pumped flow creating negative pressures to eliminate small pump

[swazimatt]

I was recently tasked with designing a small leachate pumpstation servicing an odour control biofilter. The static head is quite low at about 3m that discharges to a terminal pumpstation that receives pumped flow from multiple other wastewater pumpstations.

I got thinking that if i connected a rising main from the leachate wetwell into one of the other rising mains with a y-junction and a low head non-return valve (like a wastop valve) would the flow from the pumped pipe across the junction create enough negative pressure on the leachate pipe that it doesn't require a pump in the leachate wetwell? (in this configuration the static head could be reduced to 2m, possibly less, leachate pipe would be about 20m.

In theory i think this could work and just curious about it as i have already finished the design. but not sure how i would even calculate this. It may only work if the velocity in the pumped main was massive.

 

[swazimatt]

Chatgpt says no :-(

Why It Might Work:​

• When a high-velocity flow moves past a side branch (like at a Y-junction), it reduces static pressure at the junction (Bernoulli effect).
• This drop in pressure can create suction on the side arm—if the main flow is strong enough.
• This is the same principle behind how some chemical eductors and vacuum generators work.

---

Why It​

1. Rising mains are pressurized systems.
   Unless flow in the main is very fast (creating significant velocity head), the pressure at the Y-junction is typically positive, not negative.
2. Y-junctions don’t provide strong suction.
   The negative pressure created is often small and localized—rarely enough to lift liquid even 0.5 m, let alone 2 m.
3. Non-return valves (e.g. WaStop) are designed to prevent backflow, not to act as suction-inducing devices. They add resistance.
4. Backpressure from the rising main can block flow from your leachate pipe unless there's a higher driving pressure behind it (usually a pump).

 

[Andrew ONeill]

It could possibly work , ultimately Bernuolli is your friend here. If the velocity goes very high, the pressure goes low. This is the principle behind venturi vacuum devices, hydro-ejectors etc. Google "Venturi" and it will make sense. The principle is to massively increase the velocity in a small location, then reduce the velocity after (usually done by reducing the pipe diameter in a small section. You plug in the suction line right into the middle of the venturi high velocity / low pressure zone.

From a calculation point of view, it's streamlines and Bernoulli's principles. Need to generate a ow enough pressure in the venturi, so that the leachate line has enough pressure difference to achieve the required flow

the "but" is
- it's not "free energy", sitll need the same amount of energy to move all the fluid
- Now have to deal with a high wear component
- More difficult to control (not sure if flow rates are critical in this application)

 

[Snickster]

The pressure at the junction will be positive and flow will be towards the biofilter.

Only way to reduce pressure at junction is to increase velocity enough to cause a vacuum by reducing the flow area at the junction, This will cause a high pressure drop in main line and introduce undesirable complexities. What you proposed I never seen done and would not try it. Eductors are used only in special applications.

 

[LittleInch]

You need some form of venturi to make this work.

OR just stick your leachate tank 3m above the pipe?

 

[ChorasDen]

What is your required negative pressure to induce such flow, and what is the vapor pressure of the fluid? Is the negative pressure sufficiently high to open the one-way valve, while simultaneously being sufficiently low to not induce phase change in the fluid in question?

 

[georgeverghese]

Pumped leachate injection should be top entry into the main, so that waste water solids dont accumulate / reverse flow in this leachate line and bung up the check valve.

 

[pierreick]

Hi,
Talk to vendors of EDUCTORS.
Pierre

 

[swazimatt]

What is your required negative pressure to induce such flow, and what is the vapor pressure of the fluid? Is the negative pressure sufficiently high to open the one-way valve, while simultaneously being sufficiently low to not induce phase change in the fluid in question?

it is basically water, and i would say minimum negative pressure would be about 3m (assuming leachate suction pipe is low velocity)

 

[swazimatt]

so it seems that the theory has merit (eductors use it!), however as Andrew said, it is not free energy, there would eb a loss somewhere and existing ww rising mains are also not really ideal to add diameter restrictions to
thanks though, it has been a fun topic.

 

[Hartley Kingon]

It won't work as is. I will write a detailed answer on how to make it work soon.

Can you please confirm the diameter, Press and velocity of water in the main pipe at the junction?

 

[Hartley Kingon]

Hello, so you will need to get a special venturi built for this to increase the velocity of the main.
To calculate whether it will work, you will need to do some trial and error with a few different jet throat diameters.
The formula to make sure it works will be

(V_jet^2 - V_main^2) / (2g) > (P_1 - P_s) / (rho*g) + delta_z + h_loss,total

Where:

• V_jet = velocity in the venturi throat
• V_main = velocity in the pipe just upstream of the throat
• P_1 = static pressure in the main at the tap-point
• P_s = static pressure at the tank surface (~atmospheric if vented)
• delta_z = vertical distance from tank liquid level down to the throat (m)
• h_loss,total = sum of suction-line friction, check-valve cracking head, and throat/diffuser losses (m)

I trust that makes sense, LMK if you have any questions.