WO2014207198A1

WO2014207198A1 - An apparatus for treating raw water by microbial nitrification

Info

Publication number: WO2014207198A1
Application number: PCT/EP2014/063690
Authority: WO
Inventors: Peter Borch Nielsen
Original assignee: Krüger A/S
Priority date: 2013-06-28
Filing date: 2014-06-27
Publication date: 2014-12-31
Also published as: DK179159B1; DK201670031A1

Abstract

The present invention provides an apparatus for treating raw water by microbial nitrification. The apparatus comprises a filter inserted in a fluid flow path with a flow direction from an inlet to an outlet. The filter comprises a porous filter material and biomass. The apparatus further comprises a phosphorus dosage device configured to dose a phosphorus containing compound to the water before the filter inlet based on a control signal significant for the efficiency of the microbial nitrification, and comprises a return flow path for returning at least a portion of the water for further treatment. The apparatus additionally comprises a control unit to determine the control signal. The invention further provides a method for treating raw water.

Description

AN APPARTUS FOR TREATING RAW WATER BY MICROBIAL NITRIFICATION

Field of the invention

The present invention relates to an apparatus for treating raw water by microbial nitrification in a filter comprising a porous filter material and biomass. The invention further relates to a method of treating raw water by microbial nitrification.

Background of the invention

Raw water which may be used as drinking water and process water is traditionally purified groundwater or surface water. Often the water contains ammonium in too high

concentrations. To reduce the amount of ammonium porous filters are often used, in which ammonium is oxidised into oxidation products by use of biomass in microbial processes by the overall reaction:

NH₄ ⁺ + 0₂ + C + P + micronutrients --> N0₃ ^" + biomass

However, to sufficiently reduce the amount of ammonium, water can additionally or alternatively to the microbial processes be chlorinated so that ammonium is converted into nitrogen by chemical oxidation.

To reach the required low level of ammonium, chlorine is dosed so that a chlorine residue is present in the treated water. This chlorine residue does however add an aftertaste to the treated water, and may produce unwanted traces of carcinogenic compounds in the water.

Description of the invention It is an object of embodiments of the invention to provide an improved apparatus for treating raw water by microbial nitrification, and an improved method for treating raw water.

It is a further object of embodiments of the invention to provide an apparatus and a method which reduce or even prevent the need for chlorination for ammonium removal.

According to a first aspect, the invention provides an apparatus for treating raw water by microbial nitrification, the apparatus comprising : - a filter inserted in a fluid flow path with a flow direction from an inlet to an outlet, the filter comprising a porous filter material and biomass;

- a phosphorus dosage device configured to dose a phosphorus containing compound to the water before the filter inlet based on a control signal; - a return flow path for returning at least a portion of the water for further treatment; and

- a control unit configured to determine the control signal significant for the efficiency of the microbial nitrification.

In the context of the present invention "raw water" should be understood as water to be treated to be used as drinking and/or process water, i.e. water having a sufficiently high quality to be used as drinking water and/or used as process water in processing plants.

In the context of the present invention "microbial nitrification" should be understood as processes in which ammonium is oxidised to different oxidation products, such as nitrate.

If the phosphorus content is below a predetermined minimum level, the nitrification may stop or at least be running at a very low speed. Surprisingly, it has been found that by adding a phosphorus containing compound to the water before treatment and by keeping the flow through the filter above a predefined threshold value, the biological nitrification can be improved whereby chlorination can be considerably deceased or even avoided.

Thus, the control signal being significant for the efficiency of the nitrification may be based on the amount of phosphorus in the water after the filter. Alternatively, the control signal may be determined based on the amount of phosphorus in the water after the filter by use of an alternative measure being significant for the amount of phosphorus in the water after the filter. The alternative measure may be a nitrification efficiency signal which may be generated manually or automatically based on a direct measurement of the ammonia content in treated water after the filter outlet and/or based on a measurement of the biologically removed ammonia by measurement of the dissolved oxygen content in the treated water after the filter effluent. As it can be determined by the biological nitrification reaction, about 3.6 mg/l oxygen is used per mg/l ammonia transferred to nitrate biologically.

I.e. the control signal may be based on the amount of at least one of phosphorus, ammonia, and dissolved oxygen in the water after the filter. In order to provide information to the control unit, the apparatus may comprise a measuring device configured to measure an amount of at least one of phosphorus, ammonia, and dissolved oxygen in the water after the filter.

