In this study, the side-stream heat-shock treatment was used to start up and maintain the nitritation of real sewage. Complete nitrification was obtained when the real sewage was treated in a sequencing batch reactor (SBR). Then, about 50% of the mixed sludge was collected from the SBR and treated with the heat-shock treatment at 60 °C for 40 min in another reactor every 2 weeks. After providing the heat-shock treatment for four times, the effluent nitrate in the SBR gradually decreased from 22.5 to 3.2 mg/L, while the nitrite accumulation rate increased from 4.4% to 81.8%, indicating a successful start-up of nitritation. Further, the sewage nitritation was stable with the regular side-steam heat-shock treatment for 91 days, and the ammonium removal efficiency of 80.6% and nitrite accumulation rate of 91.2% were achieved. This study suggests that the side-stream heat-shock treatment could be used to start up sewage nitritation and maintain stability for a long-term operation.
The conventional nitrogen removal process is generally based on the nitrification and denitrification, during which ammonium is oxidized to nitrate, while nitrate is reduced to dinitrogen. Compared to the conventional nitrification and denitrification processes, the removal of biological nitrogen via nitrite is a novel process, which could reduce the oxygen consumption during the nitrification by 25% and could reduce the requirement for a carbon source during the denitrification stage by 40% (Zhang et al. 2018a). In particular, nitritation could be integrated with the anaerobic ammonium oxidation, saving 100% consumption of the carbon source (Ma et al. 2016; Gao et al. 2018; Miao et al. 2018; Zhang et al. 2018b). Thus, nitrogen removal via nitrite is beneficial in the carbon-limited biological wastewater treatment plants (WWTPs).
Nitritation requires the ammonium to oxidize to nitrite without further oxidization to nitrate. Therefore, it is critical to retain ammonium-oxidizing bacteria (AOB), while suppressing nitrite-oxidizing bacteria (NOB) to achieve the nitritation. Several studies have investigated the suitable conditions for suppressing NOB, including low dissolved oxygen, short sludge retention time and high temperature (Han et al. 2016; Zheng et al. 2017; Han et al. 2018). However, a long start-up time is generally required for achieving nitritation through operational control. The start-up of nitritation could be further shortened through adding certain inhibitors such as free nitrous acid and hydrazine (Xiao et al. 2015; Wang et al. 2016a, 2016b; Wang et al. 2017; Wang et al. 2018). These inhibitors would decease both AOB and NOB activity, but AOB activity is less impacted and more quickly recovers from inhibition, inducing the rapid start-up of nitritation (Wang et al. 2016b). Although inhibitor addition is beneficial to the start-up of nitritation, the cost and the secondary pollution are still of concern for the full-scale application (Li et al. 2018). Thus, effective and economical methods to start up and maintain stable nitritation still require investigation.
Heat-shock treatment is a feasible alternative for the nitritation start-up without any inhibitors and secondary pollution (Kazuichi et al. 2008; Chen et al. 2016). When activated sludge is treated with the heat-shock, AOB and NOB activities are completely suppressed, but the recovery of AOB activity is quicker than NOB, resulting in the nitrite accumulation. Chen et al. (2016) started up nitritation by the heat-shock treatment at 70 °C for 20 min. The reactor was fed with synthetic wastewater and the nitritation was successfully established and maintained for 17 days with an average nitrite accumulation rate of 65.1%.
Although heat-shock treatment is beneficial to start up nitritation, there are still several obstacles for full-scale application. Firstly, the majority of the previous studies on the heat-shock treatment are associated with synthetic wastewater; the feasibility and performance of achieving the nitritation using real sewage are poorly understood. Secondly, the heat-shock suppresses AOB and significantly reduces the ammonium removal. The lag-phase for the AOB recovery also limits the practical application of the heat-shock treatment. Overall, the heat-shock treatment method requires further optimization to achieve a robust nitritation with a higher average nitrite accumulation rate before final application. In this context, the aim of this study was to investigate the feasibility of achieving and maintaining nitritation in a real sewage using heat-shock treatment.
MATERIALS AND METHODS
Seed sludge and wastewater
Seed sludge and wastewater used in this study were collected from GaoBeiDian WWTP, Beijing (China) from a returned sludge pipe and the primary sludge tank, respectively. The chemical oxygen demand and NH4+-N in wastewater were 346.1 ± 53.6 mg/L and 51.1 ± 5.8 mg/L, respectively, while NO2–N and NO3–N were lower than 0.8 mg/L.
