Environmental Engineering Reference
In-Depth Information
1. Anaerobic bioreactor landfills
The Anaerobic Bioreactor seeks to accelerate the degradation of waste by optimizing
conditions for anaerobic bacteria. In landfills, consortia of anaerobic bacteria are responsible
for the conversion of organic wastes into organic acids and ultimately into methane and
carbon dioxide. Anaerobic conditions develop naturally in nearly all landfills without any
intervention. The waste in typical landfills contains between 10 and 25 percent water. It is
generally accepted that to optimize anaerobic degradation moisture conditions at or near field
capacity, or about 35 to 45 percent moisture, are required. Moisture is typically added in the
form of leachate through a variety of delivery systems.
However, the amount of leachate produced at many sites is insufficient to achieve
optimal moisture conditions in the waste. Additional sources of moisture such as sewage
sludge, storm water, and other non-hazardous liquid wastes may therefore be necessary to
augment the leachate available for recirculation. As the moisture content of the waste
approaches optimal levels, the rate of waste degradation increases, this in turn leads to an
increase in the amount of landfill gas produced. Also observed is an increase in the density of
the waste. While the rate of gas production in an anaerobic bioreactor can be twice as high as
a normal landfill, the duration of gas production is significantly shorter. Because of this
accelerated production, gas collection systems at bioreactor landfills must be capable of
handling a higher peak volume but need do so for a shorter period of time. The anaerobic
biodegradation of MSW follows three sequenced biochemical reactions involving three
different groups of anaerobic bacteria which are: (1) Fermentative and hydrolystic bacteria,
(2) Acidogenic bacteria and (3) Methanogenic bacteria. In the anaerobic stage, there are four
steps involved in the bacteria groups to convert waste into biogas (CH
4
, CO
2
) as end products,
and organic acids as intermediate products. The four steps are hydrolysis, acidogenesis,
acetogenesis and methanogenesis (Jin, 2005).
Figure 1 shows a cut-away view of an anaerobic bioreactor with elevated levels of
ammonium in the leachate. Leachate is removed via pipes from the bottom of the landfill and
piped to an on-site biological leachate treatment facility. The treated leachate and other
liquids are then reinjected into the landfill. At the same time, gas generated by the
decomposing waste rises through the landfill, and is collected by pipes within the waste and
on top of the landfill. The landfill gas that is collected is used to generate energy.
Groundwater monitoring occurs at monitoring wells situated around the perimeter of the
landfill (U.S EPA 2004).
2. Aerobic bioreactor landfills
The Aerobic Bioreactor seeks to accelerate waste degradation by optimizing conditions
for aerobes. Aerobes are organisms that require oxygen for cellular respiration. In aerobic
respiration, energy is derived from organic molecules in a process that consumes oxygen and
produces carbon dioxide. Aerobes require sufficient water to function just as anaerobes do.
However, aerobic organisms can grow more quickly than anaerobes because aerobic
respiration is more efficient at generating energy than anaerobic respiration. One consequence
of this is that aerobic degradation can proceed faster than anaerobic degradation. Another
consequence is that aerobic respiration can generate large amounts of metabolic heat, which
requires significant quantities of water. In landfills aerobic activity is promoted through
