The International Information Center for Geotechnical Engineers

Monitoring of waste degradation processes for sustainable MSW landfills - Leachate Monitoring

Leachate Properties

Leachate is defined as liquid that has come into contact with waste. In a conventional landfill, leachate trickles down through the waste mass and into the leachate collection and removal system (LCRS) at the base of the landfill. In a bioreactor landfill, pipes are installed throughout the landfill to recirculate the leachate as evenly as possible throughout the waste. Leachate can be distributed horizontally or vertically, but the optimal method of even distribution depends on the characteristics of the specific landfill.

Physical and chemical leachate properties should routinely recorded as they help determine the effectiveness of the bioreactor. Some of the parameters that are commonly monitored in the leachate include:

  • Chemical oxygen demand (COD) (mg/L)
  • Biochemical oxygen demand (BOD) (mg/L)
  • Dissolved oxygen content (mg/L)
  • pH
  • Temperature
  • Redox potential (mV)
  • Ammonia, nitrate/nitrite, phosphorus levels (mg/L)
  • Heavy metals (chromium, nickel, zinc, cadmium, copper, lead, iron) (μg/L)
  • Total dissolved solids (TDS) (mg/L or ppm)

The levels of these various parameters over time indicate how long the leachate should remain in circulation before being pumped out as well as how much treatment, if any, must be provided to the leachate before leaving the landfill site. The key parameters to measure   stabilization progress are COD, BOD, level of nitrogen forms, and pH. Other parameters on this list are used to measure the amount of heavy metal sequestration brought about by recirculation. (MPCA, 2009)

BOD & COD

COD is the capacity of a body of water to consume oxygen during oxidation of inorganic chemicals (such as ammonia) and the decomposition of organic material. In contrast, BOD measures aerobic microorganisms' oxygen demand by microbial oxidation. The COD, in the case of landfills, is typically higher than the BOD, as shown in Figure 1. The higher the COD and BOD, the more the leachate is oxygen-deficient. In the initial stages after waste is deposited in a landfill, the BOD and COD quickly rise as the oxygen and nitrogen is depleted. As the organic matter is oxidized into methane and CO2 gases, the BOD and COD fall and reach a stable level. In a bioreactor landfill, stable levels are reached much faster than a conventional landfill. The lower the final BOD and COD, the less leachate treatment for organic matter is required upon leaving the landfill. However, with leachate recirculation, a potential issue that arises is large concentrations of heavy metals in the leachate. Extra treatment for heavy metal removal may need to be added post-circulation. 

COD BOD graphs

Figure 1: The process of waste stabilization over time during the aerobic and anaerobic phases of waste degradation. There is no scale on the x-axis; the amount of time it takes to reach the final mature stage is dependent on the specific parameters of the landfill. (Townsend et al. 2015)

Heavy Metals in Aerobic & Anaerobic Landfills

Heavy metal concentrations can be significantly different in an aerobic vs. anaerobic bioreactor. Landfill leachate typically contains high concentrations of heavy metals such as chromium, zinc, cadmium, copper, lead, and iron. Metals are much more likely to remain in the body of waste in an anaerobic bioreactor by sorbing onto soil particles and precipitating. In an aerobic bioreactor, the redox potential is reversed from strongly negative to positive, and metals are more likely to remain in the leachate as it exits the landfill. (Giannis et al. 2008)

Most aerobic reactors use a dual process: first acting as an aerobic reactor to accelerate the biodegradation of organic matter and then encouraging quicker methanogenesis during an anaerobic stage. Another added benefit of the dual bioreactor is more destruction of volatile organic compounds in the waste. (Waste Management Inc, 2015) Differences in leachate distribution systems between aerobic and dual systems is shown below.

 anaerobic OL

Figure 2: Process diagram for an anaerobic bioreactor with vertical leachate distribution and gas collection. (Waste Management Inc, 2012)

 anaerobic diagram OL

Figure 3: Process diagram for a dual aerobic-anaerobic bioreactor with horizontal leachate distribution, gas collection, and air injection. (Stauffer, 2006)

Monitoring BOD, COD, Heavy Metals, TDS

Tracking of leachate parameters can take place either in-situ within the landfill or within a leachate tank outside of the landfill. There has not been substantial research on leachate monitoring via sensors in the landfill; most of the current data on leachate parameters has been compiled from tests in recirculation tanks (see Figure 4). Each parameter is tested using methods described in the American Public Health Association’s (APHA) 1989 Standard Methods for the Examination of Water and Wastewater (SMWW).

One method to measure BOD and COD is using SMWW’s oxygen-consumption rate test. First, an oxygen-consumption rate device, such as a probe with an oxygen-sensitive electrode, is calibrated at a temperature similar to measured temperatures in the landfill. Then, the dissolved oxygen (DO) content of a leachate sample is increased by shaking the sample in a partially filled bottle or bubbling oxygen through it. The sample is added to a BOD bottle along with a biological suspension and a magnetic stirring bar, and the DO level is measured with the oxygen-sensitive probe and recorded. (APHA, 1989) The difference in the DO level over a period of five days represents the BOD5 value (a standard measurement of BOD).

