Remediation of PAH-Contaminated Soils and Sediments: A ...

Composting
Composting is a form of biostimulation because it consists of nutrient additions,
moisture and oxygen control in a contained system. In a properly-maintained
compost pile, temperatures in excess of 55˚C are reached which kills most
pathogens (Eweis et al., 1998) and is a technique most commonly used for
treatment of municipal solid wastes. In applications for hazardous waste,
contaminant organic compound concentrations and extremely high temperatures
(>55˚C) cannot support microbial activity in a composting environment and
additional organics must be added and temperatures controlled (Eweis et al.,
1998).
Like all bioremediation techniques, composting is effective only if implemented
properly. Optimization of four parameters is necessary for successful PAH
degradation during composting; aeration, temperature, moisture and pH.
Composting is a generally aerobic process with microsites of anaerobic activity;
therefore, the pile must be aerated through mechanical mixing or additions of
bulking agents to improve structure and porosity. Temperature of the pile must
remain between 30 and 60˚C for optimal microbial community structure, solubility
and mass transfer rates (Eweis et al., 1998). Addition of organic material, while
maintaining a C:N ratio for 25:1, facilitates higher temperatures in compost piles
and biodegradation. Moisture must be manipulated through water and
amendment additions. Most compost piles are able to reach high temperatures
with low moisture contents. To obtain optimum moisture content in the pile (50-
80%), a bulking agent (wood chips and sewage sludge) can be added (Epstein
and Alpert, 1980).
There are three types of composting systems; windrow, static piles and closed
reactors (Eweis et al., 1998). Open systems (windrow and static piles) are more
commonly used than the closed reactors. In a windrow system, the compost
mixture is placed on an impermeable layer to prevent leaching of contaminants.
The cross section of the pile must be such that high temperatures are maintained
on the interior of the pile with minimal heat losses on the outside of the pile.
Aeration is achieved through mechanical mixing (front end loader or turner). If
the pile is too large to mix, pipes can be installed to increase aeration in the pile.
Windrow piles are often covered with a high density, polyethelyne material or
wood chips in outside environments to prevent leaching from the pile during
rainfall events (Eweis et al., 1998). In static piles, compost material is placed on
an impermeable platform with perforated pipes connected to a blower system.
Piles are aeriated at either a fixed rate or variable rate depending on the
microbial activity. Static piles are also covered with plastic sheeting, but the
cover must allow air to enter the pile. Closed reactors are the least used
systems because of size limitations, but are the most controlled. Closed systems
are constructed as the name implies and contain mixing devices through drum
rotation or mixing instruments within the tank (Eweis et al., 1998).
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