Sewage Treatment
. h t t p : / / W W W . S C I E N C E - P R O J E C T S . C O M / . . . . . . .



There are few aspects that are more responsible for moving civilization out of the Middle Ages and into the Modern Age than is the treatment of our municipal and industrial sewage. This and the treatment of our drinking water (potable water) comprise THE two major advances in public health that have contributed to greatly extended life expectancies due to the control of infectious diseases. (What non-infectious disease kills the most people worldwide?) Anyway, if you do not recognize the significance of this technology to your life, go hit your head on a brick wall until you do appreciate its importance. IT IS VERY IMPORTANT, and, as a citizen, you should know about it, unpleasant as it may be to your nose.

In essence, sewage treatment has two functions:

Let's take a look at the process and equipment your locality possesses. While you follow the steps, take note of what "primary treatment" is, and what "secondary treatment" is.

  1. The first part of sewage treatment is concerned with the removal of large items that enter the treatment plant. These items can jam the system, can consist of twigs, Volkswagens, gravel, animals, tampons and condoms.

    The raw influent first goes through a self-cleaning screen (B = Bar screen), and then into one end of a shallow and rather fast moving basin (S = Sand basin) so that sand and gravel can settle out. Often skimmers rotate around the surface of the basin to remove oils that may have been flushed into the system.

  2. In order to provide for the further processes - such as the addition of flocculants, the volume of the incoming raw sewage water must be measured (M = Meter). One of the most common types of meters that has no moving parts to become clogged or jammed is the kind shown here, and is known as a Parshal Flume. As the volume of water increases, the difference in water level on each side of the constriction increases. This difference is then translated into flow-through volume. With today's electronics, signals are transferred downstream for the needed adjustments for chlorine, flocculants, etc.

  3. Finally the waste water enters the first step of sanitization and is directed into the Primary Clarifier (PC). The theory behind this device is that it creates an ecosystem that is very alien to cellular pathogens (e.g.: bacteria) of humans. It does this by supporting anaerobic growth of microbes that also digest a small amount of the dissolved organic material (BOD, see later) while they produce lots of sulfides and other chemicals that are noxious to both noses and to the pathogens of humans. The construction is a huge circular vat with a metal curtain that does not quite reach the bottom. The water comes in between the curtain and the outer wall; gently and slowly flows under the curtain and rises to spill into a receiving trough at the top of the tank. As microbes digest the organic materials they grow and floc together. Large quantities settle to the bottom of this tank. If you are lucky to find the treatment plant, which you are visiting, has spare, empty tanks, you can see scrapers on the bottom that slowly sweep the sludge into a hole. The sludge is then augered into a sludge holding tank. You have now finished what is called "Primary Treatment." The effluent is more or less safe with regard to bacterial pathogens, but it is a disaster to dump this into a stream or river because it contains so much dissolved organic material. In the receiving body of water, which is filled with lots of its own microbes, all that organic material is digested at the expense of the oxygen that is dissolved in that river or lake. This is called biological oxygen demand (BOD) which is directly correlated with the amount of rapidly biodegradable organic material. It is disasterous to dump this "high BOD" water into a lake because soon all the fish and other aerobic creatures will die, and you will get a fetid, stagnant cess pool.

  4. Rather than dumping the "primary effluent" into a beautiful natural stream, let us make an artificial lake and vigorously aerate the water to promote the rapid growth of microbes that will "eat up" the BOD before the water gets to the river. Hence we have an "aeration chamber" (AC). In the days before electric power, the water was allowed to sprinkle out over the surface of a basin that was filled with rocks. The water would trickle down through the rocks before exiting at the bottom. The water was thus exposed to a great amount of surface area, and the rocks became coated with a slime of all sorts of aerobic microbes that rapidly dropped the BOD. On the sunlit top rocks, algae vigorously grew, but just below them the slime turned to a vast microbial dog-eat-dog ecosystem of bacteria, fungi, grazing amoebae, snails.

    An alternative way of doing this is by making huge silos filled with plastic honeycombs. The water is sprayed on top to trickle down through the honeycomb while air is forced upwards through the honeycomb. Yet a third way of promoting aeration is shown in this diagram: giant "egg beaters" churn and splash the water. A fourth way is even more direct: bubble air up through the water from the bottom. All these alternative devices are with the purpose of converting the BOD to either carbon dioxide (which wafts away) or to cell matter (for the next step).

