An average person produces 200-300 liters of wastewater every day, and when this domestic wastewater is combined with industrial wastewater, an estimated 1000 km³ of wastewater is generated around the world every year. This means we discharge twice of Lake Erie’s water volume every year, and by treating only a fraction of this water costs 3% of global electricity. While wastewater is now considered as a resource to contribute to a circular and low-carbon economy, we wonder if the wastewater treatment industry itself can become carbon negative, and even more, serves as a carbon sink for CO₂ valorization.

The carbon negative wastewater treatment concept or potential has not been discussed much, as the industry’s main focus has been on increasing energy efficiency and recovery. However, because many CO₂ capture and utilization (CCU) technologies use natural or clean water as the electrolyte, we got excited about the idea of using wastewater for CCU operation, because the 1000 m³/yr volume can make a big impact on global negative carbon emission, and the wastewaters need to be treated anyway, so no major infrastructure investment is needed.

In the perspective review we discussed the major negative emissions wastewater treatment technologies, and we wanted to make it clear that they are different in that they can capture and convert additional exogenous CO₂ (beyond self-generated CO₂ from organic degradation), while most other technologies only reduce overall GHG emissions without CCU capability. We used microbial electrolysis + microalgae as examples to quantify the CCU and economic benefits and found there are indeed huge potentials if current wastewater treatment plants are upgraded to carbon valorization plants, converting tens of millions of tonnes of CO₂ and generate billions of dollars in revenue.

But is this concept realistic? We think so. There are untapped synergies on carbon management that has not been explored, and a low hanging fruit we found is the co-location of a major carbon emitter such as a power plant or refinery with a wastewater treatment plant. Interestingly, we found such co-location is pretty common. For example, the attached Google image shows stunning coal storage piles in a power plant and the large water tanks in the nextdoor wastewater facility. By using our idea, the coal pile will become a modern CO₂ refinery to produce negative-emission chemicals.

While we know there’s a long way ahead, we are confident that with the engagement of different stakeholders this dream can one day become reality. Addressing multiple challenges to help achieve U.N. Sustainable Development Goals, we are hopeful that utilities of the future will serve the society in multi-latitudes.