You’ve just watched the water go down the drain in the kitchen sink, or down the hole in the toilet. Did you know that every building in Prince George's and Montgomery County is connected to a sewer pipe?
Here is a timeline on how the sewer lines were installed in our two counties. These pipes lead to the wastewater treatment plant.
The first stop on the journey of wastewater is through a grinder or bar screen. This is where large pieces of rags, toys, trash, sticks, dead fish/small animals, money and other debris are removed from the wastewater. These objects could damage the pumps if this gets into them!
The next stop on wastewater journey is the Grit Removal Chamber. This is where sand and gravel settle out in the chamber or tank. Grit is heavy and abrasive and might scratch and damage the pumps so they won’t work anymore. This could cause the whole process to stop! These 'settled' objects and the large objects from before are all removed and sent to a landfill.
Now it’s time to get rid of the rest of the settleable solids! After the grit is removed, the wastewater flows into the primary clarifiers, big tanks where, what's now called SLUDGE, settles to the bottom and the scum floats to the top. The wastewater is now in two parts - SLUDGE and LIQUID.
The liquid still needs some cleaning so it is carried to large tanks where air is blown into it. A population of tiny microorganisms live in this tank, where they survive by feeding on the organics in the wastewater. The blowing air sets these little 'bugs' in action to start chomping away on the rest of the solids in the water. They literally eat this up! And what’s left is a little cleaner liquid. This is called "BNR (Biological Nutrient Removal)". There are also two nutrients removed in this process, nitrogen and phosphorus. The 'bugs' remove the nitrogen and chemicals (typically aluminum sulfate) are added to remove the phosphorous. These nutrients are removed because they promote algae growth, which reduces the amount of oxygen in the water. (This would not be good for fish and plants that depend on the air in water!)
Our water now reaches the point where chlorine is added to kill off any pathogenic (disease-causing) bacteria. This is similar to adding chlorine to your swimming pools. The amount of chlorine added is carefully monitored to make sure not too much is added, but if it is, then some of it will be removed. Too much chlorine can be harmful to aquatic and wildlife.
When the liquid is finally 90% clean of its original organic content, it is pumped back into a nearby stream. At this point, the water is actually cleaner than the natural stream water itself!
While the water is being cleaned up and returned to the nearby stream, the sludge is continuing on its journey to its new home. First, the sludge is dewatered by using a belt filter press, centrifuges, vacuum filters, or drying beds. Sometimes lime is added to help with the dewatering process. Now we have 'biosolids', which can be used to help fertilize landfills.