Highly variable flow phasing - Wet well sizing 1

[AnalyzingAlan]

thread161-194365

I'm designing a large residential sewer pump station. Due to development phasing, the initial flows are much lower than final flows. The retension time during initial phases is unavoidably long given pump submergence requirements. Septic conditions is a concern, is a bubbler a solution? I am concerned about cavitation. I have seen bubblers in a dry well pump station design but never on a submerged pump wet well. Any input would be much appreciated

 

[scmurphy]

We have had a a few phased in developments over the past years but never really had a problem with odor from the lift stations. We usually lower the float switches in the wet well down to very near the bottom then set the "on "switch just above. It causes the pumps to short cycle and its not in a very efficient part of the curve but it seems to limit any odor problems. Wear on the pumps might be a concern but as the flow picks up we raise the switches to better use the wet well's capacity and have never really seen any reduced pump life. Also, in very sensitive neighborhoods we've installed small exhaust fans on timers that push through carbon canisters to remove odors. This works pretty well but you have keep the canisters changed out.

 

[sjohns4]

I was discussing this with a trusted equipment rep not long ago. He claims he has had luck with a small blower in wet wells to keep the stuff from going ripe.

In the past I've been able to get away with sitting floats close but provide the depth to raise the pump on float as flow picks up.

Also consider using a pump with a trimmed impeller for the first stages of development. When flow picks up you have a larger impeller put in the pump. This is paticulaurly a good option of there will be many years between development phases.

 

[AnalyzingAlan]

Thanks for the responses. sjohns I am going to utilize just that, a float switch controled by a touch screen for easy adjustments. However the state spec (hawaii) requires a minimum of 6 inches between on and off floats, as well as maximum 30 minute retension time. The first phase will be .044 cfs peak and final buildout is .836 cfs peak 8 years later. The first phases are subdivisions with not all houses being built at once. I will look into the blower, carbon filter fans as an option, thanks again

 

[cvg]

be careful about trimming the impeller or throttling the flow. I take this to mean that you would be reducing the size of the impeller or even installing a smaller impeller which will reduce the flow capacity. the flow velocity in the force main would also be reduced, thereby possibly fouling the force main with solids. You might be better off dealing with the odor than you would be pigging your force main to clear it.

 

[AnalyzingAlan]

The benefits of reducing pump output are almost negligible on retension time because the time it takes to discharge the pumped volume will still be very small even with minimum velocity through the force main. I'm looking into maximizing the grout around the pumps as well as temporary filler or something. Also a pump model that allows the pump off level to be as low as possible.

 

[redbridge]

Use two wet wells. The first wet well would have the pumps and be deeper and a smaller diameter and the second would be for when you had more flow and could be a larger diameter and wouldn't be as deep. The two wet wells would be connected with a pipe.

 

[AnalyzingAlan]

Lburg that is a good suggestion. The problem with that is to fit two pumps of the size required, the minimum diameter is almost as big as our well diameter. I looked into a stepped cylinder, with the bottom being 4' diameter then transitioning to a 6' diameter, but the pumps need 5.5' at the bottom. That's why adding maximum grout was where I looked next. I've hit a dead end with minimizing the volume and have been looking into ways to keep it from going septic. Thanks though all participation appreciated

 

[sjohns4]

YES - i forgot to mention if you would put smaller pumps or impellers, make sure you still maintain a scouring velocity in the FM!!

6" difference is probably the minimium you could realistically get away with anyhow with a float, you might be able to get an additional inch or two with a bubbler system or transducer. I dont know that an ultra-sonic would be a good choice for fine tuning pump on/off because of the possibility of grease or other floating solids in raw wastewater.

I designed a pump station in NC, which requires between 2-8 cycles per hour in 4 phases. The first phase operated very close to 2 cycles per hour with 2 pumps (one at a time and a rudundant pump). Next phase added a third pump, third phase added a fourth pump, and final phase replaced all pumps with larger pumps and the cycle times were close to 8 cycles per hour, although the final phase also included installation of VFD's and flow matching capabilities.

If you can get away with adjusting pump on/off set points that is the simpleist.

Given the distance from the WWTP, and wheather or not you're discharging to a gravity sewer, the aeration option may provide additional benifits if it keeps the flow in the downstream sewer from going septic and H2S from attacking manholes, but that really should be looked at on a case by case baisis.