WO1995009818A1

WO1995009818A1 - Method of wastewater treatment using a chemical precipitation-microfiltration-nanofiltration hybrid system

Info

Publication number: WO1995009818A1
Application number: PCT/KR1994/000133
Authority: WO
Inventors: Chung Hak LEE; Hyun A PARK; Sang Ho Lee
Original assignee: Doosan Glass Co., Ltd.
Priority date: 1993-10-06
Filing date: 1994-10-05
Publication date: 1995-04-13
Also published as: JPH09504223A; KR950011344A

Abstract

A method of wastewater treatment comprising the steps of passing wastewater through a neutralization and/or chemical precipitation unit (1), passing the effluent from the chemical precipitation unit (1) through a microfiltration unit (2), and passing the effluent from the microfiltration unit (2) through a nanofiltration (3) unit, whereby the constituents to prevent the reuse of wastewater are removed from the concentrated wastewater containing inorganic ions is an effective, economical and simplified zero-discharge system which has the advantages that the flux decrease due to membrane fouling is prevented, the quality of treated water is improved and the washing frequency is increased.

Description

METHOD OF WASTE WATER TREATMENT

USING A CHEMICAL PRECIPITATION-MICROFILTRATION-

NANOFILTRATION HYBRID SYSTEM

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to a method of removal of the constituents which inhibit the reuse of waste water. More particularly, the present invention relates to a method of waste water treatment using a hybrid system of chemical precipitation, microfiltration, and nanofiltration to remove the constituents which inhibit the reuse of waste water.

Description of Related Arts

Waste water discharged from various industries, especially glass industry, is of strong acid which contains a large amount of fluorine and heavy metals and causes a serious environmental pollution when it is discharged without treatment. Although this waste water is discharged after treatments such as some neutralization or dilution to decrease the concentration of toxic compounds, these compounds are concentrated in land or increase the water pollution to consequently become harmful to human. Accordingly, environmental regulation about the discharge of waste water becomes more and more stringent.

However, even though the environmental regulation becomes more stringent treated waste water causes to pollute water or land so long as it is discharge. Accordingly, it is not a fundamental solution of the pollution problems. The way to fundamentally prevent the pollution is not to discharge the treated effluent into water but to reuse the treated effluent after the treatment using zero-discharge system. Especially, waste water that is neutralized and chemically precipitated by chemicals such as lime has been discharged after dilution to conform the water quality criteria. However it is not a solution for reducing the total weight of discharged contaminants. To overcome this problem, instead of discharging the water treated by chemical precipitation, it is necessary to develop the technique that such treated water is recycled to process water to prevent the contaminants from discharging outside and to reduce the total needed amount of industrial water.

However, because waste water treated by chemical precipitation has high supersaturation of inorganic salts, the precipitates are accumulated in tanks and pipes when treated water is used as process water. It is also inappropriate to use waste water treated by chemical precipitation as washing water because the residue of treated waste water after evaporation decreases the quality of products. Accordingly, in order to reuse the treated water as process water it is necessary to reduce the concentration of constituents which prevent the reuse of treated water as process water.

The various methods including ion-exchange, electro dialysis, reverse osmosis have been used to reduce the concentration of constituents. When the waste water pretreated by chemical precipitation is introduced to these apparatuses, these apparatuses are overloaded to decrease the water quality and efficiency because the influent that is pretreated by chemical precipitation has high concentration of total dissolved solids (TDS) . Accordingly, it is required to provide additional apparatuses so that the processes become complicated and the costs increase.

In the meantime, a nanofiltration is the technique to separate large size materials from water by allowing water to pass the membrane by pressure, which can almost separate materials having the radius of more than 10^"9 m from water. That is, nanofiltration is the improved method of reverse osmosis that may filtrate most of constituents except water to treat a large quantity under lower pressure and it is also a separation technique of low energy and apparatus cost. Such a nanofiltration is not overloaded by treated waste water even in case that the concentration of total dissolved solids (TDS) is comparatively high, because it has higher removability of bivalent ions that cause the precipitation than any other ions. Therefore, the waste water treated by nanofiltration has high quality and does not require additional treatment to allow the process simple, effective, and stable independent of the lo%wed volume.

However, the ion removal technique using nanofiltration membrane has a serious problem to decrease the flux by membrane fouling. If flux is low, the process has a critical drawback in the economical aspect because the volume to be treated per a constant time is decreased. In case that the retentate is continuously recycled into the nanofiltration unit to obtain a maximum permeate from a constant influent water the flux decrease become more serious and an obstacle to practical use.

