Anaerobic reactor driven by electric field and method for treating refractory organic matters by using anaerobic reactor

Abstract

The invention discloses an anaerobic reactor driven by an electric field and a method for treating refractory organics, and relates to the technical field of anaerobic fermentation devices.The anaerobic reactor driven by the electric field comprises a reactor container, the reactor container is provided with a feeding port, a gas outlet and a gas sampling hole, and the gas outlet is connected with a gas collecting chamber through a pipeline; an anode and a cathode are arranged in the reactor container, the anode and the cathode are made of insulating titanium sheets, and the anode and the cathode penetrate through the reactor container through titanium wires and are connected with an external power supply; a set distance is formed between the anode and the cathode; the insulated electrode is arranged in the reactor, and an electric field generated by the insulated electrode has a relatively strong influence effect on a microbial functional group, so that the anaerobic digestion efficiency can be effectively improved, and the methane yield is further improved; in addition, the outer surface of the electrode is sealed and insulated, the loss of the electrode can be reduced through sealing, and the reduction of the anaerobic fermentation efficiency caused by electrode enlargement can be avoided through insulation.

Description

Technical Field

[0001] The invention relates to the technical field of anaerobic fermentation reaction devices, and in particular to an anaerobic reactor driven by an electric field and a method for treating refractory organic matter thereof. Background Art

[0002] The common method of utilizing lignite for energy is to burn it to generate electricity, but long-term operation will damage the equipment, and a large amount of harmful substances will be produced during the combustion process. In addition, lignite gasification and liquefaction technology is also a high-quality process, but the processing cost is high and it is easy to cause secondary pollution. Anaerobic fermentation is currently the mainstream technology for treating organic waste. While degrading organic matter, it can also produce recyclable resources such as methane. However, there are problems such as unstable hydrolysis, slow degradation rate, and low methane production during anaerobic fermentation.

[0003] In order to obtain more efficient and stable methane, in recent years, the bioelectrochemical system (BES) has been found to improve the enrichment efficiency of electroactive microorganisms, and use circuits to achieve direct interspecies electron transfer (DIET) between different species at the cathode and anode to obtain higher electron transfer efficiency, thereby significantly increasing methane production. However, in the process of BES promoting DIET methane production, the anaerobic efficiency is low due to the increase in the size of the electrode, and the system setting cost is high. At the same time, BES mainly uses non-insulated electrodes immersed in liquid to allow related microorganisms to attach to the electrode surface to produce methane. Since the BES non-insulated electrodes are immersed in liquid for a long time, the acid and alkali in the solution will corrode the electrodes, resulting in high electrode loss, and the electrodes need to be replaced regularly, which is not very practical. In addition, according to recent studies, the amount of methane produced by the bioelectrode in the BES system accounts for less than 20% of the total amount. Therefore, the methane production effect of the bioelectrode is not obvious. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention provides an electric field driven anaerobic reactor and a method for treating refractory organic matter, which can improve the efficiency of anaerobic digestion under the action of the electric field and produce methane more efficiently and directly through the electric field stimulation of the DIET pathway.

[0005] The technical solution of the present invention is as follows: In a first aspect of the present invention, an electric field driven anaerobic reactor is provided, comprising a reactor container, wherein the reactor container is provided with a feed port, a gas outlet and a gas sampling hole, wherein the gas outlet is connected to a gas collecting chamber through a pipeline; an anode and a cathode are arranged in the reactor container, wherein the anode and the cathode are made of insulating titanium sheets, wherein the anode and the cathode pass through the reactor container through titanium wires and are connected to an external power supply; wherein the anode and the cathode are separated by a set distance, voltages are applied to the anode and the cathode respectively, and the electric field generated between the anode and the cathode stimulates microorganisms to undergo anaerobic fermentation reaction through a direct interspecies electron transfer pathway. In some embodiments of the present invention, the insulating titanium sheet comprises a titanium sheet having a titanium content of 99.99%, and an outer surface of the titanium sheet is coated with epoxy thermal conductive potting glue. In some embodiments of the present invention, the method for preparing the insulating titanium sheet comprises the following steps: First, the titanium sheet was soaked in anhydrous ethanol to remove surface impurities; Then rinse repeatedly with deionized water until there is no ethanol smell; The titanium sheet is then polished and cut into the required size; Finally, epoxy thermal conductive potting glue is repeatedly and evenly applied on the surface of the titanium sheet to seal and insulate the titanium sheet.

