WO1999002239A1

WO1999002239A1 - Method and apparatus for treating water

Info

Publication number: WO1999002239A1
Application number: PCT/US1998/013904
Authority: WO
Inventors: Jene N. Vance
Original assignee: Molten Metal Technology, Inc.
Priority date: 1997-07-08
Filing date: 1998-07-07
Publication date: 1999-01-21

Abstract

Water is treated by adding a polyelectrolyte and a filter aid to the water. The introduction of these additives permits the particulates within the water to agglomerate with one another and to agglomerate with the filter aid. The water is then directed through a filter, where the agglomerated particulates are filtered out. The filter is then backflushed with a fluid to remove the agglomerated particulates. In accordance with one aspect of this invention, the amount of polyelectrolyte added to the water is determined by a controller which enters measurements from a turbidity sensor and a conductivity sensor into an algorithm to determine the amount of polyelectrolyte needed to aggregate a targeted amount of particulates in the water stream.

Description

METHOD AND APPARATUS FOR TREATING WATER

BACKGROUND OF THE INVENTION

Water used in industrial plants, including waste water from industrial plants, must often be treated to remove a majority of the insoluble particulate contaminants before the water can be used in the plant or before it can be released to the environment. Typically, particulate contaminants are removed from the water by a filtration process. Common features of most filters are that (1) as the filter removes particles from the water it will become plugged, preventing the passage of water and (2) the filter media must either be cleaned or replaced. A goal of filtration processes is to achieve the desired effluent clarity while minimizing the cleaning and/or replacement frequency of the filter media. This goal is particularly important in filtration processes in liquid radioactive waste treatment systems where the costs for disposing of the filter media as a radioactive waste are large.

The ease or difficulty of removing particles from water by a filtration process is typically dependent upon: (a) the characteristics of the filter media; (b) the characteristics of the particles; (c) the particle size distribution; and (d) the interaction of the particles with soluble chemicals in the water. Soluble ions will be adsorbed onto the surface of small particles, causing an electrical charge on the surface of the particles and thereby preventing a natural agglomeration of particles. Without natural agglomeration, the particles are smaller in size and larger in number. As a result, filter media used to filter non-agglomerated particles must have smaller pore sizes than would be required if the particles were agglomerated. Further, the filtration of non-agglomerated particles causes the filter media to plug faster, necessitating an increase in the cleaning or replacement frequency of the media.

Experience has also shown that, because filtered particles generally have irregular shapes and surface charges, particles having a relatively small mass will adhere to the filter media, making it difficult to remove them in a backwash cleaning step from most types of filter media.

Therefore, a need exists for a method of filtering water that overcomes or minimizes the above-mentioned problems .

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of one embodiment of filtration apparatus of the invention that is suitable for practicing the method of the invention.

Figure 2 is a cross-sectional view of a filter suitable for practicing the method of the invention.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus for filtering particulates from water to purify the water for further use or for discharge to the environment .

According to a method of this invention, water is filtered by adding a polyelectrolyte and a filter aid to the water. The water is then directed through a filter, where the agglomerated particulates are filtered out. The filter is then backflushed with a liquid and/or gas to remove the agglomerated particulates from the filter.

In another method of this invention, the rate at which the polyelectrolyte is added to a water stream is determined by measuring the conductivity and turbidity of the water stream. The conductivity and tubidity of the water stream is then correlated with an amount of polyelectrolyte needed to aggregate a targeted amount of particulates, and this amount of polyelectrolyte is added to the water stream.

An apparatus of this invention includes a water conduit for transporting a water stream. Conductivity and turbidity sensors at the water conduit are provided to measure the conductivity and turbidity, respectively, of the water stream. Both sensors are electronically coupled with a controller. The controller is also electronically coupled with a means for adding a polyelectrolyte to the water stream at a location downstream from the conductivity and turbidity sensors. Downstream from the means for adding a polyelectrolyte, a backflushable filter is provided for filtering particulates from the water stream.

