Efficient Disposal of the Aqueous Products of Wet Organic Waste Hydrothermal Carbonization by Paddy Constructed Wetlands

The hydrothermal carbonization (HTC) technology has recently been broadly applied. However, disposal of the accompanying high-strength HTC aqueous products (HCAPs) largely determines the promotion and application of the HTC technology. The direct discharge of HCAPs might deteriorate the water environment, and previous studies have reported that HCAPs were insecurely removed via anaerobic fermentation or microalgae production. In this work, wet organic waste HCAPs were characterized and identified, and their removal efficiency in a paddy constructed wetland (PCW) was determined. The HCAPs were found to be rich in nutrients and mainly composed of humic acid (77.8-84.7%) or lignin (44.8-68.2%). With increasing HTC temperature, the relative humic acid content in HCAPs decreased by 4.4-6.4%, as determined by the three-dimensional fluorescence excitation-emission matrix (3D-EEM) analysis. HCAPs also underwent decomposition and polymerization due to the change in MWw and Al-mod as determined by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis. The PCW removal efficiency of NH4+-N and chemical oxygen demand (COD) reached 99.2 and 93.2% in the HCAP treatments, respectively, which were much higher than the values obtained for traditional disposal applications. On the basis of the higher soil NH4+-N content (1.6- to 6.0-fold), larger grain total nitrogen (TN) content (5.1-18.6%), and lower fluorescence intensity (5.7-88.2%), three primary processes (i.e., uptake by rice plants, microbial degradation, and soil immobilization) were proposed as PCW removal mechanisms. This study provides novel utilization prospects for the resource utilization of HCAPs in PCW systems.

Automated summary

The disposal of high-strength hydrothermal carbonization aqueous products (HCAPs) is crucial for the wide application of HTC technology. This study characterized and identified HCAPs from wet organic waste and determined their removal efficiency in a paddy constructed wetland (PCW). The results showed that PCW had higher removal rates for NH4+-N and COD compared to traditional disposal methods. This study provides new prospects for the resource utilization of HCAPs in PCW systems.