WO1998023538A1

WO1998023538A1 - Wastewater treatment process and apparatus for high flow impurity removal

Info

Publication number: WO1998023538A1
Application number: PCT/US1997/021375
Authority: WO
Inventors: Stephen D. Allen; Leonard R. Lyman
Original assignee: Microbar Incorporated
Priority date: 1996-11-26
Filing date: 1997-11-26
Publication date: 1998-06-04
Also published as: AU5360398A; US5965027A; US6312601B1

Abstract

A process and system for removing heavy metals, fluoride, silica and other contaminants from large volumes of wastewater is disclosed. In the process, a wastewater stream containing the contaminant is treated with a chemical coagulant to create a particle having a diameter greater than 5 microns. Treated wastewater is passed through a microfiltration membrane which physically separates the metal contaminant particle from the wastewater. Commercially available microfiltration membranes having a pore size from 0.5 micron to 5 microns may be used. The treated wastewater flow rate through the microfiltration membranes can range from 700 gallons per square foot of membrane per day ('GFD') to 1500 GFD for metal contaminants and from 140 GFD to 600 GFD for silica contaminants. Solids are removed from the membrane surface by periodically backflushing the microfiltration membranes and draining the filtration vessel within which the membranes are located. The dislodged solid material within the filtration vessel is flushed into a holding tank for further processing of the solids.

Description

WASTEWATER TREATMENT PROCESS AND APPARATUS FOR HIGH PLOW IMPURITY REMOVAL

Field of the Invention The present invention relates to the treatment and purification of wastewater at high flow rates. More particularly, the present invention relates to process and apparatus for removing heavy metal and certain non-metal contaminants, such as silica and fluoride, from large quantities of wastewater.

Background of Invention Many manufacturing operations generate extremely large quantities of water containing heavy metals or other contami- nants. For instance, mining drawdown wells which are used to dewater deep mining operations are known to generate up to 75,000 gallons per minute (gpm) of water. Often this water contains heavy metals or other impurities which must be removed from the water before it can be safely discharged into the environment.

Current techniques for treating drawdown wastewater include large settling ponds, clarifiers, and sand filter systems utilizing iron or aluminum chemistry with large quantities of polymer additives. Such systems are able to demonstrate 90% compliance to discharge regulations. For example, arsenic cannot be safely discharged into the environment unless its concentration is less than 50 ppb ("parts per billion") . If influent arsenic levels are greater than 300 ppb, clarifier and sand filter systems are not able to consistently provide discharge levels less than 50 ppb. To achieve this level of arsenic reduction, chemical coagulants are required to form heavy and large particles, typically greater than 200 microns in size. However, such systems are subject to biological fouling, sand settling, and upsets. Upsets result in out of compliance water. In addition, system maintenance is extensive, with very large land areas required for the system installation.

Filters have been considered to remove metal contaminants from wastewater. But traditional microfiltration membranes have flux rates that are too low to justify their use in large scale water processing systems.

Many industrial operations generate large quantities of water containing silica. For instance, chemical mechanical polishing (CMP) processes, widely used in the manufacture of semiconductor devices, produce waste water streams containing high quantities of silica. CMP processes are used to polish the silicon-based wafer surface during various stages of semiconductor manufacture. Waste streams containing the polishing slurry and silica are produced during CMP. The silica must be removed before the water can be safely discharged to the environment or recycled within the facility.

Dissolved silica in industrial cooling water is also a major problem. Silica is a scale forming material commonly found in cooling water which can foul heat exchangers, pipes, valves, pumps, and boilers. No known inhibitor, chelating agent or dispersant exists which will significantly control silica's tendency to form scale. When the silica concentration in a cooling water system exceeds its solubility limit of roughly about 150 to about 200 milligrams per liter, silica polymerizes to form scale. It may also react with multivalent cations, such as magnesium and calcium, to form scale.

Researchers have examined many different methods of removing soluble silica, including the use of ferric sulfate, calcium chloride, magnesium chloride, magnesium sulfate, magnesium oxide, aluminum hydroxide, sodium aluminate and activated alumina. Activated alumina has received much attention in processes for removing silica. See, U.S. Patent No. 4,276,180 to Matson and U.S. Patent No. 5,512,181 to Matchett. Other aluminum containing compounds such as sodium aluminate, aluminum sulfate, and aluminum chloride in an alkaline environment (pH greater than 8) have been used to remove soluble and colloidal silica. See, U.S. Patent No. 5,453,206 to Browne. However, these processes are not capable of processing large volumes of wastewater through high flow mechanical systems because of degradation of particles and particulates below 5 micron in size.

Microfiltration systems have also been considered to remove silica contaminants from wastewater. However, traditional microfiltration membranes having a pore size of about 0.5 microns rapidly clog with silica precipitated using conventional inorganic coagulants. Such particulates are consistently less than 1 micron in size. Moreover, the inorganic coagulants cannot aid in the precipitation of microfine colloidal silica. The partially formed floe will also deform and block the membrane pores, preventing flow.

