« Back to Home Page

The Physics of Wastewater Reclamation

Presidio Economics | Wednesday May 11th, 2011 | 5 Comments

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

Trending toward entropy

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.


▼▼▼      5 Comments     ▼▼▼

Newsletter Signup
  • http://www.waterlegacy.com chris

    Great Blog… there is another way to reduce water usage and save energy in the water reclamation message and that is through using your greywater from your washing machine and shower to flush your toilets. See http://waterlegacy.com. We have shown a savings of 30-40% of a household’s water use and a potential energy savings of 13%. As we assess new ways of saving energy and water, the Water Legacy system is efficient and effective.

    • http://www.facebook.com/people/Pone-Fallow/100002167033706 Pone Fallow

      Great “post”, not “great blog”, unless you are referring to the site at large? in which case, I’m sure they appreciate it!

  • Anonymous

    Chad, you seem to overlook one simple reality. Any water that is returned to the earth eventually enters the water cycle and returns as fresh water. This is facilitated by a huge virtually inexhaustable energy source called “The Sun”. Are you just trying to insult everyone’s intelligence?

    • Chad

      Anonymous, thank you for your feedback. You are right, but the post is about the rate of water use (movement towards high entropy) versus rate of the sun’s low-entropy input. The efforts to move part of a system back to a more orderly, complex and heterogeneous (low entropy) state requires a greater local increase in entropy somewhere else in the system to make up the difference. No insults intended.

  • Rick Wilson

    Hi Chad,

    Great post! We appreciate the references and links to our Cycle of Insanity video and our Indirect Potable Reuse campaign in San Diego. You might be interested in knowing that the COI video is now on YouTube:
    http://www.youtube.com/surfriderfoundation#p/a/u/0/espNKMCZ1kU