Climate change impacts of conventional sewage sludge treatment and disposal

Applicability of temperature-phased anaerobic digestion in enhancing methanation of high-solid sludge: Process performance, microbial community analysis and energy balance assessment

High-solid anaerobic digestion has been paid more attention, expected to solve the increasing amount of sewage sludge. In order to cope with the new issues of high-solid sludge digestion, recently emerging thermophilic (stage I)-mesophilic (stage II) temperature-phased anaerobic digestion (TPAD) process was employed to probe into its applicability in enhancing methanation. High-solid sludge at a total solid (TS) of above 15 % was fed to a TPAD process and a single-stage mesophilic digestion (MD) process continuously. The increasing loadings from 3.96 g chemical oxygen demand (COD)/L/d to 8.05 g COD/L/d were set by gradually shortening hydraulic retention time from 20 d to 10 d. Methane yield could be increased from 0.11 L/g CODadded to 0.15 L/g CODadded, with 10 % higher TS removal achieved in the TPAD. The reason could be attributed to improved hydrolysis of the main fraction protein. Despite acetic acid accumulation in stage I, surplus alkalinity supply rendered acid/alkalinity ratios much lower. The interaction between the 2 stages offered more diverse microbial community, which led to intensive adaptive ability to external shocking. The density of archaea for stage II /stage I increased nearly linearly with higher organic loading. As high as around 60 % Methanosarcina became the main mesophilic archaea. The dominant functional bacteria Firmicutes in stage II was also promoted. On the premise of enhanced conversion efficiency, additional energy input from heat requirement of thermophilic stage in the TPAD was proven to be compensated by improved methane production, leading to similar or even higher net energy production with the MD.

Parameter-sensitive life cycle assessment of sludge incineration technologies integrating energy balance model

Incineration is vital for safe sewage sludge treatment and resource recovery in China, using methods like mono-incineration (INC), and co-incineration in coal plant (CINP), cement kiln (CINC), and municipal solid waste incineration plant (CINM). Existing studies suffer from poor inventory quality and inaccurate quantification. To overcome these challenges, this study integrates the energy balance model with life cycle assessment to evaluate key system parameters, quantify co-incineration disturbances, and assess environmental impacts. Findings indicate critical moisture content for combustion as 70 %, 60 %, 50 %, and 80 % for the four methods, respectively, with INC exhibiting the highest environmental impact, followed by CINM. CINP and CINC yield environmental benefits by replacing coal or raw materials, achieving negative carbon effects of 34.8 % and 78.8 %, and avoiding 66.4 % and 76.1 % of environmental impacts, respectively. When sludge moisture surpasses 75 %, co-incineration results in higher carbon emissions than INC, with lower dry calorific values potentially increasing emissions up to fourfold. The study positions CINP and CINC as transitional solutions, with CINM as the future trend, while INC suits cities with high sludge output and strong economies. This research offers a basis for developing inventories for solid waste co-incineration in industrial kilns and optimizing the selection of sludge incineration technologies.

Experimental Study on Optimizing Hydrogen Production from Sludge by Microwave Catalytic Pyrolysis Using Response Surface Methodology

Catalytic pyrolysis technology is a harmless and useful solid waste treatment method. Studying the catalytic pyrolysis of sludge for hydrogen production is of practical significance. Therefore, this paper prepared a bifunctional catalyst with both microwave absorption and catalytic properties from electroplating sludge through carbonization and acid modification processes and then was characterized by XRD, BET, SEM, XPS, and FTIR. An experimental study employed a conventional-then-microwave pyrolysis method to investigate the catalytic pyrolysis process of municipal sludge. Combining central composite design (CCD) and response surface methodology (RSM) with three factors and five levels, this paper investigated the interactive effects of the conventional pyrolysis temperature, microwave irradiation time, and catalyst addition ratio on the unit hydrogen production (UHP) of sludge. A predictive model based on a second-order polynomial regression equation was developed. The results revealed that the catalyst possesses a specific surface area and pore structure and that the second-order polynomial model fits well. The conventional pyrolysis temperature, microwave irradiation time, catalyst addition ratio, and interaction between the latter two significantly affected the UHP of sludge. The optimal operation conditions of conventional pyrolysis temperature, microwave irradiation time, and catalyst addition ratio were 462.7 °C, 8.8 min, and 12.4%, respectively. Under these optimal conditions, the UHP was 13.22 mmol/g, with a relative error of only 1.12% compared to the predicted model value of 13.37 mmol/g.

