A high-salinity wastewater resource recovery system and method

Abstract

The application discloses a high-salinity wastewater resource recovery system and method, and relates to the technical field of high-salinity wastewater treatment and resource recovery. The system comprises a pretreatment system, a magnesium hydroxide crystallization and granulation unit, a calcium carbonate crystallization and granulation unit, a filtering device, a desalination device, an NF nanofiltration salt separation device, an RO concentration device, a reaction device, an evaporation crystallization device and a gypsum crystallization unit. The system synchronously recovers magnesium hydroxide, calcium carbonate, sodium chloride and gypsum, realizes waste-to-resource and full resource recovery, and does not enrich organic matters through electric adsorption desalination, thereby solving the problems of membrane pollution and evaporation system fouling from the source and improving the anti-pollution capability. The system does not need expensive pretreatment such as advanced oxidation, the seed circulation crystallization reduces the addition of reagents, the evaporation water volume is greatly reduced, and the operation cost is low. The water production rate is greater than or equal to 90%, the whole process is closed-circuit circulation, there is no mother liquor, no impure salt and no hazardous waste discharge, and the system realizes true zero emission.

Description

Technical Field

[0001] This invention relates to the field of high-salinity wastewater treatment and resource recovery technology, specifically to a high-salinity wastewater resource recovery system and method. Background Technology

[0002] High-salinity wastewater treatment is a core challenge in my country's environmental protection field. Current mainstream treatment methods aim for zero discharge and employ a process of "pretreatment + reverse osmosis concentration + nanofiltration desalination + sodium sulfate evaporation and crystallization / mother liquor dehydration and drying + sodium chloride concentration and reduction + evaporation and crystallization." However, this process has several shortcomings in practical applications:

[0003] Membrane concentration and salt separation can only separate water and total dissolved solids, but cannot efficiently separate different dissolved solids. The continuous enrichment of organic matter leads to a sharp drop in membrane flux and system failure.

[0004] Organic pollution causes problems such as increased boiling point in evaporation and crystallization systems, substandard condensate, scaling and clogging of equipment, and high production of mixed salts.

[0005] Existing advanced oxidation and adsorption processes for removing organic matter are costly to invest in and operate, significantly increasing disposal expenses.

[0006] The softening pretreatment produces a large amount of calcium magnesium sludge, which has a low resource utilization rate and is mostly disposed of by landfill, resulting in resource waste and secondary pollution.

[0007] To this end, we propose a high-salinity wastewater resource recovery system and method to achieve the full resource recovery of water, magnesium salts, calcium salts, sodium salts, and gypsum. Summary of the Invention

[0008] Therefore, the present invention provides a high-salinity wastewater resource recovery system and method to solve the above-mentioned problems in the prior art.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a high-salinity wastewater resource recovery system, comprising a pretreatment system, a magnesium hydroxide crystallization granulation unit, a calcium carbonate crystallization granulation unit, a filtration device, a desalination device, an NF nanofiltration desalination device, an RO concentration device, a reaction device, an evaporation crystallization device, and a gypsum crystallization unit. The pretreatment system is sequentially connected to the magnesium hydroxide crystallization granulation unit, the calcium carbonate crystallization granulation unit, the filtration device, and the desalination device. The concentrated water outlet of the desalination device is connected to the NF nanofiltration desalination device. The monovalent salt outlet of the NF nanofiltration desalination device is connected to the RO concentration device. The divalent salt outlet of the NF nanofiltration desalination device is connected to the reaction device. The RO concentration device is connected to the evaporation crystallization device. The mother liquor outlet of the evaporation crystallization device is connected to the reaction device. The reaction device is connected to the gypsum crystallization unit. The filtrate outlet of the gypsum crystallization unit is refluxed and connected to the calcium carbonate crystallization granulation unit.

[0010] Furthermore, the pretreatment system includes an equalization tank, a high-density tank, a dewatering device, and a filtration device. The equalization tank is used to stabilize the water quantity and quality and control the pH to ≥7. The high-density tank integrates coagulation, flocculation, sedimentation concentration, and sludge return. The dewatering device is a plate and frame dewatering machine. The filtration device adopts one of sand filter, multi-media filter, and membrane filter.

