Sewage treatment and decontamination
By post-treatment means methods and processes that complement the traditional technological schemes for wastewater treatment in cities and settlements. The possible degree of removal of contaminants in the post-treatment processes is practically unlimited and is determined by the conditions for the discharge of the treated sewage or their subsequent disposal. To the methods of post-treatment of sewage of cities and settlements include:
- a method of filtration, carried out on filters with granular loading, on reticulated drum filters;
- a biological method that is traditionally implemented in biological ponds with natural and artificial aeration, as well as on post-treatment facilities, arranged according to the principle of aeration tanks with loading;
is a flotation technique based on the ability of hydrophobic particles to adhere to gas (air) bubbles and float to the surface to form a foam;
is a sorption method for removing residual organic impurities from purified waters, for example using activated carbon;
- a method of oxidation of residual dissolved contaminants by strong oxidants (ozone, chlorine, chlorine dioxide, potassium permanganate, etc.);
- various methods of additional purification of waters from biogenic elements (reagent, ion exchange, biological, etc.);
- a combination of these methods.
To remove suspended solids and BOD, up to 6 ... 8 mg/l after biological treatment in standard air tanks, filters with granular loading are used. They do not develop microbiological processes and the content of nitrogen and phosphorus compounds does not change.
A filter is a tank loaded with granular material through which water seeps from the top down or from the bottom up. The distribution of water on the surface prior to filtration and the collection of filtered water must be uniform.
As a granular charge, as a rule, quartz sand of fineness 1.2 ... 2 mm, 1.2 ... 1.3 m layer is used; the water filtration rate is 6 ... 8 m/h. When accumulation of pollutants in the filter body, the water supply for purification is stopped and water-air flushing is carried out.
Filters with different loads are used - two-layer, frame-zasypnaya, floating of plastic chips, small chips, etc. The performance of filters is not much different, their choice should be determined by specific conditions of use - available residual pressure of water, height of buildings, degree uneven flow of sewage, etc.
Modern designs of a continuous grained sand filter, such as DynaSand, do not require a shutdown for flushing. Filtering of water on the filter is carried out from the bottom up with an average speed of 15 m/h. As loading, sand of 0,8 ... 1,2 mm is used. The principal distinctive feature of the filter is the presence of an airlift intake of the most polluted sand from the lower conical part of the filter and feeding it to the washing unit. From the washing unit, the clean sand enters the upper part of the filter, the washing water is diverted to the head of the treatment plant. The filter can be operated in the reagent filtration mode.
Reagent filtration is produced to further remove organic contaminants after the aerotanks, but mainly to remove phosphorus.
Preparation of water before filtration (mixing with reagent) is carried out in a separate chamber. The dose of coagulant is 15 ... 45 mg/l. Addition of flocculants in doses of 0.5 ... 1 mg/l allows to reduce the dose of coagulant by half. The design of filters is similar to the design of filters with granular loading for reagent-free filtration. But when reagents are used, the filtering speed is reduced to 4 ... 5 m/h, the frequency of flushing increases. The concentration of organic contaminants after the MIC and suspended matter filter reaches 4 ... 5 mg/l, phosphorus - 0,2 ... 0,5 mg/l.
The combination of filtration methods and biological post-treatment is realized on bioreactor filters. Bioreactor filters contain a charge from an inert material on the surface of which an active sludge develops, capable of forming an additional biological filter between the loading grains. To remove suspended solids and VPK, sand loading with a size of 1.6 ... 2 mm, crushed gravel crushed stone and porous slag-pumice loading can be used. It is preferable to use gravel crushed stone for nitrification processes. At the same time, the design of the bioreactor filter provides for preliminary aeration and recirculation of the treated waters to the head of the structure by pumps. To remove residual phosphorus, it is recommended to introduce a reagent (coagulant, flocculant). This structure allows to reach the values of the concentrations of purified water: by suspended substances - up to 1 ... 3 mg/l, according to BOD5 - up to 1.5 ... 4 mg/l, in ammonium nitrogen - up to 0 ... 0.5 mg/l, phosphorus by using the reagent - up to 0.2 mg/l.Ozonization as a method of deep purification of biologically treated urban wastewater, despite its high cost, is universal, as it allows simultaneous reduction of pollutant concentration by COD by 40% by VPK5 - by 60 ... 70%, the content of suspended substances - by 60%, surfactants - by 90%, phenols by 40%, nitrogen by 20%, carcinogens by 80%, and also by water 60% with simultaneous disinfection of water. The recommended dose of ozone is 10 ... 15 mg/l at the time of its contact with waste water of 15 minutes. The combination of ozonization with the process of removal of suspended matter by flotation in one facility was called ozonoflotation.
Ozonoflotator is a device into which wastewater is fed, passed the reactor to dissolve in it an ozone-air mixture. Simultaneously, the apparatus creates conditions for the formation of small bubbles of the ozone-air mixture, which carry with it a slurry and oxidize it during the ascent. The slurry forms a flotation on the surface of the liquid, which is removed for treatment.
