Abstract
The pH is one of the key factors affecting microbial activity in anaerobic systems. In this paper, the pH impact tolerance of Fe0/GO (zero-valent iron/graphene oxide) mediated anaerobic treatment system for high concentration organic wastewater was studied. The effects of a Fe0/GO mediated anaerobic system on wastewater treatment, degradation kinetics, and the physicochemical properties of sludge were studied at pHs of 5.5 and 8.5; the separate addition of Fe0 and GO and a blank system were used as the blank control. The results showed that the pH had adverse effects on the treatment of each system and the physicochemical characteristics of sludge. However, the Fe0/GO system under pH shock maintained a relatively high CODCr removal rate and gas production; the effluent volatile fatty acid content was the lowest, the effluent pH value deviation from the normal range was small, the degradation rate constant, and sludge concentration and flocculation performance of the mixed liquid were better than those of other systems. The recovery phase of Fe0/GO returned to normal in a relatively short time. These results showed that adding Fe0/GO to the anaerobic treatment of high concentration organic wastewater system can drastically improve the pH shock resistance of the system.
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Effect of pH Shock on the Treatment of High Concentration Organic Wastewater via a Fe0/GO-Anaerobic System
Yecheng Lin,a Xiangzhi Wang,a Longyu Wang,a Da Yang,a and Huixia Lan a,b
The pH is one of the key factors affecting microbial activity in anaerobic systems. In this paper, the pH impact tolerance of Fe0/GO (zero-valent iron/graphene oxide) mediated anaerobic treatment system for high concentration organic wastewater was studied. The effects of a Fe0/GO mediated anaerobic system on wastewater treatment, degradation kinetics, and the physicochemical properties of sludge were studied at pHs of 5.5 and 8.5; the separate addition of Fe0 and GO and a blank system were used as the blank control. The results showed that the pH had adverse effects on the treatment of each system and the physicochemical characteristics of sludge. However, the Fe0/GO system under pH shock maintained a relatively high CODCr removal rate and gas production; the effluent volatile fatty acid content was the lowest, the effluent pH value deviation from the normal range was small, the degradation rate constant, and sludge concentration and flocculation performance of the mixed liquid were better than those of other systems. The recovery phase of Fe0/GO returned to normal in a relatively short time. These results showed that adding Fe0/GO to the anaerobic treatment of high concentration organic wastewater system can drastically improve the pH shock resistance of the system.
DOI: 10.15376/biores.17.1.1591-1605
Keywords: Anaerobic; Fe0/GO; pH shock; Impact resistance
Contact information: a: College of Environment and Safety Engineering of Qingdao University of Science and Technology, Qingdao 266042 China; b: Guangdong Provincial Key Lab of Green Chemical Product Technology, Guangzhou 510640, China;
* Corresponding author: lanhuixia@163.com
GRAPHICAL ABSTRACT
INTRODUCTION
Anaerobic biotechnology has the following characteristics: a low energy demand, large potential for renewable energy via biogas production, low amount of surplus sludge, and low operating costs, so it is widely used in various high concentration organic wastewater treatments (Appels et al. 2008; Chan et al. 2009; Poh and Chong 2009). Under anaerobic conditions, the organic matter in wastewater is transformed into biogas to purify wastewater under the co-metabolism of bacteria. However, microorganisms under anaerobic conditions have strict requirements in terms of environmental conditions (especially methanogens). When the external environment fluctuates, the microbial activity will be inhibited, and the wastewater treatment efficiency will be considerably lowered. Therefore, it has become a hot research topic to improve microbial activity to resist adverse effects when environmental conditions fluctuate. It was found that the combined addition of iron series and carbon series enhanced the anaerobic fermentation system and achieved good research results (Zhang et al. 2020).
