How is sewage generally treated?

In essence, there are only two means of water treatment: one is separation, the other is conversion.

Separation is to separate some pollutants in sewage from the water body, the specific measures include precipitation, flocculation, centrifugation, air flotation, blowing and so on, the basic physical and chemical methods. Usually, the pollutants in sewage such as organic matter can be removed after preliminary separation and treatment, and the requirements are not high, so it can be directly discharged. This is called primary processing.

Some pollutants cannot be effectively separated, such as dissolved organic matter, ammonia nitrogen, phosphates, which need to be converted into harmless substances, or easily separated substances. The most important biochemical processes in sewage treatment are conversion work, such as the removal of dissolved organic matter by converting organic matter into carbon dioxide (basically harmless, the gas can be easily separated from the water) and biological sludge (harmful, but also easy to precipitate and separate). This is called secondary processing. There are many means of conversion, such as various advanced oxidation, acid-base neutralization and so on. The cyanide sewage produced by the Tianjin explosion accident can only be broken by the strong oxidation of hydrogen peroxide to break the C-N bond and make it harmless.

The process of domestic sewage plant is usually 1 grid 2 primary precipitation 3 biochemical treatment 4 secondary precipitation 5 disinfection. From the above classification, 124 is separation and 35 is transformation. This separate type of process, although stable and easy, but occupies a large area, high construction costs, long residence time (can be understood as a large volume of structures occupy a large area).

Now new technologies are increasingly inclined to combine separation and conversion into a set of systems to reduce costs and improve efficiency, such as membrane biological treatment process (MBR), which is the biochemical process and primary and secondary sedimentation into a pool, so obviously the footprint is greatly reduced. Although the cost of the membrane process is still high, with the advancement of technology, the cost will be lower and lower, and it will be more and more popular. This is also why the valuation of green water Source is higher than that of ordinary water treatment companies.

I'm sure you thought I worked in MBR or had something to do with the water source. In fact, it doesn't matter a dime, what I do is mainly sludge reduction. Most of the pollutants in the sewage above are eventually turned into sludge. This is actually a pollution transfer, but the treatment of biological sludge is much more difficult and costly than sewage treatment. For these sludge, it is now common practice to bury, but the big cities will soon have no place to bury, and in the end, it can only be burned, mixed with fuel. It's expensive, but we can't help it. How to solve the sludge generated in sewage treatment at a low cost is the biggest challenge in the sewage treatment industry today, and it is also an opportunity for our water environmental engineering researchers.