WO2015181208A1 - Method for removing phosphate from water fractions using an ultrafiltration membrane - Google Patents

Method for removing phosphate from water fractions using an ultrafiltration membrane Download PDF

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Publication number
WO2015181208A1
WO2015181208A1 PCT/EP2015/061654 EP2015061654W WO2015181208A1 WO 2015181208 A1 WO2015181208 A1 WO 2015181208A1 EP 2015061654 W EP2015061654 W EP 2015061654W WO 2015181208 A1 WO2015181208 A1 WO 2015181208A1
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Prior art keywords
adsorbent
water stream
holding tank
phosphate
ultrafiltration membrane
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PCT/EP2015/061654
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French (fr)
Inventor
Ferenc Horvath
Lute Broens
Arie Cornelis Besemer
Monica PARAVIDINO
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Biaqua B.V.
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Publication of WO2015181208A1 publication Critical patent/WO2015181208A1/en

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    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/28—Treatment of water, waste water, or sewage by sorption
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/14—Ultrafiltration; Microfiltration
    • B01D61/145—Ultrafiltration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
    • B01D65/08—Prevention of membrane fouling or of concentration polarisation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/0203—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04
    • B01J20/0225—Compounds of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt
    • B01J20/0229—Compounds of Fe
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/22—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising organic material
    • B01J20/223—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising organic material containing metals, e.g. organo-metallic compounds, coordination complexes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/22—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising organic material
    • B01J20/24—Naturally occurring macromolecular compounds, e.g. humic acids or their derivatives
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/30—Processes for preparing, regenerating, or reactivating
    • B01J20/34—Regenerating or reactivating
    • B01J20/3425—Regenerating or reactivating of sorbents or filter aids comprising organic materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/30—Processes for preparing, regenerating, or reactivating
    • B01J20/34—Regenerating or reactivating
    • B01J20/345—Regenerating or reactivating using a particular desorbing compound or mixture
    • B01J20/3475—Regenerating or reactivating using a particular desorbing compound or mixture in the liquid phase
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • C02F1/444—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by ultrafiltration or microfiltration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2311/00—Details relating to membrane separation process operations and control
    • B01D2311/16—Flow or flux control
    • B01D2311/165—Cross-flow velocity control
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2311/00—Details relating to membrane separation process operations and control
    • B01D2311/25—Recirculation, recycling or bypass, e.g. recirculation of concentrate into the feed
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2311/00—Details relating to membrane separation process operations and control
    • B01D2311/25—Recirculation, recycling or bypass, e.g. recirculation of concentrate into the feed
    • B01D2311/252—Recirculation of concentrate
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2311/00—Details relating to membrane separation process operations and control
    • B01D2311/26—Further operations combined with membrane separation processes
    • B01D2311/2626—Absorption or adsorption
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2321/00—Details relating to membrane cleaning, regeneration, sterilization or to the prevention of fouling
    • B01D2321/04—Backflushing
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/28—Treatment of water, waste water, or sewage by sorption
    • C02F1/281—Treatment of water, waste water, or sewage by sorption using inorganic sorbents
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/28—Treatment of water, waste water, or sewage by sorption
    • C02F1/286—Treatment of water, waste water, or sewage by sorption using natural organic sorbents or derivatives thereof
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/28—Treatment of water, waste water, or sewage by sorption
    • C02F1/288—Treatment of water, waste water, or sewage by sorption using composite sorbents, e.g. coated, impregnated, multi-layered
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2101/00—Nature of the contaminant
    • C02F2101/10—Inorganic compounds
    • C02F2101/105—Phosphorus compounds
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2301/00—General aspects of water treatment
    • C02F2301/04—Flow arrangements
    • C02F2301/046—Recirculation with an external loop
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2303/00—Specific treatment goals
    • C02F2303/16—Regeneration of sorbents, filters

Definitions

  • the present invention pertains to a method for removing phosphate from water fractions using an ultrafiltration membrane .
  • Phosphate is present in many water fractions, including waste water and water derived from water cleaning operations.
  • phosphate is an important nutrient for microorganisms, its presence may contribute to the growth of microorganisms.
  • a particular problem with bacterial growth is that even minor growth of microorganisms, a process which is sometimes also indicated as biofouling, may interfere with further
  • water streams are often subjected to treatment in membrane operations, e.g., membrane filtration or reverse osmosis.
  • membrane operations e.g., membrane filtration or reverse osmosis.
  • the occurrence of even minor amounts of biofouling in apparatus provided with a membrane will severely affect operation thereof.
  • invention provides such a method.
  • the invention pertains to a method for removing phosphate from a phosphate-containing water fraction comprising the steps of
  • step b) periodically stopping step b) , and providing a water stream through the membrane, countercurrent to the withdrawal direction in step b) to form an adsorbent-containing water stream,
  • adsorbent concentration in the adsorbent collection vessel is at least 3 and at most 100 times the adsorbent concentration in the holding tank.
  • adsorbent in the form of a suspension in a holding tank the use of a specific ratio between amount of water passing through the membrane and the amount of water passing over the membrane, and a specific membrane cleaning operation makes it possible to combine efficient operation and low water
  • WO91/04791 describes a method for removing contaminants, e.g., nitrate, from water by mixing the water with small particles of ion exchange resin to form a
  • NL1018870 describes a process for cleaning waste water wherein the waste water to which a particulate material has been added is passed over a membrane in a crossflow operation mode. It is indicated that the addition of particulate material may be used to decrease the crossflow velocity.
  • the ratio required for the present invention of at least 4:1 is far removed from the ratios conventionally applied in crossflow operations, which are of the order of 1:5, i.e., at least 20 times lower.
  • the specific membrane cleaning operation is also not disclosed or suggested.
  • FR2801809 describes a membrane filtration process for the removal of organic contaminants wherein a particulate
  • adsorbent is added to the liquid to be treated.
  • the invention focuses on the provision of a membrane which has a geometry which is such that Dean vortex flow is induced.
  • the majority of the water provided to the membrane filtration unit passes through the membrane, with a
  • concentration factor of 50 being exemplified, which is much higher than that applied in the present invention.
  • this reference does not disclose or suggest the specific regeneration step of the present invention wherein the adsorbent concentration in the adsorbent collection vessel is at least 3 and at most 100 times the adsorbent concentration in the holding tank.
  • FIG. 1 provides an overview of one embodiment of the present invention.
  • the water fraction to be treated with the process according to the invention is a phosphate-containing water fraction.
  • the water fraction has a phosphate content of at least 10 ppb, in particular at least 20 ppb, more in
  • the maximum for phosphate content is not critical. A suitable maximum value may be at most 50000 ppb (50 ppm) . In one embodiment, the phosphate content may be at most 2000 ppb, specifically at most 1000 ppb, more specifically below 500 ppb.
  • phosphate encompasses organic and inorganic phosphates, including orthophosphate and polyphosphate.
  • the phosphate content can be determined using the phosphate-molybdenum method, which is well known in the art.
  • the water fraction to be treated will generally have a pH around 7, e.g. in the range of 6 to 7.5.
  • the water fraction to be treated may have a variable salt content. Its conductivity is generally in the range of 20-100 mS/m, in particular in the range of 40-70 mS/m.
  • the water fraction to be treated may, e.g., have a nitrate content in the range of 0.1 to 50 mg N/1, in particular 1-20 mg N/1.
  • the water fraction to be treated may originate from various sources. In one embodiment it is derived from a waste water treatment plant.
  • the water fraction can be subjected to conventional pretreatment steps to remove contaminants.
  • An example of a suitable pretreatment step includes filtration to remove large particle size contaminants.
  • the operation mode of the method according to the invention encompasses the steps of
  • the cleaning mode of the method according to the invention comprises the steps of
  • step b) periodically stopping step b) , and providing a water stream through the membrane, countercurrent to the withdrawal direction in step b) to form an adsorbent-containing water stream,
  • adsorbent concentration in the adsorbent collection vessel is at least 3 times the adsorbent concentration in the holding tank and at most 100 times.
  • a phosphate containing water fraction continuously provided to a holding tank, where it is contacted with an adsorbent in the form of a suspension.
  • the holding tank may have any suitable configuration. It may, e.g., be a stirred tank, in particular a continuous stirred tank reactor such as a CSTR.
  • a holding tank has been found to be advantageous, because it makes it possible to handle the ratio between the gross flowrate (i.e. the flow rate of the process during operation mode) and the nett flowrate (i.e. the flow rate of the overall process,
  • the adsorbent is in the form of a suspension makes it possible to have a short contact time due to the high accessibility of the adsorbent for the phosphate.
  • a suspension-form adsorbent also allows easy handling of the adsorbent and periodic partial regeneration of the adsorbent, as will be discussed in more detail below.
