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flooded splitter chamber

flooded splitter chamber

(OP)
thread161-148604: septic tanks

I came across this helpful comment by Ussuri as I need to split flow from a gravity sewer evenly into two settling tanks in parallel.

"In certain cases (septic tanks in parallel) the invert of the inlet is below the invert of the outlet, and a flooded splitter chamber is provided upstream to ensure a even split of flow. The invert entering the splitter is higher than the outlet of the tanks (to maintain the hydraulic gradient), but the chamber has a bottom exit. This ensure the head above the outlets is the same, and solids do not settle out during the split. Soin this arranngement you cannot vent using the inlet pipe."

I'm wondering if anyone can describe or point me towards an example of flooded splitter chamber that would work this way.

RE: flooded splitter chamber

Your gravity flow sewer has to come into the flow splitter box such that the maximum water elevation in the sewer is above the maximum water elevation in the splitter box. If not, the water may back up into the sewer changing the flow regime in the sewer to full pipe flow. Air may become trapped in the sewer causing unsteady flow.

Because you must come into the splitter box above the elevation of the splitter box, the flow entering the splitter box will generate velocity currents and splashing that will cause uneven flow across weirs (and interfere with flow splitting weirs when you are trying to evenly split the flow).

To avoid this problem, you will need to add an additional section on the splitter box to transition and calm the entering water. The water entering this calming section will flow downward under a partition wall and then come upward into a weir section. The bottom should not have corners but should be filleted so that solids will not collect. Oversizing will also cause solids to collect. The weir must be above the water elevation in the settling tank such that water does not back up to the weir.

In the weir section, the water will be rising and then overflowing flow splitting weirs into a third section. The third section will be divided into two compartments, one for each settling tank. Bottom outlets in the third section should be filleted to minimize solids deposition. The bottom outlets in the third section will allow full pipe flow into the settling tanks.

If you need to stop the flow to the settling tanks, use weir gates instead of fixed weirs.

RE: flooded splitter chamber

(OP)
Thank you bimr for this clear and thorough description! This seems like it would work nicely in an application where the inflows have been screened of debris.
Due to the small scale of the project I'm working on (design peak flow 35gpm) and a design criteria to keep the O&M burden low, I'm not sure I can provide a screening device. My concern is that the weirs would not function properly when debris (diapers, sanitary napkins, etc.) find their way into the splitter box.
Any thoughts on alternative flow splitting methods and/or inexpensive low maintenance screening methods?

RE: flooded splitter chamber

You probably have two options.

The first is to install an oversized manual screen. These usually incorporate a screen installed at a flatter slope than normal. This will allow solids to build up over a greater area with less head buildup.

The second option is a macerator. While the macerator will shred the materials, it will not remove the materials. A macerator will have more maintenance and cost as well.

I would think that the materials would tend to foul the downstream equipment more than a weir box. I am sure that you are aware that it is best to screen these materials out up front. However, for a small treatment plant, there is not much that is affordable.

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