3.1 Factors Analysis on the Hydrolysis Acidification Process
In order to study the adaptability of the printing and dyeing wastewater to the hydrolysis acidification process, analysis on the influencing factors including dissolved oxygen, pH, temperature and sludge concentration were conducted.
3.1.1 Dissolved Oxygen
The dissolved oxygen (DO) concentration in the three sets were controlled as 0 mg/L, 0.5 mg/L, 1.5 mg/L, representing anoxic, micro-aerobic, and aerobic conditions respectively. Water samples were taken after 2 h, 4 h, 6 h, and 8 h for COD and nitrogen analysis during the operation of the hydrolysis acidification process.
It can be seen from Fig. 2 that DO has influence on the COD and nitrogen removal. In Fig. 2 (a), within 5 h of the reaction, when the DO concentration was 0 mg/L and 0.5 mg/L, the removal rate was 33.5% and 31.3%, respectively; when the DO concentration was 1.5 mg/L, the removal efficiency of COD was poor, and the removal rate was only 24.2%. From Fig. 2(b), it can be found that concentration of NH3-N increased as DO decrease, the increase rate of NH3-N was 24.8%, 21.2%, and 16.9% when DO was 0 mg/L, 0.5 mg/L, and 1.5 mg/L, respectively. The source of the NH3-N could be from a small amount conversion of NO3-N and NO2-N to NH4; the ammoniating of organic nitrogen into NH3-N (protein is decomposed by ammoniating bacteria), and certain amount of NH3-N release from sludge. From Fig. 2(c), it can be found that TN has been removed certain level after the reaction is completed, mainly because the removal of nitrate nitrogen in the wastewater leads to the reduction of TN. When the concentration of DO is greater than 1.0 mg/L, the activity of anaerobic bacteria will be suppressed to a certain extent, and the removal effect of TN will also be reduced. When the DO concentration was 0 mg/L, 0.5 mg/L, and 1.5 mg/L, the removal rate of TN was 24.7%, 24.5%, and 15.8%, respectively. From Fig. 2(d), it can be found that when DO concentration was greater than 1 mg/L, the removal of NO3-N was inhibited, and the removal rate after 8 h was only 50.7%; while in anoxic and micro-oxygen conditions, the removal rates of NO3-N were 87.6% and 79.6%, respectively.
From Figure (e), it can be found that in anoxic and micro-oxygen condition, NO2-N was almost completely removed, and the removal rate was as high as 90%. When the DO concentration is greater than 1 mg/L, there was a limited removal of NO2-N as 55.8%. In summary, when the DO concentration is greater than 1 mg/L, the removal effect of each index is poor, which is not beneficial to the growth of anaerobic sludge; while when micro-oxygen is compared with anoxic condition, although some indexes have relatively poor treatment effects, but it speeds up the conversion of TN. On the one hand, it greatly reduces the reaction time and increases the hydraulic load bearing capacity; on the other hand, the micro-aeration in the hydrolysis acidification process can also prevent the pipeline from being damaged. In view of operating costs and effects, the use of micro-oxygen hydrolysis acidification process is more economical than the completely enclosed hydrolysis acidification process, and it also has higher operational feasibility (Fan et al. 2021, Gu et al. 2018).
3.1.2 pH value
Relevant studies have shown that pH of wastewater in hydrolysis acidification process should be maintained within a certain range (Ma et al. 2016). It is well known that hydrolysis acidification is divided into three stages. The first stage is the hydrolysis stage, where large particles are degraded into small particles. The second stage is the acidification stage, where acetic acid is produced under the action of bacteria, and the pH value will decrease in this stage. The third stage is the methane production stage, during which the pH value will rise.
In the experiment, effect of different pH values on hydrolysis and acidification were studied. The pH values in the system were 5.5, 6.5, and 7.0 respectively, and 10% sodium hydroxide solution or hydrochloric acid solution was used to adjust the pH value of wastewater. COD and ammonia nitrogen were used as indicators to study the influence of pH on the hydrolysis and acidification process.
