Techno-economic analysis of a combined anaerobic digestion and hydrothermal carbonization system

Sewage sludge is the solid byproduct from wastewater treatment plants, 90% of it is sent to the landfill or incinerated, and 10% is treated by anaerobic digestion (AD). AD produces biogas and digestate, which can be treated by hydrothermal carbonization (HTC) at high temperatures and pressures to produce hydrochar. The objective was to assess the techno-economics of treating sewage sludge and producing hydrochar for soil amendment through a combined AD-HTC system. HTC of digestate from sewage sludge was conducted between 180°C to 260°C for 30 to 50 minutes. Parameters evaluated included temperature, time, and initial pH; the response variables included hydrochar yield and elemental composition. To determine the economic feasibility of the combined AD-HTC system, a techno-economic analysis was performed. The hydrochar produced along with other chars were amended to soil at different rates. Soil responses, lettuce emergence, and biomass production were analyzed based on char sources and rates. Temperature was the most influential parameter in producing hydrochar. Higher temperatures resulted in lower hydrochar yields. The capital investment for the combined AD-HTC system was ~$32 M, with a payback time of <6 years and an operating cost of $1,382/ton of hydrochar. Soil amended with hydrochar had higher pH, phosphorus content, and cation exchange capacity compared to other chars. All char types resulted in germination rates higher than 80%, as well as higher dry-weight values. The treatment of sewage sludge and production of hydrochar for soil amendment through a combined AD-HTC system has the potential to be technically and economically feasible.