KR100385706B1 - Process for Removing Silica from Wastewater - Google Patents

Abstract

Disclosed herein are methods and systems for removing silica from large amounts of wastewater. In this process, the wastewater stream containing silica is treated with a flocculant such as epichlorohydrin / dimethylamine polymer to produce clustered agglomerated spherical particles having a diameter of at least 5 microns. Treated wastewater is passed through a microfiltration membrane that physically separates silica contaminant particles from the wastewater. Commercially available microfiltration membranes with pore sizes of 0.5 microns to 5 microns can be used. The flow rate of the treated wastewater passing through the microfiltration membrane is from 150 gallons per square foot of membrane per day (GFD) to 600 GFD. Solids are removed from the surface of the membrane by periodically backwashing the microfiltration membrane and intermittently draining the filtration vessel in which the membrane is placed. The solid material in the filtration vessel is transferred to a holding tank for further processing.

Description

Process for Removing Silica from Wastewater

Many industrial operations generate large amounts of wastewater containing silica. For example, wastewater streams containing large amounts of silica are generated from chemical mechanical polishing (CMP) processes that are widely used in the manufacture of semiconductor devices. This CMP process is used to polish silicon-based wafer surfaces during various steps for the fabrication of semiconductor devices. During this CMP a waste stream containing a polishing slurry and silica is generated. This silica must be removed before the wastewater is released into the environment and recycled to the plant.

Dissolved silica present in industrial cooling water is a major problem. Silica is a scale forming material that is commonly present in cooling water and can clog heat exchangers, piping, valves, pumps, and boilers. No inhibitors, chelating agents or dispersants are known which can significantly inhibit the tendency of silica to form scales. When the concentration of silica in the cooling water system exceeds the solubility limit of about 150 to about 200 mg / l, the silica polymerizes to form scale. The silica may also react with polyvalent cations such as magnesium and calcium to form scale.

The researchers studied a number of methods for removing soluble silica using ferric sulfate, calcium chloride, magnesium chloride, magnesium oxide, aluminum hydroxide, sodium aluminate, and activated alumina. Among these methods, much attention has been focused on using activated alumina (see US Pat. No. 4,276,180 to Matson and US Pat. No. 5,512,181 to Matchett). Other alkaline containing compounds such as sodium aluminate, aluminum sulphate and aluminum chloride have been used to remove soluble and colloidal silica in alkaline environments (above pH 8) (see Browne Patent, US Pat. No. 5,453,206). However, these methods cannot break down large amounts of wastewater through high-flow mechanical systems because the silica particles are degraded down to 5 microns in size.

Microfiltration systems have been considered for the removal of silica contaminants from wastewater. However, representative microfiltration membranes having a pore size of about 0.5 microns are rapidly blocked by silica precipitated by conventional inorganic flocculants. Most of these silica particles have a size of 1.0 micron or less. In addition, the inorganic coagulant does not help precipitation of fine colloidal silica. Partially formed flocs also impede the flow by deforming or blocking the pores of the membrane.

Thus, there is a need in the art for an improved method for removing silica from wastewater.

Such methods and systems are disclosed and claimed herein.

The present invention relates to the treatment and purification of wastewater containing silica at high flow rates. More specifically, the present invention relates to a method and apparatus for removing silica from large quantities of wastewater using organic polymers with filtration membranes.

Figure 1a is a photomicrograph of 24,000 magnification of the precipitated silica particles produced in Example 3.

Figure 1b is a 49,000 magnification photomicrograph of the precipitated silica particles produced in Example 3.

Figure 2a is a photomicrograph of 20,000 magnification of the precipitated silica particles produced in Example 4.

Figure 2b is a 40,000 magnification photomicrograph of precipitated silica particles produced in Example 4.

3 is a schematic representation of one wastewater pretreatment system.

4 is a schematic diagram of one wastewater microfiltration apparatus for high velocity removal of impurities.

