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Life in the pits

October 11, 2010
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The original six-bay carwash that we have now was built eight years before we bought it, so the decision on how to build and pipe the pits was not up to us.

We thought the previous owners did a good job though, so when we built a new six-bay beside the original we installed the pits the same way with a few improvements.

The original six bay

The floors in the original six-bay are sloped 4-1/2 inches in 6 feet, which gives it a “self-cleaning effect.” As the water flows to the pit, it carries some of the solids with it.

The original six-bay pits are 16 feet long, 30 inches wide and mostly 30 inches deep. The truck bay is a little deeper and the first bay, a little less.

They have open grates for the full length. The piping is 4 inch PVC between each pit, about 4 inches below the surface of the concrete.

There is little to no slope to the pipes and the outlet has a 90 degree “L” on the end, so any oil floating on top of the water will stay in the pit. If the solids get up to the underside of the “L,” flow will stop, not allowing sludge into the pipe.

They all drain to the third bay in the middle and then to the final pit in front of the carwash.

The final pit is about 4 feet square and 4 feet deep, with a concrete barrier in the middle and a 4 inch hole, 2 feet up off the floor. I think that this hole should have a 90 degree “L” also, but doesn’t.

The water from the third bay flows into one side, then through the barrier into the second side and out to the sanitary sewer.

The inlet to the sewer has a 90 degree “L” also, in order to contain the oil and solids in the last and final pit.

The thinking is to contain the oil and solids in the pits, and not allow it to flow into the sanitary sewer. Most of the oil and solids will settle out in the wash bay’s pits and the final bay should catch any that get beyond them.

The original six-bay works very well. In 18 years, the carwash has never plugged up the sanitary sewer; so it may be safe to assume the system is holding the solids as designed.

We have been required to test the sludge for about the last 10 or 12 years, and have failed only once and that sample passed on re-testing.

If any oil does get to the sanitary sewer, it is diluted by an average 240,000 gallons of water monthly through the 12 bays.

The solution to pollution is dilution.

The new 6 bay

I mentioned that we tried to improve the new six-bay pits when we built them in 2001. The improvements we made to the original pit design were:

  1. Sloped the pipes between the bays so we wouldn’t get any run back.
  2. Increased the slope of the floor (8 inches in 6 feet) to increase the self cleaning effect. (I was a little worried about the footing for the customers with more slope, but it has worked well with no complaints.)
  3. Increased the depth to 3 feet in the back of the bay and 4 feet in the front, but maintained pit-size, 16 feet by 30 inches.

Our thinking was that it would be better for the vacuum truck to suck out the bay if more of the sludge would flow to the front of the pit. This improvement did not work.

The solids are almost even in depth from front to back, even though we have the one foot difference in the floor. The back of the pit just fills up faster.

The new six-bay has worked well for almost four years with no trouble.

There are probably many different pit and piping designs out there. I’ve described ours that I know works and works well.

There are a few things that I know don’t work.

Any slope is better than none

We bought an existing seven-bay carwash in 1992. The wash was in fairly good shape for a 12-year-old facility.

We put about $50,000 into new equipment, caught up on maintenance and repaired a leak in the floor heat.

We also bricked up the bay next to the equipment room, giving us a door in front and badly needed storage space.

The floors in the bays were heated, but perfectly level; absolutely no slope whatsoever.

This was a terrible mistake. Not only did you have to wash the solids toward the pits, you had to wash the water also.

This same carwash had small pits, about 4 feet square and only about 2 1/2 feet deep. They didn’t have a final pit outside, so all the small pits inside were divided in half and the sewer outlet side had a solid cover over it so the dirt could not be washed into it.

We were constantly shoveling out the pits because they were so small. After the city had to clean out their lines about three times they made us install a final pit in front of the carwash.

Another wash we bought had very little slope to the pit and no heated floor. We put in a heating system on top of the old floor and then poured new concrete with as much slope as possible.

That same wash had a pit that was about 6 feet square and 6 feet deep. It was much too big.

The solids sat on the bottom for so long that they hardened. We had to use the high pressure pump to loosen the solids before the vacuum truck could pick them up. It was too deep for a shovel to be of any real use either.

A picture’s worth a thousand words

The pits aren’t just a headache for owners; they can sometimes cause anxiety for customers.

One time our attendant arrived at the carwash to find one of the bays had all 16 feet of grate taken out of the pit and placed on the floor.

He could find nothing wrong, so he put the grates back in and went about his duties.

At about 10 a.m. the attendant got a call from a man who said his daughter had lost her keys in the pit the night before and would like to know if our cameras had caught it.

The video showed that the girl came in about 7 p.m. and, while washing her car, dropped her keys in the bay. She stepped into the next bay and talked to another person she was with.

They both came back, removed the end grate behind the car and attempted to reach in for the keys. When they realized that the pit was too deep (4 feet), they went to another customer and borrowed a shovel.

They dug and dug in the pit for 15 or 20 minutes until they decided to push the car back out of the bay and remove the rest of the grates.

Unfortunately, the car now blocked part of the activity in the bay from the camera. They dug around in the pit for another hour in between various heated discussions, probably about what to do.

Eventually, the girl took off her shoes and stepped into the pit. The pit on the back end is 3 feet deep and 4 feet deep in the front.

The water was probably about 2 1/2 feet and 31/2 feet deep with sludge being about half of that. The temperature of the mixture would have been about 40 degrees.

We couldn’t see much because of the car, but eventually she crawled out of the pit, wiped herself off as best as she could, hopped in the car and left.

She left the grates off, but I can hardly blame her; she had to be frozen.

Sometimes, the pits can be funny.

Dennis Ryan has been in the carwash business since 1988 and the construction business for 40 years. At one time he owned and operated five self-service carwashes. Currently he owns and operates American Pride Carwash in Casper/Evansville, WY. He can be contacted at

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