Challenges to Accurate Estimation of Methane Emission from Septic Tanks with Long Emptying Intervals

Integration of steel slag and zeolite enhances simultaneous nitrification and autotrophic denitrification in ultra-low carbon/nitrogen ratio wastewater: Remodeling microbiota and iron metabolism

Constructed wetlands (CWs) are widely used for nitrogen pollution control in rural aquatic environments, yet their nitrogen removal efficiency often remains suboptimal. This study firstly examines how zeolite robustly stimulates Fe-utilization of steelmaking waste (i.e., steel slag) to improve nitrification and autotrophic denitrification of low carbon-to-nitrogen (C/N) ratio wastewater (C/N ≈ 1). Steel slag, by providing alkalinity for nitrification, also serves as an electron donor for denitrification due to its low-valent iron content. As a result, the total nitrogen (TN) removal efficiency was increased by 153.5% compared to the control group. Zeolite reshaped the microbial consortia, enriching iron autotrophic denitrifying bacteria and aerobic denitrifying bacteria. More importantly, zeolite facilitated microbial iron utilization by enhancing transmembrane iron transport and intracellular iron oxidation to boost nitrification and autotrophic denitrification without additional aeration, external carbon sources, or pH regulation. Our work advances understanding the development of low carbon technologies for wastewater nitrogen removal.

Barriers and enablers for group-based manual emptying services for onsite sanitation facilities in Nairobi, Kenya: a qualitative study

Onsite sanitation, such as pit latrines, is essential to achieving universal access to safe sanitation, as outlined in Sustainable Development Goal target 6.2. However, manual emptying for pit latrines in low-income areas is often unhygienic, posing health and environmental risks. Enhancing the safety of these services increases costs, yet affordability for customers is essential. Thus, reducing service costs is a key priority. Group-based approach, where emptiers visit multiple toilets consecutively, has potential to improve both service efficiency and affordability. However, few studies have investigated its applicability to manual emptying. This paper aims to identify barriers and enablers for group-based manual emptying services in low-income neighbourhoods of Nairobi. We conducted 12 focus group discussions with landlords, tenants, and manual emptiers in the Korogocho informal settlement in Nairobi and interviewed 20 key informants from relevant sectors in Kenya. We identified five categories of barriers and enablers that affected general and group-based manual emptying: 1) funding, 2) expertise and equipment, 3) social and commercial habits, 4) physical conditions, and 5) regulatory systems. Of these, a norm that pits are not emptied until they get full, operating time constraints, fair and transparent pricing, and an organiser who can arrange group-based emptying from Category 3, and transport capacity from Category 4 specifically affected group-based manual emptying. Given that the barriers have a cascade structure, addressing some primary barriers such as capacity building and recognising manual emptiers' role in the sanitation policies could be effective ways to ensure safe and affordable emptying services.

A review-based estimation of GHG emissions of China's wastewater management system

Under China's "Dual Carbon Goal", the wastewater treatment system plays a crucial role in the country's efforts to reduce greenhouse gas (GHG) emissions. However, a lack of baseline emissions data poses challenges for decarbonization efforts. This study aims to profile and diagnose the GHG emissions of China's entire wastewater system and identify key contributing factors. Our findings show that China's wastewater system, including wastewater treatment plants (WWTPs) and septic tanks, is responsible for significant emissions, with baseline estimates at 108.26 ± 47.37 Mt CO2-eq/a. Septic tanks and WWTPs emerged as the major GHG hotspots, contributing the most to the total emissions. This study highlights the variability in emission results from previous literature, stressing the need for consistent accounting methods and scientific emission factors. Additionally, current on-site monitoring practices in China show gaps, which hinder the accurate determination of baseline emissions. To guide future emission reduction strategies, regulatory frameworks and improved monitoring practices are recommended for the wastewater sector in China.

Indicators to complement global monitoring of safely managed on-site sanitation to understand health risks

Halfway through the Sustainable Development Goal (SDG) period, there has been little research on the criteria for monitoring safely managed sanitation under SDG target 6.2. For reporting against SDGs, global indicators are necessarily limited and exclude many safety aspects from a public health perspective. Primary survey data from 31,784 households in seven countries in Asia and Africa were analysed, comparing estimates of safely managed on-site sanitation based on global indicators with five complementary indicators of safety: animal access to excreta, groundwater contamination, overdue emptying, entering containments to empty and inadequate protection during emptying. Application of additional criteria reduced the population with safely managed sanitation by 0.4-35% for specific indicators, with the largest impact due to the risk of groundwater contamination, animal access, and containments overdue for emptying. Combining these indicators across the service chain, excluding transport and treatment, found almost three-quarters of on-site systems currently assessed as safely managed with global indicators were considered unsafe based on complementary indicators. A more comprehensive assessment of safety of on-site sanitation can be achieved through these indicators, which could be integrated into national monitoring systems and used to inform sanitation investments that address local health-related risks.

Greenhouse gas emissions from on-site sanitation systems: A systematic review and meta-analysis of emission rates, formation pathways and influencing factors

Onsite sanitation systems (OSS) are significant sources of greenhouse gases (GHG) including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). While a handful of studies have been conducted on GHG emissions from OSS, systematic evaluation of literature on this subject is limited. Our systematic review and meta-analysis provides state-of-the- art information on GHG emissions from OSS and identifies novel areas for investigation. The paper analyzes GHG emission rates from different OSS, the influence of various design, operational, and environmental factors on emission rates and proffers mitigation measures. Following the Preferred Reporting Items for Systematic reviews and Meta-analysis (PRISMA) guidelines, we identified 16 articles which quantified GHG emissions from OSS. Septic tanks emit substantial amounts of CO2 and CH4 ranging from 1.74 to 398.30 g CO2/cap/day and 0.06-110.13 g CH4/cap/day, respectively, but have low N2O emissions (0.01-0.06 g N₂O/cap/day). CH4 emissions from pit latrines range from 0.77 to 20.30 g CH4/cap/day N2O emissions range from 0.76 to 1.20 gN2O/cap/day. We observed statistically significant correlations (p < 0.05) between temperature, biochemical oxygen demand, chemical oxygen demand, dissolved oxygen, storage period, and GHG emissions from OSS. However, no significant correlation (p > 0.05) was observed between soil volumetric water content and CO2 emissions. CH4 emissions (expressed as CO2 equivalents) from OSS estimated following Intergovernmental Panel for Climate Change (IPCC) guidelines were found to be seven times lower (90.99 g CO2e/cap/day) than in-situ field emission measurements (704.7 g CO2e/cap/day), implying that relying solely on IPCC guidelines may lead to underestimation of GHG emission from OSS. Our findings underscore the importance of considering local contexts and environmental factors when estimating GHG emissions from OSS. Plausible mitigation measures for GHG emissions from OSS include converting waste to biogas in anaerobic systems (e.g. biogas), applying biochar, and implementing mitigation policies that equally address inequalities in sanitation service access. Future research on GHG from OSS should focus on in-situ measurements of GHGs from pit latrines and other common OSS in developing countries, understanding the fate and transport of dissolved organics like CH4 in OSS effluents and impacts of microbial communities in OSS on GHG emissions. Addressing these gaps will enable more holistic and effective management of GHG emissions from OSS.