Actually, the plant is in the city of Renton, which is very close to Seattle. I took a tour there Saturday, June 6. Here is the summary of their work (PDF) that they present in diagram form.
Public wastewater treatment is the process of channeling used water from private homes, businesses, factories, and all public plumbing into the city’s sewage system. The city’s sewers start with narrow pipes at individual locations then gauge up as they merge flow into huge pipes large enough to drive vehicles through. These huge pipes divert the wastewater to treatment plants.
The first step of treatment is the removal of trash. This is done by putting a grate inside that last huge pipe. The bars of the grate will be spaced about 2cm and are regularly raked clean by a robotic arm. The trash collected here goes to the landfill, and in fact should never have gotten into the water at all. Paper that is not toilet paper, solid feminine hygiene products, condoms, dental floss, cotton, and other trash should never be put into a toilet, because those things all create problems here and have to be removed.
The next step is that grit and dirt are removed from the wastewater. The water from the huge pipes is diffused into a shallow flow over bumpy declining planes. The grooves on these planes collect grit, which also is raked and then brought to landfills.
Next oil and large biosolids are removed from the water. The processes so far have depended on the water’s movement to separate waste; this step requires water to be as still as possible. Water is separated into fields of great vats 2-3 meters deep. Oil will float to the top of still water, and biosolids will sink to the bottom. A skimmer removes the floating oil and a rake removes everything that has sunk to the bottom.
At this point there will still be small biosolids in the water and some bacteria. The types of bacteria are monitored so that some bacteria are allowed to thrive. The water is moved into another area where it is churned somewhat. The bacteria eat the small biosolids and release them as larger waste products. Then there is a rake for collecting the foamy scum that the bacteria produce.
Next the water goes into another area where it is chlorinated to kill the bacteria. After that it goes into another area where it is dechlorinated to remove the chlorine. At this point the water is pumped to a water reservoir. In the case of Seattle, this means Puget Sound. The water exits from diffusing sprinklers near the bottom of the reservoir so as to minimize disruption of plant and animal life as water reenters the ecosystem. The sound itself connects to local aquifers. Water cycles by rain back to the Cedar River, which is the watershed that supplies Seattle with its drinking water.
Notice that during this process, there are no steps to remove chemicals from the water. When any chemical gets into the sewer, those chemicals go directly into our ocean without treatment. It is extremely expensive to remove any chemicals from water and in practice this is not done. It is important that people be careful not to introduce toxic soaps, cleaners, oils, pharmaceuticals, solvents, or anything else other than body waste, biologically friendly soaps, or toilet tissue into the water.