RU2207987C2 - Method for purifying drain water of solid domestic waste polygons - Google Patents

Abstract

FIELD: purification of drain water. SUBSTANCE: method involves preliminary electrochemical treatment of drain water for removal of contaminants with conversion of at least 25 wt% of ammonia nitrogen into nitrate, with active chlorine produced upon treatment process facilitating disinfection of water under treatment; filtering drain water and subjecting to reverse osmosis separation; providing final cleaning of permeate on sorbent; returning part of concentrate (up to 35 wt%) into polygon body and feeding at least 65 wt% of concentrate into evaporator and accumulating vessel-crystallizer; discharging crystalline salt from vessel-crystallizer for utilization. EFFECT: increased efficiency in purifying drain water of solid domestic waste polygons from ammonia nitrogen, heavy metals, mineral salts, improved ecology control on area occupied with polygon. 2 cl, 1 dwg, 4 tbl

Description

 Urban landfills or landfills for solid household waste (MSW) pose a particular danger to groundwater and surface water sources. The most dangerous is the filtrate that forms in the body of the landfill during the interaction of waste with infiltrating precipitation. The filtrate contains numerous components of the decay of organic and mineral substances, therefore, the waste water generated in the landfill is very different in composition and concentration, their composition also depends on the duration of the burial and the operating mode of the burial place.

The present invention relates to a technology for the treatment of drainage water from solid domestic waste landfills. The versatility of the substances polluting the drainage water, the complexity of their composition (Table 1) causes difficulties in their cleaning. As a rule, practically all traditional methods of wastewater treatment are used for the treatment of wastewater generated in landfills: biological, electrochemical, membrane, sorption, reagent, etc., as well as their combinations. As the main methods, combinations of 3 methods that are currently used on an industrial scale around the world are especially considered:
- biological, chemical and / or physical methods;
- membrane and chemical, physical and membrane methods;
- thermal methods.

 To improve the performance of solid waste landfills, a group of Russian specialists from Ecotech-Moscow LLP, MSTU im. Bauman, Institute of Physical Chemistry of the Russian Academy of Sciences proposed a technology for wastewater treatment near Moscow solid waste landfill "Timokhovo" [1].

The purification method is based on a bio-treatment method with an aerobic and anaerobic reactor. Pre-drainage water is rendered harmless by a combined technology:
- flocculation and coagulation with milk of lime Ca (OH) ₂ ;
- sedimentation and filtration of sediment;
- subsequent blowing in cooling towers of ammonium nitrogen;
- treatment of the filtrate with ultraviolet light and final finishing filtration.

After bio-purification at the last stage, sorption purification on granular activated carbons and carbon fiber materials is supposed. Such comprehensive cleaning, however, does not reduce the content of toxic impurities, mainly heavy metals and mineral salts, to standard values. For these purposes, dilution of purified runoff with surface water is used. In addition, the scheme has the following disadvantages:
- biological methods do not give the desired cleaning effect from heavy metals and mineral salts;
- a large and costly problem is the removal of ammonia;
- the combination of many energy-intensive stages (compressors, photochemical processing, etc.) makes this process expensive in general;
- huge areas for bioponds, cooling towers and filters are required.

The company "Laval Service" (France) has developed and implemented the technology for cleaning the drainage water of the landfill for the disposal of household waste with strict preliminary sorting [2]. The technology combines two methods: bio-treatment and physico-chemical treatment. The forming filtrate is collected in pools, where it is purified both biological (in the aeration pool) and physicochemical (coagulant Al ₂ (SO ₄ ) ₃ and flocculant are added). After that, the water enters the after-treatment pool (sump). The entire cleaning cycle is 2.5-3 months. Purified water is discharged into a pond. The evolved gas is collected and, using an electric generator, processed into electricity or burned. The method allows to obtain purified water in accordance with the MPC, however, it has the following disadvantages:
- the introduction of additional reagents in significant quantities;
- a large number of toxic sludge;
- large occupied areas;
- increase the salt content of the source water.

 An alternative to biological treatment methods and combined methods based on bio-treatment is the option of using membrane filtration for wastewater treatment in landfills. These methods in the world practice are considered the most promising for polygons under construction.

