1. den Elzen, M. et al. The Emissions Gap Report 2017 (United Nations Environment Programme 2017). A detailed quantitative report that demonstrates the need for negative emission.

2. Fuss, S. The 1.5°C Target, Political Implications, and the Role of BECCS (Oxford Univ. Press, 2017).

3. Minx, J. C. et al. Fast growing research on negative emissions. Environ. Res. Lett. 12, 035007 (2017).

4. Rogelj, J. et al. Energy system transformations for limiting end-of-century warming to below 1.5 °C. Nat. Clim. Change 5, 519–527 (2015).

5. Rau, G. H. et al. Direct electrolytic dissolution of silicate minerals for air CO 2 mitigation and carbon-negative H 2 production. Proc. Natl Acad. Sci. USA 110, 10095–10100 (2013).

6. Alex, S. Tapping sewage as a source of useful materials. Chem. Eng. News 95, 30–34 (2017).

7. Mohan, S. V. et al. Waste biorefinery: a new paradigm for a sustainable bioelectro economy. Trends Biotechnol. 34, 852–855 (2016).

8. Mateo-Sagasta, J., Raschid-Sally, L. & Thebo, A. in Wastewater: Economic Asset in an Urbanizing World (eds Drechsel, P., Qadir, M. & Wichelns, D.) Ch. 2, 15–38 (Springer, 2015).

9. Sato, T. et al. Global, regional, and country level need for data on wastewater generation, treatment, and use. Agric. Water Manag. 130, 1–13 (2013).

10. McCarty, P. L., Bae, J. & Kim, J. Domestic wastewater treatment as a net energy producer–can this be achieved? Environ. Sci. Technol. 45, 7100–7106 (2011).

11. Li, W.-W., Yu, H.-Q. & Rittmann, B. E. Chemistry: reuse water pollutants. Nature 528, 29–31 (2015).

12. Rosso, D. & Stenstrom, M. K. The carbon-sequestration potential of municipal wastewater treatment. Chemosphere 70, 1468–1475 (2008).

13. Shahabadi, M. B., Yerushalmi, L. & Haghighat, F. Impact of process design on greenhouse gas (GHG) generation by wastewater treatment plants. Water Res. 43, 2679–2687 (2009).

14. IPCC Climate Change 2014: Mitigation of Climate Change (eds Edenhofer, O. et al.) (Cambridge Univ. Press, 2014).

15. Global Anthropogenic Non-CO 2 Greenhouse Gas Emissions: 1990–2030 (USEPA, 2012).

16. Griffith, D. R., Barnes, R. T. & Raymond, P. A. Inputs of fossil carbon from wastewater treatment plants to US rivers and oceans. Environ. Sci. Technol. 43, 5647–5651 (2009).

17. Inventory of US Greenhouse Gas Emissions and Sinks: 1990–2015 (USEPA, 2017).

18. Shizas, I. & Bagley, D. M. Experimental determination of energy content of unknown organics in municipal wastewater streams. J. Energy Engin. 130, 45–53 (2004).

19. Heidrich, E., Curtis, T. & Dolfing, J. Determination of the internal chemical energy of wastewater. Environ. Sci. Technol. 45, 827–832 (2010).

20. Bradsher, K. & Friedman, L. China unveils an ambitious plan to curb climate change emissions. New York Times (19 December 2017).

21. Rogelj, J. et al. Paris Agreement climate proposals need a boost to keep warming well below 2 °C. Nature 534, 631–639 (2016).

22. Salek, S. S. et al. Mineral CO 2 sequestration by environmental biotechnological processes. Trends Biotechnol. 31, 139–146 (2013).

23. Wang, H. & Ren, Z. J. A comprehensive review of microbial electrochemical systems as a platform technology. Biotechnol. Adv. 31, 1796–1807 (2013).

24. Lu, L., Huang, Z., Rau, G. H. & Ren, Z. J. Microbial electrolytic carbon capture for carbon negative and energy positive wastewater treatment. Environ. Sci. Technol. 49, 8193–8201 (2015). Demonstrated carbon-negative MECC process for wastewater treatment.

25. Lu, L. et al. Self-sustaining carbon capture and mineralization via electrolytic carbonation of coal fly ash. Chem. Eng. J. 306, 330–335 (2016).

