Decentralized wastewater treatment has different treatment scales according to the different water volumes and collection methods, such as in rural wastewater treatment, which can be divided into single-household wastewater decentralized collection and treatment mode, joint-household wastewater decentralized collection and treatment mode, and village wastewater centralized collection and treatment mode (Libralatoetal.,2012;Wang Yang et al., 2015). The choice of sewage treatment facilities varies due to different local settlement conditions and economic situations .
Current treatment processes such as biofilm method, stabilization ponds, oxidation ponds, and artificial wetlands, which are commonly used for small-flow decentralized wastewater, are useful for pollutant reduction, but they also face many problems. For example, stabilization ponds cover a large area and sludge is easy to silt (Liu Yunguo et al., 2014); artificial wetlands are generally not suitable for direct treatment of higher concentrations of domestic wastewater and suffer from problems such as low hydraulic loading, large area, and susceptibility to climate and temperature influences (Liu Fung et al., 2010; Sun Zongjian et al., 2007); and the johkasou process, although it has a relatively good COD, BOD, and NH4+-N in wastewater treatment effect, the removal of TN and TP is less considered in the design (Wang Chang et al., 2009); and the high cost of filler in biological contact oxidation method increases the investment, in addition to the requirement of uniformity of water and gas distribution in the biological contact tank (Zhao Xianhui et al., 2010). With the introduction of more stringent effluent discharge standards, the treatment requirements for total phosphorus, total nitrogen and other pollutants have also been further enhanced, and the previous decentralized wastewater treatment facilities are difficult to meet the new requirements.
Continuous flow intermittent aeration process has been studied at home and abroad, relative to the traditional activated sludge process, intermittent aeration process can reduce the demand for carbon sources during denitrification, suitable for denitrification of low C/N sewage (Haoetal., 1996; Fulazzakyetal., 2015).Insel et al. (2006) concluded that aeration stopping aeration cycle time and the percentage of aeration time in it have important influence on the whole process of reaction nitrogen removal. Domestic research on the intermittent aeration process is mostly focused on the upgrading of existing wastewater treatment plants and the control parameters of the treatment process, such as Zhang Wen et al. (2013) studied the effect of intermittent aeration and continuous aeration on the nitrogen removal performance of the complete mixed reactor, pointing out that when intermittent aeration, denitrification is carried out more thoroughly due to the anaerobic stage is favorable for the metabolic activities of heterotrophic parthenogenetic anaerobic bacteria, and therefore the removal rate of total nitrogen The removal rate of total nitrogen could be maintained above 70%. Jin Chunji et al. (2003) carried out intermittent aeration process for low C/N wastewater, examined the effect of intermittent aeration cycle on the nitrogen removal of wastewater, and concluded that the aeration time should be kept above 0.5h according to the ammonia nitrogen load of influent, and the stirring anoxia time should be controlled at about 1h. Qiao Haibing et al. (2006) through the study of continuous flow intermittent aeration oxidation ditch, pointed out that the smaller the cycle period, the higher the frequency of aerobic and anoxic alternation, the relatively high level of DO in the system, which is conducive to nitrification, but also conducive to the elimination of the effect of the short flow of the stopping period; with the decrease in the percentage of the aeration time, stopping time increases, the organic matter of the feed water into the ditch as a denitrification of the additional carbon source, thus accelerate the denitrification rate. However, there are few reports on the application of decentralized intermittent aeration activated sludge process to decentralized wastewater treatment. Due to the constraints of treatment cost and water quality, it is of great practical significance to study the treatment efficiency of intermittent aeration reactor with small treatment capacity and low energy consumption.
This paper examines the effectiveness of the bioreactor in removing COD, nitrogen, and phosphorus through the production test of an intermittent aeration bioreactor applied to decentralized wastewater treatment, with a view to providing suggestions for its application in the decentralized wastewater treatment process.
