Smelly drain pit in garage

[slybry]

My new house has a driveway that slopes towards the house (not one of my favorite features). There is a drain at bottom of driveway just outside garage door. The water from driveway pours into this drain and it also has the corregated pipes from french drains coming in on either side. This drain has a basin. When it fills up the water exits a 3 inch pipe that runs under garage floor into another drainage pit inside the garage.

This garage floor drainage pit might hold about 20 gallons of water all the times. When water rises to a certain level it flows out to the main sewage drain to city's system. The entrance to the main sewage system is a 4 inch plsstic pipe with and elbow so the pipe is turned down below the water surface at all times.

The trouble I am having is that the water sitting in garage pit becomes stagnant and starts to smell like sewage. I have confirmed that the sewage pipe entrance is still submerged so the smell is not coming from the sewer. It is the stagnant water.

During winters here in Montreal there will be very little water pouring into this drain (as everything is frozen). This water will just sit for months.

I have only been in house since August and have twice cleared all the water from this pit and re-filled it to get rid of the smell.

I was wondering if there is a chemical I can put in this drain periodically to stop the water from stagnating and avoid this smell.

Thanks.... new house and many many questions

[HayZee518]

Chlorox bleach may cut down on the smell.

[LazyPup]

The ground water from your footer drains or washing in off the driveway is technically classified as "Storm Runoff" and as such the plumbing codes expressly prohibit discharging it into a sanitary sewer.

Storm runoff may discharge into a storm sewer, storm drainage ditch or directly into the watershed however EPA restrictions prohibit discharging oil or chemical sludge into the watershed.

The collector at the garage door is catching water that is washing in off the driveway, and there is a high probability that the water might contain slight amounts of oil which has dripped on the driveway surface from vehicles so they have installed a simple "Oil Separator" pit. The discharge line from the separator pit has a 1/4bend pointing downward so that as the water level in the pit rises to the point of flowing into the discharge line, the actual intake of that line is below the surface of the water. In theory, oil is lighter than water and would therefore be floating on the surface of the water and is restricted from flowing out the discharge line.

As the water flows in from the driveway in addition to the minute amounts of oil that it may pick up, it will also contain trace amounts of organic matter such as leaves or grass clippings. If left in the pit for an extended period the organic matter will decompose and produces sewer gases which accounts for the odor.

The immediate solution is to clean the pit and sanitize it with common household bleach to kill the bacteria that supports the decomposition. Once that is done you could periodically pour a small amount of bleach in the pit or you could put one of those hanging toilet tank chlorinator tablets in the pit where it will slowly dissolve into any residual water that remains in the pit.