The present invention relates to a method for removing silica from a large amount of wastewater. In this process, a wastewater stream containing silica is treated with an organic polymer as a flocculant. This flocculant reacts with silica to form spherical particles that clump into clusters having a size of about 5 microns or more. The term "wastewater stream" as used herein includes raw water containing silica as well as process water stream containing silica.

Organic and inorganic flocculants which can be used to form the desired particles include polyacrylamide (cationic, nonionic and anionic), a molecular weight of 25,000 to 1,500,000, preferably a molecular weight of 100,000 or more, more preferably 100,000 to 300,000 Epi-dma (epichlorohydrin / dimethylamine polymer) having a molecular weight, DADMAC (polydiallyldimethylammonium chloride), a copolymer of acrylamide and DADMAC, natural guar, and the like. The stoichiometric ratio of flocculant to silica is preferably optimized so that the removal of acceptable silica is achieved with minimal flocculant cost. The required flocculant concentration depends on several factors, including the concentration of silica contaminants, wastewater flow rate, silica contaminant allowance, and flocculant / contaminant reaction rates. In the case of silica contaminants, the ratio of silica to flocculant is generally from 20: 1 to 50: 1, preferably about 40: 1, depending on the system. If a small amount of silica can be present in the effluent stream, the ratio of silicon to flocculant can be at least 120: 1. The optimum molar ratio may also vary depending on the flocculant used. For example, low molecular weight epi-dma and ultra high molecular weight epi-dma may need three to five times the dose to agglomerate silicon.

The organic flocculant was found to form silica as spherical particles which agglomerate in the form of particles having a size of about 10 to 90. Since these silica particles are easily separated from the microfiltration membrane, the silica can be efficiently removed without decomposition of the membrane.

Optionally, small amounts of coagulant may be used with the organic and polymeric flocculant to optimize the removal of silica. Examples of representative coagulants include aluminum chloride ("ACH", Al _n OH _2n-m Cl _m , for example Al ₄ OH ₆ Cl _{2 with} a typical ratio of Al to Cl ₂ : 1), sodium aluminate (NaAlO). ₂ ), aluminum chloride (AlCl ₃ ), and polyaluminum chloride (“PAC”, Al ₆ OCl ₅ ). A representative molar ratio of silica to the inorganic flocculant is about 25: 1.

Treated wastewater is passed through a microfiltration membrane that physically separates silica contaminants from the wastewater. Suitable microfiltration membranes are W.L. Commercially available from manufacturers such as Gore, Koch, and National Filter Media (Salt Lake City, Utah). For example, the GOR-TEXR membrane used in the present invention consists of polypropylene felt with a spray coating of Teflon. The Teflon coating is for promoting the passage of water through the membrane. These microfiltration membrane materials have been found to be useful in many wastewater treatment systems. However, when the microfiltration membrane is used in a system for removing fluoride or silica, it has been found that the aggregated particles adhere to the inner and outer surfaces of the membrane and block the membrane. In this case, backflushing was found to be ineffective.

Such microfiltration membranes are used in the form of tubular "sock" to maximize their surface area. This type of membrane is placed over a slotted tube to prevent it from collapsing during use. A net material is disposed between the membrane and the grooved tube to facilitate flow between the membrane and the groove of the tube. In order to handle very large volume flow rates, a number of membrane modules each containing a filter sock are used.

Such microfiltration membranes preferably have a pore size of 0.5 microns to 5 microns, preferably 0.5 microns to 1.0 microns. By controlling the ratio of flocculant to silica contaminant, 99.99% of the precipitated contaminant particles may be at least 5 microns. Thus, microfiltration membranes with larger pore sizes can be used. 0.5 to 1 micron flow rate of the treated waste water is passed through a small amount of filtration membrane of pore size was 150 GFD; was found to be (gallon / ft ² membrane / one gallon per foot of membrane per day) to 600 GFD.

It is desirable to remove the solids from the membrane surface by periodically backwashing the microfiltration membrane and draining the filtration vessel in which the membrane is placed. By this periodic, short-term countercurrent wash, any accumulated contaminants are removed from the walls of the microfiltration membrane sock. This removed solid material present in the filtration vessel is discharged into a holding tank for further processing.

