EPANET - Residence Time "Age" analysis

[johnyt79]

I am currently modeling a watersystem with a pump with a continuous fresh source to a reservoir joined by a common main (i.e. no dedicated line to the reservoir). The system feeds a single family development.

I want to check the quality (age) of the water within the reservoir, theoreticaly the age of the water should taper off at around 4 days or so. However in the analysis, the age is proportional at 1:1 with the time lasped over the analysis (i.e. a 500 hours analysis spits out a water quality of 500 hours). This occures only at the reservoir.

At the demand nodes throughout the development, there are rediculous fluctuations. As when the pump is on, which means the line is being replaced with fresh water, the quality is very good (4 hours at most), and then when the pump flips off the system is fed by the reservoir which has a very poor quality as explained above.

Viewing the reservoir during the pump cycles, it shows both flow in and flow out, but quality never changes.

This should be purely an issue with the way i setup the EPANET analysis.

Can anyone help? more information required?

Thanks!

 

[hydrae]

How have you modeled the tank on the mixing parameter?
There are 4 mixing modes, single inlet outlet with a low velocity entering the tank would uses the 2 compartment mixing.

From a operation standpoint, not modeling, increasing the dead band on the pump will freshen the tank up a bit. Otherwise, re plumb the tank to separate inlets and outlets, even if it is through check valves inside the tank

Hydrae

 

[johnyt79]

Thanks for the comment Hydrae!

I've set it up as "FIFO" or First in First Out. This should give the highest water quality as turnover time would be the lowest. However, i've tried the other three options as well without any noticeable difference. I've also tried adjusting so that pump turn on is when the reservoir is fully drained.

Explain: The reservoir maximum top of water level is 5.6m (datum=reservoir slab) and normal pump turn on level is 5.0m. I've tried adjusting the analysis so that the pump turn on level is a 0.0m. This would suggest that the reservoir is fully turned over every pump cycle. This would give the maximum dead band on the pump, and this has no effect on quality inside the reservoir (weird since its empty and then replenished!).

Theoretically the model makes sense, there are things like pump start/stop levels, and pump rates that i can adjust to fine tune the system, but because of the rediculous skew of results, it would suggest that the problem that lies here seems to be within the modeling software. It doesnt appear that im missing any parameters, but from the results, im obviously missing something...

Thanks again.

 

[hydrae]

the age will also be affected by the size and length of the pipe between the pump and reservoir,
I actual cases the reservoir is mixing during the fill cycle
which depends upon the inlet velocity and internal configuation
FIFO is an unmixed model, I assume you do not have any demands near the reservoir the age will be older that anywhere else,
is the volume of the pipe between the pump and tank larger than the volume of the resevoir?
Hydrae

 

[johnyt79]

The pipe volume between the pump and the reservoir is not greater than the reservoir volume. However, the pipe volume is greater than the reservoir cycle volume. This indicates that each pump cycle, the entire volume in the pipe is replaced, and then water from the pump will be filling the reservoir.

However, it should be easy to see that the MODEL is not working properly and it is not a design issue. The age of the water in the reservoir is the same age as the test. i.e. if i run the test for 10,000 hours, the age of the water in the reservoir is 10,000 hours, even though the reservoir says it is receiving water from the pump....

Thanks

 

[hydrae]

Do you know the reservoir internals?
is FIFO the correct model for that reservoir?

FIFO may be only recording the oldest water in the reservoir thus all water is considered the oldest and since it never goes empty it would then matche the age

I suggest switching to a mixed model the most closely matches the reservoir internal piping. a low velocity inlet would be a 2 compartment model, where a high velocity would use a fully mixed.

Hydrae