Patent Application
20110210080
Not applicable.
The invention relates to an effective water treatment process for removal of organics in liquid, especially potable water. The process combines ceramic porous membranes having photo-catalytic properties to oxidise the organic matter or photo-catalysts in the form of particulate with micro- and nanocapsules that will deliver strong oxidising agents at the membrane surface.
Chlorine is widely used in the treatment of potable water today. According to toxicological studies and reports, some disinfection by-products (e.g. trihalomethane (THM), haloacetic acid (HAA), chlorite, chlorate, bromate) are possible human carcinogens. Most of the chlorine demand in unpolluted drinking waters is exerted by natural organic matter (NOM).
The optimum selection of treatment processes to remove organics depends on the character of the organics present and on the required final quality of the treated water. Generally alum is the best performing inorganic coagulant for NOM, colour and turbidity removal under conventional pH conditions (6-7). However, there is a portion of the organic matter that cannot be removed by coagulation processes and will require additional treatment. The residual NOM after treatment affects the disinfectant demand, the formation of disinfection by-products and biofilm formation in the distribution system. Removal of biodegradable organics will reduce disinfectant decay and biofilm growth in distribution systems.
The selection of a treatment process to remove organics will be dependant on the character of the organics and the extent of removal required. The need to remove NOM for improving water quality beyond what is achievable by coagulation alone will require additional treatment. For the treatment of water for potable use a number of advanced treatment techniques have been developed worldwide. These generally fall into three categories: oxidative processes, adsorbents and membrane filtration.
Oxidative Process:
UV-treatment of NOM leads to progressive reduction in its molecular weight, the demand of organic carbon and eventual mineralization. The product water from the VUV/BAC process presents low potential health risks in terms of THM, HAA, nitrite, hydrogen peroxide, bromate, cytotoxicity and mutagenicity.
A process involving a polymer adsorption resin incorporating iron was specifically designed for the removal of DOC from drinking water (Morran et al, 1996). This process combined with powdered activated carbon (PAC) and coagulant treatment was found to improve the amount of DOC removed by between 82-96%, to decrease chlorine demand, and to significantly decrease THM. Bacterial regrowth was however increased, highlighting the critical difference between using a treatment to reduce NOM concentration and changing NOM character.
Adsorbents:
When activated carbon is applied for the removal of problem microcontaminants, such as taste and odour compounds, algal toxins or pesticides, NOM affects significantly its effectiveness. Strong competition for adsorption sites results in higher dose requirements for powdered activated carbon (PAC) and shorter lifetimes for granular activated carbon (GAC) filters. NOM character also plays a role in the competitive effect, with the NOM in the molecular weight range similar to the target compound causing the greatest competition, and therefore the greatest effect on adsorption.
Membrane Filtration:
Microfiltration/Ultrafiltration membranes remove little NOM as the size of the molecules is usually smaller than the pore size of the membranes (see Table 1). However, NOM fouls low pressure membranes and chemical cleaning is required to restore the flux. Composition of NOM has a strong impact on the rate of fouling: hydrophilic neutral compounds with high molecular weight appear to have a large influence on the fouling rate.
Coagulants almost always lower the rate of membrane fouling. Addition of particles, such as magnetite, with a coagulant may improve membrane performance by increasing the porosity of the filter cake. UV-degradation of NOM prior to membranes lowers the fouling rate of membranes.
Application of photo-catalysis to liquid treatment has been limited due to the difficulties in having efficient contact between photo-catalysts and reactants in liquid and in supplying sufficient light to the photo-catalysts. Dispersing photo-catalysts having fine powder form in liquid increases the contact between photo-catalysts and reactants in liquid. However, separating the fine photo-catalysts that can have size from a few nanometres to sub-micrometers from the liquid is difficult. In addition, the powder in liquid reduces the light strength in the depth direction quickly. Accordingly, large part of the photo-catalysts can be in short of light supply. As photo-catalytic activity depends on the light power, the configuration of dispersing photo-catalysts may not give the optimum oxidation performance of the photo-catalytic material. Photo-catalytic microspheres of size about 10 μm to about 200 μm improve not only the recycle of the microspheres by membrane filtration but also could improves the photo-catalytic ability than photo-catalyst in powder form (WO 2008/076082).
Immobilised catalyst is preferred in the sense that the process does not require extra facility to separate out the catalyst. In addition, catalyst loss during the treatment can be negligible which is important when expensive catalysts are employed in the system. Furthermore, light will be supplied evenly to the photo-catalyst independently on the photo-catalyst position in the liquid. In such immobilised catalyst system, liquid can flow over the catalyst (cf. U.S. Pat. No. 5,779,912) and through the catalyst layer (“Photo-catalytic membrane reactor using porous titanium oxide membranes, Tsuru, T; Toyosada, T; Yoshioka, T, et al., J. Chem Eng. Japan, Vol. 36 (9), p. 1063-1069 (2003)). Extra gas can be added to the reaction field by using membrane (photo-WaterCatox, WO 02/074701.)
