RU2852001C2 - Method for utilisation of concentrate from leachate treatment plants of municipal solid waste (msw) landfills - Google Patents

Abstract

FIELD: municipal solid waste.

SUBSTANCE: invention relates to the field of purification of landfill leachate from municipal solid waste. The method includes purification of MSW leachate using nanofiltration and reverse osmosis and obtaining a concentrate containing organic substances and a salt concentrate. The permeate of the first stage nanofiltration is fed to the second stage nanofiltration membrane. The concentrate of the first and second stages of nanofiltration and the permeate of the second stage of nanofiltration are fed into the first mixing tank. After dilution, the concentrate enters the third stage of nanofiltration, from where the concentrate containing organic substances is withdrawn, and the permeate of the third stage is fed into the second mixing tank. From where it is fed by a pump to the fourth stage of nanofiltration, from where the concentrate is discharged into the first mixing tank, and the permeate is directed to the first stage reverse osmosis apparatuses. The concentrate of the first stage reverse osmosis apparatus is fed to nanofiltration apparatuses to obtain a salt concentrate, and the desalinated water is fed to the second and third stages of reverse osmosis. The concentrates of each of the second and third stages of reverse osmosis are returned to the inlet of the previous reverse osmosis stage. Purified water is obtained at the outlet of the third stage reverse osmosis. The concentrate containing organic substances is mixed with soil or sludge from a natural and wastewater treatment plant and sent to the landfill. The salt concentrate is evaporated to dry salts.

EFFECT: reduction of the concentrate volume and its utilisation.

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Description

The invention concerns the treatment of wastewater (leachates) from municipal solid waste (MSW) landfills containing high levels of salts, including elevated ammonium levels, and high organic matter content (as measured by the COD indicator). The invention improves the efficiency and cost-effectiveness of the reverse osmosis treatment method for MSW leachates by reducing concentrate consumption and its disposal.

Separating organics and salts is crucial when disposing of reverse osmosis system concentrates. Currently, one of the most accessible solutions for concentrate disposal is evaporation. The presence of organics interferes with water evaporation and salt crystallization. By separating organics and salts, it's possible to achieve greater reduction in concentrate volume due to the fact that organic solutions have 4-5 times lower osmotic pressure than electrolyte solutions of the same concentration. Separating the solution into an organic solution and a saline solution not only reduces the volume of concentrate but also reduces disposal costs.

State of the art

A plant for cleaning the filtrate of solid waste landfills is known, comprising a pre-treatment unit, which contains a sequentially arranged electrochemical treatment unit with an electrolyzer, a filter press, a two-stage mechanical filtration unit with a pressure sand filter at the first stage and a cartridge filter at the second stage, a two-stage treatment unit with reverse osmosis membranes and an adsorption unit (Patent RU 2207987, C02F 9/10, C02F 1/04, 2003).

The disadvantage of the known installation is the high energy consumption of the electrochemical processing unit when the filtrate contains a high concentration of difficult-to-oxidize organic matter and toxic compounds, which leads to high operating costs for replacing the sorbent of the adsorption unit.

Also known is a plant for processing filtrate of municipal solid waste landfills using reagent precipitation of organic matter and utilization of concentrate by multiple reduction of its volume, comprising a filtrate receiving tank, a reagent processing unit with units for feeding a coagulant and flocculant, a settling unit containing a settling tank with thin-layer modules, a two-stage filtration unit, a reverse osmosis unit, a sludge dewatering unit containing a storage tank and a mechanical dewatering device, a sludge collection collector, a disinfection unit and a pipeline for removing purified filtrate, the reagent processing unit is equipped with an acid supply unit, two parallel-installed filtrate neutralization reactors and a reactor with a low-speed mixer, the two-stage filtration unit contains at the first stage a first-stage storage tank and a contact clarifier, at the second - a second-stage storage tank and a disk microfilter, the reverse osmosis unit is equipped with a concentrate storage tank and is made in at least four stages, wherein at the first stage of reverse osmosis a nanofiltration membrane is used, and in the second and subsequent stages, low-pressure reverse osmosis membranes are used, each stage of the reverse osmosis unit is equipped with a hydroaccumulator tank and a pump, the permeate outlet of the previous reverse osmosis stage is connected to the inlet of the subsequent stage, while the permeate outlet of the last stage is connected to the purified filtrate pipeline, the concentrate outlet of the first and second stages of the reverse osmosis unit are connected to the inlet of the concentrate storage tank, the concentrate outlet of all subsequent stages is connected to the inlet of the previous stage, and the outlet of the concentrate storage tank is connected to the sediment collection collector.