Collecting flotopenes from the surface of a liquid during ozone flotation is one of the most important elements of the design process, which in practice is rather difficult to implement, and this is one of the obstacles to the implementation of the method.Sorption is carried out, usually after the filters, if necessary to reduce the BOD to 2 ... 3 mg/l. As a sorbent, activated carbons of various classes or natural sorbents, in particular, bentonites, are used. Sorption is carried out in filters in a static mode (filtration through a fixed charge from a granular granular material) or in a dynamic mode (contact with vigorous stirring with a powdery material).
The speed of filtration through fixed loading is 5 ... 8 m/h, the duration of contact in a dynamic mode is up to 15 min. The design of the sorption filter is similar to the construction of sand filters. When the sorption capacity is lost (exhausted), the loading material is replaced. Coal can be regenerated in a thermal way, mineral sorbents are removed to landfills.
The effect of sorption increases significantly when combined with ozonation. The effect of sorption deep purification during preliminary ozonation of water increases by 30 ... 60%, depending on the dose of ozone in the range from 3 to 14 mg/l. With the joint conduct of sorption and ozonization, the efficiency of sorption on coal is 1.5 ... 3 times higher than without prior oxidation.
Disinfection of sewage is produced to destroy the pathogenic bacteria and viruses contained in them and to eliminate the danger of contamination of the reservoir by these microbes when draining the treated sewage. Pathogenic microbes can not be completely removed either during sedimentation or in biological treatment of wastewater. In plants for artificial biological treatment (in biofilters and aerotanks), 91 to 98% of such bacteria are eliminated, therefore, after mechanical and artificial biological treatment, it is necessary to decontaminate sewage before they are released into reservoirs. In cases of soil purification of waste waters on irrigation fields or filtration fields, disinfection is generally not required, since up to 99.9% of bacteria are eliminated.
The current Rules for the Protection of Surface Waters from Pollution by Sewage require that the effluent does not contain pathogens. In view of the complexity of the direct determination of the content of pathogenic bacteria in wastewater, a method for evaluating the effectiveness of their disinfection but the titre of E. coli is usually used. Disinfection of waste water can be recognized as sufficient if the number of them is brought to 100.
In order to disinfect sewage, chlorination with liquid chlorine, chlorine lime or sodium hypochlorite, obtained by electrolysis. The chlorine dose necessary for water disinfection depends on the number of pathogenic bacteria, organic and inorganic substances, capable of oxidation, which are in the waste water. The essence of the decontaminating action of chlorine is the oxidation and inactivation of the enzymes that make up the protoplasm of bacterial cells, as a result of which the latter die.
In disinfection, chlorine should be well mixed with disinfected water and be at least 30 minutes in contact with it. Contact of chlorine with sewage is carried out in structures called contact reservoirs, as well as in trays and pipes before draining the water.
For preliminary calculations in projects, the estimated chlorine doses should be taken: for standing sewage, 30 g/m3; for not completely purified sewage in aerotanks or high-load biofilters - 15 g/m3; for completely purified sewage water - 5 ... 10 g/m3.
Decontamination of large masses of water, as a rule, is carried out by liquid chlorine or sodium hypochlorite; with small amounts of sewage (up to 1000 m3/day) use of chlorine lime or sodium hypochlorite. The plant for disinfection of waste water consists of a chlorinator, a mixer and contact tanks.
Ozonization involves the use of ozone, which is one of the most powerful oxidants. The decontaminating effect of ozone is based on its high oxidizing ability, which is explained by the ease with which an active oxygen atom is released to it. The oxidation-reduction potential of ozone is 1.9 V, chlorine is 1.36 V, oxygen is 1.23 V. The ozone-air mixture obtained in the ozonizer reacts with water in the contact tanks. The completeness of using ozone depends on the degree of dispersion of the ozonated air in the water. The most complete use of ozone is achieved when the air is dispersed by filters, porous tubes and ejectors.
Due to its high oxidation potential, ozone energetically interacts with many mineral and organic substances, including plasma microbial cells. Ozone acts on bacteria faster than chlorine and is used in smaller doses - 0.5 ... 5 mg/l, depending on the water content of oxidizable substances. The water temperature and pH value have a much smaller effect on the ozonation effect of water than on chlorination. An important advantage of ozonization is that ozone dosage does not require as much care as chlorine dosing.In ozonization, along with disinfection, the true dissolved and colloidal organic impurities of water are oxidized and destroyed, which leads to a decrease in color and odor and excludes special treatment for this purpose, thereby simplifying the water purification scheme. Ultraviolet Disinfection (UV) refers to modern methods of disinfection of treated waters. It was observed that chlorination of water leads to the formation of dangerous by-products. An analysis of alternative chlorination technologies of disinfection has shown that all oxidative technologies of disinfection lead to the formatting of certain by-products, most of which are dangerous for people's health. The second important factor in promoting UV technology was the inefficient chlorination efficiency against a number of microorganisms, in particular Cryptosporidium parvum. Ultraviolet disinfection proved to be the ideal solution for both these problems, which was the reason for the turbulent the development of UV technology around the world.
