Environment
Effects of the Type of Micro-Organism on the Water Purification Ability of Micro-Organism Supported Crystallization Type Phosphorus Removal Materials
2023 Volume 64 Issue 5 Pages 1078-1082
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2023 Volume 64 Issue 5 Pages 1078-1082
In order to prevent water pollution in closed waters such as inner bays and lakes, it is essential to establish a method to remove phosphorus and organic substances which cause water pollution. We have tried to give organic substances removal ability to a crystallization type phosphorus remover composed of gypsum and CaCO3 by supporting useful microorganisms on the surface of the remover. In this study, the effects of types of microorganisms derived from activated sludge or culture solution on the water purification ability of the phosphorus remover supported with microorganisms were investigated. The porous phosphorus removal material was obtained from solidified gypsum containing NaCl by washing to dissolve the NaCl, silica coating and carbonating. The phosphorus removal material supported with micro-organisms by immersing in activated sludge showed high organic matter removal ability, and there was almost no change in phosphorus removal ability before and after the micro-organisms were supported. In contrast, when microorganisms were supported by immersing materials in culture solution, the phosphorus removal capacity was significantly lower than that before immersing in the culture solution. In addition, the removal capacity of organic substances was lower than that of the materials immersed in activated sludge. The decrease in phosphorus removal ability and low organic matter removal ability of the materials immersed in the culture solution are caused by the detachment of the silica coating layer and CaCO3 on the surface of the phosphorus removal material. Therefore, activated sludge as the type of microorganisms is more effective than culture solution to prepare the water purification material which has both the phosphorus removal and organic substances removal capabilities.
Water pollution that occurs in closed water bodies such as inner bays and lakes has become an environmental problem, which is caused by the inflow and accumulation of organic matter and nutrients such as phosphorus and nitrogen contained in industrial and domestic wastewater.
Eutrophication, a phenomenon in which an excess of nutrients exist in the water, causes phytoplankton to proliferate, resulting in the formation of blue-green algae and red tides. This causes environmental problems such as the death of aquatic organisms and water pollution, so it is necessary to remove the causative agents of nutrient salts. In particular, phosphorus is a cause of environmental problems, but it is also an important component of agricultural fertilizers.1)
In addition to nutrients contained in wastewater, the inflow of organic matter into closed waters increases chemical oxygen demand (COD), a typical indicator of water pollution.2) This can cause the death of aquatic organisms and foul odors, so the removal of organic matter is just as important as phosphorus removal.
Crystallization-type dephosphorization is one of the methods expected to remove and recover phosphorus. The method directly precipitates calcium phosphate crystals on the surface of the material. Compared to the commonly used coagulation-precipitation method, phosphorus removal takes a long time with the crystallization-type method, but no sludge is generated, and phosphorus recovery is easier. Therefore, phosphorus removal by immersing directly in water is expected to be a promising method.3–5) In previous studies, the authors have found that the removal capacity is significantly improved by the addition of gypsum or PFBC (Pressurized Fluidized Bed Combustion) ash as a Ca2+ elution source to a CaCO3–SiO2 composite.6–8) Furthermore, it has been shown that CaCO3 acts as a crystallization site for calcium phosphate crystals.9) In addition, by using microbial metabolism, which is commonly used in the removal of organic matter, it has been shown that organic matter can be removed at the same time as phosphorus by supporting microorganisms on phosphorus removal materials.10)
In this study, crystallization-type phosphorus removal materials based on gypsum were supported with microorganisms by using activated sludge or culture solution (Ehime AI-2). Activated sludge is environmental purification microorganisms used for water purification at sewage treatment plants. Culture solution is also environmental purification microorganisms containing lactic acid bacteria, yeast bacteria, and natto bacillus developed by the Ehime Prefectural Industrial Technology Research Institute. Using those two types of microbial groups, the effects of the microbial species loaded on the materials on their phosphorus removal and organic matter removal capacities were compared.
A mixture of 30 g of a gypsum powder and 10.8 g of NaCl was mixed with ion-exchanged water and stirred for 2 minutes to form a slurry. After stirring, the obtained slurry was poured into a rubber mold 10 mm in diameter and 5 mm high and allowed to dry to produce a hardened body. The prepared sample was immersed in ion-exchanged water and agitated slowly for 30 minutes to dissolve the NaCl from the surface. This procedure was repeated three times and the surface was made porous. The porous sample was immersed in silica sol for 1 hour and then dried to form a silica layer on the surface. This operation was performed twice. After coating, the sample was immersed in Na2CO3 solution (0.3 mol L−1) for 3 hours to precipitate CaCO3 on the sample, which acts as a crystallization site for calcium phosphate.
2.2 Support with micro-organism 2.2.1 Micro-organism loading from activated sludgeActivated sludge was obtained from the Toba Water Quality and Environmental Conservation Center and stored at room temperature with constant aeration.
The prepared porous gypsum bodies were immersed in activated sludge at a rate of 15 specimens/500 mL for 1, 3, and 5 days with aeration at room temperature, and then the excess activated sludge was washed off with ion exchanged water.
