More adventures with the science club - Water treatment plant

Last weekend I chaperoned the science club trip to a local water treatment plant. Here's the general start-to-finish process.
It begins with taking river water from a branch just before it feeds into the Karnaphuli. This is a relatively clean river that serves as a huge, important fish breeding ground for the area. Here's the intake portion of the plant:
After the water comes from the river, they pump it into huge settling ponds so the larger sediment and particles can settle out via gravity:
After it spends 24-36 hours in the first clarifying process, it's sent to the larger clarifying and filter tanks. First, aluminum sulfate is added to aid in flocculation (creating solid particles to settle out) and chlorine is added to kill some bacteria. This first picture is the building where the chemicals are stored--here's Professor Ibrahim in front of piles of aluminum sulfate chunks:
And the room where the chlorine is automatically metered into the water flow:
Don't be fooled by the 'modern' look of that--here's where the chlorine is stored--in huge, rusty tanks in a dirty old building:
They only use 3 chemicals here--chlorine for bacteria and algae control, lime (calcium hydroxide) for pH control and aluminum sulfate for flocculation. So the chemicals are mixed into the water flow in the area underneath where all the students are standing in the next photo. This photo shows the large settling/clarifying tanks. There were still lots of small fish and a few big crabs in these ponds, so not so much chlorine to kill them, I guess.
At the end of these settling tanks is sand and gravel filter (so no baby fish getting through there). The solids and sludge that is removed from all this settling is dumped straight back into the river. It would be interesting to find out what kind of aluminum levels are in the area.....
Finally, the water is pumped into the city for people's use. Here's one (of a series) of the giant pumps that is needed to lift all that water to the holding tanks at various locations around the city.
Here's their "ultra-modern" monitoring room where they can see if various pumps aren't working. There are tiny lights that blink if there are problems at various locations.
Finally, we got to see my favorite part, the chemistry lab! Here's where they test the water for all kinds of things--fecal coliform, e.coli, heavy metals, and of course pH, hardness, residual chlorine, etc. The most exciting part for me was to see their equipment and chemicals and it revived my hope of getting a few decent pieces of equipment for our labs!

After our little tour, we got a typical Bangladeshi welcome with a piece of fruit and cracker snacks. In place of the usual tea, we got water from their treatment plant! Yum! They also had a pretty interesting video where they very graphically depicted what the 3rd world war would be fought over--water, of course.
Overall, a very interesting day and the weather was fairly reasonable, so it was nice to be outside. As far as we know, there is no wastewater treatment in all of Chittagong (a city of over 4 million people), so it would be really interesting to study some of the ditches and creeks that serve as sewers that end up in the rivers and ultimately this treatment plant....