3.1 Microbial biomass in the UASB and the aerotank
Over the 30 day operational period, the MLSS and MLVSS of anaerobic sludge at a depth of 1.3 m in the UASB tank decreased by 47.2 ± 17.9 to 45.0 ± 2.0 g/L and 22.4 ± 8.9 to 22.0 ± 5.8 g/L, respectively. The MLVSS/MLSS index rose from 0.47 to 0.58. The MLSS and MLVSS of anaerobic sludge, at a depth of 2.4 m, both decreased from 18.1 ± 0.4 to 12.7 ± 0.3 g/L and 8.68 ± 0.3 to 6.25 ± 0.1 g/L, respectively. The MLVSS/MLSS index varied between 0.47 and 0.51, between 1.3 and 2.4 m depth. According to Li et al. (2011), the biomass (MLSS) concentration in the UASB tank during the first 45 days was approximately 30 g/L. Fukuzaki et al. [12] reported that the initial 30-day increase in both MLSS and MLVSS was limited to 5 to 10 g/L. The reduction in microbial diversity was attributed to the high organic content, i.e. the organic loading rate (OLR) in the anaerobic reactors [13]. The growth of specific microorganisms was impeded by the increase in volatile fatty acids produced due to an abrupt increase in the OLR [13, 14]. Li et al. [14] also provided evidence that an increase in the C/N ratio caused by the high OLR in rice paper effluent inhibited the biodegradation rate and removal of specific pollutants. According to Li et al. [11], an MLVSS/MLSS ratio greater than 0.75 signifies increased anaerobic bacterial activity in the sludge. The results from this study revealed a marginal reduction in the growth of biomass present in the UASB, at depths of 2.4 and 1.3 m. In the UASB tank, the MLVSS/MLSS ratio varied between 0.47 and 0.51 (below 0.75), suggesting that the anaerobic sludge bacteria had a low metabolic activity [11].
<Table 1>
Table 1
The growth of anerobic microorganisms in the UASB tank (30 days)
Indicators | Depth (m) | Microbial growth period (days) |
5 | 10 | 15 | 30 |
MLSS | 1.3 | 47.2 ± 17.9 | 47.5 ± 14.4 | 52.2 ± 13.0 | 45.0 ± 2.0 |
2.4 | 18.1 ± 0.4 | 14.1 ± 0.4 | 13.8 ± 1.0 | 12.7 ± 0.3 |
MLVSS | 1.3 | 22.4 ± 8.9 | 22.6 ± 6.4 | 22.2 ± 6.9 | 22.0 ± 5.8 |
2.4 | 8.7 ± 0.3 | 6.7 ± 0.4 | 7.1 ± 0.3 | 6.3 ± 0.1 |
MLSS/MLVSS | 1.3 | 0.47 | 0.48 | 0.42 | 0.49 |
2.4 | 0.48 | 0.47 | 0.51 | 0.49 |
pH | | 7.9 ± 0.1 | 7.9 ± 0.1 | 8.0 ± 0.1 | 8.0 ± 0.1 |
Bacillus cereus (TB19), Bacillus sp. (TB31), and Bacillus thuringiensis (TB17) were the three predominant bacterial species found in the aerotank (Fig. 2; Appendix 1). Some previous research has established that the proliferation of aerobic bacteria in WWTPs was primarily dominated by these species [15, 16, 17]. The Bacillus species demonstrate a good capability for the removal of COD, BOD5, TN, and TP from the rice paper production wastewater [18]. Some Bacillus species have the ability to be active in either an aerobic or facultative anaerobic state [15, 18]. Li et al. [18] assert that the capacity of these species to generate spores enables them to effectively withstand a variety of abiotic stresses. This attribute is frequently employed in the formulation of mixed biocatalysts that can be used for the restoration of polluted environments. According to Reddy et al. [15], Bacillus species have the potential to improve the nitrification process through the conversion of total ammonia nitrogen to nitrite-N and nitrate-N. Bacillus species also possess the capability to chemotrophically and heterotrophically utilize ammonia, thereby potentially enhancing the removal of ammonium from wastewater [15]. According to Ndao et al. [16], the production of endotoxins, reproduction, and development in Bacillus thuringiensis were dependent on the utilization of adenosine triphosphate (ATP) generated through the oxidative phosphorylation and carbon metabolism via the Krebs cycle.
