WO2001039870A2

WO2001039870A2 - Determination of the efficiency of a filter

Info

Publication number: WO2001039870A2
Application number: PCT/NL2000/000871
Authority: WO
Inventors: Stephan Cornelis Johannes Maria Van Hoof; Franciscus Nicolaas Marie Knops
Original assignee: Norit Membraan Technologie B.V.
Priority date: 1999-11-30
Filing date: 2000-11-29
Publication date: 2001-06-07
Also published as: WO2001039870A3; NL1013701C2; AU2557901A

Abstract

The invention relates to a method for determining the effectiveness of at least one filter (7) for filtration of solids from a flow of liquid (4), substantially comprising of: determining the quantity of solids in the flow (9) behind the filter and, at least immediately prior to determining the quantity, adding to the flow (4) a quantity (5) of at least one known solid, characterized by adding a much greater number of particles of the known substance than the solids present in the liquid in the flow (4) in front of the filter, wherein the quantity to be added (5) is known precisely, such that total numbers of particles prior to filtration can be at least approximately equated to the number of added particles.

Description

Determination of the efficiency of a filter

The present invention comprises a method for determining the effectiveness of at least one filter for filtration of solids from a flow of liquid.

Such a method is generally known, wherein the quantity of solid in the flow is determined prior to filtering with the filter, for instance by counting the particles of these substances, and then doing the same in the flow direction behind the filter. The effectiveness of removal of the filter is found from the ratio between the amount of substance prior to filtration and the amount of substance after filtration.

A drop in the effectiveness of removal of the filter points to a defect in the filter.

A drawback of the known reliability tests for filters is that if there are very few or no solids present in the flow for filtering at the time of a measurement or determination, the comparison for the purpose of assessing the efficiency of a tested filter will be very unreliable. This is a particular problem in the case of membrane filters, for which tests have been developed which are based on maintaining pressure or maintaining a vacuum and/or which are based on diffusion of an air flow. For such tests the membrane filter must however be taken out of operation, which decreases productivity. Such tests moreover do not produce any direct relation between the data measured therein and the filter efficiency.

The present invention has for its object to obviate the above stated drawbacks of the known art, and a method is provided for this purpose which comprises of: determining the quantity of solids in the flow behind the filter and, at least immediately prior to determining the quantity, adding to the flow a quantity of at least one known solid, and which is characterized by adding a much greater number of particles of the known substance than the solids present in the liquid in the flow in front of the filter, wherein the quantity to be added is known precisely, such that total numbers of particles prior to filtration can be at least approximately equated to the number of added particles. The above stated problems of the known art are obviated with the method according to the present invention. For a membrane filter the test can be performed in situ and during operation of an installation in which the filter is incorporated. For filters in general it is the case that the determination can be performed even if at the moment of this determination the quantity of solid in the flow for filtering is very small or even zero.

By adding a much greater number of particles of the known solid than the solids present in the liquid in the flow in front of the filter, wherein the quantity of the added known solid is known precisely, the number of particles prior to filtration can be at least approximately equated to the number of added particles. A measurement of the number of particles in the flow in front of the filter can hereby be dispensed with, whereby only a measurement of the number of particles in the flow behind the filter is necessary.

The filter is preferably a membrane filter. In contrast to the known art, an operational test according to the present invention can be performed with the membrane filter during operation.

The solid preferably comprise particles with dimensions corresponding with micro-organisms. By testing for dimensions corresponding with micro- organisms, which is for instance important in the case of water for consumption, the effectiveness of the filter can be determined in respect of these micro- organisms. The dimensions preferably amount to 1-5 μm. In a practical situation this corresponds closely with the dimensions of micro-organisms.

In a method according to the present invention solid is preferably used wherein the known solid can be filtered without interaction with the filter. Such an interaction is undesirable, since the operation or effectiveness of the filter after the test can hereby be influenced. The solid preferably comprises active carbon.

Active carbon can be readily detected in the flow both in front of and behind the filter, but has no adverse side-effects. This could for instance be important in the testing of drinking-water treatment plants, where a less than optimally functioning filter could allow through a considerable quantity of the active carbon which could be consumed. The active carbon is not hazardous when consumed.