  • the residence time in the holding tank generally resides between 2 minutes and 2 hours, in particular between 5 minutes and one hour, more in particular between 10 minutes and 40 minutes. If the residence time is too short, very high adsorbent volumes are required to obtain the desired
  • the residence time can be calculated by dividing the volume of the holding tank by the throughput of the process.
  • the volume of the holding tank is of course the volume of the tank in as far as it is filled with water and adsorbent.
  • the gross or nett flowrate can be adapted to the volume of the holding tank.
  • the amount of adsorbent used in the holding tank will depend on the adsorption capacity of the adsorbent, on the residence time, on the phosphate content of the feed fraction, and on the phosphate content of the product.
  • a suitable amount may, e.g., be in the range of 0.05 - 20 mg adsorbent per liter water in the holding tank, preferably 0.1 - 5 mg adsorbent per liter water.
  • Contacting conditions are not critical, and encompass a contacting temperature of 0-100°C, in particular 1-50°C, more in particular 1-30°C.
  • the pressure may vary between wide ranges, e.g. from 0.1 to 10 bar. Atmospheric pressure is generally suitable.
  • the nature of the adsorbent is not critical to the process according to the invention, as long as the adsorbent is capable of adsorbing phosphate under the conditions prevailing in the holding tank, and as long as the adsorbent can be in the form of a suspension under the conditions prevailing in the holding tank.
  • the particle size and the density of the adsorbent may be of importance. It is within the scope of the skilled person to determine whether an adsorbent is suitable for use in the form of a suspension, and to adapt product and process properties if this is required.
  • the particle size and density of the adsorbent also influence the settling rate of the adsorbent. A higher settling rate may be preferred in where the stream is in contact with the ultrafiltration membrane. On the other hand, a lower settling rate may be preferred in the holding tank .
  • the adsorbent used in the process is adsorbent used in the process
  • the particle size of the adsorbent used in the present invention can vary within wide ranges, e.g., in the range of 50 nm to 5 mm. In one embodiment, the particle size of the adsorbent is at most 10 micron, preferably less than 5 micron, more preferably less than 1 micron. This helps to ensure that a suitable slurry is obtained. The lower limit is prescribed by the pore size distribution of the membrane.
  • the particle size generally is at least 50 nm, in particular at least 100 nm.
  • particle size refers to the Dv50, which is the median diameter of the particle size distribution, where 50% of the volume of particles in a sample has a diameter above the median particle diameter, and where 50% of the volume of particles in a sample has a diameter of at most the median particle diameter.
  • the adsorbent used in the present invention preferably has a phosphate adsorption capacity of at least 15 mg/g. Higher values, e.g., at least 30 mg/g, in particular at least 40 mg/g are considered preferred. There is no maximum for the phosphate adsorption capacity for the adsorbent to be suitable for use in the process according to the invention. As a general attainable maximum a value of 300 mg/g may be mentioned. Examples of suitable adsorbents which may be used in the present invention, optionally after modification, e.g., particle size adjustment, include ion exchange resins like Indion ASM, ResinTech ASM-10, HP, Hydrolite ZGS820,
  • BoijeAS600 Lewatit F036.
  • Examples also include ferritin- based materials, e.g., as described in US2008/0223789, in particular iron-containing ferrit in-based materials.
  • the adsorbent comprises Fe(III) on a carrier.
  • Fe(III) is well known in the art for its capacity to adsorb phosphate.
  • Suitable carriers include the materials known in the art .
  • a complex of Fe(III) and cellulose is used as adsorbent in the method according to the invention. It is noted that the term "complex” does not place a limitation on the chemical relationship between the iron (III) and the cellulose, as long as the iron is chemically or physically bonded with the cellulose.
  • lignocellulose-based anion adsorbing material which is obtained by the steps of pelletising a lignocellulose, and contacting it with an acidic solution of an iron or aluminium salt, followed by an alkaline treatment to fixate the compounds onto the lignocellulose .
  • the inventors have also published an article on this subject (Kim et al . , Journal of Environmental Science and Health Part A, 41:87-100, 2006).
  • Eberhardt et al . (Bioresource Technology 97 (2006) 2371-2376) describe the removal of phosphate from stormwater runoffs by contacting it with refined aspen wood treated with
  • a particularly preferred adsorbent for use in the method according to the invention is an adsorbent comprising a complex of Fe(III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent carboxylate per gram. It has been found that this particular type of adsorbent combines a high adsorption capacity with good suspension formation properties.
  • the adsorbent is economically attractive, it can be regenerated in an efficient manner to allow re-use, and it is based on a biobased, degradable material.
  • Adsorbents of this type are described in European patent application No. 14167550.4 filed on May 8, 2014, and the PCT application claiming priority therefrom. The description of these applications as regards the properties and the nature of the adsorbent and its methods for manufacture are
  • an adsorbent comprising a complex of Fe(III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent (peq) carboxylate per gram, wherein the complex of Fe(III) and oxidised cellulose preferably has an Fe(III) content of at most 70 wt.%, expressed as metallic iron, per gram of oxidised cellulose.
  • oxidised cellulose with a carboxylate content of at least 200 peq/g (microequivalent carboxylate per gram) gives good results. It is preferred for the carboxylate content to be higher, e.g. at least 300 peq/g, in particular at least 400 peq/g, as a higher
  • carboxylate content makes for increased phosphate adsorption.
  • a value of at most 2000 peq/g may be mentioned, more in particular a value of at most 1000 peq/g.
  • the carboxylate content may be determined by methods known in the art, e.g., by conductometric titration or FTIR (Fourier Transform Infrared spectroscopy) .
  • Oxidised cellulose is known in the art, and commercially available. It can be obtained by subjecting a cellulose, or a cellulose-based material to an oxidation step. This can be effected using various types of oxidising agents in various ways, e.g., using oxidising agents like hydrogen peroxide or other peroxide compounds, nitrogen tetroxide, whether gaseous or in solution such as phosphoric acid solution, periodate, leading to the corresponding dialdehyde derivative, followed by oxidation to the dicarboxy cellulose with sodium chlorite, optionally in the presence of hydrogen peroxide,
  • oxidising agents like hydrogen peroxide or other peroxide compounds, nitrogen tetroxide, whether gaseous or in solution such as phosphoric acid solution, periodate, leading to the corresponding dialdehyde derivative, followed by oxidation to the dicarboxy cellulose with sodium chlorite, optionally in the presence of hydrogen peroxide,
  • hypochlorite, hypobromite, or hypoiodite compounds organic oxidising agents like TEMPO ( ( 2 , 2 , 6 , 6-tetramethylpiperidin-l- yl ) oxidanyl ) , or 4-hydroxy-TEMPO, and combinations thereof. Suitable combinations are reaction with hypochlorite, hypobromite, or hypoiodite, or other oxidising compound in the presence of TEMPO.
  • TEMPO ( 2 , 2 , 6 , 6-tetramethylpiperidin-l- yl ) oxidanyl )
  • 4-hydroxy-TEMPO 2-hydroxy-TEMPO
  • oxidising agent is also possible. Methods for manufacturing oxidised cellulose require no further elucidation here.
  • the complex of Fe(III) and oxidised cellulose generally has an Fe(III) content of at least 1 wt.%, expressed as metallic iron, per gram of oxidised cellulose. If the Fe(III) content is too low, the adsorption capacity of the adsorbent will be insufficient. It may be preferred for the Fe(III) content to be at least 5 wt.%, expressed as metallic iron per gram of oxidised cellulose, more in particular at least 10 wt.% expressed as metallic iron per gram of oxidised cellulose. In general, the Fe(III) content will be at most 90 wt.%, expressed as metallic iron, per gram of oxidised cellulose. When more iron is present, the accessibility of the
  • additional iron will be limited, and it will therefore only have a limited contribution to the phosphate removal. It may be desired to use adsorbents with a lower Fe(III) content, e.g., at most 70 wt.%, in particular at most 60 wt.%, more in particular at most 50 wt.%, expressed as metallic iron per gram of oxidised cellulose. It may be preferred to use even lower Fe(III) contents, e.g., at most 50 wt.%, more in particular at most 30 wt.%, in some embodiments at most 20 wt.%.
  • adsorbents with a lower Fe(III) content, e.g., at most 70 wt.%, in particular at most 60 wt.%, more in particular at most 50 wt.%, expressed as metallic iron per gram of oxidised cellulose. It may be preferred to use even lower Fe(III) contents, e.g., at most 50 wt.%, more in particular at
  • the particle size is at least 100 microns, as discussed in more detail below, it may be particularly preferred for the Fe(III) content to be in the range of 1-30 wt.%, in particular 5-20 wt .
  • the complex of Fe(III) and oxidised cellulose can be obtained by conventional methods.
  • oxidised cellulose may be contacted with an aqueous solution of an Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt, resulting in adsorption of Fe(III) onto the cellulose. It may be preferred for this adsorption step to be carried out at acidic pH .