The pH is a key factor affecting anaerobic biological treatment technology. The optimal pH range of different microorganisms is different, and the sensitivity of different microorganisms to pH is different. In the process of wastewater treatment by anaerobic fermentation system, the pH value is constantly changing, and the fluctuation range of system pH value is related to wastewater quality. The cost of wastewater treatment is related to the frequency of pH regulation. The adverse effects of improper pH on the growth and reproduction of microorganisms are predominantly manifested in the following aspects: first, the decrease of pH will induce positive and negative charges on the surface of microorganisms, and then it will alter the absorption of nutrients by microorganisms; secondly, too high or too low pH value will induce ionization of organic compounds, which indirectly affects microorganisms; last but not least, an unsuitable pH reduces enzyme activity, which in turn affects biochemical processes within microbial cells. Therefore, it is of great research value to reduce the influence of system pH fluctuation on anaerobic fermentation bacterial community and to ensure the stable operation of anaerobic fermentation system, which can reduce the cost of anaerobic fermentation system to treat wastewater. Braguglia et al. (2017) found that the fermentation bacteria could remain active in a pH range of 4 to 8.5, while the optimal pH for methanogens was 6.5 to 7.2. Lu et al. (2020) found that the concentration, composition, intermediate products, and metabolic state of volatile fatty acids (VFAs) produced via the anaerobic digestion of potato peel waste were different under different pH conditions. In addition, the activities of acetate kinase and butyrate kinase were slightly inhibited at a pH of 5.0 and a pH of 11.0, which resulted in relatively low production of VFAs. It was found that adding ferrous materials and carbonaceous materials can stimulate the growth and reproduction of microorganisms, enhance the activity of related enzymes, and maintain high microbial activity even when the pH is beyond the appropriate range (Liu et al. 2012; Teng et al. 2017). This alleviates the adverse effects caused by pH fluctuation and provides the system with a certain amount of pH shock tolerance.
A cheap and green reducing agent, zero-valent iron (Fe0) is converted into Fe2+ in an anaerobic system, which does the following: supply pollutant electrons, accelerate the degradation of organic substances, reduce the oxidation reduction potential (ORP), promote electron transfer between species, and create more favorable conditions for anaerobic digestion (Liu et al. 2012). In the development of microbial fuel cell sensors, Jia et al. (2017) used Fe0 to promote acetic acid conversion and inter-species electron transfer to delay excessive acidification and reduce the impact of excessive acidification on sensor performance, thus making the system more resistant to pH shocks. At the same time, Fe0 is also one of the essential micronutrient elements of methanogens and an indispensable component of the prosthetic group of methanogens. Fe0 can increase the enzyme activity of anaerobic microorganisms, reduce the inhibitory effect of pH shock on an enzyme, and considerably increase the methane production in the anaerobic system (Wu et al. 2015). Kong et al. (2016) put forward the idea of adding Fe0 to an anaerobic digestion system to inhibit the over acidification of food waste. During the experiment, it was found that the pH of the reactor without Fe0 ranged between 5.2 and 5.4, while the pH of the reactor with Fe0 was maintained between 7.5 and 8.0. These results showed that Fe0 can effectively inhibit the excessive acidification of the anaerobic digestion process and enhance the ability of the system to resist pH shock. However, Fe0 can easily accumulate at the bottom of the reactor and therefore is not fully effective. As such, fixing Fe0 to the supporting medium can prevent it from gathering together and losing its effectiveness (Stefaniuk et al. 2016). Carbon materials with large specific surface areas, e.g., activated carbon, carbon nanotubes, mesoporous carbon, and graphene, have been proposed as carriers (Teng et al. 2017). Graphene oxide (GO), as a two-dimensional monolayer with abundant functional groups (epoxy, hydroxyl, and carboxyl), is a very promising carrier (Perreault et al. 2015). Many oxygen-containing functional groups, e.g., ·OH, -O-, C=O, and more abundant hydrophilic groups, are introduced on its surface. These hydrophilic groups provide a large number of active sites for the connection of various organic molecules, macromolecules, and biomolecules, improving the possibility of GO surface functionalization and contributing to its dispersion in various solutions (Konios et al. 2014; Smith et al. 2019). At the same time, due to its large specific surface area, layered structure, and good conductivity, GO can effectively adsorb pollutants and promote interspecific electron transfer (DIET) as well as effectively reduce the impact of pH fluctuation on an anaerobic system and improve the methane production in an anaerobic system (Fan et al. 2018). Consequently, it is hypothesized here that the addition of Fe0 and GO in the wastewater treatment process can upgrade the enzyme activity of anaerobic microorganisms and strengthen the pH tolerance of the key enzymes of anaerobic fermentation, that is, sustain a high activity in the non-optimal pH range (6.7 to 7.5). It is expected that Fe0, particularly the complex combined with GO, can give full play to their respective advantages in the anaerobic fermentation system, make up for each other, enhance the pH shock tolerance of the system, and make the system more secure and more effective in eliminating pollutants.
