Cow-dung is a widely used stabiliser applied in traditional earthen buildings with one objective to improve water resistance. However, most research has focused on explaining its mechanical strength, with only one study suggesting water resistance mechanism via formation of insoluble compounds at high pH, a phenomenon uncommon in natural cow dung and soil mixtures. This article investigates the water-resistance behaviour of cow-dung stabilised compressed earthen blocks (CD-CEBs) through an extensive experimental programme to understand the influence of cow-dung and soil related factors and to characterise the components of cow-dung responsible for its water resistance. It was found that the small-sized microbial aggregates (SSMA) present in cow-dung, which are negatively charged hydrophobic aggregates of low specific surface area, are responsible for enhanced water resistance of CD-CEBs. The insights gained from experiments are compiled to recommend the following strategies for improved performance of CD-CEBs: (i) The use of wet cow-dung is advised over dry cow-dung as it provided over 80 times better water resistance; (ii) Adopting a higher compaction liquid content (by 3%) improved the water resistance by over 40 times; (iii) The water resistance of CD-CEBs was improved over 30 times by using soils rich in low-swelling clay minerals such as kaolinite. A case study applying these findings demonstrates the successful scaleup from the lab to field showcasing potential of cow-dung and soil in low-carbon construction.

Activated sludge (AS) wastewater treatment generates substantial excess sludge which needs to be discarded and thereby increasing operational costs. Extracellular polymeric substances (EPS) within AS present a potential resource for recovery, reducing sludge volume and mass while adding value. Achieving this goal requires a better characterization of EPS, as the relationship between its composition and the microbial communities responsible for its production remains insufficiently understood. Here, we analysed extracted EPS from 16 wastewater treatment plants across 13 countries and 5 continents and found that alkaline extractable EPS yields varied widely (2.81–18.5 wt.% VSS). The microbial community composition of abundant species varied across plants and particularly across continents and did not correlate to the EPS yield. Only sludge retention time had a significant correlation with the EPS yield (p < 0.005). Traditional colorimetric assays failed to detect compositional trends of the EPS, but Fourier Transform Infrared (FTIR) analysis indicated that extracted EPS from biological phosphorus removal systems had higher lipid and polysaccharide content, while chemical phosphorus removal systems had higher relative protein content. Thus, FTIR proved effective for distinguishing extracted EPS composition, demonstrating its potential as a high-throughput characterization tool. These findings highlighted that the wastewater treatment design and operation may shape the functional groups in EPS when using the alkaline method. More investigations are needed to find possible correlations between the composition of extracted EPS and the microbial community structure. Overall, the study presents a baseline for the amount and overall composition of biopolymers that can be extracted from global AS plants for recovery.

Various C₁–C₂ compounds are increasingly available through electrochemical reduction of CO₂. Although not always suitable as a sole substrate, these compounds can supplement a primary substrate like glucose to enhance microbial growth. Yet, the mechanisms underlying the effects of dual substrate consumption on growth rate and growth yield remain poorly understood. We developed a generalized, species-agnostic thermodynamic framework to partition anabolic and catabolic fluxes for various glucose/secondary substrate combinations, predicting maximum growth rate and growth yield as a function of the substrate ratio. The optimal strategy is to use the secondary substrate as electron donor, conserving the most efficient carbon source, glucose, for assimilation. Because many substrates yield similar energy per electron, biomass yield remains constant until glucose becomes limiting for anabolism. When further lowering the glucose fraction, additional assimilation of the auxiliary carbon source reduces the yield. The growth rate follows similar trends. Dual substrate growth enables generalists to produce more biomass from the total resource pool than a combination of specialists, conferring a competitive edge under substrate-limiting conditions. These theoretical observations align with experimental observations of lower residual substrate concentrations and dominance of generalists in natural and engineered oligotrophic environments.

