In what situations could "dirty water leak into" municipal water pipes besides a catastrophic loss of pressure?

Question

Under an answer to How is the quality of tap water in Thailand? I commented:

Usually water in pipes is pressurized so that it comes out of the tap when you open it; is "dirty water making its way into cracks in the clean water pipes" really a known thing? Does it really happen, can you cite an authoritative source for this phenomenon? Sure during a major weather event water treatment plants can be overwhelmed, and breakage of pipes resulting in complete loss of pressure can allow inflow, but I don't think heavy rainfall itself can push dirty water into the pipes against normal levels of pressurization. I'm certainly happy to be proven wrong if you find something.

and someone replied

water coming out with pressure is due to gravity because the tank is on the roof. Usually tanks are filled with water from municipal sources and then that tank water flows through pipes into the building's houses.

to which I replied:

Sure that's quite common, but at what point is there negative pressure such that the dirty water can leak into the pipes? I know this is a DIY SE or Physics SE rather than a travel question, I'm just really curious if this is really a thing, or just something we imagine.

Now those pumps that "pull" water up to the tank on the roof are worrying me; can they in fact be so aggressive that they drop the pressure in a small diameter rural municipal water pipe so much that dirty water could leak in?

Are there other things that can happen, short of catastrophic pressure loss that can lead to this happening?

Answer 1

When there is no flow in an underground water supply pipe under pressure, a small crack or a loose joint in the pipe can only leak water. Contaminants can't get in.

However, as soon as there is flow, there is the potential for contaminants to enter the pipe due to the venturi effect. This occurs when there is a restriction, reduction or change in direction of the pipe, such as at an elbow or tee. And as @fraxinus points out, pipe corrosion can easily create both a restriction and an opening at the same place.

The U.S. Environmental Protection Agency publishes a guide (PDF) that discusses contamination of drinking water in distribution systems, and it includes this paragraph:

If a pipe with cracks or leaking joints is exposed to a wet environment, negative pressure can cause water to be drawn in (or to intrude into) the distribution system through backsiphonage (Kirmeyer et al., 2001). A separate issue paper addresses risks from intrusion due to pressure transients.

I was told by a foreman at a sewer construction site in New York City that buried water supply lines that run in the same trench as sewer lines must always be above the sewer line, never below. It's because of the potential for contamination from the venturi effect.

You'll understand how difficult it may be to comply with that rule in densely populated areas considering what it looks like under the streets. (Photo credit: davidgalbraith.org)

Answer 2

Backsiphonage and backpressure

The situation you ask about in your question is called backsiphonage, where pressure demand (suction) in a line that isn't adequately pressurized pulls in contaminants. The converse of that is a backpressure situation, where a pollutant source that's at higher pressure than the city water gets connected to it without any sort of non-return mechanism, causing water and pollutants to flow backwards from the pressure source into the city mains. (Note that in North America, it's very common to get a harmless backpressure situation when water main pressures are low due to thermal expansion in hot water heater tanks -- this is less of an issue in countries that don't use tanked hot water heaters, though.)

Since both phenomena pose a serious risk to public health, backflow prevention and cross-connection control (since you can't have harmful pollutants getting into the supply without a source for them) are major tasks for water utilities the world over. Generally speaking, sources of pollution (such as the wine bottler in DDS' answer, or more mundane sources such as hose bibbs, lawn sprinklers, and the like) have backflow preventers installed in their water supplies to prevent this from happening. (Lawn sprinklers, specialty sinks, and hose bibbs/outdoor taps use vacuum breaker devices, while more industrial applications will use double check valve or reduced pressure type devices.)

The US EPA has a handy manual that discusses this topic in more detail for those who wish to go further.

Answer 3

Here happened that a factory (wine bottling facility) pushed pollutant (wine in our case) in the city water mains. Authorities said the pressure used by the facility was higher than the acquaduct's and a failure of a facility valve.

Answer 4

It would depend on the pump and the municipal water line. It's certainly not inconceivable for a pump to pull suction on a low-pressure supply line.

Also, in many cases the roof tank is also because the municipal water line is only pressurized SOME of the time, and at other times has no pressure.

In other cases the municipal water line does not exist, but there's a water truck that drives around and delivers.

Answer 5

Besides cracks and pressure as explained by other answers, also it is common during pipe repair works and in case of negligence in the purifying stations.

Or just too polluted water that the purifying equipment can't cope with. Unfortunately that happens around here especially in small villages.