Force main - initial low flows 2

[joepavu]

Recently, we've completed a design for a 300 gpm submersible pump station which will discharge into a 5,000 lf 6" diameter force main. Initally, there will only be a few homes discharging to the gravity sewer which feeds to the wet well / pump station. We have a concern that there will be odor problems at the discharge manhole and also potential clogging on the gravity side untill the subdivision is close to build out. What to do?

The gravity sewer is designed with a minimum of 0.50% to 3.5% slope to the wet well.

We have considered perodic flushing possibly as much as 4 times a year untill anticipated build out in 5 years.

Other options were to feed some type of chemical to minimize the anerobic conditions in the force main.

Possible aeration to the wet well and maximize the detension time (on/off) levels.

Any input is apprecated - I' sure this is a common problem but one our small firm has run across for the first time.

 

[RWF7437]

One thing you might consider is artifically reducing the volume of the wet well. This will cause the pumps to cycle on and off more often and thereby reducing the time the wastewater remains in the wet well. Raw domestic wastewater will go septic in about three to four hours so you'd want the cycle to be less than that. This will cause a little more wear on the pumps but might avoid odor problems. The method of reducing the volume might be as simple as partially filling it with, say, bricks which can be removed as flows increase.

 

[stanier]

You could install a small diameter PE pipe inside the main and use only that until the demand increases to the point where the increased diameter can be used.

 

[semo]

Reducing the wetwell size will shorten the pump run time and and shorten the cycle time. This can be done through the controls. Placing the lead and off floats or levels closer together will accomplish this.

Do not shorten the cycle time too much or you will incur too many cycles and burn out the starters quicker. Let the pump run for a few minutes if possible.

I'd be careful if placing something in the wetwell. Don't want a pump to suck it up and have a bigger problem.

Adding chemicals (hydrogen peroxide, permanganate, chlorine) to add oxygen or disinfect the waste can be expensive and not operator friendly. If you are discharging to a receiving stream with adequate flow, you probably won't have a bad odor problem. If not, you might need to do something.

I would set the levels close and see what happens with the odors (keep it in mind for the developer; but, save the money until it is needed). You basically have to design for the higher flows which with low flows will give the longer detention times in the forcemain no matter what.

You can flush the gravity system; but, most of the solids will probably make it without problems anyway. Most of the residential sewage is water which helps flush the previous solids.

 

[SanEnr1]

I agree with SEMO regarding wet well float locations and conercens regarding motor starters.

I have experienced this problem before and rely mainly upon regular maintenance of the system by our field crews to keep solids from depositing in the gravity system and excessive odors from developing in the forcemain.

Based upon the length of your forcemain, you may also experience odor complaints from areas near the forcemian discharge. If there is a means to flush the forcemain with clean water (either by utilizing a by-pass pump port or filling the wet well with clean water) this will help to decrease odors in the forcemain. There are also odor masking agents available that can be used to "cover-up" the odors until the station is at sufficient operating capacity.