 Unlike traditional methods of purification, membrane filtration provides simultaneous purification of wastewater from organic and inorganic components, from bacteria, viruses and other types of pollution.

 Another important aspect is that during membrane filtration no additional chemicals enter the waste water and, thus, along with the cost-effectiveness of the method, an absolutely environmentally friendly technology is provided. When combining membrane methods with traditional methods, the cleaning efficiency for various components reaches 95-100%, while ensuring the compactness of the installation, reliable operation with fluctuating compositions, ease of operation and quality control of drainage water treatment.

 For example, a new biomembrane method has been developed that combines bio-purification, ultrafiltration-based membrane filtration with activated carbon post-treatment [3]. Bioremediation is carried out continuously in bioreactors using a special nutrient medium. This method is distinguished by the advantages inherent in the membrane technology: compactness, a high degree of wastewater treatment, the absence of newly introduced reagents, the possibility of processing wastewater with a wide variation in the initial composition. However, this method does not provide requirements for the degree of purification for compounds containing ammonium ion.

 The closest technical solution to the technical nature and the achieved result is a method of deep cleaning the filtrate of solid waste landfills using a combination of biological treatment and membrane filtration (ultrafiltration and nanofiltration) [4], which consists of a biological treatment stage, an ultrafiltration stage for separating the silt mixture and a stage nanofiltration for the purification of wastewater from organic products of biological destruction of the initial pollution. The concentrate from the nanofiltration stage is subjected to preliminary treatment at the adsorption stage with thermal regeneration of activated carbon and is returned to the biological treatment operation.

 However, the known method has disadvantages inherent in biological purification methods: inefficiency of purification in relation to ammonium nitrogen, heavy metals and mineral salts, problems of disposal of sludge, large footprint.

 The aim of the proposed method is to increase the degree of purification of drainage water from solid waste landfills in relation to ammonia nitrogen, heavy metals, mineral salts, to improve the environmental situation in the area occupied by the landfill.

 The problem to which the invention is directed is to purify and desalinate the drainage water of the solid waste landfill to the MPC value for the main pollutants, at the same time to disinfect the treated water and remove the utilized salts from recycling.

This goal is achieved due to the fact that in the known method, including pretreatment followed by membrane filtration with separation of the feed stream into permeate and concentrate, adsorption purification step, drainage water is subjected to electrochemical purification of contaminants with the conversion of ammonium nitrogen in an amount of at least 25 (wt.%) In nitrate form. In this case, water is disinfected by the active chlorine formed, then a two-stage mechanical filtration and reverse osmosis are carried out. After reverse osmosis, permeate is refined on a sorbent, concentrate in an amount of up to 35 (wt.%) Is returned to the body of the landfill, and the remaining part in an amount of at least 65 (wt.%) Is fed to the evaporator using a jet pump using the energy of a reverse osmosis concentrate and the storage tank is a crystallizer, from where the resulting crystalline salt is diverted for disposal. Electrochemical treatment of drainage water is carried out at a chloride ion content of 0.8-4.5 g / l and anode current density of 10-15 A / dm ² for 8-20 minutes.

 A comparative analysis of the claimed solution with the prototype shows that the claimed method differs from the known one in that the drainage water is subjected to preliminary treatment by electrochemical oxidation. In this case, under the influence of direct current supplied to the electrodes of the electrolyzer, firstly, the decomposition of organic complexes of ammonium nitrogen and oxidation of ammonium nitrogen into nitrate form is carried out, and secondly, the oxygen released and active chlorine serve as effective oxidizing agents for additional purification from a number of contaminants and simultaneous disinfection of water. After electrochemical treatment, a two-stage mechanical filtration and reverse osmosis are carried out into permeate and concentrate. The permeate is further purified on a sorbent and poured onto the terrain.

