Integration of sludge digestion and microalgae cultivation for enhancing bioenergy and biorefinery

Sludge generated from wastewater treatment plants causes severe environmental problems, which can be significantly reduced using anaerobic digestion. However, CO2, anaerobic digestate, and the residues from anaerobic digestion process still need to be treated. Accordingly, some energy-rich microalgae can grow well in sludge digestate contained wastewater, which enhances its economic feasibility for biofuel production. Therefore, in this review, the integration of sludge digestion and microalgal cultivation is proposed for enhancing the performance of bioenergy and biorefinery industries. Microalgae can simultaneously utilize sludge digestate and CO2 as the carbon sources for biofuel production and biogas upgradation. This comprehensive review mainly analyzes different compositions of anaerobic digestate, sludge pretreatment methods, favorable environmental factors, and different CO2 concentrations, which influence the growth of microalgae and the accumulation of bioenergy. Finally, the waste residues from sludge digestion and microalgae cultivation are converted to bio-gas, bio-oil, and biochar, which can be used as biofuels, supercapacitors, adsorbents, and catalysts through thermal conversion. This review indicates that the pyrolysis process has a positive net energy production and hydrothermal treatment can be chosen as the wet biomass conversion method. The aim of this review is to provide useful information for combining the sludge digestion and microalgal cultivation to simultaneously reduce the pollutants and produce bioenergy.