Water purification process and plant

Abstract

It is the object of the present invention a plant and a process for the purification of wastewater comprising: a) electrocoagulation; b) electro-oxidation under UV light at a wavelength in the range of 275-380 nm, where the water to be treated constitutes the electrolyte.

Description

[0001] Water purification process and plant

[0002] Background art

[0003] Wastewater purification plants typically comprise a series of components adapted to remove, with a process in series, the contaminants from waste of urban and / or industrial origin.

[0004] The products that exit from a purification plant are:

[0005] - clarified water, discharged into channels adjacent to the plant;

[0006] - sludge, which can be sent, according to its organic load, to a composting plant, special landfills or disposed of in agriculture;

[0007] - reject products such as leaves, debris, toxic and non-toxic waste, separated from the main products and disposed of in an appropriate place.

[0008] During the design of a purification plant, one of the first discriminating criteria for the overall choice of which technologies to adopt is the type of wastewater. A standard distinction made by the legislator is as follows:

[0009] - domestic wastewater: water coming from residential type settlements and from services mainly deriving from the human metabolism;

[0010] - urban wastewater: domestic wastewater, runoff rainwater conveyed into sewer systems, even separate, and coming from a conurbation;

[0011] - industrial wastewater: any type of wastewater discharged from buildings or plants where commercial activities or the production of goods take place, other than domestic wastewater and runoff rainwater.

[0012] The plants typically in use provide for chemical-physical processes, membrane separation and vacuum evaporation.

[0013] The object of the present invention is to provide a separation technique which guarantees advantageous performance levels in terms of management, sustainability and cost effectiveness.

[0014] Description of the invention

[0015] It is the object of the present invention to provide a process and a plant for implementing it.

[0016] Description of the drawings

[0017] Figure 1 : (A) process diagram according to the present invention; (B) process diagram according to another embodiment.

[0018] Figure 2: schematic representation of the plant according to an embodiment.

[0019] Figure 3: A, B, C, diagram representative of the yields of three processes implemented according to the present invention.

[0020] Figure 4: process diagram in the embodiment comprising a photooxidation step implemented during electro-oxidation, with recovery and re-introduction into the tank of the photocatalyst crystal.

[0021] Figure 5: schematic representation of an embodiment of a portion of the plant. (A) front view. (B) side view. (C) perspective view.

[0022] Figure 6: schematic representation of a detail of the portion of plant in the embodiment of figure 5. (A) perspective view. (B) front view. (C) side view.

[0023] The process according to the present invention is characterized in that it comprises two steps:

[0024] Step a) electrocoagulation;

[0025] Step b) electro-oxidation under UV light at a wavelength in the range of 275-380 nm, where the water to be treated constitutes the electrolyte.

[0026] In an embodiment, said process further comprises:

[0027] Step c), addition in said electro-oxidation step, of a photocatalytic semiconductor selected from the group comprising TiO2, ZnO, SnO2, WO3, ZnS or mixtures thereof.

[0028] In an embodiment, said photocatalytic semiconductor is added in an amount comprised between 0.03 and 0.30 g / l, or 0.05-0.10 g / l.

[0029] Said step a), of electrocoagulation enables the formation of a colloid system, in which the solid phase tends to separate off forming a flocculate. The electrocoagulation step is performed with Iron and / or Aluminum electrodes. At the anode, anodic oxidation takes place with the release of metal ions in the form of hydrates which absorb the pollutant material creating flocs. At the cathode, anodic reduction takes place, with the formation of hydrogen gas which keeps the flocs that has formed afloat. The effect of the electric field is such as to also cause the large molecules present in the water to be treated to break up into fractions with a lower molecular weight, reducing the soluble COD (chemical oxygen demand). Thus, a large part of the pollutants is removed in the form of gases (N2, CO2) and the soluble compounds are converted into insoluble compounds (silica, heavy metals, organic substances). The precipitates tend to form a sludgy sediment, kept in suspension through the distribution of air-filled microbubbles from below. The microbubbles guarantee both continuous contact between the electrodes and suspension, and a uniform distribution of M27M3+throughout the suspension.

[0030] This step is followed by filtration, preferably with a 0.2-micron MBR (membrane bioreactor) immersion membrane, to clarify the wastewater. Also in this case, to prevent the membranes from clogging, air-filled microbubbles are distributed from below.