[0006] In some embodiments of the present invention, the gas collecting chamber includes an outer shell and an inner shell, the outer shell is filled with a saturated NaOH solution acidified with sulfuric acid, the inner shell is placed in the outer shell with a certain gap between the outer shell and the inner shell, the inner shell opening extends downward into the solution, a sealed space is formed between the inner shell and the liquid, and an air inlet and an air outlet are arranged at the top of the inner shell.

[0007] In some embodiments of the present invention, the reactor container comprises a bottle body and a bottle cap connected by threads, a stainless steel gasket is arranged in the bottle cap, and a silicone gasket is added at the connection between the bottle cap and the bottle body.

[0008] In some embodiments of the present invention, the feed port, gas outlet and gas sampling hole are all opened on the bottle cap, the titanium wire passes through the bottle cap and is connected to an external power supply, and the contact section between the titanium wire and the stainless steel gasket in the bottle cap is insulated by a heat shrink tube.

[0009] In a second aspect of the present invention, a method for treating refractory organic matter using an anaerobic reactor driven by an electric field is provided, and when treating low-rank coal, the method comprises the following steps: An electric field strength of 0.2-3.2 V / cm is provided between the cathode and the anode, and the sewage treatment bacteria are placed in a reactor container to cultivate the electroactive bacteria; The low-rank coal to be processed is added into the reactor container from the feed port. At the same time, anaerobic sludge particles are added into the reactor container to assist in completing the electric field driven anaerobic fermentation and methane production process. The generated methane enters the gas collecting chamber through the gas outlet.

[0010] In some embodiments of the present invention, during the anaerobic fermentation to produce methane, the feed port is blocked by a plug, and the gas sampling hole is connected to a gas detection device for detecting the biogas concentration.

[0011] In some embodiments of the present invention, before adding the low-rank coal into the reactor container, the lump lignite is first crushed, and then the powdered lignite below 200 mesh is screened out, and then the surface of the lignite is rinsed to remove surface impurities, and finally dried.

[0012] In some embodiments of the present invention, the anaerobic sludge particles are filtered through an 80-mesh screen to retain only intact granular sludge, and then rinsed three times with deionized water to remove surface impurities.

[0013] One or more technical solutions of the present invention have the following beneficial effects: (1) The electric field driven anaerobic reactor provided by the present invention has an insulated electrode arranged in the reactor. The electric field generated by the insulated electrode has a strong influence on the microbial functional group, which can effectively improve the anaerobic digestion efficiency and thus increase the methane production. In addition, the outer surface of the electrode is sealed and insulated. The sealing can reduce the loss of the electrode and extend the service life of the electrode. Compared with the bioelectrochemical system, the insulation can act on the microorganisms in the entire reaction solution, and can also avoid the increase of the electrode leading to a decrease in the anaerobic fermentation efficiency.

[0014] (2) The present invention utilizes an electric field-driven anaerobic reactor to process lignite, and controls the electric field strength within the range of 0.2-3.2 V / cm. Experimental verification shows that within this electric field strength range, methane production can be maximized. The research results of the electric field-enhanced low-rank coal methanation process will open up a new type of organic waste gasification technology, which can fill the supply gap in the traditional natural gas market and better meet the needs of expanding urbanization.

[0015] (3) The present invention uses direct interspecies electron transfer to complete the methanogenesis of low-rank coal, which does not require a redox medium and is therefore more advantageous in terms of thermodynamics and energy conservation. The application of an electric field can effectively activate DIET in the reaction solution, and this pathway has a significant effect on increasing methane production. Therefore, in the biomethanogenesis of lignite, the methane production is increased by stimulating the DIET process through an external electric field. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1It is a schematic structural diagram of an anaerobic reactor driven by an electric field of the present invention; Figure 2 It is a schematic structural diagram of the gas collecting chamber of the present invention; Figure 3 The cumulative amount of methane produced by the reactor under different electric field strengths in Example 2 of the present invention.