The wastewater treatment system disclosed herein is designed to meet silica contaminant emission limits. By this wastewater pretreatment, insoluble silica contaminant particles are efficiently removed by the microfiltration membrane.

The present invention relates to a method for removing silica contaminants from a large amount of wastewater. In the practice of the present invention, the wastewater is collected and pretreated with at least one organic polymer flocculant, whereby the silica reacts with the flocculant (s) to form spherical particles that cluster into clusters having a size of at least 5μ. do. Such flocculants are preferably mixed with the wastewater using reaction vessels or static in-line mixers, although other mixing methods may be used.

The treated wastewater is then passed through a microfiltration membrane having a pore size of 0.5 microns to 5 microns to remove silica contaminant particles. In such a system, wastewater flow rates from 150 gallons per square foot of membrane per day (GFL) to 600 GFD are possible. This microfiltration membrane is periodically backwashed to remove solids from the membrane surface. This removed solid is gravity collected at the bottom of the filtration vessel and discharged in a settling tank in a time cycle to further treat the sludge.

The microfiltration membrane is preferably provided in a cassette-shaped module. These microfiltration membranes provide positive particle separation in a high recovery dead head filtrat ion array. This dead head filtration is effectively performed at low pressures (4 psi to 15 psi, preferably 5 psi to 10 psi) and high flow rates so that 100% of the feed water can be discharged without the need for a transfer pump. Solids that settle on the wall of the membrane during filtration are periodically backwashed to remove them from the membrane surface and gravity settling to ensure a clean filtration area continuously. By designing these individual modules in a cassette shape, membrane modules can be easily replaced.

Filter sock, which is generally preferred and useful in the present invention, contains a Teflon coating coated on a polypropylene or polyethylene felt backing material. The filter inside is W.L. Available from Gore. Another generally preferred filter core manufactured by National Filter Media, Salt Lake City, Utah, consists of a polypropylene membrane coated on a polypropylene or polyethylene felt substrate. The "failure" of the membrane occurs mainly because of flux loss, not because of mechanical failure. In many operations, it is believed that replacing the membrane sock is more cost effective than cleaning contaminants from the membrane.

The life of the membrane is important in terms of the continuous operation of the filtration system and the cost of the operation. W.L. Membranes manufactured by Gore and National Filter Media have been found to be tough at temperatures above 160 ° F and pH above 13 and do not cause significant failures. The operating conditions expected in the present invention are room temperature and a pH of 5-11. Generally the preferred pH range is about 7.3 to 9.3, but good results are obtained in the pH range of ± 1.0 of this pH range. It is generally desirable to adjust the pH before adding the inorganic flocculant. Membranes used according to the invention are believed to have a lifespan of at least 18 months. This filtration system is operated at low pressure, preferably at a pressure of 4 to 15 psi. It is also possible to operate at higher pressures, but the higher the pressure, the faster the flux loss of the membrane. Although the preferred operating pressure is generally about 25 psi or less, good results have also been obtained when using organic coagulants with commercially available high pressure microfiltration systems operated at pressures of 30 to 80 psi. Dramatically improved performance can be obtained by adapting existing microfiltration systems using conventional inorganic flocculants for use with organic flocculants.

The following examples are presented to further illustrate the present invention. These examples are purely illustrative and are not intended to limit the invention.