  • the oxidised salt e.g., a sulphate, nitrate, or chloride salt
  • cellulose onto which iron (III) has been adsorbed can be removed from the aqueous solution, and dried if so desired. It may be preferred within this embodiment to subject the
  • Suitable bases include, e.g., sodium carbonate, sodium hydroxide, potassium
  • a complex of Fe(III) and oxidised cellulose is used which is prepared as follows: Oxidised cellulose is contacted with an aqueous suspension comprising one or more Fe ( III ) oxide,
  • the aqueous suspension comprising one or more
  • Fe ( III ) oxide, Fe ( III ) hydroxide, and Fe(III) oxyhydroxide can suitably be obtained by adding a water soluble inorganic base, in solid form, or in the form of an aqueous solution, to a solution of an inorganic Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt.
  • an inorganic Fe(III) salt e.g., a sulphate, nitrate, or chloride salt.
  • the water soluble inorganic base is, e.g.,
  • This method may be particularly suitable for oxidised cellulose with a particle size below 100 microns to ensure a good Fe(III) distribution.
  • a complex of Fe(III) and oxidised cellulose which is prepared as follows: Oxidised cellulose is contacted simultaneously with an aqueous solution of an inorganic Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt, and an aqueous solution of a water soluble inorganic base, e.g., selected from one or more of sodium carbonate, sodium hydroxide, potassium
  • Fe ( III ) oxide, Fe ( III ) hydroxide, and Fe(III) oxyhydroxide are formed in the presence of the oxidised cellulose. This may lead to an improved distribution of the Fe(III) (hydr) oxide compound on the oxidised cellulose. This method may also be particularly suitable for oxidised cellulose with a particle size below 100 microns to ensure a good Fe(III) distribution.
  • a complex of Fe(III) and oxidised cellulose is used which is prepared as follows: In a first step, oxidised cellulose is contacted with an aqueous solution of an inorganic Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt. If necessary, excess water is removed, e.g., by filtration. Preferably, the resulting Fe ( I I I ) -containing oxidised cellulose is dried.
  • an inorganic Fe(III) salt e.g., a sulphate, nitrate, or chloride salt.
  • excess water is removed, e.g., by filtration.
  • the resulting Fe ( I I I ) -containing oxidised cellulose is dried.
  • the Fe ( I I I ) -containing oxidised cellulose is contacted with an aqueous solution of a water soluble inorganic base, e.g., selected from one or more of sodium carbonate, sodium hydroxide, potassium carbonate, and potassium hydroxide, followed by removal of water.
  • a water soluble inorganic base e.g., selected from one or more of sodium carbonate, sodium hydroxide, potassium carbonate, and potassium hydroxide
  • one or more Fe ( III ) oxide, Fe ( III ) hydroxide, and Fe(III) oxyhydroxide are formed after the Fe(III) salt has been adsorbed onto the oxidised cellulose. This may lead to an improved distribution of the Fe(III) (hydr) oxide compound on the oxidised cellulose, especially, where the oxidised cellulose has a particle size of at least 100 microns. It can of course also be applied on oxidised cellulose with a smaller particle size.
  • complex of Fe(III) and oxidised cellulose does not place a limitation on the chemical relationship between the iron(III) and the oxidised cellulose, as long as the iron is (chemically or physically) bonded with the cellulose.
  • the adsorbent used in the present invention may comprise additional components to the complex of Fe(III) and oxidised cellulose.
  • additional components are bonding agents. Suitable bonding agents may, e.g., be cellulose type materials. It is preferred for the adsorbent to be made up for at least 50 wt . % of the complex of Fe(III) and oxidised cellulose, in particular for at least 70 wt.%, more in particular for at least 80 wt.%, in some embodiments for at least 90 wt.%.
  • the reason for this preference is that the complex of Fe(III) and oxidised cellulose is responsible for the phosphate adsorption.
  • the presence of other components will increase the volume of the adsorbent without
  • the amount of other components is preferably limited.
  • a further particularly preferred adsorbent for use in the method according to the invention is an adsorbent comprising a complex of Fe(III) and starch. It has again been found that this particular type of adsorbent combines a high adsorption capacity with good suspension formation properties.
  • the adsorbent is economically attractive, it can be regenerated in an efficient manner to allow re-use, and it is based on a biobased, degradable material.
  • Adsorbents of this type are described in European patent application with the title “Method for removing phosphate from water fractions” filed on May 28, 2014 in the name of BiAqua B.V., and in the PCT application claiming priority from this European application. The description of these applications as regards the properties and the nature of the adsorbent and its methods for manufacture are incorporated herein by reference.
  • an adsorbent is used which comprises a complex of Fe(III) and starch.
  • Starch or amylum is a carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as an energy store. It is contained in large amounts in plants like potatoes, wheat, maize (corn), rice, and cassava.
  • Starch The starch industry extracts and refines starches from seeds, roots and tubers, by wet grinding, washing, sieving and drying. Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two. The resulting fragments are known as dextrins. Starch is
  • oxidised starch is used. It has been found that, as compared to unoxidised starch, oxidised starch may result in an adsorbent with a higher phosphate adsorption capacity in mg/g.
  • oxidised starch is used with a carboxylate content of at least 200 peq/g (microequivalent carboxylate per gram) . It is preferred for the carboxylate content to be higher, e.g. at least 300 peq/g, in particular at least 400 peq/g, as a higher carboxylate content makes for increased phosphate adsorption. As a maximum for the carboxylate content, a value of at most 2000 peq/g may be mentioned, more in particular a value of at most 1000 peq/g.
  • Oxidised starch is known in the art, and commercially
  • the oxidised starch e.g., has a degree of oxidation between 1 and 30%. If the degree of oxidation is below 1 wt . % the advantageous effect of using oxidised starch may not be obtained. If the degree of oxidation is above 30 wt.%, the integrity of the starch may be affected. It may be preferred for the oxidised starch, if used, to have a degree of oxidation in the range of 3-20 wt.%.
  • the complex of Fe(III) and starch generally has an Fe(III) content of at least 1 wt.%, expressed as metallic iron, per gram of starch. If the Fe(III) content is too low, the adsorption capacity of the adsorbent will be insufficient. It may be preferred for the Fe(III) content to be at least 5 wt.%, expressed as metallic iron per gram of starch, more in particular at least 10 wt.% expressed as metallic iron per gram of starch.
  • the Fe(III) content will be at most 90 wt.%, expressed as metallic iron, per gram of starch. When more iron is present, the accessibility of the additional iron will be limited, and it will therefore only have a limited contribution to the phosphate removal. More specifically, the Fe(III) content may be at most 60 wt.%, more in particular at most 50 wt.%, expressed as iron per gram of starch.
  • Fe(III) contents e.g., at most 50 wt.%, more in particular at most 30 wt.%, in some embodiments at most 20 wt.%. In one embodiment of the
  • the particle size is at least 100 microns, as discussed in more detail below, it may be particularly preferred for the Fe(III) content to be in the range of 1-30 wt.%, in particular 5-20 wt .
  • the complex of Fe(III) and starch can be obtained by
  • first prepare the oxidised starch and then provide the iron(III) are also possible. For example, it is possible to first add the iron to the starch, and then carry out the oxidation step. This is less
  • the adsorbent comprising Fe(III) and starch may comprise additional components to the complex of Fe(III) and starch.
  • a further step in the method according to the invention is withdrawing a purified water stream from the holding tank through an ultrafiltration membrane and passing a water stream over the ultrafiltration membrane and recycling it to the holding tank, wherein the volume ratio between the water stream withdrawn per second through the ultrafiltration membrane and the water stream passed over the ultrafiltration membrane per second is at least 4:1 and at most 20:1.
  • a purified water stream is withdrawn through the ultrafiltration membrane.
  • the purified water stream has a phosphate content which is lower than that of the phosphate contacting water fraction used as starting material.
  • the phosphate content of the purified water stream is less than 50% of the phosphate content of the starting phosphate-containing water fraction, in particular less than 25%, more in particular less than 10%. In one embodiment, the phosphate content of the purified water stream is less than 100 ppb, in particular less than 50 ppb, more in particular less than 20 ppb, or even less than 10 ppb.
  • the phosphate content of the purified water stream is
  • adsorbent is entrained in the stream which is withdrawn. As this adsorbent will not pass through the ultrafiltration membrane, it causes built-up of an adsorbent cake on the membrane. Passing a water stream over the ultrafiltration membrane will lead to reduction of the formation of an adsorbent cake. The resulting adsorbent- containing water stream is recycled to the holding tank.
  • ultrafiltration membrane per second is at least 4:1. It is the intention that the majority of the water passes through the membrane, and a limited amount of water is used to clean the membrane.
  • the process according to the invention differs from conventional dead-end operation where all potential effluent is passed through the membrane.
  • the process also differs from conventional cross- flow operation, where the flow of the feed is parallel to the direction of the membrane, and the volume ratio between the water stream withdrawn through the ultrafiltration membrane and the water stream passed over the ultrafiltration membrane is much lower, e.g., of the order of 1:5.
