Escaping Historical Lock-in─Redesigning Wastewater Treatment Plants and Their Microbiomes for the 21st Century

Wastewater treatment plants (WWTPs) have gradually, over the last hundred years, been designed and extended to deal with a sequence of problems, including a) odor, b) suspended solids, c) organics, d) ammonia, e) nitrate and phosphate, and f) recalcitrant pollutants. The line of historical developments was piecemeal rather than holistic and did not focus on sustainability, resource recovery, and water reuse. On the contrary, microbial processes that accelerated the removal of nitrogen were incorporated and heralded as a positive part of the "cleanup" agenda, despite their relatively large energy consumption and substantial production of nitrous oxide, a potent greenhouse gas. The time has come to examine the historical, technological, and microbiological lock-in present in today's WWTPs, so that a more coherent integrated system can be developed for future generations. Some disruptive strategies are outlined, and a categorization of processes in terms of their potential for the future is formulated.