Water … down the dirty drain!
By Madeline Seveland
Out of sight, out of mind is common with waste water leaving our homes.
Every day we send thousands of gallons of water down our sinks, tubs, toilets, etc., and it’s pretty safe to say most of that water is dirty. However, even though it’s out of our minds, the water is only in the middle of its journey. Most wastewater leaving our homes travels to a nearby wastewater treatment facility (WWTF) to be cleaned, disinfected, and returned to a useful state.
WWTF’s treatment process is similar to nature’s own purification process but goes at a much faster rate. The treatment can be broken down into five basic steps; varying slightly from plant to plant. The whole system relies on gravity to move the wastewater which is why most WWTF are located on low ground.
After you flush, take a shower, use a sink or do laundry, that wastewater is transported from your house to WWTF via sewer pipes. As the wastewater enters the preliminary treatment it flows through bar screens to remove trash and debris before passing into a grit tank. While in the grit tank, sand and heavy particles (ex. coffee grounds) settle out, are removed and usually taken to landfills.
Next the wastewater moves to primary treatment where it sits undisturbed for hours in sedimentation tanks. Here, solids (or sludge) sink to the bottom of the tank and are removed by being scrapped off; greases and oils, and soaps float to the top and are removed with large rotating skimmers. Both are sent to large heated and closed tanks called digesters. The sludge is kept here for 20-30 days so bacteria can break down the material reducing its volume, odor and potentially disease causing organisms.
The secondary treatment takes the water and winds it through a chain of concrete basins where water is mixed up and oxygen is added to begin the aeration process. In this environment, active bacteria feed on the incoming waste solids and dissolved organic matter thus speeding up the treatment process. Then the water flows to a clarifying tank where these bacteria settle out, although some are recirculated to keep the bacterial process alive.
Then it’s on to disinfection! At this time harmful bacteria and other organisms are killed by adding chlorine or running the water past high intensity ultraviolet lights. In some plants, water then moves on to an advanced treatment where the nutrients phosphorus and nitrogen are removed. These nutrients lead to algae growth in lakes and rivers to its important to remove them before releasing the newly treated water into a water body.
For more WWTF information, visit www.epa.gov/npdes/pubs/what_happens_after_the_flush.pdf.
Not everyone is connected to a city water system like this though. Many homes have septic systems. A septic system is a scaled down WWTF. The sludge sinks to the bottom where it decomposes, the oils and grease rise to the top and create scum, and both are pumped from the tank at regular intervals. The water then flows into absorption trenches where it’s dissipated to surrounding ground, air and vegetation.
So next time you hop in the shower or wash dishes remember the journey the water must take after it leaves you. Regardless of whether your house is connected to a septic system or wastewater treatment facility, the less water you use, the less wastewater needs to be treated. Check out www.co.carver.mn.us/water and visit the “Homeowner Tips” to learn more about conserving water.