2Materialandmethods
2.1Experimental setup
Continuous-flow intermittent aeration anoxic bioreactor (hereinafter referred to as the "bioreactor") was designed and processed according to the previous research results of the group (Liuetal. , 2017; Liuetal., 2017), as shown in Figure 1. The bioreactor is installed in a container, with a total volume of 27.6 m3, of which, the mixing tank is 3.2 m3, the intermittent aeration tank is 19 m3, the sludge retention tank is 2.2 m3, and the final settling tank is 1.9 m3.The wastewater enters into the mixing tank for mixing and then enters into the intermittent aeration tank. Intermittent aeration tank using dissolved oxygen meter online control device and the central control circuit (PLC) to control the aeration intensity and aeration time ratio. Intermittent aeration tank and mixing tank connected through the internal reflux pipeline, by adjusting the reflux flow control mixture reflux ratio. Sewage flow through the intermittent aeration tank, through the folding plate or thin tube and sludge retention tank connected to the mud-water mixture in the interception tank for mud-water separation and clarification, the supernatant flows into the final settling tank for further clarification and discharge, the retained sludge through the sludge reflux device to return to the intermittent aeration tank, so that the intermittent aeration tank to maintain a high sludge concentration. The final settling tank is equipped with a sludge discharge device, which discharges all the remaining sludge from the settling. By controlling the sludge discharge time, the sludge residence time can be controlled.
Figure 1 Schematic diagram of the bioreactor
Bioreactor intermittent aeration tank through the PLC automatic control of aeration and de-aeration time, to achieve intermittent aeration. Dissolved oxygen concentration in the aeration phase is controlled by the dissolved oxygen meter (Model: WTWIQSensorNet2020XTController). When the concentration of dissolved oxygen after aeration reaches the set upper limit (e.g. 2.5mg˙L-1), the aeration fan automatically stops aeration, at this time the mixing device is automatically turned on, the biological reactor in the biological consumption of dissolved oxygen. When the dissolved oxygen concentration decreased to the lower limit (e.g., 0.5 mg˙L-1), the aeration fan was automatically turned on to carry out aeration. In this study, we investigated the COD, nitrogen and phosphorus removal effect of the bioreactor by adjusting the combination of working conditions such as aeration time ratio, mixture reflux ratio and HRT. Each working condition was maintained for at least 15 d, of which, working condition VI was maintained for more than 30 d and working condition VII was maintained for 3 months. The working conditions are shown in Table 1.
2.2 Experimental water
The experimental wastewater was taken from the aeration and sand sedimentation tank of a municipal wastewater treatment plant in Rizhao City, Shandong Province, and entered into the reaction device through the lifting pump. The inoculated sludge of the bioreactor was taken from the oxidation ditch of this sewage plant. Reactor influent water quality indicators shown in Table 2.
2.3 analytical items and methods
Water inlet and outlet water samples are mixed homogeneously to determine its total COD, total nitrogen (TN), ammonia nitrogen (NH4 + - N), nitrate nitrogen (NO3 - N), total phosphorus (TP), the above indicators used Hach water quality analytical method of the serial number of 8000, 10072, 10031, 10020, respectively, 8190. The sludge concentration (MLSS) in the reactor was determined by the gravimetric method, and pH was determined by a portable pH meter (WTWMulti3220).
3Resultsanddiscussion
3.1Dissolved oxygen concentration in the bioreactor
Dissolved oxygen in the intermittent aeration pool of the bioreactor varied over time in an intermittent aeration cycle as shown in Figure 2. During the aeration phase, the average dissolved oxygen concentration in the pool is indicated by the horizontal dotted line in the figure. Condition Ⅰ, for example, the beginning of aeration, the concentration of dissolved oxygen in the pool rises, when it reaches the aeration upper limit of 2.5mg ˙ L-1, the aeration pump stops working; when the dissolved oxygen reaches the set lower limit of 0.5mg ˙ L-1, the aeration pump is automatically turned on. So the cycle repeats until the aeration cycle stops, the average concentration of dissolved oxygen in the pool is 1.64mg˙L-1. When the aeration stage is over, enter the stop aeration and mixing stage, the dissolved oxygen needs to be consumed for 10-20min to enter the anoxic stage. The traditional activated sludge method requires that the dissolved oxygen concentration in the aeration tank should not be less than 2.0mg˙L-1 to ensure that the nitrification reaction is complete. Studies have shown that lowering the DO concentration in the reactor can reduce the aeration energy consumption, such as controlling the aeration DO concentration at 0.5mg˙L-1, which is estimated to save 10% of the operational energy consumption (Liuetal., 2013). At the same time, low dissolved oxygen concentration can induce changes in the bacterial population in the reactor, promote synchronous nitrification and denitrification, and improve the TN removal rate (Lv Xi-wu et al., 2001; Wu Chang-yong et al., 2012; Liuetal., 2013).
