Natural coagulants enhance struvite collection efficiency after phosphorus recovery from poultry litter

Agricultural runoff is the major contributor to eutrophication. To address this problem, some have advocated for nutrient recovery from agricultural waste. We have previously reported phosphorus recovery from poultry litter using a sequencing batch reactor with CO₂-assisted nutrient extraction and NaOH-based precipitation of the slow-release struvite and potassium struvite fertilizers. In our patented process, US Patent US11104617B2, the process effluent was recycled to generate the slurry for the next batch. Pilot-scale studies suggested that precipitated particles were being recirculated during reuse of the process effluent, reducing overall recovery. The objective of this work was to improve struvite settling through the addition of natural coagulants and flocculant aids. Jar tests were conducted with liquid extracts generated from 20 g L^-1 poultry litter slurries. The solution pH was adjusted to 9.0 to precipitate struvite, and then variable chitosan and bentonite concentrations were dosed into the jars under rapid mix conditions at 120 rpm for 3 min. The particle size distributions showed that chitosan and bentonite formed larger particles. The 50th percentile struvite particle size increased from 0.07 µm without coagulant and flocculant aids to ∼400 µm with 100 mg L^-1 of chitosan and bentonite. When 500-1000 mg L^-1 of chitosan and bentonite were added, large, uniform flocs formed and settled within 1 h. The chitosan-bentonite system had optimal performance with 25 mg L^-1 chitosan and 10 mg L^-1 bentonite and 50 mg L^-1 chitosan and 50 mg L^-1 bentonite. The chitosan-alginate system generated larger flocs with 75 mg L^-1 alginate, but the addition of chitosan diminished performance. Alginate-only systems were most effective at aggregating fine struvite particles produced from poultry litter extracts and have the added benefit of providing an environmentally friendly alternative to conventional coagulants as a soil amendment that provides controlled nutrient release.