<Figure 2>
Bacteria release chemical energy by oxidative phosphorylation, a key metabolic pathway that produces ATP [19]. The microorganisms present in the Aerotank decompose organic matter into dissolved organic components while producing ATP [16, 19]. Nitrogen and organic carbon are the building blocks of Bacillus cereus, a heterotrophic nitrifying/aerobic denitrifying bacterium [11]. The capacity of heterotrophic nitrifying and denitrifying bacteria to convert NH4+-N into nitrite or nitrate was demonstrated by Liu et al. [11] during nitrification in a bioreactor, and to convert the nitrate (NO3−) into molecular nitrogen (N2) during the denitrification step.
<Table 2>
Table 2
The growth of aerobic microorganisms in the Aerotank (7 days)
Time (days) | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
MLSS (g/L) | 1.62 ± 0.06 | 1.70 ± 0.11 | 1.96 ± 0.05 | 2.34 ± 0.05 | 2.71 ± 0.04 | 2.95 ± 0.06 | 3.23 ± 0.05 |
MLVSS (g/L) | 0.76 ± 0.04 | 0.87 ± 0.04 | 1.17 ± 0.06 | 1.51 ± 0.03 | 1.97 ± 0.06 | 2.24 ± 0.04 | 2.51 ± 0.04 |
MLVSS/ MLSS | 0.47 | 0.51 | 0.6 | 0.65 | 0.73 | 0.76 | 0.78 |
pH | 7.9 ± 0.1 | 7.9 ± 0.1 | 8.0 ± 0.1 | 8.0 ± 0.1 | 8.2 ± 0.1 | 8.3 ± 0.1 | 8.4 ± 0.1 |
The MLSS increased from 1.62 ± 0.06 to 3.23 ± 0.05 g/L over a 7-day period, while the MLVSS increased from 0.76 ± 0.04 to 2.51 ± 0.04 g/L. There was a gradual increase in the microbial concentration over the 7-day period. The wastewater from rice paper manufacturing facility is rich in carbon sources, and the aerobic microorganisms can thrive in the aerotank because of the oxygen supplied from the aerated system [20, 21, 22]. In their study, Fan et al. [20] reported that, when high levels of total phosphorus (TP), suspended solids (SS), and chemical oxygen demand (COD) were present in the wastewater, the MLSS was positively correlated, while the influent ammonia levels were negatively associated with the MLSS. In a similar study, after 30 days of operation, the MLSS concentrations increased from 1000 to 4215 mg/L in the aeration tank [21]. Over the course of seven days, the aerotank's aerobic sludge's MLSS/MLVSS ratio rose from 0.47 to 0.78. The bacterial metabolic activity in the aerobic sludge increased after 6 days when the MLSS/MLVSS ratio was greater than the 0.75 threshold [11].
3.2 Pollutants removal in the sedimentation tank (1#)
Figure 3 shows that over the 30 days of treatment in the sedimentation tank (1#), the following removal efficiencies were obtained for the different wastewater quality parameters: 43.3–50.1% TSS, 22.7–33.7% COD, 55.6–62.3% BOD5, -6.7-6.2% TN, and − 5.4–18.9% TP. Table 3 compares the performance of the sedimentation tank (1#) (period 30 days), UASB tank (2#) (period 30 days) and the Aerotank (3#) (period 7 days) and it is evident that the sedimentation tank efficiently removes BOD5, TSS, and COD, while TN and TP were not efficiently removed.
Table 3
Performance of the sedimentation tank (1#) (period 30 days), UASB tank (2#) (period 30 days) and the Aerotank (3#) (period 7 days) - Comparison of different wastewater quality parameters
Water quality index | pH | TSS (mg/L) | COD (mg/L) | BOD5 (mg/L) | TN (mg/L) | TP (mg/L) | DO (mg/L) |
Input flow | 4.0 ± 0.1 | 591 ± 25 | 2858 ± 273 | 1754 ± 75 | 19.5 ± 1.0 | 3.7 ± 0.2 | 0.4 ± 0.1 |
Sedimentation tank (1#) (period 30 days) | 6.7 ± 0.1 | 314 ± 13 | 2048 ± 106 | 1177 ± 60 | 19.1 ± 0,8 | 3.5 ± 0,3 | 0.5 ± 0.1 |
UASB tank (2#) (period 30 days) | 7.1 ± 0.3 | 135.5 ± 28.5 | 657.2 ± 162.6 | 355.5 ± 98.2 | 17.5 ± 0.8 | 3.1 ± 0.2 | - |
Aerotank (3#) (period 7 days) | 8.1 ± 0.2 | 45.6 ± 11.2 | 138.4 ± 9.7 | 43.3 ± 9.3 | 1.1 ± 0.2 | 0.3 ± 0.1 | 2.4 ± 0.2 |
The organic nitrogen and organic phosphorus present at the bottom of the sedimentation tank were assimilated by the microorganisms or converted into their dissolved forms [23, 24]. According to Islam et al. [23], the following steps are involved during the decomposition of organic nitrogen and organic phosphorus: Ammonia and ammonium ions are generated during the ammonification process. Certain species of bacteria facilitate the additional oxidation of the ammonium ions, transforming them into nitrite and nitrate, while the enzymatic degradation of organic phosphorus compounds results in the formation of inorganic phosphate species. As organic and inorganic compounds are transformed/biodegraded in the sedimentation tank, they release nitrogen and phosphorus into the water column; therefore, the TP and TN concentration in the tank also increases [23].