U.S. Patent No. 4,994,882 and U.S. Patent 4,981,594 have disclosed a zero-discharge system using nanofiltration. U.S. Patent No. 4,944,882 in the title of "Hybrid membrane separation system" discloses the system to combine any of the solvent removal processes such as reverse osmosis, nanofiltration, membrane distillation, pervaporation and the like with any one or more of the solute removal process for example, pervaporation, supported liquid membrane, coupled transport membrane, membrane contactor and the like to separate the solute from the solvent.

U.S. Patent No. 4,981,594 in the title of "Waste water purification system" discloses the hybrid system of a sand filtration unit, a disinfection unit and a nanofiltration unit to purify the waste water from cooling tower. However, in this system the flux is decreased due to membrane fouling because the retentate from nanofiltration is recycled directly into the module. To prevent the problem of the flux decrease, it has been suggested to add antiscalent chemicals. However, when this antiscalent chemicals are used to zero-discharge system, these are accumulated in the waste water, which is a critical drawback to the efficiency of chemical precipitation process.

Also, the above described two systems are not directed to separation of volatile organic solvents and solutes so that they can not remove supersaturated salts and solids which are the constituents inhibiting the reuse of waste water. Accordingly, these two systems can not decrease ion concentrations of permeates and can not prevent under zero-discharge system the decrease of the flux due to chemical precipitation on the surface of nanofiltration membrane, either.

SUMMARY OF INVENTION

The inventors of the present invention suggest simple and effective zero-discharge recycle system using nanofiltration which is chemical precipitation- microfiltration-nanofiltration.

It is the object of the present invention to provide a zero discharge recycle system which removes the saturated salts and solid particles in chemically treated waste water that prevent the reuse of treated water as process water, and simplifies the treatment process, and produces industrial water with the high quality in a short time.

It is an another object to provide a zero discharge recycle system using a combination of microfiltration and nanofiltration which prevents the flux decrease due to membrane fouling to be far superior to the system using antiscalent chemicals for preventing the precipitation.

The present invention provides a method of waste water treatment comprising the steps of passing waste water through a neutralization and/or chemical precipitation unit, passing the effluent from the chemical precipitation unit through a microfiltration unit, and passing the effluent from the microfiltration unit through a nanofiltration unit, whereby the constituents to prevent the reuse of waste water are removed from the waste water containing high concentration of inorganic ions.

Also, the present invention provides a chemical precipitation-microfiltration-nanofiltration hybrid system comprising a first means for neutralizing and/or chemically precipitating waste water, a second means for microfiltrating the effluent from the first means, a third means for nanofiltrating the effluent from the second means.

The nanofiltration unit of the present invention preferably contains a polymer membrane having a high removability of bivalent ions which operates at the pressure of 5 to 20 psi, and the module of which is plate type or screw type.

The microfiltration unit of the present invention preferably contains a polymer membrane or a cartridge filter having the pore size of 0.2 to 50 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of a hybrid system according to the present invention;

Fig. 2 is a graph showing flux of the waste water treated according to one embodiment of the present invention with concentration factor;

Fig. 3 is a graph showing flux of the waste water treated according to another embodiment of the present invention with concentration factor.

DETAILED DESCRIPTION OF THE INVENTION

In the instant invention, the retentate from chemical precipitation unit is recycled into nanofiltration unit after treatment by microfiltration instead that recycled directly into nanofiltration without microfiltration. Therefore, the flux decrease due to membrane fouling is prevented, the water quality of treated water is increased and the washing frequency of membrane is increased to improve the processibility and economical effectiveness dramatically. The cartridge filter or polymer membrane used for microfiltration has the pore size of 0.2 to 50 μm dependent on retentate.

The operation pressure to improve the processibility of nanofiltration membrane and to maintain the high flux value of permeate is preferably 5 to 20 psi and the module of the nanofiltration membrane is preferably plate type or screw type.

The following specific examples are illustrative of the present invention, but are not to be considered limiting thereof in any way.

EXAMPLE 1

A waste water discharged from surface polishing process of crystal glasses manufactured and sold by Doosan Glass Co., Ltd. located at Kwangju, Kyonggi-do, Korea was passed through a neutralization and chemically precipitation unit (1) of Fig. 1 with lime and the effluent from a chemical precipitation unit (1) was passed through a microfiltration unit (2) of Fig. 1, followed by passing the effluent from the microfiltration unit (2) through a nanofiltration unit (3) of Fig. 1. The flow rate of membrane surface was 1.9 m/sec, the operating pressure was 10 psi and the temperature was 30 °C. The module of nanofiltration was a plate type. The pore size of cartridge filter was 0.45 μm.