[0017] In the figure: 1, reactor container; 2, gas sampling hole; 3, feeding port; 4, gas outlet; 5, cathode; 6, anode; 7, gas collecting chamber; 701, outer shell; 702, inner shell; 703, sealed space; 704, air inlet; 705, air outlet; 8, external power supply; 9, anaerobic sludge particles. DETAILED DESCRIPTION

[0018] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0019] Example 1 In a typical embodiment of the present invention, an electric field driven anaerobic reactor is provided, comprising a reactor container, such as Figure 1 As shown, the reactor container 1 is provided with a feed port 3, a gas outlet 4 and a gas sampling hole 2, and the gas outlet 4 is connected to the gas collecting chamber 7 through a pipeline; an anode 6 and a cathode 5 are arranged in the reactor container 1, and the anode 6 and the cathode 5 are made of insulating titanium sheets, and the anode 6 and the cathode 5 pass through the reactor container 1 through titanium wires and are connected to an external power supply 8; the anode 6 and the cathode 5 are separated by a set distance, and voltages are applied to the anode and the cathode respectively, and the electric field generated between the anode and the cathode stimulates the microorganisms to undergo anaerobic fermentation reaction through a direct interspecies electron transfer pathway.

[0020] In this embodiment, the insulating titanium sheet includes a titanium sheet with a titanium content of 99.99%. The outer surface of the titanium sheet is coated with epoxy thermal conductive potting glue. The epoxy thermal conductive potting glue is applied to the titanium sheet to seal and insulate it. By sealing the titanium sheet, the loss of the electrode can be reduced and the service life of the electrode can be extended. By insulating the titanium sheet, the electrode can act on the microorganisms in the entire reaction solution, while reducing the loss of the electrode.

[0021] In this embodiment, the method for preparing the insulating titanium sheet includes the following steps: First, the titanium sheet was soaked in anhydrous ethanol to remove surface impurities; Then rinse repeatedly with deionized water until there is no ethanol smell; The titanium sheet is then polished and cut into the required size; Finally, epoxy thermal conductive potting glue is repeatedly and evenly applied on the surface of the titanium sheet to seal and insulate the titanium sheet.

[0022] In this embodiment, if Figure 2 As shown, the gas collecting chamber 7 includes an outer shell 701 and an inner shell 702, the outer shell 701 is filled with a saturated NaOH solution acidified with sulfuric acid, the inner shell 702 is placed in the outer shell 701 and there is a certain gap between the outer shell and the inner shell, the opening of the inner shell 702 extends downward into the solution, and a sealed space 703 is formed between the inner shell 702 and the liquid, and an air inlet 704 and an air outlet 705 are arranged on the top of the inner shell, the air inlet 704 is connected to the gas outlet 4 through a hose to realize the collection of methane, and the air outlet 705 is connected to the methane storage tank through a hose to store the collected methane; the gas collecting chamber 7 is filled with a saturated NaOH solution (20 mol / L) acidified with sulfuric acid (pH<2) to prevent methane from dissolving. Since methane is a colorless gas, by filling the gas collecting chamber with a saturated NaOH solution, when methane enters the inner shell of the gas collecting chamber, the methane production is calculated according to the rising scale of the inner shell 702 to mark the methane production.

[0023] In this embodiment, the reactor container 1 includes a bottle body and a bottle cap connected by threads, a stainless steel gasket is arranged in the bottle cap, and a silicone gasket is added at the connection between the bottle cap and the bottle body, and the connection between the bottle cap and the bottle body is sealed by the silicone gasket.

[0024] Furthermore, the feed port 3, gas outlet 4 and gas sampling hole 2 are all opened on the bottle cap, the titanium wire passes through the bottle cap and is connected to the external power supply 8, and the contact section between the titanium wire and the stainless steel gasket in the bottle cap is insulated by a heat shrink tube to avoid contact and conduction between the titanium wire and the stainless steel gasket.

