RU2294794C2 - Clarified water production process - Google Patents

Abstract

FIELD: water treatment.

SUBSTANCE: invention relates to treating water and effluents and can be used to produce clarified water for feeding water-rotation cycles in ammonia production. Process comprises taking initial water, clearing it on 200-μm gauze filters, flocculation, clarification, and supplying clarified water to plant, wherein demineralized water is produced via nanofiltration and reverse osmosis. Initial water can be biologically purified chemical enterprise waste waters, storm flows, mine effluents, and other effluents or mixtures thereof having total salt contents 3-4 g/L, suspended substances content up to 50 mg/L, total microbial number up to 10 thousand units in ml, and total hardness up to 15 mg-eq/L. Clarification of water is accomplished through ultrafiltration to separate dissolved impurities on ultrafiltration membranes characterized by filtration spectrum from 50 to 1000 Å under pressure 0.05 to 0.5 MPa to achieve full elimination of suspended substances in permeate and detention of all microbiological impurities. Prior to performing clarification, water is supplemented by flocculant, in particular 14% of FeCl₃. As membrane elements are becoming polluted, they are being cleaned by receiving cleaning solutions: 50% sulfuric acid and 42% alkali or 20% sodium hypochlorite, followed by ageing for a specified time.

EFFECT: enabled full removal of microbiological impurities and suspended particles to provide water suitable for nanofiltration and reverse-osmosis plants.

1 dwg, 4 tbl, 3 ex

Description

The invention relates to the field of water and wastewater treatment using ultrafiltration and can be used in chemical and other industries to produce clarified water for replenishing water circulation cycles of ammonia production and other critical technological cycles.

One of the urgent problems of the Donetsk region is, on the one hand, the presence of a large amount of saline mine water that needs to be desalinated, and on the other hand, the lack of fresh water in the Seversky Donets-Donbass canal. The basic principle of rational use of water resources in case of their shortage is the creation of closed water systems of industrial enterprises and the use of modern methods of water purification.

The authors know a method for producing clarified water, which consists in liming and coagulation with subsequent clarification on mechanical filters [1. L.A. Kulsky, V.F.Nakorchevskaya, Water chemistry. Kiev, "Vishka school", 1983, p.105, 109-112].

The disadvantages of the known method of producing clarified water is the low cleaning efficiency. In the process of clarification and discoloration, water is not completely purified from suspended solids and microbiological contaminants. The degree of disinfection does not exceed 98-99%. The disadvantages also include the limited or inability to use natural waters with high salinity. A large amount of algae, organic components and other sedimentary substances leads to blocking of filters during the operation of the installation. Significant costs of chemicals and the formation of large volumes of wastewater adversely affect the environment. Simultaneously with the increase in reagent costs, the specific costs for own needs of water, steam and electricity also increase.

In light of the above disadvantages, the reagentless membrane method of water purification - ultrafiltration, is of great interest.

Ultrafiltration membranes and ultrafiltration are used to treat natural and waste water. In particular, coagulation and filtration, clarification of water are replaced by ultrafiltration. Recently, ultrafiltration methods as elements of an integrated water purification technology are widely used for dehydration of wastes that are generated during biological treatment of water (activated sludge, destructive microorganisms), as well as during dehydration of coagulant sludges that are formed as a result of water purification by hydrolysis products salts of multiply charged metals.

The authors are aware of technical solutions when ultrafiltration is used in the separation of predominantly spent water-oil emulsions in the petrochemical industry [2. Patent of Russia No. 2050178, 01 D 65/06, published on December 20, 1995, bull. No. 35], in the wastewater treatment of textile and knitwear industry [3. RF application No. 5024225/26, C 02 F 1/44, B 01 D 61/00 publ. 10/10/1996]. Currently, certain chemical enterprises and industries have already accumulated positive experience in this area. However, the domestic experience in the development of this important area in chemical production has not been sufficiently studied, is rarely applicable in practice, and has not found industrial application in the preparation of feed water for vaporization systems in ammonia production.

