Methods of water purification
The problem of cleaning up the pod covers the issues of physical, chemical and biological changes during processing in order to make it suitable for drinking. This is not only about eliminating undesirable and harmful properties of water (purification), but also about improving its natural properties by enriching the missing ingredients. Therefore, it is more correct to treat water treatment as a process of improving its quality.
The degree and methods of improving the quality of water and the composition of water treatment facilities depend on the properties of natural water and on the requirements that the user places on the quality of water. The main methods of water treatment for household and drinking water supply are clarification, discoloration and disinfection.
Clarification of water, ie. removal of suspended solids from it can be achieved by sedimentation in sedimentation tanks, by centrifugation in hydrocyclones, by passing it through a layer of a previously formed suspended sediment in so-called clarifiers, by filtering water through a layer of granular or particulate filter material in filters or by filtering through nets and fabrics .
In order to achieve the desired effect of clarification of water in sedimentation tanks, clarifiers and filter devices with granular filter loading, impurities of water must be coagulated, i.e. to the action of salts of polyvalent metals. At the same time, there is a significant discoloration of water.
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Discolouration of water, ie. elimination or discoloration of various colored colloids or true solutes can be achieved by coagulation, the use of various oxidizing agents (chlorine and its derivatives, ozone, potassium permanganate) and sorbents (active carbon, artificial resins).
Disinfection of water is produced to destroy the pathogenic bacteria and viruses contained in it. To do this, chlorination of water is most often used, but other methods are also possible - ozonization, bactericidal irradiation, etc.
In addition to these basic methods of water treatment, other special methods for cleaning both domestic and industrial water can be used.
Basic technological schemes of water purification
The combination of the necessary technological processes and facilities makes a technological scheme for improving the quality of water. Technological schemes used in the practice of water treatment can be classified according to the following main characteristics: reagent and reagent, on the effect of clarification, on the number of technological processes and the number of steps of each of them, on the nature of the water being treated.
Reagent and non-reagent process flows are used to prepare water for both domestic and drinking purposes, and for industry. Reagent technological schemes differ significantly in the design and size of water treatment plants and the conditions of their operation.
Processes of water treatment with the use of reagents take place (Figure 7.1, a) many times faster and sometimes much more efficiently. So, to precipitate the bulk of suspended solids in the first case, it takes 2 ... 4 hours, and in the second - several days. With the use of reagents, filtration is carried out at a rate of 5 ... 12 m/h (and more), and without reagents (slow filtration) - 0.1 ... 0.3 m/h.
Figure 7. 1. Reagent technological schemes for improving water quality with sedimentation tanks (a ), clarifiers with a layer of suspended sediment (b), microfilters and contact brighteners (in):
1.11 - supply the source and drain the treated water; 2 - contact camera; 3 - installation for carbonation and fluoridation of water; 4 - chlorinator; 5 - coagulant tanks; 6 - a vertical mixer; 7 - a flocculation chamber; 8 - a horizontal sump with built-in thin-layer modules; 9 - fast filter; 10 - a tank of clean water; 12 - a clarifier with a layer of suspended sediment and its recirculation; 13 - microfilter; 14 - contact clearing agent KO-3
When treating water using reagents, water treatment facilities are much smaller in volume, more compact and cheaper in construction, but more difficult to operate than constructions of a non-reagent scheme. Therefore, reagentless technological schemes (with hydrocyclones, wash-up and slow filters) are generally used for water supply to small water consumers with the color of the initial water to 50 ° of the platinum-cobalt scale.
Reagent schemes (Fig. 7.2) are widely used for rough clarification of water in water supply of some industrial facilities. Sometimes for these purposes one settling or one filtration is used on fast coarse-grained filters or straining through grids.
Fig. 7.2. Non-reactive technological schemes for improving the quality of water with a hydrocycle (a), acoustic (b) and slow (c) filters:
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1.5 - supply of source water and waste water disposal; 2 - hydrocyclone; 3.4 - fast filters of the I and II stages; 6 - acoustic filter; 7 - intermediate capacity; 8 - a two-stream dual stage II stage filter; 9 is a slow filter; 10 - a reservoir of clean water; 11 - the pump; 12 - Sludge treatment
The clarification effect distinguishes technological schemes for full or deep clarification of water and for incomplete lightening. In the first variant the purified water meets the requirements of drinking water GOST 2874-82 Drinking water and SanPiN 4630-88. In the second variant, the content of suspended matter in purified water is many times more - up to 50 ... 100 mg/l.
Technological schemes for deep clarification of water are used for both domestic and drinking water, and for many industrial water pipelines, where high standards are imposed on the quality of industrial water. Schemes for incomplete clarification of water are usually used for the preparation of industrial water, for example for cooling.
According to the number of technological processes and the number of steps each of them, technological schemes are divided into single, double and multi-process. The improved flow chart shown in Fig. 7.1, b, is a two-process. There are two main technological processes: treatment of water in a layer of suspended sediment (ie contact coagulation with sedimentation) and filtration. Both processes are carried out sequentially, and the filtration is performed twice (in two steps).
In the event that one of the main technological processes is carried out twice or more times, the technological scheme is called two-, three- or multistage. For example, in a single-stage two-stage process scheme with contact clarifiers (Figure 7.1, c), the main process-filtration-is performed twice.
Obviously, the number of technological processes and the number of stages of each process are dictated by the requirements for the quality of the water required by the consumer, and depend on the degree of contamination of the source water. Thus, for coarse clarification, it is possible to confine oneself to one precipitation process or only by filtration. When processing high-pitched waters for domestic and drinking purposes, they resort to sedimentation in two stages, followed by filtration into one stage and the like.
According to nature of the movement of treated water technological schemes are divided into gravity (non-pressure) and pressure. At urban and large industrial waterworks, the movement of the source water from the building to the construction is carried out by gravity. In this case, the mark of the water mirror in each subsequent building is lower than the mark in the previous one. The difference in marks determines the pressure required to overcome the hydraulic resistances inside the structure and in communications from one structure to another.
With the pressure technological scheme, the process water moves from the building to the building under pressure above atmospheric pressure, therefore, individual structures can be located at the same elevation. It is pertinent to note that with the use of pressure technological schemes, clean water tanks and pumping station II of lifting can not be arranged. Purified water under the pressure of pumps I lift is transmitted directly to the consumer's network. In the case of a free flow of water through sewage treatment plants, two pumping stations and clean water tanks are required (see Figure 7.1).
The choice of one or another technological scheme for improving water quality is dictated not only by the quality of the source water and the requirements of the consumer, but also by the amount of water consumed.
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