Hydrothermal alkaline conversion of sewage sludge: optimization of process parameters and characterization of humic acid

Co-Thermal Oxidation of Lignite and Rice Straw for Synthetization of Composite Humic Substances: Parametric Optimization via Response Surface Methodology

In this study, Baoqing lignite (BL) and rice straw (RS), which were the representatives of low-rank coal and biomass, were co-thermally oxidized to produce composite humic substances (HS), including humic acid (HA) and fulvic acid (FA). Taking HS content as the output response, the co-thermally oxidizing conditions were optimized through single factor experiment and response surface methodology (RSM). The structures of HA and FA prepared under optimized conditions were analyzed by SEM, UV, and FTIR. Results showed that HS content was clearly influenced by the material ratio, oxidation time, and oxidation temperature, as well as their interactions. The optimized co-thermal oxidization condition was as follows: BL and RS pretreated with a material ratio of 0.53, oxidation time of 59.50 min, and oxidation temperature of 75.63 °C. Through verification, the experimental value (62.37%) had a small relative error compared to the predicted value (62.27%), which indicated that the developed models were fit and accurate. The obtained HA had a tightly packed block structure; FA had a loosely spherical shape. The molecular weight of FA was 2487 Da and HA was 20,904 Da; both had a smaller molecular weight than that reported in other literature. FA showed strong bands at 1720 cm^-1, thus confirming the presence of more oxygen-containing functional groups. The appearance of double peaks at 2900~2980 cm^-1 indicated that HA contains more aliphatic chains. The co-thermal oxidation of BL and RS gives a new method for the synthesis of HS, and the optimization of co-thermal oxidation conditions will provide fundamental information for the industrialization of composite HS.

BOF slag as a new alkalizing agent for the stabilization of sewage sludge

This study assessed, for the first time ever, the use of basic oxygen furnace (BOF) slag as alkalinizing material during the sludge conditioning, as an environmentally-friendly alternative to CaO and other conventional alkalis. Its effects on the dewatering, solubilisation and stabilization of sewage sludge were studied, testing increasing dosages of BOF from 0 to 6 gBOF/gTSS₀ at room temperature and under constant mechanical agitation was evaluated. Results revealed that the addition of BOF slag to sewage sludge produced similar degrees of solubilisation to those obtained using lime, reaching a maximum of 34% of total COD for 3.00 gBOF/gTSS₀. The use of BOF slag also involved a low solubilisation of either nitrogen, carbon or phosphorous, a negligible mobilization of heavy metals and a positive effect on its biological hygienisation. A Class A biosolid for doses of 4.50gBOF/gTSS₀ or higher was achieved, which can be applied directly to the soil for agricultural purposes in accordance with current legislation.

Hydrothermal treatment of food waste for bio-fertilizer production: Formation and regulation of humus substances in hydrochar

Food waste (FW) poses serious challenges to incineration and composting. Hydrothermal treatment (HTT) is a promising method to produce carbon-rich materials from biomass, including humus substances. In this study, FW containing cellulose, starches, and proteins was treated by HTT to study the formation and regulation of three kinds of humus (i.e., humin, humic acids [HAs], and fulvic acids [FAs]). Ultimate analysis and proximate analyses were conducted to explore the material composition, which was very similar to natural humus. Three kinds of humus were quantified. Optimal temperature (200 °C) and residence time (30 min) for production of HAs were determined based on HAs yield (14.60%). In addition, formation and regulation of humin, HAs and FAs was discussed. The amino acids, peptides, monosaccharides, and HMF obtained by hydrolysis of FW produced important precursors of humus. Moreover, the transfer of nutrient elements was revealed. Nearly 90% of K was dissolved in water. Recovery of N (60%) was relatively stable in hydrochar. Up to 67.61% of P deposited in hydrochar with 12 h.