Method of treating aqueous waste and chemical treatment agent therefor

Abstract

A chemical treatment agent for treating aqueous waste containing detergent, said chemical treatment agent comprising a blend of a polyelectrolyte coagulant and calcium chloride in aqueous solution. Th

Description

FIELD OF THE INVENTION This invention relates to method of treating aqueous waste containing spent detergents and to a chemical treatment agent therefor. BACKGROUND OF THE INVENTION In the field of waste water treatment it is known to treat aqueous-based waste water streams using various chemical and physical treatment processes. The chemical treatment of waste water typically comprises three main steps, namely coagulation, neutralisation and flocculation, as follows: •Coagulation - a chemical coagulant is added at low pH to the waste, to eliminate suspended solids; •Neutralisation - pH is increased using an alkali to bring the overall pH in line with a given discharge permit; •Flocculation - a long chain polymer is added to increase weight of any insoluble particles, decreasing their settlement time. Occasionally it is necessary to treat an aqueous waste water stream that has derived from a cleaning process, and as such will contain spent detergent / cleaning agents. This causes an issue for known chemical treatment processes. When adding a chemical coagulant, detergent present in the waste water can interact with the coagulation process, for example, preventing the elimination of negative charge on suspended solids in the waste. This results in the flocculation process not working correctly. Generated floc will be very fine and lightweight and the treated water will not be clarified and settlement times will be longer. SUMMARY OF THE INVENTION In a first aspect, the present invention provides a chemical treatment agent for treating aqueous waste containing detergent, said chemical treatment agent comprising a blend of a polyelectrolyte coagulant and calcium chloride in aqueous solution. The calcium chloride may have a purity of at least 94%. The calcium chloride is provided in prill form and may have a particle size of between 1mm and 5mm before addition to the solution. The polyelectrolyte may be selected from one of a cationic, nonionic or anionic agent. In one embodiment the polyelectrolyte may comprise an anionic polyacrylamide in aqueous solution. The polyelectrolyte may be provided in solid form and dissolved in water at a concentration of between 2g / l and 4g / l. In a preferred embodiment the polyelectrolyte is dissolved in water at a concentration of 3g / l. The concentration of calcium chloride in the blend may be between 5g / l and 120g / l, more preferably between 98g / l and 102g / l, added to the selected polyelectrolyte in aqueous solution. Preferably the calcium chloride is dissolved into the polyelectrolyte solution. In a second aspect there the present invention provides a method of treating aqueous waste containing detergent comprising the steps of passing the aqueous waste into a reaction vessel and adding a chemical treatment agent thereto in accordance with the first aspect of the invention. Preferably the calcium chloride in the chemical treatment agent is dissolved into the aqueous polyelectrolyte solution before the chemical treatment agent is added to the aqueous waste. The chemical treatment agent may be added to the aqueous waste in the reaction vessel by means of a dosing pump. The problem address by the present invention is the reduction in the effectiveness of a polyelectrolyte coagulant when treating a waste stream containing a detergent in a reaction vessel or batch reactor. This problem is solved by creating a blend of calcium chloride and polyelectrolyte flocculant in aqueous solution prior to adding the blend to the reaction vessel. Adding the calcium chloride to the polyelectrolyte solution before it is added to the reaction vessel has been found to greatly increase treatment efficiency, while reducing the amount of calcium chloride otherwise required to deal with detergent, and also reduces or eliminates the need for additional alkali reagents after treatment. The calcium chloride used is preferably 94-97% purity. The calcium chloride may be supplied in prill form before being dissolved into the polyelectrolyte solution. Particle size of the prills may vary from 1 -5mm. A prill is a small pellet of a material, most often a dry sphere, typically formed by spray crystallization. “Prilled” is a term used in mining and manufacturing to refer to a product that has been pelletised. When treating aqueous waste containing spent detergents, the spend on calcium chloride added directly into a reaction vessel in order to deal with detergent in a waste water stream using known techniques is typically in the region of £62k per annum. When using a blend comprising a polyelectrolyte coagulant and calcium chloride in aqueous solution in accordance with the present invention, this additional cost can be reduced to around £1500 due to the large reduction in the amount of calcium chloride required. DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION A method of treating aqueous waste containing spent detergents in chemical treatment facility (CTF) in accordance with a preferred embodiment of the present invention comprises the following processes: Coagulation Waste water can have insoluble material dispersed throughout which may be in suspension, colloidal dispersion or as inert material than can readily settle out. Colloids and suspended solids will not settle out by themselves and will be kept afloat in the waste water due to electrostatic effects or simply through Brownian motion. To allow settlement of this material, any net charge of the particles must be eliminated to facilitate agglomeration and thus end in successful settlement. This is achieved through the use of a coagulant, such as Aluminium Sulfate, Ferric / Aluminium Sulfate (FAS) or Ferric Chloride. A preferred coagulant is Ferric Chloride, added to the waste water in a treatment reactor of the CTF. Neutralisation The above coagulation process occurs at a low pH to increase the concentration of positively charged ions in solution, increasing the likelihood of the negatively charged suspended solids having their net charge reduced to zero and thereby promoting agglomeration and settlement. Discharge consent via a discharge