Co-hydrothermal carbonization of agricultural waste and sewage sludge for product quality improvement: Fuel properties of hydrochar and fertilizer quality of aqueous phase.

Hydrothermal carbonization (HTC) is a promising carbon-neutral technology for converting sewage sludge (SS) and agricultural waste into energy. However, HTC-generated aqueous phase (AP) impedes the development of the former. This study investigated the potential of SS with rice husk (RH) and wheat straw (WS) co-HTC to form hydrochar and AP as substitutes for fuel and chemical fertilizer, respectively. Compared with single SS hydrochar, the yield of co-HTC-based hydrochar and higher heating value significantly increased by 10.9%-21.6% and 4.2%-182.7%, reaching a maximum of 72.6% and 14.7 MJ/kg, respectively. Co-HTC improves the safe handling, storage and transportation, and combustion performance of hydrochar. The total nitrogen concentration in AP-SS was 2575 mg/L, accounting for 67.7% of that found in SS. Co-HTC decreased and increased the amine and phenolic components of AP, respectively. AP-SS-RH and AP-SS-WS significantly increased pakchoi dry weight by 45.5% and 49.4%, respectively, compared with AP-SS. The results of the hydroponic experiments with AP instead of chemical fertilizers revealed that AP-SS did not reduce pakchoi dry weight by replacing <20% chemical fertilizers. However, AP-SS-RH or AP-SS-WS replaced 60% chemical fertilizers. Therefore, the co-HTC of SS and agricultural waste increased the AP substitution of chemical fertilizer from 20% to 60%. These findings suggest that the co-HTC of agricultural waste with SS is a promising technology for converting SS into renewable resource products for fuels and N-rich liquid fertilizer while significantly improving fuel and fertilizer quality.

Effects of persulfate-assisted hydrothermal treatment of municipal sludge on aqueous phase characteristics and phytotoxicity.

Hydrothermal technology (HT) has received much attention in recent years as a process to convert wet organic waste into hydrochar. The aqueous phase (HTAP) produced by this process is still a burden and has become a bottleneck issue for HT process development. In this study, we provide the first investigation of the HTAP characteristics, phytotoxicity, and their correlation with persulfate (PS) (PS, 2.0 mmol/g TS)-assisted municipal sludge HT. The results showed that PS accelerated the hydrolysis of protein substances and increased the concentration of NH4+ by 13.4% to 190.5% and that of PO43- by 24.2% to 1103.7% in HTAP at hydrothermal temperatures of 120 to 240 °C. PS can reduce the phytotoxicity of HTAP by reducing aldehydes, ketones, N heterocyclic compounds, and particle size and by increasing its humification index. The maximum values of the root length and biomass of pakchoi (Brassica chinensis L.) seedlings occurred when electrical conductivity was 0.2 mS/cm of HTAP. This work provided a new strategy for the selection and design of HTAP management strategies.

Innovative insights into organic nitrogen degradation through protein family domains analysis in chicken and pig manure composting using metagenomic sequencing.

The nitrogen loss in composting is primarily driven by the transformation of organic nitrogen, yet the mechanisms underlying the degradation process remain incompletely understood. This study employed protein family domains (Pfams) analysis based on metagenomic sequencing to investigate the functional characteristics, key microorganisms, and environmental parameters influencing organic nitrogen degradation in chicken manure and pig manure composting. 154 Pfams associated with ammonification function were identified. Predominant Pfams: proteolytic peptidase, followed by chitin/cell wall degraders, least involved in nucleic acid degradation. Ammonifying microbial diversity was basically consistent among compost types, particularly in the thermophilic stage with the peak of abundance of dominant ammonifying microorganisms. Viruses played an important role in ammonification process, especially Uroviricota. 26 key ammonifying genera were identified by the microbial network. pH dominated the metabolic activity of ammonifying microorganisms in various manure compost types, primarily consisting of protein-degrading bacteria with stable community structures.

Effect of aqueous phase from hydrothermal carbonization of sewage sludge on heavy metals and heavy metal resistance genes during chicken manure composting.

Livestock manure is often contaminated with heavy metals (HMs) and HM resistance genes (HMRGs), which pollute the environment. In this study, we aimed to investigate the effects of the aqueous phase (AP) produced by hydrothermal carbonization (HTC) of sewage sludge (SS) alone and the AP produced by co-HTC of rice husk (RH) and SS (RH-SS) on humification, HM bioavailability, and HMRGs during chicken manure composting. RH-SS and SS increased the humic acid content of the compost products by 18.3 % and 9.7 %, respectively, and significantly increased the humification index (P < 0.05) compared to the CK (addition of tap water). The passivation of HMs (Zn, Cu, As, Pb, and Cr) increased by 12.17-23.36 % and 9.74-15.95 % for RH-SS and SS, respectively, compared with that for CK. RH-SS and SS reduced the HMRG abundance in composted products by 22.29 % and 15.07 %, respectively. The partial least squares path modeling results showed that SS and RH-SS promoted compost humification while simultaneously altering the bacterial community and reducing the bioavailability of metals and host abundance of HMRGs, which has a direct inhibitory effect on the production and distribution of HMRGs. These findings support a new strategy to reduce the environmental risk of HMs and HMRGs in livestock manure utilization.

Co-hydrothermal carbonization of municipal sludge and agricultural waste to reduce plant growth inhibition by aqueous phase products: Molecular level analysis of organic matter.

The organic matter molecular mechanism by which combined hydrothermal carbonization (co-HTC) of municipal sludge (MS) and agricultural wastes (rice husk, spent mushroom substrate, and wheat straw) reduces the inhibitory effects of aqueous phase (AP) products on pak choi (Brassica campestris L.) growth compared to HTC of MS alone is not clear. Fourier-transform ion cyclotron resonance mass spectrometry was used to characterize the differences in organic matter at the molecular level between AP from MS HTC alone (AP-MS) and AP from co-HTC of MS and agricultural waste (co-Aps). The results showed that N-bearing molecules of AP-MS and co-Aps account for 70.6 % and 54.2 %-64.1 % of all molecules, respectively. Lignins were present in the highest proportion (56.3 %-78.5 %) in all APs, followed by proteins and lipids. The dry weight of co-APs hydroponically grown pak choi was 31.6 %-47.6 % higher than that of the AP-MS. Molecules that were poorly saturated and with low aromaticity were preferentially consumed during hydroponic treatment. Molecules present before and after hydroponics were defined as resistant molecules; molecules present before hydroponics but absent after hydroponics were defined as removed molecules; and molecules absent before hydroponics but present after hydroponics were defined as produced molecules. Large lignin molecules were broken down into more unsaturated molecules, but lignins were the most commonly resistant, removed, and produced molecules. Correlation analysis revealed that N- or S-bearing molecules were phytotoxic in the AP. Tannins positively influenced the growth of pak choi. These results provide new insights into potential implementation strategies for liquid fertilizers produced from AP arising from HTC of MS and agricultural wastes.

Nitrogen loss, nitrogen functional genes, and humification as affected by hydrochar addition during chicken manure composting.

This study investigated the effect of hydrochar addition on nitrogen (N) transformation, N functional genes, and humification during chicken manure composting. The addition of 10 % hydrochar reduced cumulative ammonia (NH3) and nitrous oxide emissions by 55.24 % and 45.30 %, respectively, and N losses by 32.07 %. Further, it increased the relative abundance of amoA while decreasing that of nirK, nirS, and nosZ in compost. Hydrochar reduces NH3 emissions during composting owing to its acid-carbon properties that lower the pH of the composting pile and promote ammonia oxidation. Moreover, hydrochar addition enhances the humification of the composting pile and significantly increases the content of humic substances. Moreover, after hydrochar addition, the germination index of the compost product reached >80 % 10 days earlier. The results demonstrate that hydrochar is a suitable composting additive for reducing N loss and shortening the composting time.

Energy and nutrient recovery by spent mushroom substrate-assisted hydrothermal carbonization of sewage sludge.

Hydrothermal carbonization (HTC) has been recognized as a promising sewage sludge (SS) treatment technology for effective pathogen elimination, bioenergy recovery, organic contaminant destruction and volume reduction. However, the solid product (hydrochar) of SS after HTC as fuel has the problems of high ash content, high nitrogen content and low calorific value. The aqueous phase (AP) produced is still considered a burden and has become a bottleneck in the development of HTC. In this study, co-HTC of SS with spent mushroom substrate (SMS) is conducted, and the fuel properties of hydrochar and the quality of the AP as a liquid fertilizer are investigated. In comparison with hydrochar of single SS, the energy yield and higher heating value of the hydrochar from co-HTC were significantly increased by 12.1-44.8 % and 33.2-137.8 %, respectively, reaching their maximum of 72.75 % and14.98 MJ/kg, respectively. Co-HTC can improve safe handling, storage and transportation, and combustion performance of hydrochar. Furthermore, the AP of co-HTC could significantly increase the biomass of pakchoi, which was 140.9 % and 90.7 % of AP from single SS and Hoagland nutrition solution (represents commercial fertilizer), respectively. The AP of co-HTC as fertilizer can recover 62.03-64.65 % nitrogen from SS and SMS. These findings suggest that co-HTC of SMS with SS is a promising technology for the conversion SS into renewable resource products for fuels and N-rich liquid fertilizer while also significantly improving fuel and fertilizer quality.

