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Research Summaries

Manure Treatment

Manure Economics

Research Purpose

The goal of this research is to evaluate the utilization of a struvite crystallizer for removing phosphorus (P) from liquid from an ambient-temperature, covered earthen digester for swine manure. It is desired to find the optimal conditions of pH adjustment and magnesium (Mg) addition for most efficient and economical phosphorus removal into the struvite product.

Activities

Laboratory tests have been conducted with digester liquid at 12 combinations of pH and Mg to determine potential P removal. Lab tests were followed with field tests at 24 combinations of pH and Mg. Based on the field tests conducted for 30 min. each for 24 conditions, one condition (+0.5 pH and + 40 mg Mg/L) was selected for longer-term tests each of 2-hr duration. Seventeen 2-hr tests have been conducted using anhydrous ammonia for increasing pH and six 2-hr tests have been conducted using sodium hydroxide (NaOH) for increasing pH. All tests used magnesium chloride hexahydrate MgCl2•6(H2O) for increasing Mg. Field tests were conducted at 5.4 L/min. (1.43 gal./min.) influent flow rate.

What We Have Learned

In lab tests of digester liquid using pH increases up to 1.0 pH unit and Mg increases up to 60 mg/L, the reductions of P varied from 74 % to 95 %. Field tests with a cone-shaped crystallizer using 5.4 L/min. flow rate of influent and various conditions of pH and Mg resulted in P removal up to 80 %. About 65 % of P was removed with the combination of 0.5 pH unit increase and addition of 40 mg/L of Mg. To test performance over longer periods, this combination of pH and Mg was utilized in 23 2-h tests and P reductions were consistently about 60 %.

P Reduction for 30-min. field tests with a struvite crystallizer with flow rate of 5.4 L/min.

An estimate of cost for a struvite crystallizer for a 1,000 sow farrow-to-finish operation was $0.0196/kg ($0.0089/lb) of pork produced.

Why is This Important?

Swine manure digester effluent and lagoon effluent have a higher P to N ratio than typically used for crop fertilization. The struvite crystallizer offers an option to remove P from the effluent into a product that can be easily dried and handled, and has nitrogen (N) and P content of approximately 6 % and 12 % respectively.

For More Information

P. W. Westerman, K. E. Bowers, K. D. Zering, and M. E. Adcock. 2008. Phosphorus recovery from covered digester effluent with a continuous-flow struvite crystallizer. ASABE Paper No. 083892, Presented at ASABE International Meeting, Providence, RI.

By Philip Westerman, Kelly Zering, Mike Adcock, NC State University Keith Bowers, Multiform Harvest, Inc.

This report was prepared for the 2008 annual meeting of the regional research committee, S-1032 "Animal Manure and Waste Utilization, Treatment and Nuisance Avoidance for a Sustainable Agriculture". This report is not peer-reviewed and the author has sole responsibility for the content.