Effectiveness of Torrefaction By-Products as Additive in Vacuum Blackwater under Anaerobic Digestion and Economic Significance

Abstract

Blackwater (BW) is a vital source of bio-energy and nutrients for the sustainable development of human society in the future owing to its organic and nutrient-rich properties. Therefore, biomass and water must be used properly to avert environmental challenges and promote the viable development of nutrient recovery and bioenergy production. Moreover, vacuum-collected BW (VCBW) as a renewable source can offer outstanding potential in bioenergy and nutrition sustainability. This review reports previous and present investigations on decentralized wastewater, water conservation, the recovery of nutrients, and the ecological implications and economic significance of integrating torrefaction with anaerobic digestion (AD), notably the continuous stirred tank reactor. The mixtures (torrefied biomass and VCBW) can be converted into valuable materials by combining torrefaction and AD technology for environmental and economic gains. This way, the heat and energy used in the process could be reused, and valuable materials with high energy contents could be obtained for financial gain. The economic evaluation shows that the minimum selling price of the torrefied biomass to reach breakeven could be reduced from 199 EUR/t for standalone torrefaction to 185 EUR/t in the case of torrefaction integrated with AD. The concept can be applied to an existing waste- or wastewater-treatment facility to create a cleaner and more efficient BW with biomass recycling. However, a comprehensive techno-economic analysis must be conducted: (1) Application of tor-biochar towards vacuum BW in AD process is feasible; (2) Digestate as a soil conditional to improve soil condition is effective; (3) Mesophilic and thermophilic conditions are applicable on AD vacuum BW; (4) Economic significance indicates technological feasibility.