Phosphorus and nitrogen forms in liquid fraction of digestates from agricultural biogas plants

The novelty of the presented research is the determination of the nitrogen and phosphorus fraction in the liquid fraction of digestate from agricultural biogas plants. This information is important because it can help in proposing possible further liquid fraction management or developing of new technologies for their purification. The research covered digestates from agricultural biogas plants, obtained from the fermentation of three different groups of waste, i.e. agricultural lignocellulosic waste (where corn silage was a monosubstrate or a dominant co-substrate), food waste (where fruit and vegetable waste or distillery residue were monosubstrate or dominant co-substrate) and livestock manure (where cow manure was a monosubstrate or a dominant co-substrate). Concentrations of nutrients in the liquid fraction of digestates varied within a wide range (230.9–649.1 mg PO₄³⁻/L and 1363–3211 mg N/L), and their content was determined by the feedstock characteristic. The highest concentrations of organic phosphorus were found in the liquid fraction of digestates from the fermentation of distillery brew and livestock manure, and the lowest in the fermentation of fruit and vegetable waste. In the liquid fraction of digestates from agricultural biogas plants, regardless of the composition of the feedstock, the dominant nitrogen form was ammonium nitrogen (from 60% to 90% TN). Organic nitrogen was dominated by CON fraction, which was from 35% to 54% of ON. It was 1.3–1.6 times higher than the DON fraction.