This paper studied the effect of a Fe0/GO anaerobic system on treating high-concentration organic wastewater under pH shock conditions in order to explore the growth and reproduction of microorganisms under different pH conditions. In addition, this study explored the impact resistance of an Fe0/GO anaerobic system under pH fluctuations and provided a basis for the practical application of the new anaerobic system.
Wastewater and Inoculated Sludge
Simulated citric acid wastewater was used in the experiments. The chemical oxygen demand based on K2Cr2O7testing (CODCr) was approximately 8000 mg/L, and the pH was 4.0 to 5.0. Ammonium sulfate ((NH4)2SO4) and potassium dihydrogen phosphate (KH2PO4) were added as the nitrogen and phosphorus sources with a CODCr:N:P weight ratio of 200 to 5 to 1. The inoculated sludge was obtained from an upflow anaerobic sludge bed (UASB) reactor from a brewery in Qingdao, China.
Experiment Methods
Preparation of the Fe0/GO
Graphene oxide was prepared according to a modified Hummer’s method (Chen et al. 2013). Fe0 and GO at a mass ratio of 5 to 1 (1.0 g of Fe0 and 0.2 g of GO) were placed in a 100 mL beaker, 20 mL of deionized water was added, and ultrasonic treatment was conducted for 20 min under nitrogen protection. Then, after ultrasound treatment, the material was put into a vacuum drying oven and dried at a temperature of 105 °C to get the Fe0/GO composite. The amount of composite combined with deionized water was 0.06 g. The activity of the prepared Fe0/GO composite was stable for at least 6 months, and at the end of use, the Fe0/GO composite were easily recovered with a magnet. A scanning electron microscopy (SEM) image of the prepared Fe0/GO material is shown in Fig. 1. The GO sheets were coated with a large number of small particles. The particles were evenly dispersed, and agglomeration was not obvious, which indicated that GO and Fe0 were fully and evenly compounded. The average particle size of the iron powder used was 37.4 μm. The BET surface areas of the Fe0, GO, and the Fe0/GO composite are shown in Table 1. Compared with Fe0 and GO, the BET surface area of the Fe0/GO composite was greatly increased, which was conducive to the adsorption of organic matter in the system and provided growth sites for microorganisms. This promoted the growth and reproduction of microorganisms and improved the microorganism activity, thus enhancing the wastewater treatment effects.
Fig. 1. SEM image of the Fe0/GO composite
Table 1. The BET Surface Areas
Four 500 mL anaerobic reactors were taken, and 200 mL of acclimated anaerobic activated sludge was added into each reactor. The four reactors were labeled as follows: the blank group; GO group; Fe0 group; and Fe0/GO group. Then, 0.2g/L of GO, 0.1g/L of Fe0, and 0.12g/L of the Fe0/GO composite were added into the GO group, Fe0 group, and Fe0/GO group, respectively, in addition to the 200 mL of experimental influent.