Due to the return flow path, it becomes possible to fine-tune the water treatment by returning at least a portion of the treated water to the inlet to keep the volumetric loading of the filter above the predefined threshold value. Increasing the water velocity in this way may minimize the depth of the stagnant water film around the biofilm, which may reduce the diffusion way for ammonia, oxygen, carbon, and nutrients to the biomass so that the nitrification in the filter becomes more effective at lower concentrations of these compounds. This way of optimising the environment for the nitrifying biomass by changing the content of phosphorus and providing a return flow has proven more efficient and flexible than the alternative of increasing or decreasing the size of the filter, and particularly, it allows for an easier change in the phosphorus content by changing the portion of water in the return flow and may minimize the necessary addition of phosphorous as it is easier transported by diffusion to the biofilm.

The biomass may be microorganisms in a biofilm which may be built up on the outer surface of the particles of the porous filter material, i.e. a biofilm comprising microorganisms which can remove ammonium by nitrification. The biofilm may comprise a layer of iron hydroxides in which the biomass/microorganisms are situated. This layer may also comprise other inorganic constituents, such as Mn, Ca, and Mg. The biomass may be added to the filter. Alternatively, the biomass may grow naturally in the filter.

The control signal may be determined based on a manual or an automatic measurement in the treated water after the filter outlet.

In one embodiment, a water sample is collected manually and the content of at least one of phosphorus, ammonia, and dissolved oxygen is determined in this sample. Alternatively, the content may be determined directly in the treated water by a measurement carried out after the filter.

In another embodiment, the apparatus comprises a measuring device arranged in the flow path after the filter to determine the amount of at least one of phosphorus, ammonia, and dissolved oxygen. This measuring device and the control unit may form a single unit whereby the measuring device may be configured to generate the control signal. This control signal may automatically be transmitted to the phosphorus dosage device which may subsequently dose a phosphorus containing compound to the water before the filter inlet based on this signal. Alternatively, the control signal may manually be read into the phosphorus dosage device.

Dependent on the use of the water after treatment, it may be an advantage if the

phosphorus content does not exceed a predetermined maximum level. This maximum level may be based on different regulations. Thus, the phosphorus dosage device may be configured to dose the phosphorus containing compound so that the phosphorus content at the filter outlet is in the range of 0.02-0.15 mg P per litre.

The phosphorus dosage device may be configured for continuously dosing of a phosphorus containing compound in response to the control signal, so that the dosage is continuously adapted in response to this signal.

As mentioned above, the control signal may be based on the amount of phosphorus in the water directly or indirectly by the nitrification efficiency signal, and may in addition be based on a measurement of dissolved oxygen. Thus, if the ammonia content is below the required level or the dissolved oxygen content is above a required level no phosphorous is dosed. If ammonia is above a required level or dissolved oxygen is below a required level,

phosphorous dosing is initiated. The dosing may be manually or automatically adjusted so the measured content of phosphorous in the filter effluent, i.e. in the water after the filter, is within a predetermined range. Phosphorous dosing may be continued until the ammonia and/or dissolved oxygen content in the effluent falls below the required level again. The phosphorus dosage device may be configured for discontinuously dosing of a phosphorus containing compound, so that dosing of the phosphorus containing compound is stepwise whereby an amount of the phosphorus containing compound is dosed and added to the water before the filter based on the control signal. The next dosing of the phosphorus containing compound may then be added after a predetermined time period which period may also depend on the control signal, whereby the steps may be of different lengths. Alternatively, the time periods may be of equal length. In the latter embodiment, the amount of the phosphorus containing compound dosed may vary.

The return flow path may return a portion of the treated water before the filter so that the portion of water being returned is fully filtered. In one embodiment, the return flow path returns the water between the phosphorus dosage device and the filter. In an alternative embodiment, the return flow path returns the water before the phosphorus dosage device.

In a simple embodiment, the return path can be opened or closed so the portion of water being return is either zero or a predetermined portion being non-variable. In another embodiment, the portion of water being returned is variable so that it can be zero, a maximum amount, or one or more different amounts in between. The portion of water being returned may be manually varied, e.g. by opening or closing a valve or controlling the speed of a return pump, or may alternatively be automatically varied based on different

parameters, such as phosphorus content, flow of untreated water, content of other particles in the untreated water, such as iron, etc.

