Sewers Could Help Clean the Atmosphere

In an article ​analyzing ​several ​possible ​technical ​approaches in ​the journal ​Nature ​Sustainability ​on Dec. 18, ​2018, ​researchers at ​Princeton ​University ​concluded that ​sewer plants ​serving ​municipalities ​worldwide offer ​a major option ​for capturing ​carbon dioxide ​and other ​greenhouse ​gases. Although ​cautioning that ​research and ​development is ​needed before ​the systems ​could be ​deployed, the ​team identified ​several ​potentially ​viable paths to ​using sewage as ​a carbon sink ​— that is,​ sewer plants ​could clean the ​atmosphere as ​they clean ​water. ​

“The ​water industry ​could play a ​big role in ​tackling ​climate change,​” said ​senior author ​Jason Ren, ​professor of ​civil and ​environmental ​engineering and ​the Andlinger ​Center for ​Energy and the ​Environment. ​“It is a ​very exciting ​idea because ​people always ​think about ​energy or ​transportation, ​but water has ​not been ​considered as a ​major factor in ​carbon ​reduction.​” ​

Sewer plants ​are massive ​industrial ​operations that ​use a variety ​of techniques ​to remove ​pollutants ​before ​wastewater ​returns to the ​environment. ​Although most ​people never ​think about the ​systems, the ​volume of water ​is staggering. ​New York City, ​for example, ​runs 14 sewer ​plants and ​processes 1.3 ​billion gallons ​of water daily (​enough to fill ​about 22,000 ​Olympic pools.) ​

In the past ​few years, ​researchers ​have proposed ​methods to use ​that wastewater ​to capture ​enough carbon ​to offset the ​amount ​generated to ​power heavy ​equipment used ​to run sewer ​plants. They ​discovered that ​some techniques ​not only would ​allow the ​plants to ​balance their ​own emissions (​cleaning water ​requires ​considerable ​energy use), ​they could also ​absorb extra ​carbon that ​operators ​pumped into the ​sewage as it ​moved through ​the plants. ​

“If you ​consider it as ​a resource, you ​could convert ​part of the ​waste material ​including the ​CO2 into ​products,”​ Ren said. ​“You ​could actually ​make money.​” ​

Generally, the ​operators would ​use pipes to ​pump carbon ​dioxide gas ​into the sewer ​water in the ​plants. They ​would then use ​a variety of ​techniques to ​convert the gas ​into carbonate ​minerals, ​biofuels or a ​sludge-based ​fertilizer ​called biochar. ​

The researchers ​reviewed a ​range of ​techniques ​including: ​

Microbial ​electrolytic ​carbon capture ​

This ​techniques uses ​a combination ​of bacteria and ​a low ​electrical ​charge to ​change the ​water’s ​alkalinity and, ​with the ​addition of ​silicates, ​convert carbon ​dioxide to ​solid carbonate ​and bi-​carbonate. In ​addition to the ​solids, which ​can be used by ​industry, the ​process creates ​large amounts ​of hydrogen gas.​ The researchers ​noted that this ​technique is ​currently used ​in the ​laboratory and ​additional work ​is needed to ​show whether it ​is economical ​and applicable ​at the ​industrial ​level. ​

Microbial electrosynthesis

Microbial ​electrosynthesis ​is similar to ​the microbial ​electrolytic ​technique ​except that the ​process relies ​on bacteria to ​directly ​capture carbon ​dioxide and ​convert it into ​other organic ​compounds such ​as ethanol or ​formic acid. ​The researchers ​noted that the ​technology is ​promising but ​major ​breakthroughs ​are needed to ​fully develop ​the process. ​

Microalgae cultivation

Microalgae ​cultivation ​could be used ​as a complement ​to other ​processes. ​Algae and ​bacteria use ​the carbon ​dioxide, ​nitrogen and ​phosphorous in ​the wastewater ​to grow. ​Operators then ​harvest the ​algae, which ​can be used as ​animal feed, ​for soil ​treatment or in ​biofuel ​production. The ​researchers ​said work is ​going forward ​on identifying ​the best local ​microbial ​communities, ​small and ​intensive ​bioreactors, ​and efficient ​techniques for ​separating ​solids and ​liquids. ​

Biochar production

This method ​converts ​wastewater ​sludge and ​microalgae into ​material that ​improves ​soil’s ​ability to ​retain water ​and nutrients. ​The technique, ​which removes ​pathogens, is ​usually self-​sufficient in ​terms of energy,​ although most ​biochar is now ​made from dry ​plants. The ​researchers ​said using ​wastewater ​sludge to make ​biochar may ​require more ​energy or ​additional ​steps to ​account for the ​additional ​water content. ​

Ren said that ​in many ​locations, ​sewer plants ​are already ​located near ​industrial ​facilities that ​emit large ​amounts of ​carbon dioxide ​such as power ​plants, cement ​factories and ​refineries. He ​said using the ​sewer systems ​to capture the ​carbon could ​provide an ​economic return ​for these ​companies in ​the form of ​carbon credits. ​He also said ​the technique ​could be used ​by industries ​that already ​run their own ​wastewater ​treatment ​systems such as ​oil and gas ​producers, ​brewers, and ​distillers. ​When analyzing ​the potential ​environmental ​and economic ​benefits of ​such operation, ​they found ​millions of ​tons of CO2 ​could be ​captured and ​utilized, while ​billions of ​dollars in ​revenue could ​be generated in ​both the U.S. ​and China, the ​world’s ​two largest CO2 ​emitters. ​

The researchers ​cautioned that ​while many ​techniques are ​promising, ​“the ​concept is ​still in its ​infancy.” ​They said that ​full use of the ​technology will ​require work of ​not only ​scientists, but ​also regulators,​ investors and ​industry. ​

Jerald Schnoor,​ an engineering ​professor at ​the University ​of Iowa, said ​national ​leaders should ​consider ​wastewater ​treatment as ​part of efforts ​to decrease the ​country’s ​carbon ​footprint in ​coming decades. ​

“​Wastewater ​treatment is ​one of the ​largest energy ​users and ​greenhouse gas ​emitters of a ​municipal ​spreadsheet,​” said ​Schnoor, who ​was not ​involved in ​this research. ​“​Technologies ​exist at pilot ​scale to ​achieve zero ​carbon and ​energy ​footprints, but ​they are not ​proven to be ​scalable or ​cost-effective ​at the current ​time. As the ​country now ​embarks on ​‘green ​infrastructure’​ initiatives, ​as being ​discussed by ​the new ​Congress, this ​should be a ​high priority.​” ​

In addition to ​Ren, the ​authors include ​first author Lu ​Lu, an ​associate ​research ​scholar at ​Princeton; ​Catherine ​Peters, a ​professor of ​civil and ​environmental ​engineering at ​Princeton; ​Jeremy Guest, ​of the ​University of ​Illinois at ​Urbana-​Champaign; Greg ​Rau, of the ​University of ​California, ​Santa Cruz; and ​Xiuping Zhu, of ​Louisiana State ​University. ​Support for the ​project was ​provided in ​part by the ​National ​Science ​Foundation. ​

Reference:

Lu Lu, Jeremy S. ​Guest, ​Catherine A. ​Peters, Xiuping ​Zhu, Greg H. ​Rau & Zhiyong ​Jason Ren, "​Wastewater ​treatment for ​carbon capture ​and utilization"​, Nature ​Sustainability, ​December 2018, ​DOI: ​10.1038/s41893-018-0187-9

Source: Princeton University