Example 1

An experimental scale system of 15 gpm was used to treat the fluoride containing wastewater and the combined flow of fluoride and silica. 38% sodium aluminate in an amount such that Al: F is 0.23: 1 to aid in the removal of fluoride, totally dissolved solids (TD S), totally suspended solids (TSS), and other salt forms of some present. A solution and a 35 ppm dose of 50% aluminum hydrochloride was used. A medium charged (25 ± 5 mol%) medium molecular weight polyacrylamide polymer was used to flocculate the precipitate to facilitate filtration or sedimentation of the precipitate. By this procedure, very low or undetectable fluoride effluent values and silt density indexes below 3.0 were obtained. The filtration membrane was a 0.5 micron polypropylene bonded membrane obtained from National Filter Media. When the operating pressure of the vessel was less than 9 psi, the flux of the membrane measured was between 650 and 900 GFD. The results measured for F in the inflow and outflow wastewater and F + SiO ₂ in the combined flow are reported in Tables 1 and 2, respectively, in Tables 1 and 2 given in parts per million (ppm).

term F inflow (ppm) F outflow (ppm) 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 at least 60.93 0.39 maximum 206.25 21.7

term F + SiO ₂ Inflow (ppm) F + SiO ₂ outflow (ppm) 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 at least 89.0 0.24 maximum 264.0 0.39

Example 2

An experimental scale system of 15 gpm was used to treat silica-containing wastewater. To aid in the removal of silica, TDS and TSS, a 38% sodium aluminate solution in an amount such that the Al: Si ratio is 0.45: 1, 50 ppm aluminum chloride at 25 ppm dose, and 20% at 0.25 to 1.0 ppm dose Epichlorohydrin / dimethylamine polymer (highly charged low molecular weight cationic epi-DMA product) was used. This material was used to form particles that are easy to filter or settle. This procedure yielded very low or undetectable fluoride effluent values and a Silt Density Indices of 3.0 or less. The filtration membrane was obtained from National Filter Media and was 0.5μ polypropylene felt with a PTFE (polytetrafluoroethylene) coating. The membrane flux was 175-400 GFD when the vessel's operating pressure was 9 psi or less. The measurement results for SiO ₂ inflow and outflow are reported in Table 3 below, where the units are given in parts per million (ppm).

term SiO ₂ inflow (ppm) SiO ₂ outflow (ppm) A 140 0.443 B 160 0.33 C 125 0.37 D 153 0.39 E 177 0.36 F 165 0.29 Average 153 0.364 at least 125 0.29 maximum 177 0.443

Example 3

Wastewater containing silica was treated using a 3-5 gpm bench scale system. This silica containing wastewater stream was obtained from a commercially available CMP slurry known as ILD 1300 sold by Rodel. The ILD 1300 slurry was diluted according to the manufacturer's instructions and measured by graphite furnace atomic absorption method and contained about 1380 ppm of Si, and by ion chromatography, it was found to contain about 70 ppm of ammonium (NH ₄ ). . One liter of wastewater weighed approximately 993.7 g. The silicon was present as dissolved and colloidal silica in wastewater. A small amount of sodium hydroxide and sulfuric acid were added to this wastewater stream to adjust the pH to about 8.58. At this time, the wastewater stream was adjusted to pH while mixing for about 3 minutes. 1 liter of 20 wt% solution of 2.09 g of epi-DMA (Epichlorohydrin / dimethylamine polymer; EnChem Lot I-1397 / 423 / MIC) having a molecular weight of 250,000 ± 50,000 and 0.19 g of dry aluminum chloride It was added to the wastewater stream and mixed for about 20 minutes.

The reaction mixture was pumped to a pressure of about 6 psi through a 2 foot long filter sock having a diameter of about 3.5 inches. Membrane flux was measured at 189 GFD. The filtration was equipped with a GOR-TEX ^R membrane (Lot. No. 66538-3-786) obtained from WL Gore. The membrane had a PTFE (polytetrafluoroethylene) coating coated on a polypropylene felt with 0.5μ pore size (1.5μ absolute).

The effluent of the filter membrane was collected and measured by graphite furnace atomic spectroscopy, and contained about 15.5 ppm of Si, and by ion chromatography, it was found to contain about 70 ppm of ammonium (NH ₄ ). Solids were collected from the surface of the filter and air dried for 24 hours. The solid thus recovered was spherical particles easily removed from the surface of the filtration membrane. The solids were dried and milled to analyze the results in weight percent in Table 4 below.