  • the volume ratio between the water stream withdrawn through the ultrafiltration membrane per second and the water stream passed over the ultrafiltration membrane per second may be at least 5:1, in particular at least 6:1.
  • the ratio will generally be less than 20:1, more in particular less than 10:1.
  • the adsorbent concentration in the stream as it is withdrawn from the membrane and recycled to the holding tank may vary within wide ranges. It can, e.g., have a concentration which is between 3 and 15 times the adsorbent concentration in the holding tank, for example between 4 and 10 times.
  • the membrane used in the present invention is an
  • Ultrafiltration membranes generally have a pore size range of 0.010 micron to 0.10 micron.
  • a layer of adsorbent will build up on the membrane over time. While this process is delayed by the water stream which is passed over the membrane and recycled to the holding vessel as discussed above, there will be a point in time where the layer of adsorbent built up on the membrane is such that the flow of water through the membrane is detrimentally affected to an unacceptable extent. Therefore, in the process according to the invention, the step of continuously withdrawing a purified water stream through the ultrafiltration membrane is periodically stopped, and water stream is provided through the membrane,
  • This step can, e.g., be effected when the trans-membrane pressure loss is between 20 and 60 Kpa.
  • the adsorbent-containing water stream is withdrawn from the membrane, and collected in a collection vessel.
  • the purpose of this step is to remove adsorbent build-up from the surface of the membrane, so that the flow of purified water stream to the membrane is restored.
  • the adsorbent is withdrawn to a collection vessel.
  • the water stream provided through the membrane is generally a fraction of the purified water stream generated when the process according to the invention is in operation mode. If so desired it is possible to add membrane cleaning compounds to the water stream, as long as they do not interfere with the further processing of the adsorbent-containing water stream.
  • the amount of water used in this step is such that the adsorbent concentration in the adsorbent collection vessel is at least 3 times the adsorbent concentration in the holding tank. As will be clear to the skilled person, this is
  • the adsorbent concentration in the adsorbent collection vessel may be at least 5 times the adsorbent concentration in the holding tank, in particular at least 10 times the adsorbent concentration in the holding tank. To allow a reasonable flowability of the material in the collection tank, a maximum value of 100 times the
  • adsorbent concentration in the holding tank may be mentioned.
  • the adsorbent-containing water fraction in the collection vessel may be disposed of as desired. In one embodiment, the adsorbent-containing water fraction is disposed of. In a preferred embodiment of the present invention, the adsorbent in the collection vessel is regenerated, and the regenerated adsorbent is provided to the holding tank.
  • the adsorbent is regenerated by contacting it with a regeneration solution, and withdrawing a phosphate- containing regeneration solution from the adsorbent.
  • concentration of adsorbent in the collection vessel is relatively high. This makes it possible to have an efficient regeneration process, in a relatively small reactor volume.
  • the regeneration solution preferably is an alkaline aqueous regeneration solution, more specifically an aqueous solution with a pH above 11.5.
  • the pH preferably is above 12. In general, the pH will not be above 14.
  • alkaline compound in the aqueous alkaline solution is not critical.
  • Alkali metal hydroxides are
  • hydroxide is considered preferred.
  • a solution comprising both an alkali metal hydroxide and a dissolved inorganic salt, in particular an alkali metal salt, e.g., NaCl or KC1.
  • an alkali metal salt e.g., NaCl or KC1.
  • a salt concentration of 0.05 to 1 M/l may be mentioned as suitable.
  • the regenerated adsorbent is provided to the holding tank.
  • the adsorbent is provided to the holding tank in the form of a suspension.
  • the suspension can be formed with regeneration solution, or with water, e.g., water derived from the
  • the suspension preferably is relatively concentrated, in that it has an adsorbent concentration which is at least 3 times the adsorbent concentration in the holding tank, in particular at least 5 times, more in particular at least 10 times. To ensure that the suspension is processable, it may be
  • the adsorbent concentration is at most 100 times the adsorbent concentration in the holding tank.
  • adsorbent may be added to the holding tank in the method according to the invention.
  • adsorbent is withdrawn from the system in the membrane cleaning operation, it is preferred to add adsorbent during the process. This can be regenerated adsorbent as described above, fresh adsorbent, or a combination of both.
  • the purified water stream generated in the method according to the invention can be processed as desired. In one
  • FIG. 1 An embodiment of the present invention is illustrated in Figure 1.
  • a fraction is continuously provided to a holding tank (2) comprising an adsorbent in the form of a suspension.
  • the suspension is provided through line (3) to a membrane filtration unit (4) .
  • a purified water stream is continuously withdrawn from the membrane filtration unit (4) through line (5) .
  • a water stream is passed over the ultrafiltration membrane (13), withdrawn from the membrane filtration unit (4), and recycled to holding tank (2) through line ( 6 ) .
  • the adsorbent in the collection vessel (9) is regenerated by contacting it with a regeneration solution provided through line (10) .
  • the used regeneration solution is withdrawn through line (11), and the regenerated adsorbent is provided to the holding tank (2) through line (12) .

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Abstract

The invention pertains to a method for removing phosphate from a phosphate-containing water fraction comprising the steps of a) continuously providing a phosphate-containing water fraction to a holding tank comprising an adsorbent in the form of a suspension, b) continuously withdrawing a purified water stream through an ultrafiltration membrane and passing a water stream over the ultrafiltration membrane and recycling it to holding tank, wherein the volume ratio between the volume of the water stream withdrawn through the ultrafiltration membrane per second and the volume of the water stream passed over the ultrafiltration membrane per second is at least 4:1 and less than 20:1, c) periodically stopping step b), and providing a water stream through the membrane, countercurrent to the withdrawal direction in step b) to form an adsorbent-containing water stream, d) collecting the adsorbent-containing water stream in a collection vessel, wherein the adsorbent concentration in the adsorbent collection vessel is at least 3 and at most 100 times the adsorbent concentration in the holding tank. It has been found that the specific combination of an adsorbent in the form of a suspension in a holding tank, the use of an ultrafiltration membrane with a limited amount of recycle, and a specific membrane cleaning operation makes it possible to combine efficient operation and low water circulation with obtaining a low phosphate content in the product.

Description

Method for removing phosphate from water fractions using an ultrafiltration membrane
The present invention pertains to a method for removing phosphate from water fractions using an ultrafiltration membrane . Phosphate is present in many water fractions, including waste water and water derived from water cleaning operations. As phosphate is an important nutrient for microorganisms, its presence may contribute to the growth of microorganisms.
Therefore, phosphate removal is required to prevent bacterial growth.
A particular problem with bacterial growth is that even minor growth of microorganisms, a process which is sometimes also indicated as biofouling, may interfere with further
processing of the water streams. For example, water streams are often subjected to treatment in membrane operations, e.g., membrane filtration or reverse osmosis. The occurrence of even minor amounts of biofouling in apparatus provided with a membrane will severely affect operation thereof.
Therefore, there is need in the art for methods for removing phosphate from aqueous water streams to prevent or reduce the occurrence of biofouling, in particular in subsequent
membrane operations.
Various methods to effect phosphate removal from water fractions are known in the art.
For example, G. Akay et al . (Phosphate Removal from Water by Red Mud Using Crossflow Microfiltration, Water Research, Vol. 32, Issue 3, 1 March 1998, Pages 717-726) describes
adsorption of phosphorus on Red Mud (which comprises
particles of silica, calcium, iron, and aluminium, and is a waste product from bauxite ore processing) in combination with dead-end or crossflow microfiltration .
There is need in the art for a method for removing phosphate from phosphate-containing water fractions which combines efficient operation with the prevention of excess water circulation and the possibility to obtain purified water streams with very low phosphate contents. The present
invention provides such a method.
The invention pertains to a method for removing phosphate from a phosphate-containing water fraction comprising the steps of
a) continuously providing a phosphate-containing water fraction to a holding tank comprising an adsorbent in the form of a suspension,
b) continuously withdrawing a purified water stream through an ultrafiltration membrane and passing a water stream over the ultrafiltration membrane and recycling it to holding tank, wherein the volume ratio between the volume of the water stream withdrawn through the ultrafiltration membrane per second and the volume of the water stream passed over the ultrafiltration membrane per second is at least 4:1 and less than 20:1,
c) periodically stopping step b) , and providing a water stream through the membrane, countercurrent to the withdrawal direction in step b) to form an adsorbent-containing water stream,
d) collecting the adsorbent-containing water stream in a collection vessel, wherein the adsorbent concentration in the adsorbent collection vessel is at least 3 and at most 100 times the adsorbent concentration in the holding tank.
It has been found that the specific combination of an
adsorbent in the form of a suspension in a holding tank, the use of a specific ratio between amount of water passing through the membrane and the amount of water passing over the membrane, and a specific membrane cleaning operation makes it possible to combine efficient operation and low water
circulation with obtaining a low phosphate content in the product .