It would be a significant advancement in the art to provide a process and system for removing metals and other contaminants such as silica and fluoride from large quantities of wastewater. Such processes and systems are disclosed and claimed herein.

Summary of the Invention The present invention is directed to a process for removing metal and certain non-metal contaminants from large volumes of wastewater. In the process, a wastewater stream containing a contaminant is treated with a chemical coagulant. Typical metal contaminants found in mining and other industrial wastewater streams include silver (Ag) , arsenic (As) , gold (Au) , barium (Ba) , cadmium (Cd) , chromium (Cr) , copper (Cu) , mercury (Hg) , nickel (Ni) , lead (Pb) , zinc (Zn) , fluoride (F) , and silica (Si0₂) . The present invention can readily be adapted for removing other metals and contaminants found in wastewater by using suitable coagulant chemistry. The coagulant reacts with the contaminant to form a particulate having a size greater than about 5 μ. As used herein, a wastewater stream includes raw water as well as process water streams containing the identified contaminant. Known and novel chemical coagulants are available to achieve the desired particulate formation. For instance, ferric sulfate, ferrous sulfate, aluminum sulfate, sodium aluminate, and aluminum and iron polymers are well known inorganic coagulants. Organic and polymeric coagulants can also be used, such as polyacrylamides (cationic, nonionic, and anionic) , epi- dma's (epichlorohydrin/dimethylamine polymers) , DADMAC's (polydiallydimethylammonium chlorides) , copolymers of acrylamide and DADMAC, natural guar, etc. Some coagulants, such as boro- hydrides, are selective for certain metals. The stoichiometric ratio of coagulant to metal or non-metal contaminant is preferably optimized to result in acceptable contaminant removal at minimum coagulant cost.

The required coagulant concentration will depend on several factors, including contaminant influent concentration, wastewater flow rate, contaminant effluent compliance requirement, coagulant/contaminant reaction kinetics, etc. For metal contaminants, the ratio of coagulant to metal contaminant is typically in the range from 3:1 to 16:1. Arsenic, for example, requires a 6:1 to 10:1 (ferric coagulant: arsenic) ratio, lead requires a 3:1 to 8:1 coagulant:metal ratio, zinc uses about 4:1 coagulant:metal ratio, while copper typically requires a coagulant:metal ratio in the range from 3:1 to 8:1. Fluoride typically has a ratio of coagulant to contaminant in the range from 2:1 to 30:1, depending on the system. For silica contaminants, the ratio of silicon to coagulant is typically in the range from 20:1 to 50:1, depending on the system, an preferably about 40:1. If small amounts of silica can remain in the effluent stream, then the ratio of silicon to coagulant can be 120:1 or even higher. The optimum mole ratio will also vary depending on the coagulant used. For instance, low molecular weight epi-dma (25,000 to 100,000) and very high molecular weight epi-dma (1,000,000 to 1.5 million) require from 3 to 5 times the dose to flocculate silica. It has been found that organic coagulants cause the silica to form well defined spherical particles having a typical particle size in the range from 0.05 μ to 0.15 μ. The particles tend to agglomerate to form larger clusters having a typical size in the range from 10 μ to 300 μ. The silica particles are easily separated from microfiltration membranes enabling efficient silica removal without membrane degradation.

Small amounts of a supplemental coagulant can optionally be used in combination with the organic and polymeric coagulant to optimize the silica removal. Examples of typical supplemental coagulants include, aluminum chlorohydrate ("ACH," Al_nOH_2n-mCl_m, e.g., Al₄OH₆Cl with a typical A1:C1 ratio of 2:1), sodium aluminate (NaA10₂) , aluminum chloride (A1C1₃) , and polyaluminum chloride ("PAC," A1₆0C1₅) . The typical mole ratio of silica to inorganic coagulant is about 25:1. Treated wastewater is passed through a microfiltration membrane which physically separates the metal, silica or fluoride contaminant from the wastewater. Suitable microfiltration membranes are commercially available from manufacturers such as W.L. Gore, Koch, and National Filter Media (Salt Lake City, Utah) . For instance, one GOR-TEX® membrane used in the present invention is made of polypropylene felt with a sprayed coating of teflon. The teflon coating is intended to promote water passage through the membrane. Such microfiltration membrane material has been found to be useful for many wastewater treatment systems.