The introduction of the polyelectrolytes and filter aid using the method and apparatus of the invention causes particulates within water to agglomerate. Preferably, the polyelectrolyte is added to the water first. By waiting at least about 10 seconds, the polyelectrolyte is allowed to bind to the particulates and to cause aggregation. The filter aid is then added to the water. Over the course of the next 10 or more seconds, the filter aid agglomerates with the aggregated particulates forming larger particles. The relatively-large agglomerated particles are then separated from the water by passing the water through a filter.

In accordance with one aspect of the invention, the filter includes a pair of backflushable cartridges. Where two backflushable cartridges are used, the water is first passed through a comparatively coarse filter and then through a comparatively fine filter such that the comparatively fine filter can be reserved exclusively for fine-particle screening. In accordance with another aspect of the invention, the polyelectrolyte is added from a polyelectrolyte addition tank to the conduit which transports the water stream. The rate at which the polyelectrolyte is added is governed by a controller which determines the amount of polyelectrolyte needed to aggregate a targeted amount of particulates within the water stream based on the turbidity and conductivity of the water stream.

This invention provides numerous advantages. As noted, the addition of the polyelectrolyte and the filter aid to the water stream before the water is filtered causes agglomeration of the particulate matter in the water. The larger particle size of the agglomerated particles enables the use of filters with larger pore sizes than would otherwise be required to produce the same water clarity in the filter effluent. In short, the agglomeration of particles causes the particles to be trapped at the entrance to relatively-large pores through which the individual particulates which comprise the agglomerate would otherwise readily flow. The use of relatively-large- pore-size filter media for the removal of the particulates and filter aid materials also enables longer run lengths, as measured by throughput in gallons between backwash cycles, and more effective backwashes to clean the filter media. Over a range of pore sizes, the agglomerated particles, because of their aggregate size, will be removed from the water in greater percentages than if left untreated. Further, the agglomeration of particles will cause fewer particles to penetrate into the pores and become fixedly trapped therein. With less clogging, the useful life of the filter is extended. The step of backwashing, in conjunction with particle agglomeration, further allows for repeated reuse of the filter. If greater clarity of effluent is required, a second, finer filter may be used in series with the coarser filter. The finer, downstream filter will experience longer run lengths because the coarse, upstream filter can be used to remove the majority of insoluble particulates.

With the methods of this invention, the run length of filtration will generally decrease with increasing body feed concentration until an optimal concentration is reached. Where two filters are used in accordance with this invention, the run length of the coarse, upstream filter will decrease with increasing body feed concentration, while the run length of the fine, downstream filter will increase. The run length of the fine filter increases because the addition of the polyelectrolyte and body feed according to the method of this invention causes many of the fine particulates to agglomerate and be trapped by the coarse filter, leaving a cleaner effluent to be passed through the fine filter. Accordingly, the shorter filter run lengths produced (in at least the upstream filter) by the methods disclosed herein demonstrates that the addition of body feed, as disclosed, advances the objective of increasing the amount of particulates trapped within the filter.

DESCRIPTION OF PREFERRED EMBODIMENTS

The features and other details of the method of the invention will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. Numbers that appear in more than one figure represent the same item. For example, the representation 10/12 in Figure 2 corresponds to either of items 10 or 12 in Figure 1. It will be understood that the particular embodiments of the invention are shown by way of illustration and not as limitations of the invention. The principle features of this invention can be employed in various embodiments without departing from the scope of the invention.

The term "polyelectrolyte" used in the present invention refers to polymeric organic compounds which are soluble in water and have a plurality of positive charge sites in order to form a bond with the insoluble particulates and the filter aid material, and thereby offset the negative charge found on most insoluble particulates and on the filter aid materials. By offsetting the negative surface charge on the particulates and the filter aid materials, natural agglomeration is promoted. Such compounds are normally referred to as organic cationic polyelectrolytes . These polyelectrolytes are well known in the art and a variety are commercially available. Examples of such polyelectrolytes include polyalkylene imines, polyalkylene polyamines, polyvinylbenzyl quaternary ammonium salts, polyvinylbenzyl tertiary amines, vinylbenzylsulfonium polyelectrolytes, etc. It should be understood that this list is not exhaustive and that other cationic polyelectrolytes can be employed.

