Optimizing resource efficiency through hydrothermal carbonization and engineered wetland systems: A study on carbon sequestration and phosphorus recovery potential

Engineered wetland systems (EWS) are globally recognized as efficient wastewater treatment solutions with the potential for sustainable biomass production. The need arises to understand hydrothermal carbonization's (HTC) potential in nutrient and resource recovery from this biomass. While willows are well-studied in bioenergy, their utility in HTC remains understudied. We investigated the unexplored HTC of EWS-sourced willows. Here, we demonstrate that combining the robust biomass yield of a zero-discharge willow system—56 t DM⋅ha−1 over a three-year rotation—with HTC yields hydrochars, enabling phosphorus recovery (13–52 kg P⋅ha−1) and carbon sequestration (9–17 t C⋅ha−1). In a broader perspective, 10-ha willow systems can sequester about 117-tons of carbon annually, with hydrochars contributing 90-tons and an additional 27-tons attributed to the root system. Moreover, recirculating HTC-process water through hydrochar can increase the recovery to about 130–280-kg of phosphorus annually. We noted HTC-reactivity variations between a natural Salix variant and a wastewater-optimized clone. We used custom-engineered reactors and a two-factor experimental design that varied the HTC temperatures (225, 250, 275, and 300 °C) and reaction times (10, 20, 30, and 60 min), maintaining a 1:10 dry biomass-to-water mass ratio and heating rate at ∼59 °C⋅min−1. Hydrochars show improved fuel properties and energy content—Higher heating value of 22–28 MJ⋅kg−1, aligning closely with mineral coals. These hydrochars also showcased enhanced thermal stability, heightened aromaticity, and increased fixed carbon content (14–45 %). Remarkably, phosphorus densification in hydrochars reached up to 100 % during HTC reactions. Chemical P-extractions indicated a high P-bioavailability in the willow-woodchips and a low potential for phosphorus leaching in hydrochars, with 87 % of total-P extractable using HCL. Our findings offer a preliminary framework for bridging EWS in the domains of hydrothermal processes as an opportunity for future integration in biorefineries. [Display omitted] •Engineered wetland yield up to 31 t DM⋅ha−1⋅a−1 of biomass and efficiently treat wastewater.•TGA suggests hydrochars from a 10-ha willow may sequester 90 t C⋅a−1, with ≥10 % expected error.•High aromaticity and up to 45 % fixed carbon in willow hydrochars.•Remarkable phosphorus densification with near-complete phosphorus retention in willow hydrochars.