Figure 2 Changes in dissolved oxygen concentration in the aeration zone with different aeration and de-aeration durations
3.2 Changes in sludge concentration (MLSS) and sludge volume index (SVI) in the bioreactor
The bioreactor was not actively discharging sludge from the reactor zone during the operation period, and the MLSS in the system was four times higher than that of the conventional activated sludge wastewater plant, and it could be stabilized up to 10000 mg˙L-1 or more (Figure 3). The sludge is settled in the retention tank and returned to the aeration tank through the sludge return device, so that the heavier sludge is retained in the bioreactor by automatic gravity selection. The final settling tank only clarifies the effluent and produces a small amount of sludge that can be discharged through the sludge discharge device.The MLSS starts to rise rapidly after inoculation and reaches about 10,000 mg˙L-1 in about 20d. In condition III, the sludge volume decreased due to the re-movement of the equipment, but then it quickly reached a stable state again. The sludge volume index gradually increased and stabilized at 80~100mL˙g-1, showing good sludge settling performance. In working conditions V and VI, the average water temperature in the bioreactor decreased to below 10°C, and there was no sludge expansion, which was consistent with the results of the previous study (Liuetal.,2017). Working condition VII entered spring and summer, the temperature rebounded, MLSS reached more than 12000 mg˙L-1, and its sludge volume index decreased slightly with the increase of sludge volume.
Figure 3 Comparison of sludge concentration and sludge settling index in the bioreactor
3.3 Removal effect on COD
The removal effect of the bioreactor on COD is shown in Fig. 4, and the effluent COD of each condition is shown in Table 3.It can be seen that the influent COD fluctuates a lot, but the bioreactor's removal rate of COD reaches more than 90% stably in the operation period. The bioreactor can maintain a high sludge concentration, which ensures that it has a good adaptability in the face of fluctuations in water quality. Adjusting the operating conditions had little effect on the COD removal effect, probably because heterotrophic bacteria have a stronger affinity for dissolved oxygen than autotrophic bacteria, therefore, in the state of low dissolved oxygen, the heterotrophic bacteria will take the lead in utilizing the oxygen to carry out metabolic activities, which can better metabolize the COD in the water (Yin Jun et al., 2013).
Figure 4 COD and removal rate of inlet and outlet water of the bioreactor
3.4 Removal effect of nitrogen
For the removal of NH4+-N, the bioreactor achieved a good nitrification effect within a short period of time after inoculation (Figure 5a). The average dissolved oxygen concentration in the aeration stage of condition Ⅰ was 1.64mg˙L-1 for 60min, and the good nitrification effect showed that the aeration volume was sufficient, so that the ammonia and nitrogen in the wastewater in the aeration stage could be fully transformed. In the stop aeration and mixing stage (duration 60min), the ammonia nitrogen in the influent water did not accumulate in the effluent water because of the dilution effect of the bioreactor, so that the ammonia nitrogen reached a better removal efficiency of more than 90%. However, the effluent TN was not well removed from the system due to the conversion of NH4+-N to NO3--N, and the TN effluent concentration was around 20 mg˙L-1 (Table 3), with a removal rate of around 40% (Figure 5c). Subsequently, the aeration stopping time was adjusted to 90 min (Case Ⅱ), at which time the aeration time ratio was reduced to 0.47 (Table 1), and the NH4+-N removal fluctuated slightly but could still be maintained at more than 90%, and the nitrogen removal efficiency was slightly improved. When adjusted to condition Ⅲ, the deaeration time increased to 150min, and the aeration time ratio further decreased to 0.33. The longer deaeration time and the change of sludge volume affected the nitrification reaction in the bioreactor, and the NH4+-N of the effluent was increased, while the NO3-N was further reduced. Due to the enhanced denitrification in the bioreactor, the nitrogen removal efficiency was further increased to 50%. When the influent flow rate is certain, the denitrification efficiency can be improved by adjusting the aeration time ratio and increasing the stopping time, which in turn improves the nitrogen removal efficiency. It should be noted that too short aeration time will cause insufficient oxidation of NH4+-N and increase the concentration of NH4+-N in the effluent, while too long aeration time will cause the denitrification stage to have insufficient carbon source for denitrification.