<Figure 3>
<Table 3>
<Table 4>
Table 4
Performance of the UASB and Aerotank after 20 weeks of operation – Comparison of different wastewater quality parameters
Water quality index | pH | TSS (mg/L) | COD (mg/L) | BOD5 (mg/L) | TN (mg/L) | TP (mg/L) | DO (mg/L) |
Input flow | 4.0 ± 0.1 | 591 ± 25 | 2858 ± 273 | 1754 ± 75 | 19.5 ± 1.0 | 3.7 ± 0.2 | 0.4 ± 0.1 |
UASB tank | 7.2 ± 0.1 | 86 ± 4 | 490 ± 76 | 247 ± 79 | 16.1 ± 0.2 | 2.8 ± 0.1 | - |
Aerotank | 8.3 ± 0.1 | 31 ± 2 | 134 ± 2 | 35 ± 2 | 0.8 ± 0.2 | 0.3 ± 0.1 | 2.4 ± 0.2 |
The results from this study indicated that the sedimentation tank's settling process played a major role in the elimination of TSS, COD, and BOD5 from rice paper effluent. The deposition of dispersed solids and organic materials in a tank occurs during the settlement phase, facilitated by hydrogen bonding, Van der Waals interactions, and gravitational force [7, 25]. As stated by Madariaga and Aguirre [7], the gravitational attraction caused the larger particles in the oxidized starch (i.e. present in the wastewater) to decrease at the rate of 1.5 cm/min. The importance of hydrogen bonding and Van der Waals interactions in the formation of flocs consisting of fine-grained suspended particles and organic matter was also discussed and reported Deng et al. [25]. These flocs managed to amass sufficient bulk to promote the process of sedimentation.
The treatment of high strength wastewater requires a considerable amount of oxygen to oxidize both the inorganic and organic compounds; consequently, the effluent produced during the production of rice paper contains elevated concentrations of COD and BOD5 [7, 26]. As a consequence of the sedimentation tank's successful reduction in suspended particle and organics, the effluent from rice paper production exhibited low levels of TSS, COD, and BOD5 [26]. In a previous work, the sedimentation pond #2 of a starch industry wastewater treatment process exhibited removal efficiencies of 10.5 ± 6.8% for COD, 8.6 ± 6.2% for BOD5, and 18.0 ± 10.9% for TSS, according to Rajbhandari and Annachhatre [26]. Annachhatre and Amatya [27] reported that approximately 70–75% of the dispersed particles in starch effluent could be eliminated via gravity settling.
In addition, the rice paper production effluent in the sedimentation tank experienced an increase in pH from 4.0 ± 0.1 to 6.7 ± 0.1 upon the addition of NaOH. A decrease in pH was caused by the excessive organic load present in the effluent from the rice paper manufacturing process [13]. By augmenting precipitation through the introduction of NaOH, i.e. by changing the pH, particle agglomeration and precipitate formation are stimulated, resulting in a more advantageous reduction of phosphorus, potassium, and nitrogen during the treatment step [28, 29]. The efficacy of NaOH precipitation in decontaminating vinasse effluent was assessed by Prazeres et al. [28] through the elimination of various wastewater components: calcium (80%), phosphorus (74%), magnesium (64%), nitrogen (24%), and potassium (19%). This process yields sludge that is rich in nutrients and organic matter, including Mg, K, P, and Na. As reported in previous studies, the addition of NaOH to wastewater for promoting/facilitating coagulation and flocculation is an important step for the removal of impurities and suspended particles [29].
