EFFICIENT CONTAMINANT REMOVAL FROM LIQUID DIGESTATE OF PIG MANURE BY CHEMICAL PRECIPITATION AND CO$lt;inf$gt;2$lt;/inf$gt; MINERALIZATION USING ALKALINE ASH

Abstract

● LFD was treated by fly ash-based chemical precipitation and CO₂ mineralization. ● > 93% COD and > 98% TP removal efficiency, and < 2 mS·cm⁻¹ EC was achieved. ● COD and TP removal was achieved by co-precipitation during CO₂ mineralization. ● CO₂ mineralization neutralized the alkaline LFD and removed heavy met. Chemical precipitation is a widely applied approach for a liquid fraction of digestate (LFD) of agricultural waste but its large-scale application requires low-cost and efficient precipitating agents and novel process design. This study evaluated novel approach for the efficient removal of contaminants from the LFD using fly ash-based chemical precipitation, followed by filtration and CO₂ mineralization. The technical feasibility of this approach was evaluated using pH and electrical conductivity (EC), and removal efficiencies of total phosphorus (TP), chemical oxygen demand (COD) and heavy metals during the treatment. The fly ash used in this study showed a promising performance as a chemical precipitation agent for COD and TP removal from the treated LFD involving complex effects of precipitation and adsorption. CO₂ bubbling after fly ash-based chemical precipitation provided further COD and TP removal by carbonation reactions between CO₂ and the excessive alkaline minerals in fly ash. Although addition of fly ash to untreated LFD increased pH from 8.3 to 12.9 and EC from 7.01 to 13.7 mS·cm⁻¹, CO₂ bubbling helped neutralize the treated LFD and reduce the EC, and concentrations of toxic ions by carbonation reactions. The fly ash-based chemical precipitation and CO₂ mineralization had > 93% COD and > 98% TP removal efficiencies, and resulted in an EC of < 2 mS·cm⁻¹ and a neutral pH in the treated LFD, as well as the high purity calcite product.