WO2013010548A1

WO2013010548A1 - A method for purifying a liquid containing ammonium as a pollutant, and an apparatus for purifying a liquid containing ammonium

Info

Publication number: WO2013010548A1
Application number: PCT/DK2012/050269
Authority: WO
Inventors: Karsten Poulsen
Original assignee: Oht A/S
Priority date: 2011-07-18
Filing date: 2012-07-17
Publication date: 2013-01-24

Abstract

The invention relates to a method and an apparatus for purifying liquid containing ammonium. The apparatus includes a mixing tank (10) with a mixing device (20, 22, 24), a separation unit (30) and a regenerating unit (60), a supply (S) of said liquid being connected to said mixing tank (10), level (LI) of said liquid being inside said mixing tank (10), said mixing device (20, 22, 24) being for mixing supplied liquid in said mixing tank (10) with a reagent, such as a granular reagent containing at least one salt containing at least phosphate and a metal, to form a precipitate of an ammonium containing salt, such as struvite in granular form, said apparatus (1) being configured for establishing during said mixing a generally upwardly directed flow (Fl) inside said mixing tank (10) of said liquid and said precipitate, where subsequently the liquid with said precipitate is discharged from said mixing tank (10) and supplied to said separation unit (30) for separating said precipitate from said liquid, said regenerating unit (60) being configured for regenerating said reagent from said precipitate by heating said precipitate and for discharging said ammonium, said separation unit (30) being connected with said regenerating unit (60). The apparatus (1) moreover includes a first pipe string (15) connected with said mixing tank (10) at said level (LI) and with said separation unit (30), to allow for said discharge of liquid with said precipitate to occur as a spill-over from said mixing tank (10) at said level (LI), said first pipe string (15) being arranged to provide for a flow of said discharge of liquid with precipitate by gravity only to said separation unit (30).

Description

A method for purifying a liquid containing ammonium as a pollutant, and an apparatus for purifying a liquid containing ammonium.

FIELD OF THE INVENTION

The present invention concerns a method and an apparatus for purifying a polluted liquid containing ammonium, involving a mixing tank with a mixing device, a separation unit and a regenerating unit, a supply of the liquid being connected to the mixing tank and the liquid being contained inside the mixing tank up to a giv- en level, the mixing device mixing the polluted liquid with a reagent to form a precipitate of an ammonium containing salt, such as struvite in granular form. During the mixing a generally upwardly directed flow is set up inside said mixing tank and the liquid with the precipitate is them discharged from the mixing tank and supplied to the separation unit for separating the precipitate from the liquid, the regenerating unit regenerating the reagent from the precipitate by heating the precipitate and also discharging the ammonium. The method and the apparatus according to the invention may be configured to run continuously.

BACKGROUND OF THE INVENTION

From chemical reaction plants it is commonly known to add reagents to a polluted liquid and subsequently to separate precipitate products from the liquid. It is known to remove ammonium by nitrification/denitrification where ammonium is released as free nitrogen to the atmosphere.

From EP 490 396 and WO 03/006384 are known a method and an apparatus for purifying a liquid containing ammonium, where this polluted liquid is supplied with a first reagent in a first chemical process, wherein this first reagent may contain magnesium. The process causes a chemical reaction between the ammonia and the reagent. By reaction, at least a first ammonium-containing salt is precipitated, typically in the form of what is known as Struvite. In a subsequent second process the liquid is separated from the precipitate. In a third process, the precipitate is decomposed into a granular product, being the original reagent, and ammonium; the granular product is then circulated back to form part of the first chemical pro- cess, where it is used as the first reagent. The decomposition of precipitation products into the original reagent and ammonium occurs at a high temperature, where ammonium together with water vapour is separated by drying or by steam stripping. From EP 0 915 058 A2 is known a similar method for purifying ammonium- containing waste water. The decomposition of precipitation products may also occur here by vapour/air precipitation.

It is a general requirement that apparatuses as mentioned above should be capa- ble of running continuously without the need for too many maintenance stops. This is particularly so where the apparatus forms a part of a city sewage system where such operational disruptions require the incorporation of buffer tanks. It is, however, considered unavoidable to schedule regular maintenance stops. The present inventor has found that the maintenance stops referred to above, even where they occur for no more than a few hours, as such give rise to problems requiring in themselves further future maintenance stops and arising from encrustations formed in not readily accessible parts, such as inside pumps, of the apparatuses during the maintenance stops. Struvite mentioned above forms par- ticularly hard encrustations during periods of inoperation, and these encrustations when occurring in pumps cannot be readily removed without a time-consuming replacing of the pumps of the apparatus; this in itself also adds to the operational costs. OBJECT OF THE INVENTION

