Composting has emerged as a suitable method to convert or transform organic waste including manure, green waste, and food waste into valuable products with several advantages, such as high efficiency, cost feasibility, and being environmentally friendly. However, volatile organic compounds (VOCs), mainly malodorous gases, are the major concern and challenges to overcome in facilitating composting. Ammonia (NH3) and volatile sulfur compounds (VSCs), including hydrogen sulfide (H2S), and methyl mercaptan (CH4S), primarily contributed to the malodorous gases emission during the entire composting process due to their low olfactory threshold. These compounds are mainly emitted at the thermophilic phase, accounting for over 70% of total gas emissions during the whole process, whereas methane (CH4) and nitrous oxide (N2O) are commonly detected during the mesophilic and cooling phases. Therefore, the human health risk assessment of malodorous gases using various indexes such as ECi (maximum exposure concentration for an individual volatile compound EC), HR (non-carcinogenic risk), and CR (carcinogenic risk) has been evaluated and discussed. Also, several strategies such as maintaining optimal operating conditions, and adding bulking agents and additives (e.g., biochar and zeolite) to reduce malodorous emissions have been pointed out and highlighted. Biochar has specific adsorption properties such as high surface area and high porosity and contains various functional groups that can adsorb up to 60%-70% of malodorous gases emitted from composting. Notably, biofiltration emerged as a resilient and cost-effective technique, achieving up to 90% reduction in malodorous gases at the end-of-pipe. This study offers a comprehensive insight into the characterization of malodorous emissions during composting. Additionally, it emphasizes the need to address these issues on a larger scale and provides a promising outlook for future research.

Biochar amendment for landfill soil cover has the potential to enhance methane removal efficiency while minimizing the soil depth. However, there is a lack of information on the response of biochar-mediated soil cover to the changes in configuration and operational parameters during the methane transport and transformation processes. This study constructed three biochar-amended landfill soil covers, with reduced soil depths from 75 cm (C2) to 55 cm (C3) and 45 cm (C4), and the control group (C1) with 75 cm and no biochar. Two operation phases were conducted under two soil moisture contents and three inlet methane fluxes in each phase. The methane removal efficiency increased for all columns along with the increase in methane flux. However, increasing moisture content from 10% to 20% negatively influenced the methane removal efficiency due to mass transfer limitation when at a low inlet methane flux, especially for C1; while this adverse effect could be alleviated by a high flux. Except for the condition with low moisture content and flux combination, C3 showed comparable methane removal efficiency to C2, both dominating over C1. As for C4 with only 45 cm, a high moisture content combined with a high methane flux enabled its methane removal efficiency to be competitive with other soil depths. In addition to the geotechnical reasons for gas transport processes, the evolution in methanotroph community structure (mainly type I methanotrophs) induced by biochar amendment and variations in soil properties supplemented the biological reasons for the varying methane removal efficiencies.

Aluminum (Al)-based drinking water treatment residue (DWTR) has often been attempted to be recycled as dominant ingredient to produce sintered ceramsite for water treatment. This study aimed to determine the long-term performance of DWTR-based ceramsite in treating domestic wastewater based on a 385-d biofilter test and by using physicochemical, metagenomic, and metatranscriptomic analyses. The results showed that the ceramsite-packed biofilter exhibited high and stable capability in removing phosphorus (P) and chemical oxygen demand (COD), with removal efficiencies of 92.6 ± 3.97% and 81.1 ± 14.0% for total P and COD, respectively; moreover, 88-100% of ammonium-nitrogen (N) was normally converted, and the total N removal efficiency reached 80-86% under proper aeration. Further analysis suggested that the forms of the removed P in the ceramsite were mainly NH4F- and NaOH-extractable. Microbial communities in the ceramsite biofilter exhibited relatively high activity. Typically, various organic matter degradation-related genes (e.g., hemicellulose and starch degradations) were enriched, and a complete N-cycling pathway was established, which is beneficial for enriching microbes involved in ammonium-N conversion, especially Candidatus Brocadia, Candidatus Jettenia, Nitrosomonas, and Nitrospira. In addition, the structures of the ceramsite had high stability (e.g., compressive strength and major compositions). The ceramsites showed limited metal and metalloid pollution risks and even accumulated copper from the wastewater. These results demonstrate the high feasibility of applying ceramsite prepared from Al-based DWTR for water treatment.