The term, "filter aid material, " is used to refer to those materials that are conventionally used in filtration processes to aid the filtration performed by the filter. Such filter aid materials are often referred to as "body feed" materials and a filtration process using such materials is referred to as "body feed filtration." Most such materials are characterized by a negative surface charge in an aqueous medium. Examples of suitable filter aid materials include cellulose fibers, diatomaceous earth, charcoal expanded perlite, asbestos fibers, polyacrylonitrile fibers and the like. One embodiment of an apparatus of this invention is illustrated as apparatus 10 in Figure 1. Water to be treated by the method and apparatus of the invention is supplied to line (conduit) 12 from source 14. The turbidity and conductivity of the water in line 12 can be measured from grab samples or by conductivity sensor 16 and turbidity sensor 18. An example of a water source that is suitable for treatment by the method and apparatus of this invention is waste water collected from floor drains, such as waste water collected from floor drains at a nuclear power plant.

During a filter run, a suitable polyelectrolyte and a suitable body feed material are directed from addition tanks 20 and 22, respectively. Raw polyelectrolyte is added to polyelectrolyte addition tank 20 and suspended in water. Metering pump 24 is used to direct the suspended polyelectrolyte through line 26 into the water stream in line 12. The polyelectrolyte is mixed into the water stream in line 12 by mixer 28 and, typically, a residence time of ten seconds or more is instituted after the injection of the polyelectrolyte into the water. Preferably, the residence time is at least 20 seconds. The requisite residance time is reached during transport through piping 30 downstream from mixer 28. Other devices, such as a baffled vessel, can also be employed to achieve a preferred residence time. The optimum polyelectrolyte dosage is defined as that dosage which produces the highest water effluent clarity after filtration. The optimum polyelectrolyte dosage will vary according to the particular water source and will depend, in part, upon the chemical composition and concentration of particulate material in the water to be treated. The pump rate of metering pump 24 is controlled to achieve a preferred polyelectrolyte dosage in the water to be treated that approximates the predetermined optimum polyelectrolyte dosage.

Preferably, the optimum polyelectrolyte dosage for any specific application is approximated empirically by preliminary bench-scale jar or filter testing. The particulate concentration within the water stream is indicated by the turbidity of the water stream. The charge on the particulates is indicated by the water stream's conductivity. The preferred polyelectrolyte dosage for agglomerating a given particulate composition is empirically related to both the turbidity and the conductivity of the water stream to be treated by the method of the invention. Through regression analysis, an algorithm is fit to determine the relationship between the preferred polyelectrolyte dosage and the turbidity and conductivity of the water stream over a range of measured turbidity and conductivity values. The equation for the algorithm has been shown to take the following form:

Polyelectrolyte dosage = (a) (C^b) (T^c) ,

where dosage is measured in parts per million (ppm) and,

a,b,c = constants

C = conductivity (μmho/cm) T = turbidity (NTU) ,

wherein "μmho" is micromhos and "NTU" is nephelometer turbidity units . During treatment of water according to the method of the invention, conductivity and turbidity are measured during the filtering process by means of conductivity sensor 16 and turbidity sensor 18, these measured values are fed to controller 32 which enters the measured values into the predetermined algorithm, using the previously estimated constants, to estimate the optimum polyelectrolyte dosage for the specific water stream being filtered. Controller 32 then regulates metering pump 24 to adjust the polyelectrolyte addition to obtain the estimated optimum dosage. The polyelectrolyte acts to destabilize any existing colloids, thereby allowing the colloids to agglomerate and form larger particles.