From Fig. 3(a), it can be found that pH has a significant effect on the hydrolysis and acidification process. Low pH will inhibit the activity of anaerobic bacteria (denitrifying bacteria, etc.), and the removal of COD decreased. At different pH values, most of the reactions were completed within the first 4 h, and stabilized after 8 h. At pH 5.5, the removal rate of COD in the first 4 hours was 19.3%, and the removal rate of COD after 8 hours was 29.8%; at pH 6.5, the removal rate of COD in the first 4 hours was 35.9%, and the removal rate of COD after 8 hours was 35.9%. The removal rate was 43.0%. At pH 7.0, the removal rate of COD in the first 4 hours was 31.9%, and the removal rate of COD after 8 hours was 38.1%. Therefore, As for COD removal at this stage, pH 6.5 was the best. In Fig. 3(b), it can be seen that as pH rises from 5.5 to 7.0, ammonia nitrogen got increased slightly. It should be noted that extra acid will be also produced in the later hydrolysis and acidification process, therefore, the slightly acidic environment pH 5.5 is not beneficial to microorganism growth. Also, a higher ammonia nitrogen would not conducive for its subsequent removal. Therefore, pH 6.5 was also the best choice at this stage as regards to ammonia nitrogen concentration variation.
3.1.3 Temperature
Temperature is also an important indicator affects the sludge microorganism activity during hydrolysis and acidification. In the experiment, 10°C, 25°C, and 35°C were selected to represent three different seasons (winter, spring and autumn, and summer) in Suzhou City in a year. results are shown in Fig. 4.
It can be seen from Fig. 4(a) that different temperatures have different effects on the COD and ammonia nitrogen removal. COD removal rate was the highest when temperature was 25℃, and COD was lowest at 10℃. After 8 hours, at 10℃, the removal rate of COD was 22.4%; at 35℃, the removal rate of COD was 28.4%; while at the most suitable temperature 25℃, COD removal rate of sludge was the highest, reaching 38.8%. It is worth noting that the sludge activity at low temperature (10°C) is worse than that at higher temperature (35°C). In practical operation of wastewater plant, more attention should be paid to the variations in the effluent index in winter. While as for the effect of temperature on ammonia nitrogen removal, from Fig. 4(b), taking 25°C as a benchmark, it can be seen that the increase or decrease of temperature will inhibit the increase of ammonia nitrogen. After 8 hours, 7.9% of the ammonia nitrogen increased at 10℃; 20.1% of ammonia nitrogen increased at 25℃; 12.2% of ammonia nitrogen increased at 35℃.
3.1.4 Sludge Concentration
To study the effect of sludge on the treatment effect of the hydrolysis and acidification process, the analysis indicators are COD and ammonia nitrogen. The only variable in the reaction is the sludge concentration. The sludge concentrations of test were 2 g/L, 4 g/L, and 6 g/L, respectively. The water sample was taken after 2 h, 4 h, 6 h, and 8 h.
It can be seen from Fig. 5(a) that the sludge concentration has a significant impact on COD and NH3-N removal in the hydrolysis and acidification section. COD removal rate were 26.3%, 38.7%, 41.3% when the sludge concentration in the reaction system were 2 g/L, 4 g/L and 6 g/L, respectively. The higher the sludge concentration, the higher the COD removal rate; Fig. 5(b) shows that when the sludge concentration in the reaction system was 2 g/L, 4 g/L, and 6 g/L, the effluent increase rate of NH3-N was 18.0%, 52.8%, 66.7%, respectively. When the sludge concentration is 2 g/L, the removal of COD and the increase of NH3-N are less, when the sludge concentration is 6 g/L, the COD removal rate increases a bit with too fast increase of the NH3-N. Therefore, it is recommended to control the sludge concentration at about 4 g/L during operation (Shengquan et al. 2008, Song et al. 2019).
3.2 Effect of Aniline and Antimony on the Removal of COD and NH3-N
3.2.1 Effect of Aniline on COD Removal
To evaluate the toxic effect of aniline on micro-oxygen hydrolysis and acidification, the inhibitory effect of different concentrations of aniline on the removal of COD was investigated (Cheng et al. 2021, Xie et al. 2016).