[0011] Furthermore, the magnesium hydroxide crystallization and granulation unit includes a crystallization reaction zone, a granulation zone one, a particle classification zone one, a sedimentation zone one, and a dehydration zone one. The pH of the crystallization reaction zone is controlled at 9.5~11. The granulation zone one adopts one of a fluidized bed, a stirred crystallizer, or a DTB crystallizer, with a seed crystal reflux ratio of 10%~50%. The dehydration zone one adopts a filter press or a centrifuge to produce magnesium hydroxide granular filter cake.

[0012] Furthermore, the calcium carbonate crystallization and granulation unit includes a reaction crystallization zone, a second granulation zone, a second particle classification zone, a second sedimentation zone, and a second dehydration zone. Sodium carbonate or sodium bicarbonate is added to the reaction crystallization zone, and the pH is controlled between 8.0 and 10.0. The second granulation zone uses fluidized bed crystallization and granulation, and the second dehydration zone uses a filter press or centrifuge to produce calcium carbonate granular filter cake.

[0013] Furthermore, the desalination device is an electro-adsorption CDI or EST device, and the electrode plate is composed of a current collector, an active adsorption layer, and a ceramic diaphragm. The active adsorption layer contains activated carbon, reduced graphene oxide, manganese dioxide, polyaniline, and rare earth materials. The desalination device has a desalination rate of 50%~99% and a water production rate of ≥75%.

[0014] Furthermore, the NF nanofiltration salt separation device separates monovalent salts and divalent salts, the RO concentration device concentrates the monovalent salt solution, and the evaporation crystallization device is selected from one of multi-effect evaporation, falling film evaporator, forced circulation evaporator, and MVR evaporation crystallizer to produce sodium chloride crystals.

[0015] Furthermore, the pH of the reaction device is controlled at 7-9, and calcium chloride is added to react with sulfate to produce gypsum dihydrate. The gypsum crystallization unit includes flocculation, sedimentation, and separation. A plate and frame filter press is used to obtain gypsum filter cake, and the seed crystals are returned to the reaction device.

[0016] A method for resource recovery of high-salinity wastewater includes the following steps:

[0017] <1> After being treated by the pretreatment system's equalization tank, high-density tank, dewatering device, and filtration device, the high-salinity wastewater enters the magnesium hydroxide crystallization and granulation unit to recover magnesium hydroxide.

[0018] <2> The effluent enters the calcium carbonate crystallization and granulation unit to recover calcium carbonate, then is filtered before entering the desalination unit. The desalinated water is reused, and the concentrated water enters the NF nanofiltration unit for salt separation.

[0019] <3> The monovalent salts separated by the NF nanofiltration unit are concentrated by the RO concentration unit and then enter the evaporation and crystallization unit to obtain industrial sodium chloride. The distilled water is reused, and the divalent salts enter the reaction unit.

[0020] <4> The mother liquor from the evaporation crystallization unit enters the reaction unit, reacts with divalent salts, and then enters the gypsum crystallization unit to recover gypsum. The filtrate is recycled to the calcium carbonate crystallization granulation unit for further processing.

[0021] Furthermore, the gypsum crystallization unit has a gypsum slurry concentration of 5% to 10% and a gypsum filter cake moisture content of 40% to 50%, and is used as a cement retarder or building material additive.

[0022] This invention has the following advantages: it simultaneously recovers four products—magnesium hydroxide, calcium carbonate, sodium chloride, and gypsum dihydrate—turning waste into treasure and achieving full resource recovery; electro-adsorption desalination does not accumulate organic matter, solving the problems of membrane fouling and scaling in the evaporation system from the source, and has strong anti-fouling capabilities; it does not require expensive pretreatment such as advanced oxidation, and the seed crystal circulation crystallization reduces the amount of reagents added, significantly reducing the amount of evaporating water and lowering operating costs; the water production rate is ≥90%, the entire process is a closed-loop circulation, with no mother liquor, no impurities, and no hazardous waste discharge, achieving true zero discharge. Attached Figure Description

[0023] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0024] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0025] Figure 1 This is a schematic diagram of a high-salinity wastewater resource recovery system according to the present invention.