The sequencing batch reactors (SBRs) with a working volume of 10.4 L were used in this study. These reactors were made of polymethyl methacrylate. Air diffusers (HAILEA, China) were placed at the bottom of the reactors for oxygen supply. Airflow meters were used to control the aeration rate. A mechanical stirrer (IKA, Germany) was used to provide mixing during the anoxic phase. Temperature sensors and electric heaters (Xi Long, China) were used to maintain the wastewater temperature at 25 ± 2 °C. The reactors were also equipped with a pH and dissolved oxygen (DO) probe (WTW Company, Germany).
Heat-shock and activity recovery experiments
After the settlement of the activated sludge and discharge of the supernatant, the concentrated sludge was transferred into another reactor for the heat-shock treatment. The reactors were continuously stirred for 40 min at a specific temperature in a water bath and the treated sludge was transferred to another reactor after cooling down to 25 °C. This reactor was fed with the real sewage (5–8 L) and continuously aerated for the activity recovery. The aeration was stopped when ammonium was lower than 1 mg/L during the recovery period. The settled concentrated sludge was washed with deionized water and returned to the initial reactor (Ma et al. 2015).
Experimental set-up and operation
Determination of the appropriate heat-shock temperature and direct heat-shock treatment
Batch tests were conducted at 25, 40, 45, 50, 60, and 80 °C to select a suitable heat-shock temperature based on the variations of nitrification activity. Then two SBRs were used for long-term operation and the heat-shock treatment was conducted when the nitrification was stable. After the heat-shock treatment (50 and 60 °C), the SBRs were fed with real sewage and re-started. The nitrogen removal performance was measured during the following operation. The reactors were operated in three cycles per day, each consisting of 20 min feeding phase (5.2 L), 90 min anoxic reaction phase, 2 h aeration, 30 min settling, 20 min decanting phase (5.2 L) and an idle period.
Sewage treatment using the side-stream heat-shock treatment
The side-stream heat-shock treatment was investigated with the purpose of mitigating the negative effect of the heat-shock treatment on the ammonium oxidation. The operational procedure was identical to that described in the preceding text. The side-stream heat shock treatment was conducted fortnightly, in which 50% of the mixed sludge was collected and given a heat-shock at 60 °C for 40 min.
Liquid-phase samples were taken every 2 days and were immediately filtered through disposable Millipore filter units (0.22 μm) for the analyses of ammonium, nitrite and nitrate. The mixed liquid suspended solids (MLSS), mixed liquid volatile suspended solids and sludge volume index were measured twice a month. All the analyses were performed according to the standard methods (APHA 1995). Temperature, DO, pH and oxidation-reduction potential were monitored using the WTW pH/oxi340i meter (WTW Company, Germany). Microbial nitrification activity was measured according to the method described by Liu et al. (2017).
RESULTS AND DISCUSSION
Starting up sewage nitritation by a single heat-shock treatment
At the heat-shock temperature of 40 and 45 °C, the activity of AOB and NOB was not significantly impacted (Table A in the supplementary material, available with the online version of this paper), whereas the AOB and NOB activities were completely suppressed at the heat-shock temperature higher than 50 °C. As previously reported, AOB would quickly recover from suppression of heat-shock, inducing the start-up of nitritation (Kazuichi et al. 2008). For the activity recovery of AOB, longer lag-phase was observed at higher temperatures. Since the lag-phase for AOB for the heat-shock at 50 °C and 60 °C is 9 and 12 days, respectively, these temperatures were selected for further experiments instead of 80 °C, which has a lag-phase of 23 days.
During the long-term experiment, a complete nitrification performance (Figure 1) was quickly achieved after inoculation in two reactors and the ammonium removal efficiencies were 91.3% and 94.5%. Nitrite accumulation was insignificant at the rates of 2.5% and 1.8%. Then, the heat-shock treatment at 50 °C and 60 °C was conducted in two SBRs separately. It was found that the nitrification activities were completely suppressed. For SBR1 with the heat-shock treatment at 50 °C, the ammonium removal efficiency gradually increased to 98.3% and the nitrite accumulation rate was 94.5% after a lag-phase of 9 d. Similarly, the ammonium removal efficiency increased to 97.9% and nitrite accumulation rate gradually increased to 93.2% in SBR2 after a lag-phase of 12 d. These results indicate that the heat-shock treatment is an effective method for the sewage nitritation (Kazuichi et al. 2011).
