The ANAMMOX reactor under transient-state conditions: Process stability with fluctuations of the nitrogen concentration, inflow rate, pH and sodium chloride addition

Bioresource Technology 119 (2012) 166–173

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The ANAMMOX reactor under transient-state conditions: Process stability with fluctuations of the nitrogen concentration, inflow rate, pH and sodium chloride addition Jin-Jin Yu, Ren-Cun Jin ⇑ Department of Environmental Sciences, Hangzhou Normal University, Hangzhou 310036, PR China

h i g h l i g h t s " The performance of an ANAMMOX UASB under extreme stresses was investigated. " The ANAMMOX reactor was susceptible to the alkaline wastewater. " The efficiency of the process increased during load shocks. " Electrical conductivity is a convenient signal for reactor control.

a r t i c l e

i n f o

Article history: Received 17 April 2012 Received in revised form 22 May 2012 Accepted 23 May 2012 Available online 1 June 2012 Keywords: ANAMMOX Overloads pH fluctuation Sodium chloride addition Electrical conductivity

a b s t r a c t The process stability of an anaerobic ammonium oxidation (ANAMMOX) was investigated in an upflow anaerobic sludge blanket reactor subjected to overloads of 2.0- to 3.0-fold increases in substrate concentrations, inflow rates lasting 12 or 24 h, extreme pH levels of 4 and 10 for 12 h and a 12-h 30 g l1 NaCl addition. During the overloads, the nitrogen removal rate improved, and the shock period was an important factor affecting the reactor performance. In the high pH condition, the reactor performance significantly degenerated; while in the low pH condition, it did not happen. The NaCl addition caused the most serious deterioration in the reactor, which took 108 h to recover and was accompanied by a stoichiometric ratio divergence. There are well correlations between the total nitrogen and the electrical conductivity which is considered to be a convenient signal for controlling and monitoring the ANAMMOX process under transient-state conditions. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Anaerobic ammonium oxidation (ANAMMOX) is a novel process for nitrogen removal from wastewater in which the ammonium is oxidized to dinitrogen with nitrite as the electron acceptor (Eq. (1)) (Strous et al., 1998). It is known that the ANAMMOX process would lead to a significant reduction in operational costs compared with the conventional nitrification–denitrification process. NHþ4 þ 1:32NO2 þ 0:066CO2 þ 0:13Hþ ! 1:02N2 þ 0:26NO3 þ 2:03H2 O þ0:066CH2 O0:5 N0:15 ð1Þ

In most studies on ANAMMOX reactors that treat different types of wastewater with various configurations, the focus was on the reactor performance under steady-state conditions (Strous et al., 1997; Waki et al., 2007; Tang et al., 2011). While stable ⇑ Corresponding author. E-mail address: [email protected] (R.-C. Jin). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.05.116

operating conditions were difficult to achieve in practical applications, which also affected the bacterial activity. It was reported that the external factors, such as the influent substrate concentration, inflow rate, salinity and pH, were considered to be the major factors that induced deterioration of the reactor (Borja and Banks, 1995; Fox and Suidan, 1996; Jin et al., 2012a,b) and the dominant species changed depending on the external environment (Steinberg and Regan, 2011). Transient high levels of substrate concentrations and inflow rates led to fluctuations in the reactors (Hu et al., 2011; Leitão et al., 2006), yet different mechanisms for the impact were revealed (Nachaiyasit and Stuckey, 1997). In anaerobic wastewater treatment, NaCl is thought to be the typical inhibitor; therefore, NaCl seriously hampered the digestion process when over the threshold of 10 g l1 (Kugelman and McCarty, 1965). Meanwhile, inappropriate pH levels in the wastewater (too high or too low) revealed the destructive effects of anaerobic sludge and weakened the reactor efficiency (Gao et al., 2010; Zandvoort et al., 2005). Consequently, it is necessary to investigate bioreactors that are subjected to various types of shocks. Until now,

