Out of sight, out of mind.
What many folks can't see they can indeed overlook. And all too many have never seen what happens to water that flows down the kitchen sink.
But with each load of laundry or flush of the toilet, we create wastewater that heads toward the treatment plant.
The average American makes 100 gallons of wastewater per day. While it's natural to think that sewage water is icky, it's natural - and it's even interesting from a biological point of view.
As a geologist, I have long noted that treatment plants are typically built on floodplains. That's partly so sewage can flow via gravity down to them from houses and buildings, and partly because their water processing pools require flat areas.
Of course, when a river floods the treatment plant can be inundated, one of the greatest disadvantages to putting them in floodplains.
Treatment plants start to process sewage by blowing air bubbles through it. The oxygen helps sewage decay, a key first step in treatment.
Sewage next enters tanks where sediment or sludge can accumulate on the bottom and scum on the top. The sludge is processed in digester tanks. The area around the digesters is often the smelliest part of the plant.
Digesters use heat and bacteria to break down sewage. One goal is to get rid of microorganisms that would cause disease. The solids that emerge from the digesters are often sent to landfills but are sometimes used as fertilizer.
Another product of treatment plants is methane gas that's created in the digesters. Methane is the main ingredient of what the utility company sells you as "natural gas." A few treatment plants make use of the methane, burning it for heat or to generate electricity.
The liquid portion of wastewater is often filtered through sand. This helps get rid of bacteria and fine solid particles still lingering in the water. In some places carbon filtration is used, a step that helps remove fine organic particles. Finally, chlorine is often added to kill remaining microorganisms.
Those of us who exercise in swimming pools and snort chlorine up our noses know that it's a serious chemical. So all remnants of chlorine in the water must be neutralized before the treated water flows into a river.
If you want an indoor toilet where you don't have a hookup to a municipal wastewater system, you can create your own little disposal system in the form of a septic tank. Inside the tank are bacteria that can live without oxygen. Septic systems are far from perfect, but they do help break down sewage. Water flows from the tank out under the soil, usually via a bed of gravel.
For the non-traditional, it's also possible to have indoor convenience with what's called a composting toilet. I had such a toilet for several years in a travel trailer, and it worked well for me.
I had quite an animated conversation about that toilet at a sewage treatment plant one day. The supervisor of the plant and I both shared an interest in how sewage is broken down by microorganisms. (You meet good people at treatment plants.)
Lately my water bill roughly doubled - and yours may have, too. What's at issue is better handling of storm water run-off from streets and parking lots. Such water contains oil and the like, at least in trace amounts.
As you wash a greasy frying pan tonight or start a load of laundry, take just a moment to consider that the water that leaves your home goes back to the Earth.
We face different choices about water treatment, but the condition water is in when it reaches local rivers is our ultimately our responsibility.
E. Kirsten Peters, Ph.D., is a rural Northwest native whose column is a service Washington State University. Follow her at rockdoc.wsu.edu and on Twitter @RockDocWSU.