The closest in technical essence and the achieved result is a method of producing clarified water for feeding water cycles of chemical production, which consists in taking the source of river water, subsequent reagent treatment in clarifiers using liming, coagulation, flocculation, subsequent filtration of mechanical and suspended particles, supplying the finished product on the installation for receiving demineralized water by nanofiltration and reverse osmosis [4. N. A. Yankovsky, V. A. Stepanov, B. V. Kravchenko “Creation of a closed water supply system for an industrial enterprise”, LLC “Lebed”, Donetsk - 2004, pp. 17-20].

For the needs of water treatment, Concern Stirol OJSC used scarce river water from the Seversky Donets-Donbass canal, used a reagent method of water clarification based on the use of special chemicals, the deposition process was carried out in a clarifier, to which treated water and reagent solutions were supplied: lime milk Ca (OH) ₂ and sulphate of iron (II) FeSO ₄ , polyacrylamide, soda Na ₂ CO ₃ . When liming, bound and free carbon dioxide is removed from water, the alkalinity of the dry residue is reduced, while the source water is softened. When liming with coagulation, Ca (OH) ₃ and FeSO ₄ remove the natural mechanical impurities and organic contaminants and iron compounds in the colloidal and finely dispersed state due to unlike charges of particles and the resulting flakes. Get clarified water with a nominal hardness of 1 ÷ 3 mEq / g To intensify the coagulation process, the introduction of a flocculant - a solution of polyacrylamide is provided. Sludge deposited in the clarifier is removed by periodically purging. The final clarification of water is carried out on mechanical filters and then the water is sent to the next stage of processing - nanofiltration and reverse osmosis.

The disadvantages of the known technology are:

- low efficiency of the cleaning process, the content of suspended solids after cleaning is 2 ÷ 5 mg / l, microbiological pollution contained in the source water is not completely delayed;

- low efficiency due to the high consumption of steam during pre-heating of water to t = 35 ° C in heat exchangers in front of clarifiers;

- the formation of sludge in the form of Ca (CO) ₃ , Mg (OH) ₂ , which is formed in clarifiers when using lime in combination with coagulants or flocculants, leads to high water consumption, significant costs and heat losses during boiler blowdowns;

- the use of a large number of reagents used in the purification of water in clarifiers, hence the large discharge of salts into the reservoirs of the concern with regeneration effluents of Na-cation filters in the amount of 120-180 tons of NaCl salts per month;

- low reliability, as water after treatment does not have stable properties and forms sludge crystals on equipment and mechanical filters;

- low service life of mechanical filters due to contamination of filter elements;

- high consumption of detergents for the regeneration of mechanical filters;

- technical difficulties in achieving the required quality of softened water using clarifiers and mechanical filters;

- low efficiency of water purification reduces the service life of the membranes of reverse osmosis plants, reduces the time between flushing, reduces the service life of cartridge filters on nanofiltration machines and reverse osmosis.

The basis of the invention is the task of improving the method of obtaining clarified water for feeding water cycles of ammonia production, excluding the intake of river water, using as source water highly mineralized biologically treated sewage from chemical production, storm sewage, mine water and other effluents or mixtures thereof, for cleaning water from impurities reagent-free ultrafiltration technology, achieving complete removal of microbiological contaminants suspended in the source water particles, ensuring water quality that is suitable for reliable and efficient operation of nanofiltration and reverse osmosis units used in the schemes vodopodgotovok concern.

The problem is solved in that, in the method of obtaining clarified water for feeding the water circulation cycles of ammonia production, which consists in taking the source water, subsequent clarification, flocculation, further filtering from mechanical and suspended particles and supplying the plant for receiving demineralized water with nanofiltration and reverse osmosis, according to The invention uses biologically treated chemical wastewater, storm sewage, mine wastewater and other effluents or their Mesi with a total salinity of 3-4 g / l, suspended solids up to 50 mg / l, with a total microbial number of up to 10 thousand units in ml, with a total hardness of up to 15 mEq / l, the water is preliminarily purified on 200 microns strainers, the water is clarified by ultrafiltration, and the process of separation from impurities is carried out on ultrafiltration membranes with a filtration spectrum from 50 to 1000 angstroms at a pressure of 0.05 ÷ 0.5 MPa, until the suspended solids in the permeate are completely eliminated, all microbiological contaminants are retained, a flocculant is fed into the water before clarification, using 14% iron chloride FeCl ₃ as the membrane elements are contaminated. cleaning by feeding and soaking in time washing solutions: 50% sulfuric acid and 42% alkali or 20% sodium hypochlorite.

