Sewage treatment system units
littering
One of the first and most important stages of urban wastewater treatment is the garbage collection stage. In this stage, also known as the preliminary stage, all large and solid garbage suspended in the sewage will be completely removed. Large garbage generally includes materials such as pieces of wood, cloth, and such items, which are collected by garbage collection devices that are made of net plates. It goes without saying that garbage collection devices are designed and manufactured in manual and mechanical models, the use of each of which will depend on the type of treatment plant and existing conditions.
Seeding
Granulation is considered as the second step in wastewater treatment, in which small grains will be separated from the wastewater. The presence of small grains in water can cause problems for water transmission lines, so it is necessary to remove such particles from water using methods. Usually, special seeding ponds are installed in the sewage transfer routes, but the model of these ponds will depend on the type of sewage. Also, in these ponds, granulating devices are used for this purpose, which separate all the fine particles in the water in the form of swirling and eddy currents, depending on the existing conditions.
Chemical treatment
Chemical treatment is considered as another stage of wastewater treatment, during which all organic substances, heavy metals and chemicals are completely removed from wastewater. Currently, there are many methods to perform this process, among which we can mention disinfection with UV rays, chlorination packages. The use of the mentioned methods in wastewater treatment is very practical and one of these methods is used in almost all treatment plants.
Sludge discharge
Sludge discharge is considered as the last step in wastewater treatment, during which the remaining sludge on the water floor must be completely removed. It goes without saying that in the stage of sludge discharge, drying it is considered as one of the key things, which is done by the filter press and some other available equipment.
Physical method
Purification by physical forces is called physical wastewater treatment. Now, this physical treatment can be the result of the settling of solid materials in the wastewater due to the force of gravity or the floating of fat and oil in the wastewater due to its lower density than water; But what is common among all physical methods is that no chemical reaction or biological process is used in the wastewater treatment package.
Among the other methods of physical wastewater treatment, we can mention garbage collection, granulation, flotation, filtration, etc., the common feature of all of them in the treatment process is the use of physical force for the wastewater treatment plant. Physical wastewater treatment is usually used in all treatment plants due to its convenience and lack of energy and high cost.
Chemical method
Any method in which chemicals are used for wastewater treatment is called chemical wastewater treatment. Chemical wastewater treatment has a higher price than other methods due to the use of chemicals as well as the complexity of the design and construction of these systems.
In industrial wastewater treatment, the use of chemical methods is inevitable; Because if we want to use physical and biological methods to treat this type of wastewater, we need large dimensions to build a treatment plant, and physical and biological treatment methods are not capable of treating industrial wastewater.
The methods used in chemical treatment include pH adjustment, chlorine disinfection, coagulation, flocculation and precipitation.
Biological method
Purification by means of biological processes is one of the most widely used methods in the wastewater treatment package. Biological wastewater treatment is done by living organisms called microorganisms. In this type of purification, no chemicals or physical forces are used, and only the growth and reproduction conditions are provided for the microorganisms which are the main factors of purification.
Biological treatment of industrial wastewater is divided into three sub-branches: aerobic treatment, anaerobic treatment of wastewater, and anoxic treatment of wastewater. Today, this method is used for purification in most of the treatment plants in the world. Biological treatment of wastewater is known as supplementary or secondary treatment of wastewater, and physical methods can be mentioned among the methods of preliminary treatment of wastewater.
What are the methods of biological wastewater treatment?
After primary wastewater treatment, secondary treatment, which is also known as biological treatment, is performed. In biological treatment, the basis of work is based on feeding microorganisms from organic substances (soluble and insoluble) in wastewater. Besides other substances, microorganisms need the following four substances more for their survival:
oxygen
Microorganisms in urban sewage are divided into two categories, aerobic microorganisms and anaerobic microorganisms, according to their oxygen requirement. Aerobic microorganisms supply the oxygen they need through oxygen gas dissolved in water, and anaerobic microorganisms from the structure of compounds containing nitrate, sulfate, carbonate ions, etc., or through the oxygen in the structure of organic materials to supply oxygen They use their needs.
Of course, it should be noted that some microorganisms are able to continue life and activity both in the presence and absence of oxygen, which are known as facultative microorganisms.
Carbon
Microorganisms supply their carbon needs through carbon dioxide dissolved in water and carbon-containing organic matter. In this sense, microorganisms can be classified into two groups: autotrophic and heterotrophic. Autotrophics use carbon dioxide to supply their carbon needs, and heterotrophics use carbonaceous organic materials for this purpose.
