Wastewater Wetwell Shape

[yogibear1]

We use a round wet well made by concrete precast plants. We also use duplex pump arrangements, and soemtimes triplex submersible pumps. Our wet wells range in size from 6 ft ID to 12 ft ID. We also line ours with a tough PVC liner for corrosion protection. On a recent trip to a precast plant I saw they had several other pre-cast shapes in their yard for other customers, such as the D.O.T. That gave me pause to think about why we use round wet wells. Is it because we've always done it that or or did this evolve from practical lessons learned?

I'd like some feed back from readers around the globe on what shape you have found best to use for wet wells and why. Do you use round or rectangular shaped, and why or why not. Do you add fillets to the corners to mitigate against the accumulation of debris? What other tips, and lessons learned can you share to help the O&M function?

 

[RWF7437]

There are many possible shapes depending on the number, spacing and size of pump intakes. Hopper bottoms are often used. The best way, for large stations, is to do some scaled down pilot studies of the eddy cuurents produced. For smaller stations this expense is seldom justified so operator experience is probably you best guide. Some agencies have developed good guidelines. The City of Portland Orefon in my area is such an example.

good luck

 

[LHA]

Fillets are essential, I've never seen a design or one as-built without them.

I prefer no corners to corners (ei, round). But, I must admit, I have no imperical evidence. It is, as you've suspected, probably just because "that's how I've always done it".

I was, however, once given a "go-by" plan set by a Municipal Authority - meaning what they wanted to end up with (the Authority was to ultimately own/O&M). The wetwell was rectangular, so I designed mine rectangular and sealed the design. The Auth has several rect. in service, and I just couldn't find any iron-clad reason not to give them what they asked for. However, they had no specific reason why they wanted rect. It seems to me, as you've suspected, probably just because "that's how they've always done it".

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[francesca]

The perimeter:area ratio for a circle (2/radius) is much greater than that for a square (4/length of side). Round wet wells likely has lower materials/excavation costs for the same volume of storage.

 

[yogibear1]

Thanks for the input. Rectangular wet wells have some advantages too, such as more volume per foot of depth; and more room for equipment layout. We generally have two pumps, plus an auxiliary suction pipe for emergency pump out, plus a bubbler stilling well. We put a generous fillet in the bottom to slope everything toward the pumps which are centered m.o.l. With all the pipes, plus pump rails that are in the wetwell we sometimes are a bit crowded with small round wet well. We now have a SCADA project just coming out of design that also drops a level transducer in a stilling well into the wetwell. I just wanted to see if throughout the industry there are more tools available for my tool bag. Thank you all.

 

[francesca]

Volume-per-foot of wet well depends on the plan-view area of the wet well. This is independent of geometry. A circular wet well with area 10 sq ft will have a volume of 10 cu ft per foot, as will a rectangular wet well with area 10 sq ft.

However, the perimeter of a square wet well with area 10 sq ft is 12.65 ft (longer for a rectangle), while in a round wet well with area 10 sq ft, the perimeter is 11.21 ft. On a wet well of this scale, the saving in materials cost may be trivial, of course, and the benefit of things fitting better into a square may override.

 

[yogibear1]

Yes, I over generalized on the storage volume statement of round -v- rectangular (square). An example of hurrying a response. A more accurate statement would have exampled round -v- square of equal ID, then the square would have more volume per foot of depth (because the plan area is greater for the square).

 

[RWF7437]

All of the above comments, while helpful, miss an important point. Detention time in the wet well at LOW flow may govern the design. Raw sewage goes septic in three to four hours and this is often the critical determinant of wet well volume. At high flows the volume needs to be large enough to prevent frequent pump starts and stops. At low flows, the volume needs to be small enough to require the pumps operate at least every three to four hours. This may require the installation of a third "jockey pump". The shape may be a secondary consideration.

good luck

 

[stanier]

You may want to consider the following technical paper on pump/sump fluid structure intereaction.

http://www.engdyn.com/papers/abstracts/ab91.htm