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the researches on reactor robustness have mainly explored the anaerobic digestion process. In relation to the ANAMMOX pathway, there are few reports that discuss reactors under external adverse conditions. This study evaluated the effects of drastic variations in the flow rate, substrate concentration, pH and salinity on the performance of an ANAMMOX upflow anaerobic sludge blanket (UASB) reactor. Moreover, for better control of the ANAMMOX reactor under transient-state conditions, the feasibility of using the electrical conductivity (EC) as a process diagnosis and monitoring signal was also examined. 2. Methods 2.1. Reactor and seeding sludge The UASB reactor was fabricated from polymethyl methacrylate with an effective volume of 4 l. The synthetic wastewater was fed from troughs into the reactor, and a peristaltic pump was used to control the inflow rate. The stock suspension in the feeding tank was heated to 30 ± 1 °C. The produced gas was discharged via porthole in the top of the reactor. Black fabric was used to cover the reactor to prevent inhibition by light. The seeding sludge in the reactor was from a Bardenpho system that treated monosodium glutamate wastewater with a TSS of 10.2 g l1 and a VSS of 8.8 g l1. 2.2. Experimental setup After start-up and operation for approximately 150 days, the UASB reactor was run under PSS (pseudo-steady-state) conditions at a hydraulic retention time (HRT) of 24 h, and the performance of the reactor was assayed (Table 1). The robustness of the ANAMMOX reactor was tested for four types of shocks, with a total of eleven shocks, as follows: 1) the substrate concentration was elevated 2.0- or 3.0-fold over the original level for 12 and 24 h; 2) the flow rate was increased 2.0- or 3.0-fold over the original level for 12 and 24 h; 3) the pH of the influent wastewater was changed to 4.0 or 10.0 for 12 h; and 4) 30 g l1 NaCl was added for 12 h. When the shock condition was discontinued, the operation was returned to normal. Additionally, there was an interval duration of 10 times the HRT (10 d) before the next shock. 2.3. Synthetic wastewater As needed, ammonium and nitrite were added to the mineral medium in the form of (NH4)2SO4 and NaNO2, respectively. The mineral medium was prepared according to Jin et al. (2008). 2.4. Chemical analysis and calculations The NH4+–N, NO2–N, NO3–N, TSS and VSS levels were measured by standard methods (APHA, 1998). The temperature, pH and electrical conductivity were detected by an alcohol thermom-

Table 1 The reactor performance at pseudo steady state. Parameter

Influent

Effluent

NH4+–N (mg l1) NO2–N (mg l1) NO3–N (mg l1) HRT(h) Nitrogen loading rate(kg m3 d1) Nitrogen removal rate (kg m3 d1)

280 280 0 24 0.56 0.41 ± 0.03

44.0 ± 13.1 9.3 ± 7.6 94.7 ± 9.6

eter, a pH meter (Mettle Toledo Delta 320) and a conductivity meter (Mettle Toledo FE30), respectively.

3. Results and discussion 3.1. Impact of an increased inflow rate on the ANAMMOX performance After steady-state operation, the inflow rate was increased 100% for 12 h (A) and 24 h (B) and then 200% for 12 h (C) and 24 h (D). Fig. 1 shows the performance of the reactor under the transiently high inflow rates. In shock A, a change in the inflow rate shock was characterized by an immediate increase in the effluent total nitrogen (TN) concentration. The TN increased from 153.6 to 183.8 mg l1, the ammonium and nitrite were elevated approximately 38.7% and 170%, and both of the peak levels emerged at the conclusion of the shock. Under this intense inflow rate condition, the pH was immobile at approximately 8.34 ± 0.18. The characteristics of shocks B, C and D were similar to shock A, in which a deterioration of the reactor performance occurred while the pH increased drastically. Among the four shocks, shock D, which lasted for 24 h, caused the most acute perturbation, with the effluent TN peak concentration increasing to 322.2 mg l1(118% increase from baseline). Additionally, the recovery time of the shock extended approximately 28 h beyond that of shock A. Under the inflow rate shocks, the factors causing the reactor deterioration were thought to be the increased total nitrogen loading, enhancement of the sludge washed out and less contact time between the biomass and substrates. In those shocks, due to the formation of a granular sludge and the capability of biomass retention in the UASB, the amount of sludge washed out was similar to the steady-state value; therefore, the other two reasons given may be the major factors. After cessation of the shocks, the effluent substrate concentration began to return to the original level or an even lower value. In particular, after inflow rate shocks A and C, the effluent TN concentration decreased from 153.5 to 125.0 and 122.0 to 95.9 mg l1, respectively. Nachaiyasit and Stuckey (1997) found that under transient hydraulic shocks, more efficient mixing and contact between the substrate and sludge were achieved in the anaerobic methanogenic reactor, which resulted in a better reactor efficiency. In an anaerobic digester, the inner bacterial community structures were dependent on the operating conditions, and the performance of the reactor was improved by low-intensity shocks (Bhatia et al., 1985). Another study using an ANAMMOX anaerobic baffled reactor also reported this promotion phenomenon (Jin et al., 2012 b). The performance of the ANAMMOX UASB under the inflow rate shocks is listed in Table 2. The degrees of deterioration were dependent on the duration and magnitude of the shocks and the adaptability of the ANAMMOX community. When the reactor showed a higher effluent substrate concentration, a longer recovery time was required. According to Eq. (2), the overloads of shocks B and C were both 1140 mg l1 h1, which caused diverse responses. The peak effluent TN concentration with shock B was smaller than with shock C, while in the former shock, the reactor took 35 h to recover, which was longer than the 19 h for shock C. It was considered that with the same overload, the load time was the dominant factor that determined the degree of disturbance in the reactor.