It is noted that WO91/04791 describes a method for removing contaminants, e.g., nitrate, from water by mixing the water with small particles of ion exchange resin to form a
suspension, and passing the suspension to a crossflow
filtration unit where the water is filtered off. A bleed of suspension is taken from the tank, regenerated, and recycled. This reference does not disclose or suggest the specific ratio between the amount water passing through the membrane and the amount of water passing over the membrane, and the specific recycle operation. The specific membrane cleaning operation is also not disclosed or suggested.
NL1018870 describes a process for cleaning waste water wherein the waste water to which a particulate material has been added is passed over a membrane in a crossflow operation mode. It is indicated that the addition of particulate material may be used to decrease the crossflow velocity.
However, this reference does not disclose or suggest the specific ratio between the amount of water passing through the membrane and the amount of water passing over the
membrane. In this context it should be noted that the ratio required for the present invention of at least 4:1 is far removed from the ratios conventionally applied in crossflow operations, which are of the order of 1:5, i.e., at least 20 times lower. The specific membrane cleaning operation is also not disclosed or suggested.
FR2801809 describes a membrane filtration process for the removal of organic contaminants wherein a particulate
adsorbent is added to the liquid to be treated. The invention focuses on the provision of a membrane which has a geometry which is such that Dean vortex flow is induced. In this reference, the majority of the water provided to the membrane filtration unit passes through the membrane, with a
concentration factor of 50 being exemplified, which is much higher than that applied in the present invention. Further, this reference does not disclose or suggest the specific regeneration step of the present invention wherein the adsorbent concentration in the adsorbent collection vessel is at least 3 and at most 100 times the adsorbent concentration in the holding tank.
The invention and the advantages associated therewith will be discussed in more detail below. Here reference will be made to the Figure, although the invention is not limited thereto or thereby.
Figure 1 provides an overview of one embodiment of the present invention. The water fraction to be treated with the process according to the invention is a phosphate-containing water fraction. In general, the water fraction has a phosphate content of at least 10 ppb, in particular at least 20 ppb, more in
particular at least 50 ppb, still more in particular at least 100 ppb. The maximum for phosphate content is not critical. A suitable maximum value may be at most 50000 ppb (50 ppm) . In one embodiment, the phosphate content may be at most 2000 ppb, specifically at most 1000 ppb, more specifically below 500 ppb.
Within the context of the present specification, the term phosphate encompasses organic and inorganic phosphates, including orthophosphate and polyphosphate. The phosphate content can be determined using the phosphate-molybdenum method, which is well known in the art.
Other parameters of the water fraction to be treated with the process according to the invention are generally not critical .
The water fraction to be treated will generally have a pH around 7, e.g. in the range of 6 to 7.5.
The water fraction to be treated may have a variable salt content. Its conductivity is generally in the range of 20-100 mS/m, in particular in the range of 40-70 mS/m.
The water fraction to be treated may, e.g., have a nitrate content in the range of 0.1 to 50 mg N/1, in particular 1-20 mg N/1.
The water fraction to be treated may originate from various sources. In one embodiment it is derived from a waste water treatment plant. The water fraction can be subjected to conventional pretreatment steps to remove contaminants. An example of a suitable pretreatment step includes filtration to remove large particle size contaminants.
Two "modes" can be recognised in the process according to the invention, namely an "operation mode", which yields a
purified water stream, and a "cleaning mode", where operation of the method is stopped and the membrane in cleaned.
The operation mode of the method according to the invention encompasses the steps of
a) continuously providing a phosphate-containing water fraction to a holding tank comprising an adsorbent in the form of a suspension, and
b) continuously withdrawing a purified water stream through an ultrafiltration membrane and passing a water stream over the ultrafiltration membrane and recycling it to holding tank, wherein the volume ratio per second between the water stream withdrawn through the ultrafiltration membrane and the water stream passed over the ultrafiltration membrane is at least 4:1 and less than 20:1.
The cleaning mode of the method according to the invention comprises the steps of
c) periodically stopping step b) , and providing a water stream through the membrane, countercurrent to the withdrawal direction in step b) to form an adsorbent-containing water stream,
d) collecting the adsorbent-containing water stream in a collection vessel, wherein the adsorbent concentration in the adsorbent collection vessel is at least 3 times the adsorbent concentration in the holding tank and at most 100 times.
The operation mode and the cleaning mode are alternated in the method according to the invention. This means that when steps c) and d) have been completed, steps a) and b) will be restarted . In a first step in the process according to the invention, a phosphate containing water fraction continuously provided to a holding tank, where it is contacted with an adsorbent in the form of a suspension.
The holding tank may have any suitable configuration. It may, e.g., be a stirred tank, in particular a continuous stirred tank reactor such as a CSTR. The use of a holding tank has been found to be advantageous, because it makes it possible to handle the ratio between the gross flowrate (i.e. the flow rate of the process during operation mode) and the nett flowrate (i.e. the flow rate of the overall process,
including the regeneration period where there is no product flow) . Further, by managing the residence time, the amount of adsorbent, and the ratio between adsorbent and phosphate- containing water fraction the desired phosphate content in the effluent stream can be obtained. This configuration also makes it possible to address varying compositions of the phosphate-containing water fraction.
That the adsorbent is in the form of a suspension makes it possible to have a short contact time due to the high accessibility of the adsorbent for the phosphate. A suspension-form adsorbent also allows easy handling of the adsorbent and periodic partial regeneration of the adsorbent, as will be discussed in more detail below.
The residence time in the holding tank generally resides between 2 minutes and 2 hours, in particular between 5 minutes and one hour, more in particular between 10 minutes and 40 minutes. If the residence time is too short, very high adsorbent volumes are required to obtain the desired
phosphate removal. On the other hand, long residence times require large tank volumes, and therefore large investments. The residence time can be calculated by dividing the volume of the holding tank by the throughput of the process. The volume of the holding tank is of course the volume of the tank in as far as it is filled with water and adsorbent. The gross or nett flowrate can be adapted to the volume of the holding tank. The amount of adsorbent used in the holding tank will depend on the adsorption capacity of the adsorbent, on the residence time, on the phosphate content of the feed fraction, and on the phosphate content of the product. A suitable amount may, e.g., be in the range of 0.05 - 20 mg adsorbent per liter water in the holding tank, preferably 0.1 - 5 mg adsorbent per liter water.
Contacting conditions are not critical, and encompass a contacting temperature of 0-100°C, in particular 1-50°C, more in particular 1-30°C. The pressure may vary between wide ranges, e.g. from 0.1 to 10 bar. Atmospheric pressure is generally suitable.
The nature of the adsorbent is not critical to the process according to the invention, as long as the adsorbent is capable of adsorbing phosphate under the conditions prevailing in the holding tank, and as long as the adsorbent can be in the form of a suspension under the conditions prevailing in the holding tank. For the latter, the particle size and the density of the adsorbent may be of importance. It is within the scope of the skilled person to determine whether an adsorbent is suitable for use in the form of a suspension, and to adapt product and process properties if this is required. The particle size and density of the adsorbent also influence the settling rate of the adsorbent. A higher settling rate may be preferred in where the stream is in contact with the ultrafiltration membrane. On the other hand, a lower settling rate may be preferred in the holding tank .
In one embodiment, the adsorbent used in the process
according to the invention has a density of 1.05 to 2.5 g/ml, in particular 1.2 to 1.8 gram/ml. The particle size of the adsorbent used in the present invention can vary within wide ranges, e.g., in the range of 50 nm to 5 mm. In one embodiment, the particle size of the adsorbent is at most 10 micron, preferably less than 5 micron, more preferably less than 1 micron. This helps to ensure that a suitable slurry is obtained. The lower limit is prescribed by the pore size distribution of the membrane. The particle size generally is at least 50 nm, in particular at least 100 nm. Within the context of the present
specification, particle size refers to the Dv50, which is the median diameter of the particle size distribution, where 50% of the volume of particles in a sample has a diameter above the median particle diameter, and where 50% of the volume of particles in a sample has a diameter of at most the median particle diameter. In one embodiment, the adsorbent used in the present invention preferably has a phosphate adsorption capacity of at least 15 mg/g. Higher values, e.g., at least 30 mg/g, in particular at least 40 mg/g are considered preferred. There is no maximum for the phosphate adsorption capacity for the adsorbent to be suitable for use in the process according to the invention. As a general attainable maximum a value of 300 mg/g may be mentioned. Examples of suitable adsorbents which may be used in the present invention, optionally after modification, e.g., particle size adjustment, include ion exchange resins like Indion ASM, ResinTech ASM-10, HP, Hydrolite ZGS820,
BoijeAS600, Lewatit F036. Examples also include ferritin- based materials, e.g., as described in US2008/0223789, in particular iron-containing ferrit in-based materials.