Figure 5 Bioreactor operation inlet and outlet water NH4+-N (a), NO3--N (b), TN (c) concentration and removal rate
Subsequently, the working condition Ⅳ reduces the stopping time of aeration to 90min, the aeration time ratio is 0.53, and the mixture reflux ratio is adjusted down to 1.5, and the bioreactor maintains a stable ammonia and nitrogen removal, and the efficiency of denitrification is about 55%~60%. Compared with the previous condition Ⅱ, the nitrification effect was guaranteed in this condition, and the nitrogen removal efficiency was improved. This is due to the reduction of mixed liquor reflux, the mixed liquor back to the mixing tank carries less dissolved oxygen, so that the mixing tank to maintain better anoxic conditions, enhance the denitrification effect. Contact Sewage Treasure or see more related technical documents.
Considering that the aeration is too large to affect the effect of nitrogen removal, followed by Condition V to reduce the aeration limit set to 1.5mg˙L-1, the average DO concentration of the intermittent aeration pool to 0.88mg˙L-1, while adjusting the flow rate to 50m3˙d-1, adjust the length of the stopping time of 110min, the aeration time ratio of 0.41. At this time, the effluent NH4 + -N concentration increased significantly. At this time, the effluent NH4+-N concentration increased significantly, and the average concentration in the operation stage was (10.0±4.3) mg˙L-1 (Table 3). Due to the decrease of average DO concentration, HRT and aeration time ratio, on the one hand, the nitrification reaction of NH4+-N was not complete, on the other hand, NH4+-N was accumulated in the longer anoxic period, and the concentration of NO3--N was significantly lower than that in the previous condition, and the removal rate of TN was slightly decreased. Considering the lower activity of microorganisms in winter, in order to maintain a better nitrification effect, it was adjusted to condition VI, reducing the influent flow rate and increasing the aeration time duration. Although the aeration time ratio increased to 0.48 and the HRT was increased, the nitrification was not complete, the removal effect of NH4+-N fluctuated, and the effluent TN was still maintained at 17~22mg˙L-1, and the removal rate was about 50%~70%. When the water volume changes, the size of the water volume affects the amount of nutrient delivery, under a certain sludge volume and respiratory strength, the water volume will have an impact on the effluent effect, therefore, it is necessary to properly adjust the intermittent aeration time ratio to ensure the treatment effect. The reduction of nitrogen removal efficiency in winter can be compensated by prolonging the aeration time and sludge age to improve the nitrification efficiency, but the total nitrogen removal is still affected by a certain degree, and it can be considered to add a certain amount of carbon source material for supplementation.
Subsequently, Condition Ⅶ will stop the aeration time is slightly lower, the average dissolved oxygen concentration in the intermittent aeration pool is 1.0mg˙L-1, keep the aeration time, continue to monitor the treatment effect for 3 months. With the increase in operating time, the bioreactor population to stabilize, effluent NH4 + - N, TN can reach the "urban sewage treatment plant pollutant discharge standards" (GB18918-2002) in the primary A discharge standards, NH4 + - N removal rate of more than 90%, TN removal rate of 70% ~ 80%. Comparing the working conditions V, VI, VII with the previous conditions Ⅰ, Ⅱ, Ⅱ, when the influent water flow rate increases, the nitrogen removal efficiency can be improved by increasing the aeration time ratio and the cycle length at the same time.