Following a sufficient polyelectrolyte residence time, as described above, body feed material is injected from tank 22, preferably at a substantially constant rate, by second pump 34 through line 36. Addition of body feed has at least three purposes: (1) to provide for additional agglomeration sites for the particulate material in the water to be treated; (2) to increase filter run lengths by maintaining porosity in the filter; and (3) to provide a protective layer on the filter to increase the performance of the filter following backwashing. The concentration of body feed in the water stream in piping 38 is expected to be in a range of between about 10 and about 20 ppm, with a dosage of about 15 ppm being adequate for most applications. The ultimate effect of body feed concentration can vary depending on the nature of the solids being filtered. After the body feed is injected into the water stream to be treated, second mixer 40 disperses the body feed throughout the stream. The preferred residence time for the combined body feed and polyelectrolyte typically is at least about 10 seconds and is achieved during transfer through piping arrangement 38. Upon leaving piping arrangement 38, the water stream is directed through inlet 42 of coarse filter 44 to outlet 46, and then through line 48 to inlet 50 of fine filter 52. Coarse filter 44 acts as a roughing filter, removing the bulk of particulate matter. Fine filter 52 acts as a polishing filter. Preferable absolute pore sizes for coarse filter 44 generally fall in a range of between about 10 and about 50 microns (μm) , while the preferable range of pore sizes for fine filter 52 generally is between about 0.45 and about 5 microns (μm) . Both coarse filter 44 and fine filter 52 most preferably include backflushable polypropylene cartridge filters 54, with absolute pore sizes for coarse filter 44 and fine filter 52 of about 20 microns (μm) and about 1.4 microns (μm) , respectively. A set of backflushable filter cartridges 54 within filter 44/52 is illustrated in Figure 2. Examples of suitable cartridge filters are SEPTRA® filters, commercially available from Pall Corp. (East Hills, New York) . Filter 44/52 corresponds to either coarse filter 44 or fine filter 52, both of which are shown in Figure 1. Backflushable cartridge filters 54, shown in Figure 2, are supported upon tube sheet 56. Tube sheet 56 separates vessel head 58 from the upstream side of filter cartridges 54. When filtering, water flows into filter 44/52 through inlet 42/50. The water is forced through filtration cartridges 54 into vessel head 58 of filter 44/52. The water exits vessel head 58 through outlet 46/60. Treated water is discharged from apparatus 10 through outlet 60 of fine filter 52 and then through line 61.

Backflushing is performed typically when differential pressure transmitter 62 or 64, shown in Figure 1, measures a difference in pressure across filter 44 or 52 that is greater than a predetermined maximum limit. The significance of the pressure differential is that it reflects the amount of clogging within the filter. Air receiver tank 66 is charged with pressurized air (nominal 125 psig) . To backwash, valves, not shown, at inlets 42/50 and at outlets 46/60 are closed. Water is drained from filter 44/52 by opening additional valves, also not shown, at outlets 72/74. The valves at outlets 72/74 are closed and vessel head 58 within filter 44/52 is filled with water from water source 67. Valves at outlets 72/74 are opened and backwashing occurs by releasing compressed air from air receiver tank 66 through lines 68, 70. Water is driven out of vessel head 58, shown in Figure 2, forcing air and water backward through filter 44/52, knocking the particulate material off the filter 54 and through outlets 72, 74 respectively. In one embodiment, the backflush is directed to a suitable liner, such as a POWDEX/ECODEX liner, for dewatering of the particulate materials. After backwashing, filters 44/52 are ready for reuse. The invention now will be further and more fully described by the following examples.

EXEMPLIFICATION

EXAMPLE 1

The following test was conducted in the radioactive water waste processing facility of Byron Nuclear Power

Plant (Byron, IL) . In the plant, a waste water collection tank was isolated after filling so that no water was added to the tank nor drained from the tank during the test period, except that required for the testing of the filter media. Flexible tubing was connected to a sample connection on the tank recirculation piping to supply waste water to the small-scale filter equipment. A tee connection was provided in the flexible tubing from the sample connection for the injection of BETZ® 1175 polyelectrolyte into the waste stream. A 25 foot length of the flexible tubing was provided before a second tee was installed in the tubing for the injection of ECOCOTE® (regenerated cellulose) filter aid material, commercially available from Graver Chemical Co. This 25 foot length of tubing provided a residence time of 20 seconds for the polyelectrolyte to interact with the contaminant particles in the waste water. A 12 foot length of tubing was installed between the second tee and the filter. This provided a residence time of 10 seconds for the contaminant particles to interact with the filter aid material. A backflushable pleated filter cartridge with a microporous media of 6 micron (6 μm) absolute pore size was installed in the filter equipment. The waste water was filtered through the filter at a specific area flow rate of 0.5 (gal . /min. ) /ft² until an 8 psi pressure drop was indicated by the system pressure gauges. The total water filtered was measured by an inline flow meter and totalizer. Turbidity measurements were taken on grab samples during the test. For the first test series, no polyelectrolyte and no filter aid materials were injected. When the pressure drop reached 8 psi, the filter was backwashed by forcing water backwards through the filter using 115 psig air stored in an air chamber connected to the filter outlet piping. This backwash cycle was repeated three times before the filter was returned to service. The filter was placed back into service and the procedure was repeated. The results of this test are given in the following table:

6μm -- No Polyelectrolyte and No Filter Aid Material Influent Turbidity = 12 NTU

Backwash Cycle Total Gallons Filtered 1 410 2 60 3 30 4 30 5 32 6 24 7 19 The turbidity on the filter effluent for the above test was measured as 3.7 nephelometer turbidity units (NTU), which is not considered a high clarity water. These results are indicative of filtration processes with a rapid drop off in gallons filtered as the filter media becomes increasingly plugged because of the inability to effectively clean the filter media by the backwash.

EXAMPLE 2

A second test was conducted using the same waste water and test conditions, except that a backflushable filter cartridge with a microporous media of twenty micron (20 μm) absolute pore size was installed in the filter equipment, and 16 ppm of polyelectrolyte and 15 ppm of filter aid materials were injected during the filter tests in accordance with this invention. The operating and backwash procedures were identical to those described above for the 6 micron filter test. The results of the second test are given in the following table:

20 μm — Polyelectrolyte and Filter Aid Materials Used Influent Turbidity = 12 NTU

Backwash Cycle Total Gallons Filtered 1 245 2 229 3 234 4 210 5 198 6 222 7 206 203 262 The effluent turbidity for this test was measured as 1.3 NTU. These results show that the present invention will produce a higher clarity effluent, longer run lengths and greatly improved recoverability of the filter following the filter backwash.

EXAMPLE 3

A third test was conducted using the same waste water and test conditions except without the use of a polyelectrolyte and without the use of a filter aid material. For the third test, a backflushable pleated cartridge filter with a microporous media of one micron (1 μm) was installed in the filter equipment. This was done in an attempt to produce an effluent clarity similar to the 20 μm test with the polyelectrolyte and filter aid material in order to provide a valid comparison in terms of the suspended solids removal capability. The results of the third test are given in the following table:

1 μm — No Polyelectrolyte and No Filter Aid Material Influent Turbidity = 12 NTU

ackwash Cycle Total Gallons Filtered

1 6.3

2 4.0

3 3.2

4 2.8

The filter effluent was 1.1 NTU which makes this filtering process comparable to the 20 μm test in terms of the effluent clarities. However, as shown, the run lengths were extremely short and the recovery of the filter was poor with an immediate decline in the filter run lengths after the initial test.

EXAMPLE 4

In a second commercial nuclear power plant (Dresden Nuclear Station; Morris, IL) , a bench-scale test was conducted to demonstrate the improved run lengths and filter effluent clarity by the application of the present invention. Four liters of waste water were collected for the testing. The influent turbidity of the sample was 93 NTU. The sample was split into two 2-liter samples. Bag filter media with a 2.5 μm absolute rating was loaded into the pressure filter holder. The sample was pumped at a rate of 3.4 gpm/sq. ft. through the filter. The pressure drop across the filter was recorded along with the filter run time. The filter effluent was collected to measure the turbidity of the effluent. The filter ran a total of 4 minutes before the pressure drop reached 25 psi which was the selected endpoint for the test. The effluent turbidity was 33 NTU. The filter was replaced with a new bag filter media.