  5. We take the outflow from the aeration chamber, and try to recover as much of the microbial growth as possible to feed it back into the beginning of the aeration process. This is done with another clarifier (SC = Secondary Clarifier). Many treatment plants have secondary clarifiers that are almost identical to the primary clarifier's design. But, for change of pace, here is shown one where the incoming water flows down a central shaft and slowly works its way back up to the top, where is spills over at the edges into a circular trough. Often alum or other flocculant is added to the incoming water to assist in the aggregation of the microbial cells into globs that a big enough to sink. The water that spills out of this tank has passed "secondary treatment." It is surprisingly clear water.

  6. Before allowing the effluent to reach the river or other receiving natural body of water, a final safety step is taken - the disinfection of the water. Shown here is a maze into which chlorine is added at the beginning, and remains in "contact" until it reaches the end of the tank. (CC = Contact Chamber)

    Because excess chlorine is harmful to the receiving body of water, the amount added must be carefully monitored at the outfall. Because chlorine is so extremely toxic, alternative methods are used elsewhere. Ozone is commonly used in Europe, which is generally more advanced in its treatment of sewage than is North America. (The reason is that Europe is much more crowded and the tolerance of receiving bodies of water cannot absorb overages there, while much less crowded North American watersheds can absorb and "mistakes." Other processes are being considered: ultraviolet radiation; high intensity sound (sonic disruption of cells), and very intense 1,000 cps electrical current. It is always better to add "physics" to a system because there is no residue to be monitored. It is easy to get rid of "physics" - just turn it off!

There are two other components of sewage that must be considered that often escape the above "secondary treatment" plants.

"Tertiary Treatment Plants" do one further step after the foregoing: they allow the secondary effluent to flow into large ponds where algae grows and uses up the ammonia and nitrates in the water. This is a slow process and requires very large holding ponds. The algal growth is harvested and used as a nitrogen-rich organic fertilizer.

The elimination of viruses from sewage is problematic. Addition of chlorine is known to have little effect on many types of viruses. The European usage of ozone is known to damage and thus inactivate most viruses, and so that process is better than the use of chlorine. Ultraviolet irradiation of the effluent should be highly effective, but there is a problem that even the slightest amount of any organic chemical containing double-bonds absorbs and renders UV rather ineffective.

A class project to test one's local sewage treatment plant's efficacy in eliminating viruses is rather simple. Just as public health use usually harmless E.coli as an indicator of fecal pollution, you can use those viruses that infect only E.coli as your indicator viruses. Of course, the treatment plant's raw influent waters contain huge amounts of E.coli. Therefore, these waters ought also contain even greater amounts of those viruses that infect E.coli. Those viruses have a nickname "coliphages" (after the bacteriophages of E.coli, where the term bacteriophage means a virus of bacteria).

  1. Obtain 5 ml samples of treatment plant waters taken at various stages along the process.
  2. Add 1 ml of chloroform to the samples; shake vigorously for a few minutes. The chloroform emulsion kills all the cells in the samples, but does not harm the coliphages.
  3. Allow the chloroform to settle to the bottom of the tubes.
  4. Make some dilutions of the overlying aqueous phases of these tubes.
  5. Mix 0.05 ml of those dilutions with 0.05 ml of an overnight culture of a lab strain of E.coli (such as K-12, C600, B, etc.)
  6. Transfer to the middle of the agar in a nutrient agar filled petri plate, and spread the droplet evenly over the surface of the agar. (Use a presterilized bent glass or metal rod for this purpose.) The droplet will soon be absorbed into the agar leaving the bacteria and coliphages stranded high and dry on the surface.
  7. Incubate at body temperature; results often appear within 6 to 9 hours.

As the thousands of stranded bacteria begin to grow, some cells come in contact with a coliphage, which infects the contacting cell. In about 40 minutes, that cell bursts releasing a hundred or so progeny coliphages, which infect surrounding cells. The infection and bursting continues upon the "lawn" of growing E.coli resulting in holes or "plaques" in the lawn. By holding the plate up to the light, the plaques become visible. You should see plaques in the samples derived from raw influent water. Do you see any decrease in numbers of plaques as the water moved through the treatment process? Probably. But is there complete removal of "plaque-forming units" (pfu's)? If not, what does that mean? While coliphages are not a health hazard to you, they do indicate that other viruses - flu, hepatitis, HIV - might also escape the treatment process. If you find that your treatment plant is not removing the coliphages, it is something to think about very seriously. But do not run off to the news reporters with your information. You will not accomplish anything constructive. Instead, try to work with the plant's engineers to discover a way to treat the viruses. You will make history in a constructive way. Society needs this discovery.

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