But what about the trip?

Cecil's Columns/Staff Reports
1

Una, in What happens to all the stuff that goes down the toilet? you answered what happens after the stuff makes it to the treatment plant, but not how it makes the journey there. (We’ve got low-flow toilets that are often getting clogged, so I find myself thinking about such things more often than I’d like.)

How long does it take for the solids to reach the treatment plant? What form are they in by the time they get there? For that matter, how long does it even take for them to get out of my house?

2

Those are all good questions, and the answers will vary quite a bit depending on the type of sewer system you are hooked up to, the piping and so forth between you and the treatment plant, and the distance between the plant and you.

Reputedly, it can actually take a few flushes before the human waste and other sewage (see, I didn’t say “biosolids” this time…) leaves your house, depending on the situation. My pipes go almost straight down, then do a sharp bend and decline only gradually from one end of the basement, under the flooring, and out to the street. As a result it may take several flushes or runs of water before huge masses of matter make it out to the street main. Not that one could expect huge masses of such things from dainty little me…

From that point on, however, the timing it takes to get from your house to the plant will vary wildly depending on location, piping, and very importantly, water flow. There may be statistics out there for individual measures, and a sewage expert I asked here says that tests are done with dye packets or radioactive tags (he claims they stopped using radioactive tags in his line of work not because of radiation danger but because they found so many “false positives” from other radioactive things entiering the pipes, including, I imagine the result of medical radionucleides which are ingested). :eek:

When it arrives at the treatment plant a large portion of it is not recognizeable - and there’s typically a lot of water; it’s not even close to being a slurry or sludge, it looks more like dirty or grey water. According to him human waste is only very infrequently recognizeable, as it tends to degrade and fall apart and dissolve in transit. What is recognizeable and requires skimming and catching like I said in the report are condoms, female sanitary products, plastics, leaves and stiff papers, and alligators. OK, not alligators, he’s never seen those, but he does see an inordinate amount of (dead) snakes for some reason.

Perhaps someone can give stats on bulk timings and so forth, and the results of specific tests. My source claims that tests are all over the map, depending on the variables, and range from 15 minutes to many hours before dye or other tracking means can be detected.

3

:slight_smile:

(My bolding)

So, would you suggest the phrase about bears and woods be changed to “Does an alligator crap in the sewers of New York?”

In answer to the question of how fast the trip to the treatment plant takes, here are my thoughts:

Sanitary sewer lines are installed (by accepted industry standard, at least in the North-Central part of the US) so as to flow at least 2 feet per second (fps). Generally this means that larger pipes can be placed at a shallower slope, but this is irrelevant to the answer. The 2 fps rate is chosen because liquid flowing at that rate should push solids along the pipe bottom. Human waste (crap) will, when rolled, break up into small pieces which, as Una indicated, takes the form of suspended solids or gray looking water.

So, getting back to the question of how long the trip takes - It depends on how far you are away from the waste treatment plant. Pipes in neighborhoods that collect sanitary sewer flow may typically be 8” in diameter. These collector pipes gather in larger main line pipe which gather together in larger trunk line pipe, etc. eventually flowing into sewer interceptor pipes that may be 10 feet in diameter (for the Twin Cities) or much larger for larger communities. My office is about 11 miles as the pipe flows from the Pig’s Eye treatment plant. (Minor side note - Before becoming St. Paul, the future capital of Minnesota was named Pig’s Eye as that was the nickname of Pierre Parrant, an early settler, trapper and tavern owner. Most St. Paulites are glad the name changed.) 11 miles is 58,080 feet. At a flow rate of 2 fps, it should take about 8 hours for the flush from the office to reach the sewer plant. The farthest reaches of the Pig’s Eye Wastewater Treatment Plant system are about 30 miles out. 2 fps is also a good assumption of flow rates for larger pipes, although as the capacity increases the flow rates can increase to 3 or 4 fps (or higher). Higher flow velocity increases the potential for pipe scour (erosion) though, so lower flows are preferred. 30 miles at 2 fps would take 22 hours.

Unless there is a blockage (like I had the other week - yuk!) it should take less than a minute for a flush to leave the house and get into the sewer line in the street.

4

If your low-flow toilets are not working right, then they are a poor quality design. There are high-quality designs out there, ones that work just as well as the older, high-water using ones.

See the recent Consumers Reports, which evaluated toilets, for an evaluation and listing of the brands that do work.

There seems to be a general trend to claim that low-flow toilets don’t work as well as older ones. That’s not true, but I suppose it’s easier for a salesman to say that, and blame it on government regulations, than to admit that their brand isn’t as good as the others!