The microfiltration membranes are used in a tubular "sock" configuration to maximize surface area. The membrane sock is placed over a slotted tube to prevent the sock from collapsing during use. A net material is placed between the membrane sock and the slotted tube to facilitate flow between the membrane and the slots in the tube. In order to achieve the extremely high volume flow rates, a large number of membrane modules, each containing a number of individual filter socks, are used. The microfiltration membranes preferably have a pore size in the range from 0.5 micron to 5 micron, and preferably from 0.5 micron to 1.0 micron. By controlling the ratio of coagulant to contaminant, 99.99% of the precipitated contaminant particles can be greater than 5 microns. This allows the use of larger pore size microfiltration membranes. It has been found that the treated wastewater flow rate through 0.5 to 1 micron microfiltration membranes can be in the range from 700 gallons per square foot of membrane per day ("GFD") to 1500 GFD for metal contaminants and in the range from 150 gallons per square foot of membrane per day ("GFD") to 600 GFD for silica contaminants. Solids are preferably removed from the membrane surface by periodically backflushing the microfiltration membranes and draining the filtration vessel within which the membranes are located. The periodic, short duration back flush removes any buildup of contaminants from the walls of the microfiltration membrane socks. The dislodged solid material within the filtration vessel is flushed into a holding tank for further processing of the solids. The wastewater treatment system disclosed herein is designed to provide compliance with the contaminant discharge effluent limits. Wastewater pretreatment chemistry creates insoluble metal and non-metal contaminant particulates which are efficiently removed by the microfiltration membranes.

Brief Description of the Drawings Figure 1 is a schematic representation of one wastewater pretreatment system. Figure 2 is a schematic representation of one wastewater microfiltration apparatus for high flow impurity removal.

Detailed Description of the Invention The present invention is directed to a process for removing metal and certain non-metal contaminants, such as silica and fluoride, from large volumes of wastewater. In operation, the wastewater is collected and pretreated with one or more chemical coagulants such that the contaminant reacts with the coagulant (s) to form particulates having a size greater than about 5 μ. The chemical coagulants are preferably mixed with the wastewater using reaction vessels or static in-line mixers, although other mixing methods can be used.

The treated wastewater is then passed through a microfiltration membrane having a pore size in the range from 0.5 μ to 5 μ to remove the contaminant particulates. In such a system, wastewater flow rates in the range from 700 gallons per square foot of membrane per day ("GFD") to 1500 GFD for metal contaminants and in the range from 150 gallons per square foot of membrane per day ("GFD") to 600 GFD for silica contaminants. are possible. The microfiltration membrane is periodically backflushed to remove solids from the membrane surface. The rejected solids are gravity collected in the filter vessel bottom and time cycle discharged to a settling tank for further sludge processing. The microfiltration membranes are preferably provided in a cassette arranged module. The microfiltration membranes provide a positive particle separation in a high recovery dead head filtration array. The dead head filtration operates effectively at low pressures (4 psi to 15 psi, preferably 5 psi to 10 psi) and high flow rates, allowing 100% discharge of the supplied water with no transfer pumps needed. Solids which settle on the wall of the membrane during filtration are periodically backflushed away (and gravity settled) from the membrane surface to ensure a continuously clean filtration area. The individual cassette module design allows for easy replacement of the membrane modules.

Currently preferred filter socks useful with the present invention contain a teflon coating on a polypropylene or polyethylene felt backing material. Such socks are available from W.L. Gore. Another presently preferred filter sock manufactured by National Filter Media, Salt Lake City, Utah, consists of a polypropylene membrane bonded to a polypropylene or polyethylene felt backing. Membrane "failure" is due primarily to flux rate loss, not mechanical failure. Many operations deem it more cost-effective to replace the membrane socks instead of cleaning contaminants from the membrane.

The membrane life is important to the continuous operation and operational cost of the filtration system. The membranes manufactured by W.L. Gore and National Filter Media have been found to be robust and free of catastrophic failures at a temperature of 160°F and a pH greater than 13. Anticipated operating conditions for the present invention are ambient temperature and pH between 5 and 11. For silica removal, a currently preferred operating pH range is between about 7.3 and 9.3, although good results are obtained ± 1.0 pH unit from the optimum pH. It is presently preferred to adjust the pH before adding the organic coagulant. It is expected that membranes used according to the present invention will have a life equal to or greater than 18 months. The filtration system operates at a low pressure, preferably between 4 and 15 psi. Greater pressures are possible; however, the higher the pressure, the quicker the membrane loss of flux rate. The operating pressure is preferably below 25 psi. Although the currently preferred operating pressure is below about 25 psi, excellent silica removal results have been obtained using the organic coagulants identified above with commercially available high pressure microfiltration systems which operate at pressures between 30 and 80 psi. Existing microfiltration systems using conventional inorganic coagulants can be retrofitted for use with the organic coagulants to obtain dramatically improved performance.

The following examples are offered to further illustrate the present invention. These examples are intended to be purely exemplary and should not be viewed as a limitation on any claimed embodiment.