Four anaerobic reactors were placed in a constant temperature oscillator at a temperature of 37 ℃. The optimal pH value for the growth and metabolism of anaerobic fermentation bacteria is between 6.7 and 7.5 (Zhang et al. 2020). In actual production operation, the pH value of the anaerobic system will not be lower than 5.5, and some bacteria will lose their removal capability when it is higher than 8.5. Therefore, the low pH value and high pH value were set at 5.5 and 8.5 in the pH shock experiment. The pH value of the four reactors was set at 5.5, the treatment period was 12 h, and the effluent pH, gas production and CODCr removal rate of the four reactors were measured in each cycle. At the end of the seventh cycle, PN and PS, VFAs, MLSS were determined. Then the shaking table pH value was restored to 7.2, and the above steps were repeated for 7 cycles. After the experiment, set the four reactors pH value to 8.5 and repeat the above steps.
Analysis Methods
The pH values were determined using a pH meter (PHS-3C/501, INESA Scientific Instrument Co. Ltd., Shanghai, China) with composite electrodes. The CODCr concentrations were determined using a COD analyzer (DR1010, HACH, Loveland, CO). The produced gas volumes were measured via the drainage method. The mixed liquor suspended solid (MLSS) levels were measured according to the national standard method (National EPA 2002). The dilute sulphuric acid method was used to extract extracellular polymers (Adav and Lee 2008). The phenol-sulfuric acid method was used to quantify the polysaccharides (PS) using glucose as the standard (Dubois et al. 1956). The protein (PN) contents were determined with a modified Lowry method using bovine serum albumin as the standard (Frolund et al. 1996). The VFAs (volatile fatty acids) were determined using a GC2014C gas chromatograph (Shimadzu, Kyoto, Japan) with an ID detector and a DB-FFAP capillary column with a specification of 30 mm ID × 0.32 mm ID × 0.25 μm. The capillary column flow rate was adjusted to 75 mL/min, with 2 μm for each injection, and a shunt injection at a ratio of 5 to 1. In addition, the temperature of the inlet, detector, and cylinder were set as 220 °C, 230 °C, and 100 °C, respectively. The temperature of the 6 °C/min program rose to a temperature of 130 °C and remained at this temperature for 1 min, then rose to a temperature of 190 °C at a rate of 10 ℃/min and stayed constant for 2 min.
RESULTS AND DISCUSSION
The Effect of pH Shock on Anaerobic System Performance
When the system was operated under optimal conditions of pH 6.7 to 7.5 and temperature 37℃, the average gas production of the blank, GO, Fe0, and Fe0/GO reactors was 488, 496, 505, and 511 mL, respectively. Then the four groups of reactors were subjected to a pH 5.5 and 8.5 shock treatment. The resulting gas production values of the reactors are shown in Fig. 2. When the influent was in the shock stage of pH 5.5, the gas production of blank group, GO group, Fe0 group, and Fe0/GO group was 260, 280, 320, and 345 mL, respectively. Gas production increased to 375, 395, 410, and 450 mL, respectively, when influent pH was restored to the optimal range of 6.7 to 7.5. In contrast, the gas production of the four groups reached 250, 275, 290, and 310 mL, respectively, when the influent was in the shock stage of pH8.5, and the gas production increased to 360, 390, 400, and 415 mL when the pH returned to the optimal range of 6.7 to 7.5. By analyzing the data shown in Figs. 2 and 3, it was found that the gas production of the Fe0 and Fe0/GO reactors was higher than the gas production of the blank group and GO group. As shown in Figs. 2A and 2B, the gas production treatment effect of the Fe0 and Fe0/GO reactors under a pH shock of 5.5 was better than the gas production under a pH shock of 8.5. In addition, the recovery of system gas production after pH 5.5 shock is better than that after pH 8.5 shock. By analyzing the results, it was found that when the pH shock was 5.5, the effluent pH of the system was stable, between 6.7 and 7.5 (as shown in Fig. 3A). At this time, the methanogens were at an optimal pH range and had high activity. By contrast, at a pH shock of 8.5, the water pH was greater than 7.5 (as shown in Fig. 3B), and the methanogen activity for the four groups of reactors was suppressed. It was found that the addition of Fe0 increased the concentration of Fe2+ and Fe3+ in the system, thus increasing the methane production rate (Hu et al. 2015). Moreover, Fe3+ has also been shown to accelerate the electron transfer rate by promoting the direct interspecific electron transfer of anaerobic microorganisms in a fermentation system, thus promoting methane production (Li et al. 2015). However, this may also be due to the reaction of Fe0 with water to produce H2 and OH–, as shown in Eq. 1,
This process increased the pH of the Fe0 and Fe0/GO anaerobic digestion systems, maintaining the pH of the system within a range of 6.7 to 7.5 for the methanogens, and thus promoting an increase in gas production in the anaerobic system (Ma et al. 2018). This explains the high gas production of the Fe0 and Fe0/GO reactors, and why the gas production under a pH shock of 5.5 was better than a pH shock of 8.5.
