The water quality of the geographically and culturally important Bagmati River is deteriorating due to the mixing of highly polluted tributaries and untreated domestic and industrial waste. Real-time physicochemical data was collected from the tributaries and mostly polluted sites along the Bagmati River in the Kathmandu Valley. The results showed that as compared to the tributaries, the Bagmati River was less polluted before the confluence. The oxidation-reduction potential and dissolved oxygen were recorded high up to the middle of the observation domain and reduced to extremely low adapting to hypoxic region downstream sites. The water quality index (WQI) of Bagmati River was determined using the assigned weighted value (model A) and the weight arithmetic index of water quality parameters (model B). It was as high as 400 (model A) and 130 (model B) downstream in the Bagmati River. The WQI value showed that all the rivers inside the Kathmandu Valley were highly polluted and unfit for human use. Human activities such as the direct discharge of domestic effluent, disposal of solid waste at the bank of the river, and the mixing of highly polluted tributary excessively loaded contaminants, turned the Bagmati River into a biologically dead river downstream.

Straw returning into paddy soil enhances soil organic matter which usually promotes the emission of greenhouse gases to the atmosphere. The application of sediment microbial fuel cells (SMFCs) to paddy soil activates power-generating microorganisms and enhances organic matter biodegradation. In the present study, rice straw addition in SMFCs was examined to determine its effect on CH₄ and CO₂ emissions. Columns (height, 25 cm; inner diameter, 9 cm) with four treatments: soil without and with rice straw under SMFC and without SMFC conditions were incubated at 25°C for 70 days. Anodic potential values at 7 cm depth sediment were kept higher by SMFCs than those without SMFCs. Cumulative CH₄ emission was significantly reduced by SMFC with straw amendment (p < 0.05) with no significant effect on CO₂ emission. 16S rRNA gene analysis results showed that Firmicutes at the phylum, Closteridiales and Acidobacteriales at order level were dominant on the anode of straw-added SMFC, whereas Methanomicrobiales were in the treatment without SMFC, indicating that a certain group of methanogens were suppressed by SMFC. Our results suggest that the anodic redox environment together with the enrichment of straw-degrading bacteria contributed to a competitive advantage of electrogenesis over methanogenesis in straw-added SMFC system.

A byproduct of anaerobic domestic wastewater treatment is sulfide species which, when left untreated, can result in malodor and increased pollutant load in the water environment. In this study, the intermittent contact oxidation process (ICOP) is proposed as a post-treatment process for anaerobic treatment effluent because it enables passive treatment that supports positive energy yield for anaerobic domestic wastewater treatment. The objective of this study is to evaluate sulfide oxidation via the ICOP focusing on the mechanism of sulfide oxidation with intermittent wastewater flow. A mass balance analysis of a channel with porous sponge media installed was used to evaluate sulfide, oxidized sulfur intermediate, and sulfate species during the treatment of synthetic wastewater containing sulfide. It was found through the mass balance analysis that sulfide was removed from the feed through retention and eventual oxidation to sulfate. Sulfide oxidation then occurred via biological and chemical means where biological oxidation contributed to faster oxidation. The rate of sulfide oxidation during the ICOP was evaluated on a per area basis where the sulfide removal rate was 5.5–5.7 g S m⁻² d⁻¹. The findings underscore the extent of sulfide oxidation during the ICOP which can expand post-treatment options for streams containing sulfide.

To attain mainstream nitritation in the aeration tank receiving low-strength wastewater, an external selector to reduce nitrite-oxidising organism biomass was investigated. In the selector, the highly thickened microorganisms transferred from the aeration tank via a mechanical solid-liquid separation were exposed to a poisonous compound and returned to the aeration tank, where a part of the nitrifiers were killed. Using a mixture with 1,000 mg-N L⁻¹ of nitrite and 1,000 mg-N L⁻¹ of ammonium, about 60% of nitritation based on the effluent nitrogen was successfully maintained in the lab-scale system for more than 150 days without significant accumulation of nitrate. The poisonous compound also led to the decay of ammonium-oxidising organism to a certain extent. However, by optimising recycle rate to the selector, solids retention time of the selector, and excess sludge withdrawal rate, selective wash-out of nitrite-oxidising organism was possible. From the experimental results, the preferable operational range of the selector was explored in contour plots using the modified IWA-ASM1 model. The model indicated the preferable external recycle rate and solids retention time of the selector were about 0.2–0.4 d⁻¹ and 1–2 d, respectively, when the aeration tank was operated with excess sludge withdrawal of 0.1–0.17 d⁻¹.

The inappropriate usage of antibiotics is accelerating the prevalence of antimicrobial resistant bacteria (ARB) and antimicrobial resistance genes (ARGs) in Vietnam. Wastewater surveillance is a promising approach to monitor ARB and ARGs. In this study, ARB and ARGs were evaluated in domestic wastewater samples collected from three major cities (Hanoi, Hai Phong, and Bac Ninh) in northern Vietnam in November 2022. The ratios of cefotaxime-resistant Escherichia coli to total E. coli in domestic wastewater were relatively high (8.4%–35.7%). Quantitative PCR demonstrated that the relative abundance of the class 1 integron integrase gene to prokaryotic 16S rRNA genes ranged from 6.8 × 10⁻² to 1.3 × 10⁻¹. In original wastewater samples, Arcobacter and Aeromonadaceae were dominant. Enterobacteriaceae, Enterococcaceae, and Acinetobacter became dominant after broth enrichment with meropenem, highlighting the prevalence of potential carbapenem-resistant bacteria in urban areas in Vietnam. PCR screening revealed that the enriched cultures had clinically important carbapenemase genes, such as bla_NDM and bla_KPC. A metagenomic analysis with hybrid capture enrichment revealed that the ARG compositions of influent samples in different cities were similar (i.e., similar ARGs were prevalent in urban areas). This study demonstrated the value of wastewater surveillance for evaluating characteristics of ARB and ARGs in urban areas in Vietnam.