Summer 2004
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 swine manure management in the United is temporary storage followed by land application as crop fertilizer, there is increasing interest in recovering energy and nutrients from manure prior to land application. Insufficient nutrient assimilation capacity in nearby cropland, or interest in adding value to swine 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.
Separated manure solids at dairy farm.
This is the first of a two-part series that discusses some of the options available to producers that facilitate nutrient recovery. 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 off-site, in order to avoid over-application of nutrients to cropland, heightens.
Flocculation, Coagulation and Precipitation Methods (P recovery). Chemicals commonly used for the purpose of particulate flocculation and/or coagulation include aluminum sulfate (Al2(SO4)3; alum), ferric sulfate (Fe2(SO4)3), ferric chloride (FeCl3), calcium carbonate (CaCO3; agricultural lime), calcium oxide (CaO; chemical lime or talc), ferric sulfate (FeSO4), and synthesized polyelectrolytes. Polyacrylamide polymers, another choice of flocculant, are used extensively as settling agents in wastewaters from food processing and packing, paper production, sugar extraction, mine and municipal wastewaters, potable water treatment, and as soil treatments to reduce leaching and erosion by irrigation waters. Flocculants are used to coagulate and precipitate nutrients and solids through chemical reactions. As a result, removals observed are typically greater than 80 percent of total solids (TS), 60 percent of nitrogen (N), 80 percent of phosphorous (P) and 60 percent of potassium (K). Because of the chemistry, using flocculants removes more P than either N or K while N is removed to a greater extent using separation or sedimentation techniques. Chemical precipitation has not been widely adopted for agricultural purposes in part because it requires a very dilute manure stream and require some sort of automated application. Cost of the chemicals is another inhibitor to their adoption. Solids and nutrients precipitated still need to be managed appropriately. However, as the need to move nutrients, particularly P, offsite implementation of flocculation using some of the cheaper flocculants (ie: ag lime) available may become more widespread.
The forced precipitation of struvite (the white crystalline scale on pump impellers and in pipe elbows and joints) from swine manure slurries prior to land application can reduce soluble phosphorus (SP) levels in the manure slurries, as well as offer the potential to concentrate and remove P from the system. The forced precipitation of struvite has been demonstrated to reduce the soluble phosphorus content in swine manure by as much as 90 percent in field-scale tests on commercial swine finish operations. Laboratory and field tests were conducted using magnesium to force the precipitation of struvite, which converts the soluble phosphorus in swine manure to a crystalline mineral. This mineral form of phosphorus could be less prone to move with runoff water and useful as a slow-release inorganic fertilizer. In Europe and Japan, large municipal sewage-handling facilities are recovering phosphorus as struvite using full-scale systems. While pilot scale struvite recovery systems using swine manure have been developed, no full-scale systems are currently in use with animal manure.
Solids Separation by Screening or Sedimentation. Because most of the nitrogen and phosphorus in manure are associated with manure solids, the separation of these solids can be used as a nutrient recovery technique. When cropland is available nearby, often the liquid can be more readily applied to croplands through an irrigation system while the solids are spread on croplands or more easily exported off-farm than the wet product that precedes solid-liquid separation. Additionally the removal of manure solids prior to storage will reduce the organic loading rate of a lagoon or holding pond. Reduced loading improves organic matter digestion, maintains useful volume and designed retention times much longer before cleanout is necessary, and reduces odors in effluent.
Methods of separating or concentrating solids include evaporation, mechanical separation, and sedimentation (gravity settling) with or without flocculation. Sedimentation or settling basins and mechanical separation are both widely used. Mechanical separators are available that use static screens, vibrating screens, drag flight, drum roller and centrifugal and screw press devices to achieve solids separation in manures. It is also common to find separation units that incorporate a combination of these techniques.
The amount of solids recovered using mechanical separation is highly variable depending on the type and amount of solids in the manure to be separated. Testing with dairy manure has indicated that separation efficiencies on a dry-mass basis can range from 15 to 60 percent depending on the TS content of the influent manure using the same separator. Separation efficiency with swine manures will be considerably less than dairy manure because dairy manure contains large amounts of fiber that is easily separated. With diluted slurry (5 to 6 percent TS), approximately 60 percent of the solids will settle by gravity sedimentation with 10 minutes or greater of settling time. As solids content increases above 7 percent, removal decreases dramatically. Nutrient removals by sedimentation of a dilute stream (<2 percent TS) in a settling basin are less than that of TS removal; ranging from 15 to 45 percent of influent N and 1 to 20 percent of P with as much as 60 minutes of settling time. Sedimentation alone, is more effective than screening to remove both solids and nutrients.
Mechanical separators typically range from $12,000 to over $100,000 in cost depending on unit size and complexity. Beyond the capital investment is the maintenance and operating costs of the separators. While gravity settling requires fewer mechanical parts, periodic and frequent emptying of settling basins is needed. Costs must be weighed against the variability in removal between mechanical separators and sedimentation.
Summary. When selecting a nutrient recovery option, producers need to consider the extent of nutrient recovery needed and weigh that against not only the economics, but also the intensity of management needed to employ a strategy successfully.
In the second part of this series, we will address composting and aquaculture as options to recover manure nutrients.