The disadvantage of this known method is its low efficiency and reliability, due to the high consumption of concentrate, which is unacceptable for its disposal. In addition to salts, the concentrate contains dissolved organic matter, which, due to increased osmotic pressure, makes it impossible to reduce concentrate consumption and obtain dry salt through evaporation.

The closest in technical essence and achievable technical result is the method for reducing concentrate consumption and its utilization, previously proposed by the authors. This method includes a system consisting of high-pressure pumps, a reverse osmosis unit, and a nanofiltration apparatus. The system also contains mixing tanks and additional purification stages, including pumps and membrane devices (article by A.G. Pervov et al., "Reducing the Consumption of Reverse Osmosis Concentrates Used to Treat Wastewater Containing Organic Pollutants," Problems of Collection, Preparation, and Transportation of Oil and Petroleum Products, No. 4 (132), 2021, pp. 36-48).

The system significantly alters the ratio of organic matter and ions in water, enabling efficient utilization of the concentrate. A drawback of this technology is the use of nanofiltration membranes with a salt selectivity exceeding 70%, which prevents effective separation of organic matter and salts using a single concentration cycle followed by dilution. The presence of a high residual salt concentration (over 30%) in the organic solution prevents the solution's osmotic pressure from being reduced and its subsequent concentration from occurring repeatedly.

The presence of organic matter in MSW landfill leachate creates major problems related to treatment efficiency, high operating costs, and the high discharge rate of concentrate, which is difficult to dispose of. Figure 1 shows a table of the most common MSW landfill leachate compositions.

The figures (Fig. 1-10) show:

Fig. 1. Table of compositions of leachates from municipal solid waste (MSW) landfills, including the composition of the leachate from the Alexandrov landfill.

Fig. 2. Balance diagram of the known technology for cleaning MSW filtrate using high-pressure reverse osmosis membranes (according to Patent RU 2207987).

Fig. 3. Scheme of the known technology using nanofiltration membranes (according to Patent RU 2757113 C1) - basic configuration.

Fig. 4. Scheme of a known technology with additional stages of nanofiltration and reverse osmosis (variant from RU 2757113 C1).

Fig. 5. Scheme of the proposed installation with four-stage nanofiltration, reverse osmosis and separation into organic and salt concentrates.

Fig. 6. Table of results of determination of concentrations and values of osmotic pressure in permeates and concentrates at each stage of processing.

Fig. 7. Graphs of the dependence of the difference in osmotic pressure on the concentration coefficient K at different stages of processing.

Fig. 8. Table of technical and economic calculations for comparison of technologies for cleaning MSW filtrate and concentrate utilization.

Fig. 9. Graph of the dependence of the decrease in the productivity of membrane elements on the concentration factor K.

Fig. 10. Graph of the dependence of membrane productivity on the difference in osmotic pressures between the feed water and the permeate.

Fi is the initial filtrate of solid waste; Fk is the filtrate after the first stage of concentration followed by dilution by 10 times with deionized water and the second stage of concentration; Fkrk is the filtrate after the second stage of concentration, dilution and subsequent concentration at the third stage.

Figure 2 shows a well-known and currently accepted balance scheme for separating MSW landfill leachate using the technology described in Patent RU 2207987, C02F 9/10, C02F 1/04, 2003, using high-pressure "marine" membranes. Similar to the new technology, the leachate treatment technology using nanofiltration membranes, described in Patent RU 2757113 C1, 2021, is shown in Figures 3 and 4.

The problem is solved, and the technical result is achieved, thanks to the new proposed installation using a technology for separating the concentrate solution into an organic solution and a solution of monovalent ions (a mixture of sodium chloride and ammonium chloride). Because organic solutions have 4-5 times lower osmotic pressure than electrolyte solutions of the same concentration, separate concentration of such solutions results in a higher total reduction in concentrate volume (Fig. 5). Separation of the solutions occurs due to differences in selectivity (membrane retention) between organic substances and monovalent ions of salts.

Approximate compositions of MSW filtrates are presented in the Table shown in Fig. 1.