For decontamination, the biologically active region of the spectrum of UV radiation with a wavelength of 205 to 315 nm, called bactericidal radiation, is used. The maximum inactivation efficiency of microorganisms is observed in the wavelength range 250 ... 270 nm: this part of the spectrum requires the wavelength generated by low-pressure UV lamps - 254 nm. The recommended dose of UV irradiation, which is the main criterion for the efficiency of decontamination of purified wastewater, is also measured at a length of 254 nm and is 30 mJ/cm2.
The decontaminating effect of ultraviolet is based on irreversible damage to DNA and RNA nucleic acids. When the microorganism multiplies, the nucleic acid molecule doubles. UV radiation at a length of 254 nm is effectively absorbed by nucleic acids. As a result of UV exposure, crosslinks form in the structure of nucleic acids, which make it impossible to double DNA/RNA, and therefore excludes the possibility of multiplying the microorganism. The microorganism thus inactivated does not pose a danger to living organisms.
The bactericidal ultraviolet selectively acts only on microorganisms, without affecting the chemical composition of the medium, which occurs when chemical disinfectants are used.
The advantages of UV disinfection are:
- UV irradiation is lethal for most bacteria, viruses, spores and parasitic protozoa. It destroys the causative agents of infectious diseases such as typhus, cholera, dysentery, viral hepatitis, poliomyelitis, etc. The F-radiation inactivates microorganisms that are resistant to chlorination;
- ultraviolet disinfection occurs due to photochemical reactions inside microorganisms, therefore, its change in water characteristics has a much smaller effect than when disinfected with chemical reagents. In particular, the effect of UV radiation on microorganisms is not affected by pH and water temperature;
- unlike chlorination and ozonization, after exposure to UV radiation in water, no harmful organic compounds are formed even in the case of multiple excess of the required dose;
- UV radiation does not affect the organoleptic properties of water (odor, smack);
- the disinfection time with UV-irradiation is 1, 10. 10 s in the flow mode, therefore there is no need to create contact capacitances;
- modern UV lamps and ballasts to them are produced serially, have a high operational life;
- the method is safe for people, there is no need to create warehouses of toxic chlorine-containing reagents that require special measures of technical and environmental safety, which increases the reliability of water supply and sewage systems as a whole;
- UV equipment is compact, requires minimal areas, its implementation is possible in the existing technological processes of treatment plants without their stopping, with minimal amounts of construction and installation work;
- easy operation. Only periodic cleaning of the surface of the quartz covers and replacement of the lamps are required as the resource is developed, it is not necessary to use auxiliary devices and special maintenance personnel;
- the UV decontamination process can be easily automated;
- no corrosion of process equipment;
- UV disinfection is characterized by lower operating costs than with chlorination and especially ozonation. This is due to relatively low energy costs (10 ... 30 W per 1 m3 of treated water).
Purified sewage after disinfection is diverted through a closed pipeline or open channel to the place of descent into the reservoir. The discharge channel usually ends with a shore well, from which water flows directly into the reservoir through an outlet located in places with increased flow turbulence (narrowing, tributaries, rapids, etc.). The main task at the release device is to achieve the most complete mixing of the discharged water with the water of the reservoir in order to obtain the greatest dilution of wastewater that contain some more contaminants.
Depending on the form and regime of a section of the river, when a treated sewage is discharged into it, a shore or channel release is arranged; the latter can be concentrated or dispersed. When discharge of purified liquid into the sea or reservoir, shore or deep water discharges are arranged. The shore discharge device is simpler, but the degree of dilution is smaller than for a channel outlet. A dispersed release (each issue ends with a head) provides a better mix of wastewater with the water of the reservoir.
The flow velocities in the underwater part of the outlet should be set at least 0.7 m/s to protect it from siltation. Holes of the head are placed at a distance of 0.5 ... 1 m from the bottom in order to avoid erosion of the bottom or skidding of the head. The distance from the bottom surface of the ice to the holes should be at least 0.5 ... 1 m.
When discharging sewage into the sea, the place of discharge should be located outside the residential area and selected so that the discharge of waste water from the populated area is ensured by the sea current. The length of the outlet to the established depth of its mouthpiece should be the smallest, the outlets are located at a depth of at least 1 m from the water level at low tide and at least 1 m from the sea bottom.
Release into the river is located at a certain distance (downstream) from the boundaries of the sewer populated area, water intake facilities for household and drinking purposes, water areas used for sporting purposes and bathing.
When discharging sewage into powerful rivers and especially into lakes or seas, it may be possible from a sanitary point of view and expedient by economic indicators to limit itself to mechanical cleaning and disinfection of waste water with subsequent release away from the coast instead of complete biochemical treatment of wastewater and release them near the shore. The option is chosen in each case on the basis of a technical and economic comparison.
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