[0031] Contextually to the use of Fe electrodes, hydrogen peroxide can be dosed into the plant to increase, in the acid environment (pH 3-5), the oxidizing power of the system through the Fenton reaction in the presence of Fe2+which catalyzes the decomposition the H2O2. This process is particularly useful for oxidizing hazardous organic molecules such as, by way of example, VOCs, BTEX, IPA, aniline.

[0032] Fe2++ H2O2 -^Fe3++ 0H° + 0H~

[0033] The radical OH°- reacts with the organic compounds according to the reaction:

[0034] OH° + RH R° + H2O with the formation of an alkyl radical, in the presence of oxygen dissolved in water, to provide a peroxide alkyl radical which is subsequently decomposed to carbon dioxide and water.

[0035] Step b), of electro-oxidation, removes soluble CODs and a large part of the ammonia. The electrodes used are electrodes made of titanium or graphite. In proximity to the electrodes there are UV LEDs (A=275-380 nm). Advantageously, exposure to UV light dissociates chlorine and hypochlorite in the form of chloride ion, neutralizing the formation of CI2 which would occur.

[0036] In an embodiment, said steps a) and b) are preceded by a pre-treatment for removing coarse solids suspended in the water to be treated.

[0037] In an embodiment, said step b), of electro-oxidation, is followed by an ultrafiltration and / or reverse osmosis step and / or further filtration through nano-structured adsorbent (HTS) or zeolites.

[0038] Once the cycle is complete, as well as water is purified, residual wet sludges are obtained.

[0039] In an embodiment, said wet sludges are conveyed towards an appropriate drying system, e.g., a drying belt filter, or a centrifuge, or a settling vessel, to enable dehydration and hence definitive removal. The dehydration water can be appropriately reintroduced into the treatment cycle.

[0040] The process according to the present invention advantageously exploits the following chemical reactions: Electrocoagulation step:

[0041] 3H2O + 3e 3 / 2H2+ 3OH’ or 2H2O + 2e H2+ 2OH’

[0042] Electro-oxidation step with exposure to UV light:

[0043] Cl2(g) + hv (A<400 nm) 2 CI¬

[0044] CI’ + H2O HCI + OH’

[0045] OH + OH’ H2O2

[0046] HOCI (g) + hv (A<400 nm) 2OH’ + Ch

[0047] (g) Cl- + H2O HCI + OH-

[0048] OH + OH- H2O2

[0049] The hydrogen peroxide formed with the indicated reaction cooperates in synergy with the electrodes in the oxidation process for decomposing organic substances and microorganisms. The presence of Fe2+coming from the electrocoagulation step, in combination with the production of H2O2, increases the oxidizing power of the system through the Fenton reaction.

[0050] Step c) is optionally used in synergy with step b) by advantageously exploiting H2O2produced by electro-oxidation to amplify its oxidizing action through the following photocatalytic reactions:

[0051] 1 ) photocatalytic + ho = h++ e-

[0052] 2) e- + O2= 0'2

[0053] 3) O-2+ H+= OOH

[0054] 4) H2O2+ 0’2= OH + OH- + O2 The h+, 0'2, OOH and OH forms generated by the photocatalytic reaction react with the pollutants degrading them.

[0055] In an embodiment, said process is implemented in a plant which, with reference to figure 2, comprises:

[0056] - at least a first electrocoagulation tank (1 ), which comprises Fe or Al electrodes. From said at least one electrocoagulation tank the water and the flocs and precipitates formed pass into a filter tank (2);

[0057] - At least one filter tank (2) that contains MBR (microfiltration membranes at 0.2 pm) submerged to collect suspended solids, bacteria and algae. The liquid thus clarified is conveyed into a second electrocoagulation tank (3);

[0058] - At least a second electro-oxidation tank (3) with Ti or graphite electrodes, adapted to oxidize soluble CODs, metals and nitrogen compounds. Said tank is preferably closed by a lid adapted to convey any vapors that are formed during oxidation. Said tank is illuminated with UV light, preferably high efficiency UV LED, for dissociating CI2 and hypochlorite. Where said lid is provided, said LEDs are conveniently positioned below said lid. From said second electro-oxidation tank (3) the liquid is conveyed by overflow to a further filtration section, for example a tank (4) optionally provided with MBR membranes and / or an ultrafiltration module (5);

[0059] - Optionally, one or more ammonia nitrogen adsorption modules (6) into which said liquid is passed after said second ultrafiltration, for removing any ammonia nitrogen still present;

[0060] - Optionally, one or more reverse osmosis modules (7), to be activated when values above the permitted threshold of salts, especially chlorides, are measured in the liquid;

[0061] - One or more storage tanks (8), adapted to contain the water used for counter washing, e.g., for counter washing the MBR and ultrafiltration during the process and for chemical washing during maintenance.

