WO1995033690A1

WO1995033690A1 - Phosphate removal from aqueous media

Info

Publication number: WO1995033690A1
Application number: PCT/US1995/006603
Authority: WO
Inventors: George E. Heinsohn
Original assignee: Vanson L.P.
Priority date: 1994-06-08
Filing date: 1995-05-30
Publication date: 1995-12-14
Also published as: AU2603695A

Abstract

A process for removing phosphate from water is disclosed. The process comprises: (1) forming a flocculate by adding an anionic polymer to water containing phosphate, a ferric compound, and chitosan; and (2) separating the flocculate from the water; wherein the weight ratio of chitosan to anionic polymer is 1:1 to 10:1; the weight ratio of chitosan to ferric compound is 1:2 to 1:200; and the weight ratio of chitosan to phosphate is 1:10 to 5:1. The process can be used to remove phosphate from treated effluent before it is discharged to natural waters.

Description

TITLE PHOSPHATE REMOVAL FROM AQUEOUS MEDIA

FIELD OF THE INVENTION

The invention relates to flocculating compositions that remove phosphate from water. The compositions comprise ferric salts, chitosan, and an anionic polymer.

BACKGROUND OF THE INVENTION

Phosphate is an essential nutrient to algae. Discharge of phosphate to natural water, i.e., lakes, ponds, streams, etc., causes an algal bloom that has a detrimental effect on indigenous flora by both blocking sunlight and competing for other nutrients. At the end of the algal life cycle, biodegradation of the excessive algae consumes much of the dissolved oxygen in the water with consequent deleterious effect on both indigenous flora and fauna. To reduce the environmental impact on natural waters, regulatory agencies have dramatically lowered the amount of phosphate permitted in discharged water. Consequently, methods for removing phosphate from treated effluent are required.

In other methods of phosphate removal, such as treatment with alum or with a ferric compound, either alone or in combination with cationic or anionic polymers, the phosphate is converted to an insoluble phosphate salt of the polyvalent metal ion. However, a finely dispersed suspension of very small particles that cannot be readily separated from the purified water is formed. The phosphate remains in the water unless either extremely fine filters, which often produce blinding, are used or extremely long sedimentation times, often measured in days, are allowed. Neither of these techniques is satisfactory in situations in which large volumes of water must be purified, such as in municipal treatment plants. H. Oedegaard et al., Chem. Water Wastewater Treat. II. Proc. 5th Gothenburg Symp., 1992, 97-114, investigated the separation charachteristics of different combinations of coagulants and flocculants for chemical wastewater treatment. Aluminum chloride, ferric chloride, and various polymers were investigated in an effort to reduce separation times.

SUMMARY OF THE INVENTION The invention is a process for removing phosphate from water, the process comprising:

(1) forming a flocculate by adding an anionic polymer to water containing phosphate, a ferric compound, and chitosan; and (2) separating the flocculate from the water; wherein the weight ratio of chitosan to anionic polymer is 1:1 to 10:1; the weight ratio of chitosan to ferric compound is 1:2 to 1:200; and the weight ratio of chitosan to phosphate is 1:10 to 5:1.

DETAILED DESCRIPTION OF THE INVENTION

Chitin is a polysaccharide consisting predominantly of unbranched chains of β-(l - 4)-2-acetamido-2-deoxy-D- glucose (N-acetyl-D-glucosamine). It is found in the shells of crabs, shrimp, and krill, as well as in the exoskeletons of arthropods, such as insects.

Chitosan is prepared from chitin by hydrolysis of some 2-deoxy-2-acetamidoglucose units. The term chitosan is generally applied to copolymers having greater than 65% 2- deoxy-2-aminoglucose monomeric units (i.e., greater than 65% of the acetamidoglucose units have been hydrolyzed). Processes for the conversion of chitin to chitosan are disclosed in Rigby, U.S. Patent 2,040,879, and Peiston, U.S. Patent 4,195,175, incorporated herein by reference. Chitosan may be most conveniently used as a water soluble chitosan salt, such as chitosan acetate. The molecular weight of the chitosan is not critical as long as the resulting solutions are not too viscous for easy handling. Any level of acetylation can be used, provided the chitosan is sufficiently deacetylated to dissolve in a dilute acid. A preferred technique is to dissolve the chitosan in a dilute solution of aqueous acid, such as 1% acetic acid to form the soluble salt. The ferric compound may be any water soluble compound that yields ferric ions when dissolved in water. Because of its excellent performance, ready availability, and low cost, ferric chloride is preferred.

The anionic polymer can be any polymer that affords a polyanion when dissolved in water. Typical polymers include cellulose derivatives, such as carboxymethyl cellulose and carboxyethyl cellulose; polyacrylic acids and copolymers of acrylic and/or methacrylic acid with other monomers; alginates; poly(sodium styrene sulfonate); and functionalized polyacrylamides. Because of their excellent performance and ready availability, anionic polymers based on functionalized polyacrylamide are preferred. These are commonly available from numerous vendors in grades that are approved for use in potable water and in food applications. The preferred weight ratio of chitosan to anionic polymer is from 1:1 to 10:1, preferably 1:1 to 4:1. The weight ratio of chitosan to ferric compound depends on degree of contamination of the water being treated. It can be as low as 1:2 if only phosphate is being removed. However, since ferric compounds form complexes with many types of water contaminants, much higher ratios, up to 1:200, may be necessary if the water is heavily contaminated. If other contaminants are not present in significant amounts, a ratio of 1:20 to 1:100 is preferred The weight ratio of chitosan to phosphate is from 1:10 to 5:1, preferably 1:10 to 2:1.