Fig. 2. Effect of the pH shock on the gas volume: A) pH = 5.5; and B) pH = 8.5
Fig. 3. Effect of the pH shock on the pH of the effluent: A) pH = 5.5; and B) pH = 8.5
When the system was operated under optimal conditions of pH 6.7 to 7.5 and temperature 37 ℃, the CODCr removal rates of the blank, GO, Fe0, and Fe0/GO reactors were 82.7%, 85.8%, 88.9%, and 91.8%, respectively, which basically remained above 80%. The CODCr removal rate in the Fe0/GO group was the highest, followed by the Fe0 group and the blank group. The CODCr removal rates of the four reactors after they experienced pH 5.5 and 8.5 shock are shown in Fig 4. The CODCr removal rates of the blank, GO, Fe0, and Fe0/GO groups were stable at 28.8%, 32.5%, 48.3%, and 49.6%, respectively, under a pH shock of 5.5. After the pH of the reactor returned to the optimal range of 6.7 to 7.5, the CODCr removal rates were 71.2%, 75.8%, 79.9%, and 82.5%, respectively. When the influent pH was 8.5, the CODCr removal rate was stable at 35.2%, 37.5%, 40.5%, and 42.3%, respectively. The CODCr removal rates of the reactor were 69.8%, 74.6%, 77.8%, and 80.5%, respectively, after the reactor returned to the optimal range of 6.7 to 7.5. The CODCr removal rate of the Fe0 and Fe0/GO reactors under a pH shock of 5.5 was higher than the CODCr removal rate under a pH shock of 8.5, and the recovery of the CODCr removal rate after an acid shock was also better than the recovery after an alkaline shock, which was consistent with the results of the gas production of the system.
Fig. 4. Effect of the pH shock on the CODCr removal rate: A) pH = 5.5; and B) pH = 8.5
When the four groups of reactors were subjected to pH 5.5 and 8.5 shock, the removal rate of the CODCr and gas production in the Fe0/GO group were the highest, and the gas production and CODCr removal rate in the Fe0 group were higher than those in the GO group, which indicated that the promotion effect of Fe0 on the CODCr removal rate and the gas production was better than the promotion effect of Fe0 in the GO group. Although GO has a certain adsorption effect on pollutants in wastewater, it also has a certain inhibition effect on the anaerobic methane production process. Studies have shown that in the anaerobic digestion process of sludge, the activity of corresponding enzymes in methanogens (such as coenzyme F420) will be reduced as the GO concentration increases, which seriously inhibits methane production (Dong et al. 2018). The addition of Fe0 weakened the GO inhibitory effect on the anaerobic microbes, promoted the sludge anaerobic digestion process, and considerably improved methane production (Wei et al. 2018). However, it also can effectively remove H2S, stimulate the anaerobic acidification process and the key enzyme of the methanogenesis process, and effectively reduce the oxidation reduction potential (ORP), so as to provide a comfortable environment for anaerobic digestion (Baniamerian et al. 2019). Under pH shock, the Fe0/GO reactor had the best operating effect, which was the result of the combined action of Fe0 and GO. The functional groups on the GO surface provided abundant contact sites for Fe0, which made it more evenly dispersed throughout the system, thus promoting the electron transfer rate of the anaerobic system and making the reaction more sufficient (Ren et al. 2018).