ILD 1300 result Loss upon drying 45.53% carbon 3.84% Hydrogen 1.04% nitrogen 1.41% silicon 36.74% aluminum 2.30%

Other components in the recovered solids such as sodium, potassium, and unknown proprietary components of ILD 1300 have not been analyzed.

1A is a scanning electron micrograph (SEM) of the resulting spherical silica particles taken at 24,000 magnification. FIG. 1B is a scanning electron micrograph (SEM) of the particle of FIG. 1A taken at 49,000 magnification. These particles typically have a particle size of 0.05μ to 0.15μ. These spherical particles were found to clump into large clusters that are smaller than the membrane pore size but do not pass through the membrane. These clusters have an average size of 10 μ to 300 μ. EDS analysis of this sample confirmed the presence of silicon and aluminum in the sample, and the concentration of silicon was found to be much higher than that of aluminum.

Example 4

Wastewater containing silica was treated using a 3-5 gpm bench scale system. This silica containing wastewater stream was obtained from a commercially available CMP slurry known as KLEBOSOL, which is commercially available from Hoescht. The KLEBOSOL slurry was diluted according to the manufacturer's instructions and measured by graphite furnace atomic absorption method, and found to contain about 4474 ppm of Si and about 3.2 ppm of aluminum. One liter of the wastewater weighed about 998.4 g. The silicon was present as dissolved and colloidal silica in wastewater. A small amount of sodium hydroxide and sulfuric acid were added to this wastewater stream to adjust the pH to about 9.84. At this time, the wastewater stream was adjusted to pH while mixing for about 3 minutes. 1 liter of a 20% by weight solution of 2.09 g of epi-DMA (Epichlorohydrin / dimethylamine polymer; EnChem Lot I-1397 / 423 / MIC) having a molecular weight of 250,000 ± 50,000 was added to the wastewater stream and for about 20 minutes Mixed.

The reaction mixture was pumped through a filter sock of Example 3 to a pressure of about 6 psi. The effluent of the filter membrane was collected and measured by graphite furnace atomic spectroscopy, and found to contain about 8.32 ppm of Si and less than 0.1 ppm of aluminum. Solids were collected from the surface of the filter and air dried for 24 hours. The solid thus recovered was nearly spherical particles that were easily removed from the surface of the filter membrane. This solid was found to be dry when removed from the membrane. 2A and 2B are scanning electron micrographs of the obtained spherical silica particles. These particles had a particle size of 0.05μ to 0.15μ. The solids were dried and crushed and the results are reported in weight percent in Table 5 below.

KLEBOSOL result Loss upon drying 1.91% carbon 1.41% nitrogen 0.43% silicon 40.49% aluminum 0.98%

2A is a scanning electron micrograph (SEM) of the obtained spherical silica particles taken at 20,000 magnification. FIG. 2B is a scanning electron micrograph (SEM) of the FIG. 2A particle taken at 40,000 magnification. EDS analysis of this sample confirmed the presence of silicon and aluminum in the sample, and the concentration of silicon was found to be much higher than that of aluminum. The silica particles of FIGS. 2A and 2B are very similar to the silica particles of FIGS. 1A and 1B.

3 illustrates one possible wastewater pretreatment system 10 within the scope of the present invention. The wastewater pretreatment system 10 includes a plurality of pretreatment reactors 12, 14, 16 capable of reacting the wastewater feed stream 18 with one or more flocculants. Coagulants that react with contaminants in the wastewater feed stream are introduced into the pretreatment reactor via coagulant feed streams 20, 22, 24. It is preferable to detect the pH inside the pretreatment reactor by the pH sensor 26. If desired, pH may be adjusted by adding acid or base to the pretreatment reactor via acid / base feed stream 28.

The number of pretreatment reactors may vary depending on the number of flocculants used and the number of chemicals used to form the waste particles. The size of the reactor may vary depending on the reaction time.