In one embodiment, the adsorbent comprises Fe(III) on a carrier. Fe(III) is well known in the art for its capacity to adsorb phosphate. Suitable carriers include the materials known in the art .
In one embodiment, a complex of Fe(III) and cellulose is used as adsorbent in the method according to the invention. It is noted that the term "complex" does not place a limitation on the chemical relationship between the iron (III) and the cellulose, as long as the iron is chemically or physically bonded with the cellulose.
Complexes of iron (III) and cellulose are known in the art. For example, US2008/0076956 describes a method for removal of anions such as phosphate from water fractions using a
lignocellulose-based anion adsorbing material, which is obtained by the steps of pelletising a lignocellulose, and contacting it with an acidic solution of an iron or aluminium salt, followed by an alkaline treatment to fixate the compounds onto the lignocellulose . The inventors have also published an article on this subject (Kim et al . , Journal of Environmental Science and Health Part A, 41:87-100, 2006). Eberhardt et al . (Bioresource Technology 97 (2006) 2371-2376) describe the removal of phosphate from stormwater runoffs by contacting it with refined aspen wood treated with
carboxymethyl cellulose and ferrous chloride.
A particularly preferred adsorbent for use in the method according to the invention is an adsorbent comprising a complex of Fe(III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent carboxylate per gram. It has been found that this particular type of adsorbent combines a high adsorption capacity with good suspension formation properties.
Additionally, the adsorbent is economically attractive, it can be regenerated in an efficient manner to allow re-use, and it is based on a biobased, degradable material.
Adsorbents of this type are described in European patent application No. 14167550.4 filed on May 8, 2014, and the PCT application claiming priority therefrom. The description of these applications as regards the properties and the nature of the adsorbent and its methods for manufacture are
incorporated herein by reference.
In one embodiment of the present invention, an adsorbent is used comprising a complex of Fe(III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent (peq) carboxylate per gram, wherein the complex of Fe(III) and oxidised cellulose preferably has an Fe(III) content of at most 70 wt.%, expressed as metallic iron, per gram of oxidised cellulose.
It has been found that the use of oxidised cellulose with a carboxylate content of at least 200 peq/g (microequivalent carboxylate per gram) gives good results. It is preferred for the carboxylate content to be higher, e.g. at least 300 peq/g, in particular at least 400 peq/g, as a higher
carboxylate content makes for increased phosphate adsorption. As a maximum for the carboxylate content, a value of at most 2000 peq/g may be mentioned, more in particular a value of at most 1000 peq/g.
The carboxylate content may be determined by methods known in the art, e.g., by conductometric titration or FTIR (Fourier Transform Infrared spectroscopy) .
Oxidised cellulose is known in the art, and commercially available. It can be obtained by subjecting a cellulose, or a cellulose-based material to an oxidation step. This can be effected using various types of oxidising agents in various ways, e.g., using oxidising agents like hydrogen peroxide or other peroxide compounds, nitrogen tetroxide, whether gaseous or in solution such as phosphoric acid solution, periodate, leading to the corresponding dialdehyde derivative, followed by oxidation to the dicarboxy cellulose with sodium chlorite, optionally in the presence of hydrogen peroxide,
hypochlorite, hypobromite, or hypoiodite compounds, organic oxidising agents like TEMPO ( ( 2 , 2 , 6 , 6-tetramethylpiperidin-l- yl ) oxidanyl ) , or 4-hydroxy-TEMPO, and combinations thereof. Suitable combinations are reaction with hypochlorite, hypobromite, or hypoiodite, or other oxidising compound in the presence of TEMPO. The use of 4-hydroxy-TEMPO, in
particular at a pH of 3-4 as such without any further
oxidising agent is also possible. Methods for manufacturing oxidised cellulose require no further elucidation here.
The complex of Fe(III) and oxidised cellulose generally has an Fe(III) content of at least 1 wt.%, expressed as metallic iron, per gram of oxidised cellulose. If the Fe(III) content is too low, the adsorption capacity of the adsorbent will be insufficient. It may be preferred for the Fe(III) content to be at least 5 wt.%, expressed as metallic iron per gram of oxidised cellulose, more in particular at least 10 wt.% expressed as metallic iron per gram of oxidised cellulose. In general, the Fe(III) content will be at most 90 wt.%, expressed as metallic iron, per gram of oxidised cellulose. When more iron is present, the accessibility of the
additional iron will be limited, and it will therefore only have a limited contribution to the phosphate removal. It may be desired to use adsorbents with a lower Fe(III) content, e.g., at most 70 wt.%, in particular at most 60 wt.%, more in particular at most 50 wt.%, expressed as metallic iron per gram of oxidised cellulose. It may be preferred to use even lower Fe(III) contents, e.g., at most 50 wt.%, more in particular at most 30 wt.%, in some embodiments at most 20 wt.%.
In one embodiment of the invention, where the particle size is at least 100 microns, as discussed in more detail below, it may be particularly preferred for the Fe(III) content to be in the range of 1-30 wt.%, in particular 5-20 wt .
The complex of Fe(III) and oxidised cellulose can be obtained by conventional methods. For example, oxidised cellulose may be contacted with an aqueous solution of an Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt, resulting in adsorption of Fe(III) onto the cellulose. It may be preferred for this adsorption step to be carried out at acidic pH . When the adsorption step has been completed, the oxidised
cellulose onto which iron (III) has been adsorbed can be removed from the aqueous solution, and dried if so desired. It may be preferred within this embodiment to subject the
Fe ( I I I ) -containing material to a post-treatment step with a base, in gaseous or dissolved form, to improve the bonding of the iron to the cellulose substrate. Suitable bases include, e.g., sodium carbonate, sodium hydroxide, potassium
carbonate, potassium hydroxide, NH3, and NH40H. Other methods are also possible. For example, it is possible to first add the iron to the cellulose, and then carry out the oxidation step. This is less preferred, however, as the presence of iron may interfere with the oxidation reaction. It is also possible to carry out the addition of iron and the oxidation reaction simultaneously, but this is also
considered less preferred.
In one embodiment of the present invention, a complex of Fe(III) and oxidised cellulose is used which is prepared as follows: Oxidised cellulose is contacted with an aqueous suspension comprising one or more Fe ( III ) oxide,
Fe ( III ) hydroxide, and Fe(III) oxyhydroxide, followed by removal of water, to form a complex of oxidised cellulose with iron. The aqueous suspension comprising one or more
Fe ( III ) oxide, Fe ( III ) hydroxide, and Fe(III) oxyhydroxide, can suitably be obtained by adding a water soluble inorganic base, in solid form, or in the form of an aqueous solution, to a solution of an inorganic Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt. In this way, a suspension will be obtained wherein the Fe(III) (hydr) oxide compounds have a relatively small particle size, which makes for a high dispersion of the Fe(III) (hydr) oxide compounds on the cellulose. The water soluble inorganic base is, e.g.,
selected from one or more of sodium carbonate, sodium
hydroxide, potassium carbonate, potassium hydroxide, NH3, and NH40H. This method may be particularly suitable for oxidised cellulose with a particle size below 100 microns to ensure a good Fe(III) distribution.
In another embodiment of the present invention, a complex of Fe(III) and oxidised cellulose is used which is prepared as follows: Oxidised cellulose is contacted simultaneously with an aqueous solution of an inorganic Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt, and an aqueous solution of a water soluble inorganic base, e.g., selected from one or more of sodium carbonate, sodium hydroxide, potassium
carbonate, potassium hydroxide, and NH40H, followed by removal of water. In this embodiment, one or more
Fe ( III ) oxide, Fe ( III ) hydroxide, and Fe(III) oxyhydroxide, are formed in the presence of the oxidised cellulose. This may lead to an improved distribution of the Fe(III) (hydr) oxide compound on the oxidised cellulose. This method may also be particularly suitable for oxidised cellulose with a particle size below 100 microns to ensure a good Fe(III) distribution.
In a further embodiment of the present invention, a complex of Fe(III) and oxidised cellulose is used which is prepared as follows: In a first step, oxidised cellulose is contacted with an aqueous solution of an inorganic Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt. If necessary, excess water is removed, e.g., by filtration. Preferably, the resulting Fe ( I I I ) -containing oxidised cellulose is dried. Then, in a second step, the Fe ( I I I ) -containing oxidised cellulose is contacted with an aqueous solution of a water soluble inorganic base, e.g., selected from one or more of sodium carbonate, sodium hydroxide, potassium carbonate, and potassium hydroxide, followed by removal of water. In this embodiment, one or more Fe ( III ) oxide, Fe ( III ) hydroxide, and Fe(III) oxyhydroxide, are formed after the Fe(III) salt has been adsorbed onto the oxidised cellulose. This may lead to an improved distribution of the Fe(III) (hydr) oxide compound on the oxidised cellulose, especially, where the oxidised cellulose has a particle size of at least 100 microns. It can of course also be applied on oxidised cellulose with a smaller particle size.
