A municipal wastewater treatment plant's visible infrastructure — its tanks, pipes and pumps — is really just the physical container for the process that actually cleans the water. The real cleaning work, breaking down the organic waste and many of the pollutants present in raw sewage, is done by dense, carefully cultivated communities of bacteria deliberately maintained within the treatment process, consuming that organic material as their own food source and converting it into simpler, far less harmful byproducts. Environmental and applied microbiology is the field that studies and actively manages exactly these kinds of practical, real-world uses of microorganisms to solve genuine environmental and industrial problems.
The bacteria are cultivated deliberately, not simply present by accident
The bacterial communities doing a treatment plant's actual cleaning work aren't simply whatever microorganisms happen to arrive in the incoming wastewater by chance — treatment plant operators actively cultivate and maintain a specific, deliberately balanced community of bacterial species suited to breaking down the particular kinds of organic waste and pollutants the plant is designed to treat, carefully controlling conditions like oxygen levels, temperature and how long wastewater remains in a given treatment stage to keep that bacterial community healthy and working effectively. This deliberate cultivation is a genuine, ongoing biological management task, not a passive, purely mechanical or chemical process — get the bacterial community's living conditions wrong, and treatment effectiveness can degrade meaningfully, even though every mechanical part of the plant's physical infrastructure is otherwise still working exactly as designed.
The same basic principle extends well beyond wastewater treatment
This general approach, deliberately cultivating specific microorganisms to break down or transform an unwanted substance into something less harmful, extends well beyond municipal wastewater treatment into a broader practice called bioremediation, in which specifically selected microorganisms are used to help clean up other kinds of environmental contamination, including some oil spills and certain contaminated industrial sites, by breaking down pollutants that would otherwise persist and cause lasting environmental harm. In each of these applications, applied microbiologists have to understand both the specific target pollutant's own chemistry and the specific metabolic capabilities of the particular microorganisms being deployed against it, since not every bacterial species can actually break down every kind of pollutant equally well, or fast enough to be practically useful for a given real cleanup task.
What we're still unsure about
The general principle of using cultivated bacterial communities for wastewater treatment, and the broader practice of bioremediation using selected microorganisms, are well established across decades of environmental engineering and applied microbiology practice. What remains a genuinely active area of ongoing research is identifying and effectively deploying microorganisms capable of breaking down some of the more persistent, chemically stubborn pollutants that have emerged more recently, including certain synthetic industrial chemicals that naturally occurring bacterial communities don't yet break down efficiently — researchers continue actively searching for, and in some cases genetically engineering, microorganisms with the specific metabolic capabilities needed to tackle these harder cases, without yet having a comprehensive, reliably effective microbial solution for every category of persistent modern pollutant.
This sits inside Environmental & Applied Microbiology, one of seven topics in Microbiology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.