How is sewage generally treated?

The main pollutants of industrial wastewater (industrial wastewater is not called sewage) and domestic sewage include and are not limited to the following: 1, COD (chemical oxygen demand) and BOD (biological oxygen demand) : consumption of water oxygen leads to the death of water organisms. The former is determined by chemical oxidation, and the latter by dissolved oxygen in water consumed by microbial culture, generally measured by 5 days. 2, total nitrogen and ammonia nitrogen, total phosphorus: lead to water eutrophication, aquatic plants and algae growth, consumption of oxygen in the water. 3, Ph value: This all understand Ha. 4, SS (suspended matter) : resulting in turbidity and sediment content. 5, chroma: let the water change color. The above pollutant indicators are the most concerned by sewage treatment, but there will be different increases or focuses according to the treatment goals and different types of sewage. Sewage treatment is mainly the use of physical (filtration, precipitation), chemical (chemical reaction) and biological (biological eating) methods to eliminate pollutants in the water reaction. For the convenience of understanding, this article does not expand on professional terms.

A sewage treatment plant (hydrolytic acidification +SBR process) the relative composition of domestic sewage is fixed, and after determining the approximate scale and inlet water quality, it can be done with a certain routine, but according to the specific local conditions and investment requirements to adjust and optimize. However, strictly speaking, process calculation, digital modeling and experiments are still needed before the design and construction of biochemical wastewater. Industrial wastewater due to various sources (such as paper waste water, alcohol waste water, printing and dyeing wastewater), different production processes (for example, the same printing and dyeing enterprises, the production line built in the last century and the production line built now are completely different) and the local environment, management level and other factors, Basically, the sewage of a factory (even if it is two factories that produce the same product, as long as the location is different) is a new technology development and construction, so there is no fixed model, and it can only be determined by digital modeling, small experiments, medium-sized experiments, etc. Industrial wastewater is not much to say, interested friends can look at the "outdoor drainage manual" industrial sewage volume, which has a general description.

First, the conventional domestic sewage treatment process is introduced to you: the domestic sewage generated by human life is collected through the pipe network to the underground septic tank near the building, where the initial precipitation and digestion are carried out. Fecal matter, toilet paper, and the like settle to the bottom of the septic tank due to its own density, while being decomposed (that is, eaten) into small particles and water-soluble substances by aerobic, anaerobic and aerobic bacteria (which can survive with or without oxygen). Therefore, the septic tank needs to clear the sediment for a period of time (generally half a year to a year), even if it is a high-end residential septic tank (when the taste of that...). .

Sewage treatment plant process schematic diagram Domestic sewage first through the grid, generally thick and thin two. The coarse grille has a gap of 10-30 mm, and the fine grille has a gap of about 3 mm. The floating and suspended materials in the sewage are intercepted when passing through here, and are transported to the slag deposit point by the screw conveyor or belt conveyor, and then transported to the landfill for landfill. After the coarse grid filtration (some in front of the coarse grid after the fine grid) of the sewage in order to reduce the complexity of the subsequent system and energy saving, generally through the pumping station to the highest point of the sewage plant, and then rely on gravity flow through the subsequent treatment facilities.

Fine grid and swirl sand sink (sewage is raised after the coarse grid, here is the highest point of the sewage plant) If the city has industrial wastewater mixed with the municipal pipe network (domestic general more or less mixed), the domestic sewage treatment plant needs hydrolysis acidification pool for further decomposition treatment. This kind of pool is mainly to form an anaerobic environment, so that anaerobic bacteria break the macromolecular chain in the sewage, eat some of it, and at the same time, make the sediment and other inorganic substances in the sewage precipitate part. Generally there will be hydrogen sulfide, methane and other gases produced here, and the taste is very bad.

The structure of the biochemical section of the SBR process is very large, and the general total volume is similar to the daily treatment scale of the treatment plant. For example, the treatment scale of a domestic sewage treatment plant is 100,000 tons/day, then the biochemical section of the plant will probably have a volume of about 80,000 cubic meters. Here, domestic sewage is biochemical through the action of microorganisms (anaerobic, facultative and aerobic bacteria) in the pool. The sewage is partially circulated inside and outside the structure using a pumping system. The blower blows oxygen from the outside to oxygenate the pool. The organic matter in the sewage is eaten by the bacteria in the pool, and the ammonia nitrogen and phosphorus in the sewage are converted into nitrogen and phosphate. If the structure is running properly, the water surface is a light reddish-brown, and you can smell the earthy fragrance after a thunderstorm. The following are several forms of biochemical ponds. After passing through the biochemical stage, the sewage is basically treated clean at this time. After the biochemical reaction, the sewage enters the sedimentation tank (generally called the secondary sedimentation tank), and through the action of gravity, the particles, bacteria and bacterial bodies in the water (those who die of old age, actually see the flocculent colloid) precipitate into sludge, which is dehydrated and transported to the landfill for landfill. The supernatant of the sedimentation tank spills through the overturning weir. After entering the disinfection equipment (generally for ultraviolet irradiation), it meets the external discharge standard, that is, the domestic sewage Level 1 B discharge standard.

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.