Dey et al. (2011) found that the optimal aeration time for this type of reactor should account for 50%~60% of the whole cycle, and the optimal cycle time should be controlled in the range of 2~3h, under which a better nitrogen removal effect can be achieved. In addition, higher sludge concentration can promote denitrification in the reactor. Sarioglu et al. (2009), through the study of simultaneous nitrification denitrification in MBR, proposed that when the sludge concentration in the reactor reaches a high level (25000~30000mg˙L-1), the attenuation of sludge can support endogenous denitrification; on the other hand, the higher concentration of sludge can be aggregated to form the inner anoxic zone, which can promote synchronized nitrification and denitrification. The results of this study are similar to the above findings, but the differences are mainly due to the different sludge concentrations and bacterial groups in practical applications, as well as the different experimental environments and working conditions.
3.5Phosphorus removal effect
The bioreactor was not actively discharged from the reaction area during the operation period, and the residual sludge was discharged from the final settling tank, and the SRT of the bioreactor was about 50d, calculated by the sludge production coefficient and the amount of sludge. In the operation period of the bioreactor in the fall and winter (working conditions Ⅰ to Ⅵ), the total phosphorus concentration in the effluent water was about 1.65 mg˙L-1 on average (Table 3), and the removal effect of total phosphorus was about 60%. The removal effect of total phosphorus was about 60%. In condition VII, the concentration of phosphorus in the influent water increased greatly, but the concentration in the effluent water gradually decreased to less than 1mg˙L-1, which met the national pollutant discharge standard for urban wastewater treatment plants, class 1B standard. With the increase of sludge concentration in the bioreactor, the removal rate reached more than 80%. According to the previous study, intermittent aeration can create an anaerobic and anoxic environment in the mixing tank, and the creation of an anoxic and aerobic environment in the intermittent aeration tank is favorable for the growth of phosphorus-aggregating organisms (PAOs), which, in turn, promotes the removal of phosphorus in the treatment (Liuetal.,2017). In addition, intermittent aeration reduced the concentration of nitrate nitrogen returning to the anoxic zone, which reduced the effect of nitrate nitrogen on anaerobic phosphorus release, and thus created an environment suitable for the growth of polyphosphorus bacteria (PAOs), which made phosphorus removal not only through assimilation removal, but also strengthened the performance of biological phosphorus removal (Hou Hongxun et al., 2009). In order to achieve a better total phosphorus removal effect, it can be considered to increase the regular sludge discharge from the reaction zone and supplemented with chemical phosphorus removal.
Figure 6 Total phosphorus concentration and removal rate in the inlet and outlet water of bioreactor operation
4Conclusions
1)Continuous-flow intermittent aeration anoxic bioreactor can maintain high sludge concentration and remove COD in domestic wastewater well.After stable operation, the COD removal rate can reach more than 90%.
2)In terms of nitrogen removal efficiency, when the water volume is certain, the aeration time ratio can be adjusted downward, increase the time of stopping the aeration, improve the nitrogen removal efficiency; in the aeration intensity is certain, you can adjust downward the mixture reflux ratio, improve the nitrogen removal efficiency; when the water volume rises, the aeration time ratio can be increased and the cycle time length, improve the nitrogen removal efficiency. After stable operation, the removal rate of NH4+-N can reach more than 90%, and the removal rate of TN reaches 70%~80%.
3)Through intermittent aeration, the bioreactor can achieve good phosphorus removal efficiency. After stable operation, the TP removal rate can reach more than 80%.
4)In the actual application of the project, we should scientifically investigate the water quality and quantity in the field, and build a regulating tank to balance the change of water quality and quantity during the day; adjust the appropriate aeration stopping time to achieve the designed treatment effect; and increase the discharge of mud in the reaction area according to the actual treatment requirements.
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