Silicate mediated simultaneous in-situ CO₂ sequestration and nutrients removal in anaerobic digestion

This study investigated the reactions among CO₃^2-, PO₄^3-, NH₄⁺, Mg²⁺, and Ca²⁺, under different CO₃^2- concentration and Mg²⁺/Ca²⁺ ratio, and conducted sludge anaerobic digestion (AD) with silicate addition to achieve in-situ CO₂ sequestration and nutrients removal. High CO₃^2- concentration facilitated the formation of MgNH₄PO₄, and Mg²⁺/Ca²⁺ ratio of 1:1 achieved best CO₃^2-, PO₄^3-, and NH₄⁺ removal in simulated anaerobic digestate. Supplementation of 40 g/L magnesium silicate combined with 20 g/L wollastonite decreased CO₂ content in biogas from 28.2% to 19.0%, and removed PO₄^3- and NH₄⁺ by 61.8% and 21.2%, respectively, in AD. Simultaneous in-situ CO₂ sequestration and nutrients removal was achieved by directed precipitation of PO₄^3-, NH₄⁺, and CO₂ with silicate released Mg²⁺ and Ca²⁺, to form MgNH₄PO₄ and CaCO₃. Meanwhile, methane production was improved by 51.2% with silicate supplementation. This study provides an attractive measure for CO₂ and nutrients removal as well as methane production enhancement of sludge AD.