Calcium peroxide pre-treatment improved the anaerobic digestion of primary sludge and its co-digestion with waste activated sludge

Improving water quality and mitigating CH4 and N2O production in urban landscape water simultaneously by optimizing calcium peroxide dosage

Recent studies show that greenhouse gas (GHG) emissions from urban landscape water are significant and cannot be overlooked, underscoring the need to develop effective strategies for mitigating GHG production from global freshwater systems. Calcium peroxide (CaO2) is commonly used as an eco-friendly reagent for controlling eutrophication in water bodies, but whether CaO2 can reduce GHG emissions remains unclear. This study investigated the effects of CaO2 dosage on the production of methane (CH4) and nitrous oxide (N2O) in urban landscape water under anoxic conditions during summer. The findings reveal that CaO2 addition not only improved the physicochemical and organoleptic properties of simulated urban landscape water but also reduced N2O production by inhibiting the activity of denitrifying bacteria across various dosages. Moreover, CaO2 exhibited selective effects on methanogens. Specifically, the abundance of acetoclastic methanogen Methanosaeta and methylotrophic methanogen Candidatus_Methanofastidiosum increased whereas the abundance of the hydrogenotrophic methanogen Methanoregula decreased at low, medium, and high dosages, leading to higher CH4 production at increased CaO2 dosage. A comprehensive multi-objective evaluation indicated that an optimal dosage of 60 g CaO2/m2 achieved 41.21 % and 84.40 % reductions in CH4 and N2O production, respectively, over a 50-day period compared to the control. This paper not only introduces a novel approach for controlling the production of GHGs, such as CH4 and N2O, from urban landscape water but also suggests a methodology for optimizing CaO2 dosage, providing valuable insights for its practical application.

Comparison of different sewage sludge pretreatment technologies for improving sludge solubilization and anaerobic digestion efficiency: A comprehensive review

Anaerobic digestion (AD) of sewage sludge reduces organic solids and produces methane, but the complex nature of sludge, especially the difficulty in solubilization, limits AD efficiency. Pretreatments, by destroying sludge structure and promoting disintegration and hydrolysis, are valuable strategies to enhance AD performance. There is a plethora of reviews on sludge pretreatments, however, quantitative comparisons from multiple perspectives across different pretreatments remain scarce. This review categorized various pretreatments into three groups: Physical (ultrasonic, microwave, thermal hydrolysis, electric decomposition, and high pressure homogenization), chemical (acid, alkali, Fenton, calcium peroxide, and ozone), and biological (microaeration, exogenous bacteria, and exogenous hydrolase) pretreatments. The optimal conditions of various pretreatments and their impacts on enhancing AD efficiency were summarized; the effects of different pretreatments on microbial community in the AD system were comprehensively compared. The quantitative comparison based on dissolution degree of COD (DDCOD) indicted that the sludge solubilization performance is in the order of physical, chemical, and biological pretreatments, although with each below 40 % DDCOD. Biological pretreatment, particularly microaeration and exogenous bacteria, excel in AD enhancement. Pretreatments alter microbial ecology, favoring Firmicutes and Methanosaeta (acetotrophic methanogens) over Proteobacteria and Methanobacterium (hydrogenotrophic methanogens). Most pretreatments have unfavorable energy and economic outcomes, with electric decomposition and microaeration being exceptions. On the basis of the overview of the above pretreatments, a full energy and economy assessment for sewage sludge treatment was suggested. Finally, challenges associated with sludge pretreatments and AD were analyzed, and future research directions were proposed. This review may broaden comprehension of sludge pretreatments and AD, and provide an objective basis for the selection of sludge pretreatment technologies.

Hydrothermal liquefaction of municipal sludge and its products applications

Hydrothermal liquefaction (HTL) is an effective medium-temperature, high-pressure thermochemical process to dispose municipal sludge (MS), and biocrude (a crude bio-oil) is its main product. Many efforts are continued extensively to improve conversion efficiency and to promote industrial application of this technology. This work focuses on critical influencing factors (e.g., reaction temperature, residence time, atmosphere, solvent, catalyst, and pretreatment) and fundamental transformation mechanisms of main components (i.e., lipids, proteins, and carbohydrates) in MS HTL. It also analyzes migration behavior of heavy metals during MS HTL, which can provide a reference for subsequent recovery of nutrients from HTL products. Moreover, the applications of MS HTL products are systematically expounded, and potential challenges and opportunities are highlighted as well. It is necessary to develop advanced methods of catalyst recovery and innovative biocrude upgrading methods so as to reduce HTL investment and operating costs. Reusing aqueous phase and solid phase products as reaction medium and catalyst carrier separately after MS HTL is feasible to realize resource utilization of MS. This information can provide valuable guidance to promote MS HTL industrialization.

Reduced sulfide and methane in rising main sewer via calcium peroxide dosing: Insights from microbial physiological characteristics, metabolisms and community traits

Although the biocidal effect of calcium peroxide (CaO2) has attracted increasing attention in wastewater and sludge management, its potential in the reduction of sulfide and methane from sewer is not tapped. This study aims to fill this gap through the long-term operated sewer reactors. Results showed one-time dose of 0.2% (w/v) CaO2 with 12-h exposure decreased the average sulfide and methane production by 80% during one week. The electron paramagnetic resonance and free radical quenching tests indicated free radicals from CaO2 decomposing posed a major contribution on sewer biofilms (•OH>•O2->alkali). Mechanistic analysis revealed extracellular polymeric matrix breakdown (e.g., protein secondary structure) and cell membrane damage were caused by the increased lipid peroxidation of cells and exacerbated intracellular reactive oxygen species under CaO2 stress. Moreover, the intracellular metabolic pathways, such as electrons provision and transfer, as well as pivotal enzymatic activities (e.g., APS reductase, sulfite reductase and coenzymes F420) were significantly impaired. RT-qPCR analysis unveiled the absolute abundances of dsrA and mcrA were decreased by 7.53-40.37% and 67.00-74.85%, respectively. Although this study broadens the application scope of CaO2 and provides in-depth understanding of advanced oxidation-based technology in sewer management, the pipe scale risk due to the release of calcium ions warrants further investigation.