 The stage of pre-concentration of the concentrate after reverse osmosis is carried out on the basis of a thin-layer evaporator with an open evaporation surface. At the same time, part of the concentrate is sent to the landfill, and part - for processing to evaporate water in the evaporator and crystallize salts in the crystallizer tank, while the feed jet pump uses the energy of the concentrate after the reverse osmosis operation. The resulting brine after the evaporator is returned to the crystallizer tank, and the crystalline salt is disposed of.

 Thus, the claimed method meets the criterion of "novelty."

Drainage water is heavily contaminated with nitrogen compounds in its various forms (Table 1), a large share in the balance of the source water is precisely ammonium nitrogen, which, along with the free form of ammonium ion, is in the drainage water in a difficult to determine complex form with an organic substance like NH ₄ - NR, where R is an organic radical of various structures. Such complexes are the most difficult to remove substances in the treatment of drainage water: they are not removed from the system by biological treatment. Traditional methods: flocculation, sedimentation and others - are also ineffective for cleaning from biologically non-oxidizable nitrogen complexes found in drainage waters. The original solution of the pre-treatment stage, proposed in this method using the electrochemical treatment of drainage water from a solid waste landfill with the simultaneous disinfection of water as a result of electrochemical activation (ECA), allows you to use the membrane technology to the maximum extent and achieve a high degree of ammonium purification - up to the MPC . This is due to the fact that the maximum permissible concentration (MPC) for ammonium nitrogen (ammonium), defined by the water and soil quality standard, is set for drinking water - 2.0 mg / l, water of fishery reservoirs - 0.5 mg / l, cultural water bodies and soils - 2.0 mg / l and show high demands on the quality of treatment precisely for this indicator. The requirements for the content of NO ₃ ^- in purified water (MPC) are 40-45 mg / l, which is much higher than the values for NH ₄ ⁺ . Considering that in the initial drainage water nitrates are 8-40 times less than ammonia nitrogen, and the selectivity of reverse osmosis membranes is approximately the same for NO ₃ ^- and NH ₄ ⁺ , the use of electrochemical treatment of drainage water allows you to change the ratio between NO ₃ ^- and NH ₄ ⁺ in the reverse side, thereby ensuring the purification of drainage water to the MPC for both complexes.

The essence of electrochemical activation is that highly mineralized solutions, which include drainage water from solid domestic waste landfills, become metastable as a result of unipolar electrochemical treatment. This condition is characterized by abnormal and relaxing (time-varying) physicochemical parameters and properties that reach stable values after some time from the moment of processing (from minutes to hours). The occurrence and existence over time of dissipative structures formed in the space charge region near the surface of the electrodes of both free and hydrated shells of ions, molecules, radicals, atoms gives the treated water the properties of a catalyst for a wide variety of chemical reactions, including biochemical ones, since contributes to a change in activation energy barriers between interacting components. Due to the indicated activation factor, the main impurities of iron are purified at the stage of electrochemical treatment with the oxidation of iron (II) to form iron (III), the precipitation of the formed Fe (OH) ₃ and heavy metals, with the sorption of organic compounds on it, as well as the decomposition of biologically unoxidized in the body of a polygon of nitrogen and ammonium containing organic complexes and free ammonium NH ₄ ⁺ into the inorganic nitrate form of nitrogen - NO ₃ ^- . Unstable compounds with a high oxidizing power include oxygen, ozone released on the anode, hypochlorous, chloric, chloric, perchloric acids, chlorine oxides and intermediate compounds of spontaneous decomposition of these substances, the sum of which (Сl ₂ -НСОl-ОСl ^- ), named free active chlorine, is formed under the influence of direct electric current in solutions with a salt content of NaCl in the concentration range of 0.8-4.5 g / l. It is the oxidizing properties of this solution that ensure the oxidation and disinfection of drainage water during electrochemical treatment.

Considering that ammonia compounds are in drainage waters, part of the ammonia ion is spent in ECA water on the formation of monochloramine NH ₄ Cl and dichloramine NH ₄ Cl ₂ , which also have a bactericidal effect that is less than active and free chlorine, but longer. Chlorine in the form of chloramines, unlike free (Cl ₂ + HOCl + OCl ^- ), is called bound active chlorine and enhances all the reactions of the pre ^- treatment stages in the electrolyzer.

