The sewage collection system is not only a modern convenience, but also one of the most important pillars of urban public health.
However, human destructive power is also strong enough, we keep making garbage, and then at the push of a button, it will flow into the ground, into the sewer pipes, and disappear completely from our vision.
In this issue, let's see how sewer pipes work.
It sounds easy to put a pipe on the ground and point it to the nearest sewage treatment plant. However, to really design a reasonable domestic sewage pipe, it is much more complicated than we think.
Laying underground pipes is a destructive and expensive task. They are often closely connected to roads and other pipelines underground. In order to save costs and reduce unnecessary troubles, these pipelines must be as long as possible, and we must not only design for the immediate situation, but also take into account the future development of the city.
After all, with the growth of the population, more waste water is generated, which is also a test for sewers.
Joseph Basel, the father of London's sewer design, advocated doubling the size of London's sewers. He believes that although the expansion of the size will bring economic burdens, this can effectively prevent sewage from contaminating drinking water.
Usually, the distance between the two pipes is very close.
The first step in designing a wastewater treatment system is to understand the expected amount of sewage. We can estimate an average and peak based on the population and land use of this area.
This is not enough, we also have to consider the drainage gullies, although they are separated from the sewer pipes, but this part of the water will still enter the sewage system. If heavy rains are encountered, a large amount of rainwater pours into the sewage system, which will inevitably exceed the design capacity of the system. The consequences could be that untreated sewage is flowed back to the city, causing serious pollution. Therefore, before designing the pipeline, engineers must choose the appropriate system capacity.
The next step is to design the pipeline, some people say that the most important thing for pipeline engineers is gravity and friction. In fact, almost all sewage systems collect and transport waste through gravity.
While gravity is readily available, free isn't necessarily the best, and it also has a serious problem, which is that gravity only works in the vertical direction, which constrains the design and construction of modern sewer systems more than any other factor.
If we use this sand to represent dirt, what are the factors that affect their discharge from the pipe?
First, there is the flow of the sewer pipe. If the flow rate is too fast, it is possible to damage the joints or walls of the pipe. If the flow rate is too slow, the solids will detach from the suspended solids, and over time, there is a risk of blockage.
We can't control the magnitude of gravity, we can't control the flow of wastewater, we can control the thickness and slope of the pipe. If the slope of the pipe is too small, the water flow will slow down, and over time, some solids will settle at the bottom and reduce the capacity of the pipe, eventually even blocking the pipe completely.
If you increase the slope of the pipeline without changing the flow rate, it is obvious that not only the flow rate of the water increases, but also washes away part of the precipitated solids, keeping the pipe clean.
This speed, also known as the self-cleaning speed, is about 1 meter per second for urban sewer pipes.
In addition to this, the relationship between the slope of the sewer and the ground should be taken into account. If the slope is too close to the ground, the water pipe cannot be well protected by the soil, and if the slope is too large and the ground is too far away, the excavation and installation of the water pipe will become very difficult.
The process of digging a gully is also very dangerous, and it is best to install such temporary brackets to ensure the safety of construction, or to use trenchless forms, such as tunneling, which we will talk about in a future video.
Even if you find a balance in the slope, what if the pipe is going to cross a river or street?
In general, it is not cost-effective to reduce or increase the slope of the entire pipeline. In this case, we generally adopt a structure called an inverted siphon, which allows part of the pipe to descend below the pressured terrain and then flow out from the other side of the river.
A hydraulic slope line is an imaginary line on the surface of a fluid that is higher than the ground, but the pipe carries the flow of water below the ground.
The problem with inverted siphons also lies in this, where water will be deposited and the flow rate is relatively small, so it is easy to form solid precipitation.
This is very similar to the reverse bend under the sink in our house, the bend is easy to block, even if the length of the bend pipe is lengthened, the problem is still there.
We can solve this problem by diverting multiple small tubes. Although smaller pipes reduce capacity, they can effectively increase the flow rate, which is high enough to wash away these solid sediments.
In addition to this, there is another key factor, which is ourselves. Every day we throw something into the sewers, and greasy foods congeal into solids at room temperature.
Disinfectant wipes are also becoming more and more popular, and if these two things are mixed together, even a strong sewer system can cause indigestion.
Let's compare the speed of decomposition of ordinary paper towels and wet wipes after stirring in water, and the difference is clear at a glance.
So, let's throw this solid garbage into your trash can.
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