From Fig. 6(a), it can be seen that when the influent water contains only the background concentration of aniline (0.4 mg/L), the COD removal efficiency was the highest, the COD was reduced from 385 mg/L to 264 mg/L within 4 h, and the removal rate was 31.4%. After the reaction continued for 8 h, the COD concentration was further reduced to 240 mg/L, and the removal rate was 37.7%. The effluent COD increased as continuous addition of aniline, and the anaerobic sludge was inhibited. The higher the concentration of the influent aniline, the more obvious the inhibition effect. When 5 mg/L aniline was added, the effluent COD concentration increased to 756.6 mg/L after 8 hours of aeration reaction, increased 96.5% in total. The above results indicated that aniline is toxic to anaerobic sludge and significantly inhibited the degradation of COD by the anaerobic sludge.
The rate constant k obtained by first-order reaction kinetics fitting is shown in Fig. 6(b). It can be seen that the reaction rate constant k decreases as the aniline concentration increases. When the aniline concentration in the influent was 0.4 mg/L, the COD degradation rate was 0.06 h− 1; when the aniline concentration increased to 5.4 mg/L, the COD degradation rate dropped significantly to -0.08 h− 1. The rate constant switched from positive to negative when aniline concentration changes from 2.4 mg/L to 3.4 mg/L, and the effluent COD got increased accordingly, indicating that aniline is toxic to activated sludge and inhibits COD removal, so the concentration of aniline in the hydrolysis and acidification section should be controlled.
3.2.2 Effect of Antimony on COD Removal
The inhibitory effect of antimony on micro-oxygen hydrolysis and acidification and the removal of COD were investigated. In the experiment, the background concentration (initial concentration) of influent antimony was 0.05 mg/L. Samples were taken at 2 h, 4 h, 6 h, and 8 h. Similarly, the first-order kinetic equation was used to fit the COD curve with time to study the relationship between the COD degradation rate and the influent antimony concentration.
It can be seen from Fig. 7 (a) that when the antimony concentration was 0.05 mg/L (no external antimony was added), the COD value decreased from 320 mg/L to 215 mg/L within 4 h, and the removal rate was 32.8%; while after 8 h, the COD concentration was further reduced to 200 mg/L with a removal rate of 37.5%. With the addition of antimony, the removal of COD was inhibited, and the higher the concentration of antimony in the influent water, the more significant the inhibition of COD removal. When 2 mg/L of antimony was added, the COD removal rate was only 2.9% after 8 hours’ aeration, which was much lower than that of the water without antimony added.
The rate constant k obtained by first-order reaction kinetics fitting is shown in Fig. 7(b). It can be seen that the reaction rate k decreases as the concentration of antimony increases. When the antimony concentration in the feed water was 0.05 mg/L, the COD degradation rate was 0.06 h− 1; while when the antimony concentration increased to 2.05 mg/L, the COD degradation rate dropped significantly to 0.004 h− 1, and the reaction rate was close to zero. It shows that antimony inhibits the micro-oxygen hydrolysis and acidification process and inhibits the removal of COD. The higher the antimony concentration, the more obvious the inhibitory effect.
3.2.3 Effect of Aniline on NH3-N Removal
The inhibitory effect of aniline on micro-aerobic hydrolysis and acidification and the removal of NH3-N in wastewater were investigated. Results are shown in Fig. 8. It can be seen that when the influent water contains only the background concentration of aniline (0.4 mg/L), the increase of NH3-N is greater. The NH3-N concentration rose from 9.72 mg/L to 11.34 mg/L within 4 h with a rate of 16.7%. After the reaction continued for 8 h, the NH3-N concentration further increased to 12.45 mg/L, with a growth rate of 28.1%. With the addition of aniline, the increase of NH3-N was inhibited, and the higher the concentration of influent aniline, the more significant the inhibition. When adding 5 mg/L of aniline, after 8 hours of reaction, the NH3-N concentration in the effluent is 10.26 mg/L, increased only 5.6%, which is much lower than that in the water without aniline.
The first-order reaction kinetic equation was used to fit the curve of ammonia nitrogen concentration with time, and the ammonia nitrogen growth rate constant under different influent aniline concentration conditions was obtained, as shown in Fig. 8(b). When the influent aniline concentration was 0.4 mg/L, the degradation rate constant of ammonia nitrogen was − 0.031 h− 1, indicating that the concentration of ammonia nitrogen was increasing. When the concentration of influent aniline was 5.4 mg/L, the degradation rate constant of ammonia nitrogen was only − 0.01 h− 1, the reaction almost stopped, indicating that aniline has a strong inhibitory effect on the activity of anaerobic bacteria.