26. Zhu, X., Hatzell, M. C. & Logan, B. E. Microbial reverse-electrodialysis electrolysis and chemical-production cell for H 2 production and CO 2 sequestration. Environ. Sci. Technol. Lett. 1, 231–235 (2014).

27. Zhu, X. & Logan, B. E. Microbial electrolysis desalination and chemical-production cell for CO 2 sequestration. Bioresour. Technol. 159, 24–29 (2014).

28. Huang, Z., Jiang, D., Lu, L. & Ren, Z. J. Ambient CO 2 capture and storage in bioelectrochemically mediated wastewater treatment. Bioresour. Technol. 215, 380–385 (2016).

29. Pandey, P. et al. Recent advances in the use of different substrates in microbial fuel cells toward wastewater treatment and simultaneous energy recovery. Appl. Energy 168, 706–723 (2016).

30. Lu, L. et al. Hydrogen production, methanogen inhibition and microbial community structures in psychrophilic single-chamber microbial electrolysis cells. Energy Environ. Sci. 4, 1329–1336 (2011).

31. Li, W.-W., Yu, H.-Q. & He, Z. Towards sustainable wastewater treatment by using microbial fuel cells-centered technologies. Energy Environ. Sci. 7, 911–924 (2014).

32. Lu, L. & Ren, Z. J. Microbial electrolysis cells for waste biorefinery: A state of the art review. Bioresour. Technol. 215, 254–264 (2016).

33. Sun, M. et al. An MEC-MFC-coupled system for biohydrogen production from acetate. Environ. Sci. Technol. 42, 8095–8100 (2008).

34. Lu, L. et al. Microbial photoelectrosynthesis for self-sustaining hydrogen generation. Environ. Sci. Technol. 51, 13494–13501 (2017).

35. Rozendal, R. A. et al. Towards practical implementation of bioelectrochemical wastewater treatment. Trends Biotechnol. 26, 450–459 (2008).

36. Lee, H.-S., Vermaas, W. F. J. & Rittmann, B. E. Biological hydrogen production: prospects and challenges. Trends Biotechnol. 28, 262–271 (2010).

37. Rabaey, K. & Rozendal, R. A. Microbial electrosynthesis—revisiting the electrical route for microbial production. Nat. Rev. Microbiol. 8, 706–716 (2010). Comprehensive review that revealed the potential of MES.

38. Blankenship, R. E. et al. Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement. Science 332, 805–809 (2011).

39. Claassens, N. J. et al. Harnessing the power of microbial autotrophy. Nat. Rev. Microbiol. 14, 692–706 (2016).

40. Patil, S. A. et al. A logical data representation framework for electricity-driven bioproduction processes. Biotechnol. Adv. 33, 736–744 (2015).

41. Christodoulou, X., Okoroafor, T., Parry, S. & Velasquez-Orta, S. B. The use of carbon dioxide in microbial electrosynthesis: Advancements, sustainability and economic feasibility. J. CO 2 Util. 18, 390–399 (2017)..

42. Jiang, Y. et al. Carbon dioxide and organic waste valorization by microbial electrosynthesis and electro-fermentation. Water Res. 149, 42–55 (2019).

43. Nevin, K. P. et al. Electrosynthesis of organic compounds from carbon dioxide is catalyzed by a diversity of acetogenic microorganisms. Appl. Environ. Microbiol. 77, 2882–2886 (2011).

44. Zaybak, Z., Pisciotta, J. M., Tokash, J. C. & Logan, B. E. Enhanced start-up of anaerobic facultatively autotrophic biocathodes in bioelectrochemical systems. J. Biotechnol. 168, 478–485 (2013).

45. Bajracharya, S. et al. Application of gas diffusion biocathode in microbial electrosynthesis from carbon dioxide. Environ. Sci. Pollut. Res. 23, 22292–22308 (2016).

46. Arends, J. B., Patil, S. A., Roume, H. & Rabaey, K. Continuous long-term electricity-driven bioproduction of carboxylates and isopropanol from CO 2 with a mixed microbial community. J. CO 2 Util. 20, 141–149 (2017).

47. Blanchet, E. et al. Importance of the hydrogen route in up-scaling electrosynthesis for microbial CO 2 reduction. Energy Environ. Sci. 8, 3731–3744 (2015).

48. Deutzmann, J. S., Sahin, M. & Spormann, A. M. Extracellular enzymes facilitate electron uptake in biocorrosion and bioelectrosynthesis. mBio 6, e00496–15 (2015).