Figure 5A and 5B show plots based on the kinetics equations of the CODCr degradation of the four reactors under pH 5.5 and pH 8.5 conditions, respectively.
Fig. 5. Fitted curve of the first order kinetics: A) pH = 5.5; and B) pH = 8.5
Table 2. Effect of pH Shock on Degradation Kinetics Equations
By comparing the degradation kinetics equations of each system, it was found that the degradation rate constant (k) of the Fe0/GO system was the highest for both the pH 5.5 impact and pH 8.5 impact (k = 0.0691 h-1 at a pH 5.5 impact and k = 0.0576 h-1 at a pH 8.5 impact). In addition, the degradation kinetics of the four groups of reactors was better than the degradation kinetics of the four groups under a pH 5.5 impact. This was consistent with the trend of the CODCr removal rate and the gas production.
The comprehensive analysis of Figs. 2 through 5 showed that when the system was shocked by pH 5.5 and 8.5, the gas production and CODCr removal rate of the four reactor systems decreased, and the effluent pH fluctuated. The treatment effect of the reactor adding Fe0/ Go was obviously better than the treatment effect of the other three reactors. When the pH of the reactor was restored to the optimal range of 6.7 to 7.5, the effluent pH of the four systems gradually recovered to a stable state, while the gas production and CODCr removal rate of the Fe0/GO group increased rapidly and tended to be stable faster. This data and the degradation rate constant of the kinetic equation all indicated that the addition of Fe0and Go made the system more tolerant to pH shock, and the Fe0/GO reactor had the strongest resistance to pH shock.
The Effect of pH Shock on the Effluent VFAs
The influence of pH shock on VFAs in system effluent is shown in Fig 6.
Fig. 6. The effect of pH shock on VFAs: A) pH = 5.5; and B) pH = 8.5
In an anaerobic system, the fermentation type of the microorganisms plays an important role in the process of methane production. Through the determination of the VFAs in the effluent of each system, the influence of Fe0 and GO on the anaerobic system was studied. There was little difference in the acetic acid content in the same reactor after a pH 5.5 and pH 8.5 shock as well as after recovery. However, the content of the VFAs in each group was considerably different. After the pH shock, the acetic acid content in the effluent of the blank group, GO group, Fe0 group, and Fe0/GO group were approximately 4700, 3500, 2850, and 1900 mg·L-1, respectively. After the pH returned to the optimal range of 6.7 to 7.5, the acetic acid content in the effluent of the blank group, GO group, Fe0 group and Fe0/GO group were 3000, 2900, 2600, and 1400 mg·L-1, respectively. The acetic acid content in each system always maintained at a high level (greater than 90%), which indicated that the acetic acid type climax community was always the dominant acid production and fermentation climax community in the anaerobic system. By comparing the propionic acid and butyric acid contained in the systems, it was found that under pH shock conditions, the propionic acid content in the Fe0 and Fe0/GO groups was considerably lower than the propionic acid content in the blank and GO groups, which may be because the addition of Fe0 promoted the transformation of propionic acid. Meng et al. (2013) found in their study that adding iron powder to an anaerobic digestion system can reduce the free energy required for propionic acid decomposition, increase the activity of enzymes related to acetic acid production, and improve the conversion efficiency of propionic acid to acetic acid. At the same time, the addition of Fe0 increased the number and diversity of the microbial community, especially the bacteria responsible for propionic acid transformation. Ye et al. (2021) found that iron was an important component of some coenzymes in anaerobic bacteria.