Climate change impacts of bioenergy technologies: A comparative consequential LCA of sustainable fuels production with CCUS

The use of sustainable biomass can be a cost-effective way of reducing the greenhouse gas emissions in the maritime and aviation sectors. Biomass, however, is a limited resource, and therefore, it is important to use the biomass where it creates the highest value, not only economically, but also in terms of GHG reductions. This study comprehensively evaluates the GHG reduction potential of utilising forestry residue in different bioenergy technologies using a consequential LCA approach. Unlike previous studies that assess GHG impacts per unit of fuel produced, this research takes a feedstock-centric approach which enables comparisons across systems that yield diverse products and by-products. Three technologies-combined heat and power plant with carbon capture, hydrothermal liquefaction, and gasification-are assessed, while considering both carbon capture and storage (CCS) or carbon capture and utilisation (CCU). Through scenario analysis, the study addresses uncertainty, and assumptions in the LCA modelling. It explores the impact of energy systems, fuel substitution efficiency, renewable energy expansion, and the up/down stream supply chain. All technology pathways showed a potential for net emissions savings when including avoided emissions from substitution of products, with results varying from -111 to -1742 kgCO2eq per tonne residue. When combining the bioenergy technologies with CCU the dependency on the energy system in which they are operated was a significantly higher compared to CCS. The breakpoint was found to be 44 kg CO2eq/kWh electricity meaning that the marginal electricity mix has to be below this point for CCU to obtain lower GHG emissions. Furthermore, it is evident that the environmental performance of CCU technologies is highly sensitive to how it will affect the ongoing expansion of renewable electricity capacity.

A comparative study of worm-sludge treatment reed bed planted with Phragmites australis and Arundo donax in the Mediterranean region

Sludge treatment reed bed planted (STRB) with Phragmites australis (P.australis) and Arundo donax (A.donax) was assessed in the presence of Eisenia fetida under control condition during the dry season. Worm-planted units were fed with mixed sewage sludge (dry and volatile solids of 29.44 g DS.L-1 and 24.23 g VS.L-1). Sludge loading rates (SLR) of 50, 60, and 70 kg DS m-2 year-1 were examined to assess dewatering efficiency. Surface layers in units with P.australis and A.donax achieved DS of 80 and 81% at a loading rate of 50 kg DS m-2 year-1, while their subsurface DS were 41 and 25%, respectively. Units with A.donax experienced plant loss when subjected to SLR exceeding 60 kg DS m-2 year-1. More than 10 cm of residual sludge accumulated on the top of units after a 2-month final rest. Evapotranspiration was greater in the unit with P.australis (5.23 mm day-1) compared to the unit with A.donax (4.24 mm day-1) while both were fed with 70 kg DS m-2 year-1. Water loss contributions from residual sludge layer, drained water, and evapotranspiration were 3, 46, and 51%, respectively. Units with P.australis indicated 20% higher water loss compared to units with A.donax. Although the drained water quality improved over time, it did not meet standard limits. The residual sludge layer contained macro and micronutrients, and heavy metals with a significant elemental order of N > Ca > P > S > mg > K (N:P:K = 31:8:1), Fe > Na > B > Mn > Mo and Zn > Cr > Cu > Pb > Ni > Cd. Overall, STRB could be a sustainable alternative technology to conventional sewage sludge management techniques.

Biotransformation of Pb and As from sewage sludge and food waste by black soldier fly larvae: Migration mechanism of bacterial community and metalloregulatory protein scales

The accumulation and transformation of lead (Pb) and arsenic (As) during the digestion of sewage sludge (SS) by black soldier fly larvae (BSFL) remain unclear. In this study, we used 16 s rRNA and metagenomic sequencing techniques to investigate the correlation between the microbial community, metalloregulatory proteins (MRPs), and Pb and As migration and transformation. During the 15-day test period, BSFL were able to absorb 34-48 % of Pb and 32-45 % of As into their body. Changes in bacterial community abundance, upregulation of MRPs, and redundancy analysis (RDA) results confirmed that ZntA, EfeO, CadC, ArsR, ArsB, ArsD, and ArsA play major roles in the adsorption and stabilization of Pb and As, which is mainly due to the high contribution rates of Lactobacillus (48-59 %) and Enterococcus (21-23 %). Owing to the redox reaction, the regulation of the MRPs, and the change in pH, the Pb and As in the BSFL residue were mainly the residual fraction (F4). The RDA results showed that Lactobacillus and L.koreensis could significantly (P < 0.01) reduce the reducible fraction (F2) and F4 of Pb, whereas Firmicutes and L.fermentum can significantly (P < 0.05) promote the transformation of As to F4, thus realizing the passivation Pb and As. This study contributes to the understanding of Pb and As in SS adsorbed by BSFL and provides important insights into the factors that arise during the BSFL-mediated migration of Pb and As.