49. Aryal, N., Ammam, F., Patil, S. A. & Pant, D. An overview of cathode materials for microbial electrosynthesis of chemicals from carbon dioxide. Green Chem. 19, 5748–5760 (2017).

50. Barry, A. et al. National Algal Biofuels Technology Review (US Department of Energy, Office of Energy Efficiency and Renewable Energy Bioenergy Technologies Office, 2016).

51. Moody, J. W., McGinty, C. M. & Quinn, J. C. Global evaluation of biofuel potential from microalgae. Proc. Natl Acad. Sci. USA 111, 8691–8696 (2014).

52. Shoener, B., Bradley, I., Cusick, R. & Guest, J. Energy positive domestic wastewater treatment: the roles of anaerobic and phototrophic technologies. Environ. Sci. Process. Impacts 16, 1204–1222 (2014). A quantitative, comparative review on energy recovery potential.

53. Li, Y. et al. Quantitative multiphase model for hydrothermal liquefaction of algal biomass. Green Chem. 19, 1163–1174 (2017).

54. Leow, S. et al. Prediction of microalgae hydrothermal liquefaction products from feedstock biochemical composition. Green Chem. 17, 3584–3599 (2015).

55. Geider, R. J. & La Roche, J. Redfield revisited: variability of C:N:P in marine microalgae and its biochemical basis. Eur. J. Phycol. 37, 1–17 (2002).

56. Klausmeier, C. A., Litchman, E., Daufresne, T. & Levin, S. Phytoplankton stoichiometry. Ecol. Res. 23, 479–485 (2008).

57. Tchobanoglous, G. et al. Wastewater Engineering: Treatment and Resource Recovery (McGraw-Hill, 2014).

58. Gardner-Dale, D., Bradley, I. & Guest, J. Influence of solids residence time and carbon storage on nitrogen and phosphorus recovery by microalgae across diel cycles. Water Res. 121, 231–239 (2017).

59. Valverde-Pérez, B., Ramin, E., Smets, B. F. & Plósz, B. G. EBP2R–an innovative enhanced biological nutrient recovery activated sludge system to produce growth medium for green microalgae cultivation. Water Res. 68, 821–830 (2015).

60. Becker, E. Micro-algae as a source of protein. Biotechnol. Adv. 25, 207–210 (2007).

61. Golueke, C. G., Oswald, W. J. & Gotaas, H. B. Anaerobic digestion of algae. Appl. Microbiol. 5, 47–55 (1957).

62. Laurens, L. M. et al. Development of algae biorefinery concepts for biofuels and bioproducts; a perspective on process-compatible products and their impact on cost-reduction. Energy Environ. Sci. 10, 1716–1738 (2017).

63. Mooij, P. R. et al. Survival of the fattest. Energy Environ. Sci. 6, 3404–3406 (2013).

64. Hu, Y., Hao, X., van Loosdrecht, M. & Chen, H. Enrichment of highly settleable microalgal consortia in mixed cultures for effluent polishing and low-cost biomass production. Water Res. 125, 11–22 (2017).

65. Ptacnik, R. et al. Diversity predicts stability and resource use efficiency in natural phytoplankton communities. Proc. Natl Acad. Sci. USA 105, 5134–5138 (2008).

66. Rich, L. G. Unit Processes of Sanitary Engineering (John Wiley and Sons, 1963).

67. Luo, S., Berges, J. A., He, Z. & Young, E. B. Algal-microbial community collaboration for energy recovery and nutrient remediation from wastewater in integrated photobioelectrochemical systems. Algal Res. 24, 527–539 (2016).

68. Wu, H. et al. Strategies and techniques to enhance constructed wetland performance for sustainable wastewater treatment. Environ. Sci. Pollut. Res. 22, 14637–14650 (2015).

69. Wu, S. et al. Development of constructed wetlands in performance intensifications for wastewater treatment: a nitrogen and organic matter targeted review. Water Res. 57, 40–55 (2014).

70. Mander, Ü. et al. Greenhouse gas emission in constructed wetlands for wastewater treatment: a review. Ecol. Eng. 66, 19–35 (2014).

71. Mander, Ü. et al. Gaseous fluxes in the nitrogen and carbon budgets of subsurface flow constructed wetlands. Sci. Total Environ. 404, 343–353 (2008).

72. García, J. et al. Anaerobic biodegradation tests and gas emissions from subsurface flow constructed wetlands. Bioresour. Technol. 98, 3044–3052 (2007).

