reservoir mixing? 5

[Cherie]

Hi,

Do you have any tips for the best way to promote mixing in a potable water reservoir?

In this case, the reservoir I'm designing holds 1.0 million gallons (32' shell height, 74' diameter).

We used to use a reducer angled upward at the inlet (eg. 12" to 6" reducer to increase velocity), with a separate outlet on the opposite side of the reservoir, and that would suffice.

However, lately we've been designing reservoirs for water distribution systems that use *chloramination* for treatment rather than chlorination. Mixing is more important for chloraminated water than for chlorinated water.

LA County Water Works has suggested using Red Valves or a Solar Bee, but they are looking to us for guidance, since we are the local experts (my company, not me personally) and chloramination is relatively new. Other suggestions have been to use a motorized mixer, or a pump with some kind of circulation system.

I would guess that an extended inlet with a few Red Valves along it, and a separate outlet would suffice and would be the most cost effective. Any suggestions or comments from your experience?

Thanks....

 

[dicksewerrat]

No experience but a small( 6") axial flow pump might work

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[joetbear]

I am a field mechanic/water operator, solar bee mechanical mixer, stay away from having moving parts in your tank. Your better off having something of a mixing chamber installed before the tank. Then there is contact time that needs to be address. Keep trying.

 

[BarryEng]

Several years ago, I worked for a major water utility (600,000 services) & we had mixing problems of dam water (some algae), ground water (nutients), & artesian water (30 deg C temperature). If the water was not mixed adequately, an algal bloom would occasionally develop (vertical visibility reduced at a rate of 1 m per hour). We tested several model configurations (square, rectangular & circular)in a hydraulics lab. The results were verified in actual reservoirs (up to 200 m square & 10 m deep). The conclusions were: -
* If there was a surface wind, the surface wind shear would over ride all other considerations & the reservoir would mix with the surface water moving in the direction of the wind & turning down & moving across the bottom. This was not a reliable mixing pattern because we did not achieve a 'last in, last out' condition. This condition no longer applies because reservoirs are now roofed.
* The location of the outlet had virtually no effect on mixing in any of the configurations tested. Outlets were tested from the centre of the reservoir, centre of one edge, & a corner, both at the bottom & near the top of the water level.
* The direction & velocity of the inlet water dictated the mixing pattern of the reservoirs.
* Inlet water that was directed into an inlet chamber & flowed over a sill had virtually no effect on mixing resulting in a slow overall mixing but stagnent water away from the outlet.
* Square reservoir - inlet water directed down one wall developed a large rotational pattern with a slight (& very small) counter rotational pattern in the corners. The overall pattern was influenced by the location of the inlet pipe. If the pipe was near the top, the pattern was a large circle spirally slowly down to the bottom (& hence to the outlet), & so 'last in last out' was achieved. If the pipe was near the bottom, the circular flow still persisted but there was some stratification due to the inlet water 'short circuiting' to the outlet with the water above still moving in a lage circle but without efficient mixing.
* Rectangular reservoir - inlet water directed towards the centre of the reservoir from the centre of the long side. Same result with good mixing but now with two counter rotating patterns of water. Same results with inlets near top of water & bottom of water as in last example.
* Circular tanks - best mixing with the inlets above TWL & directed tangentially. Water flows in a large helix & eventually to the outlet. Location of outlet had no effect on mixing. If inlet directed radially towards the centre of the tank, weak mixing with two counter rotating patterns of water but some stagnent water opposite the inlet.
* Circular tanks - inlet thru floor & directed upwards at 45 deg in a circumferential direction. Fairly good mixing with the whole of the tank water rotating but some cross flow in elevation (I need a sketch for this?).

I've run out of time to answer fully, so I will follow up later if there are any questions on the above.

 

[cvg]

you might try some sort of perforated pipe diffuser. I'm not sure what benefit you get from using a Red valve.

 

[rconner]

I have no experience in this field (other than likewise recognizing a noble intent!) but in reading this thread I wonder if perhaps multiple smaller, perhaps tangential and "spaced" at different elevations/clock position inlets/outlets, joined together outside the tank walls to then consolidate to larger piping etc. as necessary for going elsewhere might improve uniformity of mixing in the tank? [I realize of course this would if it helped involve some more complicated construction/fabrication of tanks and piping, as opposed to single larger inlets/outlets, and there is perhaps some trade-off of this vs. water quality/value achieved].

 

[JStephen]

Be careful specifying Red valves indiscriminantly. They seem way overpriced for what they are. Just looking at them, it doesn't look like they should cost much more than a car tire, but that's not the case.

 

[stanier]

You might consider a submersible mixer that is lowered into the tank from the roof. This enables them to be lifted out. ABS and ITT FLygt make them in stainless teeel. They are used for sewage applications in bioreactors. Bothe companies have software for modelling the mixing of a tank. See your local representative for a copy.

Red Valves also do the modelling for you but they are cagey about releasing their software. In respect of the comment about the price, yes they are mre costly than competitors, that is because they come with engineering whereas others are cheap imitations.

Same goes for Ventomat air release valves, Noreva check valves and a myriad of engineering components designed some time ago and copied by opportunists who have no idea how their "shapes" work! Sold by catalogue engineers, they leave you gasping for information they dont have.

 

[joetbear]

One of the units I worked for had some of the same problems, mixing and contact time. O&M and Engineering came up with adding sections of the pipe (48") and connected them in zig zag (best discription I could think of) causing the water flow to mix with the change in direction and increase in contact time before it reach a tank or user. Keeps it simple, no moving parts, less maintenance.

 

[JStephen]

Stanier, we have seen Red Valves specified for overflows on tanks- use Red Valve at $2,000 or standard flap valve @ $200- and there wasn't a bit of engineering done by Red Valve in that case- think it was just a consultant that hadn't bothered pricing these items.

I'm just familiar with one competitor with the duckbill-typed products- they're a bit cheaper, but not a lot- but that wasn't what I was getting at.