[0026] Figure 2 This is a schematic diagram of the pretreatment system in a high-salinity wastewater resource recovery system of the present invention.

[0027] Figure 3 This is a schematic diagram of a magnesium hydroxide crystallization granulation unit in a high-salt wastewater resource recovery system of the present invention.

[0028] Figure 4 This is a schematic diagram of a calcium carbonate crystallization and granulation unit in a high-salt wastewater resource recovery system of the present invention.

[0029] Figure 5 This is a schematic diagram of a high-salinity wastewater resource recovery method according to the present invention.

[0030] In the diagram: 1. Pretreatment system; 11. Equalization tank; 12. High-density tank; 13. Dehydration device; 14. Filtration device; 2. Magnesium hydroxide crystallization granulation unit; 21. Crystallization reaction zone; 22. Granulation zone one; 23. Particle classification zone one; 24. Sedimentation zone one; 25. Dehydration zone one; 3. Calcium carbonate crystallization granulation unit; 31. Reaction crystallization zone; 32. Granulation zone two; 33. Particle classification zone two; 34. Sedimentation zone two; 35. Dehydration zone two; 4. Filtration device; 5. Desalination device; 6. NF nanofiltration desalination device; 7. RO concentration device; 8. Reaction device; 9. Evaporation crystallization device; 10. Gypsum crystallization unit. Detailed Implementation

[0031] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] like Figures 1 to 5 As shown, a high-salinity wastewater resource recovery system includes a pretreatment system 1, a magnesium hydroxide crystallization and granulation unit 2, a calcium carbonate crystallization and granulation unit 3, a filtration device 4, a desalination device 5, an NF nanofiltration salt separation device 6, an RO concentration device 7, a reaction device 8, an evaporation crystallization device 9, and a gypsum crystallization unit 10. Specifically:

[0033] The pretreatment system 1 is sequentially connected to the magnesium hydroxide crystallization granulation unit 2, the calcium carbonate crystallization granulation unit 3, the filtration device 4, and the desalination device 5. The concentrated water outlet of the desalination device 5 is connected to the NF nanofiltration desalination device 6. The monovalent salt outlet of the NF nanofiltration desalination device 6 is connected to the RO concentration device 7. The divalent salt outlet of the NF nanofiltration desalination device 6 is connected to the reaction device 8. The RO concentration device 7 is connected to the evaporation crystallization device 9. The mother liquor outlet of the evaporation crystallization device 9 is connected to the reaction device 8. The reaction device 8 is connected to the gypsum crystallization unit 10. The filtrate outlet of the gypsum crystallization unit 10 is refluxed to the calcium carbonate crystallization granulation unit 3.

[0034] Pretreatment system 1 includes an equalization tank 11, a high-density tank 12, a dewatering device 13, and a filtration device 14, specifically:

[0035] Equalization tank 11: Collects influent, stabilizes water volume and quality, and adjusts pH to ≥7;

[0036] High-density tank 12: integrates coagulation, flocculation, sedimentation and concentration, and sludge return. PAC / PFS and PAM are added to form dense flocs with the returned sludge as crystal nuclei.

[0037] Dewatering device 13: realizes sludge dewatering, sludge cake transportation, and filtrate return to the filtration device;

[0038] Filtration device 14: Selected from sand filter, multi-media filter, and security filter, to remove suspended solids and protect downstream equipment.

[0039] Magnesium hydroxide crystallization granulation unit 2 includes a crystallization reaction zone 21, a granulation zone 22, a particle classification zone 23, a sedimentation zone 24, and a dehydration zone 25, specifically:

[0040] Crystallization reaction zone 21: pH is controlled at 9.5–11, the reaction Mg²⁺ + 2OH⁻ → Mg(OH)₂↓ occurs, generating magnesium hydroxide crystal nuclei;

[0041] Granulation Zone 12: A fluidized bed or guide tube crystallizer is used, with 10%–50% of large seed crystals recirculated, so that the newly generated colloids coat the surface of the seed crystals, resulting in dense and large-sized particles;

[0042] Particle classification zone 123: Large particles settle, while fine particles continue to grow as the effluent circulates.