The present invention has as its object to solve the aforementioned problems by providing a method and an apparatus reducing the total lasting of maintenance stops over a given period in that encrustations will occur in more easily accessible and serviceable parts of the apparatus, from which parts the encrustation can more quickly be removed, such as by a mechanical action. This is of benefit both where the apparatus is used in large cities with a heavy load of polluted water and where maintenance staff is readily available or where it is used in remote areas with perhaps a less heavy load and where properly trained maintenance staff is not readily available. SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems by providing an im- proved method where discharging the liquid with said precipitate is brought about as a spill-over from the mixing tank, by supplying the spill-over only by gravity, via a pipe system connected to the mixing tank at the level of the liquid in the mixing tank, to the separation unit, by separating the precipitate from the liquid, and supplying the separated precipitate to a regenerating unit configured for re- generating the reagent from the separated precipitate by heating and for discharging said ammonium.

According to the invention the apparatuses discussed in the introductory paragraph and known from i.a. WO03/006384 are improved in that the apparatus in- eludes a first pipe string connected with the mixing tank at the level of the liquid contained therein and with the separation unit, to allow for the discharge of the liquid with the precipitate to occur as a spill-over from the mixing tank at that level, the first pipe string being arranged to provide for a flow of the discharge of liquid with precipitate by gravity only to said separation unit.

The reagent may advantageously contain at least one salt at least containing phosphate and magnesium.

Advantageously, the separation unit may include at least one hydrocyclone. Here- by may be achieved a rapid and effective separation, and a collection of the precipitate may take place at a bottom part of the hydrocyclone, which bottom part may conveniently be configured for easy disassembly, allowing for a removal of any encrustations having formed there during standstill of the apparatus. Preferably the apparatus includes a second pipe string incorporating the regenerating unit and supplying the regenerated reagent back to the mixing tank, and a third pipe string connected with a precipitate collecting part of the separation unit and having an internal screw conveyor conveying the separated precipitate to the regenerating unit. The third pipe string preferably extends upwards from the sep- aration unit, and the screw conveyor conveys the separated precipitate up to a level above a level of the liquid in the separation unit. The screw conveyor preferably is configured to allow for a back-flow inside the third pipe string of any of the liquid remaining mixed with the precipitate, back to the separation unit. Preferably, a heating device of the regenerating unit is formed as a jacket surrounding a portion of the second pipe string and being supplied with a heating medium, such as hot oil. Preferably, the mixing device is configured to establish the upwardly directed flow through the design of mixing paddles or by having an inlet for pressurized gas arranged at the bottom of the mixing tank.

The above aspects, and other aspects of the invention will be apparent from and elucidated with reference to an embodiment described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The device according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Fig. 1 is a highly schematic representation of an apparatus according to the invention and making use of the inventive method.

DETAILED DESCRIPTION OF AN EMBODIMENT

Fig. 1 shows one embodiment of an apparatus 1 according to the invention, designed for being placed in a standard freight container, i. e. a compact and highly efficient plant or apparatus which efficiently removes ammonium from a polluted liquid which may be waste water from sludge concentrations or highly polluted in- dustrial waste water.

On Fig. 1 is shown a supply string S for the untreated polluted liquid. A magnesium containing reagent may initially be added to string S together with a phosphate-containing reagent. The supply string S has connection to a closed or open mixing tank 10 where a mixing and reaction takes place to form a precipitate. A pump (not shown) in the supply string S may accurately dose the amount of polluted liquid, and the concentration of ammonium in the supplied liquid may be measured on-line by a spectrophotometer. The measured concentration of ammo- nium (mg/1) may multiplied with the supplied amount of medium (1/min) in order to reach the value of supplied ammonium per time unit (mg ammonium/min).

The reagents, such as a phosphate of e. g. P04 and a metal (Me: e. g. Mg, Mn, Fe, K, Ca) are added to the polluted liquid, preferably for achieving a 1 : 1: 1 ratio between NH4, Me and P04. When this ratio is present, the aforementioned precipitate will appear:

NH⁺ ₄ + MgO + H₃P0₄ + 5 H₂0 -> P0₄MgNH₄, 6H₂0 + H⁺

Reaction 1

This precipitate generally has a density slightly higher than that of water and will have a tendency to settle. With the invention means 20, 22, 24 are provided to keep the precipitate suspended in the liquid in the mixing tank 10 such that the supplied liquid together with the precipitate may be discharged near the top of the mixing tank 10 and flow as a spill-over by gravity to a separation unit 30, through a pipe string 15. For this purpose this pipe string 15 is connected with the mixing tank 10 at a level LI to which the mixing tank 10 is filled and at which the apparatus 1 is run. The term "spill-over" as used herein refers to liquid with precipitate leaving the mixing tank 10 at level LI being collected to flow through pipe string 15. The pipe string 15 may in the simplest form be connected to a discharge opening in the sidewall of the mixing tank 10 at a desired height. By using gravity only no pumps are required and, hence, no encrustations will form in pumps in this part of the apparatus 1. From the separation unit 30 a screw conveyor 45 conveys separated precipitate in the form of sludge further on to a level above the level L2 of liquid in the separation unit 30, to a regenerating and heating unit 60, 62 wherein the ammonium contained in the polluted liquid supplied through string S is released from the precipitate and collected. The screw conveyor 45 is preferably designed to allow for liquid to flow back into the separation unit 30. From the regenerating unit 60 re- generated reagent corresponding essentially to the originally added reagents is preferably returned to the mixing tank 10.