It is noted that in the context of the present specification the term "complex of Fe(III) and oxidised cellulose" does not place a limitation on the chemical relationship between the iron(III) and the oxidised cellulose, as long as the iron is (chemically or physically) bonded with the cellulose.
The adsorbent used in the present invention may comprise additional components to the complex of Fe(III) and oxidised cellulose. Examples of additional components are bonding agents. Suitable bonding agents may, e.g., be cellulose type materials. It is preferred for the adsorbent to be made up for at least 50 wt . % of the complex of Fe(III) and oxidised cellulose, in particular for at least 70 wt.%, more in particular for at least 80 wt.%, in some embodiments for at least 90 wt.%. The reason for this preference is that the complex of Fe(III) and oxidised cellulose is responsible for the phosphate adsorption. The presence of other components will increase the volume of the adsorbent without
contributing to the phosphate adsorption. Therefore, the amount of other components is preferably limited.
A further particularly preferred adsorbent for use in the method according to the invention is an adsorbent comprising a complex of Fe(III) and starch. It has again been found that this particular type of adsorbent combines a high adsorption capacity with good suspension formation properties.
Additionally, the adsorbent is economically attractive, it can be regenerated in an efficient manner to allow re-use, and it is based on a biobased, degradable material.
Adsorbents of this type are described in European patent application with the title "Method for removing phosphate from water fractions" filed on May 28, 2014 in the name of BiAqua B.V., and in the PCT application claiming priority from this European application. The description of these applications as regards the properties and the nature of the adsorbent and its methods for manufacture are incorporated herein by reference. In one embodiment of the present invention an adsorbent is used which comprises a complex of Fe(III) and starch.
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as an energy store. It is contained in large amounts in plants like potatoes, wheat, maize (corn), rice, and cassava.
The starch industry extracts and refines starches from seeds, roots and tubers, by wet grinding, washing, sieving and drying. Starch can be hydrolyzed into simpler carbohydrates by acids, various enzymes, or a combination of the two. The resulting fragments are known as dextrins. Starch is
commercially available form a large number of manufacturers. In one embodiment of the present invention, oxidised starch is used. It has been found that, as compared to unoxidised starch, oxidised starch may result in an adsorbent with a higher phosphate adsorption capacity in mg/g.
In one embodiment, oxidised starch is used with a carboxylate content of at least 200 peq/g (microequivalent carboxylate per gram) . It is preferred for the carboxylate content to be higher, e.g. at least 300 peq/g, in particular at least 400 peq/g, as a higher carboxylate content makes for increased phosphate adsorption. As a maximum for the carboxylate content, a value of at most 2000 peq/g may be mentioned, more in particular a value of at most 1000 peq/g.
Oxidised starch is known in the art, and commercially
available. It can be obtained by subjecting a starch, or a starch-containing material to an oxidation step. Reference is made to what is stated above for the preparation of oxidised cellulose .
The oxidised starch, e.g., has a degree of oxidation between 1 and 30%. If the degree of oxidation is below 1 wt . % the advantageous effect of using oxidised starch may not be obtained. If the degree of oxidation is above 30 wt.%, the integrity of the starch may be affected. It may be preferred for the oxidised starch, if used, to have a degree of oxidation in the range of 3-20 wt.%.
The complex of Fe(III) and starch generally has an Fe(III) content of at least 1 wt.%, expressed as metallic iron, per gram of starch. If the Fe(III) content is too low, the adsorption capacity of the adsorbent will be insufficient. It may be preferred for the Fe(III) content to be at least 5 wt.%, expressed as metallic iron per gram of starch, more in particular at least 10 wt.% expressed as metallic iron per gram of starch.
In general, the Fe(III) content will be at most 90 wt.%, expressed as metallic iron, per gram of starch. When more iron is present, the accessibility of the additional iron will be limited, and it will therefore only have a limited contribution to the phosphate removal. More specifically, the Fe(III) content may be at most 60 wt.%, more in particular at most 50 wt.%, expressed as iron per gram of starch.
It may be preferred to use lower Fe(III) contents, e.g., at most 50 wt.%, more in particular at most 30 wt.%, in some embodiments at most 20 wt.%. In one embodiment of the
invention, where the particle size is at least 100 microns, as discussed in more detail below, it may be particularly preferred for the Fe(III) content to be in the range of 1-30 wt.%, in particular 5-20 wt .
The complex of Fe(III) and starch can be obtained by
conventional methods. Reference is made to what is stated above for the manufacture of complexes of Fe(III) and
cellulose .
Where the use of oxidised starch is aimed for, it is
preferred to first prepare the oxidised starch and then provide the iron(III) . Other methods are also possible. For example, it is possible to first add the iron to the starch, and then carry out the oxidation step. This is less
preferred, however, as the presence of iron may interfere with the oxidation reaction. It is also possible to carry out the addition of iron and the oxidation reaction
simultaneously, but this is also considered less preferred.
It is noted that in the context of the present specification the term "complex of Fe(III) and starch" does not place a limitation on the chemical relationship between the iron (III) and the starch, as long as the iron is chemically or
physically bonded with the starch.
The adsorbent comprising Fe(III) and starch may comprise additional components to the complex of Fe(III) and starch.
Examples of additional components are bonding agents. For the amount of bonding agents reference is made to what is stated above for the complex of Fe(III) and cellulose. A further step in the method according to the invention is withdrawing a purified water stream from the holding tank through an ultrafiltration membrane and passing a water stream over the ultrafiltration membrane and recycling it to the holding tank, wherein the volume ratio between the water stream withdrawn per second through the ultrafiltration membrane and the water stream passed over the ultrafiltration membrane per second is at least 4:1 and at most 20:1.
In this step, a purified water stream is withdrawn through the ultrafiltration membrane. The purified water stream has a phosphate content which is lower than that of the phosphate contacting water fraction used as starting material.
In one embodiment, the phosphate content of the purified water stream is less than 50% of the phosphate content of the starting phosphate-containing water fraction, in particular less than 25%, more in particular less than 10%. In one embodiment, the phosphate content of the purified water stream is less than 100 ppb, in particular less than 50 ppb, more in particular less than 20 ppb, or even less than 10 ppb.
A process wherein the phosphate content of the effluent water fraction is below 10 ppb is of particular interest,
especially, but not limited to, the situation where the effluent water fraction is to be provided to a reverse osmosis process, as will be discussed in more detail below.
The phosphate content of the purified water stream is
determined by the phosphate content of the starting water fraction, the nature and amount of the adsorbent, and the residence time in the holding tank. It is within the scope of the skilled person to select these parameters so that an effluent with the desired phosphate content is obtained.
As indicated above, a water stream is passed over the
ultrafiltration membrane, and recycled to the holding tank. The reason for this stream is the following. During the withdrawal of the purified water stream through the
ultrafiltration membrane, adsorbent is entrained in the stream which is withdrawn. As this adsorbent will not pass through the ultrafiltration membrane, it causes built-up of an adsorbent cake on the membrane. Passing a water stream over the ultrafiltration membrane will lead to reduction of the formation of an adsorbent cake. The resulting adsorbent- containing water stream is recycled to the holding tank.
The volume ratio between the volume of the water stream withdrawn through the ultrafiltration membrane per second and the volume of the water stream passed over the
ultrafiltration membrane per second is at least 4:1. It is the intention that the majority of the water passes through the membrane, and a limited amount of water is used to clean the membrane. Here the process according to the invention differs from conventional dead-end operation where all potential effluent is passed through the membrane. On the other hand, the process also differs from conventional cross- flow operation, where the flow of the feed is parallel to the direction of the membrane, and the volume ratio between the water stream withdrawn through the ultrafiltration membrane and the water stream passed over the ultrafiltration membrane is much lower, e.g., of the order of 1:5.
It may be preferred for the volume ratio between the water stream withdrawn through the ultrafiltration membrane per second and the water stream passed over the ultrafiltration membrane per second to be at least 5:1, in particular at least 6:1. On the other hand, as it is required in the process according to the invention that some water is passed over the membrane, and recycled to the holding vessel, the ratio will generally be less than 20:1, more in particular less than 10:1.
The adsorbent concentration in the stream as it is withdrawn from the membrane and recycled to the holding tank may vary within wide ranges. It can, e.g., have a concentration which is between 3 and 15 times the adsorbent concentration in the holding tank, for example between 4 and 10 times.
The membrane used in the present invention is an
ultrafiltration membrane. Ultrafiltration membranes generally have a pore size range of 0.010 micron to 0.10 micron.