The technical result is achieved by using a two-stage nanofiltration scheme for processing MSW filtrate, similar to the composition of the Alexandrov landfill (Fig. 1) and separating organic matter and salts to obtain two solutions of concentrates: a concentrated solution of organic substances and a concentrated solution of monovalent ions (mainly sodium and ammonium chlorides).

The method of utilizing the concentrate of the membrane unit is carried out using the unit (Fig. 5), which contains: high-pressure pumps 15-20, devices with nanofiltration membranes 1-4, 6, 7, 9, 10, 12 at the first, second, third and fourth stages of purification, intermediate tanks 5 and 8, reverse osmosis devices of the first, second and third stages.

The initial MSW filtrate with a flow rate of 100 cubic meters per day with a COD value of 5000 mg/L and a sodium chloride concentration of 4000 mg/L passes through the membrane units (1) of the first stage with low-selectivity nanofiltration membranes (the average selectivity for salts is 70%). Thus, after the first stage, we obtain a solution with a COD content of 500 mg/L and a NaCl content of 1500 mg/L. The concentrate flow rate is 3 cubic meters per day. The permeate flow rate is 97 cubic meters per day. The permeate from the first stage units (1) and the permeate from the third stage units (3) contain organic matter and are purified using low-selectivity nanofiltration membranes in the second stages (2) and (3). The concentrate from the second stage enters the mixing tank (5), where the concentrate from the first stage also enters. The permeate from the second-stage units (2) also enters the mixing tank. Thus, the concentrate is diluted fivefold in the mixing tank. Since the membranes have a higher selectivity for organic matter than for monovalent salt ions, the organic-to-salt concentration ratio in the mixture is higher than in the feed water. After dilution in tank (5), the concentrate returns to the third processing stage. After passing third-stage units (6) and (7), the concentrate contains all organic matter and only 1/4 of all salts, while accounting for 1/10 of the feed water by volume. Permeate from stage 3 enters tank (8), from where it is fed via pump (17) to stage 4 with nanofiltration membranes (9) and (10) for organic concentration. The concentrate from the fourth-stage units is returned to tank (5), and the permeate from the fourth stage is sent to the first-stage reverse osmosis units (11), producing demineralized water. The concentrate from the first-stage reverse osmosis units (11) undergoes further concentration using nanofiltration units (12), producing a salt concentrate with a sodium chloride concentration of 140 grams per liter.

After the first stage of reverse osmosis, the water undergoes two more purification stages to reduce the ammonium concentration. The concentrates from each stage are returned to the inlet of the previous stage. Thus, from the initial MSW filtrate, we obtain purified water with a salinity of 5-10 mg/L and an ammonium content of approximately 0.2 mg/L, as well as two concentrates: a salt concentrate with a salt concentration of 100-150 g/L (a mixture of sodium and ammonium salts) and an organic concentrate, a mixture of organic matter (with a COD concentration of 100 g/L) and calcium, magnesium, sodium, and ammonium salts with a concentration of 30-40 g/L. The consumption of the salt concentrate is 2% of the initial filtrate consumption, and the organic concentrate is 5%. The organic concentrate can be mixed with soil or sludge from natural water and wastewater treatment plants and sent to a landfill. The salt concentrate can be evaporated to dry salts. The table (Fig. 6) presents the results of determining the concentrations and osmotic pressures in the permeates and concentrates at each stage of dilution and subsequent concentration. The graphs (Fig. 7) show the dependence of the osmotic pressure difference on K during concentration, demonstrating maximum concentration.

For a solution with COD = 5000 mg/l and with a NaCl concentration = 4000 mg/l, technical and economic calculations are presented (Table, Fig. 8).

The data in the table presented in Fig. 8 shows a comparison of operating costs when using the various described technologies for cleaning MSW filtrate for cleaning the filtrate and recycling the concentrate.

Thus, the new technical result of the method is a reduction in the volume of the concentrate and its separation into two volumes by subjecting the filtrate to two-stage processing using low-selectivity nanofiltration membranes. The concentrate from the nanofiltration membrane units is sent to a mixing tank, and then the resulting mixture is reprocessed using third-stage nanofiltration membrane units. The permeate from the first-stage nanofiltration units is again processed using second-stage nanofiltration units for more complete organic matter extraction.