[0043] Settling Zone 124: Enhances the settling of large particles and reduces the subsequent dewatering load;

[0044] Dehydration Zone 125: High-purity magnesium hydroxide granules are produced using a filter press or centrifuge, which can be used as a desulfurizing agent, flame retardant, phosphorus and fluoride removal agent, and magnesium salt raw material; the filtrate enters the calcium carbonate crystallization and granulation unit.

[0045] Calcium carbonate crystallization granulation unit 3 includes a reaction crystallization zone 31, a granulation zone 32, a particle classification zone 33, a sedimentation zone 34, and a dehydration zone 35, specifically:

[0046] Reaction crystallization zone 31: Add sodium carbonate or sodium bicarbonate, control pH=8.0–10.0, and calcium carbonate crystal nuclei will be generated;

[0047] Granulation Zone 2 32: Water flows upward to form a fluidized state, fine crystals circulate internally, and large particles grow densely;

[0048] Settling Zone 2 (34): Increases solids content and reduces dewatering load;

[0049] Dehydration Zone 2 35: High-purity calcium carbonate granules are produced using a filter press or centrifuge, which can be used as filler, desulfurizer, and building material raw material; the supernatant and filtrate enter the filtration device 4.

[0050] Filter device 4: intercepts unprecipitated calcium carbonate fine crystals and returns them to the calcium carbonate crystallization granulation unit for further granulation, preventing the fine crystals from damaging the downstream electrochemical and membrane equipment.

[0051] Desalination device 5 is an electro-adsorption CDI or EST device, and the electrode plate consists of a current collector, an active adsorption layer, and a ceramic diaphragm. Specifically:

[0052] Activated adsorption layer: Activated carbon: Reduced graphene oxide: Manganese dioxide: Polyaniline: Rare earth = 92–96%:1%:1%:1%:1%:1–5%;

[0053] Diaphragm: Made of ceramic diaphragm, resistant to acids, alkalis and high temperatures;

[0054] Performance: Water production rate ≥75%, optimized ≥90%, desalination rate adjustable from 50% to 99%, does not accumulate organic matter and can remove some organic matter;

[0055] Permeate: Water that meets the standards is reused for production makeup water; concentrated water enters the nanofiltration desalination unit.

[0056] The NF nanofiltration salt separation device 6 separates monovalent and divalent salts;

[0057] RO concentration unit 7 concentrates the monovalent salt solution, increasing the salt concentration and reducing the amount of water evaporated; the product water is reused, and the concentrated solution enters the evaporation and crystallization unit 9;

[0058] The evaporation crystallization device 9 is selected from one of the following: multi-effect evaporator, falling film evaporator, forced circulation evaporator, and MVR evaporation crystallizer. It evaporates and crystallizes sodium chloride concentrate to produce industrial-grade sodium chloride. The distilled water is reused, and the centrifuged mother liquor enters the reaction device 8.

[0059] The pH of the reaction apparatus is controlled at 7-9. Calcium chloride is added, and the reaction Ca²⁺ + SO₄²⁻ + 2H₂O → CaSO₄・2H₂O↓ occurs, producing gypsum dihydrate microcrystals.

[0060] The gypsum crystallization unit 10 includes flocculation, sedimentation, and separation. In the flocculation zone, PAM is added to cause gypsum microcrystals to flocculate and grow. In the sedimentation zone, 5%–10% of the gypsum crystal seed crystals are returned to the gypsum reaction device 8, and the rest enters the separation zone. In the separation zone, plate and frame filter press is used to produce dihydrate gypsum filter cake, which can be used as a cement retarder and building material additive. All the filtrate is returned to the calcium carbonate crystallization granulation unit 3.