Polyelectrolyte was added to the waste water at a dosage of 5 ppm. The polyelectrolyte was allowed to mix for about one minute. Next, ECOCOTE® filter aid material was added at a dosage of 20 ppm. This mixture was allowed to mix for about one minute and was then pumped through the filter at the same flow rate of 3.4 gpm/sq. ft. The run time and pressure drop were recorded. The run time at a pressure drop of 25 psi was 53 minutes. The effluent turbidity was 0.35 NTU. These results demonstrate the dramatic improvement in the filter run lengths caused by the agglomeration process of the present invention. EXAMPLE 5

The following description is provided as an illustration of the development of the algorithm for determining the optimum polyelectrolyte dosage. The technique involves a laboratory filter apparatus that includes a peristaltic pump, tygon tubing and a pressure filter holder,

To develop an algorithm for the optimum dosage, several samples of the water to be filtered were collected at different times. The turbidity and conductivity of each sample was then measured using standard laboratory instruments. An approximation of the optimum polyelectrolyte dosage for each sample was determined using the laboratory filter apparatus. Once the turbidity, conductivity and optimum polyelectrolyte dosage were known for each sample, a regression analysis was performed to derive the empirical mathematical relationship, which serves as the algorithm.

The technique used to estimate the optimum polyelectrolyte dosage in this example involved subdividing each of the samples into a number of smaller volumes and conducting a series of filter tests with different dosages of polyelectrolyte added to the sub-volumes. Specifically, a 2-liter sample of the water to be filtered was sub- divided into 300 ml sub-volumes for each test series. The test series included a first run with no polyelectrolyte addition as a base case. In subsequent filter runs, the polyelectrolyte was added in increasing dosages. In each of these subsequent runs, the polyelectrolyte was added at the specified dosage and allowed to mix with the water for one minute. At the end of the minute, the pump was started and the water was pumped at a flux rate of 3.4 gpm/sq ft (45 ml/min) through the filter media which was a 2.5 μm bag filter media. The pump was allowed to run for four minutes. At the end of four minutes, a small sample (30 ml) was collected from the filter effluent and a turbidity measurement was made on the collected sample using laboratory turbidity instrumentation. For each test run, new filter media was placed in the filter holder. Over the course of the test series, a range of dosage values and a corresponding range of turbidity values were produced. The filter test run with the lowest measured effluent turbidity was defined as the optimum polyelectrolyte dosage.

One such test series was performed on drain waste water from the LaSalle Nuclear Plant floor. The conductivity of the waste water sample was measured at 691 μmho/cm, while the turbidity of the sample was measured at 40 NTU. The results of the test series are as follows:

Polyel. Dosage (ppm) Effluemt Turbidity (NTU)

0 3.90

2 1.40

4 0.80

8 0.76

12 0.48

16 0.55

20 0.80

The optimum dosage for this sample is 12 ppm.

A similar procedure was conducted on several samples of liquid radioactive waste from the LaSalle County Nuclear Station (IL) and Dresden Nuclear Station (Morris, IL) on samples collected from both stations over a period of two months. The results of the optimum polyelectrolyte dosage determinations for these samples are as follows:

al Tur- Init . Conductiv- Optimum y (NTU) ity (μmho/cm) Dosage (ppm)

1 40 85 4 2 96 5 .5 3 60 68 3 4 63 40 2 5 32 98 6 6 8 200 8 7 10 218 9

2.6 3 1

9 31 200 10 10 10 254 12 11 11.4 300 10 12 93 450 9 13 75 36 1.2 14 40 691 12

Logarithms were taken of the turbidity, conductivity and dosage data and were input to a multiple regression analysis with dosage as the dependent variable. Where dosage = (a) (C^b) (T^c) , as shown above, the results of the regression analysis are as follows:

a constant = 0.62 b constant = 0.622 c constant = -0.256

The correlation coefficient, r², measuring the fit between the data and the algorithm, was 0.985. Thus, the algorithm developed for the liquid radioactive waste water from the LaSalle and Dresden Nuclear Stations is as follows:

Polyelectrolyte Dosage = (0.62) (c⁰-⁶²²) (T^"°-²⁵⁶) .

EQUIVALENTS

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention to be encompassed by the following claims.

