The United Nations (UN) sustainable development goal (SDGs) # 6 is about “Clean Water and Sanitation”. The world is thriving towards the milestone that everyone accesses improved sanitation. Safely managed sanitation however goes beyond and is a matter of hygiene, health, dignity, and human rights. In 2010, the UN General Assembly explicitly recognized the human right to access improved sanitation. In 2020, 3.6 billion people lacked safely managed services. These include 1.9 billion people with access to basic services, 580 million with limited services, 616 million using unimproved sanitation facilities, and 494 million who still practice open defecation.

Access to a toilet and primary treatment options septic tanks and soak-pits is the starting point of a healthy sanitation system. Countries like China and India have built millions of toilets in the past few years, especially in rural areas, to end open defecation and provide a healthy living environment. However, building a toilet is just the first step to establish a safely managed, environmentally-friendly sanitation system. A huge area of focus is the wastewater excretion from the sewage system. This wastewater needs to be appropriately managed, and valuable resources need to be recovered. Globally, more than 80 percent of wastewater returns to the environment without any treatment. In some countries of the Global South, the share is as high as 95%. This is a major issue that leads to unsanitary living conditions and the spread of damaging diseases.

Infrastructure such as conventional centralized sewerage systems is assumed normality in the Global North. In the Global South where on-site sanitation systems are most prevalent, the management of fecal sludge and other non-sewered sanitation systems (NSSS) has started to emerge as considered optimal in view that the conventional solution is expensive and water-intensive. This Research Topic aims to identify and establish an alternative sanitation system that is affordable and accessible to the Global South, to mitigate the spread of disease and provide a healthier living environment.

Further maintenance must never be neglected to sustain the functioning of the sanitation systems. Public awareness needs to be cultivated towards optimal sanitation behavior and move towards financial sustainability. Nowadays, the worldwide intention is to develop and establish non-conventional sanitation systems that could function well and meet the SDGs. For instance, Bill & Melinda Gates Foundation launched the Reinvent the Toilet Challenge to stimulate the development of new toilet technologies that safely and effectively manage human waste. In China and India, on the other hand, Faecal Sludge Management (FSM) is gaining traction and being rapidly deployed in many states.

This collection hopes to highlight the sanitary issues in the current world and new methods and strategies to optimize sanitation systems efficiently. This collection seeks high-quality works and topics focusing on the following aspects of sanitation:

• Economically viable methods to optimize conventional centralized sanitation systems in a low/mid-income setting as the major portion of public funding goes into establishing and managing centralized conventional sanitation systems.

• Techno-financial and environmental viability of non-sewered sanitation system (NSSS)

• Water-saving technologies and strategies in sanitation systems, including optimizing water management and saving water (examples include vacuum toilet, dry toilet, reclaimed flushing water toilet, as well as reinvented toilet challenge by Bill & Melinda Gates Foundation)

• Energy- and resource recovery from sanitation systems, including nutrient recovery, blackwater/greywater/yellow water, fecal sludge, and urine source separating system.

• Sustainable technological solutions in WASH (water, sanitation, and hygiene) where sustainability is understood in relation to social, economic, and environmental and political aspects. This may include a life cycle assessment of sanitation systems.

• Sanitation technological and institutional solutions for low-income areas.

• Engineering education and sustainability concerns in WASH.