2.2.2 Micro-organism loading from culture solutionA culture solution, which containing lactic acid bacteria, yeast bacteria and natto bacillus, was prepared by mixing 4 g of dry yeast, 50 g of yogurt, 3 grains of natto and 50 g of sugar in 900 mL of ion-exchanged water and allowed to stand at 35°C for 1 week.
The prepared porous gypsum bodies were immersed in culture solution at a rate of 3 g/100 mL for 1, 3, and 5 days at 35°C, and then the excess culture solution was washed off with ion exchanged water.
2.3 Removal experiments of phosphorus and organic mattersFor a phosphorus removal experiment, a phosphorus solution (50 ppm) was prepared by dissolving KH2PO4 in ion exchanged water. One sample was immersed in 50 mL of the prepared phosphorus solution for 1, 3, 5 and 10 days, and then the solution was filtrated after removing the sample. The filtrate was diluted 10-fold, and the removing phosphorus concentration was measured using an ICP emission spectrometer (ICPE-9820, Shimadzu Co., Ltd.) to calculate the phosphorus removal ratio.
For an organic removal experiment, a model sewage with a chemical oxygen demand (COD) value of 180 ppm was prepared by dissolving D(+)-glucose and glycine in ion exchanged water, and one sample was immersed in 50 mL of the prepared model sewage for 1, 3, 5 and 10 days. After soaking for the prescribed number of days, the filtrate was diluted, and COD in the solution was measured using a simple COD meter (COD METER COD-60A, Toa DKK Co., Ltd.) to calculate COD decreasing ratio.
2.4 CharacterizationThermal decomposition behavior was examined by thermogravimetry-differential thermal analysis (TG-DTA; Thermo plus TG8120, Rigaku Co.) from room temperature to 1000°C at a heating rate of 10°C min−1 in an air atmosphere. The surface morphology of the specimen was observed with a scanning electron microscope (SEM; JSM-7600F, JEOL Ltd.) with an acceleration voltage of 15 kV. Furthermore, the distribution of elements on the sample surface was measured with an energy dispersive X-ray spectroscope (EDX; X-MAX, Oxford instruments plc.).
Figure 1 and Fig. 2 show the results of the phosphorus and the organic matter removal experiments on the specimens before and after immersing in activated sludge or culture solution.
Comparison of phosphorus removal properties of sample between before and after immersing in activated sludge or culture solution.
Comparison of COD removal properties of sample between before and after immersing in activated sludge or culture solution.
As shown in Fig. 1, as for the samples supported with microorganisms by immersing in activated sludge, there was almost no difference in phosphorus removal ratio between before and after supporting with microorganisms. On the other hand, when the specimens were supported with microorganisms by immersing in the culture solution, the phosphorus removal ratio was significantly lower than that of the specimens without microorganisms.
As shown in Fig. 2, the COD decreasing ratio of the samples supported with microorganisms by immersing them in activated sludge or culture solution increased compared to the samples without microorganisms. In particular, the samples immersed in activated sludge to support with microorganisms showed higher COD decreasing ratio.
In the following, the results of phosphorus and organic matter removal experiments shown in Fig. 1 and Fig. 2 are discussed based on the results of TG-DTA, SEM observation and EDX analysis. As shown in Fig. 3, for the sample without microorganisms supporting, two weight losses due to dehydration of gypsum dihydrate are observed at 100°C–200°C and due to thermal decomposition of CaCO3 at 600°C–800°C. The thermal decomposition behavior of the sample loaded with microorganisms using activated sludge was very similar to that of the sample before loading the microorganisms. On the other hand, the weight loss attributed to the thermal decomposition of CaCO3 did not appear in the samples using the culture medium. Table 1 compares the weight loss ratio of each sample due to the thermal decomposition of CaCO3. Table 1 shows that the weight loss ratio of CaCO3 was 2.52% in the sample before supporting microorganisms and 2.72% in the sample after immersing in activated sludge. In contrast, the weight loss ratio of the specimen after immersing in culture solution showed 0%, indicating that the amount of CaCO3 decreased after the microorganism loading.
TG curves of sample before and after immersing in activated sludge or culture solution.
Figure 4 shows the SEM images of the specimen surfaces before and after immersing in activated sludge or culture solution. Silica coating and rhombohedral CaCO3 crystals were observed on the surfaces of the specimens before and after immersing in activated sludge, whereas these were not observed after immersing in culture solution.11) Besides, Fig. 5 shows elemental mapping images of Si and Ca for the SEM image of the sample surface immersed in the culture solution. As shown in Fig. 4 and Fig. 5, after immersion in the culture solution some SiO2 coating layer remained on the surface, but the surface lost its smoothness and CaCO3 precipitated by carbonation disappeared, suggesting that the surface of the SiO2 coating layer was peeled off by immersion in the culture solution, and CaCO3 precipitated on the surface fell off at the same time.