The present invention will be further described hereinbelow on the basis of an embodiment thereof with reference to the annexed figures, in which: fig. 1 is a schematic view of a filter system according to the invention; fig. 2A and fig. 2B show graphs of the number of particles in the flow, respectively in front of and behind a filter, for a properly operating filter, and fig. 3A and fig. 3B show graphs of the number of particles in the flow, respectively in front of and behind a filter, for a defective filter. The filter system 1 shown in figure 1 comprises two stages, i.e. a first stage 2 and a second stage 3, which are schematically separated from each other v/ith a dash- dot line. An inflow 4 is supplied in the first stage 2 of filter system 1. An additive flow 5 is added to the inflow 4 from an additive buffer 6. An inflow 8 with additive is then directed to a filter 7. The inflow 8 with additive is filtered in filter 7, which is for instance a membrane filter. Particles in inflow 4 and additive from inflow 8 with additive are herein filtered. A filtered flow 9 leaves filter 7 and flows to a measuring instrument 10 where the number of particles remaining in the filtered flow 9 can be measured.

The efficiency of the filtration by filter 7, for instance a membrane filter, in the first stage 2 is determined as follows. The logarithmic removal is calculated as:

__10 __n beforefi It rat ion ■. after fil tra tion wherein η is the logarithmic removal of the filtration by filter 7, n_{before flltratlon} is the number of particles in the inflow 8 with additive and n_{after flltratιon} i-^{s tne} number of particles in the filtered flow 9 which can be measured with measuring instrument 10.

The number of particles in additive flow 5 is much greater than that in inflow 4 such that the number of particles in inflow 4 is negligible relative to the particles in additive flow 5. By introducing a precisely determined number of particles from the additive buffer 6 with the additive flow 5 into the inflow 4 in order to obtain the inflow 8 with additive, a determination of the number of particles in inflow 8 with additive is here at least approximately unnecessary. The number of particles in inflow 8 with additive is well known. Arranged in the flow behind filter 7, for instance a membrane filter, is a measuring instrument 10 which is per se well known to the skilled person in the art. Measuring instrument 10 determines n_{after fl}ι_tratlon in respect of filter 7 in the first stage 2. By then calculating the efficiency, wherein the quantity of additive in the inflow 8 with additive added from additive buffer 6 is taken for n_{before flltratlon}, this logarithmic removal can be compared to a previously obtained value or a desired value of this logarithmic removal.

When such a comparison shows that the filter 7, for instance a membrane filter, is not complying with a desired operation or has deteriorated in time in respect of the logarithmic removal thereof, it can be decided to replace or repair filter 7, this in accordance with the type of filter 7 applied and the options for cleaning, repair or replacement thereof. The flow which flows out of measuring instrument 10 is the inflow 11 for the second stage 3 of filter system 1.

The additive in the additive flow 5 from additive buffer 6 is for instance powdered activated carbon (PAC) which has no interaction with filter 7, particularly when it is a membrane filter. It has moreover been approved in accordance with all current standards (D I, KIWA, ATA etc.) and has a negligible risk when consumed. This is particularly important in drinking-water systems. A marked advantage of adding said powdered activated carbon is that the distribution in the size of the particles thereof corresponds closely with the dimensions of micro-organisms which, particularly in the case of drinking-water, have to be filtered out of the inflow 4. Cryptosporidium thus normally has a size of 2- 7 μm and Giarda a usual size of 4-12 μm. It is noted that, depending on the type of solid particles for filtering out of inflow 4, an additive can be used which has a particle size corresponding therewith. The invention is not limited to powdered activated carbon or the stated dimensions. Possible other additives are calcium particles, sludge etc.

The feeding of additive from additive buffer 6 via additive flow 5 to inflow 4 so as to obtain the inflow 8 with additive is a test; this is carried out at regular intervals. The measurement by measuring instrument 10 to determine the number of particles in the filtered flow is however preferably performed continuously in order to detect fluctuations in the number of particles in filtered flow 9. This is already a measure per se for determining whether the filtered flow 9 is fulfilling pre-imposed criteria, for instance for drinking-water. The second stage 3 of filter system 1 as implementation of a method according to the present invention is in many respects the same as the first stage 2 of the filter system; an additive flow 12 is fed from additive buffer 13 to inflow 11, which is the outflow of the first stage 2 of filter system 1, in order to obtain an inflow 15 with additive which is directed to filter 16, whereafter the filtered flow 18 is subjected with the second measuring instrument 17 to a measurement of the number of particles in the inflow with additive, whereafter an outflow 19 is discharged from filter system 1.

The difference between the first stage 2 and the second stage 3 of filter system 1 lies in the addition of measuring instrument 14 in the inflow 11 in front of filter 16 so as to determine here the number of particles in inflow 15 with additive. This is of particular importance when the number of particles in inflow 11, such as micro-organisms, is not negligible relative to the number of particles of a known solid, such as powdered activated carbon (PAC), which is supplied to inflow 11 with the additive flow 12 from additive buffer 13. In such a case use is made in the above stated formula for determining the logarithmic removal of filter 16 of the measurement result of measuring instrument 14.

