Utilization of aqueous product generated by hydrothermal carbonization of waste biomass

Abstract

Hydrothermal carbonization (HTC) is a thermochemical treatment process that allows for the conversion of relatively dilute biomass slurries into value added products which are hydrochar and filtrate. This investigation focuses on the potential for utilization of the filtrate (aqueous by-product) created via HTC. A majority of the research to date has focused on the solid HTC product (hydrochar), however little attention has been paid to the utilization of the HTC filtrate, which makes up the larger mass fraction. Finding value added products is key to making the process a viable treatment option for waste biomass and other organic by-products. The option of using HTC filtrate as a fertilizer replacement for agricultural crop production was evaluated through studies of soil microbial effects and impacts on seed germination and early plant growth. These studies confirmed bio-toxicity effects of HTC filtrate on agricultural soil microbes at high application rates. On the other hand, lower rates of application induced biodegradation of the phytotoxic components of the filtrate and released additional plant nutrients through N-mineralization. These effects are dependent on filtrate type, concentration, and post-treatment of the applied filtrate. Phytotoxicity effects on seed germination and seedling growth of corn (Zea mays L.) also showed a dependence on HTC filtrate source and concentration. Similar to the impacts observed on the soil microbes, high concentration typically inhibited seed germination and growth, but lower concentrations stimulated early corn growth. Characterization of the filtrates via a 2-dimensional gas chromatography (GC) time-of-flight mass spectrometry confirmed a very complex chemical fingerprint of the filtrates. Chemical speciation in the filtrate appeared to be a function of the feedstock. More importantly, the simple storage of filtrate in an open container for 90 days drastically alters the chemical species composition and correspondingly the observed impact on soil microbes and plant growth, leading to the conclusion that there could be chemical inhibitors present in the filtrate that are responsible for the observed effects that are eliminated though simple volatilization or microbial mineralization during storage. This work shows great promise for utilization of HTC filtrates as an agricultural fertilizer and the recycling of critical plant nutrients. Additional work is needed to fully characterize the chemical diversity present in these filtrates prior to the implementation of this renewable and sustainable source of agricultural fertilizers.