September 26, 2017
We have a man-made waterfall and a stream with small pools and fish in our backyard. The system is naturally maintained by water plants and a biological filter. Water is pumped from an underground tank at the downstream end of the stream up into another underground tank at the top of the waterfall. That tank contains two open-cell foam pads that are coated with beneficial bacteria to form the biological filter. The pads capture dirt and debris, while the bacteria trap other microorganisms, nutrients, elements such as iron, and so forth. The whole mess of stuff is called schmutzdecke, which is German for “dirty skin.”
The same principle, applied in combination with natural sand, used to be the preferred method to filter drinking water. In the filter, called a slow sand filter, water first passes through about 36 inches of sand, then through a layer of gravel, and finally exits the system via an underdrain. Schmutzdecke, similar to that in our water feature, forms on top of the sand. The sand removes particles from the water through adsorption and straining, and the schmutzdecke takes care of most of the other nasty things in the water. Whatever nasties aren’t captured are finished off by disinfectants such as chlorine and ozone.
One of the earliest applications of slow sand filtration was in Pittsburgh. During the late 19th century, the death rate from typhoid and diphtheria in Pittsburgh was significantly higher than most, if not all, other cities in the world. A slow sand filtration system was put on line in 1908, and the death rate from typhoid and diphtheria dropped immediately and dramatically.
One problem with slow sand filters is that the schmutzdecke slows down the filter flow rate, so the system must be very large. Also, when the schmutzdecke becomes too thick, the water flow will stop. Municipal filter plants periodically have to scrape away their schmutzdecke, as well as the top inch or so of sand.
During the early 20th century, rapid sand filtration began to replace slow sand filters as the preferred method of water treatment, especially in large metropolitan areas. The main difference between rapid and slow sand filtration is that rapid sand filtration utilizes physical treatment dependent on coagulation and flocculation, whereas a slow sand filter utilizes biological treatment dependent on schmutzdecke.
Particles in turbid water that are too small to settle out (called colloids) commonly have a surface charge on them. The like-charged colloids repel one another and stay in suspension. Adding coagulants to turbid water neutralizes the charges on the colloids so the colloids can coagulate into a gelatinous mass (floc) large enough to settle out of the water. Gently stirring the water (flocculation) causes the flocs to collide with one another and agglomerate into even larger masses that settle out or are filtered from the solution.
Eliminating the schmutzdecke allows a higher rate of filtration, thus a rapid sand filter system can be much smaller than the slow sand system and still process the same amount of water. Consequently, rapid sand systems tend to prevail in large municipalities where space is limited. Slow sand systems may serve the needs of smaller communities and of developing countries.
Well, schmutzdecke, coagulation, and flocculation are neat scientific topics, but water purification also depends on sand. So……
Next time you have a nice glass of clean, cold water, remember to say…
Danka schon, schmuzdecke, and sand.