During the phosphate removal process according to the
invention, a layer of adsorbent will build up on the membrane over time. While this process is delayed by the water stream which is passed over the membrane and recycled to the holding vessel as discussed above, there will be a point in time where the layer of adsorbent built up on the membrane is such that the flow of water through the membrane is detrimentally affected to an unacceptable extent. Therefore, in the process according to the invention, the step of continuously withdrawing a purified water stream through the ultrafiltration membrane is periodically stopped, and water stream is provided through the membrane,
countercurrent to the withdrawal direction above to form an adsorbent-containing water stream. This step can, e.g., be effected when the trans-membrane pressure loss is between 20 and 60 Kpa.
The adsorbent-containing water stream is withdrawn from the membrane, and collected in a collection vessel. The purpose of this step is to remove adsorbent build-up from the surface of the membrane, so that the flow of purified water stream to the membrane is restored. The adsorbent is withdrawn to a collection vessel.
The water stream provided through the membrane is generally a fraction of the purified water stream generated when the process according to the invention is in operation mode. If so desired it is possible to add membrane cleaning compounds to the water stream, as long as they do not interfere with the further processing of the adsorbent-containing water stream.
The amount of water used in this step is such that the adsorbent concentration in the adsorbent collection vessel is at least 3 times the adsorbent concentration in the holding tank. As will be clear to the skilled person, this is
determined by the amount of water provided through the membrane, and by the amount of adsorbent present in the cake built up on the membrane, which is removed in this step.
If the adsorbent concentration is lower, the amount of water used to "clean" the membrane is so large that the overall throughput of the process is reduced to a too large extent. It may therefore be preferred for the adsorbent concentration in the adsorbent collection vessel to be at least 5 times the adsorbent concentration in the holding tank, in particular at least 10 times the adsorbent concentration in the holding tank. To allow a reasonable flowability of the material in the collection tank, a maximum value of 100 times the
adsorbent concentration in the holding tank may be mentioned.
The adsorbent-containing water fraction in the collection vessel may be disposed of as desired. In one embodiment, the adsorbent-containing water fraction is disposed of. In a preferred embodiment of the present invention, the adsorbent in the collection vessel is regenerated, and the regenerated adsorbent is provided to the holding tank.
In one embodiment, the adsorbent is regenerated by contacting it with a regeneration solution, and withdrawing a phosphate- containing regeneration solution from the adsorbent.
An advantage of the present invention is that the
concentration of adsorbent in the collection vessel is relatively high. This makes it possible to have an efficient regeneration process, in a relatively small reactor volume.
The regeneration solution preferably is an alkaline aqueous regeneration solution, more specifically an aqueous solution with a pH above 11.5. The pH preferably is above 12. In general, the pH will not be above 14.
The nature of the alkaline compound in the aqueous alkaline solution is not critical. Alkali metal hydroxides are
generally preferred for reasons of availability, cost, and safety. The use of sodium hydroxide and/or potassium
hydroxide is considered preferred.
It may be preferred to use a solution comprising both an alkali metal hydroxide and a dissolved inorganic salt, in particular an alkali metal salt, e.g., NaCl or KC1. The presence of a salt has been found to improve regeneration efficiency. A salt concentration of 0.05 to 1 M/l may be mentioned as suitable. Once the regeneration step is complete, e.g., when no
phosphate is detected in the stream withdrawn from the adsorbent, the regenerated adsorbent is provided to the holding tank.
The adsorbent is provided to the holding tank in the form of a suspension. The suspension can be formed with regeneration solution, or with water, e.g., water derived from the
purified water stream generated in the process according to the invention, or with a neutralising solution, i.e., a slightly acidic aqueous solution, e.g., a solution with a pH between 4 and 6, which would bring the pH of the adsorbent suspension after regeneration to a value between 6 and 8. The suspension preferably is relatively concentrated, in that it has an adsorbent concentration which is at least 3 times the adsorbent concentration in the holding tank, in particular at least 5 times, more in particular at least 10 times. To ensure that the suspension is processable, it may be
preferred for the adsorbent concentration to be at most 100 times the adsorbent concentration in the holding tank.
It is noted that it may be desired to add fresh adsorbent to the holding tank in the method according to the invention. As adsorbent is withdrawn from the system in the membrane cleaning operation, it is preferred to add adsorbent during the process. This can be regenerated adsorbent as described above, fresh adsorbent, or a combination of both.
The purified water stream generated in the method according to the invention can be processed as desired. In one
embodiment, it provided to a reverse osmosis step, where the purified water stream is treated to form a purified effluent water fraction and a contaminant fraction. The reverse osmosis step effects contaminant removal, in particular salt removal . An embodiment of the present invention is illustrated in Figure 1.
In Figure 1, in line (1) a phosphate-containing water
fraction is continuously provided to a holding tank (2) comprising an adsorbent in the form of a suspension. In the embodiment in the figure, the suspension is provided through line (3) to a membrane filtration unit (4) . A purified water stream is continuously withdrawn from the membrane filtration unit (4) through line (5) . A water stream is passed over the ultrafiltration membrane (13), withdrawn from the membrane filtration unit (4), and recycled to holding tank (2) through line ( 6 ) .
Periodically, the flow through line (1) is stopped, and a water stream is provided through the membrane (13) through line (7), countercurrent to the withdrawal direction of stream (5) to form an adsorbent-containing water stream, which is withdrawn through line (8), and provided to
collection vessel (9) . The adsorbent in the collection vessel (9) is regenerated by contacting it with a regeneration solution provided through line (10) . The used regeneration solution is withdrawn through line (11), and the regenerated adsorbent is provided to the holding tank (2) through line (12) .
It will be clear to the skilled person that in the process according to the invention the preferred embodiments of the various steps of the process can be combined as desired.

Claims

1. Method for removing phosphate from a phosphate- containing water fraction comprising the steps of
a) continuously providing a phosphate-containing water fraction to a holding tank comprising an adsorbent in the form of a suspension,
b) continuously withdrawing a purified water stream through an ultrafiltration membrane and passing a water stream over the ultrafiltration membrane and recycling it to holding tank, wherein the volume ratio between the volume of the water stream withdrawn through the ultrafiltration membrane per second and the volume of the water stream passed over the ultrafiltration membrane per second is at least 4:1 and less than 20:1,
c) periodically stopping step b) , and providing a water stream through the membrane, countercurrent to the withdrawal direction in step b) to form an adsorbent-containing water stream,
d) collecting the adsorbent-containing water stream in a collection vessel, wherein the adsorbent concentration in the adsorbent collection vessel is at least 3 and at most 100 times the adsorbent concentration in the holding tank.
2. Method according to claim 1, wherein the adsorbent in the collection vessel is regenerated, and the regenerated adsorbent is provided to the holding tank.
3. Method according to claim 2, wherein the adsorbent is regenerated by contacting it with an alkaline solution.
4. Method according to claim 2 or 3, wherein the
regenerated adsorbent is provided to the holding tank in the form of a suspension, the suspension preferably having an adsorbent concentration which is at least 3 times the adsorbent concentration in the holding tank, in particular at least 5 times, more in particular at least 10 times and/or at most 100 times the adsorbent concentration in the holding tank .
5. Method according to any one of the preceding claims, wherein the adsorbent concentration in the adsorbent
collection vessel is at least 5 times the adsorbent
concentration in the holding tank, in particular at least 10 times the adsorbent concentration in the holding tank the adsorbent concentration in the holding tank.
6. Method according to any one of the preceding claims wherein the volume ratio between the water stream withdrawn through the ultrafiltration membrane and the water stream passed over the ultrafiltration membrane in at least 5:1 and at most 10:1.
7. Method according to any one of the preceding claims wherein the phosphate content of the purified water stream withdrawn through the ultrafiltration membrane is less than 100 ppb, in particular less than 50 ppb, more in particular less than 20 ppb, or even less than 10 ppb, in particular less than 10 ppb.
8. Method according to any one of the preceding claims, wherein the residence time in the holding tank generally resides between 2 minutes and 2 hours, in particular between 5 minutes and one hour, more in particular between 10 minutes and 40 minutes.
9. Method according to any one of the preceding claims, wherein the adsorbent has a phosphate adsorption capacity of at least 15 mg/g, in particular at least 30 mg/g, more in particular at least 40 mg/g.
10. Method according to any one of the preceding claims, wherein the adsorbent comprises Fe(III) on a carrier.
11. Method according to claim 10, wherein the adsorbent comprises a complex of Fe(III) and cellulose, preferably a complex of Fe(III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent carboxylate per gram.
12. Method according to claim 10, wherein the adsorbent comprises a complex of Fe(III) and starch.
PCT/EP2015/061654 2014-05-27 2015-05-27 Method for removing phosphate from water fractions using an ultrafiltration membrane WO2015181208A1 (en)

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CN112337439A (en) * 2020-11-06 2021-02-09 江西挺进环保科技有限公司 Rare earth chelate and phosphorus adsorbent for adsorbing phosphorus element in wastewater
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CN112337439B (en) * 2020-11-06 2023-02-17 江西挺进环保科技股份有限公司 Rare earth chelate and phosphorus adsorbent for adsorbing phosphorus element in wastewater

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