26 May From waste to wealth: Effluent from pit latrines to energy and fertiliser
In South Africa 31% of the sanitation needs of households (both rural and urban) are met by pit latrines. (Buckley et al., 2015).
These pit latrines have been constructed as a substitute for toilets with running water for a number of reasons. The most common being that residents in households cannot afford running water and sewerage connections, or that there is simply no infrastructure available in terms of running water and sewerage pipelines in their communities (Still, 2020) .
With 90 billion rand per year over 10 years being required to bridge the infrastructure and development needs for water and sanitation in South Africa (GreenCape, 2020), it is unlikely that even the households which can afford to pay for water and sanitation will be connected to main water and sewerage lines in the near-term. This becomes more apparent when considering that 56% of South African households do not pay for water either because they cannot afford to do so or they do not want to (StatsSA, 2016). This means municipalities may not have the required revenue to undertake the number of developmental and infrastructure projects required.
The faecal sludge which comes from the use of pit latrines creates a host of environmental and health problems. Over time, pit latrines without lining allow faecal sludge to leach into groundwater reserves as well as soil and crops. This leaching may result in the contamination of ground water, soil and crops with pathogens such as Hepatitis, E.Coli, Salmonella, Shigella and Protozoan Cysts to name a few (WHO, 2000). As a result of these pathogens the World Health Organisation estimates that a global total of 2.2 million people die each year from diarrhoea related diseases. In addition, 10% of the population of developing countries are severely infected with intestinal worms related to improper handling or disposal of faecal sludge. The eutrophication of water ways due to the rich nutrients in the faecal sludge also posses a problem to water quality and aquatic life.
The current methods of emptying pit latrines include vacuum pumps, transportation by tanker (where communities are easily accessible via roads) and manual emptying with spades and buckets (Buckley et al.,2015). The tankers generally transport the faecal sludge to waste water treatment works for disposal and further processing, while the other disposal methods often result in the faecal sludge being disposed of in water bodies or covered with soil in the ground.
Faecal sludge, like agricultural and food waste can be processed to biogas through anaerobic digestion (a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen) (Andriana et al., 2015).
Biogas can comprise of 50% methane and about 45-48% carbon dioxide, with the remainder being hydrogen sulphide (highly dependant on faecal properties such as human diet) (Duffy, 2017).
If faecal sludge is properly collected for the production of biogas this could reduce the improper handling and disposal of faecal matter and avoid the environmental and health problems discussed above. In addition, the process could provide communities with cheaper methods of obtaining energy.
GreenCape has shown that the cost of electricity provision by ESKOM has risen by about 300% between 2007-2019, while the cost of energy production using renewables has dropped by about 77% between 2010 and 2018 (GreenCape, 2020). This shows renewable energy sources have become more and more affordable.
As an approximation, 10,000 people/day produce about 2,500 kg of faecal waste, which can produce approximately 106 cubic metres of methane or 652Kw/day (Andriani et al.,2015). To put this into perspective, 1 cubic metre of methane can produce enough energy to power a 60-100 watt light bulb for 6 hours or cook 3 meals a day for a family of 6 (Andriani et al.,2015). This energy content can be higher if the biogas produced contains a higher percentage of methane, or if the biogas is further refined by removing the unwanted carbon dioxide fraction.
Capital costs including purchasing the required equipment (anaerobic digester, mixers, tanks, heating systems, pumps) to produce biogas from faecal sludge in the scenario above take approximately 4.6 years to recover (Moser et al., 2017). There are many business models that can be applied to make this work for communities. As an example, households can pay a surcharge to get their pit latrine waste removed and converted to energy at a centralised facility within their community in return for a certain amount of household energy. Another option is that the waste collector and energy providers can purchase the waste from these households, it is then up to the individual households how much energy they wish to purchase from the centralised facility within their community.
An additional bi-product from the production of bio-gas is pathogen free, nutrient rich fertiliser that can be used for crop production or sold for extra income within the communities.
The benefits of biogas production from faecal sludge are not just applicable for rural and urban communities, but also for waste water treatment works (WWTW). Currently WWTW consume roughly 25% of municipal electricity expenditure. They would also stand to benefit from producing and capturing their own biogas. Conservative estimates show that if they were to start doing this, the WWTW could cut down on 10-30% of their energy requirements drawn externally from ESKOM (GreenCape, 2020).
In the case of WWTW converting faecal sludge to biogas, further processing of the biogas to purify it and make it energy dense through the removal of carbon dioxide would be required. This could be done through utilising a number of readily available processes such as pressure swing absorption, membrane separation and carbon dioxide absorption (Duffy, 2017).
Opportunities exist for both the removal of waste and the beneficiation of it through energy production. This can be through the production of biogas for use at home in lanterns and stoves, or further processing the biogas to generate electricity. An additional benefit is the readily available pathogen free, nutrient rich fertiliser that is a bi-product of this biogas production process.
In conclusion, opportunities exist for private-public partnerships in rural and urban communities to tackle infrastructure, developmental, environmental and health issues by focusing on converting waste to value added products. The achievement of developmental progress in communities should not be viewed solely as government’s responsibility, but rather as a collaborative effort between the private and public sector.