COMPARATIVE EXAMPLE 1

The waste water was treated according to the same procedure to the EXAMPLE 1 except that the nanofiltration was not employed. To investigate the maximum operating conditions, the flux was observed and the water quality of the permeate and the retentate were analyzed with changing the transmembrane pressure.

Table I shows the ion concentrations in the influent and the permeate treated by EXAMPLE 1 according to the present invention and removability of ions. The ion concentrations of Table I was observed using ion chromatography, Dionex series 4500i, manufactured and sold by DIONEX. The removability was calculated by the following formula:

Re ovabi1ity (%)

Cone, of influent - Cone, of permeate x 100 Cone, of influent

TABLE I Ion Concentration & Removability

Ca so₄ K Na cone. of influent (ppm) 544 2,274 243 266 cone. of permeate (ppm) 5.1 17.5 66.8 62.7 removability (%) 99.1 99.2 72.5 76.4

Fig. 2 shows the fluxes of EXAMPLE 1 and COMPARATIVE EXAMPLE 1 with concentration factor. As shown in Fig. 2, the flux of EXAMPLE 1 according to the present invention shows very low decrease of flux, while the flux of COMPARATIVE EXAMPLE 1 is rapidly decreased. As shown in Table I, the removability of bivalent ion is more than 97 %, the removability of total dissolved solid is more than 80 %.

EXAMPLE 2

The waste water was treated according to the same procedure to the EXAMPLE 1 except that the flow rate of membrane surface was 0.6 m/sec, the module of nanofiltration was a screw type.

COMPARATIVE EXAMPLE 2

The waste water was treated according to the same procedure to the EXAMPLE 2 except that the nanofiltration was not employed.

Fig. 3 shows the fluxes of EXAMPLE 2 to 4 and COMPARATIVE EXAMPLE 2 with concentration factor. As shown in Fig. 3, the flux of EXAMPLE 2 shows very low decrease of flux, while the flux of COMPARATIVE EXAMPLE 2 is rapidly decreased.

The pore size of cartridge filter or polymer membrane for microfiltration at the pretreatment of retentate is 0.2 to 50 μm dependent of water quality of retentate and the flux of retentate is more than 40 L/m²/h under the operating condition of 10 psi, 25 to 30 "C to obtain a maximum effectiveness.

In conclusion, the way to obtain a maximum amount of permeate from a constant influent is to recycle the retentate into nanofiltration. However, this recycle of the retentate causes membrane fouling to decrease the flux and water quality. In order to protect the flux decrease and water quality decrease due to membrane fouling occurring when recycling the retentate according to the conventional process, the inventors employ the technique of recycling the retentate into nanofiltration after the pretreatment of the retentate through microfiltration in the present invention.

The present invention is a zero-discharge recycle system to obtain treated waste water of high quality independent of the loads according to each time and process and the present invention is very simple process to produce a process water because it does not require the coagulation or settling process. Accordingly, less land or less labor is required and the operation and the control become easier.

The chemical precipitation-microfiltration- nanofiltration hybrid system of the present invention may be applied when recycling the inorganic type waste water containing a large amount of fluorine and heavy metals after neutralization and chemical precipitation. Especially, it is very effective to recycle without discharging the waste water originated from the surface treatment processes in glass industry or electronic industry.

Claims

What is claimed is:

1. A method of waste water treatment comprising the steps of: passing waste water through a neutralization and/or chemical precipitation unit; passing the effluent from said chemical precipitation unit through a microfiltration unit; and passing the effluent from said microfiltration unit through a nanofiltration unit, whereby the constituents to prevent the reuse of waste water are removed from the waste water containing high concentration of inorganic ions.

2. The method of claim 1, wherein said nanofiltration unit contains a polymer membrane having a high removability of bivalent ions which operates at the pressure of 5 to 20 psi, and the module of which is plate type or screw type.

3. The method of claim 1, wherein said microfiltration unit contains a polymer membrane or a cartridge filter having the pore size of 0.2 to 50 μm.

4. A chemical precipitation-microfiltration- nanofiltration hybrid system comprising: a first means for neutralizing and/or chemically precipitating waste water; a second means for microfiltrating the effluent from said first means; and a third means for nanofiltrating the effluent from said second means.

5. The hybrid system of claim 4, wherein said third means contains a polymer membrane having a high removability of divalent ions which operates at the pressure of 5 to 20 psi, and the module of which is plate type or screw type.

6. The hybrid system of claim 4, wherein said second means contains a polymer membrane or a cartridge filter having the pore size of 0.2 to 50 μm.