Public Works is replacing what I believe to be sewer lines in my neighborhood. There is a pump constantly running and pumping water from underground into the bottom/side of a large (15’ x 15’x 4’) bladder which has water coming out of the top, running down the sides and into the storm drains. What the heck is this thing? A filter of some sort? The water coming out looks clear.
Never seen one, but through a sequence of guesses and google searches I think you’re looking at a dewatering bag or silt bag: Dewatering Bags and Tubes | Geotextile Filter Bag
One use is to comply with clean water regulations. (From the site above.)
Dewatering products have also been used by companies as a way to successfully stay in compliance with clean water regulations such as NPDES Phase II and the Clean Water Act. A dewatering bag is an excellent way to filter out sediment and allow a clean water flow from your site.
If the water coming out of the top of the bladder seems to smell less than horrifying, it almost certainly isn’t a septic sewer being serviced. Off hand I’d guess they’re working on some flavor of storm sewer/groundwater control but without more info and maybe pics, It’s hard to say with any certainty.
I’m a civil/environmental engineer who works for a public sewer utility.
It’s a device to remove sediment from water being pumped out of the trench so they can install the new sewer main in dry conditions. A dewatering system may be as simple as a pump in a sump with some crushed stone in the trench, or may be an elaborate system of temporary wells. Either way there are prohibitions against discharging pumped groundwater that contains sediment into storm drains (which typically empty into the nearest water body).
I completely agree that the water being pumped is neither from a septic system (which is unrelated to a public sanitary sewer), nor is it from a sanitary sewer. Neither of these would be pumped into a storm drain. That is strictly illegal and would subject the offender to large penalties.
(As an aside, a properly flowing sanitary sewer doesn’t smell horrifying. A sanitary sewer that is stagnant and gone septic does. 
)
Bottom line: it’s clean groundwater being treated for sediment before being pumped into the storm drain.
So what do they do with a giant bladder filled with sediment when the project is complete? Is there a way to empty the sediment somewhere?
Sure, they dump it anywhere where the residual water won’t flow into a storm drain. It’s just clean sediment. No big deal.
Ignorance fought in less than an hour! Thanks. I live on a barrier island and you don’t have to go down too far to hit water. There is a stench but I don’t think its sewerage, just decomposing muck. We get the same smell from the salt marsh during particularly low tides.
Heck, I suppose they might dump the sediment back in the trench after they’ve laid their new pipes. Or at least as much as fits.
That might pose a settling problem unless they wait for it to fully dewater then compact it before trying to pave or landscape over it.
Right, it would likely be too wet to use for backfill.
There’s always excess excavated material to be disposed of when installing utilities, because room is taken up by the new pipeline and any specialized imported backfill (such as bedding sand, or processed stone base for roadways). In addition, if the native material is unsuitable for backfill (such as clayey material) it may be replaced with imported bank-run gravel.
Anyway, any sediment from dewatering would be disposed of along with the excess excavated material.
Excellent! Thank you.
I was thinking more along the lines of repairs or upgrades, where the new stuff being buried was more or less the cross-section of whatever’s being dug up. Rather than greenfield construction where, as you say, the cross section of whatever is being buried usually dominates the volume excavated.