[0062] In an embodiment, said process is implemented in a plant where one or more of the tanks comprised in the plant of figure 2 is replaced by a multiplicity of electric hoses. With reference to figures 5 and 6, said electric hoses 50 are hoses 51 that comprise electrodes 52 inside. Said electric hoses are arranged in series, where said water to be treated enters into a first one of said hoses and then, passing into each of the electric hoses, exits from the last of said electric hoses. Advantageously, said electrodes 52 are easily removable from said hoses 51 , so as to facilitate the maintenance and replacement operations. The number of said electric hoses varies according to the flow rate.

[0063] In an embodiment, said electric hoses are arranged on top of one another, where the water to be treated enters into the first hose which is the highest one and, with a zig-zag path, crosses each of said hoses and exits from the lowest hose. By way of explanation, the embodiment of figure 5 comprises 16 electric hoses and enables a flow rate of 15 m3 / h, with an average length of time of the liquid in contact with the electrodes of about 30 minutes. In this embodiment, said electric hoses occupy a surface area of about 5m2. This solution enables considerable savings in terms of occupied surface area, where the same flow rate and length of time are obtained in the tank with a demand of at least 12m2surface area. In this embodiment, said plant for the purification of wastewater comprises:

[0064] - at least a first multiplicity of electrocoagulation electric hoses, arranged in series, each comprising Fe or Al electrodes, with an inlet hose and an outlet hose;

[0065] - at least one filter tank that contains MBR (0.2 pm microfiltration membranes) submerged to collect suspended solids, bacteria and algae;

[0066] - at least a second multiplicity of electro-oxidation electric hoses, arranged in series, each comprising Ti or graphite electrodes, adapted to oxidize soluble CODs, metals and nitrogen compounds, with an inlet hose and an outlet hose;

[0067] - optionally, one or more ammonia nitrogen adsorption modules;

[0068] - optionally, one or more reverse osmosis modules, to be activated when values above the permitted threshold of salts, especially chlorides, are measured in the liquid;

[0069] - one or more storage tanks, adapted to contain the water used for counter washing.

[0070] In a preferred embodiment, the plant also comprises process control systems, by way of example, redox sensors, level sensors, pH sensors and / or turbidimeters. When present, the plant also comprises a PLC control cabinet (9) which collects the signals from the sensors, processes them, and handles the process.

[0071] In an embodiment, said plant is conveniently housed on a movable unit. The steps that compose the process according to the present invention, depicted in figure 1A, B, are detailed below.

[0072] Step I: P re-treatment

[0073] Optionally, the incoming fluid is stored in an accumulation tank (optional). This may be a reinforced concrete tank, a silo or a tank inserted especially on the skid of the plant and it is required as protection for the suction centrifugal pump. The presence of a tank of larger dimensions is necessary where the annual flow rate has a discontinuous trend, i.e. , with peak periods with respect to the average annual flow rate.

[0074] Optionally, the incoming fluid can pass through a grille, so as to remove any solid and / or sludgy material in the wastewater to be treated.

[0075] The incoming fluid is then subjected to rough filtration, carried out by washable basket filters, self-cleaning hydrocyclones or cartridge filters, usually made of metal material, with a not excessively fine mesh. Said filters, placed upstream of the charge pump, provide for the inlet of large solid particles inside the suction of the charge pump, thus protecting its impeller and auger.

[0076] Step II: Flocculation

[0077] This is the first step inside the skid, and it can be carried out according to two different methods: a) by injection of a chemical flocculant which promotes the aggregation and precipitation of the suspended solids, causing a large reduction of the organic pollutant load and a first reduction of the metals; b) by electroflocculation, where the aggregation step is encouraged indirectly by the passage of electric current in the fluid, liberating therein negative ions coming from the anode made of metal material. This method has lower costs for the supply of reactants but requires periodic replacement of the metal plates.