In SBR2, the nitritation was maintained at an average nitrite accumulation rate of 75.6% for 26 days, while it lasted only for 18 days in SBR1, suggesting that the nitritation performance for the heat-shock treatment at 60 °C was better than that at 50 °C. It might be because NOB were more suppressed at heat-shock treatment of a higher temperature (60 °C), resulting in a more stable nitritation performance. Although direct heat-shock treatment could effectively achieve sewage nitritation through suppressing NOB, it also resulted in the temporary suppression of AOB, leading to an increase in the effluent ammonium concentrations. During the lag-phase, the effluent ammonium concentration after the heat-shock treatment was generally higher than the values prescribed by the standards for WWTPs, limiting the application of the heat-shock treatment. Consequently, the side-stream sludge treatment could be used as an alternative for the successful inhibition of NOB with little effect on AOB (Wang et al. 2018).
Stable sewage nitritation by the side-stream heat-shock treatment
A control reactor and an experimental reactor were used in the side-stream heat-shock treatment experiment. In the control reactor without the heat-shock treatment, ammonium was almost fully converted to nitrate at an average ammonium removal efficiency of 99.2% and a nitrite accumulation rate of 1.1% (Figure A in the supplementary material, available online). In the experimental reactor, the average ammonium removal efficiency and nitrite accumulation were similar to the control reactor before the heat-shock treatment. Then, the side-stream heat-shock treatment was conducted from the 10th day. As shown in Figure 2, the effluent nitrate decreased from 23.6 to 3.15 mg/L by day 81, while the effluent nitrite increased from 0.4 to 14.2 mg/L (Figure 2(b)). Meanwhile, the nitrite accumulation rate increased gradually from 22.6% to 81.8% (Figure 2(a)), indicating a successful start-up of sewage nitritation after four heat-shock treatments. Additionally, the AOB and NOB activities decreased after the side-stream heat-shock treatment. The NOB activity continuously decreased from 1.5 to 0.4 mgN/(gSS·h), while AOB activity was stable at about 2.1 mgN/(gSS·h) (Figure 3). Meanwhile, the ammonium removal efficiency of the experimental reactor varied from 65.3% to 95.4% during operation, which is significantly higher than that of the direct heat-shock treatment. Thereafter, nitritation was stable for 91 days with an average ammonium removal efficiency of 80.6% and a nitrite accumulation rate of 91.2%.
In this study, sewage nitritation was achieved after four side-stream heat-shock treatments (60 °C, 40 min). Moreover, the side-stream treatment reduce the negative effect of heat-shock on AOB activity, avoiding the significant increase of the effluent ammonium. Moreover, stable nitritation was obtained in this system, which might be attributed to the increase in the relative abundance of AOB and the decrease of NOB. A previous study reported that the NOB population was significantly reduced after a heat-shock treatment of 60 °C for 20 min while the population of AOB was barely changed even after heat-shock at 80 °C for 60 min (Kazuichi et al. 2008). In this study, abundances of AOB and NOB were measured on day 171. In the control reactor, AOB and NOB relative abundances were 1.42% and 4.42%, respectively. In the experimental reactor, AOB and NOB relative abundances were 1.44% and 0.71%, respectively. The ratio of the AOB/NOB abundance in the experimental reactor (2.03) was higher than that of the control reactor (0.32), which is beneficial for start-up and maintenance of the nitritation (Zhang et al. 2012; Ma et al. 2013).
Potential application of the side-stream heat-shock treatment in WWTPs
The side-stream heat-shock treatment is feasible to achieve stable nitritation of real sewage. Compared with other methods such as free nitrous acid addition, the side-stream heat-shock treatment started-up sewage nitritation does not require any dosing inhibitor, does not have any secondary pollution and is stable during the operation. For the application in a WWTP, the energy requirement for the heat-shock treatment could be easily achieved through the waste heat released in the anaerobic digester by the heat-pump technology (Figure 4). Therefore, the proposed novel strategy to achieve the mainstream nitritation is economically attractive and practically feasible. However, it is important to optimize the operational parameters such as treatment time, frequency and sludge volume in a pilot or full-scale plant.
The study developed a new method for achieving sewage nitritation through side-stream heat-shock treatment. The side-stream heat-shock treatment of 60 °C for 40 min could quickly start up the sewage nitritation. Moreover, stable nitrite accumulation and a high ammonium removal efficiency was simultaneously obtained, showing the potential for full-scale application.
This research was financially supported by National Natural Science Foundation of China (51608013 and 21777005), and Beijing Municipal Science and Technology Project (D171100001017) and the 111 Project (D16003).