The liquid in the mixing tank 10 is continuously stirred while setting up an up- wardly directed flow Fl of the liquid with the precipitate. Stirring and generation of this flow Fl may result from using stirrer blades of appropriate design, such as through radially extending inclined blades, or by blowing in air bubbles at the bottom of the tank 10. As mentioned, using a pipe string 15 the precipitate/liquid mixture is conducted out of the tank 10, to a level below the level LI, to the separation unit 30 which may by way of example be a settling tank but which preferably is a hydrocyclone and which separates the precipitate from the liquid. The precipitate in the form of sludge is preferably removed from the separation unit 30 by the aforementioned screw conveyor 45 extending into a bottom part of the separation unit 30 and may as required be dried and filled into bags for fertiliser purposes or, preferably, delivered to the regenerating unit 60. In the regenerating unit 60 the precipitate is heated by a heating device 62 to about 70°C. In this way, the precipitate (P0₄MgNH₄, 6H₂0) is transformed into a second product or sludge (MeHP0₄). If MgO is used as metal supply, the following reaction occurs:

P0₄MgNH₄, 6H₂0 + heat -> MgHP0₄ + NH₃ + 6H₂0

Reaction 2

The second product may be used in the first reaction 1 as substitution for P0₄ and metal supplied to the string S/mixing tank 10 and flows out of the regeneration unit 60, falling by gravity into the mixing tank 10. For possible adjustment of pH in the regeneration unit 60, base/acid may be added. Ammonia vapour together with air may be conducted through a pipe 61 to ammonia washer 70 to which sulphuric acid may be supplied and from the bottom of which a fertiliser salt may be taken out.

As will be understood, purified liquid leaves the hydrocyclone 30 through a pipe connection while precipitate is taken out at a bottom part 35 of the hydrocyclone 30. The function of hydrocyclone 30 is to separate the precipitate (P0₄MgNH₄, 6H₂0) from the liquid by means of the centrifugal force induced by the pressure of the liquid that flows to the hydrocyclone 30 by gravity through the first string 15; the hydrocyclone 30 brings the granular precipitate out at the sides of the hydro- cyclone 30 and down into a bottom part or trough 35, where it is concentrated and wherefrom the precipitate is taken out. The function of hydrocyclones is known from other sides; as shown in fig. 1, the hydrocyclone 30 has an outer annular chamber into which liquid with precipitate is introduced tangentially from first string 15, and an inner cylindrical part what which liquid is discharge and which is located above the bottom part or trough 35 in which precipitate settles.

The regenerating unit 60 is preferably made up of a jacket thermal heating unit

62 surrounding a pipe string 65 and may be connected to an ammonium washer 70. The pipe string 40 supplies precipitate to pipe string 65, and air may be sup- plied through a ventilation system. The screw conveyor 45 may extend along the combined length of the two pipe strings 40, 65, terminating near a discharge end

63 of the pipe string 65, located above level LI.

The function of the regenerating unit 60 is, by means of a thermal process to re- generate magnesium and phosphate from the precipitate (P0₄MgNH₄, 6H₂0) produced in the mixing tank 10. Compared with continuous addition of magnesium and phosphate to the mixing tank 10, the purpose of the regenerating unit 60 is to attain a substantial improvement of the economy of the entire process by regenerating magnesium and phosphate from the precipitate (P0₄MgNH₄, 6H₂0) produced in the mixing tank 10 so that it may be reused in the mixing tank 10.

It is preferred that the supply string S is connected with the mixing tank 10 at the top part thereof, and that piping 25 for the supplied polluted liquid extends inside the mixing tank 10 whereby a downward flow F2 of the polluted liquid in the pip- ing 25 will result from the general upward flow Fl of liquid in the mixing tank 10; preferably, the reagent is added to the downward flow Fl of the polluted liquid by the discharge end 63 of pipe 65 being located such that reagent is dispensed continuously into the flow F2 of liquid.

