Enhancing Phosphorus Recovery from Scraped Liquid Dairy Manure Using Process Effluent


Our long term goal is to develop a robust phosphorus (P) recovery technology suitable for use on dairy farms of all sizes through an integrated research and extension program for manure nutrient recovery. Enrichment of water bodies by P in agricultural runoff has been and remains the focus of water quality protection. Animal feeding operations (AFOs), regardless of size, especially in sensitive ecosystems and the Chesapeake Bay (Bay) in particular, have been identified as major contributors to water and air pollution. These operations are facing extra scrutiny by state and federal regulation agencies with regards to nutrient management. Although nutrient management plans have not been required for most dairy farms in Virginia, this status is changing rapidly. Nutrient reductions in unregulated small dairies has been identified as part of the strategy to clean up the Bay. Our project focuses manure P management on dairy farms and will particularly benefit dairy farms in Shenandoah Valley, Virginia and Lancaster County, Pennsylvania, where excess P in manure is a huge challenge. The dairies in these localities are predominantly small and are key drivers of local economies. About 1,928 (averaging 60 milking cows per farm) and 321 (averaging 80 cows per farms) facilities are located in Lancaster county and central Shenandoah Valley, respectively (USDA NASS, 2014). If implemented on farms, our project will help remove the excess P and allow the continued use of manure as a fertilizer in areas that may be restricted by the P-based manure application regulations. Also, it will help prevent over application of P, which is a typical result of applying manure to meet crop N needs.

What did we do? 

We use the “designer manures” approach as a guiding philosophy to manure nutrient management. Our working definition of designer manure is “a manure based product with the appropriate balance of N and P fertilizer values that meet specific requirements of different crops”. We conducted lab scale tests using chemical salts of aluminum, iron and calcium to identify the most effective chemical and method to recover manure P in scraped dairy manure. In the state of Virginia, the total solids (TS) content of manure from most dairies ranges between 4 to 6%. This TS range requires dilution to bring the levels to about 2% for chemicals to precipitate manure effectively. Dilution was achieved using supernate (process liquid) to dilute 4% TS dairy manure to recover P. Our lab studies have shown aluminum to be the best chemical.

What have we learned? 

Some of our results are shown in Figure 1 for aluminum chloride and aluminum sulfate. Using 600 mg Al/L we are able to remove over 95% of the total P (supernate liquid). No significant accumulation of metal salts or solids was observed in the supernate even after eight passes. Iron based chemicals produced too much foam and the resulting sludge with very poor settling characteristics. This project has the potential of concentrating manure P into small volumes and quantities that can be transported off farm and positively impact the nutrient management practices on farms.

Figure 1 total solids


Future Plans

Chemical precipitation has not been widely adopted for agricultural purposes because (1) they require a very dilute manure stream and require some sort of automated application, (2) cost of the chemicals may be high and not enough expertise on the farm to handle chemicals, and (3) solids and nutrients precipitated still need to be managed appropriately. We are actively seeking funds to install a demonstration unit on a farm where producers and other interested parties can come learn how to overcome these challenges and experience the technology.


Jactone A. Ogejo, Assoc. Prof arogo@vt.edu

Sampath Karunarathne, Graduate Student; Madonna Yoder, Research Assistant

Additional information

DeBusk, J.A., J.A. Ogejo, K.F. Knowlton, N.G. Love. 2008. Chemical phosphorus removal for separated flushed dairy manure. Applied Engineering in Agriculture 24(4): 499-506

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