permit issued to the operator of the CTF authorises the discharge of effluent (i.e. treated water) at a pH between 5 and 10. Therefore a neutralisation step must be performed to bring the pH of the treated water into consistency with the discharge permit. Varying the pH also has the benefit of causing any insoluble metals present in the waste water to precipitate out as hydroxide salts. For example, Chromium Hydroxide precipitates optimally at a pH of 7.5 and is readily soluble at pH values lower than this, whereas, at a higher pH, it will re-dissolve into solution. To create insoluble hydroxide salts, a neutralisation step is performed, preferably using hydroxide-based reagents, such as Sodium, Potassium, Calcium and Magnesium Hydroxides, added to the treatment reactor. Sodium Hydroxide may be used as the most financially viable reagent for completing this neutralisation step. A chemical flocculant in the form of a polyelectrolyte in aqueous solution is added to the treatment reactor. Polyelectrolytes are long chain organic polymers often having molecular weights in excess of one million and are either natural or synthetic in origin. The term “polyelectrolyte” includes polymers which, by some ion-producing mechanism, can become converted to a polymer molecule having electrical charges along its length. The electrical charges arise from the presence of ionisable functional groups along the polymer chain. Polyelectrolytes are, therefore, polymeric-electrolytes (i.e. having characteristics of both polymers and electrolytes). A high molecular weight polyelectrolyte agglomerates microfloc, increasing its size and weight, thereby increasing its settling ability and decreasing overall settlement time, facilitating shortening the overall treatment time and increasing plant throughput. A multitude of polyelectrolytes may be used in various embodiments of the treatment method of the present invention, such as cationic (positively charged) nonionic (neutral) and anionic (negatively charged) agents. The choice of polyelectrolyte can be selected depending on the waste water being treated. In accordance in accordance with the present invention, before it is added to the treatment reactor, the polyelectrolyte solution contains a predetermined amount of calcium chloride to allow the treatment process to deal with problematic waste streams containing detergents, eliminating the need for secondary treatment. The polyelectrolyte calcium chloride blend can also assist in the neutralisation of the waste water in the treatment reactor. In one embodiment the polyelectrolyte may comprise an anionic polyacrylamide, such as that sold under the trade name Flopam AN 910, supplied in solid form and dissolved in water at a concentration of 3g / l. The concentration of calcium chloride in the solution may range from 5g / l to 10Og / l. It has been found that a concentration of 10Og / l gave the best performance out of the blends tried, but potentially doping calcium chloride at higher concentrations may provide further benefits. Settlement & Clarified Water Discharge Sufficient time is given to allow the floc to settle to the bottom of the treatment reactor. This can range in time from as little as 15 minutes to several hours for problematic waste streams. Once settlement is complete, the treated “clarified” water is sampled and analysed to ensure compliance, before discharge through a sand filter (a final polishing phase which can capture any errant solids kicked up during discharge) to holding tanks, before transfer off-site. Sludge Dewatering Once the treated water has been discharged from the treatment reactor, the remaining floc (now a residual sludge) is pumped to a dedicated sludge holding tank, and may be fed through a filter press. The filter press captures any solids in the sludge, forming plates of filter cake. The water separated from the solids in the filter press may be recycled back into the system. The water may be is transferred to a general rinses holding tank, and combined with distillate from a vacuum evaporator. This mix of clean water can be used to treat other incoming effluent. Problematic waste streams can be successfully treated using this new chemical blend of polyelectrolyte and calcium chloride. Water clarification can be improved, eliminating the need for secondary treatment, which saves time and reagent cost. Floc settlement time is increased, which speeds up overall treatment time. This increases the treatment capacity of the process. A greater tonnage of waste can be treated over a given 24 hour period, with faster settlement times. The previously discussed issues with treating detergent-contaminated waste were such that it was easier to eliminate the detergent via a separate process before the remaining waste water could be processed through the CTF. This waste type would have to leave our site to go to a different facility for treatment, usually costing more than being able to deal with it in-house. Now, using the new polyelectrolyte blend, we can treat previously-rejected waste ourselves, at a lower cost per tonne than before. As previously stated, problematic detergent containing waste water streams can sometimes be improved by directly adding Calcium Chloride to the reactor, or using an alternative alkali. However, using the polyelectrolyte blend of the present invention requires significantly less Calcium Chloride than directly charging solid calcium chloride into the batch reactor. This saves on reagent costs. It also eliminates the need to switch to the more expensive alkali for the neutralisation step, further reducing treatment costs. The present invention provides the following benefits: 1. Increased treatment efficiency, eliminating time spent waiting on the solid prills to dissolve in the reactor; 2. Reduced manual labour costs - operators do not have to manually add 25kg bags of calcium chloride to the reactor; 3. Improved health &safety - calcium chloride is pre-solubilised and addition to the reactor can be automatically controlled via software and dosing pumps. This reduces potential spills of dry product on addition to the reactor and ultimately reduces the operators contact time with dry product. The invention is not limited to the embodiment described herein but can be amended or modified without departing from the scope of the present invention as defined by the following claims.