73. Brix, H., Sorrell, B. K. & Lorenzen, B. Are phragmites-dominated wetlands a net source or net sink of greenhouse gases? Aquat. Bot. 69, 313–324 (2001). Shows the complexity of constructed wetlands in greenhouse gas emissions.

74. de Klein, J. J. & van der Werf, A. K. Balancing carbon sequestration and GHG emissions in a constructed wetland. Ecol. Eng. 66, 36–42 (2014).

75. Lu, L., Xing, D. & Ren, Z. J. Microbial community structure accompanied with electricity production in a constructed wetland plant microbial fuel cell. Bioresour. Technol. 195, 115–121 (2015).

76. Qambrani, N. A. et al. Biochar properties and eco-friendly applications for climate change mitigation, waste management, and wastewater treatment: a review. Renew. Sustain. Energy Rev. 79, 255–273 (2017).

77. Hossain, M. K. et al. Influence of pyrolysis temperature on production and nutrient properties of wastewater sludge biochar. J. Environ. Manage. 92, 223–228 (2011).

78. Bird, M. I. et al. Algal biochar–production and properties. Bioresour. Technol. 102, 1886–1891 (2011).

79. Huggins, T. M., Haeger, A., Biffinger, J. C. & Ren, Z. J. Granular biochar compared with activated carbon for wastewater treatment and resource recovery. Water Res. 94, 225–232 (2016).

80. Xiao, X. et al. Insight into multiple and multilevel structures of biochars and their potential environmental applications: a critical review. Environ. Sci. Technol. 52, 5027–5047 (2018).

81. Woolf, D. et al. Sustainable biochar to mitigate global climate change. Nat. Commun. 1, 56 (2010). Demonstrates the potential of biochar for GHG sequestration.

82. Méndez, A., Gómez, A., Paz-Ferreiro, J. & Gascó, G. Effects of sewage sludge biochar on plant metal availability after application to a Mediterranean soil. Chemosphere 89, 1354–1359 (2012).

83. Hossain, M. K., Strezov, V., Chan, K. Y. & Nelson, P. F. Agronomic properties of wastewater sludge biochar and bioavailability of metals in production of cherry tomato (Lycopersicon esculentum). Chemosphere 78, 1167–1171 (2010).

84. Alhashimi, H. A. & Aktas, C. B. Life cycle environmental and economic performance of biochar compared with activated carbon: a meta-analysis. Resour. Conserv. Recycl. 118, 13–26 (2017).

85. Yu, K. L. et al. Microalgae from wastewater treatment to biochar–feedstock preparation and conversion technologies. Energy Convers. Manage. 150, 1–13 (2017).

86. Lackner, K. S. et al. The urgency of the development of CO 2 capture from ambient air. Proc. Natl Acad. Sci. USA 109, 13156–13162 (2012).

87. Monteith, H. D., Sahely, H. R., MacLean, H. L. & Bagley, D. M. A rational procedure for estimation of greenhouse-gas emissions from municipal wastewater treatment plants. Water Environ. Res. 77, 390–403 (2005).

88. Smith, A. L. et al. Perspectives on anaerobic membrane bioreactor treatment of domestic wastewater: a critical review. Bioresour. Technol. 122, 149–159 (2012).

89. Kampschreur, M. et al. Emission of nitrous oxide and nitric oxide from a full-scale single-stage nitritation-anammox reactor. Water Sci. Technol. 60, 3211–3217 (2009).

90. Miller-Robbie, L., Ramaswami, A. & Amerasinghe, P. Wastewater treatment and reuse in urban agriculture: Exploring the food, energy, water, and health nexus in Hyderabad, India. Environ. Res. Lett. 12, 075005 (2017).

91. Li, Z. et al. Exploring the impacts of regional unbalanced carbon tax on CO 2 emissions and industrial competitiveness in Liaoning province of China. Energy Pol. 113, 9–19 (2018).

92. ABO scores historic victory for carbon utilization. Algae Biomass Organization (9 February 2018).

93. Clean Power Plan (USEPA, 2014); https://www.epa.gov/stationary-sources-air-pollution/electric-utility-generating-units-repealing-clean-power-plan#rule-history

94. Gonçalves, A. L., Pires, J. C. & Simões, M. A review on the use of microalgal consortia for wastewater treatment. Algal Res. 24, 403–415 (2017).