Part 2: Nutrient recovery options
by Wendy Powers, associate professor of animal science, Robert Burns, associate professor of agriculture and biosystems engineering
While the primary method of manure management in the United States is temporary storage followed by land application as crop fertilizer, there is increasing interest in recovering energy and nutrients from manures prior to land application. Insufficient nutrient assimilation capacity in nearby crop land, or interest in adding value to manure beyond the fertilizer value, are among the reasons that alternative management strategies may be sought. Producers who consider alternative manure uses will find many options available.
This is the second of a two-part series that describes several nutrient recovery processes. Each process is explained and primary issues that a producer should consider with each process are discussed. Opportunities and approaches that enhance the ability to recover nutrients will continue to gain popularity as the need to move nutrients offsite, in order to avoid over-application of nutrients to cropland, heightens. In the first part, we addressed solids separation and flocculation and precipitation as methods of nutrient recovery. This section will cover composting and aquaculture.
A significant amount of dried manure, composted manure, composted solids separated from manure, or some combination of these is bagged and sold as organic fertilizer. For example in California, a dairy cooperative was set up to move manure off of large, intensive drylot dairies located in an urban area. The cooperative picks up the manure from the farm, takes it to a central location where it is processed, bagged, and marketed.
Composting is a logical way to process wet manure solids (but not slurries unless the slurry can be added to drier materials) when animal producers must create a product that easily moves off-farm and is stable enough so that suburban users or agricultural users near urban centers will want to use it. Composting requires routine management of the piles in order to ensure complete and timely processing. Therefore, operations usually consider the process if marketable products that will help them remove excess nutrients, especially phosphorous (P), from the farm can be generated even if income does not equal processing and handling costs.
Advantages include: aerobic composting reduces volume and converts biodegradable materials into stable, low-odor end products; thermophilic temperatures of 130o F to 160o F, achieved in this process, kill most weed seeds and pathogens. If moisture content is too high, anaerobic conditions develop and odorous compounds can be produced. Obviously, high quality compost has much greater value in horticultural and urban markets than simply assessing nitrogen (N), P, and potassium (K) value. In addition, the capital investment for manure composting can be considerably less than other options provided that equipment to turn the compost is already available on the farm. While P and K remain in the finished product, and must be managed appropriately, much of the N is volatilized, potentially creating a challenge in the face of air quality regulations. Typically a material such as wood shavings, sawdust or some other carbon source is needed to successfully compost animal manures. The bulk material needed to improve carbon/ nitrogen ratio (C/N ratio) must be locally available for the process to be reasonably costeffective.
Composting is a very well developed technology with thousands of full-scale installations using animal manures worldwide. The success of a composting system relies heavily on ensuring adequate labor is allocated to system management.
Manure nutrients can be used to produce aquatic plants, fish and other aquatic invertebrates. Typically plants such as algae and duckweed are produced and then either harvested and sold as a feedstuff for fish production, or used on-site as a nutrient source to produce fish, baitfish, crawfish or other marketable aquatic invertebrates in integrated production systems. Swine manures have been used as a nutrient source for fish production in Asia for hundreds of years. While significant research has been conducted on integrated manure wastewater treatment systems that include the production of aquatic plants and animals, no full-scale integrated aquaculture systems have been implemented, commercially, in the United States.
Some aquatic plants have the potential to be used as livestock and poultry feed as well as a feed for fish and other aquatic species. Because aquatic duckweed plants have a fast biomass production rate and contain relatively high nitrogen content (i.e., high protein content) there is recent interest in the U.S. in evaluating the potential of duckweed production as a means of treating swine manure. Use of algae or duckweed production as a waste treatment process works off the principle that nutrients from manure are recycled during the production of plant biomass. The biomass, then, can be harvested and used as a feed source in animal production. Duckweed can be produced in a grid-system that contains the duckweed mat in order to facilitate harvesting. Belt dewatering systems have shown promise for harvesting algae.
The recovery of nutrients as an additional farm product requires integrating two or more production systems and marketing additional products. Considerable dilution of excreted manure is required before use as a nutrient source in an aquaculture system. To be economically competitive, aquaculture systems require a warm climate with a long growing season. For these reasons, the greatest interest in the U.S. has occurred in southern regions where lagoon systems predominate and temperatures are warm. The primary market, to date, for aquatic plants, such as duckweed, has been fish farming which is relatively highvalue when compared to livestock feed.
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Page last updated October 5, 2004
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