Effect of particle size and doses of olivine addition on carbon dioxide sequestration during anaerobic digestion of sewage sludge at ambient and mesophilic temperatures

Biogas from anaerobic digestion of sludge at wastewater treatment plants consists of methane, carbon dioxide and trace contaminants which can be upgraded for utilization. Compared to current costly upgrading technologies, mineral carbonation has many benefits by using natural magnesium and calcium rich ores capable of sequestering CO2. The feasibility of olivine to sequester CO2 in-situ during batch anaerobic digestion of sludge was tested for (1) ambient versus mesophilic temperatures, (2) placement of olivine in the digester, and (3) olivine particle size and concentration. Increasing the temperature, increasing the olivine surface area via increased dose and decreased particle size, and elevating the olivine in the reactor increased mineral carbonation rates during anaerobic digestion. At mesophilic temperature, the elevated 5% w/v fine olivine digester had a 17.5% reduction in CO2 which equated to a 3.6% increase in methane content (%) and at ambient temperature, the same condition had a 21.7% CO2 sequestration resulting in an 8.8% increase in methane content compared to the control. Response surface methodology was applied for optimization of digestion time and olivine surface area at both temperatures.