[0078] Said flocculation step, in one embodiment, takes place in an appropriately shaped tank so as to promote the precipitation of the flocs, which are then sent to the sludge line. In an embodiment, this takes place in a multiplicity of electric hoses arranged in series.

[0079] Step III: Microfiltration (MF)

[0080] The microfiltration step removes a significant part of the turbidity caused by suspended solids and bacteria. The water leaving this step is already visibly clearer without suspended solids, although it still has a characteristic color. This water moves on to step IV.

[0081] Step IV: Electro-oxidation and photo-oxidation process

[0082] The tank or the electric hoses where said process takes place comprise non-sacrificial electrodes adapted to oxidize organic material (e.g., abatement of: COD, BOD, ammonia, plant protection products, IPA, etc..) and inorganic material (e.g., heavy metals). The presence of power LED UV (with radiated energy from 500 to 1000 w / m2) prevents the formation of gaseous chlorine. LED light is able to activate a photocatalyst capable of eliminating microorganisms and oxidizing complex organic molecules (e.g., drugs, PFAS etc..).

[0083] In an embodiment, such step also makes use of a photocatalyst. Making use of a photocatalyst in this process step, when the water is already clear and no formation of sludge is observed, guarantees maximum photocatalytic effectiveness.

[0084] In an embodiment, said photocatalyst is a constituent of the inner walls of said tank. In this embodiment, as the contact surface is smaller than the volume, said photocatalyst is not particularly effective.

[0085] In an embodiment, said photocatalyst is a constituent of the inner walls of said electric hoses. In this embodiment, the ratio of the contact surface / volume is increased with respect to the tank embodiment, clearly improving the effectiveness of the photocatalyst.

[0086] In a preferred embodiment, said photocatalyst is in suspension in said tank or in said electric hoses. Photocatalyst crystals with dimensions 1- 100 pm are introduced into the tank or into said electric hoses. At the end of the electro-oxidation process, the suspension passes through a microfiltration system (porosity 0.5 pm) where the permeate is discharged or reused in the production process whereas the photocatalyst returns into the tanks or the electro-oxidation hoses for continuing the photocatalytic action, as shown in Figure 4. This enables the reuse of the photocatalyst.

[0087] Step V: Final process

[0088] Optionally, the surfactants are removed using a foamer. This is an effective method which consists of insufflating air-filled microbubbles. The dissolved surfactants tend to form bubbles in contact with the air, and the foam thus formed is mechanically removed. The pH is corrected to restore the neutral level of the fluid before it passes through the osmotic membrane and also to keep the pH within the limits established by law for discharging.

[0089] Step VI: Conformity check

[0090] Before discharging the fluid into the provided channel, the main pollution parameters are checked on the fluid to verify its compliance with the limits established for introduction into the discharge channel. The measured parameters refer to:

[0091] - pH

[0092] - electrical conductivity

[0093] - total suspended solids (TSS)

[0094] - BOD (by total oxidation of the TOC or by UV radiation absorption spectrum)

[0095] - ammonia

[0096] Optionally, the sludges collected along the entire treatment line are conveyed onto a secondary plant for dehydration.

[0097] The plant embodiment comprising electric hoses, reducing the volume with respect to the solution comprising tanks, guarantees greater interaction between electrodes and pollutants, thus increasing the efficiency of electroflocculation and electro-oxidation.

[0098] Electrodes in hoses get less dirty than electrodes in tanks and they are easier to clean than electrodes arranged in tanks.

[0099] During the process steps, the system frequently changes the polarity of the electrodes. The polarity change is necessary in order to eliminate any induced polarization phenomena on the electrodes which would reduce the capacity to oxidize the pollutants. The polarity change also eliminates the phenomenon for which one electrode releases the cation and the other reduces it reforming the metal deposited on the electrode, a phenomenon which would make the electroflocculation process absolutely inefficient.

[0100] During said electro-oxidation and electroflocculation processes gas is formed which is conveniently conveyed into a trap by a valve placed in the highest portion of said system of electric hoses.

[0101] Advantageously, the process according to the present invention guarantees:

[0102] - absence of chemical substances that are typically used by methods of the prior art;

[0103] - reduction of sludges produced with respect to current chemical and physical methods, where a large part of the pollutants is eliminated in gaseous form (N2 and CO2);

[0104] - simultaneous removal of organic and inorganic pollutants with high efficiency;

[0105] - absence of emissions of ammonia and oxidation of NH4+ to N2

[0106] - possibility to work with notable salinity values;

[0107] - high process speed;

[0108] - high level of automation with consequent easy process management, also remotely.