The ingredients are added to the phosphate containing water. Typically, the water will be at or near neutrality and at ambient temperature. If the water has already been treated by a conventional ferric chloride treatment, the water may already contain ferric compound. Little or no additional ferric compound may be required to attain the necessary ferric compound concentration. The ferric compound is added first, followed by chitosan. Alternatively, these ingredients may be added simultaneously. A convenient method of adding these ingredients is to add a pre-mix comprising chitosan and ferric chloride in the weight ratio of 1:2 to 1:200.

Following addition, these ingredients are thoroughly mixed with the water and then allowed to react for least one minute before introduction of the anionic polymer. The total elapsed time between addition of the chitosan and the anionic polymer is dependent on the mixing equipment employed and can be readily determined by one skilled in the art. A flocculate, comprising ferric compound, chitosan, phosphate, and anionic polymer, is formed. It is easily separated from the water by conventional techniques such as sedimentation and filtration.

In one embodiment, the ferric compound and the chitosan are introduced under conditions of turbulent flow into a pipeline carrying the water to be treated. The anionic polymer is introduced at a point further downstream where the conditions noted above have been met. As soon as the anionic polymer has been mixed with the water/ferric compound/chitosan mixture, flocculation begins and is complete in 2-5 min. The flocculate can be separated from the treated water by conventional techniques, such as sedimentation, filtration, dissolved air flotation, etc.

INDUSTRIAL APPLICABILITY

The process can be used to remove phosphate from treated effluent before it is discharged to natural waters. A major advantage is that the flocculate formed can be easily separated from the treated effluent.

EXAMPLE Example 1 This test was performed at the Harrington, DE, municipal waste water plant. Analyses were performed by Evirocorp, Inc., Harrington, DE.

Influent water containing 4.0 ppm of total phosphorous was treated in a conventional ferric chloride treatment process with sufficient ferric chloride to bring the concentration of ferric chloride to about 100 ppm. A side stream of 25 gallons/min was passed through 120 ft. of 2" i.d. pipe.

Shrimp-derived chitosan with a viscosity of 400 cps (Vanson, L.P., Redmond, WA) was added as a 1% solution in 1% aqueous acetic acid. At the upstream end, chitosan was introduced at a rate sufficient to bring the chitosan concentration to 2 ppm. At the 60 ft mark along the piping, Deartek® 2421 (a medium molecular weight, low charged anionic polymer available from the Deraborn Division of W. R. Grace, 300 Genessee St., Lake Zurich, IL 60047-2458) was added as a 0.5% solution at a rate sufficient to bring the anionic polymer composition to 1 ppm. Hourly samples were collected at the exit of the pipe and allowed to settle for 5 min. The clear liquid was collected and equal volumes of the hourly samples combined to form a daily composite sample. For the two day test period, total phosphorous concentration in the daily composite samples was <0.1 ppm (limit of detection).

Claims

CLAIMSWhat is claimed is:

1. A process for removing phosphate from water, the process comprising: (1) forming a flocculate by adding an anionic polymer to water containing phosphate, a ferric compound, and chitosan; and (2) separating the flocculate from the water; wherein the weight ratio of chitosan to anionic polymer is 1:1 to 10:1; the weight ratio of chitosan to ferric compound is 1:2 to 1:200; and the weight ratio of chitosan to phosphate is 1:10 to 5:1.

2. The process of claim 1 wherein the concentration of phosphate in the water following step (2) is less than 0.1 ppm.

3. The process of claim 1 in which the ferric compound is ferric chloride.

4. The process of claim 1, or claim 3 in which the weight ratio of chitosan to anionic polymer is 1:1 to 4:1; the weight ratio of chitosan to ferric compound is 1:20 to 1:100; and the weight ratio of chitosan to phosphate is 1:10 to 2:1.

5. The process of claim 1, claim 3 or claim 4 in which the anionic polymer is a functionalized polyacrylamide.

6. The process of claim 1, claim 3, claim 4 or claim 5 additionally comprising, before step (1), wherein the water containing phosphate, a ferric compound, and chitosan is formed by adding to the phosphate containing water a pre-mix comprising chitosan and ferric chloride in the weight ratio of 1:2 to 1:200.

7. The process of claim 6 in which the anionic polymer is introduced into a pipeline carrying the water containing phosphate, ferric compound, and chitosan.

8. The process of claim 6 or claim 7 wherein the concentration of phosphate in the water following step (2) is less than 0.1 ppm.

9. A process for removing phosphate from water, the process comprising: (1) treating water containing phosphate with a ferric compound and chitosan;

(2) adding an anionic polymer, and forming a flocculate; and (3) separating the flocculate from the water; wherein the weight ratio of chitosan to anionic polymer is 1:1 to 10:1; the weight ratio of chitosan to ferric compound is 1:2 to 1:200; and the weight ratio of chitosan to phosphate is 1:10 to 5:1.

10. The process of claim 9 wherein the concentration of phosphate in the water following step (2) is less than 0.1 ppm.

11. The process of claim 9 or claim 10 in which the ferric compound is ferric chloride.

12. The process of claim 9, claim 10 or claim 11 in which the weight ratio of chitosan to anionic polymer is 1:1 to 4:1; the weight ratio of chitosan to ferric compound is 1:20 to 1:100; and the weight ratio of chitosan to phosphate is 1:10 to 2:1.