What goes down our toilet is commonly viewed as waste. This makes intuitive sense because separating people from their excreta – sanitation – is arguably the single most important public health objective, given the massive global disease burden of faecally-transmitted diseases.
Sadly 2.5 billion people still lack access to sanitation. Diarrhoeal disease is the biggest resultant travesty, killing 1.5 million children under the age of five every year. This equates to one in five children globally and accounts for more paediatric deaths than malaria, AIDS and measles combined.
Sanitation — in essence, the toilet — is an incredibly effective means of lessening the devastating impact of many faecally-transmitted diseases. The World Health Organization states that it reduces diarrhoeal morbidity by 38%.
But there is also an oft-forgotten secondary benefit arising from sanitation. Simply through providing an organised collection and distribution system, this nutrient-laden “waste” can also be seen as resource rather than a problematic substance to be disposed of.
Sanitation systems vary considerably, from simple pit latrines, to septic systems, through to large-scale reticulated sewerage systems servicing thousands of households. The types of “waste” are accordingly labelled: “sewage” for the waste being carried by a sewer and, equally imaginatively, “septage” for the contents of a septic tank.
At the end of the day though, there are just two fundamental types worth considering: a liquid component, wastewater (a broader term encompassing sewage, greywater and other sources) and a solid component, faecal sludge (more politely but misleadingly called “biosolids” in many of the more conservative Western societies).
There are various potential uses for faecal sludge and wastewater. But unquestionably the greatest potential and the largest current uses are in crop productivity improvement.
Human waste plays a vital role in providing food and sustaining livelihoods throughout the developing world. And it is becoming increasingly important in developed nations as well, particularly water-stressed countries like Australia, where wastewater irrigation schemes are now commonplace.
Faecal sludge can rejuvenate impoverished soils. Wastewater, while carrying some free fertiliser, is primarily just another irrigation source, and often the only option for the millions of poor urban and peri-urban vegetable farmers throughout Africa and Asia.
In Ghana, faecal sludge has been reported to lead to two- to three-fold increases in yield when applied to cereal crops and is highly sought after by farmers. It is also attractive to Ghanaian farmers because it is free (except for a tip of about US$2 to the truck driver per load). In contrast, a 50-kg bag of NPK fertiliser costs around US$17.
The preferred form of faecal sludge in Ghana is from septic tanks. In some instances it is “dewatered” in a drying bed: a shallow concrete basin designed to maximise evaporation. Such dewatered sludge is very high in organic matter, which means that it can be composted in conjunction with other solid organic waste to produce an excellent crop fertiliser.
Wastewater irrigation of crops is common throughout the world. But despite some very rough estimates (for example, 20 million ha), we really have no idea of its true extent.
In many African countries it is technically illegal, but authorities tend to turn a blind eye to it because they recognise its importance. Wastewater irrigation generally faces less official opposition in Asia.
The use of raw or partially-diluted wastewater is common in many developing countries, and treatment is the exception rather than the rule. This of course raises the question of disease risk, as with excreta.
These risks are real and need to be dealt with accordingly, and the WHO provides guidance on how this can be achieved. It is also crucial to consider the bigger picture of public health, and this includes the risks associated with using human waste as well as the benefits it has to offer in terms of supporting livelihoods and providing food.
There are also agricultural risks and benefits to consider. For example, the benefit of continuity of wastewater supply needs to be weighed against potential degradation of soil structure and decreased yield resulting from high salt concentrations.
Owing to its function of separating humans from their waste, the humble toilet has been rightly lauded as one of the most significant contributions to public health. But it is time for it to be viewed as a nutrient and water depository rather than a bin.