Abstract
To maintain the waste treatment capacity of anaerobic swine lagoons, excessive accumulation of sludge in the bottom of the lagoon is traditionally removed by dredging and pumping followed by dewatering prior to land application. A widespread and low-cost method of dewatering this lagoon sludge in U.S. farms is the use of geotextile bag filtration. Because of typically high phosphorus (P) contents in lagoon sludge, the dewatered sludge is eventually hauled to distant fields to reduce the environmental risk of excess P in land nearby the lagoon. The inclusion of a P recovery approach, called Quick Wash (QW), along with the geotextile bag technology could eliminate or reduce hauling costs of dewatered sludge and produce a valuable P product for use as fertilizer. The QW process uses a novel combination of acid, base, and organic polyelectrolytes to selectively extract and recover P from manure solids. The objective of this study was to evaluate the potential advantage and technical feasibility of combining the QW process and geotextile dewatering in a system to extract and recover P from lagoon sludge. Laboratory tests results showed that the amount of recoverable P from lagoon sludge depends on the pH level obtained by acidification. The highest release of P in solution, 83% of total initial P, was obtained when the lagoon sludge was acidified to pH 3. The amount of P recovered as a precipitate with lime addition at pH 10 was about 79% of the initial total P in the sludge. A mass flow balance confirmed that about 20% of the total P remained in the dewatered sludge solids. The effluent contained extremely low soluble P concentrations. The calcium phosphate product was identified as amorphous calcium phosphate (ACP) with very low heavy metal content. The recovered ACP had P grades (33.2 to 35.5 % P2O5), higher than rock phosphate, with the advantage that there is no need for additional chemical processing for its use as fertilizer. Results of this study support the technical feasibility of using the QW process in combination with geotextile bag filtration for the economic recovery of P from anaerobic swine lagoons.
Highlights
With a total inventory of 8.9 million heads, North Carolina is the second largest swine (Sus scrofa L.) producer in the United States (NASS, 2018)
A complete economic evaluation for deployment of this technology to recover P from lagoons per year still requires taking into account the annualized energy and capital or rental cost of the equipment and potential additional income from nutrient trading programs (Shortle, 2013). This laboratory study showed that the amount of recoverable P from lagoon sludge depended on the acidic pH reached after application of sulfuric acid
The highest release of P in solution, 83% of total P, was obtained when the lagoon sludge was acidified to pH 3.0
Summary
With a total inventory of 8.9 million heads, North Carolina is the second largest swine (Sus scrofa L.) producer in the United States (NASS, 2018). This intensive swine production generates large amounts of manure which is typically treated in anaerobic lagoons (Barker, 1996). In North Carolina’s swine farms, a low-cost and widespread method of dewatering lagoon sludge is the use of geotextile bag filtration with injection of a flocculant polymer (Spearman, 2017). The inclusion of a P recovery approach along with the geotextile bag technology could: (1) eliminate or reduce hauling costs of dewatered sludge, and (2) produce a valuable concentrated P product for use as fertilizer. Since P is a finite mining resource being depleted due to its global demand for agricultural production (Gilbert, 2009; Karunanithi et al, 2015), P recovery is essential to close the P loop in production agriculture and reduce the P pollution risks due to soil leaching and runoff P losses into aquatic environments (Keyzer, 2010; Szogi et al, 2015b; Jurgilevich et al, 2016)
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