Example

To demonstrate the developed technology, experiments were conducted to purify leachate from the Alexandrov landfill and separate the concentrate into a solution of organic matter. The original MSW leachate was concentrated tenfold. Its volume was reduced from 20 liters to 2 liters. The concentrate was then diluted tenfold with deionized water obtained by passing tap water through a model 100 reverse osmosis apparatus manufactured by Raifil (Russia). Following dilution, the concentrate was processed, and its volume was reduced by a factor of 20. The permeate from the second experiment was processed and concentrated tenfold. The third-stage permeate consisted primarily of a solution of ammonium chloride and sodium chloride with a total salt content of 2000 mg/L, which can be effectively processed in the fourth-stage unit in accordance with the developed technology (Fig. 5).

During the experiments, the filtrate of the Alexandrov MSW was treated. The initial filtrate was fed under a pressure of 1.6 MPa into a membrane unit, where it was separated into permeate and concentrate. The selectivity (salt retention) of the semipermeable nanofiltration membranes was 70%, while that of the reverse osmosis membranes was 96%. The specific productivity of the nanofiltration membrane units was 1.5-15 l/m² × h. To separate the solutions into stages 1, 2, and 3, we used the NME nano NF 1812-C roll-type elements manufactured by Membranium (Vladimir, Russia), and for further desalination and concentration of the permeate, we used the RE 1812-60 roll-type reverse osmosis elements and the NE 1812-70 nanofiltration elements manufactured by Raifil (Russia). The membrane area in the elements was 0.5 square meters. The filtrate yield was 0.1-0.05, which corresponded to a 10-20-fold reduction in the feed water volume in the unit. The transit flow velocity in the intermembrane space of the pressure channel was 4-10 cm/sec, which corresponded to a transit flow rate of 100-160 l/hour through the membrane apparatus. During unit operation, the total salt content of the concentrate and the COD values in the concentrate increase. Thus, organic matter accumulates in the concentrate, significantly increasing the osmotic pressure. Due to the low selectivity of the first-stage membranes for salts, high osmotic pressure is established in the first-stage permeate, reducing the difference in osmotic pressures between the concentrate and permeate, and enabling separation of the concentrate solution at relatively low operating pressures (no higher than 1.6-1.8 MPa). Due to the relatively high selectivity of nanofiltration membranes for dissolved organic matter (90%), the concentration of organic matter in the first-stage concentrate increases. The first-stage permeate undergoes additional purification using second-stage nanofiltration membranes to completely remove organic matter, and the difference between the osmotic pressures in the concentrate πc and in the permeate increases: πf: (πc - πf). The second-stage permeate is used to dilute the first-stage concentrate for subsequent purification in the third stage and to reduce the salt ion content in the concentrate. The third stage concentrate has a COD value of at least 120-160 g/l.

If necessary, the concentrate can be mixed with soil or dewatered sludge from water treatment plants, which is sent to a dedicated landfill. Sludge treatment costs are no more than 20-30 rubles per cubic meter. In this case, MSW leachate treatment can produce a sludge containing organic waste decomposition products and a concentrated solution of sodium and ammonium chlorides, which can be used as a raw material for fertilizer production.

Claims (1)

A method for utilizing concentrate from municipal solid waste (MSW) landfill leachate treatment plants, including purifying MSW leachate using nanofiltration and reverse osmosis and obtaining a concentrate containing organic matter and a salt concentrate, characterized in that the permeate of the first nanofiltration stage is fed to a second-stage nanofiltration membrane, the concentrate of the first and second stages of nanofiltration and the permeate of the second stage of nanofiltration are fed to a first mixing tank, after dilution the concentrate is fed to a third nanofiltration stage, from where the concentrate containing organic matter is removed, and the permeate of the third stage is fed to a second mixing tank, from where it is fed by a pump to a fourth nanofiltration stage, from where the concentrate is removed to the first mixing tank, and the permeate is sent to first-stage reverse osmosis devices, the concentrate of the first-stage reverse osmosis device is fed to nanofiltration devices with obtaining a salt concentrate, and the desalinated water is fed to the second and third stages of reverse osmosis, the concentrates from each of the second and third stages of reverse osmosis are returned to the input of the previous stage of reverse osmosis, at the output of the third stage of reverse osmosis, purified water is obtained, while the concentrate containing organic matter is mixed with soil or sludge from a natural and wastewater treatment plant and sent to a landfill, and the salt concentrate is evaporated to dry salts.