After the Toilet Flush, Where Does Wastewater Go?


The places that wastewater goes after we flush the toilet is not something that we often think about. So the Professor was pleased when his colleague Jim McHale, vice president of engineering at American Standard and all-around brilliant toilet scientist, shared what he learned from a trip to the Philadelphia Southeast Waste Water Treatment Plant to track down the journey from toilet flush to sewer and beyond.

Taking such an overwhelmingly odorific journey may not sound like the most appealing idea, but McHale had an interesting goal.

“My main motivation for organizing the tour was to get some insight on the question of whether or not our system of water-based toilets and sewers is really sustainable. That is, can humans continue doing this for the next thousand years without significant impact to the environment? I did not expect an answer in one visit, but thought it was a good way to start learning.”

The process begins when sewage first enters the plant, and solids like grit, sludge, grease, and scum are all gradually removed through a multi-step process of filtration and sedimentation. As McHale explained to the Professor, one of the most striking aspects of the plant was how “natural” the process is. Essentially, all the plant does is control the flow of the sewage to allow sufficient time for natural processes like sedimentation and biological metabolism to take place in a reliable manner.

The plant provides a controlled residence time and provides oxygen to help the microbes do their work removing contaminants from the waste water, but it is all essentially a natural process. The bacteria that do the work were not even specially selected: they utilize the native bacteria that flowed in with the sewage. All the plant needs to do is control their population and keep them fed. At the end of the line, the water is then treated with chlorine before it is released into the Delaware River to protect the river water for wildlife and for swimmers.

The plant McHale visited handles an unbelievable 120 million gallons of wastewater flow in a typical day. It is one of the three facilities serving Philadelphia and its suburbs, and serves the most populated part of the city. Therefore the Professor was not surprised to learn that the system can be unreliable. Philadelphia, like most of the older cities, has a combined sewer system, which means that household sewage, runoff from rainstorms, and industrial wastewater are all combined and handled by one sewer system.

The average rain storm sends enough water into the sewer system that it can overflow, releasing untreated sewage into the local waterways. McHale reported that it had rained about an inch on the day of his visit, and that the affects on the plant were evident. The water leaving the plant wasn’t as clean as it should be: flow rates were too high and residence times inside the plant too short.

One interesting potential solution to this problem is that individual households could help by releasing cleaner water into the system. Household plumbing systems could pre-treat wastewater, for example. Toilets could be designed to ease the burden on water treatment plants by separating liquids, which require less treatment, from solids that require more time in the plant. Incorporating anaerobic digestion on a household or community level is another interesting possibility. Anaerobic digestion produces methane gas, which could then be used to produce electricity or household heating while also relieving the burden on local sewage treatment plants.

Dealing with such immense quantities of wastewater is a complicated problem to tackle, but the Professor is heartened to see such great minds working on solutions.

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