News headlines often feature stories of water scarcity challenges and increasing competition for water. So it is clear that we need to improve the efficiency and effectiveness of water systems globally – especially in the realm of irrigation management, the water we use to grow our food. However, a data gap exists about evapotranspiration (ET) which, if fixed, would help us understand the amount of water available and used in irrigation and would help us to have more accurate water balances at the basin level.
In one of Mexico City’s most populated areas, Iztapalapa, there is a street named Alessandro Volta. With little knowledge about who this man was, we researched a bit and found that Alessandro Volta concluded, in 1776, that there was “a direct correlation between the amount of decaying organic matter and the amount of flammable gas produced”. Sir Humphry Davy determined 32 years later (in 1808) that methane was present in the gases produced during the anaerobic digestion (AD) of cattle manure. The first digestion plant was built at a leper colony in Bombay, India in 1859 (just 83 years later!).
Anaerobic digestion (AD) as a renewable resource has been growing since, in the international context, and has the potential to be a sustainable, affordable solution for wastewater management. In the 21st century, we are still fascinated with the idea of the benefits of biogas production. In modern times, AD is being used as a reliable energy source, and sludge resulting from AD processes can be used as fertilizer. Countries like the UK are producing enough biogas to power 1 million homes, 210 years after Sir Humphry Davy’s discoveries. In fact, according to a new report from the Anaerobic Digestion & Bioresources Association (ADBA) of the UK, in 2017 the total energy generation from anaerobic digestion plants reached 10.7 Terawatt-Hour (TWh) / year.
What do Yucatan (Mexico), Michoacan (Mexico), Karur (India), and Jan Kempdorp (South Africa) have in common? These are all places with successful stories of implementing Anaerobic Digestion (AD) for wastewater treatment. But what is AD? What are the benefits?
AD systems are installed for many different purposes, such as a waste treatment step, a means to reduce odours, a source of additional revenues, or a way to improve public image. The AD treats water and waste, reducing adverse environmental impacts. Through AD, two main by-products can be obtained: biogas—that can be used as a fuel, and sludge—that can be used as a soil amender for improve crops. These AD “by-products” are important in the context of mitigating the impacts of climate change, where environmental co-benefits come from efficient use of “by-products”. For instance, livestock enteric fermentation, livestock waste management, rice cultivation, and agricultural waste burning are all sources of methane emissions, representing between 7 and 10 percent of global methane emissions. AD not only treats water through an environmentally sustainable approach, but also contributes to produce high rates of methane for recovery and further utilization.
The region is also not homogenous. There is a large disparity in the levels of treatment per country: we see countries like Chile, which treats 90% of its wastewater, and countries like Costa Rica, which treats approximately 4% of its wastewater.
For the last six years, a power plant in San Luis Potosi, Mexico has bought water from a nearby wastewater treatment plant to use in its cooling towers (instead of using freshwater). This operation, Project Tenorio, a public-private partnership, continues today and has already resulted in the reduction of groundwater extraction of at least 48 million cubic meters (equivalent to 19,000 Olympic size pools) and increased aquifer sustainability.
This is a good example of the water and energy nexus in practice: the wastewater treatment plant covers almost all of its operating costs from this additional revenue stream and the power plant gets a more reliable water source that is also 33% cheaper than groundwater in that area.
Treated wastewater has been used to reduce the water requirements of power plants in several other countries as well, as water supply becomes more variable or disappears. In the US, for example, around 50 power plants are using treated wastewater for cooling in order to adapt to water shortages. However, innovative integrated approaches like these are still more of an exception than the norm.