Do sewers need high ratio of fluids to work?

November 25, 2007 11:48 AM

SanitationEngineerFilter: Would more efficient water use cause problems for sewer systems for individual homeowners, or for municipalities as a whole?

Sanitation/civic engineers take many things into account when desigining sewer systems. The pitch of pipes, their diameters, etc. assume that households and businesses have a waste stream comprised of a ratio of solids and fluids.

So lets say a homeowner on a municipal sewer system, not a private septic system, starts getting "smart" about water use and waste stream. He uses a "if it's yellow let it mellow, if it's brown flush it down" protocol. Maybe even starts reusing greywater, so bath and laundry water no longer goes down the toilet pipes. So the solid/liquid ratio of his sewage will change. With less frequency and volume of fluid flushes, but a similar amount of solids, could this cause some buildup/blockage in his sewer?

Suppose a city started encouraging this kind of behavior - what problems might arise?

Have any cities asked these questions of their engineers?

posted by yesster to society & culture (5 comments total)

In South East Queensland, one of the results of careful water usage (140L per person per day and gray water use) resulted in less waste/sewerage available for recycling, reducing the expected recycled water availability.

Apart from that, I've not heard any issues with sewerage problems due to reduced water. Queensland has been experiencing drought conditions for so long that water restrictions have hit level 6. No outdoor hosing at all, watering by bucket only on two hours of three days a week, no washing of cars, houses, driveways etc.

This website might provide more useful information.
posted by b33j at 1:09 PM on November 25, 2007

His home is connected to a collection pipe that is probably less than 100 feet away. Any change in his behavior would only affect his short connection.

If the entire block or an entire section of the city were to change behavior, this could change the dynamics, but I cannot tell you how. I do know that, until they reach pumping stations, municipal sanitary sewers rely on gravity to move wastewater along.
posted by yclipse at 1:14 PM on November 25, 2007

I suspect that *major* changes to water consumption would make a difference, but since the vast bulk of everything that goes down the sewer is actually liquid all I think would really happen would be a reduction in the processing volumes at the sewage plants.

I don't know the exact figures, but even the ratio of solids to liquids in the case of `flush it down' is pretty small, and most of the waste being flushed away is just water with a little detergent, such as clothes washing, showers, etc.
posted by tomble at 4:41 PM on November 25, 2007

individuals no, municipalities yes. taken as hearsay, i've heard that certain towns in germany had problems with their municipal sewers when everyone started conserving too much water.
posted by geos at 8:50 PM on November 25, 2007

such an interesting question, what made you think of it?

I don't have a whole lot to add, except to mention that the paris sewer system was originally designed for periodic cleansings by giant wooden spheres.

So maybe if there were problems due to a change in the composition of the sewage, you could schedule periodic flushes of the entire system.
posted by Chris4d at 11:30 PM on November 25, 2007