensors is essential because of the preferential flow previously mentioned, resulting in the extremely heterogeneous moisture levels within landfills.

Electrical resistance sensors are the most commonly used devices for moisture content measurement due to their low cost, their ease of installation, and their reliability. Electrical resistance sensors consist of a porous media with a pair of embedded electrodes. As moisture content in the porous media increase, the electrical resistance across the media decreases (Townsend et al. 2015). Typically, the porous media is a gypsum block or a granular matrix, and is insulated to prevent environmental conditions, such as salinity of the soil, from distorting measurements of the sensor (Reinhart et al. 2002).

Additionally, studies have attempted surface techniques for measurement of moisture contents at various depths through electrical resistivity tomography (ERT) (Reddy et al. 2015). ERT measures resistivity with depth, and shows general trends for moisture content; however, several limitations exist for this method. Resistivity measurements have limited resolution and the resolution decreases with depth, providing an average resistance for large sections of waste.  Additionally, metal objects within the waste itself can distort the resistivity measurements (Oonk et al. 2013). 

 

Temperature

Microbial activity is responsible for the biodegradation of waste and is an exothermic reaction. Consequently, while waste is undergoing degradation, landfills have significantly high internal temperatures compared to the outside air. Several studies have demonstrated temperatures as greater than 170° F (Townsend et al 2015). Monitoring the temperature in MSW at various depths provide an indication of the activity phase of the biodegradation process. Moreover, in-situ temperature measurement indicates when waste stabilization has been achieved (Lopes and Gomes 2013).

MSW landfills typically use thermistors and thermocouples to measure temperature in-situ at landfills. Both thermistors and thermocouples need to be designed to a range of expected temperatures, typically from -76 to 212°F (Townshend et al 2015). Thermistors and thermocouples can either be placed in the landfill as it is filled with MSW, or boreholes can be drilled after the MSW has been placed. Thermistors measure temperature by monitoring the variable resistance to temperature in a resistor. On the other hand, thermocouples consist of two wire strands that are made of different metals and then welded together at one end, at the junction of the wires, temperature is measured.

Additionally, techniques for measuring surface temperature can provide relative information on the level of waste degradation activity (Faisal 2011).  Faisal found that land surface temperature (LST) for landfill sites was higher than surrounding areas, and weak correlation with LST and methane emission readings. Thermal imaging techniques are based on emissivity, which is defined as the fraction of energy being emitted relative to that of a black body. A black body is defined as the perfect emitter of heat energy with an emissivity value of 1. For example, pure water has an emissivity of 0.96, soil saturated with soil is approximately 0.95, and dry soil is 0.92. Thermal infrared cameras detect the infrared energy and converts this energy into an electronic signal. This signal is then processed to produce a thermal image, and the thermal image post-processed for temperature calculations.

Pressure

 Real-time monitoring of pressure is essential for safe operation of landfills, especially those with leachate recirculation systems (LRS). Leachate injection, especially in landfill side slopes, results in excess pore fluid pressures and consequently lower shear strength. The lower shear strength compromises the slope’s stability and can result in failures at landfills (Reddy et al. 2015).  Internal pressures within a landfill consist of pore pressures and pressure from the overburden weight of waste through total earth pressure cells (TEPC). Total earth pressure cells (TEPC) consist of a pressure transducer connected to a flat plate containing fluid. More overburden weight on the transducer results in a higher pressure reading.  (Reinhart et al. 2002).  TEPC provides changes in overburden pressure as additional waste lifts are placed at a location.

fig8

Figure 9: Installation of a total earth pressure cell (Reinhart et al. 2002)  

Pore pressures are the combination of gas pressure and liquid pressure in the pore space of the MSW.  Pressures are typically measured by buried electronic pressure transducers that are connected to a data logger and power source with a cable.  Installation of pressure transducers and the accompanying cables is challenging in the landfill environment, and studies have proposed to development of wireless sensor networks to reduce the complexity and cost of monitoring (Nasipuri et al. 2006). Pressure transducers buried in waste have aided significantly in understand waste anisotropy, which results from the heterogeneous nature of landfills and results in non-uniform waste degradation (Townsend et al. 2015).

 fig9

Figure 10: Installation of pressure transducers in borehole for in-situ pore pressure measurement (Townsend et al. 2015)

Add comment

NOTE: The symbol < is not allowed in comments. If you use it, the comment will not be published correctly.

Security code
Refresh
*Please insert the above-shown characters in the field below.

The Geoengineer.org Corporate Sponsors: