Biomass-Derived Carbonaceous Materials with Graphene/Graphene-Like Structures: Definition, Classification, and Environmental Applications.

Biomass-derived carbonaceous materials with graphene/graphene-like structures (BGS) have attracted tremendous attention in the field of environmental remediation. The introduction of graphene/graphene-like structures into raw biochars can effectively improve their properties, such as electrical conductivity, surface functional groups, and catalytic activity. In 2021, the International Organization for Standardization defined graphene as a "single layer of carbon atoms with each atom bound to three neighbours in a honeycomb structure". Considering this definition, several studies have incorrectly referred to BGS (e.g., biomass-derived few-layer graphene or porous graphene-like nanosheets) as "graphene". The definitions and classifications of BGS and their applications in environmental remediation have not been assessed critically thus far. Comprehensive analysis and sufficient and robust evidence are highly desired to accurately determine the specific structures of BGS. In this perspective, we provide a systematic framework to define and classify the BGS. The state-of-the-art methods currently used to determine the structural properties of BGS are scrutinized. We then discuss the design and fabrication of BGS and how their distinctive features could improve the applicability of biomass-derived carbonaceous materials, particularly in environmental remediation. The environmental applications of these BGS are highlighted, and future research opportunities and needs are identified. The fundamental insights in this perspective provide critical guidance for the further development of BGS for a wide range of environmental applications.

[Main Influencing Factors and Strengthening of Anaerobic Transformation of Excess Sludge in China].

The anaerobic conversion efficiency of excess sludge in China is significantly lower than that in developed countries. Sludge characteristics are important influencing factors in the anaerobic degradability of sludge. The typical differences between excess sludge in China and in developed countries are mainly reflected in three aspects:long sludge age, high content of micro-sized grits, and high content of metal ions. Therefore, the effects of sludge age, micro-sized grits (represented by silica particles), and metal ions on the anaerobic digestion (AD) of excess sludge were studied systematically. Quantified comparison and targeted enhancement research were also carried out. The results showed that the negative effect of grit in the influent on VS reduction of sludge during AD was not obvious, while sludge age and metal ions were the main inhibitory factors. In addition, the inhibition level of sludge age was significantly higher than that of metal ions (P<0.05). With an increase in sludge age, the inhibition level of metal ions showed a further strengthening trend. Therefore, it could be concluded that long sludge age was the most important factor limiting the degradability of excess sludge. Moreover, with thermal hydrolysis pretreatment (160℃, 0.55 MPa, 30 min), the cumulative methane yield and VS reduction of sludge with long sludge age (40 d) almost increased to the values of sludge with a sludge age of 5 d. That is, thermal hydrolysis pretreatment has the potential to break through the bottleneck of degradability of sludge caused by long sludge age.

[Enhancement of Anaerobic Methane Production by Removal of Organic-bonding Metals from Sewage Sludge].

To improve anaerobic digestion biogas yield, the removal of organic-bonding metals by EDTA pretreatment was investigated to strengthen the anaerobic biogas effect. The results showed that after EDTA pretreatment (experimental group), the organic-bonding metals were reduced from (5.09±0.57)% to (1.37±0.20)% (calculated with TS) and dissolved organic matter increased significantly (SCOD improved 627%), suggesting that this method could remove a large amount of organic-bonding metals and enhance the dissolution of organic sludge. According to the results for the apparent activation energy (AAE) of sludge organic matter dissolution, the AAE of the experiment group was 36% lower than the control group, showing that pretreatment could effectively reduce the reaction energy barrier for organic matter dissolution. In a test of anaerobic acid production lasting 16 days, the VFAs of the experiment group were higher than the control group, with the largest upgrade of 42%. In the anaerobic methane production test (BMP) that lasted for 22 days, compared with control group, the accumulated biogas production of the experiment group increased by 48%. Through further research on the dynamics of methane production, the rate-limiting step in the experiment group was methanogenesis, while that of the control group was hydrolysis, indicating that removal of organic-bonding metals by EDTA pretreatment could effectively accelerate the hydrolysis rate.

Digested sludge-derived three-dimensional hierarchical porous carbon for high-performance supercapacitor electrode.

Digested sludge, as the main by-product of the sewage sludge anaerobic digestion process, still contains considerable organic compounds. In this protocol, we report a facile method for preparing digested sludge-derived self-doped porous carbon material for high-performance supercapacitor electrodes via a sustainable pyrolysis/activation process. The obtained digested sludge-derived carbon material (HPDSC) exhibits versatile O-, N-doped hierarchical porous framework, high specific surface area (2103.6 m2 g-1) and partial graphitization phase, which can facilitate ion transport, provide more storage sites for electrolyte ions and enhance the conductivity of active electrode materials. The HPDSC-based supercapacitor electrodes show favourable energy storage performance, with a specific capacitance of 245 F g-1 at 1.0 A g-1 in 0.5 M Na2SO4; outstanding cycling stability, with 98.4% capacitance retention after 2000 cycles; and good rate performance (211 F g-1 at 11 A g-1). This work provides a unique self-doped three-dimensional hierarchical porous carbon material with a favourable charge storage capacity and at the same time finds a high value-added and environment-friendly strategy for disposal and recycling of digested sludge.

[Effects of Free Ammonia Regulation on the Performance of High Solid Anaerobic Digesters with Dewatered Sludge].

High free ammonia nitrogen (FAN) is regarded as a main factor that inhibits biogas production in high solid anaerobic digestion systems with dewatered sludge. Two mesophilic semi-continuous anaerobic digesters fed with dewatered sludge (input total solids=15%) were operated and their performance under different FAN concentrations by two regulation methods including pH reduction and total ammonia nitrogen (TAN) promotion were investigated. Results showed that when FAN was elevated from (400±174) mg·L-1 to (526±25) mg·L-1 with the increase of TAN, the daily biogas yield declined from (11.9±0.3) L·d-1 to (10.3±0.2) L·d-1. Meanwhile, volatile solids (VS) reduction rate decreased by 33.7%. When FAN dropped from (330±99) mg·L-1 to (47±13) mg·L-1 due to pH reduction, daily biogas yield declined from (14.4±1.1) L·d-1 to (10.8±0.3) L·d-1 and VS reduction rate decreased by 26.9%. The system performance was not enhanced with the reduction of FAN. With the stop of pH regulation, FAN gradually raised again to 300 mg·L-1 and digester performance recovered to the same as the situation before regulation. The result suggested that inhibition occurred during pH reduction. By 16S rRNA pyrosequencing analysis on bacterial and archaeal diversity and quantitative PCR analysis aiming at methanogens, it was found that bacterial community structure both changed obviously with obvious performance decline in the two digesters. Excessive FAN (>500 mg·L-1) could change the main bacteria species which degrade protein from Tepidimicrobium and Proteiniborus to Anaerobranca. On the other hand, some carbohydrate and protein degrading bacteria were also inhibited by pH reduction for FAN control, leading to the decline of hydrolysis rates of organic matters. As a result, the shortage of substrate supply for the methanogens weakened the biogas production of the digestion system.

[Effects of Organic Loading Rate on Startup Performance of Anaerobic Digestion with Vinegar Residues].

Biochemical methane potential experiments were conducted to investigate the effects of organic loading rate on the performance of anaerobic digesters with vinegar residues. According to the comparisons of methane production and liquid phase compositions, as well as thermogravimetry, X-ray Diffraction and infrared spectroscopy analyses, the conclusions could be drawn that:①Lower organic loading rate was better to mitigate the accumulation of VFAs and pH decrease during the hydrolysis and acidification of organic matters, which consequently improved methane production. When the inoculum to substrate ratio was 1:1[organic loading rate of 1.78 g·(L·d)-1, pH=7.60], the cumulative methane production was the highest, reaching 2249.7 mL, and the performance of the digesters was stable. The VFAs content increased with the increase of the organic loading rate, leading to the suppression and further the stop of methane production. And when the inoculum to substrate ratio was 1:4[organic loading rate of 7.12 g·(L·d)-1, pH=5.52], the simultaneous generation of acetate and lactic acids could be achieved at 8000 mg·L-1 and 2650 mg·L-1, respectively. ② As vinegar residues were short-range ordered with microcrystalline structure or mainly contained amorphous substances, they were more biodegradable than feedstocks such as corn stalk. During the anaerobic digestion processes, the degradation rates of lignin, cellulose and hemicellulose increased with the decrease of organic loading rate.

Facile synthesis of sewage sludge-derived in-situ multi-doped nanoporous carbon material for electrocatalytic oxygen reduction.

Developing efficient, low-cost, and stable carbon-based catalysts for oxygen reduction reaction (ORR) to replace the expensive platinum-based electrocatalysts remains a major challenge that hamper the practical application of fuel cells. Here, we report that N, Fe, and S co-doped nanoporous carbon material, derived via a facile one-step pyrolysis of sewage sludge, the major byproduct of wastewater treatment, can serve as an effective electrocatalyst for ORR. Except for the comparable catalytic activity with commercial 20% Pt/C via a nearly four-electron transfer pathway in both alkaline and acid medium, the as-synthesized co-doped electrocatalyst also exhibits excellent methanol crossover resistance and outstanding long-term operation stability. The organic compounds in sewage sludge act as the carbon source and the in-situ N and S dopant in the fabrication, while the inorganic compounds serve as the in-built template and the in-situ Fe dopant. Our protocol demonstrates a new approach in the economic and eco-friendly benign reuse of sewage sludge, and also provides a straightforward route for synthesizing excellent carbon-based electrocatalysts as promising candidates for ORR directly from a type of waste/pollution.

[Development of Determination Method of Fluoroquinolone Antibiotics in Sludge Based on Solid Phase Extraction and HPLC-Fluorescence Detection Analysis].

Fluoroquinolone antibiotics (FQs), as the common pharmaceuticals and personal care products (PPCPs), are widespread in the environment. FQs contained in wastewater would be ultimately enriched in sludge, posing a potential threat to the consequent sludge utilization. To optimize the analytical method applicable to the determination of FQs in sludge, the authors selected ofloxacin (OFL), norfioxacin (NOR), ciprofloxacin (CIP) and lomefloxacin (LOM) as the target FQs, and established a method which was based on cell lysis, FQs extraction with triethylamine/methanol/water solution, Solid Phase Extraction (SPE) and HPLC-Fluorescence Detection (FLD) determination. After the investigation, phosphoric acid-triethylamine was decided to be the buffer salt, and methanol was chosen as the organic mobile phase. The gradient fluorescence scanning strategy was proved to be necessary for the optimal detection as well. Furthermore, by the designed orthogonal experiments, the effects of the extraction materials, pH, and the eluents on the efficiency of SPE extraction were evaluated, by which the optimal extraction conditions were determined. As a result, FQs in liquid samples could be analyzed by utilizing HLB extraction cartridge, and the recovery rates of the four FQs were in the range of 82%-103%. As for solid samples, the recovery rates of the four FQs contained reached up to 71%-101%. Finally, the adsorptivity of the sludge from the different tanks ( anaerobic, anoxic and oxic tanks) was investigated, showing gradual decrease in the adsorption capacity, but all adsorbed over 90% of the EQs. This conclusion also confirmed that 50% removal of FQs in the domestic wastewater treatment plant was realized by sludge adsorption.

[Carbonization of heavy metal Cu implanted sewage sludge and stability of heavy metal in the resulting char].

In this research, a new method for sewage sludge (SS) disposal was introduced, by which heavy metals were implanted into sewage sludge before pyrolysis. Cu was adopted as the representative of heavy metals to test this process and was implanted in the form of CuCl2. Effects of Cu implanting concentration and reaction temperature on the residual ratio and immobilization of heavy metals in pyrolysis char were studied. Meanwhile, two leaching methods were employed with the purpose to determine the maximum capacity of heavy metal immobilization in the char. The primary research results showed that when the Cu implanting concentration was 0.5% (mass fraction), more than 90% of Cu remained in the char after carbonization, and the leachability of heavy metals in the char was related to pyrolysis temperature. Cu leaching from the char increased with increasing pyrolysis temperature. There was also a limitation for Cu implanting concentration in the sewage sludge, which was determined by the destination of the pyrolyzed char. If it went to sanitary landfill, the limitation would be 0.5%. The primary results showed that sewage sludge could be kneaded with other wastes containing heavy metals before pyrolysis to achieve co-processing.

[High-solids anaerobic co-digestion of sludge and kitchen garbage under mesophilic conditions].

At solid retention time (SRT) of 20 days, biogas production, volatile solid (VS) degradation and system stability in co-digestion systems of dewatered sludge (DS) and kitchen garbage (KG) were investigated in semi-continuous completely mixed reactors numbered R1-R5 (the DS/KG of their feeding substrate based on wet mass was 1:0, 4:1, 3:2, 2:3 and 0:1, respectively). The results showed that, with larger proportion of KG in feeding substrate, higher methane yield and biogas yield were obtained with lower methane content. For certain reactor at given SRT, KG addition could significantly improve the organic loading rate (OLR) and volume biogas production. System with more KG addition favored higher hydraulic constant k and VS reduction. The hydraulic constant k was 0.25 d(-1), 0.61 d(-1), 1.09 d(-1) and 1.56 d(-1), and the VS reduction was 37.4%, 50.6%, 60.7% and 68.2% for R1-R4, respectively, indicating higher hydrolysis rates with more KG addition, which led to increased VS reductions. With larger KG proportion in feeding substrate, pH, total alkalinity (TA), total ammonia nitrogen (TAN) and free ammonia nitrogen (FAN) showed decreasing trend. As KG addition increased by 60%, pH, TA, TAN and FAN decreased by 6%, 16%, 22% and 75%, respectively. FAN and Na+ respectively were potential inhibitory chemicals that threatened the stability of the mono-system of DS and KG. In comparison with the mono-system of DS or KG, the co-system showed higher stability by diluting toxic chemicals like ammonia or Na+ to much lower levels.

Greenhouse gases emissions accounting for typical sewage sludge digestion with energy utilization and residue land application in China.

About 20 million tonnes of sludge (with 80% moisture content) is discharged by the sewage treatment plants per year in China, which, if not treated properly, can be a significant source of greenhouse gases (GHGs) emissions. Anaerobic digestion is a conventional sewage sludge treatment method and will continue to be one of the main technologies in the following years. This research has taken into consideration GHGs emissions from typical processes of sludge thickening+anaerobic digestion+dewatering+residue land application in China. Fossil CO(2), biogenic CO(2), CH(4,) and avoided CO(2) as the main objects is discussed respectively. The results show that the total CO(2)-eq is about 1133 kg/t DM (including the biogenic CO(2)), while the net CO(2)-eq is about 372 kg/t DM (excluding the biogenic CO(2)). An anaerobic digestion unit as the main GHGs emission source occupies more than 91% CO(2)-eq of the whole process. The use of biogas is important for achieving carbon dioxide emission reductions, which could reach about 24% of the total CO(2)-eq reduction.

[Effects of mild thermal pretreatment on anaerobic digestibility of sludge with low organic content].

The effects of mild pretreatment at temperature of 100 degrees C on the solubilization anP anaerobic digestibility of high solid sludge with low organic content were studied with the variation of heating times. Experimental results show soluble organic concentrations in supernatant increase with the prolonging of thermal pretreatment time rapidly, and slowly after 30 min. The dissolution rates of COD, protein and carbohydrate with 30 min of thermal pretreatment at 100 degrees C were 10.5%, 11.6% and 8.2%, respectively. Mild thermal pretreatment not only enhanced total methane yield, but also advanced the peak time of methane production. The methane production ratio with 30 min of thermal hydrolysis was 136 mL.g-1 (VS) at day 10 of anaerobic digestion, with an 86% increase over the control group. VS reduction ratio after 30 days anaerobic digestion o also increased to 33.3% with 30 min of thermal pretreatment at 100 degrees C compared with 19.1% in control group. In addition, studies on enzymatic activity indicated the activities of four key enzymes (protease, acetokinase, phosphotransacetylase and coenzyme F420) involved in anaerobic digestion were all enhanced by mild thermal pretreatment.