When liquid is supplied over the catalyst with parallel flow to the catalytic layer, turbulent flow and narrow liquid layer thickness over the catalytic layer are preferred to facilitate the contact between reactants in the liquid and photo-catalyst, and to maintain the light power as strong as possible and to increase the catalyst/reactant ratio. On the contrary, if the liquid goes through the catalytic layer, reactants will be transported to the catalyst by diffusion and also by the flow. This configuration is beneficial for the transport. However, it also can cause problems: the permeation pathways have nanometer order size in the catalytic layer and the pathways can be plugged after certain time of operation with particles or molecules in the liquid. Plugging causes pressure increase at the liquid side and reduction of flux going through the layer. In addition, higher pressure, such as 5-100 bars, is required to obtain sufficient flux going through the catalytic layer as the permeation is reduced both by the resistances of catalyst layer and porous supporting layer.
The encapsulation of the photo-catalyst (for example, TiO2) could in many cases enhance the optical/photo-catalytic properties. Photo-catalyst can also be coated by e.g. porous silica layers to prevent deterioration of a base material that has a close contact with photo-catalyst (JP 09225321 A1).
However, the inventors of present invention have found that the encapsulation of oxidizing agents such as H2O2 in TiO2 capsules could further enhance the photo-catalytic properties. The controlled release of the oxidizing agents could significantly improve the efficiency of the photo-catalysts. The advantages of encapsulation of oxidizing agents inside the photo-capsules, can be listed as follows:
The present invention provides a process for removal of organics in liquids, especially dilute, toxic organics in water, by contacting the liquid with microcapsules 2 containing oxidising agents, in combination with a photo-catalytic membrane 3.
The present invention further provides a system for removal of organics in liquids, especially dilute, toxic organics in water, comprising a photo-catalytic membrane 3 in combination with microcapsules 2 containing oxidising agents.
The microcapsules can be made of porous materials, e.g. mesoporous or microporous materials or can be made of dense materials.
Further, the microcapsules can be made of photo-catalytic material, e.g. TiO2. JP 2003096399 describes use of a photo-catalytic microcapsule of TiO2. A photo-catalyst support for use in water treatment based on photo-catalyst coated particles having core-shell structure is reported in JP 2006247621.
The microcapsules can also be made of inorganic materials like metal oxides or organic and inorganic hybrid composite materials or organic materials.
The microcapsules can be either dispersed or fixed on a mesh filter that has much smaller transport resistance than the porous support, having nano to micrometer-sized pores with mm thickness. The mesh filter can be hydrophobic or hydrophilic and has affinity to reactants in liquid.
Capsules in liquid increase the overall transport of reactants to the catalytic surface. In addition, the pressure drop in the reactor is smaller compared to the case where flow is passing through the membrane and high pressure is not required providing a simpler and cheaper unit design. Furthermore, the combination of light source and nets where capsules are immobilized gives better light supply to the photo-catalyst that exists at the surface of the capsule.
The advantages of this configuration are: less transport resistance, better mass transport of reactant(s) to the catalytic surface, fixed catalyst, short distance from the light source to the catalyst, supply of the oxidising agent from the capsules.
Left: hollow photo-catalyst (18). The particle consists of photo-catalytic layer (shell), which can be dense or porous. The capsule is hollow.
Middle: The core (19) of the capsule (18) is filled with liquid, solid or gas that has oxidising property by itself or generating oxidants by photo-catalysis. Capsules can also be filled with reducing agents or other chemicals.
Right: The central void of the shell has two or multi mixture of gas, liquid and/or solid.
Potassium Permanganate KMnO4 (Carus® Chemical Company) and paraffin wax were used as oxidant and capsule material, respectively. The powder of the oxidant was portion by portion added to the melted wax with continuous stirring and heating to form homogeneous mixture containing 45% of oxidant. After stirring the mixture for some time, the molten wax with dispersed oxidant was added drop-wise slowly to water. The molten wax solidified immediately when the droplet of wax got in contact with water.
Formed capsules were weighed and poured to predetermined volume of water.
KMnO4 has a solubility of 6.4 g/100 ml water at room temperature. Accordingly, if KMnO4 was dispersed as powder, it will dissolve immediately. On the contrary, the concentration of KMnO4 increased slowly but continuously with time when capsules were added. The results clearly show that the encapsulation can control the dissolution of KMnO4.
Sodium Persulfate Na2S2O8 (Sigma-Aldrich®) and paraffin wax were used as oxidant and capsule material, respectively. The powder of the oxidant was portion by portion added to the melted wax with continuous stirring and heating to form homogeneous mixture containing 37% of oxidant. After stirring the mixture for some time, the molten wax with dispersed oxidant was added drop-wise slowly to water. The molten wax solidified immediately when the droplet of wax got in contact with water.
Na2S2O8 has a solubility of 55.6 g/100 ml water at room temperature. Accordingly, if Na2S2O8 was dispersed as powder, it will dissolve immediately. On the contrary, the concentration of Na2S2O8 increased slowly but continuously with time when capsules were added. The results clearly show that the encapsulation can control the dissolution of Na2S2O8.