3.3 Pollutants removal in the UASB tank (2#)
As shown in Table 1, the rice paper production effluent exhibited removal efficiencies of 66.5–82.9% for COD, 81.2–92% for BOD5, and 69.2–82.2% for TSS during the 30-day period of operation in the UASB. The integrated pilot scale UASB and Aerotank system removed TSS, COD, and BOD5 concentrations from the rice paper production wastewater with respective efficiencies of 80.6–90.3%, 82.2–83.3%, and 85.6–86.4% over the course of 20 weeks of operation (Fig. 4; Table 4). The performance of the UASB for the removal of TSS, COD, and BOD5 is illustrated in Tables 3 and 4.
With regard to the operational mechanism of an UASB, the anaerobic sludge granules would ascend due to the hydraulic pressure produced by the wastewater flow through the pipe discharge located in the lower section of the UASB [30]. This would cause the sludge and wastewater to be thoroughly mixed. Increasing the upflow velocity will result in an increased occurrence rate of collisions between the sludge granule and the suspended particles [31]. The influence of extracellular polymeric substances (EPS) generated by bacteria on the aggregation and attachment of suspended particles in wastewater has been reported in the literature [31]. In their study, Luostarinen et al. [30] observed that the incorporation of a recycling system in the UASB improved the interaction between sludge and wastewater, leading to enhanced physical and biological processes for the removal of dissolved chemicals and suspended particles. In previous studies, the settling of suspended solids, facilitated by the flocculation mechanism in the UASB has also contributed to the removal of TSS, COD, and BOD5 in the effluent [30, 31].
The anaerobic degradation process generates CH4 and CO2, from the decomposition of organic compounds [32, 33]. In their study, Gagliano et al. [32] provided evidence that spherical biofilm granular sludges are rich in EPS, which serve as a habitat for the microbial trophic groups responsible for the anaerobic decomposition of organic matter in an UASB. By promoting the formation of hydrophobic properties, EPS may enhance the stability and aggregation of granules, thereby enabling microorganisms to adapt to environmental fluctuations. In their study, Utami et al. [33] reported the treatment of wastewater from an ethanol production facility (9360 mg/L for COD, 4013 mg/L for BOD, and 317.5 mg/L for TSS) using an UASB and removal efficiencies of 55.6%, 67.8%, and 66.6% for COD, BOD5, and TSS, respectively, over a 36-hour hydraulic retention time (HRT) was observed. Likewise, at a HRT of 72 hours, the corresponding removal efficiencies for BOD5, TSS, and COD were 74.5%, 84.1%, and 66.8%, respectively. Li et al. [18] ascertained that the UASB system effectively eliminated approximately 80% of the COD in an effluent containing high concentrations of COD (6000 ± 500 g/L) within the initial 45 days of operation. The results of this study indicate that the anaerobic decomposition of organic matter has a reduced impact on the removal of COD, BOD5, and TSS from the effluent, as evidenced by the decrease in MLSS and MLVSS concentrations (Table 1). Consequently, flocculation of suspended particles was necessary to remove COD, BOD5, and TSS from the wastewater.
The effluent from the rice paper production facility showed a reduction of 5.1–17.9% TN and 8.1–24.3% TP concentrations; thus, the application of the UASB had no substantial impact on the efficacy of TN and TP removal [34]. The inadequate MLVSS/MLSS ratio in the UASB tank (0.47–0.51) signifies a metabolic activity deficiency among the anaerobic microorganisms, leading to less removal of TN and TP from the rice paper production facility wastewater. In a previous work, the primary mechanism by which TN and TP are eliminated from wastewater was reported to be via the organic nitrogen and insoluble phosphorus mineral particle settlement process within an UASB [35]. In their study, Tian et al. [35] discovered that organic nitrogen, organic phosphorus, and COD particulates were effectively separated/removed (> 90%) by anaerobic bacteria in an UASB via bioflocculation and the production of EPS.
Anaerobic bacteria facilitate the degradation and assimilation of organic matter, leading to a decrease in the concentrations of organic nitrogen and organic phosphorus [24, 35]. Anaerobic bacteria, according to Tang et al. [24], transforms the solid organic matter into its soluble form. In their study, Antwi et al. [5] utilized an UASB to treat wastewater from a potato starch processing facility. The authors reported that the effluent contained 2.21 times more NH4+-N than the influent, which contained only 109 mg/L. Nevertheless, the effluent and influent contained comparable concentrations (45 mg/L) of TP, indicating that only a negligible amount of P species was removed, irrespective of the applied HRTs.