 The conversion of ammonia nitrogen into the nitrate form provides the optimum ratio of these compounds in the purified water for the subsequent effective operation of the reverse osmosis separation stage. Such a redistribution of concentration fields allows the reverse osmosis stage with a membrane selectivity of at least 99% (0.15% NaCl solution) to ensure separation of the streams with purification by ammonium and nitrates in accordance with the requirements of regulatory documents on MPC.

With insufficient oxidation of ammonia nitrogen (NH ₄ ⁺ ) from the organic form to the nitrate form - (NO ₃ ^- ), the necessary purification ^is not achieved at the subsequent stage of reverse osmosis from ammonia nitrogen, the MPC for which is much lower than for nitrates (see table. 2).

 It was experimentally established that the optimal degree of conversion of organic complexes of ammonium nitrogen and free ammonium into an inorganic nitrate form should be at least 25 (wt.%).

 The main parameters of the electrochemical cleaning stage are the current density and the duration of the treatment in the electrolyzer, required for oxidation and removal of the main impurities with the simultaneous effect of disinfection with active chlorine, which is formed, as noted above, in the apparatus when a constant electric current is applied to the electrodes.

With a decrease in current density and residence time in the electrolyzer, the required cleaning quality is not ensured: the complete removal of Fe ^total , color, is not formed in a sufficient amount of active oxidizing agents, and, as a result, the required degree of conversion of ammonia nitrogen from organic complexes (N ^org ) to inorganic form (NO ₃ ^- ), which makes the subsequent cleaning operations ineffective. An increase in the current density leads to an increase in the concentration of active chlorine in the treated solution, intensive gas evolution at the electrodes, which reduces the current efficiency during the passage of the main electrochemical reactions, contributes to an increase in the temperature of the solution, and, ultimately, leads to an increase in energy consumption and to an unjustified increase in the cost of the process whole. It was experimentally determined that the current density of 10-15 A / dm ² is optimal.

The process time also determines the boundaries of the process. Reducing the duration of the electrochemical treatment also leads to a decrease in the quality of treatment (Fe ^total , color) and the effect of disinfection due to the insufficient amount of active chlorine formed.

 Increasing the process time increases the salt content of nitrates, increases the content of active chlorine in the solution, which leads to an increase in the norms of the dechlorinating agent in the subsequent stage of dechlorination. At the same time, an increase in the processing time leads to an increase in the consumption rates for energy, which is impractical. It was experimentally determined that the optimal duration of the operation of electrochemical processing is 8-20 minutes

Landfill drainage water always contains chlorine ion, since sodium chlorides are introduced into the body of a landfill for household waste and household waste, which is confirmed by static data on the composition of the drainage water of solid waste landfills (Table 1). With an insignificant concentration of chlorides in the treated water, even with the optimal parameters of the electrochemical treatment, the required amount of active chlorine will not be released and the quality of treatment will not be ensured according to a number of indicators. In this case, additional introduction of NaCl with adjustment of the composition by chlorine ion is required. With an increase in Cl ^- - ions in the source water, the salt content of the source drainage water increases, which gives an additional load to the stage of reverse osmosis desalination. In addition, during electrolysis, an excess of active chlorine is released that is not involved in oxidation reactions, which requires an increased consumption of dechlorinating reagent. It was experimentally established that the optimal content of chlorine ion for the proposed technology is in the concentration range of 0.8-4.5 g / l.

 The use of two-stage mechanical filtration after electrochemical treatment ensures the removal of the main types of mechanical impurities with a particle size of more than 5 microns from the drainage water.

 As a result of subsequent water purification by membrane filtration, a reverse osmosis unit produces two streams: purified water to the required parameters - permeate and concentrate - a stream enriched with organic impurities and mineral salts (chlorides, sulfates, phosphates, etc.). Permeate is purified on a sorbent from low molecular weight organics and improved organoleptics (removal of odors) and poured onto the terrain.

 The comprehensive cleaning technology involves either completely returning the concentrate stream to the body of the landfill or diverting it for further processing to remove salts from the water circulation and prevent their accumulation in circulating drainage waters and the body of the landfill.