SEM photographs for surface of sample between before and after immersing in activated sludge or culture solution.
SEM photograph and EDX mappings of a sample after immersing in culture solution.
This difference in the surface condition of the materials depending on the species of microorganisms could be due to the influence of the pH of activated sludge or culture solution at the time of loading the microorganisms. Since activated sludge is a neutral solution with a pH around 6, the silica coating is not affected, and so CaCO3 as a crystallization site is retained and the microorganisms are supported. On the other hand, because culture solution was an acidic solution with a pH around 4, the coating layer on the sample surface became brittle and the surface layer of SiO2 coating peeled off. As a result, CaCO3 and attached microorganisms probably dropped off with the surface layer of SiO2 coating. In addition, this difference in pH effecting on the surface condition could be attributed to metabolic reactions of the respective microbial species. Metabolic reactions are processes to obtain energy for microorganisms to live and are one of the most representative elements describing the characteristics of microorganism species. In these reactions, various substances are synthesized as byproducts. The yeast and lactobacillus bacteria present in culture solution produce acids such as pyruvic acid and lactic acid, so the optimal solution pH for the microorganisms is acidic.12,13) On the other hand, the mud in activated sludge is more stable at neutral pH than at acidic pH, because H2S generated at acidic pH in activated sludge causes the sludge to decompose.14) Thus, activated sludge would be maintained at neutral pH by microbial metabolic reactions. For these reasons, differences in the surface condition of the specimens were caused by differences in the optimum solution pH due to the function of microbial species.
As described above, when the materials were supported with microorganisms by immersing in activated sludge, the phosphorus removal ratio was higher than that of the materials after immersion in culture solution, because the coating layer on the surface of the specimens did not peel off by immersing in activated sludge, and CaCO3 was present as before the microbial loading. In addition, the COD decreasing ratio is considered to be much higher than that of the specimens before and after immersion in culture solution due to the presence of the microorganisms.
3.2 Effects of microbial loading period on phosphorus and organic matter removal capacities of materialsFigures 6 and 7 show the results of the phosphorus removal experiment and the organic matter removal experiment after 10 days of removal for the specimens with different number of days of microbial loading (1, 3, and 5 days), and Table 2 shows the weight loss ratio of CaCO3 read from the TG curves of the specimens with different number of days of microbial loading.
Effect of immersing time in activated sludge or culture solution on the decreasing ratio of phosphorus of the samples after immersing in a test solution.
Effect of immersing time in activated sludge or culture solution on the decreasing ratio of COD of the samples after immersing in a test solution.
Figure 6 and Table 2 show that the phosphorus removal ratio and the weight loss ratio of CaCO3 as a crystallization site did not change regardless of the number of days of immersing in activated sludge, whereas the removal ratio and weight loss ratio of CaCO3 decreased with the number of days of immersing in culture solution.
Figure 7 shows that the COD decreasing ratio increased with the number of days in the samples immersed in activated sludge, while it decreased in the samples immersed in culture solution.
These results suggest that the differences in phosphorus removal ratio and COD decreasing ratio with respect to the number of days of immersing materials in activated sludge or in culture solution were due to changes in the amount of CaCO3 and loaded microorganisms of the materials. In the case of the specimens immersed in culture solution, the amount of CaCO3 and supported microorganisms decreased as the coating layer of the specimens dropped off with increasing number of days of loading. On the other hand, in the case of the samples immersed in activated sludge, the coating layer was not shed and the amount of CaCO3 did not change even when the loading days were varied, and the amount of loaded microorganisms increased with the number of days of supporting.
Crystallization-type phosphorus removal materials were supported with microorganisms by using activated sludge or culture solution, and the effects of the microbial species loaded on the materials on their phosphorus removal and organic matter removal capacities were compared. Experimental results show that the dephosphorization materials supported with microorganisms by immersing in activated sludge have higher phosphorus removal and organic matter removal capacities than the materials immersed in culture solution. This difference in the water purification ability could be due to the effect of the surface condition of the materials attributed to the pH of activated sludge and culture solution. The culture solution is an acidic solution with a pH around 4, and the silica coating on the surface of the material becomes brittle and drops off due to immersing in culture solution. On the other hand, the activated sludge is a neutral solution with a pH around 6, and the surface of the material is not affected by immersing in activated sludge, so CaCO3 as a crystallization site is retained and the microorganisms are supported.
The phosphorus removal capacity of the material after immersion in activated sludge was independent of the number of days of microbial loading, and the organic matter removal capacity increased with the number of loading days. On the other hand, the phosphorus removal and organic matter removal capacities of the material after immersion in culture solution decreased with increasing the number of days of supporting microorganisms. This could be because the surface of the material drops off as the number of days of immersing in culture solution increases, while the surface of the material immersed in activated sludge does not drop off.
In conclusion, activated sludge as the type of microorganisms is more effective than culture solution to prepare the water purification material which has both the phosphorus removal and organic substances removal capacities.
The authors declare no competing financial interest.