[0061] A method for resource recovery of high-salinity wastewater includes the following steps:

[0062] <1> After being treated by the equalization tank 11, high-density tank 12, dewatering device 13, and filtration device 14 of the pretreatment system 1, the high-salt wastewater enters the magnesium hydroxide crystallization and granulation unit 2 to recover magnesium hydroxide.

[0063] <2> The effluent enters the calcium carbonate crystallization and granulation unit 3 to recover calcium carbonate, then passes through the filtration device 4 before entering the desalination unit 5. The desalinated water is reused, and the concentrated water enters the NF nanofiltration and salt separation unit 6.

[0064] <3> The monovalent salt separated by the NF nanofiltration salt separator 6 is concentrated by the RO concentration unit 7 and then enters the evaporation and crystallization unit 9 to obtain industrial sodium chloride. The distilled water is reused, and the divalent salt enters the reaction unit 8.

[0065] <4> The mother liquor from the evaporation crystallization device 9 enters the reaction device 8, reacts with the divalent salt, and then enters the gypsum crystallization unit 10 to recover gypsum. The filtrate is recycled to the calcium carbonate crystallization granulation unit 3 for further processing.

[0066] The gypsum crystallization unit 10 has a gypsum slurry concentration of 5% to 10% and a gypsum filter cake moisture content of 40% to 50%, and is used as a cement retarder or building material additive.

[0067] Example

[0068] Experimental conditions:

[0069] Apparatus: CDI electroadsorption module; Voltage: 1.2V; Flow rate: 10mL / min; Water sample: NaCl simulated saline; Initial conductivity: C0 = 1850μS / cm (approximately 1000 mg / L); Effluent conductivity: C 10 Experiment time: 10 min;

[0070]

[0071] Conclusion: Under the conditions of 1.5 V voltage, 10 mL / min flow rate, and initial conductivity of 1850 μS / cm, by changing the material ratio of the electroadsorption electrode, the conductivity of the effluent decreased to 1090, 1040, 965, 900, and 890 after 10 min of operation of the electroadsorption CDI device, and the desalination rates were 41.1%, 43.8%, 48.1%, 51.4%, and 51.9%, respectively. In this experiment, the ratio of activated carbon: reduced graphene oxide: manganese dioxide: polyaniline: rare earth = 93%: 1%: 1%: 1%: 4% showed the most significant desalination effect.

[0072] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

[0073] The terms such as "upper," "lower," "left," "right," and "middle" used in this specification are merely for clarity of description and are not intended to limit the scope of the invention. Any changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

Claims

1. A high-salinity wastewater resource recovery system, characterized in that, The system includes a pretreatment system (1), a magnesium hydroxide crystallization and granulation unit (2), a calcium carbonate crystallization and granulation unit (3), a filtration device (4), a desalination device (5), an NF nanofiltration and salt separation device (6), an RO concentration device (7), a reaction device (8), an evaporation and crystallization device (9), and a gypsum crystallization unit (10). The pretreatment system (1) is sequentially connected to the magnesium hydroxide crystallization and granulation unit (2), the calcium carbonate crystallization and granulation unit (3), the filtration device (4), and the desalination device (5). The concentrated water outlet of the desalination device (5) is connected to the NF nanofiltration and salt separation device (6). The monovalent salt outlet of the NF nanofiltration and salt separation device (6) is connected to the RO concentration device (7). The divalent salt outlet of the NF nanofiltration and salt separation device (6) is connected to the reaction device (8). The concentration device (7) is connected to the evaporation crystallization device (9), the mother liquor outlet of the evaporation crystallization device (9) is connected to the reaction device (8), the reaction device (8) is connected to the gypsum crystallization unit (10), and the filtrate outlet of the gypsum crystallization unit (10) is refluxed to the calcium carbonate crystallization granulation unit (3).

2. The high-salinity wastewater resource recovery system according to claim 1, characterized in that, The pretreatment system (1) includes an equalization tank (11), a high-density tank (12), a dewatering device (13), and a filtration device (14). The equalization tank (11) is used to stabilize the water volume and quality and control the pH ≥ 7. The high-density tank (12) integrates coagulation, flocculation, sedimentation concentration and sludge return. The dewatering device (13) is a plate and frame dewatering machine. The filtration device (14) adopts one of sand filter, multi-media filter and membrane filter.

