Energy Positive Waste Water Treatment Plants

Profitable waste water treatment: a new strategy to face global warming

A very ambitious objective is being pursuit by the French startup Nxo which promotes and develops innovative eco-solutions using microalgae catalyzers to treat sewage and make it profitable. Nxo’s team has designed a new Waste Water Treatment Plant (WWTP) scheme based on biological oxidation in algae reactors, flotation cells, anaerobic digestion and an earthworms-compost process to transform the residual digestate into a bio-fertilizer. The whole setup offers the possibility to overcome 3 global challenges simultaneously: water scarcity (recycling/reuse), circular economy (zero organic waste) and global warming (a negative carbon balance).  

Rebalancing water treatment and energy cost

Even if the idea of an energy producing WWTP is not new, today (2/2017) there is no real-scale demonstration plant achieving it. Encouraged by the progress made by its Spanish colleagues of FCC Aqualia through the All-Gas project, César NARVÁEZ, CEO and co-founder of Nxo put together a strategy to make this vision a reality: converting WWTP into hydrocarbon (methane) production facilities. What we call an energy-positive WWTP. While studying this new perspective closely, he had a conversation with a colleague that gave him an idea. He states: “I was discussing with a friend about losing weight, for him there were two strategies: either you eat less (you intake less energy than your body consumes) or you eat like always but you do some sport to burn the extra calories (you increase your metabolism energy demand). This obvious fact made me realize that accomplishing the energy-positive concept would not pass by improving digesters performance (which produces methane from waste). We need to focus on the energy demand of established technologies in order to find energy efficient alternatives without compromising performance”.  

Standard waste water treatment VS Nxo’s technology

WWTP has adopted activated sludge as the standardized technology for almost one century but the drawback is the energy cost. Treating a cubic meter of water consumes around 0.8 kWh. Nxo’s microalgae bioreactors operate efficiently by consuming only 0.11 kWh for a better depuration result and without using chemical products to eliminate phosphates. They also offer an extra huge advantage: the reclaimed water meets French regulations for recycling (in irrigation or other activities). Algae reactors present a minor OPEX (operational expenditure) as they require only a low-energy propeller to keep them in suspension. On the other hand, activated sludge units need constant large amounts of air provided by air compressors, thus electricity, to keep bacteria oxidizing organic compounds. The energy demand ratio is approximately 1 to 8 in favor of algae. These bioreactors can operate with a fraction of the electricity obtained by the biogas.  

Environmental perspective

In France particularly, this technological package seduces citizens who are well aware that nearly 40% of the potable water price today is due to sewage sanitation before rejection to the environment. Adapting this eco-improvement in WWTP will reduce at least by 3 the cost of their vital commodity: potable water. Concerning the environmental frame, Nxo’s reactors foster CO₂ bioremediation thanks to microalgae. In the CEO’s perspective, WWTP have the potential to become “climate change mitigators or CO₂ managers” consuming more CO₂ than generated even after the combustion of the produced biogas (a negative carbon balance). This is the will of the European Commission for the Horizon 2020.  

Social impact

As a final point, there is also a social envelope considered in the strategy. Local farmers will profit by having access to reclaimed water and bio-fertilizers, either freely or by a commercial transaction. A win/win approach for municipalities, producers and again for the environment thanks to this zero organic waste formula. In a nutshell, while water stakeholders are joining efforts to reach energy neutrality, Nxo’s philosophy remain that WWTP should operate as a profitable structure. Adopting this breakthrough innovation, where the energetic demand is lower than the production, is going to create collateral revenues by the commercialization of reclaimed water, bio-fertilizers and the exceeding natural gas (public transportation or reinjection to the city network).  

Nxo’s CEO’s vision

Nxo’s CEO firmly believes that when a new global challenge arises, there is also room for development and innovation. He states: “…In every crisis, there is an opportunity. I’ve recently read that the current president of the Occitanie’s Region, Carole Delga, has expressed a very ambitious objective: to become the first Energy-Positive Region in Europe by the horizon 2050. I totally share her vision. We need inspiring goals to make our economy move forward. This is an opportunity to reexamine our past convictions and to conceive more efficient technologies that, in the end, will raise people’s life quality and preserve our planet”.