From Toilet to Wheels: Powering Transportation with Human Waste

Every person produces 1.1 to 2.25 kilograms of human waste each day, and with the global population rising, this creates a growing environmental challenge. After we flush, waste travels to large treatment plants for physical, chemical, and biological processing. The treated liquid is released into rivers, lakes, or the ocean, while the remaining solids become wastewater sludge, which still requires further treatment.This sludge contains high levels of organic matter, disease‑causing microorganisms, heavy metals, and micro‑pollutants such as hormones, pharmaceuticals, and flame retardants. As populations grow, sludge volumes increase, and traditional biological treatment methods, such as anaerobic digesters (AD) that produce biogas, can no longer keep up. These systems are also ineffective at removing micro‑pollutants, allowing many chemicals to persist in the environment.To address this challenge, a recently published study examined hydrothermal liquefaction (HTL), a technology that uses high temperature and pressure to break down wet organic waste. HTL mimics the natural formation of petroleum, but in minutes instead of millions of years. Its main product, biocrude oil, can be refined into cleaner fuels, including sustainable aviation fuel. The liquid by‑product contains additional organics that can generate biogas when HTL is paired with AD, and the solid by‑product, hydrochar, is rich in phosphorus and can be used as a soil amendment.While HTL has long been tested for microalgae and plant biomass, municipalities are now exploring it as a solution for wastewater sludge. The study showed that an integrated HTL–AD system can recover up to 85% of the bioenergy in municipal sludge, nearly double the ~45% achieved by conventional AD alone. Importantly, the optimized HTL conditions (332°C, 17 minutes) remained robust despite regional and seasonal variations in sludge composition.These findings highlight HTL–AD as a strong and resilient approach for maximizing energy recovery and improving pollutant degradation in modern wastewater treatment.