November / December, 2002
Adapting Municipal Wastewater Treatment Methods to Animal Manure
Sustainability,
nutrient management, and zero discharge are key issues facing livestock
producers in the state of Michigan today. Social, environmental, and economic
pressures are driving livestock producers to adapt alternative manure
treatment methods that reduce odor and minimize the potential for air,
water, and ground pollution. The municipal wastewater treatment industry
traditionally has been a source of technologies adapted to treat animal
manure to improve handling, storage, transportation, and application.
Aeration and solid/liquid separation are two examples of technologies
that have successfully been used to treat animal manure. The difficulty
in adapting such treatment technologies is that municipal waste is dilute
and low in nutrients where as animal manure is concentrated and contains
high levels of nutrients. Utilizing municipal treatment technologies to
treat animal manure will not only improve environmental conditions and
nutrient management, but may also create a profitable business enterprise
centered on manure management. Anaerobic digestion, phosphorus separation,
and wetland treatment are three technologies that researchers in the Agricultural
Engineering Department at Michigan State University are currently studying.
Anaerobic digestion is one technology being evaluated. Municipal treatment plants have used digestion to strip nitrogen and phosphorus from wastewater. During digestion, organic matter is converted to carbon dioxide, methane, trace gases, and water. A stabilized slurry results. Digestion of animal manure over the years has proven to be a useful tool for stabilizing the manure and reducing odor. An additional benefit is the production of methane, a green energy that can be converted to heat, steam, or electricity. Digester effluent, a stabilized manure slurry, can be treated using other municipal treatment options to better manage nutrients and reduce pathogens.
Biological and chemical phosphorus separation techniques are other municipal treatments that may feasibly be used to treat animal manure. Anaerobic digestion is essential for biological phosphorus separation and improves the efficiency of chemical separations. Biological phosphorus separation is the cycling of anaerobic and aerobic conditions, during which the microbial population consumes a "luxury" amount of phosphorus. The cycling of aerobic and anaerobic conditions also removes nitrogen from manure via a nitrification/denitrification process. Chemical phosphorus separation describes two distinct processes; chemical precipitation and struvite formation. Chemical phosphorus separation involves the addition of a coagulant that precipitates phosphorus, the coagulated phosphorus particles aggregate into "flocs". Struvite formation is the process of crystallizing soluble phosphorus. Bound phosphorus, whether it is by biological or chemical means, can be separated passively or mechanically.
The products of phosphorus separation include a liquid effluent low in phosphorus and a concentrated phosphorus sludge representing 10 to 20% of the original volume. The phosphorus sludge can be land applied to areas deficient in phosphorus or composted and sold. In the case of struvite, it may be marketed as a slow release fertilizer or secondary phosphate ore. Chemical phosphorus precipitation systems are currently being installed on two Michigan dairy farms.
Man-made wetlands are useful for treating dilute manures, such as digester and phosphorus separation effluent, for nitrogen and fecal bacteria reduction. Wetlands may also be used to treat runoff from open lots. For manure treatment, wetlands will not serve as stand alone treatment options, but will act as a final polishing step for a manure treatment system. Wetland plants do utilize some nutrients, but, more importantly, create anaerobic/aerobic zones that break down organic matter into simple end products (CO2, N2, CH4, etc). Reusing wetland effluent at the production facility may reduce demand and/or dependency on fresh water.
Utilizing municipal wastewater treatment technology can improve the management of manure nutrients and may create a profit center focused on manure. Useful and renewable end products can be sold or used to offset the cost of fertilizer and energy inputs. More importantly, the adaptation of municipal treatment technologies to animal manure will improve nutrient utilization and reduce the threat of pollution.
Agricultural Engineering
Michigan State University
A.W. Farrall Hall
East Lansing, MI 48824-1323
(517) 355-4720
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