by: Stewart J. Smith, M.S. and Harold W. Bentley, Ph.D.

The degradation rate of landfill municipal waste is influenced by:

• the landfill’s moisture content and distribution;
• waste composition and biodegradability;
• the waste’s physical state (soluble or insoluble, shredded or unshredded, etc);
• temperature; and
• presence of inhibitory compounds (particularly certain metals).

Degradation rates in anaerobic landfills control the rates of landfill gas (LFG) generation and the length of time over which LFG will be generated. Both are important considerations in any landfill where LFG is actively recovered to generate power, minimize subsurface gas migration, or limit emission of non–methane organic compounds. The greenhouse effects of LFG emission, particularly methane, are an additional concern.

...aerobic treatment can reduce biodegradable
mass within the landfill up to 10 times
faster than anaerobic treatment.

Figure 1: Surface Topographies (upper and lower surfaces of numerical model)

Active management of anaerobic landfills enhances waste degradation and LFG generation, improves the economics of power generation, and reduces the concentration of organic compounds in leachate and the time period over which the LFG is generated. This type of management is suitable for larger landfills where power generation or continuous flare operation is feasible. The most effective method to enhance degradation rates is to increase moisture content by supplying additional water or by collecting and reapplying leachate. Because biodegradation takes place in the aqueous phase, increasing moisture content enhances degradation rates in two ways: 1) the higher moisture content will support a larger microbial population; and 2) more of the soluble, biodegradable material can dissolve and become available for microbial attack. Predicting the enhanced degradation rate resulting from increasing moisture content requires knowing the rate-limiting quantities of oxygen, moisture, and nutrients, and how each affects the growth and decay of microbial populations. For these predictions we used the computer code TRAMP1. Figure 1 shows the surface topographies used in the simulation and Figure 2 (below) shows the simulated reduction in biodegradable waste with time at different moisture contents and temperatures.

Figure 2: Simulated Anaerobic Mass Reduction at Two Moisture Contents and Temperatures

Smaller landfills do not have the potential to generate sufficient LFG to make power generation or continuous flare operation feasible. For these landfills, aerobic treatment may be the preferred method to reduce the amount of time over which LFG is generated. Bringing air into a landfill will suppress anaerobic bioactivity and LFG production, and stimulate the much faster aerobic processes. With sufficient moisture and the absence of inhibiting substances, aerobic treatment can reduce biodegradable mass within the landfill more than 10 times faster than anaerobic treatment. However, aerobic treatment of landfills requires careful management of moisture and air supply to keep temperatures below levels that can kill the microbial population or cause landfill fires. The simulated results of aerobic treatment of a landfill, using an air and water management system designed with TRAMP¹, are shown in Figure 3 (below). The differences between aerobic and anaerobic degradation rates under similar conditions of moisture content are dramatic.

Figure 3: Simulated Mass Reduction Under Aerobic and Anaerobic Conditions

Once the biodegradable portion of the waste has been destroyed by aerobic treatment, LFG production and waste settling resulting from decomposition will no longer occur to a significant extent. Thus long–term maintenance and monitoring costs are reduced and post closure development of the landfill can occur much quicker. Should the landfill remain active, its life can be extended by mining its aerobically treated portions to accept more waste.

For more information, contact Stewart Smith at (520) 293-1500 ext. 122 (stewarts@hgcinc.com).

¹Bentley, H. and B. Travis. 1989. A 3–dimensional finite difference computer code that simulates biodegradation and liquid and gas transport in variably saturated porous media, under both aerobic and anaerobic conditions.