WO2014089796A1

WO2014089796A1 - Method for treating high concentration wastewater such as ro brine

Info

Publication number: WO2014089796A1
Application number: PCT/CN2012/086515
Authority: WO
Inventors: Jianwen ZHAI; Joseph T. Szczepanski; Rihua Xiong
Original assignee: General Electric Company
Priority date: 2012-12-13
Filing date: 2012-12-13
Publication date: 2014-06-19

Abstract

High strength industrial water, including reverse osmosis brine, is treated with a nanofiltration (NF) membrane unit. In one system and process, NF retentate is recirculated to the NF unit through a softener. Feedwater may also flow to the NF unit through the softener. Sodium carbonate is added to the softener to form calcium carbonate precipitates which are removed from the softener effluent by a microfiltration membrane. The hardness of water flowing to the NF unit is reduced only as much as required to avoid scaling in the NF unit. In another system and process, an aqueous solution comprising sulfate is added to feedwater flowing to an NF unit. Sulfate may also be added to wastewater fed to the first system and process.

Description

METHOD FOR TREATING HIGH CONCENTRATION WASTEWATER SUCH AS RQ

BRINE

FIELD

[0001] This specification relates to wastewater treatment and to membrane filtration.

BACKGROUND

[0002] Membranes, such as nanofiltration (NF) membranes and reverse osmosis

(RO) membranes, can be used to treat wastewater to remove salts from the wastewater. However, NF membranes have limited rejection of some salts and do not always provide high removal rates. RO membranes have higher rejection but are prone to scaling or fouling at high concentration factors. Accordingly, RO systems must be operated at limited recovery rates when treating some industrial wastewaters and produce a significant amount of brine. Lime softening and ion exchange may be used to reduce the hardness of membrane feedwater and reduce scaling, but these processes generate a slurry and regeneration waste stream respectively.

INTRODUCTION

[0003] High strength industrial water, including reverse osmosis brine, is treated with a nanofiltration (NF) membrane unit. In a first system and process, NF retentate is recirculated to the NF unit through a softener. Feedwater may also flow to the NF unit through the softener. Lime or soda is added to the softener to form calcium carbonate precipitates which are removed from the softener effluent by a microfiltration membrane. The hardness of water flowing to the NF unit is preferably reduced in the softener as required to avoid scaling in the NF unit.

[0004] In a second system and process, an aqueous solution comprising sulfate is added to feedwater flowing to an NF unit. Permeate from the NF unit may optionally be treated in an RO unit. Sulfate may also be added to wastewater fed to the first system and process.

BRIEF DESCRIPTION OF THE FIGURES

[0005] Figure 1 shows a schematic process flow diagram of a wastewater treatment system having a step of calcium removal. [0006] Figure 2 shows a schematic process flow diagram of a wastewater treatment system having a step of softening with an NF membrane.

DETAILED DESCRIPTION

[0007] Figure 1 shows a wastewater treatment system 10 for treating wastewater

1. Wastewater 1 may have a chemical oxygen demand (COD) concentration of over 100 mg/L or a total dissolved solids (TDS) concentration over 2000 mg/L. The wastewater 1 may also have one or more of hardness, alkalinity and colloids. For example, the wastewater 1 may be: high strength industrial wastewater such as coking wastewater, landfill leachate, or textile plant wastewater; or brine produced by an RO plant such as an industrial wastewater re-use plant.

[0008] The wastewater treatment system 10 has a softener 2, a filter 4, and a nanofiltration (NF) unit 6. In a primary treatment path, the wastewater 1 passes through the softener 2, filter 4 and NF unit 6 in series. The wastewater 1 is fed to the softener 2. The softener 2 produces a softener effluent 3 which is fed to the pre-filter 4. The filter 4 produces a filtrate 5 which is fed to the NF unit 6. The NF unit 6 produces a permeate 7. The permeate 7 may be discharged as the final effluent from the wastewater treatment system 10 or it may be treated further if required to meet any particular discharge or reuse requirements.

[0009] In a first recycle loop, some of a solids steam 9 separated by the filter 4 is returned to the softener 2 directly or by mixing with the wastewater 1. In a second recycle loop, some of the NF concentrate 8 is also returned to the softener 2 directly or by mixing with the wastewater 1 .

