Environmental Engineering Reference
In-Depth Information
food source for the fastest-acting bacteria (in the center of the pile) has been largely consumed.
When turning the pile does not cause a temperature rise, it brings no further advantage. When all of
the material has turned into dark brown crumbly matter, it is ready to use.
32.3.1.5 Industrial and agricultural composting
Industrial and agricultural composting systems are increasingly being installed as a waste
management alternative to landfills, along with other advanced waste processing systems. Industrial
and agricultural composting or anaerobic digestion combined with mechanical sorting of mixed
waste streams is called mechanical biological treatment increasingly used in Europe because of
stringent new regulations controlling the amount of organic matter allowed in landfills. Treating
biodegradable waste before it enters a landfill reduces global warming from fugitive methane;
untreated waste breaks down anaerobically in a landfill, producing landfill gas that contains
methane, a greenhouse gas even more potent than carbon dioxide.
Most commercial, industrial, and agriculture composting operations use active composting
techniques (Yadav et al. 1982; Baeten and Verstraete 1988; Georgacakis et al. 1996; Imbeah 1998;
Mohaibes and Heinonen-Tanski 2004; Zhu et al. 2004; Wakase et al. 2008). These ensure that
the process does not get out of control, especially with the high throughput demand imposed
by contracted, incoming waste. This means that as short as possible a processing time must be
maintained to keep the facility properly functioning. Partly for this reason composters have declined
to support compost maturity standards if it would increase the required holding time. The greatest
amount of technological control of composting is seen in systems that use an enclosed vessel and
control their temperature, airflow, moisture, and other parameters.
32.3.2 a
naEroBic
d
igEStion
Anaerobic digestion (Wikipedia 2008a) is a series of processes in which microorganisms break down
biodegradable material in the absence of oxygen. It is widely used to treat wastewater sludges and
organic wastes because it provides volume and mass reduction of the input material with biogas
as byproduct. As part of an integrated waste management system, anaerobic digestion reduces the
emission of landfill gas into the atmosphere. Anaerobic digestion is a renewable energy source
because the process produces a methane and carbon dioxide rich biogas suitable for energy production
helping replace fossil fuels. Also, the nutrient-rich solids left after digestion can be used as fertilizer.
32.3.2.1 Biochemical reactions in anaerobic digestion
There are four key biological and chemical stages of anaerobic digestion (Figure 32.1):
1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis
In most cases, biomass is made up of large organic compounds. For the bacteria in anaerobic
digesters to access the chemical energy potential of the organic material, the organic matter molecular
chains must first be broken down into their smaller constituent parts. These constituent parts or
monomers such as sugars are readily available to bacteria to process. The process of breaking these
chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysis
of these high molecular weight molecules is the necessary first step in anaerobic digestion. Through
hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and
fatty acids. Hydrolysis can be biological (using hydrolytic microorganisms), biochemical (using
extracellular enzymes), chemical (using catalytic reactions), as well as physical (using thermal
energy and pressure) in nature.
