Phosphorus speciation during anaerobic digestion and subsequent solid/liquid separation

Area of Science:

• Environmental Science
• Biogeochemistry
• Waste Management

Background:

• Phosphorus (P) is a critical nutrient, and its speciation affects bioavailability and environmental fate.
• Anaerobic digestion (AD) is a key process for organic waste treatment, potentially altering nutrient forms.
• Understanding P transformation during AD is crucial for nutrient recovery and environmental risk assessment.

Purpose of the Study:

• To investigate the impact of anaerobic digestion on phosphorus (P) species.
• To determine the distribution of P species within digestate liquid and solid fractions.
• To assess the influence of ingesta composition on P speciation in digestate.

Main Methods:

• Analysis of P fractionation using sequential extraction with NaHCO3, HCl, and NaOH-EDTA.
• Identification of P species via 31P Nuclear Magnetic Resonance (NMR) spectroscopy.
• Investigation across six full-scale anaerobic digestion plants with representative biomass sampling.

Main Results:

• Anaerobic digestion showed minimal impact on overall P speciation, which was primarily dictated by ingesta P fractions.
• High NaHCO3-extractable P in ingesta correlated with higher NaHCO3 fraction in digestate, indicating inherited speciation.
• Increased pig/cow slurry in feed reduced readily soluble P and increased P bound to organic matter and Fe/Al in digestate.

Conclusions:

• Digestate P-fraction composition largely reflects ingesta P speciation, with AD having a limited direct effect on P species.
• Feed composition significantly influences P distribution, with slurry content affecting P solubility and association with solid phases.
• Orthophosphate is the dominant inorganic P species, while monophosphate esters are the primary organic P compounds in digestate.