Proper disposal of human waste, particularly in developing countries, has always been a challenge for researchers because of the inadequacies in infrastructure and funding for treatment on a large scale. In 2011, the Bill & Melinda Gates Foundation began the “Reinvent the Toilet” initiative that sought to fund innovations in affordable wastewater treatment to allow for the sustainable disposal of human waste for around 4.2 billion people worldwide who lack basic sanitation.
A group of engineers from Duke University conducted studies on a large-scale, real-world field trial of their off-grid sanitation system and published their results in the journal Science of the Total Environment.
Brian Hawkins, research scientist in the Duke Center for Water, Sanitation, Hygiene and Infectious Disease (WaSH-AID), said that the first step in their reinvented affordable wastewater treatment system was to separate the solid and liquid waste through a conveyor belt made of rubber bands. “It works great in India where they have a washing culture, but in South Africa, where they have a wiping culture, the toilet paper got into the mix and gummed up all of the gears. That resulted in the system needing cleaning every couple of days, which is not sustainable,” Hawkins said.
The Gates Foundation established criteria that needed to be met for their affordable wastewater treatment system to be successfully considered for further funding. It not only needed to remove pathogens from human waste and recover resources such as energy, clean water and nutrients, but it must also accomplish without the need for external electricity or water sources. Finally, the systems must also cost less than five cents per user, per day which amounts to $10 per 200 users.
One major concern of the researchers was how long the system can run before critical maintenance was required. To address this issue, the Duke researchers installed a prototype waste treatment system at two locations—a women’s dormitory at a textile mill in Coimbatore, India and a communal ablution block at the edge of Durban, South Africa. After eight months, both systems managed to serve up to 50 potential users at any given time, processing more than 11,000 liters of waste throughout the field trials.
In their system, the solids are separated from the liquids and are pushed through a large activated carbon filter. Although the resulting liquid is free of biosolids, there is still the problem of disinfection and salt accumulation. For the latter, the team found that running the wastewater through an electric current which broke down the molecular bonds of the remaining salts while resulting in the production of chlorine oxidants which act as a potent disinfectant.
The field trial results proved the potential of their system’s performance. Duke’s sanitation system met all of the biological criteria but only managed to meet three out of the five chemical standards for liquid effluent according to recently released stand-alone sewage treatment requirements from the International Organization for Standardization (ISO).
They are currently figuring out a process that would remove the nitrogen and phosphorus from the effluent in an efficient and affordable manner. Meeting all these requirements would eventually enable them to expand their project even further.
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