Heavy metals in landfills often come from industrial waste, incineration waste, mine ash, batteries, paints, inks, and other hazardous household products. In a 2007 study in Greece, it was determined that the amount of heavy metals in the leachate vs. the amount remaining sorbed in the landfill was highly pH-dependent (Giannis et al. 2007). One of several ways to measure heavy metal concentrations is using ASTM D3987-85 shake extraction of solid waste with water procedure. A shaking device agitates leachate samples, and the samples are left to settle. The aqueous phase and solid metals are then separated by centrifugation, decantation, or filtration through a coarse paper. The remaining liquid is pressure filtered through a 0.45 μm filter paper. Different heavy metals can then be analyzed in the solid extract. (ASTM, 2004)

To determine the TDS of the leachate, standard methods consist of filtration to remove suspended solids, drying, and then measurement of the remaining mass. The bulk of TDS is made up of dissolved inorganic ions and organic matter. The inorganic ion (Cl-, SO42-, HCO3-, Na+, K+, Ca2+, Mg2+, etc) concentrations are measured using ion chromatography. The dissolved ion concentrations can provide information on the strength of the leachate; the concentrations generally increase over time as the leachate becomes less diluted with repeated circulation. (Townsend et al. 2015) The pH and temperature of the leachate can be measured either in the recirculation tank or within the landfill with simple pH and temperature meters, probes, and calibration fluids.

bioreactor diagram 2

 Figure 4: Diagram of leachate circulation and leachate control tank in a lab-scale bioreactor cell. (Abdallah, Kennedy, 2013)

Leachate Recirculation Considerations

Leachate is either immediately sent off to a wastewater treatment plant or receives partial on-site treatment before getting transported to a treatment plant. Some of the treatment can even occur in the leachate recirculation tank. The fraction of leachate that is re-circulated and the fraction that is removed from the waste mass for treatment should be carefully controlled. Some issues that may arise from over-circulation include ponding and overly acidic conditions. (Townsend et al. 2015). Overly acidic conditions disrupt the leachate treatment process, and pools of leachate within the landfill could be difficult and costly to remove.

The leachate can continually circulate through a section of the landfill even after the section is closed off to new waste until full stabilization in achieved. When the leachate COD is less than 1,000 mg/L and BOD is less than 100 mg/L, and the BOD:COD ratio is less than 0.1, and the gas production drops to 5% of its peak value, the waste is officially considered to be stabilized (Townsend et al. 2015).

Though it is difficult to achieve adequately even leachate circulation throughout the entire landfill, evenly spacing out leachate distribution pipes throughout ensures that the maximum possible amount of the waste mass comes in contact with the re-circulated leachate. At the same time, the pipes must be placed in such a way that lowers the risk of ponding and leaking. For example, in the state of Minnesota, leachate pipes must be at least 20 feet above the base of the waste and at least 50 feet from exterior slopes to minimize the risk of leachate seeps. (MPCA, 2009) 

Leachate Blankets - Case Studies

Leachate is circulated through two conventional approaches: either vertical pipes or horizontal trenches, spaced out to minimize dry zones. A newer development in leachate distribution technology is the use of permeable circulation blankets. The high fluid conductivity blankets are composed of a nonwoven geotextile layer, two feet of shredded scrap tires, and another nonwoven geotextile on top. A variety of sensors are embedded in the blanket layer to monitor various parameters:

  • Moisture content sensors
  • Load sensor to monitor weight/stress of the waste
  • Digital pressure gage to measure leachate head
  • Magnetic flow meter

Sensor data is logged in an automatic data collection system. This technology was piloted at full scale in Jackson, MI in 2004, and the results indicated that the blankets were both hydraulically efficient for through leachate recirculation and cost-effective (Khire, 2004).

jackson mi 1 jackson mi 2

Figure 5: Testing permeable leachate circulation blankets in Jackson, MI. The blankets were created with shredded recycled tire scraps (left) and part of the automated sensor monitoring system is shown on the right.

In 1994, Yolo County, California began a field-scale test that compared anaerobic cells to aerobic cells in a landfill utilizing leachate recirculation. To distribute the leachate, horizontal trenches filled with shredded vehicle tires were installed throughout the landfill. The results indicated that the aerobic reactor allowed for greater organic decomposition than the anaerobic reactor because both aerobic and anaerobic reactions were able to take place throughout the waste mass. This was partially due to dead zones where flow did not reach, as well as "preferential flow pathways" where the tire-filled trenches were not able to evenly distribute the leachate (Townsend et al. 2015). Air was added to the aerobic section by using a vacuum system in the gas collection pipes to pull air in through a permeable soil cover. However, a major issue that arises from having a permeable cover is the seepage of water into the landfill during rainfall events, uncontrollably increasing the amount of circulated leachate.

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