Example 1 Using a 50 gallon per minute (gpm) pilot scale system, actual mine draw-down wastewater containing arsenic contaminant was processed according to the present invention. Ferric sulfate (at a ratio of 8:1 Fe:As) was used as the coagulant. DADMAC ( (poly)diallyldimethylammonium chloride) and a copolymer of acrylamide and DADMAC were used at a concentration of 1 ppm (parts per million) . The DADMAC was used as a 20% liquid and the DADMAC acrylamide copolymer was used as a 10% liquid. The membrane was obtained from W.L. Gore having a teflon coating and a nominal pore size range of 0.5 μ. The flux rate ranged from 430 to 600 GFD at an operating pressure less than 10 psi. The results are reported below in Table 1.

Table 1

All Values are in Parts Per Billion (ppb) Time Arsenic Influent Arsenic Effluent

Period Mean High Low Mean High Low

A 331 429 247 13.3 82 0

B 270 375 165 5.3 15 0

C 279 369 231 7.0 24 0

D 278 278 278 2.7 7 0

E 244 268 197 4.9 14 0

Example 2 A 15 gpm pilot scale system was used to process wastewater containing fluoride and a combined flow of fluoride and silica. A 38% sodium aluminate solution at a ratio of 0.23:1 A1:F and 50% aluminum chlorohydrate at a dose of 35 ppm to aid in the removal of the fluoride, total dissolved solids (TDS) , total suspended solids (TSS) , and some of the other present salt forms. The precipitate was flocculated with a medium charge (25±5 mole percent) , medium molecular weight anionic polyacrylamide polymer for ease of filtering or settling. This yielded very low to non-detectable effluent values of fluoride and Silt Density Indices (SDI) below 3.0. The filtration membrane was a 0.5 μ polypropylene bonded membrane obtained from National Filter Media. The membrane flux rate was measured at 650 to 800 GFD at a vessel operating pressure less than 9 psi. The results are reported below in parts per million.

Period Influent F Effluent F

A 130.0 1.86

B 191.5 21.7

C 142.2 2.13

D 120.0 0.72

E 156.5 1.41

F 125.7 0.79

G 60.93 0.97

H 206.25 0.95

I 133.3 0.39

J 112.9 0.85

K 78.2 3.96

L 133.5 3.96

Average 132.6 3.8

Min 60.93 0.39

Max 206.25 21.7

Time

Period Influent F + SiO_? Effluent F + SiQ₂

A 264.0 0.24

B 172.0 0.26

C 140.0 0.31

D 153.0 0.39

E 98.0 0.36

F 89.0 0.29

Average 152.7 0.31

Min 89.0 0.24

Max 264.0 0.39

Example 3 A 15 gpm pilot scale system was used to process wastewater containing silica. The silica was present in dissolved and colloidal silica form in the waste stream. A 38% sodium aluminate solution at a ratio of 0.45:1 Al:Si, 46% aluminum sulfate at constant dose of 45 ppm, 50% aluminum chlorohydrate at a dose of 25 ppm, and a 20% epichlorohydrin/dimethylamine (a high charged, low molecular weight cationic epi-DMA product) at a dosage of 0.25 - 1.0 ppm to aid in the removal of the silica, TDS and TSS. This formed a well defined particle for filtering or settling. This yielded very low to non-detectable effluent values of the silica and Silt Density Indices (SDI) below 3.0.

The filtration membrane was a 0.5 micron polypropylene felt with a PTFE (polytetrafluoroethylene) coating obtained from W.L. Gore. The membrane flux rate ranged from 175 GFD to 400 GFD at a vessel operating pressure less than 15 psi. The results are reported below in parts per million.

Time

Period Influent Si0₇ Effluent Si0₇

A 140 0.443 B B 1 16600 0.33

C 125 0.37

D 153 0.39

E 177 0.36

F 165 0.29

Average 153 0.364

Min 125 0.29

Max 177 0.443

Example 4

A 15 gallon per minute (gpm) pilot scale system was used to process wastewater containing copper and lead in a combined waste flow. The copper and lead removal system employed the use of a blend of sodium thiocarbonate and sodium aluminate which was fed at a ratio of 3.2:1 (thiocarbonate to combined metal concentration of copper and lead as measured by atomic absorption) . The precipitate was flocculated with a medium charge, medium molecular weight polyacrylamide polymer for ease of filtering or settling. This yielded a very low to non- detectable effluent values of copper and lead in the effluent. The membrane was a 1.0 micron polypropylene needled monoelement obtained from National Filter Media. The membrane flux rate was estimated to be 1000 GFD at vessel pressures from 4.5 to 6.0 psi. The results are reported below in parts per million: Time Lead Copper

Period Influent Effluent Influent Effluent

A 3.2 0.11 28.0 N.D.

B 2.85 0.14 32.98 0.032

C 3.66 0.109 21.31 0.045

D 2.45 0.15 23.0 0.023

E 3.0 0.10 28.0 N.D.

F 2.4 0.09 35.0 N.D.

G 3.8 N.D. 35.11 0.07

H 2.76 0.10 33.0 0.055

I 4.12 N.D. 27.27 0.11

J 2.65 0.12 24.6 N.D.