[0025] A 250 ml brown reagent bottle is used as the reactor container. The reactor container is made of acrylic resin. The production process of the electric field driven anaerobic reactor is as follows: (1) First, prepare the anode and cathode with insulating titanium sheet as the electrode skeleton. Specifically: first soak the titanium sheet with a titanium content of 99.99% in anhydrous ethanol for 24 hours to remove surface impurities; after 24 hours, rinse it repeatedly with deionized water until there is no ethanol smell, and then polish it to provide conditions for the subsequent bonding of materials. Then, cut the titanium sheet into 2 cm × 5 cm with a thickness of 1 cm. Then evenly apply epoxy thermal conductive potting glue on its surface, and repeat the pasting after it cools down to ensure sealing and insulation. The two completed electrodes are placed in the anaerobic reactor with a spacing of 2.5 cm as cathode and anode. As part of the EFAD (electric field driven anaerobic digestion) reactor, the electrodes are connected to the titanium wire and retained at a certain length.

[0026] (2) A gas sampling hole, a feeding port, a gas outlet and a through hole for the titanium wire to pass through are opened on the bottle cap of the reagent bottle, the gas outlet is connected to the gas collecting chamber through a pipeline, and an internal pipe is inserted into the feeding port and the gas sampling hole. During the anaerobic fermentation and methane production process, the feeding port is blocked by a plug, and the gas sampling hole is connected to a gas detection device for detecting the biogas concentration.

[0027] (3) Pass the titanium wire through the through hole and connect it to the external power supply. Since the titanium wire needs to pass through the bottle cap and contact the stainless steel gasket, the titanium wire in the contact section is insulated with a heat shrink tube. Finally, the bottle cap and the bottle body are sealed.

[0028] The method for using the electric field driven anaerobic reactor provided in this embodiment is: Providing a suitable electric field strength between the cathode and the anode, while placing the sewage treatment bacteria in the reactor container to cultivate the electroactive bacteria; The low-rank coal to be processed is added into the reactor container 1 from the feed port 3. At the same time, anaerobic sludge particles 9 are added into the reactor container 1 to assist in completing the electric field driven anaerobic fermentation and methane production process. The generated methane enters the gas collecting chamber 7 through the gas outlet.

[0029] The mechanism of anaerobic fermentation reaction in the above electric field driven anaerobic reactor is: The device uses direct interspecies electron transfer (DIET) to complete the methanogenesis of low-rank coal. The direct electron transfer between electroactive microorganisms through cytochrome C and conductive pili can solve the limitations of indirect interspecies electron transfer (IIET) using intermediates such as hydrogen or formic acid as electron carriers. At the same time, DIET does not require redox mediators (intermediates, referring to polarized electrodes, that is, non-insulated electrodes), so it is more favorable in terms of thermodynamics and energy saving. The application of an electric field can effectively activate DIET in the reaction solution, and this pathway has a significant effect on improving methane production. Therefore, in the biological methanogenesis of lignite, the DIET process is stimulated by applying an external voltage to stimulate and drive methanogens to produce methane. The types of related methanogenic microorganisms are more obviously enriched under the action of the electric field, thereby increasing methane production.

[0030] Example 2 In a typical embodiment of the present invention, a method for treating refractory organic matter using an anaerobic reactor driven by an electric field is provided, and when treating low-rank coal, the method comprises the following steps: An electric field strength of 0.2-3.2 V / cm is provided between the cathode and the anode, and the sewage treatment bacteria are placed in a reactor container to cultivate the electroactive bacteria; The low-rank coal to be processed is added into the reactor container from the feed port. At the same time, anaerobic sludge particles are added into the reactor container to assist in completing the electric field driven anaerobic fermentation and methane production process. The generated methane enters the gas collecting chamber through the gas outlet.

[0031] By controlling the electric field strength within the range of 0.2-3.2 V / cm, the cost of using the electric field can be reduced while maximizing the methanogenesis yield.

[0032] Furthermore, during the anaerobic fermentation and methane production process, the feed port is blocked by a plug, and the gas sampling hole is connected to a gas detection device for detecting the biogas concentration.

[0033] Furthermore, before the low-rank coal is added to the reactor container, the lump lignite is first crushed, and then the powdered lignite with a mesh size of less than 200 is screened out, and then the surface of the lignite is rinsed to remove surface impurities, and finally dried. After drying, the sample is packed into a sample bag and placed in a dryer for standby use to prevent contact with air and minimize moisture absorption. It is then pre-treated and kept for standby use.

[0034] Furthermore, the anaerobic sludge particles are filtered through an 80-mesh screen to retain only complete granular sludge, and then rinsed three times with deionized water to remove surface impurities.