[0010] The softener 2 may be a lime or soda softener. The softener 2 includes a mixing tank for receiving the wastewater 1. A precipitant 1 1 , for example soda (sodium carbonate, Na₂C0₃), lime (calcium hydroxide Ca(OH)₂), or a combination of lime or soda and NaOH, are dosed into the tank and mixed with the wastewater 1 . NaOH is used if required to produce a pH of about 10-1 1 in the softener 2. Soda is preferred over lime. In the softener 2, the precipitant 1 1 reacts with hardness in the wastewater 1 to produce precipitates comprising calcium carbonate.

[0011] The precipitates, and other suspended solids in the wastewater 1 , are carried in the softener effluent 3 to the pre-filter 4. The filter 4 is preferably a microfilter with a pore size of about 10 microns or less, preferably 2 microns of less. The filter 4 removes suspended solids from the softener effluent 3. Since the suspended solids include calcium carbonate, the filtrate 5 has less hardness than the wastewater 1. [0012] The filter 4 may comprise a membrane made, for example, of a non-woven fabric or tube. One suitable filter 4 is a One-Pass™ membrane filter sold by GE Water & Process Technologies. These filters use a bundle of candle filters held in a tubesheet within a pressure vessel. The candles have a membrane film made of a 3-dimensional web-like structure made of expanded polytetrafluoroethylene (PTFE). The membrane has a pore size of 0.5 to 1.5 microns. The filter can be backwashed with air or water.

[0013] Optionally, one or more coagulants or flocculants 12 may be added to the softener effluent 3 to help with the operation of the filter 4, for example to reduce fouling, or to help remove very small suspended solids. Recycling a portion of the solids stream 9 produced by the filter 4 to the softener 2 can improve the precipitation and coagulation process.

[0014] The filtrate 5 produced by the filter 4 flows to the NF unit 6. An acid 13, such as H₂S0₄, may be added to the filtrate 5 to reduce its pH, for example to about 9-10. A portion of the NF concentrate 8 is recycled to the NF unit 6 as required to produce a selected recovery ratio or concentration factor. For example, the NF unit 6 may be operated at a recovery of about 60 to 80%, or 65 to 70%, per pass. However, the NF concentrate is recycled through the softener 2 rather than directly to the inlet to the NF unit 6. The NF unit 6 removes most of the COD and bivalent ions such as S04^2", Ca²⁺ and Mg²⁺ to produce permeate 7 with reduced concentrations of these items. The recovery of permeate 7 relative to wastewater 1 may be 95% or more or 98% or more.

[0015] An NF unit 6 can typically filter water with a COD concentration as high as a few thousand ppm. However, with most wastewater 1 CaS04 would cause scaling before the NF unit 6 could reach its COD limit. In this case, CaS04 scaling limits the amount of permeate 7 that can be recovered from the wastewater 1. In the water treatment system 10, softening the recirculating concentrate 8 inhibits CaS0₄ scaling to allow for a higher recovery rate. However, the recirculating concentrate 8 is only partially softened. The wastewater 1 is also only partially softened. Optionally, particularly if the wastewater 1 is not at least near CaS0₄ saturation, the wastewater 1 may bypass the softener 2 and filter 4 to be blended with the filtrate 5. The softener 2 and filter 4 preferably operate such that the filtrate 5, or a blend of filtrate 5 and wastewater 1 bypassing the softener 2, is near the maximum concentration of CaS0₄ that can be tolerated in feedwater to the NF unit 6. For example, the filtrate 5, or filtrate 5 and wastewater 1 blend, may have an ion product of the concentrations of calcium and sulfate ions ([Ca²⁺][S0₄ ^2"]) at about 30-70%, or about 50%, of saturation. The retentate 8 may be saturated or supersaturated with CaS0₄. [0016] For at least some wastewater 1 , the amount of Na₂C0₃ that is required in the wastewater treatment system 10 is less than if the wastewater 1 were fully softened to an extent required to operate the system 10 at the same recovery rate without recycling concentrate 8 to the softener 2. Reducing the amount of Na₂C0₃ and avoiding conventional clarifiers also reduces the amount of waste slurry that is produced.

Providing only small amount of acid 13 inhibits CaC0₃ saturation with a small acid dosing cost while still providing a basic (pH greater than 7) filtrate 5. Filtering a basic filtrate 5 helps avoid fouling in the NF unit 6 if the wastewater 1 contains organic compounds.