Average 3.09 0.09 28.82 0.0335

Min 2.4 N.D. 21.31 N.D.

Max 4.12 0.15 35.11 0.11

Exampl e 5

A 3-5 gpm bench scale system was used to process wastewater containing silica. The silica-containing waste stream was obtained from a commercially available CMP slurry sold by Rodel, known as ILD 1300. The ILD 1300 slurry was diluted according to manufacturer's instructions, and it was found to contain about 1380 ppm Si, measured by graphite furnace atomic absorption, and about 70 ppm ammonium (NH₄) , measured by ion chromatography. One liter of the waste stream weighted about 993.7 grams. The silicon was present in the waste stream as dissolved and colloidal silica. The waste stream was adjusted to a pH of about 8.58 by adding small amounts of sodium hydroxide and sulfuric acid. The waste stream was mixed for about 3 minutes while the pH was adjusted. 2.09 g of a 20% by weight solution of epi-DMA, an epichlorohydrin/dimethylamine polymer having an average molecular weight of 250,000150,000 (EnChem Lot I- 1396/423/MIC) and 0.19 g of dry aluminum chlorohydrate were added to one liter of the waste stream and mixed for about 20 minutes.

The reaction mixture was pumped at a pressure of about 6 psi through a two foot long filter sock having a diameter of about 3.5 inches. The membrane flux was estimated at 189 GFD. The filter sock contained a GOR-TEX® membrane (Lot. No. 66538-3- 786) obtained from W.L. Gore. The membrane had a PTFE (polytetrafluoroethylene) coating on polypropylene felt having a 0.5 μ pore size (1.5 μ absolute). The filter membrane effluent was collected, and it was found to contain about 15.5 ppm Si, measured by graphite furnace atomic absorption, and about 70 ppm ammonium (NH₄) , measured by ion chromatography. The solids were collected from the filter surface and air dried for 24 hours. The recovered solids formed well defined spherical particles which were easily removed from the filter membrane surface. The dried and ground solids were analyzed, and the results are reported below in weight percent. ILD 1300 Results

Loss on Drying 45.53 %

Carbon 3.84 %

Hydrogen 1.04 %

Nitrogen 1.41 % Silicon 36.74 %

Aluminum 2.30 %

Other ingredients in the recovered solid, such as sodium, potassium, and unknown proprietary ingredients of ILD 1300, were not analyzed. Scanning electron micrographs (SEM) of the resulting spherical silica particles show the particles to be spherical, having a typical particle size in the range from 0.05 μ to 0.15 μ. Although the spherical particles are smaller than the membrane pore size, it has been found that the particles agglomerate to form large clusters that do not pass through the membrane. The clusters have an average size in the range from 10 μ to 300 μ. EDX analysis of the sample indicated the presence of silicon and aluminum in the sample, wherein the concentration of silicon was much greater than the concentration of aluminum.

Example 6 A 3-5 gpm bench scale system was used to process wastewater containing silica. The silica-containing waste stream was obtained from a commercially available CMP slurry sold by

Hoescht, known as KLEBOSOL. The KLEBOSOL slurry was diluted according to manufacturer's instructions, and it was found to contain about 4474 ppm Si and about 3.2 ppm aluminum by graphite furnace atomic absorption. One liter of the waste stream weighed about 998.4 grams. The silicon was present in the waste stream as dissolved and colloidal silica. The waste stream was adjusted to pH 9.84 by addition of small amounts of NaOH and

H₂S0₄. The waste stream was mixed for about 3 minutes while the pH was adjusted. 2.09 g of a 20% by weight solution of epi-dma, an epichlorohydrin/dimethylamine polymer having an average molecular weight of 250,000150,000 (EnChem Lot I-1396/423/MlC) was added to one liter of the waste stream and mixed for about

20 minutes.

The reaction mixture was pumped through the filter sock of

Example 3 at a pressure of about 6 psi. The filter membrane effluent was collected, and it was found to contain about 8.32 ppm Si and < 0.1 ppm aluminum by graphite furnace atomic absorption.

The solids were collected from the filter surface and air dried for 24 hours. The solids formed were well defined spherical particles which were easily removed from the filter membrane surface. The solids appeared dry as they were removed from the membrane. The dried and ground solids were analyzed, and the results are reported below in weight percent.

KLEBOSOL Results Loss on Drying 1. 91 %

Carbon 1. 41 % Nitrogen 0. 43 % Silicon 40 . 49 % Aluminum 0. 98 % Scanning electron micrographs (SEM) of the resulting silica particles show the particles to be spherical, having a typical particle size in the range from 0.05 μ to 0.15 μ. The spherical silica particles were remarkably similar to the silica particles of Example 5. EDX analysis of the sample indicated the presence of silicon and aluminum in the sample, wherein the concentration of silicon was much greater than the concentration of aluminum.

