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.

 

[HTURKAK]

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.

So you know the theory. The pressure at y branch would be positive with this set up. I would like to remind the air lift pumps theory.

 

[bimr]

Looks like a maintenance headache.

Low pressure check valves are not reliable.

System only works when you have flow from the wetwell.

 

[swazimatt]

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?

there are multiple rising mains going to this pumpstation chamber so can choose between a range from 500mm to 160mm diameter. Assuming these are designed the way i would they are close to their discharge location so pressure will be less than 5m (likely around 1-2m) and velocity around the 1-2m/s

The leachate pipe will be 40mm id

It could also connect to the rising main from the WWPS if it is better to have higher pressure, and being close to the pumps their duty is 200l/s and 20m, 54m and 32m TDH for the 3 pumps

I imagine that adding a venturi will require added head to the pumped systems

With a venturi, would there still be a requirement for a non-return valve? I suppose as the pumps wind down and velocity reduces there would be a point where the venturi no longer "sucks" and when it stops completely ww in the rising main would slowly drain into the leachate wetwell, so NRV required

When the wetwell is empty i would thin that this system would then suck air into the remaining length of rising main (could help oxygenate the ww and reduce septicity?)