3.2.4 Effect of Antimony on NH3-N Removal
The effect of antimony on the inhibition of micro-oxygen hydrolysis and acidification and the removal of NH3-N in wastewater was studied to evaluate the toxicity of antimony. Results showed that NH3-N increased to a certain extent during the micro-aerobic hydrolysis and acidification process, and the increase of NH3-N concentration was mainly in the first 4 hours. When the antimony concentration in the micro-oxygen hydrolysis and acidification system was 0.05 mg/L, the NH3-N concentration increased from 10.5 mg/L to 12.8 mg/L at 4 h with a growth rate of 21.9%; after 8 hours, the NH3-N concentration further increased to 13.6 mg/L, with an increase rate of 29.5%. When 2 mg/L of antimony was added, after 8 hours of reaction, the NH3-N concentration in the effluent was 10.7 mg/L, with only 1.9% of increase rate.
The rate constant k obtained by first-order reaction kinetics fitting was shown in Fig. 9(b). It can be seen that the reaction rate k decreases with the increase of the antimony concentration. When the antimony concentration in the influent was 0.05 mg/L, the increase rate of NH3-N was 0.03 h− 1. When the antimony concentration increased to 2.05 mg/L, the increase rate of NH3-N dropped sharply to 0.002 h− 1, indicating that the concentration of antimony is negatively related to the reaction rate, and antimony inhibits the micro-oxygen hydrolysis and acidification process.
3.3 Effect of Aniline and Antimony on Sludge Penetration
By studying the penetration rate of aniline to the hydrolyzed and acidified sludge, the effect of micro-aerobic hydrolysis and acidification on the removal of aniline can be judged.
Figure 10(a) showed that the sludge in the hydrolysis acidification section has a certain removal effect on aniline. When the influent aniline was 0.4 mg/L, the effluent aniline concentration was 0.61 mg/L, and the penetration rate of aniline to activated sludge is 153.0%. As the concentration of influent aniline increases, the penetration of aniline to activated sludge gradually decreases. When the concentration of aniline exceeds 2.4 mg/L, the penetration rate of aniline to activated sludge stabilizes was around 120%. Figure 10(b) showed that the removal rate of antimony by anaerobic sludge was low.
When the antimony concentration in the aerobic section was 0.05 mg/L, the antimony concentration in the effluent was 0.042 mg/L. The penetration rate of sludge is 84.0%. As the concentration of antimony in the influent increases, the penetration rate of antimony to anaerobic sludge gradually increases, which means that it is difficult to remove antimony at this stage. When the antimony concentration was 2.05 mg/L, the penetration rate of antimony to anaerobic sludge was 97.4%.
3.4 Effect of Aniline and Antimony on Sludge Concentration Change
The concentration change of sludge in the reaction is an important parameter to reflect the treatment performance. By studying the sludge concentration change before and after micro-aerobic hydrolysis and acidification, the toxic effect on the sludge can be characterized.
It can be seen from Fig. 11(a) that the remaining amount of anaerobic sludge was significantly related to the influent aniline concentration. The greater the aniline concentration, the slower the increase in the residual sludge concentration and the worse the treatment effect. When the aniline concentration changed from 0.4 mg/L to 5.4 mg/L, the concentration of anaerobic sludge decreased from 5.58 g/L to 3.44 g/L, and the growth rate dropped from 39.6% to -14.0%. The growth rate below 0 indicates that the anaerobic sludge in the micro-aerobic hydrolysis and acidification section was greatly inhibited.
From Fig. 11(b), it can be found that the residual anaerobic sludge was also significantly related to the influent antimony concentration. Similar trend was found as the relationship between aniline concentration and the residual sludge concentration; the greater the antimony concentration, the slower the increase in the residual sludge concentration. When the antimony concentration changed from 0.05 mg/L to 2.05 mg/L, the concentration of sludge decreased from 5.73 g/L to 4.26 g/L, and the growth rate dropped from 43.2–6.5%. The high concentration of antimony also inhibited anaerobic sludge. In addition, when the concentration of antimony in the influent was 2.05 mg/L, the concentration of the sludge after the reaction was still above 4 g/L, indicating that although the removal rate of antimony by anaerobic sludge was very low, the sludge did not cause a large number of microorganism deaths, the antimony may just cause inhibition effects on the sludge (Hargreaves et al. 2016).