The Physics of Wastewater Reclamation

This post is part of a blogging series by economics students at the Presidio Graduate School's MBA program. You can follow along here.

By: Chad Reese

My father recently called me to say "They say by not turning off the tap, I'm wasting water. How can I waste water if it can't be destroyed?"

He has a good point, albeit a misdirected one. He was referencing the first law of thermodynamics, which states matter and energy can't be created or destroyed. And he was right.

I didn't take my dad's political bait by asking who they were; instead, I took to higher ground and trumped his first law argument with the second law of thermodynamics.

The second law introduces the concept of entropy, which is the tendency of matter and energy toward disorder. Or, to be more technical, the second law recognizes the irreversible processes of a system toward less-ordered states of matter and energy.

But back to my dad’s running water tap

The accumulated used water from my dad, other residences and industry in his town runs down their respective drains and travels down a series of municipal sewer pipes to wind up in an industrial wastewater treatment plant. The water is then filtered through a series of screens and basins and several treatment processes requiring large amounts of energy to remove harmful pathogens, chemicals and other pollutants. Finally, oxygen and beneficial bacteria are added to the treated water before it (and potential viruses, chemicals and other nutrients that slip through the cracks) is released back into oceans, lakes and streams.

(Let's not even discuss the doomed and monumentally misdirected idea of desalination, which uses gobs of energy and gazillion dollars to extract salt from water that we previously dumped in the ocean. This has been called the water "cycle of insanity" and deserves no more attention.)

Now let’s suspend disbelief and assume a best-case scenario where the local municipality actually recycles its water so that it's drinkable again. The indirect potable reuse (IPR) system, dubbed "toilet-to-tap" by clever opponents of recycled water, also requires a sophisticated water-processing infrastructure.

Even a greywater system, which is a low-tech way to recycle laundry, dish or bathing water on site by irrigating landscape, still drives our precious natural resource – fresh water – from a low-entropy to a high-entropy state. This comes at the expense of potential energy consumed and converted into unavailable energy. The satisfaction derived from running the tap and wasting water comes at the cost of infrastructure, energy and misspent resources. It would have been much easier to just turn off the tap, conserve our precious and limited resource, and derive satisfaction some other way.

Waste not, want not.

Chad Reese is an MBA candidate in Sustainable Management at Presidio Graduate School. His professional focus is the nexus of energy, water and food production. Follow Chad on Twitter.