As an environmental problem researcher:especially about water issues (2/2)
A Book written by Jun Ui(宇井純)
Sewage treatment technology introduced by Jun Ui: Oxidation Ditch
Mr.Jun Ui, who is now dead and was a leader in pollution problems (and one of my teachers: an assistant in the Department of Urban Engineering, Faculty of Engineering, University of Tokyo), was a versatile person, but he went to see a sewage worker in Delft, the Netherlands. He visited the treatment plant, became interested in the facility: Oxidation Ditch (translated into Japanese, "Oxidation Groove"), and stayed for several days to observe it. According to the engineer who developed it, "Many Japanese researcher visit this facility, but no one else stays and observes for a long time like you."
Jun Ui understood the principle of this oxidation ditch, and he himself built a sewage treatment plant for individuals and groups that he understood in order to spread this technology to Japan, and conducted demonstration experiments. I have been involved in it for a while.
The basis of sewage treatment technology is the decomposition of "organic matter" in water. Organic matter is a molecule formed by carbon atom (C) as a nucleus, but there are almost infinite types of this molecule. Sugar, soy sauce, gravy, manure ... there is no limit to the number. Anyway, it is a process of sending oxygen (O) in the air into water, reacting it with organic matter, converting it to carbon dioxide (CO2), and returning carbon to the air. It is called aeration or oxidation.
This method, now called the "activated sludge method," introduces healthy microorganisms in the soil into the water and completes the oxidation by the action of these microorganisms. Certainly, organic matter can be decomposed fairly well in this process. However, sewage cannot be completely purified by itself. Pollutants other than carbon include nitrogen (N) and phosphorus (P). These are difficult things. Even if organic matter is removed and the water looks clean at first glance, if N, P, etc. remain, they will come out into the environment, and algae etc. will propagate using these as minerals, and organic matter will be produced again. The water is polluted and the precious oxidation becomes meaningless.
At this time, the sewage treatment technology shows two directions. One is the "continuous" process and the other is the "batch type" process. The continuous method is the current mainstream sewage treatment method in which sewage flows out of the plant and new sewage always flows into the pond. Therefore, multiple ponds are set up, and one pond "has only one function." On the other hand, the batch type has multiple functions in one pond. It has four functions of "inflow, stirring, precipitation, and outflow". Jun Ui chose this batch type, not to mention the oxidation ditch (oxidation groove).
The process of removing C is called (primary processing), but in the case of a continuous system, the process of removing N and P is processed in the second and third ponds that specialize in N and P. (Secondary and tertiary processing). In particular, in the case of N, activated sludge deprives HNO3 of O from N, which has changed from ammonia (NH3) to nitric acid (HNO3) as a result of treatment and oxidation without adding O, and N becomes N2 in the air. It will come out (denitrification). This is a chemical reaction called reduction, which is the opposite of oxidation. To remove N, it is necessary to repeat these two reactions. However, in the case of the continuous type, since a large amount of O has already been added and aerated, there is a lot of waste in removing it, and oxygen deficiency and oxygen excess are repeated, resulting in a high cost of sewage treatment. It will jump up. Indeed, sewage treatment is a gold eater. Think about it, the power that an air conditioner takes to drive air is enormous, but if it's water, the cost of sewage treatment is even higher.
Denitrification
In the case of the batch type, depending on the operation, all of the first, second and third processes can be performed in one pond. By performing the steps of aeration (stirring) and precipitation, the above-mentioned denitrification controls the oxygen concentration to the limit of oxygen concentration that can be changed into two steps at any time, and the pond is as shallow as a "groove". Aeration can be done with the "eel farming water wheel". Also, because it is shallow, it will not drown even if a child gets stuck. A major feature of the batch system is that the inflowing sewage and the substances and microorganisms contained in it will "always" remain in the pond, although they will gradually dilute. These are favorable features in terms of the stability of the treated water. And here is the advantage over continuous process.
More troublesome than N is P. Basically, the most basic and reliable treatment method for P, which cannot be flown in the air, is to take out a certain amount of activated sludge itself, which contains a large amount of it, from the pond and separate it from the water. Primitive is primitive, but if sludge can be returned to the soil, this is the best. Well, the same is true for C and N. In that sense, neither the continuous type nor the batch type can fully function under the current situation where sewage contains a lot of harmful substances. Things that cannot be returned to the soil.
A word of the day: C, N, P are originally fertilizers for growing plants. If you return the wastewater and manure to the farmland without modern sewage treatment, no problems will occur. In this respect, humanity's thinking is insanely stupid.