Phosphorus (P) is an essential macronutrient for crops. However, widespread use of phosphorus has unbalanced the global phosphorus cycle. High quality supplies of minable phosphate may be exhausted this century and many regions currently lack access to critical phosphorus resources, while others experience environmental problems associated with phosphorus pollution. The latter is the case in Ohio and the Great Lakes region, where widespread use of phosphorus in industry and agriculture has resulted in excess loading of phosphorus in lakes, impacting water quality.
Nutrient-rich animal manure is commonly applied to crops, typically in quantities to satisfy the nitrogen (N) requirement of the crop. However, due to low N:P ratios in typical manures, this practice results in application of phosphorus well in excess of crop needs, saturating agricultural soils with phosphorus over time. From the field, rain and erosion carry excess phosphorus to lakes and rivers, causing eutrophication and harmful algal blooms (HABs).
In Ohio waters, the yearly recurrence of HABs due to phosphorus pollution has damaged ecosystems, endangered human health, and disrupted lake-dependent businesses. Although not the only contributor, runoff from phosphorus-rich animal manure is an important contributor to the eutrophication of Ohio waters. One of the key strategies for reducing the impact of animal agriculture on water quality is to remove and recover phosphorus from animal manure prior to land application. Efficient recovery of phosphorus from animal manure would also provide an alternative to unsustainable phosphate rock mining.
Technologies for phosphorus removal from waste streams, including animal manure, generally fall into one of three categories: physical, chemical or biological methods. Different technologies are often used in combination to provide effective treatment. Many of these strategies have been widely applied for several decades at municipal wastewater treatment facilities, where public funding enables significant investment in phosphorus recovery. However, distinct challenges exist for application of these technologies to animal manure, particularly for the treatment of manure on the farm.
In an agricultural context, these techniques are most applicable to highly concentrated point sources of phosphorus, such as slurry from a liquid manure storage lagoon or effluent from an anaerobic digester. Efforts to recover phosphorus from animal manure on the farm have been limited largely by logistical and economic barriers. High equipment and input costs quickly become prohibitive, especially for small farms. It is necessary to develop a phosphorus removal and recovery system suitable for animal manure application.
Physical treatment for phosphorus removal is solid-liquid separation, which is designed to concentrate phosphorus in the solid stream while producing a liquid stream for further treatment. Solid-liquid separation can be accomplished by centrifugation or use of mechanical presses such as filter, belt, or screw presses. Polymer flocculants are often utilized to enhance the separation process.
For manure, solid-liquid separation is a common first step in the treatment process to remove particle-bound phosphorus as well as fibrous solids or organic matter that may interfere with subsequent treatment. Chemical treatments exploit the natural affinity that exists between phosphorus and certain metal salts, namely iron, aluminum and calcium compounds. Iron (FeSO4, FeCl3) and aluminum compounds (alum, Al(OH)3) can precipitate phosphate quickly and effectively. However, the resulting solids have limited reuse value in agriculture due to toxicity concerns.
Lime products (CaO, Ca(OH)2) and, less commonly, magnesium salts (MgO, MgCl2) can also be used to precipitate soluble phosphorus as various calcium/magnesium phosphates at alkaline pH. The resulting solids are relatively insoluble, but may be applicable as slow-release fertilizers. High removal efficiencies (greater than 95%) have been reported from animal manure using lime, however, the requirement of high dosages of lime may result in high chemical input/handling costs and excessive sludge generation. Another emerging technique is phosphorus removal via struvite crystallization. Struvite (MgNH4PO4), which can simultaneously recover both N and phosphorus from animal manure as insoluble crystals under controlled conditions, has been reported to have greater than 95% recovery from animal manure. Struvite technology is nearing commercialization, but again chemical input costs may be prohibitive.
Quasar energy group, a regional leader in the waste-to-energy industry in Ohio, recently developed a mobile, pilot scale system for removal and recovery of phosphorus from dairy manure. The process involves two solid-liquid separations and one chemical treatment. First, manure slurry from a storage lagoon is centrifuged with the aid of polymer flocculants. The P-rich solids can then be economically exported from a watershed with excess phosphorus and potentially used as fertilizer on cropland, which needs additional P. The liquid stream is further treated with hydrated lime (Ca(OH)2) to remove phosphorus as Ca-P compounds. To complete treatment, the lime slurry is then pumped through a filter press, resulting in another solid product and a treated liquid with low P concentration (less than 1 part per million) suitable for irrigation. Further research and development will focus on improving system efficiency and economic feasibility.