Claims

CLAIMSI claim:

1. A method for treating water by filtering particulates from the water, comprising the steps of: a) adding a polyelectrolyte to the water to be filtered; b) adding a filter aid to the water; and thereafter c) directing the water through a filter; and d) backflushing the filter after the water has passed through the filter.

2. The method of Claim 1, wherein the polyelectrolyte is added to the water to be filtered before the filter aid is added to the water.

3. The method of Claim 1, wherein the water is directed through a filter that includes a pair of backflushable cartridges, the pair including a first filter and a second filter, the first filter having a greater absolute pore size than the second filter, and the water being directed through the first filter before being directed through the second filter.

4. The method of Claim 2, wherein the polyelectrolyte and the water to be filtered are mixed for about 10 seconds before the step of adding the filter aid to the water.

5. The method of Claim 2, wherein the filter aid is mixed with the water to be filtered for about 10 seconds before the step of passing the water through the filter.

6. The method of Claim 2, wherein: a) the polyelectrolyte and the water to be filtered are mixed for about 10 seconds before adding the filter aid to the water; and b) the filter aid is mixed with the water to be filtered for about 10 seconds before passing the water through the filter.

7. A method for treating a water stream by filtering particulates from the water stream, comprising the steps of: a) adding a polyelectrolyte to the water stream; b) adding a filter aid to the water stream; c) directing the water stream through a filter at a location downstream from where the polyelectrolyte and the filter aid were added; and d) backflushing the filter with a fluid.

8. The method of Claim 7, wherein the water stream is a waste stream.

9. The method of Claim 8, wherein the polyelectrolyte is added to the water stream at a location upstream from where the filter aid is added to the water stream.

10. The method of Claim 7, wherein the water is directed through a filter that includes a pair of backflushable cartridges, the pair including a first filter and a second filter, the first filter having a greater absolute pore size than the second filter, and the water stream being directed through the first filter before being directed through the second filter.

11. The method of Claim 9 wherein the polyelectrolyte is mixed with the water stream for at least about 10 seconds before adding the filter aid to the water stream.

12. The method of Claim 9 wherein the filter aid is mixed with the water stream for at least about 10 seconds before directing the water through the filter.

13. The method of Claim 7 wherein: a) the polyelectrolyte and the water stream are mixed for at least about 10 seconds before adding the filter aid to the water stream; and b) the filter aid is mixed with the water stream for at least about 10 seconds before directing the water stream through the filter.

14. A method for controlling a rate at which a polyelectrolyte is added to a water stream, comprising the steps of: a) measuring conductivity of the water stream; b) measuring turbidity of the water stream; c) correlating the conductivity and the turbidity of the water stream with an amount of the polyelectrolyte needed to aggregate a targeted amount of particulates within the water stream at the measured conductivity and turbidity; and d) adding the polyelectrolyte in the amount correlated to achieve the targeted amount of aggregation of particulates within the water stream.

15. The method of Claim 14 further including the step of adding a filter aid to the water stream after adding the polyelectrolyte to the water stream.

16. The method of Claim 15, further including the step of passing the water stream through at least one filter after adding the polyelectrolyte and the filter aid to the water stream.

17. An apparatus for adding a polyelectrolyte to a water stream at a preferential rate comprising: a) a water conduit for transporting a water stream; b) a conductivity sensor at said water conduit for measuring the conductivity of the water stream; c) a turbidity sensor at said water conduit for measuring the turbidity of the water stream; d) a controller electronically coupled with both the conductivity sensor and the turbidity sensor; e) a means for adding the polyelectrolyte to the water stream downstream from the conductivity sensor and the turbidity sensor, the means for adding polyelectrolyte being electronically coupled with and controlled by the controller; and f) at least one backflushable filter at the water stream downstream from the means for adding the polyelectrolyte to the water stream.

18. The apparatus of Claim 17, further including a means for adding a filter aid to the water stream upstream from the backflushable filter.

19. The apparatus of Claim 18, wherein the backflushable filter includes a first backflushable cartridge and a second backflushable cartridge, wherein the second backflushable cartridge has a finer absolute pore size than the first backflushable cartridge, and where the second backflushable cartridge is positioned downstream from the first backflushable cartridge.