 The degree of return of the concentrate is determined by the salt balance factor in the body of the landfill. With the complete removal of the concentrate for processing into the evaporator, there is no accumulation of salts. With an increase in the volumes of concentrate discharged into the body of the solid waste landfill, an increase in the concentration of salts in the initial drainage water and, as a result, in purified permeate and concentrate is observed. A further increase in the amount of concentrate discharged to the landfill body leads to an increase in salt content in the initial drainage water to values at which the use of the reverse osmosis process becomes impractical. In addition, the resulting permeate does not meet the requirements of the MPC. It was experimentally determined that the optimal degree of return of the concentrate to the landfill is in the range up to 35%, and at least 65 wt.% Of the concentrate is sent for processing.

 The main parameters and results of experiments on the treatment of drainage water from the solid waste landfill are given in table. 3.

The proposed method for the treatment of drainage water from solid domestic waste landfills is implemented in an installation operating according to the technological scheme shown in figure 1, where:
1 - sump;
2 - pump for supplying drainage water;
3 - electrolyzer;
4 - DC rectifier (VAK);
5 - collection tank;
6 - pump;
7 - filter press;
8 - pressure sand filter;
9 - cartridge filter;
10 - collection tank;
11 - high pressure pump;
12 - metering pump;
13 - capacity for dechlorinating agent;
14 - reverse osmosis module 1 stage;
15 - adsorber;
16 - collecting capacity of the concentrate;
17 - pump;
18 - reverse osmosis module 2 steps;
19 - jet pump;
20 - storage capacity-crystallizer;
21 is a thin-layer evaporator with an open evaporation surface.

Drainage water from the sump 1 in an amount of 2.5 m ³ / h pump 2 is fed into a flowing electrolyzer 3, the electrodes of which are supplied with voltage from a DC rectifier 4. Under the influence of a direct electric current field as a result of electrochemical reactions occurring in the near-electrode volumes (at the anode and cathode), water is purified and disinfected, thereby enhancing the biocidal ability of the water being purified. Ruthenium oxide electrodes (ORTA) are used as the anode, and stainless steel plates are used as the cathode. The process is carried out at the following parameters: voltage - 15 V, current magnitude - 3.5 kA, anode current density - 10 A / dm ² , processing time - 10 min, medium temperature - 30 ^o C.

 When the content of NaCl salts in the initial drainage water is 1040 mg / L and the above parameters provide a concentration of active chlorine of 53 ± 5 mg / L.

 Moreover, the degree of oxidation of ammonium nitrogen to nitrates is maintained in the range of 84-86%.

 The results of stepwise treatment of drainage water are given in table. 4, which shows the composition of the drainage water before treatment and the content of polluting components after the main stages of treatment: electrochemical treatment and two-stage mechanical filtration, reverse osmosis separation, adsorption.

A characteristic feature of the operation of electrochemical treatment and activation of drainage water in the electrolyzer 3 is a high rate of flocculation and deposition of coagulated particles of iron hydroxides, heavy metals, organics formed during the process. The suspension obtained after electrochemical treatment is collected in a container 5 and pumped to a mechanical filtration unit 6 to separate suspended particles and sediment. Filtration of the suspension is carried out on a filter press (1st stage of mechanical filtration) 7 with an area of filter material of 10 m ² , while ensuring the maximum delayed ability for suspended solids with a particle size of at least 15 microns.

 The clarified water after filtration on pressure sand filters 8 and fine filtration on cartridge filters 9 (2nd stage of mechanical filtration) is collected in a tank 10 for further deep purification and desalination on a two-stage membrane reverse osmosis module OOM. Why water from the tank 10 is fed to the suction line of a high-pressure pump 11 and then directly to the membrane apparatuses equipped with roll elements of the type ERO-KM-200-1016. OOM installation includes two stages of concentration.

