pumped wastewater attenuation tank design

[swazimatt]

I have been tasked with designing an interim pump station that will discharge into an attenuation tank. I have just started looking into the design and see a few possible pitfalls.
The pump station is an interim pump station that will be upgraded in the future when the development grows and the receiving gravity reticulation has been upgraded. The pump will need to pump at about 65l/s to a high point (this high point will ultimately become an air valve location and the rising main extended) however the receiving gravity retic at this point only has capacity for 13 l/s. The prelim design (carried out by another firm, who will also be reviewing our design for the council)put forward is to have an attenuation tank at this point that will hold the peak flows and discharge at 13 l/s max.

Now looking at this i see that i can determine the orifice size using bernoulli's equation but if i set the flow at 13 l/s at tank full I will have a lower flow at lower tank levels (probably not a problem until i start calculating the tank size?). I am think i will need to have an overflow system in case of pipe blockages as my initial sizing had the orifice at about 75mm diameter, so possibly a larger diameter orifice at a higher level. I suspect i will need a Tee inlet to the orifice to reduce the solids and scum entering the outlet pipe (similar to the outlet from a septic tank) and an air vent out the top of the tank.
The intention is to also try to design this tank in such a way that it can be repurposed as an air valve chamber in the future.

Thinking about it now i will investigate the options of using the pump station emergency storage as an alternative and have the pump set to 13 l/s however i suspect this may result in too many pump cycles

is there anything that I should keep in mind going forward. Any of you had to design something similar?

 

[bimr]

Could you post a sketch with distances, heights, etc.?

[ul]
[li]Very unusual to use an orifice plate on wastewater.[/li]
[li]How do you plan to keep the solids mixed in the tank?[/li]
[li]Air valves are not a good idea on wastewater force mains.[/li]
[li]What time period is interim?[/li]
[li]Why can't you use a force main instead of the attenuation tank?[/li]
[li]What about the odors from the attenuation tank?[/li]
[/ul]

 

[LittleInch]

I think you really either want to do as per your second scheme or make the tank at the high point more like a true break tank and install a pump there sized for 13 l/sec.

How big is the tank going to be?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[swazimatt]

@bimr

Could you post a sketch with distances, heights, etc.? WILL TRY TO PUT UP A SIMPLIFIED SKETCH

Very unusual to use an orifice plate on wastewater. I WAS CONCERNED ABOUT THIS, BUT NOT SURE HOW ELSE TO DO IT. WAS THINKING MORE ALONG THE LINES OF JUST A SMALLER PIPE CONNECTION, BUT THE ISSUES ARE PROBABLY THE SAME
How do you plan to keep the solids mixed in the tank? THIS WAS MY CONCERN FOR ABOVE. ONCE I GET A BIT FURTHER INTO THE DESIGN I WILL HAVE A BETTER UNDERSTANDING OF THE STORAGE REQUIRED AND THE TIME IT WILL TAKE TO EMPTY
Air valves are not a good idea on wastewater force mains. BUT BETTER THAN AN AIR LOCK. I UNDERSTAND THE JURY IS STILL OUT REGARDING THE POSSIBILITY OF PUMPING PUSHING THE AIR OUT
What time period is interim? I BELIEVE IT WILL BE A FEW YEARS BUT ALL DEPENDS ON HOW QUICKLY THE AREA DEVELOPS
Why can't you use a force main instead of the attenuation tank? IT NEEDS TO INITIALLY PUMP INTO THE EXISTING GRAVITY NETWORK. THE NEXT STAGE WILL EXTEND THE RISING MAIN TO A DIFFERENT GRAVITY NETWORK WITH HIGHER CAPACITY (STILL BEING DEVELOPED) AND THE FINAL STAGE WILL BE A NEW RISING MAIN (AS FUTURE DEVELOPMENTS WILL BE FLOWING TO THIS PUMP STATION BY THE) TO YET ANOTHER NETWORK LOCATION
What about the odors from the attenuation tank? THAT WE CAN DEAL WITH ODOUR FILTERS AND POSSIBLE DOSING AT PUMP STATION, COUNCIL WOULD WANT THEM ON THE AIR VALVES AS WELL

 

[swazimatt]

@LITTLEINCH

will get back to you once i have a better understanding of flows and hours

I think the problem is with the riser being sized for future flows requires 65 l/s to acheive 1m/s pipe velocity. My thoughts are to have a temporary pipe that will pump at 13 l/s and this becomes redundant when changing to the full rising main at stage 2. I doubt the cost would be much more considering the trenching etc will already be getting done for the larger pipe

 

[swazimatt]

So I've had a closer look at this and due to the phasing of this development it is not really an option to do it any other way. we are doing this as an interim option for about 2 years and then after that the tank will become the air valve and order will be restored.

based on some prelim calcs the pump will need to pump at 65 l/s to acheive the required velocities in the rising main. This will be about 6m3 per cycle. The only way i can see to attenuate the flows here to 13 l/s is to reduce the size of the tank outlet pipe. Using a 2.5m dia tank at about 1.2m full I would need an outlet of about 65mm dia to ensure we do not exceed 13 l/s and i really don't want to have an outlet of 65mm for wastewater.

I am proposing we go with 100mm diameter and this way we will average the flow rate for one cycle to 13 l/s (max of 22 and min of about 5) and it should empty in about 8 minutes so hopefully not really long enough to cause any real issues

I am using bernoulli's equation to calculate flow out of the tank, but what i am inable to confirm is if wastewater has a significantly different Cv (velocity coefficient) 0.97 for water is what i have used, but a little bit more reading has shown that we will actually have a short tube outlet so Cv=0.8 and Cc=1

 

[LittleInch]

One thing I was thinking was could you insert a smaller pipe within the bigger one for the two years or so to keep your velocities up and then remove the smaller pipe when you get to your 65l/s period?

A PE line should be easy to remove? No idea about the length of this new line.

One problem with "future" things is that sometimes they don't happen....

Also what is there to stop your tank overflowing if the outlet get blocked or restricted? How will you know?

A 100mm outlet which is actually dumping 22l/sec into your other pipe sounds a bit high to me

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

An interim option for about 2 years is really not practical if you consider that it probably will take 3-6 months for design and 6-9 months for construction.

 

[swazimatt]

@LittleInch

I did offer this as an option but the cost of the smaller pipe and pumps for such a short period was not accepted (this line is about 2000m).

The 22 l/s would be when the tank is full and then it will drop down to about 5ls for the last 100mm depth but the average would be close to 13 l/s and pumping could be programmed to be off peak (it is quite possible since this is an industrial development pumping into a residential area). I would actually prefer to have a 150 and then just work on daily limit of 1,100m3/day (13 l/s) the only repurcussion of this is that the pipes downstream may surcharge a bit but doubt they will fill to the top.

Anyway my plan for a blockage is to firstly allow for telemetry in the tank to alert council if it is not draining, have 100% extra capacity in the tank and then also to have a high level overflow of 150mm pipe

I am now in New Zealand and there is a real shortage of developable land so pretty low chance of it not being developed