Currently, membrane ultrafiltration technology in conditions of water scarcity is the most preferable for water treatment in terms of technical and economic indicators. Its use in water treatment will allow the use of mineralized wastewater: biologically treated chemical wastewater, mine water, stormwater, drainage, and others, or a mixture thereof without the intake of river water.

The ultrafiltration separation method is used at the stage of preliminary water purification instead of traditional liming schemes with coagulation and further post-treatment of water on mechanical filters. Ultrafiltration membranes applicable to all types of process water at Concern Stirol OJSC can operate in the range of 90-95% recovery of purified water with 100% retention of all microbiological contaminants in the water and 100% retention of suspended solids.

Ultrafiltration is one of the most effective and multifunctional methods for pre-treatment of water before it is fed to reverse osmosis desalination and purification. The ultrafiltration method is sufficient to obtain clarified water of the necessary conditions, since the filtrate contains practically no suspended particles, microorganisms, emulsified substances, and also organic compounds.

The process of ultrafiltration wastewater treatment is used as a preliminary stage before the removal of low molecular weight inorganic salts and molecular organic compounds (alcohols, sugars, humic and fulvic acids, solvents, low molecular weight compounds, pesticides, etc.) using nanofiltration or reverse osmosis.

Implementation of the claimed method will allow you to create a modern complex of water purification according to the scheme: ultrafiltration, nanofiltration and reverse osmosis.

The use of ultrafiltration separation in industrial technology and the wastewater treatment system is due to the following advantages:

- reduction of heat costs, because this process is carried out at ambient temperature, there is no need to heat water in front of clarifiers, ultrafiltration membranes can work even at a source water temperature of 5 ° C;

- replacement of inefficient equipment of clarifiers and mechanical filters with more efficient - reagent-free baromembrane;

- high separation efficiency, as all microbiological and mechanical impurities contained in the source water are completely retained, the iron content is reduced;

- increase in profitability since steam consumption and heat loss during boiler blowdown are eliminated by stopping the formation of sludge in the form of Ca (CO) ₃ , Mg (OH) ₂ in clarifiers when using lime in combination with coagulants or flocculants;

- reducing the cost of production due to the cessation of the use of chemicals used in water treatment in clarifiers;

- increase in reliability, because water after cleaning on ultrafiltration membranes has stable properties and does not form sludge crystals on equipment and fine filters of nanofiltration and reverse osmosis plants, which increases their service life and reduces the consumption of detergents;

- an increase in the inter-washing period due to the reduction of membrane contamination;

- the possibility of creating a recycling of selected substances and water, which will allow to exclude the intake of river water for the needs of the concern;

- reduction in the amount of discharged wastewater and sludge into the reservoirs of Concern Stirol OJSC with regenerative effluents of Na-cation exchange filters;

- simplification of the staff, as the operation of the ultrafiltration unit is fully automated.

For the process of ultrafiltration of industrial wastewater, it is advisable to use membranes with pore sizes from 50 to 1000 angstroms, performing separation at a working pressure of 0.05-0.5 MPa. For membranes with a smaller pore size, which have a higher initial productivity, an intensive decrease in productivity during operation is observed (as in the case of an increase in working pressure).

Pretreatment of the source water with an optimally selected FeCl ₃ flocculant will make it possible to organize a stable ultrafiltration process and provide a reliable and efficient operating mode for reverse osmosis membranes without precipitation on them with an increase in the interwashing period.

Pre-filtration of the source water through a mesh filter with a pore size of 200 μm will allow you to remove coarse suspensions, scales, algae from the source water, to avoid blocking the operation of the ultrafiltration module.

The drawing shows a schematic diagram of the implementation of the claimed method.

The scheme includes a mesh prefilter 1, a source water supply tank 2, a centrifugal source water pump 3, an ultrafiltration module 4, consisting of one membrane filter element, with a productivity of 2.5-4.5 m ³ / h, depending on the quality of the source water, permeate stock tank 5, flocculant stock tank 6, flocculant metering pump 7, acid stock tank 8, acid metering pump 9, alkali stock tank 10, alkali metering pump 11, permeate pump for backwashing of the membrane 12, on-board computer of the control system .

The method is as follows.