A sulfur-based biofilter enhanced by phosphate modified activated carbon as particle electrodes was constructed to simultaneously remove total nitrogen (TN) and estrogen from low carbon-to-nitrogen ratio (C/N) wastewater containing 1 mg/L 17-alpha-ethinylestradiol (EE2). Results showed that the enhanced biofilter achieved outstanding performance in EE2 removal (93.2 %) and TN reduction (effluent < 5 mg/L), demonstrating robustness against C/N fluctuations. It was noteworthy that it successfully reduced both acute toxicity (59.5 %) and estrogenic activity (88.6 %). Comprehensive characterization investigations and microbial community structure analysis revealed that enhanced electron transfer and increased microbial abundance likely contributed to improved biofilter performance. Core microorganisms, such as Pseudomonas and Chryseobacterium were identified as key contributors to synergistic estrogen degradation and denitrification. This study presented a feasible and promising strategy of combined process with three-dimensional electrodes and sulfur-based biofilter, highlighting substantial potential for advanced purification and safe reuse of wastewater.

Microbially mediated NO3--N and Cr(VI) reduction is being recognized as an eco-friendly and cost-effective remediation strategy. Iron sulfide mineral, as a natural inorganic electron donor, has a strong influence on NO3--N and Cr(VI) transformation, respectively. However, little is known about the simultaneous nitrate and chromium removal performance and underlying mechanism in an iron sulfide mineral-involved mixotrophic biofilter. This study demonstrated that the NO3--N and Cr(VI) removal efficiencies were stable at 62 ± 8% and 56 ± 10%, and most of them were eliminated in the 0-100-mm region of the biofilter. Cr(VI) was reduced to insoluble Cr(III) via microbial and chemical pathways, which was confirmed by the SEM-EDS morphology and the XPS spectra of biofilm and pyrite particles. SO42- was as a main byproduct of pyrite oxidation; however, the bacterial SO42- reduction synchronously occurred, evidenced by the variations of TOC and SO42- concentrations. These results suggested that there were complicated and intertwined biochemical relations between NO3--N/Cr(VI)/SO42-/DO (electron acceptors) and pyrite/organics (electron donors). Further investigation indicated that both the maximal biomass and greatest denitrifiers\' relative abundances in microbial sample S1 well explained why the pollutants were removed in the 0-100-mm region. A variety of denitrifiers such as Pseudoxanthomona, Acidovorax, and Simplicispira were enriched, which probably were responsible for both NO3--N and Cr(VI) removal. Our findings advance the understanding of simultaneous nitrate and chromium removal in pyrite-involved mixotrophic systems and facilitate the new strategy development for nitrate and chromium remediation.

In this study, an acclimated manganese-oxidizing bacteria (MnOB) consortium, QBS-1, was enriched in an acid mine area; then, it was used to eliminate Mn(Ⅱ) in different types of wastewater. QBS-1 presented excellent Mn removal performance between pH 4.0 and 8.0, and the best Mn-removal efficiency was up to 99.86% after response surface methodology optimization. Unlike other MnOB consortia, the core bacteria of QBS-1 were Stenotrophomonas and Achromobacter, which might play vital roles in Mn removal. Besides that, adsorption, co-precipitation and electrostatic binding by biological manganese oxides could further promote Mn elimination. Finally, the performance of the Mn biofilter demonstrated that QBS-1 was an excellent inoculant, which indicates good potential for removing Mn contamination steadily and efficiently.

