Environmental Engineering Reference
In-Depth Information
Wastewater streams produced at food-processing industries, which contain high
concentrations of suspended solids, are treated either in wastewater treatment plants or lagoons
or sent to public works. Emissions from treatment come from
1. Direct emissions from fermentation that produces carbone dioxide, methane, nitrous
oxide, VOCs, and ammonia.
2.
Indirect emissions from running pumps and aeration equipment that are used to promote
oxygenation and the development of aerobic bacteria.
In the case of lagoons, they are highly anaerobic systems that produce large amounts of
methane, which is 23 times more powerful than carbon dioxide as a GHG for a 100-year
horizon.
The best way to mitigate direct and indirect emissions from wastewaters is to migrate from
open-to-the-air anaerobic or aerobic approaches to close anaerobic systems capable of
recovering methane gas that could be subsequently used for production of energy or processing
heat. This methane gas, which is mixed with carbon dioxide and other compounds, is
considered to be “biogenic” and therefore produces net-zero emissions once burned in a boiler
or generator.
Solid wastes from many food-processing plants, especially operations dealing with fruits
and vegetables, can use the inedible by-products for animal feed, which is a great alternative
to landfills. In this case, the main impact comes from transporting the residues to the farm, and
the magnitude of the impact is proportional to the distance from the processing plant to the
point of consumption. This will be covered more in Chapter 10.
By-products of the meat industry
Meat- and poultry-processing plants generate inedible solid waste materials that are treated as
by-products and processed in rendering plants. Also, there are edible solid parts that are also
subjected to rendering to recover fat in the pork and beef industries, but these will not be
discussed here.
Rendering is probably one of the oldest recycling methods in the food industry. All the ined-
ible parts, such as internal organs, trims, parts with no market value, and condemned animals
or parts, are send to special rendering plants. There, the materials are cooked and fat is sepa-
rated from the solids (protein and bones), which are dried, ground, and sold as animal feed.
Rendering plants have a high impact on air emissions. Direct emissions of carbon dioxide
come mainly from transportation and burning natural gas for generation of steam in boilers.
Also, the process itself produces VOCs, especially odors. Indirect emissions happen mainly as
a result of purchased electricity.
Depending on the area where these plants are located, odor complaints are frequent;
therefore, rendering plants have huge air-handling systems that suck the air of the entire plant
and redirect it to scrubbers that capture VOCs and odors. These systems are effective but at a
high cost of running big motors for the air-handling systems and pumping. Typical VOCs from
rendering plants include organic sulfides, disulfides, C-4 to C-7 aldehydes, trimethylamine,
C-4 amines, quinoline, dimethyl pyrazine, other pyrazines, C-3 to C-6 organic acids, and to a
lesser extent C-4 to C-7 alcohols, ketones, aliphatic hydrocarbons, and aromatic compounds
(EPA, 1995).
A second source of VOCs and odors happens before the rendering process even starts.
By-products are trucked for long distances from the processing facilities to the rendering
plants in regular semi-trucks. Clearly, decomposition of the “raw” material starts during the
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