Potassium Permanganate KMnO4 (Carus® Chemical Company) or Sodium Persulfate Na2S2O8 (Sigma-Aldrich®) were used as oxidants. Sylgard® resin (Aldrich®) was used as capsule material, respectively. Sylgard® resin was mixed with the Sylgard® curing agent and powdered oxidant then vigorously mixed to obtain uniform mixture. Obtained mixture of the resin with oxidant was poured on the special matrix. The matrix was made from a stainless steel foil. In the flat foil many indentations (few millimeters in diameter) of the semispherical form were made.
After pouring the mixture of the resin with the oxidant on the matrix the excess mixture was wiped out, only the mixture contained in the indentations was left. The matrix was left at the room temperature for 24 hours. The capsules were withdrawn and used. In the case of both oxidants the concentration of the oxidant in the mixture was 45%.
Similar to examples 2 and 3, the concentration of KMnO4 increased slowly but continuously with time when capsules were added. The results clearly show that the encapsulation can control the dissolution of KMnO4.
Potassium Permanganate KMnO4 (Carus® Chemical Company) or Sodium Persulfate Na2S2O8 (Sigma-Aldrich®) were used as oxidants. The inner void of hollow particles consists of porous silica shell and having size of 2-5 μm (Washin Chemical, Japan) was filled with oxidant as follows.
Persulfate and permanganate anions are negative, so to facilitate adsorption of oxidant in silica capsules silica powder was first treated in aqueous solution of PEI (Polyetylene imine, m.w. 70000, Polyscience®) of the concentration 2000 ppm for 1 hour with continuous stirring. The capsules were separated by centrifugation, washed with water and dried at the room temperature. Dried silica was poured to the saturated solution of sodium oxidant for 24 hours. Finally the silica powder with oxidant was washed and dried.
The dissolution of KMnO4 is faster than in the examples 1 to 3. This is because the shell of the capsule is porous in this case, while the shell in examples 1 to 3 was dense. The dissolution of KMnO4 was limited, showing the possibility to control the release by the pore structure of the shell material.
The comparison of the results of examples 1 to 4 also show that changing type of capsule material and amount of oxidant in capsule one can control effectively the rate of oxidant release.
Hollow particles consists of porous silica shell was purchased from Washin Chemical, Japan. As an example of photo-catalyst, TiO2 was deposited on the surface by two methods. In the first method, commercial TiO2 powder (P25, Evonic, former Degussa®) and hollow particles were dispersed in water or in ethanol. The pH of the solution was controlled to 2<pH<5, so that silica and TiO2 have opposite surface charge. In the second method, the hollow silica particles were dispersed into a mixture solution of 2% titanium isopropoxide and 98% ethanol. In both cases, the dispersion was stirred for 1 hour, and then the particles were removed from the solution, washed, dried and calcined at 250-600° C. for one hour.
TiO2 was deposited on the hollow particle by both methods.
Humic acid sodium salt (HANa) was dissolved in water with the concentration of 50 mg/l. The oxidant and the photo-catalyst were mixed with the HANa solution and the mixture solution was exposed to either visible light (VIS) or UV light for one hour. Na2S2O8 was used as oxidant and TiO2 (Degussa®, P25) was used as photo-catalyst. Halogen lamp and Xenon lamp were used as VIS and UV sources, respectively. The concentration of HANa before and after applying light was measured by UV-VIS spectrometry. The absorbance at 254 nm was used to follow the HANa concentration.
Table 1 summarise the results. HANa is stable and was not decomposed by either UV or VIS irradiation when no oxidant or TiO2 was present in the solution. Oxidant (Na2S2O8) and irradiation decomposed HANa as shown in the table 1 but only to a limited extent. The concentration of HANa decreased more with UV light than with VIS light, that might be due to a formation of stronger oxidant under UV. Photocatalyst (TiO2) alone can also decompose HANa under the irradiation. As TiO2 is activated with UV light, the removal rate is again higher with VIS light. The decomposition of HANa in one hour was less than 3% and 20% under visible light and UV, respectively, in the case when only oxidant or only TiO2 was present in the solution, showing the difficulty to oxidise HANa by oxidant and by photo-catalyst.
On the contrary, when both oxidant and photo-catalyst were added to the solution, the HANa decomposition rates dramatically increased. More than 90% of HANa was removed after exposing the solution to UV light for 1 hour. The combination of oxidant and photo-catalyst also decomposed HANa under VIS light. More than 30% of the HANa was removed after exposing the solution to VIS light for 1 hour.
The results clearly show the synergy effect of mixing oxidant and photo-catalyst.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008 3578 | Aug 2008 | NO | national |
This application is a 35 U.S.C. §371 national stage application of PCT/NO2009/000291 filed Aug. 18, 2009, which claims the benefit of Norwegian Application No. 20083578 filed Aug. 18, 2008, both of which are incorporated herein by reference in their entireties for all purposes.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO2009/000291 | 8/18/2009 | WO | 00 | 5/13/2011 |
