Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Changeover from joint sanitary/storm sewer to separate storm and sanitary lines.

jhrost Member Posts: 57
By way of background, the street I live on was reconstructed a number of years ago. The former system was a combined storm and sanitary and now they are supposedly separate. To me the main difference I have noticed is that during periods of very heavy rain sometimes the water would back up a bit through the main cellar drain (which is at the low point of the cellar near the street) with the old system. Even with very heavy flooding rains it hasn't since the reconstruction.

For this reason I was surprised when a neighbor on the other side of the street had his cellar flooded with about 16 inches of sewer water, even though there wasn't even any rain that day, and the house itself hadn't been used(no one had been living there using water or sewer) for a number of years. The municipality admitted they had been working on a problem on an intersecting larger street at the time, which was apparently the cause of the back up.

I should point out that the cellar drains in my and my neighbor across the streets house (both about 100 years old) originally had some type of backflow preventer on those drains which was no longer functional. After what happened to him I put one on my drain, even though I haven't had any issues since the reconstruction.

It is hard to get information from the city, my neighbor has since died, even though I don't have the specifics of the situation on my particular street I thought maybe people on this site might give me some incite into the typical and the ideal situations in municipal plumbing.

First off I believe that before the reconstruction work that the main sewerline from the house and the line from the cellar drain entered the common sewer at different places, though I guess the cellar drain could have connected into the main drain before it emptied both into the street sewer at one point. Which arrangement would be typical? Assuming that the main house sewer and the cellar drain line were separate, when they created separate sewers for the sanitary and storm drainage would a cellar sewer typically be run into the sanitary sewer or the storm sewer , which would be ideal ?

My next question involves the connection with the intersecting street, which I believe has never been reconstructed and still has a joint storm/sanitary sewer. My assumption is that the city anticipates eventually reconstructing all the streets, and as it does creating separate storm and sanitary sewers so that the sewage treatment plant doesn't have to process all the rainwater, which can just got to the river. In the meantime though, the separate storm and sanitary lines on my street it would seem just end up being mixed back together when they flow into the larger intersecting street which has the old combined sewer. Would it be good practice or standard practice to put in some type of backflow preventer either at the intersection of these two streets drainage systems or maybe at the point on the reconstructed street where the cellar drains emptied into either the now separate sanitary or storm sewers? Is this ever done , always done, or just depend on the budget of the particular job? Any incites on this would be appreciated. Sorry for the lengthyness of this post.


  • Alan (California Radiant) Forbes
    If your cellar drain is below the closest downstream manhole in the street, you need a backflow preventer on the cellar drain.

    Yes, the new storm drain in your street should have a backflow device before entering the old, adjoining combined sewer, but this may be too much of an added expense for the city, especially if the old combined sewer is going to be upgraded to separate piping in the near future.

    Here in San Francisco, there's one sewer for rainwater and sanitary which means that all drains (fixtures, floor drains, roof drains, etc.) have to be trapped and vented.

    Did I answer your question(s)?
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Jamie Hall
    Jamie Hall Member Posts: 23,436
    In principle, the cellar drain should have been connected to the main lateral coming from your house to the combined sewer, and when the two were separated it should, then, be connected to the sanitary sewer. In principle. Didn't always happen that way, but that's the way it is supposed to be.

    There are occasions where cellar drains and roof leaders were connected in a separate line to the street. The cellar drain shouldn't have been, but... and the records aren't always clear on that. Roof leaders would be routed to the storm sewer, and if the cellar drain was hooked to them, it could also have been transferred to the new storm sewer.

    In a situation where the newly separated sewers enter an old combined sewer, it would be not that unusual, really, for there to be a backup on the new sanitary sewer if the combined sewer surcharged in heavy rain. As @Alan (California Radiant) Forbes said, a backflow preventer on your cellar floor drain, if it's low, isn't a bad idea at all.

    The question of a backflow preventer on either the new sanitary of storm sewer is slightly vexed. Frankly, when I was working with these things, I never would have put one on, as they are a maintenance nightmare. Different engineers might have different opinions on that -- and it would be opinion and professional preference, not a matter of right or wrong.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • jhrost
    jhrost Member Posts: 57
    Thank you Alan and jamie for your insights. I suspect Alan may be correct in the assumption that they plan on eventually doing the other street in the near future anyway, so might not bother with backflow prevention. If it can be a maintenance headache as well, all the more reason not to bother.

    I guess the best bet is to have a backflow preventer on your own drain and keep it in good repair. You can get sewage backup insurance in your policy as well - my neighbor found this out but he didn't have it - unless you are in a very low lying area prone to backups you probably never think of it. Conditions around you change to . In 2011 here we had major flooding. I happened to talk to some folks who got flooded not from streams or rivers but their sewage system backing up. A major retailer had just built a large store adjacent to their street - because it was in a low lying area a procession of trucks brought in fill to raise the sight about 6 feet or so. Whether it was this or not they all felt that this grade change was the cause of their woes, as long time residents claimed it had never happened to them in the past when the sight had formerly been occupied by factories.

    It's interesting that Jamie mentions the roof leaders being connected to the sewer sometimes. On mine their discharge just runs across the sidewalk. When the street was being redone I asked someone about routing these to the sewer with a connection and was told it was no longer permitted by code (I've seen them in other parts of the city so I guess they are grandfathered in ). The problem I have with the present system is that during the winter water often trickles and freezes across the sidewalk necessitating application of melter to avoid a treacherous condition.
  • Intplm.
    Intplm. Member Posts: 2,006
    Many municipalities are upgrading their infrastructure systems by having two separate drainage systems. One being the raw sewage system the other a storm water system.

    Rain water overloads some of the current single pipe systems.
    When this happens, many systems have difficulty treating the sewage for safe discharge from the treatment plants.
    There is also the the concern about overflow carried into the environment.

    @jhrost You post a very good and interesting story. To answer the question about intersecting streets? As the work progresses some areas under ground will have the upgrades that you mention, other areas will have the old system in place. It all depends on the process of how the work is progressing. ( This might be why your neighbor had the flood because they were still on the old system ? )

    An FYI.....Commonly a backflow preventer is installed on a potable or, drinking water main.

    What they are likely installing on the rain and sewer drains is something a bit different called a backwater valve. Just a terminology FYI.