During the stage of hydrolytic acidification, the activities of dehydrogenase, acetic kinase, phosphotransacetylase, and butyrate kinase were increased by 47.8% to 88.1% with the addition of Fe0, which was conducive to the conversion of propionate and butyrate to acetate. Therefore, a large amount of acetic acid produced via hydrolytic acidification can be rapidly utilized by methanogens. It can be seen from Fig. 6 that the Fe0 group was better than the GO group in promoting acetic acid decomposition, and the effluent VFAs content of the Fe0/GO group was much lower than the effluent VFAs content of the other three groups after pH shock and after recovery. This was primarily due to the addition of Fe0 and GO, which promoted the synthesis of key enzymes in the process of methane production as well as improved enzyme activity, enabled the anaerobic system to maintain enzyme function under the impact of pH, sped up the conversion of VFAs to methane, and reduced the accumulation of VFAs in the system. At the same time, Fe0 promoted the transformation of methanogenic dominant bacteria from Methanothrix to Methanosarcina thermophila (its acetic acid utilization rate was 3 to 5 times higher than Methanothrix) in the anaerobic reactor, thus accelerating the utilization of acetic acid (Wei et al. 2018). These two effects accelerated the conversion of VFAs to methane in the anaerobic system, while the addition of GO made the Fe0 dispersion more uniform, which further improved the treatment effect of the Fe0/GO system on high concentration organic wastewater. Combined with various effluent indicators, the changes in the VFAs content in each system further verified that the system showed a stronger impact tolerance to pH when Fe0 and GO are added.
Influence of pH Shock on System Mixed Liquor Suspended Solid (MLSS)
The impact of pH 5.5 and 8.5 shock on the MLSS of the system is shown in Fig 7. The charge on the surface of the microorganisms changed as the four reactors were subjected to pH shock, which affected the activity of the enzymes of the microorganisms, which in turn affected the growth of the organisms. After a pH shock of 5.5, the blank, GO, Fe0, and Fe0/GO systems was reduced to 7.40, 7.41, 7.80, and 7.85 g·L-1; after a pH shock of 8.5, the MLSS was reduced to 7.30, 7.33, 7.70, and 7.75 g·L-1. The microbial activity gradually increased while the pH of the inlet water returned to the optimal range of 6.7 to 7.5. At the same time, MLSS concentration in Fe0 and Fe0/GO groups increased to slightly higher than that in blank group and GO group, the MLSS in the Fe0/GO group changed the most. After recovery, the MLSS increased by 0.38 g·L-1 (pH 5.5) and 0.29 g·L-1 (pH 8.5) compared with the shock. These results were consistent with the CODCr removal rate and gas production results. The MLSS value of the Fe0/GO group in the pH shock phase and the recovery phase was the highest among the four reactors, which indicated that the growth and reproduction of the microorganisms in this reactor was the best, which was consistent with the CODCr removal rate and production of the reactor with Fe0/GO. This is because iron plays an important role in the metabolic mechanism of a variety of anaerobic microorganisms, which can accelerate the growth and reproduction of anaerobic microorganisms while promoting the conversion of complex organic matter to biogas, and thus shows the growth of MLSS (Yekta et al. 2014). However, under the action of Fe0, a large number of anaerobic microorganisms took advantage of the large specific surface area of GO to enrich and grow, so that anaerobic activated sludge can maintain a dense structure under pH impact and improve the sludge floc and microbial population structure to a certain extent (Liang et al. 2017). These results indicated that the Fe0/GO reactor had strong impact resistance. Under the impact of pH, the degree of damage to the system and the degree of sludge loss were lower than other reactors and the MLSS in the system was higher. In addition, the pollutant degradation effect was better. Therefore, adding Fe0/GO to the reactor can enhance the anti-pH shock ability of anaerobic microorganisms.
