Force main - initial low flows
Force main - initial low flows
(OP)
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.
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.





RE: Force main - initial low flows
RE: Force main - initial low flows
RE: Force main - initial low flows
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.
RE: Force main - initial low flows
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.