To remove residual active chlorine, a solution of sodium pyrosulfite reagent with a concentration of 80 mg / l (based on active chlorine) is introduced into the purified water from the tank 13 by the metering pump 12. Next, the pump 11 creates the necessary working conditions for desalination at the first stage of the reverse osmosis module 14: working pressure up to 3.0 MPa, supply to the roll element of at least 1.5 m ³ / h. In the process, under the influence of working pressure, the initial stream is divided into two streams: desalted and purified to the required parameters permeate and enriched with salts and other impurities concentrate. The permeate of the first stage is sent to the final purification in the adsorber 15 and then poured into the soil.

 The concentrate of the first stage is subjected to pre-concentration at the 11th stage of the OOM installation - reverse osmosis module 18. The pre-concentration is carried out at a pressure of 6.0 MPa, while the permeate is returned to the suction line of the pump 11, and the salt concentrate is constantly diverted to the tank 16 for strengthening.

Part of the concentrate in an amount of 0.06 m ³ / h (30% of the total flow) is returned to the body of the landfill, where natural biochemical reactions occur. Another part of the concentrate in an amount of 0.14 m ³ / h (70% of the total flow) is injected with a jet pump 19 into the distributor of a thin-layer evaporator 21 with an open evaporation surface, the area of which is 330 m ² . The jet pump 19 uses the free energy of the reverse osmosis concentrate exiting the OMM with a pressure of about 30 MPa. The injection coefficient K = 4.5. In a thin-layer evaporator 21, water is evaporated from the concentrate in film mode under natural conditions: due to contact of the falling concentrate film with air (irrigation density 0.02 m ³ / m ² h) and natural water evaporation due to the temperature gradient Δt _cf = 20- 30 ^° C. The intensity of the process and the temperature gradient depend on the time of the year and weather conditions - humidity, time of day, year, wind speed, etc. The amount of water removed is 0.6 kg / m ² h. The brine formed in the evaporator 21 flows into the storage tank-mold 20 pre-filled with concentrate, where salt crystals are precipitated from the brine in an amount of 16 kg / h (in terms of hard salt with a humidity of 20%). Periodically, as accumulation, crystalline salt is diverted to disposal.

 After installation, purified demineralized water (permeate) from the solid waste landfill does not require dilution to be drained onto the terrain, it is environmentally friendly and its discharge to the soil is not accompanied by negative effects on the surrounding flora and fauna.

List of references
1. Cleaning the filtrate of solid waste landfills, Doctor of Technical Sciences Skvortsov L.S., Ph.D. Kamrukov AC, Seliverstov A.F., Journal: Clean City, 2, April-June 1998, pp. 2-6.

 2. On the treatment of wastewater leaching waste water. / Nadeau I., / Environ.mag. - 1996 - N1546 - c. 34, 36. - Fp.

 3. Operating experience of the installation for the treatment of drainage effluents of a special landfill in Billigheim, / Halder U., Dogu H., Entsorg. Prax. -1994, N3, - p. 52, 54th.

 4. Deep purification of the filtrate of landfills using membrane biology and nanofiltration - the results of the operation of the installation at the main landfill of Luneburg.

 5. Schalk Ingeborg, Timm Gomelia, Ringe Hubert. Korrespond. Abwasser - 1999, 46, N8, p. 1253-1258 - prototype.

Claims (2)

 1. A method for treating drainage water of solid domestic waste landfills, including pretreatment followed by membrane filtration with separation of the feed stream into permeate and concentrate, an adsorption treatment step, characterized in that at the pretreatment stage the drainage water is subjected to electrochemical purification of contaminants with the conversion of ammonia nitrogen to amount of at least 25 wt.% in the nitrate form and simultaneous disinfection of water by the resulting active chlorine followed by a two-stage mechanical filtration by reverse osmosis and separation, the obtained permeate is further purified on a sorbent, the concentrate in an amount of up to 35 wt.% is returned to the body of the landfill, and the remaining part, in an amount of at least 65 wt.%, is fed to the evaporator using a jet pump using the energy of the concentrate and storage tank - crystallizer, from where crystalline salt is diverted for disposal. 2. The method according to claim 1, characterized in that the electrochemical treatment of drainage water is carried out at a chloride ion content of 0.8-4.5 g / l and an anode current density of 10-15 A / dm ² for 8-20 minutes.

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