[0035] The lignite from a coal mine was treated by the device in Example 1 and the method in Example 2, wherein the anaerobic sludge particles came from a sewage treatment plant. After preliminary preparation, 0.1 kg of anaerobic granular sludge was weighed, deionized water was added to the 200 ml scale line of the EFAD reactor, and then 0.4 g of lignite, 1 ml of minerals and 2 ml of vitamins were added. Nitrogen was introduced for about 5-10 minutes to remove oxygen in the reactor to ensure an anaerobic environment. The reactor was placed on a shaker and operated at 100 r / min, and a heating pad was added between the reactor and the shaker to keep the reactor solution at 35°C.

[0036] Six reactors with insulated electrodes with different electric field strengths of 0, 0.2 V / cm, 0.4 V / cm, 0.8 V / cm, 1.6 V / cm, and 3.2 V / cm were used to treat lignite. The height of the gas collecting chamber was recorded every day, the volume of gas produced by the reactor was measured, and the daily methane production under standard conditions was calculated. The calculated daily methane production was summed to obtain the cumulative methane production under standard conditions.

[0037] Specifically, a CV9310-VI gas chromatograph equipped with a thermal conductivity detector (TCD) was used to measure the methane content produced in the reactor. The specific measurement conditions were as follows: the model of the filling column was TDX-01, the carrier gas was nitrogen, the flow rate was 30 mL / min, the split ratio was 1:1, the temperatures of the injector, column box, and detector were 150°C, 120°C, and 180°C, respectively, and 1 mL was injected each time. The daily methane production needs to be converted into the volume under standard conditions using the following formula.

[0038]

[0039] Where Vm (at T) refers to the volume of methane collected (mL), T is the room temperature of 25 °C, WVP is the saturated vapor pressure of water at 25 °C, and VB is the methane production of the blank reactor (mL). The calculation method of Vm (at T) is as follows:

[0040] Where hi refers to the height of the gas collecting chamber scale line on the i-th day (cm), and S represents the cross-sectional area of ​​the inner cylinder of the gas collecting chamber ( ), γi is the methane content measured on the i-th day (%).

[0041] After the anaerobic fermentation reaction is completed, the cumulative amount of methane produced by the reactor under different field intensities is as follows: Figure 3 As shown in the figure, in the EFAD system, the presence of the electric field stimulates the efficiency of direct interspecies electron transfer and accelerates the generation of methane. Methane production is an important indicator for evaluating EFAD. During the 40 days of the reaction, the gas composition produced in the reactor was detected on time every day. Figure 3As shown in the figure, the cumulative methane production of the control reactor (0 V / cm) was only 31.13 ml / g lignite, but the cumulative methane production of the other reactors with electric fields was higher than that of the control reactor. The cumulative methane production of the reactor with an electric field strength of 0.2 V / cm increased to 44.31 ml / g lignite, followed by 41.14 ml / g lignite at 0.4 V / cm and 53.79 ml / g lignite at 0.8 V / cm. It was found that with the increase of electric field strength, the methane production also increased continuously, indicating that the electron transfer efficiency of methane production increased with the enhancement of electric field. Among them, the cumulative methane production of 1.6 V / cm and 3.2 V / cm was significantly higher than that of the reactors in other groups, and the best performance was 99.89 ml / g lignite at 1.6 V / cm, and it was also better than the other groups in terms of methane production rate. It is speculated that the substrate is more easily biodegraded due to the excessively high potential, and the direct interspecies electron transfer efficiency of electroactive microorganisms may be enhanced. However, the cumulative methane production in the reactor with an electric field strength of 3.2 V / cm is not as high as that with an electric field strength of 1.6 V / cm. This may be because as the electric field strength increases, the growth of microorganisms is inhibited. When the electric field strength is greater than the range that the organism can withstand, the biological cells may be inactivated and lose their processing capacity. In addition, an electric field strength of 3.2 V / cm means that more electricity is needed to maintain operation, and it is impossible to achieve an economic benefit balance. Therefore, in this experiment, it is believed that the methanation of lignite can achieve the best benefits at an electric field strength of 1.6 V / cm.