A contracted & baffled final settler, with a low H (height)/D (diameter) ratio and a high hydraulic selective pressure (HSP) imposed by elevating up-flow velocity by influent or effluent, was applied to promote in-situ granulation in a continuous biological nutrient removal (BNR) process. Under observational HSP_obs = 0.9–3.2 m3/(m2·h), baffles could retain potential granules, and wash out flocs via 4–5 times higher up-flow velocity through baffle gaps. Continuous granulation was efficient within 44 d (HSP_obs = 0.9–1.9 m3/m2·h): granulation increased to 42%, and SVI₃₀ decreased to 52 mL/g from 97 mL/g, with COD ≥ 85%, TN ≥ 90% and TP ≥ 90%. A higher HSP_obs (1.9–3.2 m3/m2·h) produced bigger granules (700 μm) and a higher granulation (95%). Extracellular polymeric substances (EPS), interfacial thermodynamics analysis indicated the settler selectively enriched granules with enhanced compactness. Microbial analysis revealed the enrichment of floc-forming bacteria and filamentous bacteria. This study provides an efficient and simple method for continuous granulation.

The authors regret that an inconsistency was identified between the results presented in Fig. 6 and the inventory data reported in Tables S.11 and S.12 of the Supplementary Information. This discrepancy arose because an additional scenario from a previous version of the manuscript was inadvertently retained in the Supplementary Information, although it was not included in the final published article. As a result, the scenario numbering in the Supplementary Information did not correspond to the scenarios discussed in the main text, leading to apparent inconsistencies for Climate change and Marine ecotoxicity results for Scenario 3. The Supplementary Information has now been corrected by removing the tables related to the excluded scenario and aligning the remaining scenario numbering with the final version of the article. The results presented in the main article remain unchanged. The authors would like to apologise for any inconvenience caused.

Population growth, climate change, and urbanisation significantly contribute to environmental stress, particularly through the depletion of finite resources like clean, easily accessible freshwater. In the water industry, the supply chain must become more independent, shifting from the prevailing linear delivery model to a circular economy. This shift can be achieved by adopting advanced treatment methods to ensure high-quality treated water and minimising waste and emissions. A transition to a circular economy can offer an opportunity to address sustainability issues in multiple sectors. For example, the water and energy nexus recognises that these two sectors are inextricably linked. Integrating green hydrogen production and wastewater treatment (WWT) has been identified as a promising strategy as part of the water-energy nexus, which advances the circular economy. When the green hydrogen economy uses treated wastewater as a feedstock, contributing to water reuse, the water industry can further enhance the sustainability of this approach by utilising co-products from hydrogen synthesis, such as high-purity oxygen. This oxygen can then be employed in various stages of WWT, including aeration and producing key reagents such as ozone and hydrogen peroxide, aiming to improve treatment efficiency and reduce emissions. Accordingly, this study examines how such applications can enhance circularity within the water sector. The principal findings were: (i) integrating green hydrogen production with WWT offers promising environmental and economic benefits but requires deeper technical, regulatory, and stakeholder alignment; (ii) optimising co-product oxygen utilisation in aeration and advanced treatment can help enhance WWT performance and economic viability; (iii) future research should prioritise techno-economic assessments, pilot-scale demonstrations, and system-wide integration studies to enable successful implementation of this circular and sustainable approach.

Riverbank filtration is a nature-based water treatment strategy known for its effective removal of organic micropollutants. Yet, the mechanisms governing their biodegradation, especially the role of redox transitions in mediating biotransformation, remain insufficiently understood. Here, we integrate metagenomic profiling with chemical analytics in a 10 m simulated riverbank filtration system to demonstrate how sequential oxidizing–reducing degradation enhances organic micropollutant transformation. Oxygen stratification structured distinct microbial and enzymatic pathways: oxidizing zones (>+200 mV redox potential) facilitated cytochrome P450-mediated oxidation (oxidizing condition, OXD), while subsequent redox shifts to reducing conditions (←400 mV, sequential oxidizing–reducing (SOR) conditions) activated reductive transformations (e.g., via nitronate monooxygenase and aldehyde dehydrogenase) and conjugation pathways. These SOR conditions significantly enhanced the removal of recalcitrant compounds, including irbesartan (+25.3%), benzotriazole (13.4%), and gabapentin (+9.7%). Metagenomic analysis revealed redox-driven microbial specialization, with Pseudomonadota and Nitrospirota dominating in oxidizing zones and reducing microzones enriched in pathways associated with nitrotoluene and ethylbenzene degradation, providing genomic evidence for sequential organic micropollutant breakdown. These findings establish a mechanistic framework for harnessing oxidizing–reducing microbial partnerships to amplify organic micropollutant removal in nature-based water treatment systems, which can be used for riverbank filtration site selection and well field construction and optimization.