After passing through the required pretreatment reactor, the wastewater feed stream enters feed tank 30 for holding the pretreated wastewater. If desired, additional flocculant may be added directly to feed tank 30 via flocculant feedstream 31. As shown in FIG. 4, the pretreated wastewater enters one or more of the filtration vessels 32, 34, 36 through the filtration vessel feedstream 38. The size of the feed stream 38 may depend on the design flow rate of the filter vessel. For example, a 24 inch supply line is suitable for a system with five filter vessels each handling 2500 gpm. Each of the filtration vessels 32, 34, 36 is a filtration device installed vertically alone. The number and size of each filter vessel can vary depending on the capacity requirements of the system. The filtrate is removed from the filtrate via filtrate stream 40.

Each filtration vessel preferably has a mounting platform for 9 to 49 filtration cassette modules. One generally preferred filter cassette module has sixteen filter sock each comprised of 0.5 micron filtration membranes. The flow rate evaluated is 0.9 gpm / membrane area (ft 2). Each cassette module has a membrane area of 64 ft ² . Each cassette module is also rated at 58 gpm and has a differential pressure of less than 15 psi. It is preferable that a lifting device is provided to remove and replace the filtration cassette module.

The filtration membrane is periodically backwashed by the filtrate to remove solids from the membrane surface. During this countercurrent washing process, the filtration vessel is shut off from the line and the wastewater is discharged from the filtration vessel and flows into the countercurrent tank 44 through the countercurrent discharge stream 42. This backwash tank 44 stores the wastewater briefly before the backwashed wastewater is transported to the feed tank 30 via the backflow return stream 46. It is believed that 400 to 500 gallons of water is used for a 2500 gpm filter vessel during a typical counter flow cycle. It is preferred that a vacuum beaker 48 be installed for pressure equalization in the individual filter vessels 32, 34, 36 during the countercurrent process. Aeration / decompression stream 49 is provided for draining or releasing excess or excess pressurized wastewater.

The filtrate side of the filtration vessels 32, 34, 36 is exposed to the atmosphere. The filtrate is collected at the top of the filter vessel and enters the filtrate stream 40. This filtrate acts as a positive head. This positive head is associated with a negative head that drains the pressure side of the vessel through the backflow outlet stream 42 to generate a pressure gradient sufficient to backwash the filtration membrane.

After sufficient sludge has settled at the bottom of the filtration vessels 32, 34, 36, the sludge is discharged through the discharge stream 50. When this sludge is removed, the filtration membrane is preferably washed with water in the water washing stream 52. The collected sludge is removed from the system for further processing or storage.

Periodically the membrane may require dipping to remove traces of organic material. It is desirable to perform the wash as needed or as part of a regular maintenance program. The drain of the vessel is opened so that all contaminants are removed through the sludge discharge stream 50. Wash solution is introduced into each filter vessel through wash feed stream 54. Representative washing solutions include acids, bases and surfactants. In some cases, the filter vessel may be operated again without draining and washing. If rinsing of the membrane is required, the contents of the filtration vessels 32, 34 and 36 are removed via the wash effluent stream 56 for further processing.

As shown in Figure 4, it is preferable to use a plurality of filter vessels in parallel to provide the required flow rate. However, the filtration vessels can be operated in series to provide primary filtration and secondary filtration. Since the filter vessel is shut off from the line during the countercurrent wash, it is desirable to use an additional filter vessel to ensure the required discharge flow rate. These additional filtration vessels are installed to stay off-line and the rest of the system meets flow rate requirements.

The wastewater treatment system preferably includes means for accessing various process streams to enable sampling and analysis. It is preferable to install valves, pumps, and sensors used in the art to safely control the fluid flowing through the filtration vessel. In addition, these valves, pumps, and sensors allow for process automation.

As described above, the present invention provides a method for removing contaminants from wastewater using a positive physical barrier to precipitated particles. With this positive separation barrier, the emissions have a lower concentration limit than conventional clarification / sand filtration systems.