The measuring instrument 14 can be the same as the measuring instrument 17 in the flow behind filter 16, and both measuring instruments 14 and 17 can be the same as measuring instrument 10 which is arranged in the first stage 2 of filter system 1. The operation of second stage 3 of filter system 1 is otherwise the same as that of first stage 2.

The measuring instrument 17 in the flow behind filter 16, for instance a membrane filter, is preferably also in continuous operation. Measuring instrument 14 can however be rendered inoperative until the moment at which a test of the effectiveness of filter 16 is desirable, i.e. when additive flow 12 is added to the inflow 11 of second stage 3. When inflow 11 contains very few particles, for instance micro-organisms, the effectiveness of filter 16 can be determined with a high degree of accuracy by adding the additive flow 12.

By increasing a number of particles in an inflow which leads to a filter an accurate determination of the efficiency of the filter, and therefore of the effectiveness thereof, can be provided according to the present invention, even if the inflow itself contains only few particles. Also when an inflow contains a considerable number of particles which is not negligible relative to the added flow of a known solid, the contrast between the number of particles in the inflow, increased by the added number of particles in an additive flow, is hereby increased relative to the number of particles in a filtered flow, whereby a more precise determination of the effectiveness of a filter is possible, irrespective of the distribution in the measurement and calculation results.

When the inflow to a filter contains 800-1200 particles per millilitre and the filtered flow contains 0.03-0.05 particles per millilitre, the logarithmic removal as defined above displays a distribution of 4.4 ± 0.2, wherein 4.2 = ¹⁰log (800/0.05) and 4.6 = ¹⁰log (1200/0.03) . By adding 2 x 10" particles/ml the logarithmic removal or the calculated result of removing particles can be increased to between 5.6 and 5.8, which produces an average of 5.7 with a distribution of 0.1. In such a case the first stage 2 of filter system 1 can be applied because the number of particles in inflow 4 is at least approximately negligible relative to the number of particles in additive flow 5. Higher or lower quantities are likewise possible within the scope of the present invention, depending on the measuring capacity of the measuring instruments, in particular measuring instrument 14. Owing to the above described greater accuracy through addition of an additive of a known solid such as powdered activated carbon (PAC from Norit®) , a change in the effectiveness of a filter 7, 16 can be determined with a greater degree of accuracy. The effectiveness of the filter can also be determined even if in the inflov/ thereto there is occasionally a very small number of particles present, such as micro-organisms in the case of drinking-water. More or less additive can be added subject to the desired logarithmic measure of purification of the flow flowing to a filter. The higher the desired logarithmic value, the greater the use of the known solid.

Fig. 2A, 2B, 3A and 3B show the test results for a correctly functioning filter and a filter with a defect. In this case the defect may be a broken fibre in a membrane filter.

Fig. 2A shows the progression of the number of particles in an inflow being carried to a filter. The inflow contains per se about 850 particles/ml, and this is increased at the time of a test to 13800 particles/ml. Fig. 2B shows the response, i.e. the measurement result of n_{a£ter filtration}. It can be seen here that the number of particles in the filtered flow as according to fig. 2B approximately hardly varies. The results of fig. 2B can be obtained with measuring instruments 10 and 17 in fig. 1, while the measurement results of fig. 2A can only be obtained with measuring instrument 14.

The measurement results in fig. 2 relate to a correctly functioning membrane filter. Without the addition of a known solid the logarithmic removal amounts to η = ¹⁰log (850/0.05) = 4.2, but with addition the logarithmic removal amounts to η = ¹⁰log (13800/0.03) = 5.7.

Similar measurement results are shown in fig. 3 for a defective filter, with for instance a broken fibre in the case of a membrane filter.

In fig. 3 the measurement results in the case of measurement without addition of known solid show no difference whatever from the measurement results shown in fig. 2. The calculation of the logarithmic removal is therefore the same and produces once again a value of η = 4.2.

At the time of the test the content of particles in the inflow is increased to 13650, wherein a content of 0.08 particles/ml is detected as being present in the filtered flow. This results in a logarithmic removal of η = ¹⁰log (13650/0.08) = 5.2. This is a marked decrease in the efficiency relative to fig. 2, where a logarithmic removal of 5.7 was achieved in the test with addition. Without the addition of known solid the defect in the filter is therefore not detected, while addition of the known solid results in a marked fall in the logarithmic removal of fig. 3 compared to fig. 2. It can be concluded herefrom that fig. 3 relates to a defective filter. The test without addition does not show such a fall in the logarithmic removal, so that the conclusion that the filter of fig. 3 is defective will not follow.