A higher filter velocity may result in a more efficient nitrification, as a higher filter velocity may reduce the thickness of a water film around the biofilm on the porous particles so that phosphorus may be more efficient transported to the biomass. Thus, the filter velocity may be in the range of 2-30 m³ water per m² filter per hour, such as 5-30, 10-30, or 20-30 m³/m² per hour, where the amount of water is the total amount, i.e. untreated water plus the amount of water being returned for retreatment.

To facilitate a high and more efficient filtering of the water and thus a more efficient nitrification, the portion of water being returned may be adjusted based on a predetermined filter velocity, thereby achieving a filter velocity which is above a predetermined threshold value. In one embodiment this threshold value may be a substantially constant filter velocity. Thus, in periods with a low flow rate of untreated water to the filter, a higher portion of treated water may be returned for further treatment to ensure a high filter velocity.

In one embodiment, a depth of the porous filter material is in the range of 0.5-3 metres. In the content of the present a filter depth should be understood a size of the filter in the flow direction. The depth of the porous filter material may depend on the origin of the raw water, the amount of water to be treated, the actual number of filters, etc.

The porous filter material may comprise particles having an effective size in the range of 0.4- 4 millimetres. The particles may be quartz sand, anthracite coal, limestone, expanded clay, activated carbon, or other similar particles of natural or human made origin.

It should be understood, that the apparatus comprises at least one filter, but may comprise two ar more filters arranged in parallel or in series or as a combination of both.

The apparatus may further comprise an iron dosage device arranged in the flow path. This iron dosage device may be configured to dose dissolved iron to the water before the filter inlet. This may facilitate accumulation of a stabile iron hydroxides layer on the particles of the filter material, in which iron oxide-hydroxide layer the biomass/microorganisms may exist. The layer may additionally comprise other inorganic constituents, such as Mn, Ca, and Mg. The control unit may be configured to control dosage of dissolved iron. In an alternative embodiment, the dosage of dissolved iron may be controlled by a separate control unit.

To avoid clogging of the filter e.g. due to biofilm growth and accumulation of different impurities, and thus avoid decreased efficiency, the filter may comprise a backwash structure for reversing the flow direction of the filter thereby enabling that water is forced through the filter in a direction opposite to the flow drirection from the inlet to the outlet. The apparatus may comprise a nephelometer configured to determine a turbidity of the water after the filter, so that backwash can be controlled based on the determined turbidity. The control unit may be configured to control backwash. In an alternative embodiment, backwash may be controlled by a separate control unit.

According to a second aspect, the invention provides a method for treating raw water by microbial nitrification, the method comprising the steps of:

- providing a filter in a fluid flow path from an inlet to an outlet, the filter comprising a porous filter material and biomass; - providing a phosphorus dosage device in the flow path, the phosphorus dosage device being configured to dose a phosphorus containing compound to the water before the filter inlet;

- providing a control unit configured to determine a control signal significant for the efficiency of the microbial nitrification;

- returning at least a portion of the water for further treatment; and - controlling dosage of the phosphorus containing compound to the water before the filter based on the control signal.

It should be understood, that the method for treating raw water by microbial nitrification may be used in connection with the apparatus for treating raw water according to the above- described first aspect of the invention may. Thus, the features of the first aspect of the invention may be applicable in relation to the method for treating raw water according to the second aspect of the invention.

It should be understood, that the raw water may be pre-filtered to remove solid particles before the raw water is treated according to the above method. An additional pre-filtering step may depend on the origin of the raw water. Brief description of the drawings

Embodiments of the invention will now be further described with reference to the drawings, in which:

Fig. 1 schematically illustrates an embodiment of an apparatus for treating raw water; Fig. 2 illustrates a porous filter particle with biomass; and

Fig. 3 illustrates a part of the porous filter material and biomass.

Detailed description of the invention

It should be understood that the detailed description and specific examples, while indicating embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Fig. 1 illustrates an apparatus 1 for treating raw water by microbial nitrification. The apparatus comprises a filter 2 inserted in a fluid flow path with a flow direction from an inlet 3 to an outlet 4. The flow direction is indicated by the arrow 5. The filter 2 comprises a porous filter material 6 and biomass 7 (see Fig. 2).

The apparatus 1 further comprises a phosphorus dosage device 8 which is configured to dose a phosphorus containing compound to the water before the filter inlet 3 based on a control signal being significant for the efficiency of the microbial nitrification, i.e. a signal which represents the amount of phosphorus and/or ammonia and/or dissolved oxygen in the water after the filter outlet 4. The control signal is determined by the control unit 9 based on a measurement of the amount of the phosphorus and/or ammonia and/or dissolved oxygen in the treated water after the filter outlet 4.