Fig. 7. The effect of pH shock on the MLSS: A) pH = 5.5; and B) pH = 8.5
The Effect of pH Shock on the Extracellular Polymers
The effect of pH 5.5 and 8.5 shock on the extracellular polymers is shown in Fig. 8. Extracellular polymeric substances (EPS) contain three-dimensional, gel-like, highly hydrated, charged characteristics. As the skeleton of microbial aggregates, EPS can considerably promote the aggregation of microorganisms and maintain the stability of microbial aggregates (Zhang et al. 2019). Extracellular polymeric substances consist of proteins (PN), polysaccharides (PS), nucleic acids, and humic acid (HA) (Desmond et al. 2018). The flocculation performance of sludge usually is reflected by the content and ratio of PN and PS. Proteins can maintain the integrity and stability of the sludge through the three-dimensional structure formed by bridges with cations because the binding capacity of PN and cations is greater than the binding capacity of PS. Therefore, the higher the PN content, the stronger the sludge flocculation ability, which is beneficial to the separation of sludge and water (Campo et al. 2018). As shown in Fig 8, at the system was subjected to pH 5.5 and 8.5 shock and recovery to the optimal range of 6.7 to 7.5, the PN and PS contents in the sludge extracellular polymer of each reactor under different pH shock conditions were not much different. However, the PN/PS values of the blank, GO, Fe0, and Fe0/GO groups were quite different during the pH5.5 and 8.5 shock and after recovery. Whether during the pH 5.5 and 8.5 shock or after the system recovery, the PN/PS value of the Fe0/GO system was considerably higher than the other three systems, which indicated the addition of Fe0 promoted the growth of PN in the EPS, thereby improving the flocculation performance of sludge. The Fe0/GO group took full advantage of Fe0 and GO, making the PN/PS value considerably higher than the other three groups. Zhang et al. (2018) found that the addition of Fe0 increased the concentration of Fe3+ in the system. Fe3+ acts as a chelating agent to promote the production of EPS and contributes to an increase in the PN content in EPS. At the same time, Fe3+ also promotes PS secretions from microorganisms. Therefore, the addition of Fe0 and GO is beneficial for promoting the increase of the PN and PS content of the sludge, enhancing the anti-pH shock ability of the system, and reducing the negative impact caused by the pH shock. Thus, the integrity and stability of the sludge are maintained, the efficiency of sludge-water separation increases, and the wastewater treatment effect is improved.
Fig. 8. The effect of pH shock on the EPS: A) pH = 5.5; and B) pH = 8.5 (Note: the bar chart shows the content of the EPS, while the line chart shows the PN/PS; S represents the PN, PS, and PN/PS content after the pH shock; and R represents the content of the PN, PS, and PN/PS content after the pH recovery.)
- In the impact experiment, the reactor with Fe0 was less affected by pH shock. Compared with the reactor without Fe0, the reactor with Fe0 had higher gas production, higher CODCr removal rate and lower effluent VFAs content. Nevertheless, the promotion effect of GO was not evident in the pH impact test.
- In the recovery experiment, the gas production and CODCr removal rate of the Fe0/GO reactor were higher than the gas production and CODCr removal rate of the other three reactors; the effluent VFAs content was the lowest, and the recovery time was shorter.
- Through the analysis of the physical and chemical characteristics of anaerobic activated sludge, it was found that the increase rate of MLSS and PN/PS value in Fe0/GO group were better than the other three groups in both pH shock stage and recovery stage, which indicated that after a pH impact, the synergy of Fe0 and GO ensured the anaerobic system had relatively good flocculation performance and greater recoverability of the system.
ACKNOWLEDGMENTS
The authors are grateful for the Research Fund Program of Guangdong Provincial Key Lab of Green Chemical Product Technology (Grant No. GC202112).
Competing Interests
The authors declare that they have no conflict of interest.
Author Contribution Statement
Yecheng Lin and Huixia Lan conceived and designed the research. Yecheng Lin and Xiangzhi Wang conducted the experiments. Da Yang contributed new reagents or analytical tools. Yecheng Lin and Longyu Wang analyzed the data. Yecheng Lin wrote the manuscript. All authors read and approved the manuscript.
Consent to Participate
All authors consented to participate in the manuscript.
Consent to Publish
All authors consented to publish this manuscript.
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Article submitted: September 2, 2021; Peer review completed: November 14, 2021; Revised version received and accepted: January 10, 2022; Published: January 14, 2022.
DOI: 10.15376/biores.17.1.1591-1605