After a 13-mile underground trip from the far reaches of the District, your sewage will meet its destiny in enormous concrete vats built 10 stories deep into the soil in Southeast Washington.
There, at DC Water’s Blue Plains treatment plant, they begin to remove toxins and flotsam from the messy mass of stuff from your dishwasher, your laundry water, your shower, your toilet flushes and everything that washes off the streets into the sewer.
When it reaches Blue Plains, the waste will be pumped into the facility’s existing treatment system or it will go through two new vats.
The two vats — properly named the surge or screening shaft and the dewatering pump station — already have been built, and right now one of them is serving as the entry point for the tunnel construction project.
Each is a massive concrete cylinder, and they sit side by side. The screening shaft is 81 feet around, and the pump station is 139.
What the screening shaft does is as simple as its name. Sewage flushing down a tunnel 23 feet in diameter can carry some large stuff with it. For example, shopping carts and the carcasses of large animals are not uncommon. The screening shaft catches those in a two-inch coarse screen. Then it passes the rest of the sewage onto the dewatering pump, where it goes through a quarter-inch fine screen.
The pump shaft is the deeper of the two, at 187 feet, and a second deep cylinder is at its core. That core cylinder is surrounded by several rings of platforms on which DC Water workers will walk to monitor the pump’s performance.
The dewatering pump uses something called a ballasted clarification process to separate water from particle matter.
With the help of fine sand and chemicals, it causes most of the pollutants to sink to the bottom of the tank, where they are removed. Grit also is removed using centrifugal force units.
The last step is disinfection with bleach to kill microorganisms followed by dechlorination.