A highly pure biomethane stream (≈97% CH₄) was produced continuously under halo-alkaline conditions (pH > 9, 0.6 M Na⁺) from complex alkaline organic waste residue originating from biopolymer extraction from sewage sludge. During the proof-of-concept operation, the substrate was degraded with similar efficiency (40% of the volatile solids, VS) compared to neutral conditions (36% of the VS). Operational data was utilised in a technical evaluation to identify bottlenecks for full-scale implementation at an early stage of process development and for comparison to conventional biogas upgrading using pressure swing and membranes. Initially identified bottlenecks for alkaline fermentation were related to overcautious assumptions, while others could be technically solved. Alkaline fermentation offers an attractive method for supplying increasingly needed high-purity biomethane using various recalcitrant substrates that have undergone alkaline pre-treatment. This is more feasible than the conventional ex-situ biogas upgrading. Next, upscaling steps for alkaline fermentation should be pursued. Strategies for integrated CO₂ sequestration and nutrient recovery are outlined, which will offer additional benefits in the future.

The recovery of C, N, and P elements by sludge biorefinery potentially reduces operation costs and increases the extra benefits. Herein, we analyzed the elemental stoichiometry of C, N, and P and functional microbiome involved in enzymatic anaerobic fermentation. Enzymatic hydrolysis was observed to increase the release of C, N, and P into the sludge supernatants by 21.8 %–26.3 %. Metatranscriptome analysis indicated that enzymatic pretreatment enhanced the metabolism of the organic carbon degradation, ammonium conversion, and P solubilization in subsequent fermentation. Specifically, enzymatic pretreatment enhanced endogenous carbon hydrolase activity by 48.4 %–72.7 % and upregulated intra-C metabolic pathways, such as glycolysis and pyruvate metabolism. Ammonium transport and conversion were significantly increased by 4–6 fold, stimulating the synthesis of glutamine and endogenous amino acids. Additionally, enzymatic hydrolysis promoted phosphatase secretion and enhanced bacterial P uptake. These effects improved the recovery of C, N, and P as dentification carbon source and struvite by 13.7 %–41.8 % and the dry sludge production was reduced by 24.3 %–28.1 %. Life cycle assessment (LCA) indicated the shift of CO₂ emissions from net positive to net negative levels as compared to the conventional A²/O process. This study offers valuable insights into the redistribution and metabolism of various elements involved in the enzymatic anaerobic fermentation, and proposes the potential strategy to recovery C, N, and P from sewage via sludge biorefinery.

The current study investigates the application of extracellular polymeric substances (EPS), recovered from municipal wastewater sludge, as sustainable flame-retardant materials. While EPS show significant fire-safety potential, their compositional complexity requires a clear understanding of fire performance. The primary objective of this research is to elucidate how key EPS components, such as proteins (PN), polysaccharides (PS), lipids (LP), and humic-like substances (HS), govern thermal decomposition and flammability. Thermogravimetric analysis reveals that PN and HS have higher activation energies than those of other components, enhancing EPS thermal stability. PS yield the highest char residue (48.7% at 900 °C), while PN achieve the highest graphitisation degree, highlighting their crucial roles in char formation of EPS. Pyrolysis gas analysis indicates that PN and HS are main sources of nitrogenous compounds, diluting oxygen. Moreover, flammability tests show PS have the lowest peak heat release rate (29.7 W/g), underlining the important role in increasing flame-retardancy of EPS. However, TGA-FTIR analysis indicates lipid as a primary source of combustible gases likely due to their hydrocarbon chains. This study provides valuable insight into the role of major EPS components in the flame-retardant properties and suggest a sustainable approach to enhance flame retardancy through targeted component optimisation.