Claims (27)

As a method for removing a large amount of silica from wastewater, (a) treating the wastewater stream containing silica with an organic polymerized coagulant to cause the flocculant to react with the silica to form agglomerated spherical silica-based particles having a size of at least about 5μ; ; (b) passing the treated wastewater through a microfiltration membrane having a pore size of 0.5 μm to 5 μm to remove the silica from the wastewater passing through the microfiltration membrane; (c) periodically backwashing the microfiltration membrane to remove solids from the surface of the membrane. The method of claim 1 wherein the molar ratio of silica to the flocculant is from 20: 1 to 50: 1. The method of claim 1 wherein the molar ratio of silica to the flocculant is from 35: 1 to 45: 1. The method of claim 1, further comprising adjusting the pH of the wastewater stream to a pH of about 5-11. The method of claim 1 wherein said flocculant is an epichlorohydrin / dimethylamine polymer. The method of claim 1 wherein said flocculant is an epichlorohydrin / dimethylamine polymer having a molecular weight of 25,000 to 1,500,000. The method of claim 1 wherein said flocculant is an epichlorohydrin / dimethylamine polymer having a molecular weight of 200,000 to 300,000. The method of claim 1 wherein said flocculant is a DADMAC (polydiallyldimethylammonium chloride) polymer. The method of claim 1 wherein the flocculant is a copolymer of acylamide and DADMAC (polydiallyldimethylammonium chloride). The method of claim 1 wherein said membrane consists of PTFE (polytetrafluoroethylene) coated on a polypropylene felt. The method of claim 1 wherein the membrane consists of PTFE (polytetrafluoroethylene) coated on polyethylene felt. The method of claim 1, wherein the treated wastewater is passed through the microfiltration membrane at a pressure of about 80 psi or less. The method of claim 1, wherein the treated wastewater is passed through the microfiltration membrane at a pressure of about 25 psi or less. The method of claim 1 wherein the treated wastewater is passed through the microfiltration membrane at a pressure of about 4 psi to 15 psi. The method of claim 1, wherein the treated wastewater is passed through the microfiltration membrane at a pressure of about 5 psi to 10 psi. The method of claim 1, wherein the treated wastewater is passed through the microfiltration membrane at a flow rate between 150 gallons per square foot of membrane per day (GFD) and 600 GFD. As a method for removing silica from a large amount of waste water, (a) treating the wastewater stream containing silica with an organic flocculant to cause the flocculant to react with the silica to form agglomerated spherical particles in the form of clusters having a size of at least about 5 microns; (b) passing the treated wastewater through a microfiltration membrane having a pore size of 0.5 μm to 5 μm at a flow rate of 150 GFD to 600 GFD to remove the silica from the wastewater passing through the microfiltration membrane; (c) periodically backwashing the microfiltration membrane to remove solids from the surface of the membrane. The method of claim 17, wherein the molar ratio of silica to the flocculant is from 20: 1 to 50: 1. The method of claim 17, wherein the molar ratio of silica to the flocculant is from 35: 1 to 45: 1. 18. The method of claim 17, wherein said flocculant is an epichlorohydrin / dimethylamine polymer. 18. The method of claim 17, wherein said flocculant is an epichlorohydrin / dimethylamine polymer having a molecular weight of 100,000 to 1,500,000. 18. The method of claim 17, wherein said flocculant is an epichlorohydrin / dimethylamine polymer having a molecular weight of 200,000 to 300,000. 18. The method of claim 17, wherein said flocculant is a DADMAC (polydiallyldimethylammonium chloride) polymer. 18. The method of claim 17, wherein the flocculant is a copolymer of acylamide and DADMAC (polydiallyldimethylammonium chloride). Spherical silica precipitates prepared by reacting a wastewater stream containing dissolved or colloidal silica with an epichlorohydrin / dimethylamine polymer having a molecular weight of 100,000 or greater, wherein the molar ratio of silicon to the polymer is from about 20: 1 to about 60: 1. . 26. The spherical precipitate of claim 25, which is clustered into clusters having an average size of 10 microns to 300 microns. The silica precipitate of claim 25 having a silica content of at least 30% by weight.