Additionally, the apparatus 1 comprises a return flow path 10 for returning at least a portion of the water for further treatment. The amount of water returned may be variable and is controlled by the pump 11. The pump 11 may be in communication with the control unit 9 to facilitate control of the amount of water to be returned.

In the illustrated embodiment, the apparatus 1 comprises a nephelometer 12 which is configured to determine a turbidity of the water after the filter 2, so that backwash can be controlled based on the determined turbidity. The nephelometer 12 may likewise be in communication with the control unit 9 to facilitate control of the backwash.

The depth of the porous filter material is in the range of 0.5-3 metres. The depth is illustrated by the arrow d. The porous filter material 6 comprises particles having an effective size in the range of 0.4-4 millimetres, the effective size being illustrated by the arrow 13 in Fig. 2.

Fig. 2 illustrates a particle 6' of the porous filter material. The biomass 7 is microorganisms in a biofilm 14 which is built up on the outer surface of the particle 6'. The biofilm 14 comprises a layer of iron hydroxides and/or other inorganic constituents, such as Mn, Ca, and Mg, in which the biomass/microorganisms 7 are situated. A constant thickness water film 15 surrounds the biofilm 14.

Fig. 3 illustrates a part of the porous filter material 6 comprising a number of particles 6'. In the cavities between the particles 6' are small chunks 16 of iron oxide-hydroxide.

Claims

1. An apparatus for treating raw water by microbial nitrification, the apparatus comprising:

- a filter inserted in a fluid flow path with a flow direction from an inlet to an outlet, the filter comprising a porous filter material and biomass; - a phosphorus dosage device configured to dose a phosphorus containing compound to the water before the filter inlet based on a control signal;

- a return flow path for returning at least a portion of the water for further treatment; and

2. An apparatus according to claim 1, wherein the control signal is determined from a measurement in the flow path after the filter.

3. An apparatus according to claim 1 or 2, wherein the control signal is based on the amount of at least one of phosphorus, ammonia, and dissolved oxygen in the water after the filter.

4. An apparatus according to any of the preceding claims, further comprising a measuring device configured to measure an amount of at least one of phosphorus, ammonia, and dissolved oxygen in the water after the filter.

5. An apparatus according to any of the preceding claims, wherein the phosphorus dosage device is configured to dose the phosphorus containing compound so that the phosphorus content at the filter outlet is in the range of 0.02-0.15 mg P per litre.

6. An apparatus according to any of the preceding claims, wherein the phosphorus dosage device is configured for continuously dosing of the phosphorus containing compound.

7. An apparatus according to any of claims 1-5, wherein the phosphorus dosage device is configured for discontinuously dosing of the phosphorus containing compound.

8. An apparatus according to any of the preceding claims, wherein the return flow path returns the treated water before the filter.

9. An apparatus according to any of the preceding claims, wherein control unit adjust the portion of water being returned based on a predetermined filter velocity.

10. An apparatus according to any of the preceding claims, wherein a depth of the porous filter material is in the range of 0.5-3 metres.

11. An apparatus according to any of the preceding claims, wherein the porous filter material comprises particles having an effective size in the range of 0.4-4 millimetres.

12. An apparatus according to any of claims, further comprising an iron dosage device arranged in the flow path, and wherein the control unit is configured to control dosage of dissolved iron to the water before the filter inlet.

13. An apparatus according to any of the preceding claims, wherein the filter comprises a backwash structure for reversing the flow direction of the filter.

14. An apparatus according to claim 13, further comprising a nephelometer configured to determine a turbidity of the water after the filter, wherein the control unit controls backwash based on the determined turbidity.

15. A method for treating raw water by microbial nitrification, the method comprising the steps of:

- providing a filter in a fluid flow path from an inlet to an outlet, the filter comprising a porous filter material and biomass;

- providing a phosphorus dosage device in the flow path, the phosphorus dosage device being configured to dose a phosphorus containing compound to the water before the filter inlet;

- providing a control unit being configured to determine a control signal significant for the efficiency of the microbial nitrification;

- returning at least a portion of the water for further treatment; and

- controlling dosing of the phosphorus containing compound to the water before the filter based on the control signal.