[0109] Examples

[0110] Example 1 :

[0111] A pilot plant was built according to the present invention, adapted to treat 12 cubic meters of wastewater per day.

[0112] Three different types of wastewater were treated according to the method of the present invention in said pilot plant. The quantitative results are those shown in diagrams A, B and C of Figure 3.

[0113] In particular, said wastewater has undergone step a) of electrocoagulation. Following the electro-oxidation step, filtration is carried out and the wastewater thus clarified moves on to step b), of electro-oxidation. Should the parameters of the treated water, detected by the sensors after electro-oxidation, not be compliant for the final use, the same returns to the start of the process for a further treatment. The sludge obtained from the process is conveyed for dehydration. This wet sludge can be conveyed towards a drying belt filter (example A), a centrifuge (example B), a settling vessel (example C) or towards another drying system for facilitating the dehydration and thus the definitive elimination thereof. The dehydration water is reintroduced into the treatment cycle.

Claims

CLAIMS1. A process for the purification of wastewater comprising: a) electrocoagulation; b) electro-oxidation under UV light at a wavelength in the range of 275-380 nm, where the water to be treated constitutes the electrolyte.

2. The process according to claim 1 , wherein said electrocoagulation is carried out with iron and / or aluminum electrodes.

3. The process according to claims 1 or 2, wherein hydrogen peroxide is added during said electrocoagulation step.

4. The process according to one of claims 1 to 3, which also comprises a pre-treatment for the removal of coarse solids.

5. The process according to one of claims 1 to 4, which also comprises a reverse osmosis and / or ultrafiltration step after said electro-oxidation step.

6. The process according to one of claims 1 to 5, wherein said electro- oxidation step is carried out in the presence of one or more photocatalysts selected from the group comprising TiO2, ZnO, SnO2, WOs, or mixtures thereof.

7. A plant for the purification of wastewater, where said plant comprises:- at least a first electrocoagulation tank (1 ), which comprises Fe or Al electrodes;- at least one filter tank (2) that contains MBR (microfiltration membranes at 0.2 pm) submerged to collect suspended solids, bacteria and algae;- at least a second electro-oxidation tank (3) with Ti or graphite electrodes, adapted to oxidize soluble CODs, metals and nitrogen compounds;- optionally, one or more ammonia nitrogen adsorption modules (6);- optionally, one or more reverse osmosis modules (7), to be activated when values above the permitted threshold of salts, especially chlorides, are measured in the liquid;- one or more storage tanks (8), adapted to contain the water used for counter washing.

8. A plant for the purification of wastewater, where said plant comprises:- at least a first multiplicity of electrocoagulation electric hoses, arranged in series, each comprising Fe or Al electrodes, with an inlet hose and an outlet hose;- at least one filter tank that contains MBR (0.2 pm microfiltration membranes) submerged to collect suspended solids, bacteria and algae;- at least a second multiplicity of electro-oxidation electric hoses, arranged in series, each comprising Ti or graphite electrodes, adapted to oxidize soluble CODs, metals and nitrogen compounds, with an inlet hose and an outlet hose;- optionally, one or more ammonia nitrogen adsorption modules;- optionally, one or more reverse osmosis modules, to be activated when values above the permitted threshold of salts, especially chlorides, are measured in the liquid;- one or more storage tanks, adapted to contain the water used for counter washing.

9. The plant for the purification of wastewater according to claim 7 or 8, where said tanks and / or said electric hoses are at least partially coated internally by photocatalysts selected from the group comprising TiO2, ZnO, SnO2, WO3, or mixtures thereof.

10. The plant for the purification of wastewater according to claim 7 or 8, where photocatalyst crystals are introduced into said tanks and / or electric hoses.

11. The plant according to claim 10, where said tanks and / or electric hoses into which said photocatalyst crystals are introduced are electro-oxidation tanks and / or electric hoses.

12. The plant according to claim 7, where said at least a first electrocoagulation tank (1 ) is closed by a lid adapted to convey any vapors that form during oxidation.

13. The plant according to claim 7 or 8, also comprising process control systems, by way of example redox sensors, level sensors, pH sensors and / or turbidimeters.

14. The plant according to claim 8, wherein said multiplicity of electric hoses also comprises a gas discharge valve.

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