A little update on the cistern we uncovered in Alexandria. This poor old cistern, which was installed during wicked slave trader Joseph Bruin's occupation of the site, has been here since around 1840. It's a water collection and purification system, considered quite state-of-the-art at the time of its construction. To appreciate it completely, imagine it without the modern concrete all over it, and with its brick dome intact.
It's a big cylinder, 10 feet in diameter, made of mortared brick. To make it watertight, it has been "parged", which just means that the interior has been coated with a thin mortar layer.
The dividing wall in the center is particularly nifty. The floor of the cistern was canted slightly. There is a small hole in the base of the wall, and a filtration box, filled with charcoal, was placed at the mouth of this opening. As water collected via the opening in the dome, it would flow into the second chamber, and the charcoal would filter the water. Water was then pumped out of the closed chamber.
In order to decide how to proceed with its excavation, I had to go monitor the removal of the modern concrete from the top of the cistern. Since it was "retired" (probably with the introduction of city water in Alexandria), it has been filled with refuse, most of which seems to be from the 1920s. Which makes sense, since a small garage was built on top of it at some point during the 1920s. In 1958, the garage was demolished, and an office building was built in its place. I recently watched that office building be demolished, and now this little old cistern is holding up a pretty major construction undertaking.
While I was visiting it the other day, I discovered an amazing piece of construction machinery. It's called a "Soil Mec":
They use it to augur insanely deep holes and then install enormous iron reinforcing bars into the earth.I also enjoyed seeing the 30-pound pneumatic chipping hammer in action. It's so petite! I must have one. Here it is, in action:
Some cisterns are filled with amazing things, like coins and Civil War treasure. This one is filled with tabasco ketchup bottles from Pennsylvania and little glass inkwells. Apparently, Alexandria in the 1920s was all about liquid ink and tabasco ketchup. I guess I should say, "Catsup", since the bottles were from the P.J. Ritter Catsup company.
Attention looters: there is nothing in good condition and nothing of value in this cistern. Also, there is very tight security so abandon hope of stealing the 1920s ketchup/catsup bottles.
Here it is, mostly freed from its concrete covering. As you can see, re-bar was placed inside to give support to the massive concrete blob that was poured into the cistern. It's astonishing that the cistern is as intact as it is! Abandoned, filled with garbage, partially knocked in to make way for a garage, filled with inkwells and catsup, then 30 or so years under a garage. It survived the garage demolition, had part of its dividing wall knocked down to make way for re-bar and a concrete building footer, then covered in 1-inch gravel and an office building. THEN the office building is demolished with a trackhoe! And then I came along with a backhoe and scraped all the gravel off. Then it had archaeologists poking at it for months.
Then we spent a day hammering away at the concrete with a chipping hammer. Not bad for 168 years of hard living...
3 comments:
I wonder if that dividing wall in the cistern could be made of pervious concrete, possibly creating an active filtration system for the whole neighborhood.
http://universalwaterfilter.blogspot.com/
Do you mean that as a speculative question about building new cisterns to solve modern drinking water problems?
I read your blog entry about the pervious concrete and it sounds ingenious to me - of course, I'm just a humble archaeologist and I'm always terribly impressed by engineering.
As far as I know, the basic technology behind this cistern is essentially sound. I think the water pumped from this particular one was used in a wash house- a laundry facility that was built directly on top of the cistern.
Water from the cistern was pumped up into brass washtubs in the wash house, where it was heated and used for clothes-washing. It may have also been used for drinking water. Many cisterns of this type were.
The filtration systems I've heard of in historic cisterns used either the charcoal box technique, or some sort of small, pea-sized gravel that the water was pumped through in order to enter the "clean" chamber (which sounds a bit like your pervious concrete idea, I think...)
"Do you mean that as a speculative question about building new cisterns to solve modern drinking water problems?"
Perhaps, at least in the short and mid term (next few decades). Most of the world's cities are just starting to get a few modern treatment plants and aren't even close to having the modern piping necessary to keep the water clean on its way to residences. The water leaves the plant completely potable and quickly becomes toxic as it flows through corroded pipes that receive infiltration by sewage and other waste.
The filtration techniques in the cisterns back then might have been adequate for removing soild from water and the systems that used charcoal would have handled other pollutants. But back then people didn't know about pollutants like arsenic, lead, bacteria, viruses, etc that can occur naturally in addition to human and animal waste.
Waste from blacksmiths, tanners, etc would have also been present and even charcoal would have not been very effective unless you know how much you need.
The design process for my filter does just that, calculating the approximate length of filter needed to remove certain concentrations of pollutants.
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