Genome-level insights into the operation of an on-site biological wastewater treatment unit reveal the importance of storage time

Review of linear and circular approaches to on-site domestic wastewater treatment: Analysis of research achievements, trends and distance to target

This comprehensive review critically assesses traditional and emerging technologies for domestic wastewater treatment and reuse, focusing on the transition from conventional centralised systems to innovative decentralised approaches. Through an extensive literature search on domestic wastewater systems serving a population equivalent of less than or equal to 10, the study juxtaposes linear and circular methods and highlights their impact on urban water management and the environment. The papers reviewed were classified into five categories: Environmental studies, economic studies, social studies, technological studies, and reviews and policy papers. The analysis was carried out separately for linear and circular approaches within each category. In addition, the maturity of the technology (lab/pilot or full-scale application) was taken into account in the analysis. The research landscape is shown to be evolving towards circular methods that promise sustainability through resource recovery, despite the dominance of linear perspectives. The lack of clear progress in decentralised technologies, the scarcity of circularity assessments and the challenges of urban integration are highlighted. Operational reliability, regulatory compliance and policy support are identified as key barriers to the adoption of decentralised systems. While conventional pollutants and their environmental impacts are well addressed for linear systems, the study of emerging pollutants is in its infancy. Conclusions on the impact of these hazardous pollutants are tentative and cautious. Social and economic studies are mainly based on virtual scenarios, which are useful research tools for achieving sustainability goals. The conceptual frameworks for assessing the social dimension need further refinement to be effective. The paper argues for a balanced integration of centralisation and decentralisation, proposing a dual strategy that emphasizes the development of interoperable technologies. It calls for further research, policy development and widespread implementation to promote decentralised solutions in urban water management and pave the way for sustainable urban ecosystems.

Bioprospecting uncultivable microbial diversity in tannery effluent contaminated soil using shotgun sequencing and bio-reduction of chromium by indigenous chromate reductase genes

The tannery industry generates a consequential threat to the environment by producing a large amount of potentially toxic metal-containing waste. Bioremediation has been a promising approach for treating potentially toxic metals, but the efficiency of remediation in microbes is one of the factors limiting their application in tanneries waste treatment. The motivation behind the present work was to explore the microbial diversity and chromate reductase genes present in the tannery effluent-contaminated soil using metagenomics approach. The use of shotgun sequencing enabled the identification of operational parameters that influence microbiome composition and their ability to reduce Chromium (Cr) concentration. The Cr concentration in Kanpur tannery effluent contaminated soil sample was 700 ppm which is many folds than the approved permissible limit by World Health Organisation (WHO) for Cr is 100 ppm. Metagenomic Deoxyribo Nucleic Acid (DNA) was extracted to explore taxonomic community structure, phylogenetic linkages, and functional profile. With a Guanine-Cytosine (GC) abundance of 54%, total of 45,163,604 high-quality filtered reads were obtained. Bacteria (83%), Archaebacteria (14%), and Viruses (3%) were discovered in the structural biodiversity. Bacteria were classified to phylum level, with Proteobacteria (52%) being the dominant population, followed by Bacteriodetes (15%), Chloroflexi (15%), Spirochaetes (7%), Thermotogae (5%), Actinobacteria (4%), and Firmicutes (1%). The OXR genes were cloned and checked for their efficiency to reduce Cr concentration. Insitu validation of OXR8 gene showed a reduction of Cr concentration from 700 ppm to 24 ppm in 72 h (96.51% reduction). The results of this study suggests that there is a huge reservoir of microbes and chromate reductase genes which are unexplored yet.

Spatial Variation of the Microbial Community Structure of On-Site Soil Treatment Units in a Temperate Climate, and the Role of Pre-treatment of Domestic Effluent in the Development of the Biomat Community

The growth of microbial mats or "biomats" has been identified as an essential component in the attenuation of pollutants within the soil treatment unit (STU) of conventional on-site wastewater treatment systems (OWTSs). This study aimed to characterize the microbial community which colonizes these niches and to determine the influence of the pre-treatment of raw-domestic wastewater on these communities. This was achieved through a detailed sampling campaign of two OWTSs. At each site, the STU areas were split whereby half received effluent directly from septic tanks, and half received more highly treated effluents from packaged aerobic treatment systems [a coconut husk media filter on one site, and a rotating biodisc contactor (RBC) on the other site]. Effluents from the RBC had a higher level of pre-treatment [~90% Total Organic Carbon (TOC) removal], compared to the media filter (~60% TOC removal). A total of 92 samples were obtained from both STU locations and characterized by 16S rRNA gene sequencing analysis. The fully treated effluent from the RBC resulted in greater microbial community richness and diversity within the STUs compared to the STUs receiving partially treated effluents. The microbial community structure found within the STU receiving fully treated effluents was significantly different from its septic tank, primary effluent counterpart. Moreover, the distance along each STU appears to have a greater impact on the community structure than the depth in each STU. Our findings highlight the spatial variability of diversity, Phylum- and Genus-level taxa, and functional groups within the STUs, which supports the assumption that specialized biomes develop around the application of effluents under different degrees of treatment and distance from the source. This research indicates that the application of pre-treated effluents infers significant changes in the microbial community structure, which in turn has important implications for the functionality of the STU, and consequently the potential risks to public health and the environment.

Natural Treatment Systems and Importance of Social Cost Benefit Analysis in Developing Countries: A Critical Review

This review article attempts to analyse the social issues that impact the performance of natural treatment systems (NTSs). An NTS is a decentralised wastewater treatment system found to be appropriate in developing countries due to its affordability and lower technicity. However, if socio-economic and institutional issues of community are ignored then NTSs may turn out to be unsuitable for developing countries. The article also takes a critical view on the extant literature which ignores the social cost of NTSs. The social cost of NTSs may be high as a decentralised system requires the engagement of various governmental agencies, research institutes and the community. The cost of engagement may make NTSs a socio-economically unattractive proposition. The article discusses the variables to be considered for the social cost-benefit analysis. It also discusses the implications of social cost-benefit analysis for appreciating the incentives and net benefits for collective actions at the community level. Social cost-benefit analysis can help overcome the initial difficulty of high financial cost and usher sustainability.

Chemical safety assessment of transformation products of landfill leachate formed during the Fenton process

Organic chemicals identified in raw landfill leachate (LL) and their transformation products (TPs), formed during Fenton treatment, were analyzed for chemical safety following REACH guidelines. The raw LL was located in the metropolitan region of Campina Grande, in northeast Brazil. We elucidated 197 unique chemical structures, including 154 compounds that were present in raw LL and 82 compounds that were detected in the treated LL, totaling 39 persistent compounds and 43 TPs. In silico models were developed to identify and prioritize the potential level of hazard/risk these compounds pose to the environment and society. The models revealed that the Fenton process improved the biodegradability of TPs. Still, a slight increase in ecotoxicological effects was observed among the compounds in treated LL compared with those present in raw LL. No differences were observed for aryl hydrocarbon receptor (AhR) and antioxidant response element (ARE) mutagenicity. Similar behavior among both raw and treated LL samples was observed for biodegradability; Tetrahymena pyriformis, Daphnia magna, Pimephales promelas and ARE, AhR, and Ames mutagenicity. Overall, our results suggest that raw and treated LL samples have similar activity profiles for all endpoints other than biodegradability.