Climate Change

Mystery Unclogged Part I

by Sarah at ProgressiveKid

Spoiler alert: If you have maintained any illusions that your drains are magical tubes that “disappear” inconveniences, I’m about to ruin them for you. Look away! Flush and run!

It is more pleasant not to think about what happens to the expired Children’s Motrin you pour down the drain, what the Liquid Plum’r really does, or where the Tide with Bleach Powder goes. But there are so many of us on the planet sharing a shrinking space that we all need to think about what we are dumping and where. If you change your own oil on your asphalt driveway, you have to consider where the spillage goes. If you are cleaning off the brushes you used to stain your floor, ask yourself where the petroleum-based distillates are going to end up. If you’re spraying an herbicide in your yard, take a second to think about where the herbicide will eventually settle. Knowing how to think about all these things requires a (a) basic awareness of the most common problematic contents of wastewater and (b) general understanding of our plumbing and wastewater treatment systems.

Wastewater Contents

Your wastewater includes nitrogen and phosphates. You will recognize these names from fertilizers. That’s because they encourage growth. Unfortunately, they also encourage the growth of algae. And when algae overgrows, it blocks sunlight which can degrade water quality and make it unsuitable for sustaining animal life.

Your wastewater also includes organic material. Bacteria decompose organic materials, and when they do, they consume oxygen. When there’s a lot of organic material, bacteria consume a lot of oxygen, which means that water is depleted of oxygen, which means that it can’t sustain animal life.

And your wastewater includes disease-causing microbes or pathogens.

Wastewater Treatment

If you’ve got drains in your home, the two most likely scenarios are that (1) you’re on a septic system or (2) you’re on a public sewer system.

1. Septic System

If you’re on a septic system and you don’t have any supporting graywater system*, then everything that goes down the drain goes to the same place. All the pipes lead away from your home to a concrete or steel tank buried or freestanding in your yard. Everything that comes into the tank separates into three layers:

• The top layer or floating layer is scum (including oil and grease).
• The middle layer, which is clear water, contains bacteria and nitrogen, phosphorous, and maybe some other fertilizers.
• The bottom layer is sludge, or heavier and partly solid materials.

Baffles prevent scum and sludge from leaving the tank. Gases that form with decomposition are vented away from the tank usually through a vent pipe protruding from your roof.

What goes down your drains flows into the tank where the water that was already there is displaced into your drain field. The drain field is nothing more than perforated pipes buried in gravel trenches. It functions as a filter. Water is slowly absorbed into the ground through the holes in the pipes and through the gravel.

The system works using gravity unless you don’t have the option of using gravity and then you need the help of a pump to move the water from your house into the tank and/or from the tank into the drain field.

(*Graywater is water that doesn’t contain human waste products. It is the water that comes from laundry, footing drains, gutters, and possibly shower drains. Some homes include a graywater system that diverts this water from the septic system as a way of minimizing the work the septic systems needs to do, and some systems divert it as a way to reclaim it for certain uses, thereby conserving water.)

2. City Sewer System

The city sewer system takes via pipe all of the wastewater from an urban area and deposits it at a wastewater treatment facility where some or all (depending on the sophistication level of the system) of the following happens:

• Primary Treatment: A screen filters out solids. Additional solids are filtered out by a series of pools where the water sits long enough for heavier material to sink to the bottom. This type of treatment might take care of between 30 and 50 percent of the solids and organic materials. The rest are treated with chlorine unless there are additional treatment steps in the system.
• Secondary Treatment: The water flows from the pools into large aerated tanks where bacteria get to work on the remaining organic materials and nutrients. From there the water flows into settling tanks to separate out the bacteria. Secondary treatment might take care of up to 90 percent of solids and organic materials.
• Tertiary Treatment: This stage might include filter beds for filtering out the remaining organic materials. But typically it involves flowing the water through a tank where chlorine is added to kill any leftover bacteria.

(You can read more about wastewater treatment systems here.)

In Part II of this two-part series on our national wastewater problem we will examine the effectiveness of these systems in cleaning our wastewater and consider the implications.

Image by tnarik (Eduardo), 2008, Creative Commons license

©2008 ProgressiveKid