Effect of hydrochar on antibiotic-resistance genes and relevant mechanisms during chicken manure composting.

The reduction of enhanced antibiotic resistance genes (ARGs) in compost is important to mitigate the risk of ARG transmission in agricultural production. Hydrochar is used in many applications as a functional carbon material with adsorption and catalytic properties. This study investigated the effects of hydrochar addition on bacterial communities, mobile genetic elements (MGEs), and ARGs in chicken manure composting. The addition of 2%, 5%, and 10% hydrochar (dry weight) reduced the total numbers of target ARGs and MGEs in the compost products by 40.13-55.33% and 23.63-37.23%, respectively. Hydrochar changed the succession of the bacterial population during composting, lowering the abundance of potential pathogens and promoting microbial activity in amino acid and carbohydrate metabolism. A significant possible microbial host for ARGs was found to be Firmicutes. Hydrochar was found to affect the host microorganisms and MGEs directly by altering environmental factors that indirectly impacted the ARG profiles, as shown by partial least squares pathway modeling analysis. In conclusion, the addition of hydrochar to compost is a simple and effective method to promote the removal of ARGs.

Macrogenomic analysis of the effects of aqueous-phase from hydrothermal carbonation of sewage sludge on nitrogen metabolism pathways and associated bacterial communities during composting.

The effects of aqueous phases (AP) formed from hydrothermal carbonation of sewage sludge (with or without rice husk) as moisture regulators of nitrogen metabolism pathways during composting are currently unclear. Macrogenomic analyses revealed that both APs resulted in notably changes in bacterial communities during composting; increased levels of nitrogen assimilation, nitrification, and denitrification metabolic pathways; and decreased levels of nitrogen mineralization metabolic pathways. Genes associated with nitrogen assimilation and mineralization accounted for 34-41% and 32-40% of the annotated reads related to nitrogen cycling during composting, respectively, representing them as the most abundant nitrogen metabolism processes. The gudB and norB were identified as key genes for nitrogen mineralization and nitrous oxide emission, respectively. This research offers a better understanding of the effects of additional nitrogen sources on nitrogen metabolism pathways during composting.

The transformation of different dissolved organic matter subfractions and distribution of heavy metals during food waste and sugarcane leaves co-composting.

As a heterogeneous fraction, dissolved organic matter (DOM) in a compost is the most active because of its direct supply of energy sources for microbes. Also, the transfer and distribution of heavy metals in the DOM fraction attract many attentions of researchers. To this end, the dynamics of humic acids (HA), fulvic acids (FA), hydrophobic neutrals (HoN), and hydrophilic (Hi) fractions derived from DOM was investigated in this study, and the transformation of different DOM subfractions and distribution of heavy metals during food waste and sugarcane leaves co-composting were assessed by excitation-emission matrix fluorescence (EEM-FL) and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The results revealed that HA transformed from polycyclic aromatic humic acid-like to polycarboxylic humic acid-like and FA changed from soluble microbial products (SMP) to humic acid-like; Hi and HoN composed mainly of SMP substances; FA showed more abundant compositions, such as SMP, humic acid-like and tryptophan. Heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) were redistributed among different DOM subfractions in the thermophilic phase of composting. Compost DOM and its subfractions showed obvious effects on germination index (GI), biomass, root length, shoot length and healthy index of Chinese cabbage seedlings. These findings shed some novel lights into the dynamic composition and characteristics of DOM subfractions and their impacts on heavy metals distribution in a composting process.

Exploring the characteristics of dissolved organic matter and succession of bacterial community during composting.

The objective of this study was to explore the relationships among physico-chemical parameters, dissolved organic matters (DOM), and bacterial community during composting to better understand composting performances. The results showed total Kjeldahl nitrogen (TKN) (57%), temperature (39%), and pH (3%) were main factors driving the succession of bacterial communities. Firmicutes was a crucial phylum degrading organic matters for DOM formation, whereas the aromaticity and humification of DOM were closely related to Luteimonas (R2 = 0.971, p < 0.05) and Sphingobacteriaceae (R2 = 0.931, p < 0.05). Additionally, total phosphorus (TP), total potassium (TK), and TKN increased by 34.84%, 43.66%, and 65.91%, respectively, while organic matter decreased by 61.79%. The final compost had a C/N of 6.91 (<15) and a germination index of 97.81% (>80%), indicating that compost reached maturity and could be safely applied for soil amendment.