<Figure 4>
These results also indicate that that the effectiveness of an UASB system in eliminating TN and TP from effluent is restricted. Aslan and Ŗekerdağ [34] assert that the efficiency of UASB reactors in eliminating TKN and TP from high-strength wastewaters is comparatively low, ranging from 5–25%. The results of this study are consistent with prior research in that the implementation of the UASB did not result in a statistically significant decrease in the levels of TN and TP in the effluent of a rice paper manufacturing facility.
3.4 Pollutant removal in the aerotank (3#) and the integrated pilot scale systems (3#)
During the 7-day operational period, the Aerotank demonstrated the following removal efficiencies: 89.0-94.1% for TSS, 94.5–95.5% for COD, 96.8–98.1% for BOD5, 91.8–95.4% for TN, and 89.2–94.6% for TP (Fig. 5a; Table 2). On the other hand, the integrated pilot-scale UASB and aerotank system operated over a period of 20 weeks, showed a significant reduction in the concentrations of TSS, COD, BOD5, TN, and TP (94.2–95.3%, 95.2–95.4%, 97.8–98.1%, 94.4–96.9%, and 89.2–95.4%, respectively) (Fig. 5b). Table 3 and Table 4 provide data on the elimination efficiencies of TSS, COD, and BOD5 subsequent to the aerotank treatment process. In the literature, Viet et al. [34] examined the application of a laboratory-scale upflow sludge blanket filtering system, which consisted of anoxic and aerobic compartments, to treat effluent from the rice paper processing industry. The removals for SS, BOD5, COD, TKN, and TP were as follows: 29.7%, 98.0%, 97.2%, 76.3%, and 51.8%, respectively. According to Vu et al. [6], the probiotic-treated activated sludge demonstrated removal efficiencies of 81% for BOD5, 82% for COD, and 55.7% for TKN under anoxic and aerobic conditions.
<Figure 5>
3.5. Literature reports on the role of microorganisms to remove pollutants from different types of agro-food processing wastewater
The microorganisms present in the activated sludge have shown to eliminate nutrients and organic matter from the wastewater via organic oxidation and nitrification [17, 36]. Liu et al. [17] utilized Bacillus cereus to treat wastewater from a livestock facility and reported that the concentrations of NH4+-N and COD decreased from 552.3 mg/L and 862.6 mg/L, respectively, to 256.6 mg/L and 221.8 mg/L, resulting in removal efficiencies of 53.3% and 46.1%, respectively. Thuy et al. [36] conducted a study to assess the efficacy of the aerobic process in treating seafood processing wastewater. The results indicated that, at 0.29–0.33 g COD/g MLVSS.day, the removal efficiencies of COD and NH4+-N were 80–83% and 65–68%, respectively. In previous studies, Reddy et al. [15] showed that Bacillus sp. was able to treat > 90% BOD and COD from an effluent of an aquaculture processing facility, while Yang et al. [22] reported that, Bacillus sp. could remove NH4+-N at rates varying between 0.70 and 1.67 mg/h/L.
3.6. Correlation between biomass concentration and the removal of pollutants
As shown in Fig. 6, the correlation coefficients (R2) between biomass concentration and the removal efficiencies of TSS, COD, BOD5, TN, and TP were as follows: 0.8335, 0.6428, 0.9036, 0.5421, and 0.5338. Fan et al. [20] reported that, when the MLSS concentration increased from 1,000 to 2,000 mg/L, the COD and NH3 removals also increased linearly. Conversely, the removal of SS and TP exhibited a progressive rise accompanied by notable oscillations. Palmarin and Young [37] ascertained how MLSS affected the performance of a hybrid membrane bioreactor, and reported that, an increase in MLSS concentrations from 1000 to 4000 (HRT: 8 h) resulted in enhanced removal of turbidity, BOD5, COD, NH3, TKN, and TN.
<Figure 6>
3.6. Future research and practical applications
In the future, this research will examine the viability of employing the integrated pilot-scale UASB and aerotank system treat a composite of wastewater originating from rice paper production and animal husbandry facilities. The management of animal husbandry wastewater can present some difficulties as a result of its varied composition, i.e. it contains suspended solids, nutrients, organic matter, and pathogens. The treated effluent will be applied to land or utilized for the irrigation of non-food crops.