Claims

1. A chemical treatment agent for treating aqueous waste containing detergent, said chemical treatment agent comprising a blend of a polyelectrolyte coagulant and calcium chloride in aqueous solution.

2. A chemical treatment agent as claimed in claim 1, wherein said calcium chloride has a purity of at least 94%.

3. A chemical treatment agent as claimed in claim 1 or claim 2, wherein said calcium chloride is provided in prill form before addition to the aqueous solution.

4. A chemical treatment agent as claimed in claim 3, wherein said calcium chloride has a particle size of between 1mm and 5mm.

5. A chemical treatment agent as claimed in any preceding claim, wherein the polyelectrolyte is selected from one of a cationic, nonionic or anionic agent.

6. A chemical treatment agent as claimed in any preceding claim, wherein the polyelectrolyte comprises an anionic polyacrylamide.

7. A chemical treatment agent as claimed in claim 6, wherein the polyelectrolyte is provided in solid form and dissolved in water at a concentration of between 2g / l and 4g / L8. A chemical treatment agent as claimed in any preceding claim, wherein the concentration of calcium chloride in the blend is between 5g / l and 120g / l.

9. A chemical treatment agent as claimed in any of claims 1 to 7, wherein the concentration of calcium chloride in the blend is between 98g / l and 102g / l.

10. A chemical treatment agent as claimed in any preceding claim, wherein the calcium chloride is dissolved into the polyelectrolyte solution.

11. A method of treating aqueous waste containing detergent comprising the steps of passing the aqueous waste into a reaction vessel and adding a chemical treatment agent thereto in accordance with any of claims 1 to 9.5 12. A method as claimed in claim 10, wherein the calcium chloride in the chemical treatment agent is dissolved into the polyelectrolyte solution before the chemical treatment agent is added to the aqueous waste.

13. A method as claimed in claim 12, wherein the chemical treatment agent is 10 added to the aqueous waste in the reaction vessel by means of a dosing pump.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:-CLAIMS1. A method of making a chemical treatment agent for treating aqueous waste containing detergent, said chemical treatment agent comprising a blend of a polyelectrolyte coagulant and calcium chloride in aqueous solution, wherein said calcium chloride has a purity of at least 94%, said calcium chloride being provided in prill form before addition to the aqueous solution and wherein said calcium chloride has a particle size of between 1 mm and 5mm.

2. A method as claimed in any preceding claim, wherein the polyelectrolyte is selected from one of a cationic, nonionic or anionic agent.

3. A method as claimed in any preceding claim, wherein the polyelectrolyte comprises an anionic polyacrylamide.

4. A method as claimed in claim 6, wherein the polyelectrolyte is provided in solid form and dissolved in water at a concentration of between 2g / l and 4g / L5. A method as claimed in any preceding claim, wherein the concentration of calcium chloride in the blended aqueous solution is between 5g / l and 120g / l.

6. A method as claimed in any of claims 1 to 4, wherein the concentration of calcium chloride in the blended aqueous solution is between 98g / l and 102g / L7. A method as claimed in any preceding claim, wherein the calcium chloride is dissolved into the polyelectrolyte solution.

8. A method of treating aqueous waste containing detergent comprising the steps of passing the aqueous waste into a reaction vessel and adding a chemical treatment agent made in accordance with the method of any of claims 1 to 7.

9. A method as claimed in claim 8, wherein the calcium chloride in the chemical treatment agent is dissolved into the polyelectrolyte solution before the chemical treatment agent is added to the aqueous waste.CM10. A method as claimed in claim 9, wherein the chemical treatment agent is added to the aqueous waste in the reaction vessel by means of a dosing pump.