The properties and dynamic changes of DOM subfractions during food waste and sugarcane leaves co-composting.

The purpose of this research was to evaluate the properties and dynamic changes of humic acids (HA), fulvic acids (FA), hydrophobic neutrals (HoN), and hydrophilic (Hi) fractions of dissolved organic matter (DOM) during food waste and sugarcane leaves co-composting process. The pools of HA, FA, HoN, and Hi were separated from DOM by fractionation method, and characterized using spectroscopic (UV-vis, FTIR) and pyrolysis-GC/MS analyses. The least peaks in the HA pool were found in FTIR spectra with the simple structure in HA. The highest value of SUV254 was observed in HA, indicating that the HA pool played a dominant role in aromaticity of DOM. Hydrophobic compounds (HA, FA, HoN) had higher percentages of alkanes and cyclo-alkanes at the end of composting, while lower contents in the Hi pool. Both DOM and its subfractions increased the Chinese flowering cabbage (Brassica campestris L.) seed germination rate (SGR), whereas HA had a significant effect on promoting the root growth.

Properties and evolution of dissolved organic matter during co-composting of dairy manure and Chinese herbal residues.

Composting is an effective method in treating solid organic wastes, in which dissolved organic matter (DOM) plays an important role in transformation of organic matter and microbial activity. Therefore, an understanding of the properties and evolution of DOM during composting is crucial. In this study, DOM was studied using elemental analysis, spectroscopic analysis (UV-vis, FTIR, and pyrolysis-GC/MS), and colloidal analysis during a 120-day composting. Results showed that the content of N and O in DOM increased while C and H content declined progressively over the composting time. Aliphatic C-H stretching, aromatic C=C or C=O stretching of amide groups, and C-O stretch (carbohydrates) showed an obvious decrease, while COO- and C-N groups had a significant increase. The evolution of DOM indicated a gradual decrease of the lipid and polysaccharide fractions, whereas an increase of aromatic and nitrogenous compounds was observed. The DOM also showed a more stable status, and an accumulation of small molecular compounds occurred with composting proceeded. Taken together, these results shed a good insight into the properties and evolution of DOM during a composting process.

Impacts of adding FGDG on the abundance of nitrification and denitrification functional genes during dairy manure and sugarcane pressmud co-composting.

To investigate the impacts of flue gas desulphurization gypsum (FGDG) amendment on the nitrification and denitrification during composting, dairy manure and sugarcane pressmud co-composting with FGDG (CPG) and without FGDG (CP) were conducted in this work. The physico-chemical parameters and the copies of nitrification and denitrification functional genes with real-time quantitative polymerase chain reaction (qPCR) during composting were analyzed. FGDG amendment displayed an inhibitory effect on the copies of 16S rDNA and delayed the occurrence of the highest gene copies of amoA during composting. The nxrA gene copies was inhibited by FGDG amendment during the mature phase. The addition of FGDG increased the relative content of narG and nirS during composting, contributing to more NO3(-)-N being reduced to NO2(-)-N. The amoA showed significant negative correlation with OM and NH4(+)-N, and positive correlation with NO3(-)-N. The nxrA displayed a negative correlation with temperature. These results demonstrated FGDG amendment significantly affected the copies of nitrification and denitrification functional genes, which changed the nitrogen flux of composting. Taken together, these data shed an insight into FGDG amendment affecting the nitrogen transformation during composting on a molecular level.

The influence of flue gas desulphurization gypsum additive on characteristics and evolution of humic substance during co-composting of dairy manure and sugarcane pressmud.

For the purpose of evaluating the effect of flue gas desulphurization gypsum (FGDG) additive on characteristics and evolution of humic substance (HS) during composting, HS from composts with FGDG (CPG) and without FGDG (CP) were extracted and assessed with respect to their particle size, elemental analysis, FTIR and UV-vis spectroscopy, and the molecular composition of HS was characterized via pyrolysis-GC/MS as well. The particle size of HS ranged between 300 and 600nm, representing a bimodal distribution. As composting proceeded, the C/H of HS increased, and C/N decreased. The FTIR and UV-vis spectroscopy indicated that the aromatization of HS was promoted over the composting process. Adding FGDG increased the unsaturated degree and aromatization of HS. Pyrolysis-GC/MS showed the level of alkane decreased, and the level of benzene and nitrogen compounds increased upon the addition of FGDG. The nitrogen compounds of HS in CPG was significantly higher than that in CP.