The source water supplied to the ultrafiltration system is a mixture of industrial wastewater, water from coal mines, surface water, biologically treated wastewater, etc. Water is collected in a reservoir. The qualitative composition of the source water is given in Table 1.

The untreated water from the reservoir is preliminarily passed through a strainer (200 μm) 1 to remove coarse suspensions, scale, algae, and sedimentary substances and, after filtration, is collected in the source water tank 2. By the circulation pump 3, the source water is supplied to the ultrafiltration module 4. Between pump 3 and module 4 is mounted flocculation unit 6. a solution of flocculant FeCl ₃ is injected via a dosing pump 7. Water, having passed through the ultrafiltration unit 4 is supplied to the reserve tank 5. The permeate permeate is fed to a partial demineralization a nanofiltration unit and further to obtain deep demineralised water using reverse osmosis. Part of the permeate is used for systematic washes and chemical cleanings. The concentrate is discharged into a sump and then into a pond for biochemical treatment, from where a pumping station serves the concern for reuse. The water cycle closes.

With a decrease in the productivity of the ultrafiltration unit, the membranes are washed with acid, alkali and sodium hypochlorite by depressurizing and recycling the initial solution with further treatment with washing solutions, which makes it possible to fully resume membrane productivity. To do this, acid pump from the reserve tank 8 and alkali from the reserve tank 10 are supplied with a metering pump 9 and an alkali metering pump 11 and hypochlorite (not shown). Permeate for washing is supplied by pump 12. The frequency of membrane regeneration depends on the degree of pollution of wastewater and is carried out after 5-10 days. The spent cleaning solution, which contains a surfactant, is sent back to the container with contaminated wastewater, which is subject to ultrafiltration treatment.

During the operation of the installation, work options were tested depending on the use of various flocculants, such as Fe ₂ (SO ₄ ) ₃ , FeCl ₃ , Al ₂ (SO ₄ ) ₃ . The operating parameters of flocculation filtration are shown in Table 2. The quality of the raw water and its preliminary treatment before ultrafiltration play a decisive role. Ultrafiltration of water, without any flocculant, can only be carried out together with frequent chemical backwashing (<2 hours) and with chemical cleaning (<1 week).

As can be seen from table 2 for rational and successful clarification of water, the preferred option is ultrafiltration with flocculation of FeCl ₃ , because With this flocculant, you can get a stable ultrafiltration process with an increased filter cycle. When water is supplied to the second stage of reverse osmosis treatment, the use of Al ₂ (SO ₄ ) _{3 is} avoided, since precipitation of Al ₂ (SO ₄ ) ₃ at pH 6.8–7.2 on the membranes is possible, which requires more frequent cleaning .

It is undesirable to use a solution of Fe ₂ (SO ₄ ) ₃ , because when using this flocculant, the filter cycle decreases, the module does not provide a stable mode of operation without frequent chemical washes.

Creating optimal ultrafiltration conditions using flocculation will allow the unit to work with a stable load, and the membrane system can provide at least 90 ÷ 95% of the clarified water yield.

Examples of the method.

Example 1 (prototype). The initial river water from the Seversky Donets-Donbass canal in the amount of 246 t / h is supplied to clarifiers of the VTI-250 I type, pre-heated to t = 35 ° C in heat exchangers. The quality of the source river water is shown in table 3. The capacity of the plant for clarified water is 240 m ³ / h. The technology for producing clarified water is carried out by the method of softening river water in clarifiers according to the scheme of liming and coagulation, followed by clarification on mechanical filters.

In a clarifier, lime milk Ca (OH) ₃ 5% is dosed into water, coagulant is iron sulfate (II) FeSO ₄ 6% in the form of solutions in the ratio of 1 ÷ 2.4, and flocculant is a 0.1% polyacrylamide solution. Of the clarifiers, lime-coagulated water with a residual hardness of up to 5 mEq / l, alkalinity of 1.2-0.7 mEq / l, instability of 0.1 mEq / l, suspended solids 15 mg / l, pH 10.5 served on mechanical filters loaded with anthracite chips, where the water is clarified to a suspended matter content of 5 mg / L. Sludge from clarifiers in the form of sludge is discharged by continuous and periodic purges. Purge bleach is about 2%. The softening of clarified water to 10 μg-eq / L is carried out on Na-cation exchange filters. Sulfuric acid is metered into softened water in order to lower the pH of the water to 8.5 after these filters.