[0017] Figure 2 shows a second wastewater treatment system 20 for treating wastewater 1. Wastewater 1 is treated in an NF unit 6. The NF unit 6 removes hardness from the wastewater 1 . However, NF membranes may be more selective for anions than for cations. The hardness removal of an NF membrane can therefore vary with the composition of the feedwater.

[0018] Sulfate concentration is increased in the wastewater 1 by dosing sulfate 22 into the wastewater 1. The sulfate may be provided, for example, in an aqueous solution of sodium sulfate or sulfuric acid. The dosage results in a molar concentration of S0₄ ^2" ions in the wastewater being 1 to 50% or more than the total hardness. The hardness removal efficiency of the NF unit 6 is then approximately equal to the sulfate removal efficiency, which may be 98% or more. The NF unit 6 also removes COD, suspended solids and some salinity. Optionally, the NF permeate 27 may be further treated with an RO unit. Sulfate is added to the wastewater 1 if the molar concentration of sulfate ions is less than the sum of the molar concentrations of Ca²⁺ and Mg²⁺ ions. An amount of sulfate may be added to the wastewater 1 such that the molar concentration of sulfate ions is approximately equal to, for example within 20% of or within 10% of, the sum of the molar concentrations of Ca²⁺ and Mg²⁺ ions. If the molar concentration of sulfate ions in the wastewater 1 already equal to or greater than the sum of the molar concentrations of Ca²⁺ and Mg²⁺ ions, then adding sulfate is not required.

[0019] Sulfate may also be added to wastewater 1 as a pre-treatment before wastewater 1 enters the wastewater treatment system 10 of Figure 1 . In this case, the pre-treated wastewater 1 is preferably blended with the filtrate 5 without first passing through softener 2.

[0020] The wastewater 1 may contain alkalinity as well as hardness. In this case a de-aerator 25 may be added before the NF unit 6 to remove carbon dioxide from the wastewater 1 . [0021] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

CLAIMS:

1. A wastewater treatment system comprising,

a) a lime softening reactor;

b) a microfilter; and

c) a nanofiltration membrane unit,

wherein,

d) concentrate from the nanofiltration membrane unit is recycled to the nanofiltration unit through the lime softening reactor and microfilter.

2. The wastewater treatment system of claim 1 wherein feedwater flows to the nanofiltration unit through the lime softening reactor and microfilter.

3. The wastewater treatment system of claim 1 or 2 having a chemical dosing unit adapted to add sulfate to feedwater flowing to the nanofiltration membrane unit.

4. The wastewater treatment system of any of claims 1 to 3 having a chemical dosing unit for adding a coagulant to effluent form the lime softening reactor upstream of the microfilter.

5. The wastewater treatment system of any of claims 1 to 4 having a recycle loop adapted to return a portion of solids separated by the microfilter to the lime softening reactor.

6. A process for treating wastewater comprising steps of,

a) flowing the wastewater to a nanofiltration membrane unit to produce a permeate and a concentrate;

b) softening the concentrate; and,

c) flowing softened concentrate to the nanofiltration membrane unit.

7. The process of claim 6 wherein step a) comprises softening the feed wastewater.

8. The process of claim 6 or 7 wherein steps a) and c) comprises flowing water at between 80 and 100% of the maximum hardness specified for feedwater to the nanofiltration membrane unit.

9. A process for treating wastewater comprising,

a) adding sulfate to the wastewater; and,

b) after step a), flowing the wastewater through a nanofiltration membrane unit.

10. The process of claim 9 wherein in step a) an amount of sulfate is added to the wastewater such that a molar concentration of sulfate ions in the wastewater is within 20% of a sum of the molar concentrations of Ca²⁺ and Mg²⁺ ions in the wastewater.

1 1 . The process of claim 9 or 10 wherein sulfate is added to the wastewater by adding an aqueous solution of sodium sulfate or sulfuric acid to the wastewater.

12. The process of any of claims 9 to 1 1 further comprising a step of de-aerating the wastewater.

13. The process of any of claims 9 to 12 further comprising steps of softening concentrate from the nanofiltration membrane unit and flowing the softened concentrate through the nanofiltration membrane unit.