Reference is made to Figure 1 which illustrates one possible wastewater pretreatment system 10 within the scope of the present invention. The illustrated wastewater pretreatment system 10 includes a plurality of pretreatment reactor vessels 12, 14, and 16 which enable the wastewater feed stream 18 to chemically react with one or more chemical coagulants. Chemical coagulants which react with contaminants in the wastewater feed stream 18 are introduced into the pretreatment reactor vessels via chemical coagulant feed streams 20, 22, and 24. The pH within the pretreatment reactor vessels is preferably monitored with a pH sensor 26. Acid or base can be added to the pretreatment reactor vessels, if necessary, to adjust the pH via acid/base feed stream 28.

The number of pretreatment reactor vessels can vary depending on the number of chemical coagulants being used and the reaction chemistry used to form the waste particulates. The size of the reactor vessels can be varied to provide different reaction times.

After flowing through the necessary pretreatment reactor vessels, the wastewater feed stream flows into a feed tank 30 for holding the pretreated wastewater. Additional chemical coagulants can be added directly to the feed tank 30, if necessary, via a chemical coagulant feed stream 31. As shown in Figure 2, the pretreated wastewater is directed to one or more filtration vessels 32, 34, and 36 via filtration vessel feed stream 38. The size of feed stream 38 will depend on the designed flow rate of the filtration vessel. For example, in a system having 5 filtration vessels, each handling 2500 gpm, a 24 inch feed line to the system is suitable. Each filtration vessel 32, 34, and 36 is a stand alone filtration device. The number and size of each filtration vessel can vary depending on the system capacity requirements. The filtrate is removed from each filtration vessel via a filtrate stream 40.

Each filtration vessel preferably provides a mounting platform for from 9 to 49 filter cassette modules. One currently preferred filter cassette module contains 16 individual sock filters configured with 0.5 micron filtration membranes. The rated flow rate is 0.9 gpm per square foot of membrane area. Each full cassette module has 64 square feet of membrane area and is rated at 58 gpm with a differential pressure less than 15 psi. A lifting mechanism is preferably included to allow removal and replacement of the membrane cassette modules. The filtration membranes are periodically backflushed with filtrate to remove solids from the membrane surface. During the backflush procedure, the filtration vessel is taken off line and wastewater is drained from the filtration vessel via a backflush exit stream 42 to a backflush tank 44. The backflush tank 44 provides temporary storage before the backflushed wastewater is conveyed to the feed tank 30 via backflush return stream 46. It is estimated that 400-500 gallons of water will be used during a typical back flush cycle for a 2500 gpm filtration vessel. A vacuum breaker 48 is preferably provided to allow equalization of pressure within the respective filtration vessel 32, 34, or 36 during the backflush procedure. A vent/relief stream 49 is provided to allow venting or release of excess or over- pressurized wastewater. The filtrate side of the filtration vessel 32, 34, 36 is open to the atmospheric pressure. The filtrate is collected in the top of the filtration vessel and allowed to drain into the filtrate stream 40. This volume of water provides the positive head which, when coupled with the negative head of draining the pressure side of the vessel via backflush exit stream 42, produces enough positive pressure gradient to backflush the filtration membrane.

After sufficient sludge settles within the bottom of the filtration vessel 32, 34, 36, the sludge is removed via a sludge discharge stream 50. While the sludge is removed, the filtration membranes are preferably rinsed with water from a water rinse stream 52. The collected sludge is removed from the system for further processing or storage.

Periodically, the membranes will require soaking to remove trace amounts of organics. Cleaning preferably occurs as needed or as part of a regular maintenance program. The vessel drain opens to remove all contaminant via the sludge discharge stream 50. The cleaning solution is introduced into each filtration vessel through cleaning supply stream 54. Typical cleaning solutions include acids, bases, and surfactants. In some cases the filtration vessel can be returned to operation without draining and rinsing the filtration membranes. If membrane rinsing is necessary, the contents of the filtration vessel 32, 34, 36 are removed via cleaning discharge stream 56 for further processing.

As shown in Figure 2 , multiple filtration vessels are preferably used, in parallel, to provide for the required flow rate. However, the filtration vessels can be operated in series to provide primary filtration and secondary filtration. Because filtration vessels are taken off line during the backflushing, additional filtration vessels and capacity are preferably used to ensure that the require discharge flow is maintained. An additional filtration vessel may be supplied to provide for off- line maintenance while the remainder of the system meets the flow rate requirements.

The wastewater treatment system preferably includes access to the various process streams to allow for sampling and analysis. The valves, pumps, and sensors customarily used in the art to safely control the described fluid flow to and from the filtration vessels are preferably provided. Such valves, pumps, and sensors also allow for automation of the process.

From the foregoing, it will be appreciated that the present invention provides a process for removing contaminants from wastewater utilizing a positive physical barrier to precipitated particles. The positive separation barrier permits discharge having lower concentration limits than conventional clarifier/sand filter systems.