Nitrogen and boron co-doped lignin biochar for enhancing calcium peroxide activation toward organic micropollutants decontamination in waste activated sludge and related microbial structure dynamics

This study synthesized dual heteroatom nitrogen and boron-co-doped lignin-based biochar (NB-LGBC) for calcium peroxide (CP) activation to enhance the removal of organic micropollutants (OMPs), namely, 4-nonylphenol (4-NP) from waste activated sludge (WAS). NB-LGBC/CP enhanced 4-NP degradation by arriving at 83 % removal in 12 h. The NB-LGBC/CP system degraded 4-NP via a synergistic interaction (HO•, O₂^•- radicals, and singlet oxygen) and electron transfer due to the N-B-C bonding configurations. Results of fluorescence excitation-emission matrix (FEEM) analysis revealed significantly increase in biodegradable organics from treated WAS mixture. NB-LGBC/CP treatment enriched alkaliphilic bacterium associated with the predominance of the genus Desulfonatronum within the phylum Proteobacteria in the WAS, which improved the biological treatment capacity of 4-NP. Thus, NB-LGBC in HR-CAOP will be a novel approach for WAS decontamination.

Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

The potential of periodate (PI) in sludge anaerobic digestion is not tapped, although it has recently attracted great research interest in organic contaminants removal and pathogens inactivation in wastewater treatment. This is the first work to demonstrate significant improvement in methane generation from waste activated sludge (WAS) with PI pretreatment and to provide underlying mechanisms. Biochemical methane potential tests indicated that methane yield enhanced from 100.2 to 146.3 L per kg VS (VS, volatile solids) with PI dosages from 0 to 100 mg per g TS (TS, total solids). Electron spin resonance showed PI could be activated without extra activator addition, which might be attributed to the native transition metals (e.g., Fe²⁺) in WAS, thereby generating hydroxyl radical (•OH), superoxide radicals (•O₂ ^-), and singlet oxygen (¹O₂). Further scavenging tests demonstrated all of them synergistically promoted WAS disintegration, and their contributions were in the order of •O₂ ^- > •OH > ¹O₂, leading to the release of substantial biodegradable substances (i.e., proteins and polysaccharides) into the liquid phase for subsequent biotransformation. Moreover, fluorescence and ultraviolet spectroscopy analyses indicated the recalcitrant organics (especially lignocellulose and humus) could be degraded by reducing their aromaticity under oxidative stress of PI, thus readily for methanogenesis. Microbial community analysis revealed some microorganisms participating in hydrolysis, acidogenesis, and acetoclastic methanogenesis were enriched after PI pretreatment. The improved key enzyme activities and up-regulated metabolic pathways further provided direct evidence for enhanced methane production. This research was expected to broaden the application scope of PI and provide more diverse pretreatment choices for energy recovery through anaerobic digestion.

Potassium permanganate pretreatment effectively improves methane production from anaerobic digestion of waste activated sludge: Reaction kinetics and mechanisms

As a powerful oxidizing agent, potassium permanganate (KMnO₄) has attracted widespread interest in sludge treatment and contaminant removal. However, its effect on the anaerobic digestion of waste activated sludge (WAS) is ambiguous. This investigation was designed to provide perspectives into this problem. In comparison with the control, 0.3 g KMnO₄/g TSS pretreatment enhanced the methane production by 78.82 %. Model analysis demonstrated that the KMnO₄ pretreatment enhanced the biochemical methane potential (B₀) of WAS. Mechanistic studies elucidated that the KMnO₄ pretreatment process generated reactive radicals such as ·OH, ·O₂^- and ¹O₂, which contributed to sludge disintegration and biodegradation process of dissolved substances, thus resulting in more substances available for subsequent methane generation. Enzyme activity analysis indicated that KMnO₄ pretreatment facilitated the activities of key enzymes associated with anaerobic digestion to various degrees. Microbial analysis illustrated that the relative abundance of functional microorganisms was significantly elevated after KMnO₄ pretreatment, which was conducive to methane production.

Enhanced technology for sewage sludge advanced dewatering from an engineering practice perspective: A review

The increasing production of sludge poses significant environmental risks. Sludge disposal and transport are costly because of the high water content (WC). Reducing the WC of sludge is the most efficient strategy to decrease treatment costs. However, the sludge contains a large amount of hydrophilic organic matter, causing poor dewaterability. Therefore, research on preconditioning and mechanical dewatering has great significance for advanced sludge dewatering. In this study, the features of sludge, the advantages and disadvantages of preconditioning methods, and the action mechanisms (including physical, chemical, and biological preconditioning) are thoroughly described. In addition, the dewatering principle and engineering applications of mechanical dewatering techniques are introduced in this manuscript, especially the application of vacuum preloading as an in-situ dewatering technology in sludge. Finally, cost analysis of different conditioning and mechanical dewatering methods is conducted to explore their application feasibility. This manuscript provides new insights for engineering applications of preconditioning methods and mechanical dewatering technology.