Claims

A method for purifying a liquid containing ammonium as a pollutant, and for subsequent regeneration of a reagent, said method including the steps of: supplying said liquid with said pollutant to a mixing tank (10), to establish a level (LI) of liquid therein, mixing said supplied liquid in said mixing tank (10) with a reagent, such as a granular reagent containing at least one salt containing at least phosphate and a metal, to form a precipitate of an ammonium containing salt, such as struvite in granular form, establishing during said mixing a generally upwardly directed flow (Fl) inside said mixing tank (10) of said liquid and said precipitate, discharging the liquid with said precipitate as a spill-over from said mixing tank (10), supplying said spill-over only by gravity, by a pipe system (15) connected to said mixing tank (10) at said level (LI), to a separation unit (30), separating said precipitate from said liquid, and supplying said separated precipitate to a regenerating unit (60) configured for regenerating said reagent from said separated precipitate by heating and for discharging said ammonium.

The method according to claim 1, comprising the step of conveying said separated precipitate to said regenerating unit using a screw conveyor (45), to a level above a level (L2) of said liquid in said separation unit (30) while allowing for a back-flow of any of said liquid to said separation unit (30).

The method according to claim 1 or 2, comprising the step of delivering said discharged ammonium to a volume of water (70).

The method according to any of claims 1-3, said separation unit (30) being a hydrocyclone, wherein said separated precipitate is collected at a bottom part (35) of said separation unit (30), and wherein a cleaning of said bottom part (35) is performed at regular intervals.

An apparatus (1) for purifying a liquid containing ammonium as a pollutant, and for regeneration of a reagent, said apparatus (1) including a mixing tank (10) with a mixing device (20, 22, 24), a separation unit (30) and a regenerating unit (60), a supply (S) of said liquid being connected to said mixing tank (10), a level (LI) of said liquid being inside said mixing tank (10), said mixing device (20, 22, 24) being for mixing supplied liquid in said mixing tank (10) with a reagent, such as a granular reagent containing at least one salt containing at least phosphate and a metal, to form a precipitate of an ammonium containing salt, such as struvite in granular form, said apparatus (1) being configured for establishing during said mixing a generally upwardly directed flow (Fl) inside said mixing tank (10) of said liquid and said precipitate, where subsequently the liquid with said precipitate is discharged from said mixing tank (10) and supplied to said separation unit (30) for separating said precipitate from said liquid, said regenerating unit (60) being configured for regenerating said reagent from said precipitate by heating said precipitate and for discharging said ammonium, said separation unit (30) being connected with said regenerating unit (60), characterised in said apparatus (1) including a first pipe string (15) connected with said mixing tank (10) at said level (LI) and with said separation unit (30), to allow for said discharge of liquid with said precipitate to occur as a spill-over from said mixing tank (10) at said level (LI), said first pipe string (15) being arranged to provide for a flow of said discharge of liquid with precipitate by gravity only to said separation unit (30).

The apparatus according to the preceding claims, including a second pipe string (65) for supplying said regenerated reagent to said mixing tank (10).

The apparatus according to claim 5 or 6, including a third pipe string (40) connected with a precipitate collecting portion (35) of said separation unit (30), said third pipe string (40) having an internal screw conveyor (45) for conveying said separated precipitate to said regenerating unit (60) and extending upwards from said separation unit (30), said screw conveyor (45) conveying said separated precipitate to a level above a level (L2) of said liquid in said separation unit (30).

The apparatus according to the preceding claim, said screw conveyor (45) extending into said second pipe string (65).

The apparatus according to any of claims 7-8, said screw conveyor (45) allowing for a back-flow of any of said liquid to said separation unit (30).

10. The apparatus according to any of claims 5-9, said regenerating unit including a heating device (62) arranged for heating said separated precipitate, and a vapour discharge (61) for discharging ammonium.

11. The apparatus according to the preceding claim, said heating device (62) comprising a jacket surrounding at a portion of said second pipe string (65) and being supplied with a heating medium.

12. The apparatus according to any of the preceding claims 5-11, said regenerating unit (60) including a heat exchanger (63) receiving said discharged ammonium and providing for a preheating of said separated precipitate.

13. The apparatus according to any of claims 5-12, characterised in that the separation unit (39) includes a hydrocyclone, a bottom part thereof (35) receiving said separated precipitate.

14. The apparatus according to any of claims 5-13 wherein said mixing device (20, 22, 25) comprises a set of paddles or an inlet for pressurized gas arranged adjacent a bottom of said mixing tank (10) and establishing said generally upwardly directed flow (Fl).

15. The apparatus according to any of claims 5-14 wherein said supply (S) is connected with said mixing tank (10) at a top part thereof, piping (25) for said supplied polluted liquid extending inside said mixing tank (10), a downward flow (F2) of said polluted liquid in said piping (25) resulting from said upward flow (Fl), said reagent being added to said downward flow (Fl) of said polluted liquid.