The apparatus for removing contaminants from wastewater occupies less space than conventional clarifier/sand filter systems. The apparatus is easily expandable.

The chemical pretreatment achieves particle formation based on size, not weight. As a result, chemical pretreatment costs are lower than those typically required for a clarifier/sand filter.

Claims

CLAIMS :

1. A process for removing metal contaminants from large volumes of wastewater comprising the steps of:

(a) treating a wastewater stream containing a metal contaminant with an organic or inorganic coagulant, wherein the coagulant reacts with the metal contaminant to form a particulate having a size greater than about 5 μ;

(b) passing the treated wastewater through a microfiltration membrane having a pore size in the range from 0.5 μ to 5 μ, wherein the treated wastewater flow rate is in the range from 700 gallons per square foot of membrane per day ("GFD") to 1500 GFD, such that the metal contaminant is removed from water passing through the microfiltration membrane; and (c) periodically backflushing the microfiltration membrane to remove solids from the membrane surface.

2. A process according to claim 1, wherein the metal contaminant is selected from the group consisting of Ag, As, Au, Ba, Cd, Cr, Cu, Hg, Ni, Pb, and Zn.

3. A process according to claim 1, wherein the metal contaminant is selected from the group consisting of Ag, As, Au, Ba, Cd, Cr, Hg, and Pb.

4. A process according to claim 1, wherein the mole ratio of coagulant to metal contaminant is in the range from 3 : 1 to 16:1.

5. A process according to claim 1, wherein the treated wastewater flow rate is greater than 5,000 gallons per minute ("gpm") .

6. A process according to claim 1, wherein the treated wastewater flow rate is greater than 10,000 gallons per minute

("gpm") .

7. A process according to claim 1, wherein the metal contaminant is arsenic and the coagulant includes ferric sulfate at a Fe:As mole ratio of from 6:1 to 10:1, DADMAC ((poly)di- allyldimethylammonium chloride) and a copolymer of acrylamide and DADMAC.

8. A process according to claim 1, wherein the metal contaminant is copper and the coagulant is a blend of sodium thiocarbonate and sodium aluminate which was fed at a ratio of from 3:1 to 8:1 thiocarbonate to total metal contaminant concentration.

9. A process according to claim 8, further comprising the step of adding from 20 to 30 mole percent, relative to the metal contaminant content, medium molecular weight anionic polyacrylamide polymer.

10. A process according to claim 1, wherein the metal contaminant is lead and the coagulant is a blend of sodium thiocarbonate and sodium aluminate which was fed at a ratio of from 3:1 to 8:1 thiocarbonate to total metal contaminant concentration.

11. A process according to claim 10, further comprising the step of adding from 20 to 30 mole percent, relative to the metal contaminant content, medium molecular weight anionic polyacrylamide polymer.

12. A process according to claim 1, wherein the microfiltration membrane comprises polypropylene felt with a coating of polytetrafluoroethylene (PTFE) .

13. A process according to claim 1, wherein the microfiltration membrane comprises polypropylene woven membrane bonded to a polypropylene or polyethylene felt backing.

14. A process according to claim 1, wherein the treated wastewater is passed through the microfiltration membrane at a pressure less than 25 psi.

15. A process according to claim 1, wherein the treated wastewater is passed through the microfiltration membrane at a pressure in the range from about 4 psi to 15 psi.

16. A process according to claim 1, wherein the treated wastewater is passed through the microfiltration membrane at a pressure in the range from about 5 psi to 10 psi.

17. A process according to claim 1, wherein the coagulant is a polyacrylamide.

18. A process according to claim 1, wherein the coagulant is a epichlorohydrin/dimethylamine (epi-dma) polymer.

19. A process according to claim 1, wherein the coagulant is a DADMAC (polydiallydimethylammonium chloride) polymer.

20. A process according to claim 1, wherein the coagulant is a copolymers of an acrylamide and DADMAC (polydi- allydimethylammonium chloride) .

21. A process according to claim 1, wherein the coagulant is a natural guar.

22. A process for removing fluoride from large volumes of wastewater comprising the steps of:

(a) treating a wastewater stream containing fluoride with a coagulant, wherein the coagulant reacts with the fluoride to form a particulate having a size greater than about 5 μ;

(b) passing the treated wastewater through a microfiltration membrane having a pore size in the range from 0.5 μ to 5 μ, wherein the treated wastewater flow rate is in the range from 700 gallons per square foot of membrane per day ("GFD") to 1500 GFD, such that the fluoride is removed from water passing through the microfiltration membrane; and

(c) periodically backflushing the microfiltration membrane to remove solids from the membrane surface.

23. A process according to claim 22, wherein the mole ratio of coagulant to fluoride is in the range from 2:1 to 30:1.

24. A process according to claim 22, wherein the coagulant is aluminum chloride, aluminum chlorohydrate, and sodium aluminate.