Two other essential substances for the survival of microorganisms are nitrogen and phosphorus, although in addition to oxygen, carbon, earth and phosphorus, microorganisms also need elements such as potassium, sulfur, magnesium and calcium, although in smaller amounts.
Note that microorganisms, especially bacteria, may be poisoned in some situations due to the presence of some organic substances such as high concentration of phenol, the presence of heavy metals, and the presence of very high concentration of minerals.
In some cases, experts manually add a certain amount of nitrogenous and phosphorous substances to the wastewater to provide the amount of nitrogen and phosphorus required by the microorganisms.
There are many methods for biological treatment, including old methods and some methods based on new technologies. Here we examine three common methods for biological wastewater treatment, including the use of lagoon systems, trickling filter systems, and activated sludge systems.
Features and some important points about the lagoon system:
Wastewater retention time or stoppage time
Wastewater retention time is one of the most important factors in evaluating the performance of any lagoon system and is derived from the relationship T=V/Q. In this relationship, T represents the retention time and includes the time each drop of wastewater spends in the lagoon. Other parameters related to this relationship, namely V and Q, represent the volume of the lagoon and the sewage flow rate. In ideal conditions, the residence time for all wastewater droplets has the same value; Ideal conditions mean conditions where there is no mixing in the sewage flow and this flow is piston-like.
For example, consider that the sewage flow of a residential complex enters the lagoon system at the rate of 5 cubic meters per hour. If the length and width of the pit or lagoon is 900.2 meters and the depth of the incoming sewage is 2.5 meters, the sewage stopping time is calculated as follows:
The volume of the lagoon is equal to the multiplication of the length and width of the rectangular pit by the height or depth of the sewage system, which is equal to 2.5x900x2.5 cubic meters. divided by the hour. In the end, the answer to the problem, which is equal to the time that the sewage remains or stops, is 900 hours, equivalent to 37.5 day and night.
Longer residence time means better wastewater stabilization
In the wastewater treatment process, the lagoon is considered as a place to break down the wastewater and reduce its organic pollution. Certainly, at this stage, the longer retention time means the reduction of volatile organic substances and better stabilization of wastewater. This time usually lasts about 3 to 8 months, and most of the experts in the field of wastewater treatment seek to reduce it to speed up the treatment process and also to treat a larger volume of wastewater.
The bad smell of the lagoon environment is caused by the decomposition of organic matter by bacteria
Aerobic bacteria in the lagoon pit produce CO2 and SO2 gases with the oxide of organic materials, and anaerobic bacteria produce CH4 and H2S gases by decomposing organic materials. CH4 and H2S gases are the cause of unpleasant smell in the lagoon environment, and therefore, more efforts are made to decompose organic matter by providing the necessary conditions through aerobic bacteria in the lagoon environment.
For example, the shallowness of the lagoon is one of the factors that can cause the decomposition of organic matter in the lagoon by aerobic bacteria, because at a shallow depth, oxygen in the air penetrates into the sewage and causes the activity of aerobic bacteria for decomposition. to be
Keep in mind that in the case of very concentrated wastewater treatment, even considering the shallow lagoon pit cannot provide enough oxygen for the activity of aerobic bacteria, and solutions such as diluting the wastewater (with less contaminated water) to solve the problem Consider.
Drip filter system
The performance of trickling filters for wastewater treatment can be considered as water treatment filters, with the difference that these filters have much larger dimensions than water filters. In this system, the sewage is spread downwards (through a nozzle) from the bed containing sand and stone particles, and air is blown into the filter from the bottom of the bed. In this case, aerobic bacteria use the oxygen in the air for their activity to decompose the organic substances in the wastewater and carry out the decomposition process and a huge part of the wastewater treatment.
In this system, the finer the bed particles (sand and stones) are, the contact surface of wastewater with air oxygen increases and this improves the performance of bacteria and other aerobic microorganisms. Of course, it should be noted that the use of too small particles over time can cause blockage of pores due to the creation of a biological layer on the particles of the bed surface, and it is not suitable.In the trickling filter system, in order to implement the best performance in wastewater treatment, large particles are placed at the top and finer particles are placed at the bottom of the bed.
Some important parameters in the performance of the drip filter system:
The height of the drip filter
In the trickling filter system, the longer the contact time between air and sewage particles, the BOD decreases; So, the height of the drip filter is effective in the optimal performance of the system.
Sewage inlet load
The sewage input load parameter can be defined as the sewage discharge per unit of filter cross-section of this system, which affects the performance of the trickling filter system and must be controlled.