The characteristics of the products - clarified water are given in table 3.

When processing river water of 1000 m ³ with a consumable coefficient of 1.025, 93 kg of FeSO _{4 were} consumed, quicklime CaO - 236 kg, industrial polyacrylamide - 14.3 kg. A total of 343.3 kg of chemicals, heat 113 GJ, and electricity 1920 MJ were spent. The amount of liquid waste in the form of sludge water was 25 m ³ per 1000 m ^{3 of} lime-coagulated water. The quantity of effluents is 240 m ³ / day [4]. Sludge water after clarifiers is discharged to the neutralization plant.

The cost of 1000 m ^{3 of} lime-coagulated water amounted to 152.32 US dollars [5. Calculation of the cost of ICV for November 2003 OJSC Concern Stirol]. Comparative data are given in table 4.

Example 2 (prototype). Source water is a mixture of effluents and river water. The flow of river water amounted to 100-250 t / h and mineralized effluents after averaging and neutralization of 150-250 t / h.

The softening process is carried out according to the scheme: lime-soda with coagulation of scale formation, clarification on mechanical filters, followed by 2-stage sodium cation. The deposition process is similar to that described in example 1.

The quality of the source water is shown in table 3.

Among other things (analogously to example 1), a solution of soda ash Na ₂ CO ₃ - 5% is introduced into the clarifier for deeper softening of water.

The quality of the clarified water obtained is given in table 3. The consumption of the main types of materials and the cost of the obtained lime-coagulated water are given in table 4.

Example 3 (the claimed technology). To obtain clarified water, an installation with ultrafiltration membranes with a productivity of 2400 m ³ / h was used. The pre-treatment included 6 modules with a capacity of 400 m ³ / h. As the source water, a mixture of industrial wastewater, water from coal mines and surface waters, biologically treated wastewater, etc. with a salinity of up to 3.5 g / l in an amount of 2667 m ³ / h without the intake of river water is used. The quality of the source water is given in table 1.

As a result, clarified water was obtained with the quality shown in Table 1. As can be seen from table 1, the ultrafiltration system provides a better purification of source water from iron, suspended solids and microbiology than the existing pre-treatment. The output showed that ultrafiltration membranes can operate in the range of 90 ÷ 95% recovery of purified water with 100% retention of all microbiological contaminants in the water and 100% retention of suspended solids.

For the production of 1000 m ^{3 of} purified water, sodium hypochloride of 0.002 tons, electricity of 0.5 thousand kW / h, heat energy of 1.75 Gcal was used. Slurry water is not available.

The ultrafiltration unit has an annual saving of lime - 5145 tons, coagulant 1770 tons. Production cost amounted to 105.39 US dollars per 1000 m ^{3 of} purified water, which is 46.93 dollars cheaper than in the prototype. Comparative data are given in table 4.

As can be seen from table 4, the technology for producing clarified water using ultrafiltration in its economic and environmental indicators significantly surpasses the technology using traditional liming schemes with coagulation.

Claims (1)

A method of producing clarified water for feeding the water circulation cycles of ammonia production, which consists in collecting the initial water, its subsequent clarification, flocculation, filtration from mechanical and suspended particles and supplying the plant for the production of demineralized water with nanofiltration and reverse osmosis, characterized in that the source water is used biologically treated chemical wastewater, storm sewage, mine wastewater and other effluents or mixtures thereof with a total salt content of 3-4 g / l, content of suspended substances up to 50 mg / l, with a total microbial number of up to 10 thousand units in ml., with a total hardness of up to 15 mg · equiv / l, water is preliminarily purified on 200 microns strainers, the water is clarified by ultrafiltration, carrying out the separation process from dissolved impurities on ultrafiltration membranes with a filtration spectrum from 50 to 1000 angstroms at a pressure of 0 , 05-0,5 Mpa, until the problem of suspended solids in the permeate, detention of microbiological contaminants in water before clarification is fed flocculant, which is used as ferric chloride 14% FeCl _3, the extent of contamination of the membrane element in their cleaning is carried out by feeding and exposure time of washing solutions: 50% sulfuric acid and 42% caustic or 20% sodium hypochlorite.