In figures 2 and 3 the "back-wash" peaks must be ignored when determining the effectiveness. This is a normal phenomenon during operation of the devices, and is carried out for instance at intervals of 15-240 minutes . Application example

The value of applying the method according to the invention becomes particularly apparent when the method is used to monitor a filter specifically installed to remove micro-organisms in drinking-water preparation. For such filters it is of particular importance to be able to test the effectiveness of removal of particles of micro-organism size. The filters are after all installed to protect the population from the presence of these micro-organisms in the drinking-water. A number of governments have recently even adopted new rules wherein the drinking-water producer is obligated to demonstrate the effectiveness of removal of particles the size of bacteria. Furthermore, these filters are preferably not taken out of use for the purpose of testing the integrity of the filter.

One filter, wherein a method according to the present invention has been tested in secret, is situated in the municipality of Keldgate, near Hull, in the United Kingdom. The filter has a net production capacity of 3750 m³/h and provides the local population with drinking-water. The installation consists of eleven separate units, wherein each unit has a gross production capacity of 440 m³/h. Two measurements were performed on each unit. One measurement without addition of active carbon and one measurement with the addition. Both measurements were performed to demonstrate the limitations of the system without application of the present invention. In both cases the number of particles in the unfiltered feed to the installation was counted, as well as the number of particles in the filtrate of the installation.

Table 1 gives an example of typical and representative results of measurements on the above described installation. Forty measurements were carried out during the test, on both the feed side of the filter and the filtrate side of the filter. The averages for both measurements were then calculated, whereafter it was possible to determine the measured effectiveness of removal. It is clearly apparent that the measured effectiveness of removal can be described as low, which can be attributed to the very limited number of particles in the feed suspension of the filter.

Table 1: measurements without addition of particles

The effectiveness of removal can be calculated as follows :

.10 'log befσrefil ra t ion .10- 152

^Jlog =3.01 n afterfil t ra t ion 0.15

For the situation where particles were added to the feed of the filter a series of comparable measurements were performed. It was established beforehand here that the membranes were actually integral. The results are shown in Table 2.

The effectiveness of removal can here also be calculated:

.10 l ₁ og ( , beforefil tra tion _\ ) .10 138358 log ) =5.91 n afte fil tra tion 0.17

It will be apparent that the filter forms virtually an absolute barrier for the added particles. When the installation is supplied with a suspension with many times more particles than in the first experiment, the measurement of the filtrate remains practically the same. The measured effectiveness of removal is however much higher, simply because the measurement in the first: test was unable to perform the measurement properly. Both measurements make it clear however that by applying the present invention the effectiveness of removal of a filter installation can be measured accurately.

It is also apparent from tables 1 and 2 that the chosen quantities of particles added to the feed are so high (compare the second columns relating to particles in feed of both tables 1 and 2) that the initial quantities (before addition) present in the feed (table 1) are negligible relative to these chosen numbers (table 2) . Measurement of quantities of particles in the feed can thus be dispensed with if desired, provided the quantities to be added are well known.

The present invention is in no way limited by the above described embodiment of a filter system as implementation of the method according to the invention, but is limited solely by the appended claims. The measuring instruments 10, 14 and 17 can thus be based on ass-spectrography or comprise their own filtering process. Particle counters are preferably used. A possibly slightly less accurate alternative could be a turbidimeter . The size of the added known solids can correspond with a wide range of micro-organisms in the case of drinking-water, or be based on distribution in the dimensions of only a few, for instance very harmful micro-organisms. The present invention can also be applied in the filtering of inflows with other elements for filtering therefrom, and the invention is not limited to drinking-water. After examination of the foregoing, the skilled person will appreciate that many alternatives or additional embodiments are possible in respect of the above described embodiment, all of which must be deemed as lying within the scope of the present invention, being defined as the new combination of measures particularly as according to the appended main claim.

Claims

1. Method for determining the effectiveness of at least one filter for filtration of solids from a flow of liquid, substantially comprising of: determining the quantity of solids in the flow behind the filter and, at least immediately prior to determining the quantity, adding to the flow a quantity of at least one known solid, characterized by adding a much greater number of particles of the known substance than the solids present in the liquid in the flow in front of the filter, wherein the quantity to be added is known precisely, such that total numbers of particles prior to filtration can be at least approximately equated to the number of added particles.

2. Method as claimed in claim 1, wherein the filter is a membrane filter.

3. Method as claimed in claim 1, wherein the known solid comprises particles with dimensions corresponding with micro-organisms.

4. Method as claimed in claim 3, wherein the dimensions amount to 1-5 μm.

5. Method as claimed in claim 1, wherein the knov/n solid can be filtered without interaction with the filter.

6. Method as claimed in claim 1, wherein the knov/n solid can be applied without risks to health.

7. Method as claimed in claim 1, which comprises active carbon as known solid.