Phosphorus recovery as struvite from hydrothermal carbonization liquor of chemically produced dairy sludge by extraction and precipitation

Phosphorus (P) recovery from dairy wastewater involves its accumulation into phosphorus-rich sludge using a physico-chemical or biological process. The high iron content in chemical sludge decreases its usability in agricultural activities. The hydrothermal carbonization (HTC) is an option used to treat the sludge to obtain hydrochar for various applications, including its use as an energy source and as a carbon-dense material. The HTC process leaves a bigger volume of nutrient-rich liquor, which phosphorus (P) purification was the subject of this work. By direct precipitation, the product iron content was 17.96%, a value higher than accepted limits for phosphate fertilizers. Thus, P extraction followed by struvite precipitation was studied. The use of oxalic acid extracted 86.7% of P from HTC liquor, while 86.6% of iron was removed. The process conditions of pH 9, and salt dosage of 1.73:1.14:1 for Mg:NH₄⁺:P mole ratio for struvite precipitation were obtained with a P recovery of 99.96%, and the effluent P concentration below 2 mg∙L⁻¹. The quality of products as fertilizers was tested by both in-vitro and in-vivo assays. High iron content in the product demonstrated a negative effect on plant germination, whilst the precipitation product from P extract demonstrated an advantage of P purification into struvite for plant macro and micronutrient availability. The used method of P extraction followed by struvite precipitation is useful for both P and iron recovery into two separate products with agricultural and chemical applications, respectively.