Biological nitrogen removal has received increasing attention in wastewater treatment. A bacterium with excellent nitrogen removal performance was isolated from biofilters of recirculating aquaculture systems (RAS) and identified as Pseudomonas chengduensis BF6. It was indicated that inorganic nitrogen is transformed into gaseous and biological nitrogen by the metabolic pathways of denitrification, anammox, and assimilation, which is the main nitrogen removal pathway of strain BF6. The strain BF6 could effectively remove nitrogen within 24 h under the conditions of ammonia, nitrate, nitrite, and mixed nitrogen sources with maximum total nitrogen removal efficiencies reaching 97.00 %, 61.40 %, 79.10 %, and 84.98 %, respectively. The strain BF6 exhibited total nitrogen removal efficiency of 91.14 %, altered the microbial diversity and enhanced the relative abundance of Pseudomonas in the RAS biofilter. These findings demonstrate that Pseudomonas sp. BF6 is a highly efficient nitrogen-removing bacterium with great potential for application in aquaculture wastewater remediation.

Biofilters inoculated with activated sludge are widely used for odor control in WWTP. In this process, biofilm community evolution plays an important role in the function of reactor and is closely related to reactor performance. However, the trade-offs in biofilm community and bioreactor function during the operation are still unclear. Herein, an artificially constructed biofilter for odorous gas treatment was operated for 105 days to study the trade-offs in the biofilm community and function. Biofilm colonization was found to drive community evolution during the start-up phase (phase 1, days 0-25). Although the removal efficiency of the biofilter was unsatisfactory at this phase, the microbial genera related to quorum sensing and extracellular polymeric substance secretion led to the rapid accumulation of the biofilm (2.3 kg biomass/m3 filter bed /day). During the stable operation phase (phase 2, days 26-80), genera related to target-pollutant degradation showed increases in relative abundance, which accompanied a high removal efficiency and a stable accumulation of biofilm (1.1 kg biomass/m3 filter bed/day). At the clogging phase (phase 3, days 81-105), a sharp decline in the biofilm accumulation rate (0.5 kg biomass/m3 filter bed /day) and fluctuating removal efficiency were observed. The quorum quenching-related genera and quenching genes of signal molecules increased, and competition for resources among species drove the evolution of the community in this phase. The results of this study highlight the trade-offs in biofilm community and functions during the operation of bioreactors, which could help improve bioreactor performance from a biofilm community perspective.

Groundwater serving as a drinking water resource usually contains manganese ions (Mn2+) that exceed drinking standards. Based on the Mn biogeochemical cycle at the hydrosphere scale, bioprocesses consisting of aeration, biofiltration, and disinfection are well known as a cost-effective and environmentally friendly ecotechnology for removing Mn2+. The design of aeration and biofiltration units, which are critical components, is significantly influenced by coexisting iron and ammonia in groundwater; however, there is no unified standard for optimizing bioprocess operation. In addition to the groundwater purification, it was also found that manganese-oxidizing bacteria (MnOB)-derived biogenic Mn oxides (bioMnOx), a by-product, have a low crystallinity and a relatively high specific surface area; the MnOB supplied with Mn2+ can be developed for contaminated water remediation. As a result, according to previous studies, this paper summarized and provided operational suggestions for the removal of Mn2+ from groundwater. This review also anticipated challenges and future concerns, as well as opportunities for bioMnOx applications. These could improve our understanding of the MnOB group and its practical applications.

Highly chlorinated benzenes usually have a low efficient degradation in environment. Here we proposed a synergistic removal strategy of pentachlorobenzene (PeCB) using Pseudomonas sp. JS100 coupled with immobilized nanoscale zero-valent iron (NZVI). The structural and textural features of the synergistic system were characterized by X-ray powder diffraction, field emission scanning electron microscopy, and a specific surface area and pore size analysis. Nanoscale zero-valent iron particles were dispersed and attached to the biofilter, which increased the specific surface area to 34.5 m2 g-1. The batch experiment revealed that the removal efficiency of PeCB reached 80.2% in the synergistic system within 48 h. The degradation followed pseudo-first-order reaction kinetics, and the reaction rate constant was measured to be 0.0336 h-1. In the degradation mechanism, PeCB was degraded by NZVI to lower chlorobenzenes, which were utilized by Pseudomonas sp. JS100 as nutrients, thereby achieving rapid removal of PeCB.