[0042] It can be seen from the above experimental results that the methane content in the reactor with applied electric field strength is higher than that in the reactor without applied electric field. This may be because the electric field is mainly conducive to the growth of electroactive methanogens. Therefore, the methane production can be increased by applying an electric field. At the same time, by limiting the electric field strength to the range of 0.2-3.2V / cm, preferably 1.6V / cm, the highest efficiency methanation of lignite can be achieved.

[0043] Although the above describes the specific implementation mode of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without creative work are still within the scope of protection of the present invention.

Claims

1. An anaerobic reactor driven by an electric field, characterized in that: The invention comprises a reactor container, on which a feed port, a gas outlet and a gas sampling hole are arranged, and the gas outlet is connected to a gas collecting chamber through a pipeline; an anode and a cathode are arranged in the reactor container, and the anode and the cathode are made of insulating titanium sheets, and the anode and the cathode pass through the reactor container through titanium wires and are connected to an external power supply; the anode and the cathode are separated by a set distance, voltages are applied to the anode and the cathode respectively, and the electric field generated between the anode and the cathode stimulates microorganisms to undergo anaerobic fermentation reaction through a direct interspecies electron transfer pathway.

2. The electric field driven anaerobic reactor according to claim 1, characterized in that: The insulating titanium sheet comprises a titanium sheet with a titanium content of 99.99%, and the outer surface of the titanium sheet is coated with epoxy thermal conductive potting glue.

3. The electric field driven anaerobic reactor according to claim 2, characterized in that: The method for preparing the insulating titanium sheet comprises the following steps: First, the titanium sheet was soaked in anhydrous ethanol to remove surface impurities; Then rinse repeatedly with deionized water until there is no ethanol smell; The titanium sheet is then polished and cut into the required size; Finally, epoxy thermal conductive potting glue is repeatedly and evenly applied on the surface of the titanium sheet to seal and insulate the titanium sheet.

4. The electric field driven anaerobic reactor according to claim 1, characterized in that: The gas collecting chamber includes an outer shell and an inner shell. The outer shell is filled with a saturated NaOH solution acidified with sulfuric acid. The inner shell is placed in the outer shell with a certain gap between the outer shell and the inner shell. The opening of the inner shell extends downward into the solution. A sealed space is formed between the inner shell and the liquid. An air inlet and an air outlet are arranged on the top of the inner shell.

5. The electric field driven anaerobic reactor according to claim 1, characterized in that: The reactor container comprises a bottle body and a bottle cap connected by threads, a stainless steel gasket is arranged in the bottle cap, and a silicone gasket is added at the connection between the bottle cap and the bottle body.

6. The electric field driven anaerobic reactor according to claim 5, characterized in that: The feed port, gas outlet and gas sampling hole are all opened on the bottle cap, the titanium wire passes through the bottle cap and is connected to an external power supply, and the contact section between the titanium wire and the stainless steel gasket in the bottle cap is insulated through a heat shrink tube.

7. A method for treating refractory organic matter using an anaerobic reactor driven by an electric field as claimed in any one of claims 1 to 6, characterized in that: When processing low-rank coal, the following steps are included: An electric field strength of 0.2-3.2 V / cm is provided between the cathode and the anode, and the sewage treatment bacteria are placed in a reactor container to cultivate the electroactive bacteria; The low-rank coal to be processed is added into the reactor container from the feed port. At the same time, anaerobic sludge particles are added into the reactor container to assist in completing the electric field driven anaerobic fermentation and methane production process. The generated methane enters the gas collecting chamber through the gas outlet.

8. The method for treating refractory organic matter using an electric field driven anaerobic reactor as claimed in claim 7, characterized in that: During the anaerobic fermentation and methane production process, the feed port is blocked by a plug, and the gas sampling hole is connected to a gas detection device for detecting the biogas concentration.

9. The method for treating refractory organic matter using an electric field driven anaerobic reactor as claimed in claim 7, characterized in that: Before the low-rank coal is added to the reactor container, the lump lignite is first crushed, and then the powdered lignite with a mesh size below 200 is screened out, and then the surface of the lignite is rinsed to remove surface impurities, and finally dried.

10. The method for treating refractory organic matter using an electric field driven anaerobic reactor according to claim 7, characterized in that: The anaerobic sludge particles are filtered through an 80-mesh screen to retain only complete granular sludge, and then rinsed three times with deionized water to remove surface impurities.

Images