25. A process according to claim 22, wherein the coagulant is a sodium aluminate solution at a ratio of from 0.2:1 to 5:1 A1:F and aluminum chlorohydrate at a dose of from 30 to 40 ppm.

26. A process according to claim 25, further comprising the step of adding from 20 to 30 mole percent, relative to the fluoride content, medium molecular weight anionic polyacrylamide polymer.

27. A process for removing silica from large volumes of wastewater comprising the steps of:

(a) treating a wastewater stream containing silica with an organic polymer coagulant, wherein the coagulant reacts with the silica to form a spherical, silica-based particulate which agglomerated to form a cluster having a size greater than about 5 μ;

(b) passing the treated wastewater through a microfiltration membrane having a pore size in the range from 0.5 μ to 5 μ, such that the silica is removed from water passing through the microfiltration membrane; and

28. A process according to claim 27, wherein the mole ratio of silica to coagulant is in the range from 20:1 to 50:1.

29. A process according to claim 27, wherein the mole ratio of silica to coagulant is in the range from 35:1 to 45:1.

30. A process according to claim 27, further comprising the step of adjusting the pH of the wastewater stream to a pH in the range from about 5 to 11.

31. A process according to claim 27, wherein the coagulant is an epichlorohydrin/dimethylamine polymer.

32. A process according to claim 27, wherein the coagulant is an epichlorohydrin/dimethylamine polymer having a molecular weight in the range from 25,000 to 1.5 million.

33. A process according to claim 27, wherein the coagulant is an epichlorohydrin/dimethylamine polymer having a molecular weight in the range from 200,000 to 300,000.

34. A process according to claim 27, wherein the coagulant is a DADMAC (polydiallydimethylammonium chloride) polymer.

35. A process according to claim 27, wherein the coagulant is a copolymers of an acrylamide and DADMAC (polydiallydimethylammonium chloride) .

36. A process according to claim 27, wherein the membrane has a PTFE (polytetrafluoroethylene) coating on polypropylene felt.

37. A process according to claim 27, wherein the membrane has a PTFE (polytetrafluoroethylene) coating on polyethylene felt.

38. A process according to claim 27, wherein the treated wastewater is passed through the microfiltration membrane at a pressure less than 80 psi.

39. A process according to claim 27, wherein the treated wastewater is passed through the microfiltration membrane at a pressure less than 25 psi.

40. A process according to claim 27, wherein the treated wastewater is passed through the microfiltration membrane at a pressure in the range from about 4 psi to 15 psi.

41. A process according to claim 27, wherein the treated wastewater is passed through the microfiltration membrane at a pressure in the range from about 5 psi to 10 psi.

42. A process according to claim 27, wherein the treated wastewater is passed through the microfiltration membrane at a flow rate is in the range from 150 gallons per square foot of membrane per day ("GFD") to 600 GFD.

43. A process for removing silica from large volumes of wastewater comprising the steps of:

(a) treating a wastewater stream containing silica with an organic coagulant, wherein the coagulant reacts with the silica to form a spherical particulate which agglomerates to form a cluster having a size greater than about 5 μ;

(b) passing the treated wastewater through a microfiltration membrane having a pore size in the range from 0.5 μ to 5 μ, wherein the treated wastewater flow rate is in the range from 150 gallons per square foot of membrane per day ("GFD") to 600 GFD, such that the silica is removed from water passing through the microfiltration membrane; and

44. A process according to claim 43, wherein the mole ratio of silica to coagulant is in the range from 20:1 to 50:1.

45. A process according to claim 43, wherein the mole ratio of silica to coagulant is in the range from 35:1 to 45:1.

46. A process according to claim 43, wherein the coagulant is an epichlorohydrin/dimethylamine polymer.

47. A process according to claim 43, wherein the coagulant is an epichlorohydrin/dimethylamine polymer having a molecular weight in the range from 25,000 to 1.5 million.

48. A process according to claim 43, wherein the coagulant is an epichlorohydrin/dimethylamine polymer having a molecular weight in the range from 200,000 to 300,000.

49. A process according to claim 43, wherein the coagulant is a DADMAC (polydiallydimethylammonium chloride) polymer.

50. A process according to claim 43, wherein the coagulant is a copolymers of an acrylamide and DADMAC (polydiallydimethylammonium chloride) .

51. A spherical silica precipitate prepared by reacting a waste stream containing dissolved or colloidal silica with an epichlorohydrin/dimethylamine polymer having a molecular weight greater than 25,000, wherein the molar ratio of silicon to polymer is in the range from 20:1 to about 60:1.

52. A silica precipitate according to claim 51, wherein the spherical silica precipitate agglomerates to form clusters having an average size in the range from 10 μ to 300 μ.

53. A silica precipitate according to claim 51, wherein the silica precipitate has a silica content greater than 30% by weight.