Removal of suspended solids from biogas slurry through alkaline calcium precipitation

Biogas Slurry (BS) contains large amounts of suspended solids (SS) that negatively impact the subsequent biological nitrogen removal process. Conventional coagulation methods, which require large quantities of coagulants, often fail to effectively treat ultra-stable BS colloids, limiting their effectiveness and hindering sustainable plant operation. In this work, a simple and efficient alkaline calcium precipitation method was developed. This method involves adjusting the pH and adding low-cost CaCl<sub>2</sub>. At pH 12, a CaCl<sub>2</sub> dosage of 20 mg/L provided the best results for SS removal. The floc sedimentation velocity was 0.0468 m/h, and the SS removal rate reached 47.47%. Additionally, the removal rates of NH<sub>4</sub><sup>+</sup>-N and PO<sub>4</sub><sup>3-</sup> reached 48.18% and 98.90%, respectively. The analysis of the formed flocs showed that calcite was the active species involved in SS removal. The mechanism involves the neutralization of the negative charge on SS by Ca<sup>2+</sup>, which destabilizes the colloid. At pH 12, bicarbonate in BS was converted to carbonate, which promoted calcite formation and contributed to SS capture. This approach provides a practical and economical pretreatment for BS that reduces reliance on toxic coagulants, offering a potentially more sustainable alternative for biogas plants where sludge valorization is feasible.