Environmental Engineering Reference
In-Depth Information
Composting can be conducted either on site or off site, which is contingent to space avail-
ability, local regulations, staff resources, use for the compost, and adequate infrastructure
(“Composting,” n.d.). For on-site composting, the best approach is to use an “in-vessel” com-
poster, which is an enclosed piece of equipment with controlled temperature, moisture, and
aeration rate. In-vessel composters come in variable shapes and capacities. Most common
shapes include rotating drums, concrete bunkers, and silos; however, many other forms are
available. Aeration in in-vessel composters is achieved by means of a mixing mechanism or
by air injection.
For large volumes, “pile composting,” also known as “aerated windrow,” is the preferred
method, which takes place in off-site operations. Pile composting consists of forming long
piles of material with heights between 1.2 and 2.4 meters and widths of 4.2 to 4.8 meters. The
pile is periodically turned with mechanical devices to promote aeration (“Aerated [turned]
windrow composting,” n.d.).
Composting has two major environmental benefits: recycling nutrients back to the soil
and preventing the disposal of biodegradable material in landfills. On the downside, com-
posting has negative environmental impacts that need to be considered to mitigate the
impact. Composting produces gases that include carbon dioxide and volatile organic carbon
as well as methane in anaerobic pockets and ammonia in poorly managed facilities.
Composting also generates liquid effluents (i.e., composting leachates) containing nutrients
and bacteria that when are not properly managed can contaminate surface and underground
water.
Biogas production
The anaerobic fermentation of food solid waste produces biogas in a
similar way to what was presented in Chapter 9 for anaerobic treatment of wastewater. In
the case of solid waste, though, different types of reactors are required to deal with high
contents of solids. Several technologies are available to digest anaerobically organic solid
wastes. However, they all have in common that they need solids disintegration and mixing
with water to produce a high solid-content slurry that to feed a one-stage or two-stage
reactor. In a one-stage reactor, all the phases of the anaerobic fermentation (see Chapter 9)
take place in a single reactor, whereas in a two-phase system, the hydrolysis and
methanogenesis take place in different reactors so that conditions can be optimized for
each phase.
The final product of the anaerobic digestion is biogas and a slurry, “digestate,” that needs
dewatering before continuing for further processing. Solids recovered during dewatering can
be aerobically treated to convert them into compost, and the liquid returned to the beginning
of the process to suspend fresh material or as liquid fertilizer because it is rich in nutrients.
The biogas produced contains a mixture of approximately 55 to 70 percent of methane and
30 to 45 percent of carbon dioxide, which depends on the starting material (Jensen and
Jensen, 2000). The volume of biogas produced is about 404 m
3
/tonne (367 m
3
/ton) of food
waste, which is significantly higher than the 27.5 m
3
/tonne (25 m
3
/ton) of cattle manure, or
132 m
3
/tonne (120 m
3
/ton) of biosolids from anaerobic wastewater treatment (“The benefits
of anaerobic digestion of food waste at wastewater treatment facilities,” n.d.). See Table
10.3. After purification, the biogas can be used in the production of thermal or mechanical
energy and converted into electricity. Depending on the volume and type of waste generated,
the amount of energy generated is usually enough to run the digesters and have a surplus
that can be sold to the grid in places where the practice is allowed. The main drawback of
anaerobic digestion of wastes combined with energy generation is the need of substantial
capital investment and maintenance costs. The cost of the anaerobic installation depends on
the volume of organic waste to treat. As the volume increases, the cost of the systems per