3. The high-salinity wastewater resource recovery system according to claim 1, characterized in that, The magnesium hydroxide crystallization granulation unit (2) includes a crystallization reaction zone (21), a granulation zone (22), a particle classification zone (23), a sedimentation zone (24), and a dehydration zone (25). The pH of the crystallization reaction zone (21) is controlled at 9.5~11. The granulation zone (22) adopts one of a fluidized bed, a stirred crystallizer, or a DTB crystallizer, with a seed reflux ratio of 10%~50%. The dehydration zone (25) adopts a filter press or a centrifuge to produce magnesium hydroxide granular filter cake.

4. The high-salinity wastewater resource recovery system according to claim 1, characterized in that, The calcium carbonate crystallization granulation unit (3) includes a reaction crystallization zone (31), a granulation zone two (32), a particle classification zone two (33), a sedimentation zone two (34), and a dehydration zone two (35). Sodium carbonate or sodium bicarbonate is added to the reaction crystallization zone (31), and the pH is controlled at 8.0 to 10.

0. The granulation zone two (32) adopts fluidized bed crystallization granulation. The dehydration zone two (35) adopts a filter press or centrifuge to produce calcium carbonate granular filter cake.

5. The high-salinity wastewater resource recovery system according to claim 1, characterized in that, The desalination device (5) is an electro-adsorption CDI or EST device. The electrode plate is composed of a current collector, an active adsorption layer, and a ceramic diaphragm. The active adsorption layer contains activated carbon, reduced graphene oxide, manganese dioxide, polyaniline, and rare earth materials. The desalination device (5) has a desalination rate of 50%~99% and a water production rate of ≥75%.

6. The high-salinity wastewater resource recovery system according to claim 1, characterized in that, The NF nanofiltration salt separation device (6) separates monovalent salts and divalent salts, the RO concentration device (7) concentrates the monovalent salt solution, and the evaporation crystallization device (9) is selected from one of multi-effect evaporation, falling film evaporator, forced circulation evaporator, and MVR evaporation crystallizer to produce sodium chloride crystals.

7. The high-salinity wastewater resource recovery system according to claim 1, characterized in that, The pH of the reaction device (8) is controlled at 7-9. Calcium chloride is added to react with sulfate to generate gypsum dihydrate. The gypsum crystallization unit (10) includes flocculation, sedimentation and separation. A plate and frame filter press is used to obtain gypsum filter cake. The seed crystals are returned to the reaction device (8).

8. A method for resource recovery of high-salinity wastewater, employing the system described in any one of claims 1-7, characterized in that, Includes the following steps: <1> After being treated by the equalization tank (11), high-density tank (12), dewatering device (13), and filtration device (14) of the pretreatment system (1), the high-salt wastewater enters the magnesium hydroxide crystallization and granulation unit (2) to recover magnesium hydroxide. <2> The effluent enters the calcium carbonate crystallization and granulation unit (3) to recover calcium carbonate, and then enters the desalination unit (5) after filtration by the filtration device (4). The desalinated water is reused, and the concentrated water enters the NF nanofiltration desalination device (6). <3> The monovalent salt separated by the NF nanofiltration salt separation device (6) is concentrated by the RO concentration device (7) and then enters the evaporation crystallization device (9) to obtain industrial sodium chloride. The distilled water is reused, and the divalent salt enters the reaction device (8). <4> The mother liquor from the evaporation crystallization device (9) enters the reaction device (8), reacts with divalent salts, and then enters the gypsum crystallization unit (10) to recover gypsum. The filtrate is recycled to the calcium carbonate crystallization granulation unit (3) for recycling.

9. A method for resource recovery of high-salinity wastewater according to claim 8, characterized in that, The gypsum crystallization unit (10) has a gypsum slurry concentration of 5% to 10% and a gypsum filter cake moisture content of 40% to 50%, and is used as a cement retarder or building material additive.

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