NO ¼ ðNSHO  NNOR Þ  V  Dt

ð2Þ 1

1

where NO: The nitrogen overload, mg l h ; NSHO: the nitrogen load in the shock condition, mg l1; NNOR: the nitrogen load in the normal condition, mg l1; V: volume of reactor, l; Dt: duration time, h.

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Fig. 1. The performance of the UASB under a 2.0-fold hydraulic shock for 12 h (A) and 24 h (B) and a 3.0-fold hydraulic shock for 12 h (C) and 24 h (D). h: total nitrogen, d: pH, : electrical conductivity.

Table 2 The reactor performance under overloads. Shock No.

A

B

C

D

E

F

G

H

Original TN load (mg l1 d1) Inflow rate increment (%) Substrate concentration Increment (%) TN overloads (mg l1) Shock duration (h) Peak NH4+–N (mg l1) Peak NO2–N (mg l1) Peak NO3–N (mg l1) Recovery time (h)

560 100 – 560 12 81.0 43.2 82.3 7

560 100 – 1140 24 86.0 47.9 84.8 35

560 200 – 1140 12 101 87.0 83.2 19

560 200 – 2280 24 140 121 87.2 35

560 – 100 560 12 78.6 42.0 118 14

560 – 100 1140 24 191 122 115 95

560 – 200 1140 12 143 100 121 55

560 – 200 2280 24 325 246 148 106

The mass balance was also calculated and is listed in Table 3. The nitrogen removal rate (W), which is a symbol of the reactor performance, varied depending on the various intensities of the shocks. With shock C, the W values of NO2–N and NH4+–N had peak levels of approximately 375 and 409 mg l1 d1, respectively. The sequence of W was as follows: Wshock C > Wshock A > Wshock D > Wshock B > Wpps. It was found that with the high effluent substrate concentrations, the W under the shock conditions was superior to that at the original level.

Under PSS, the stoichiometric ratio of the ammonium and nitrite removed (Rs) was 1:1.15 with a ratio of ammonia utilization to nitrate production (Rp) of approximately 0.40, which was similar to the stoichiometric ratio of ANAMMOX of 1:1.32:0.26 (Strous et al., 1998). During the shock period, the Rs and Rp levels deviated from the stoichiometric ratio, which also showed fluctuations in the ANAMMOX pathway. The anaerobic digestion process is a chain of interconnected biological reactions in which the organic matter is transformed

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J.-J. Yu, R.-C. Jin / Bioresource Technology 119 (2012) 166–173 Table 3 Calculated values of the influent, effluent and removed nitrogenous matter mass under the nitrogen overloads. Shock No.

MI(g) +

A B C D E F G H

ME(g) 

+

MR(g) 

+



MP(g)

W (mg l1 d1) NH4+–N

NO2–N

345 324 375 350 343 286 363 293

407 380 409 382 374 342 395 359

NH4 –N

NO2 –N

NH4 –N

NO2 –N

NH4 –N

NO2 –N

NO3–N

1.45 3.31 2.57 4.33 1.91 3.78 3.17 6.21

1.45 3.31 2.57 4.33 1.91 3.78 3.17 6.21

0.356 0.773 0.635 1.59 0.25 1.06 0.51 2.06

0.160 0.335 0.459 1.34 0.10 0.53 0.27 1.13

1.09 2.54 1.94 2.74 1.66 2.72 2.66 4.15

1.29 2.98 2.11 2.99 1.81 3.25 2.9 5.08

0.362 0.85 0.66 1.06 0.53 1.07 0.85 1.84

RS

RP

1.18 1.17 1.09 1.09 1.09 1.19 1.09 1.22

0.33 0.33 0.34 0.39 0.32 0.39 0.32 0.44

where MI: total influent substrate mass; ME: total effluent substrate mass; MR: removed substrate mass; MP: mass of product NO3–N.

into methane, carbon dioxide and anaerobic biomass. The main bacteria, acid-forming microorganisms, differed from the methane-forming microorganisms in terms of physiology, nutritional needs and sensitivity to environmental conditions. The overloads immediately weakened the acid step by an accumulation of volatile fatty acids that decreased the pH, which disturbed the reactor operation (Borja and Banks, 1995). However, the ANAMMOX pathway does not have multistage reactions similar to anaerobic digestion and can be completed by a single bacterium. Therefore, the impaired efficiency due to a high inflow rate in the ANAMMOX reactor was not as severe as in a digestion reactor. 3.2. Impacts of an increased substrate concentration on the ANAMMOX performance With the substrate shocks, the inflow nitrogen concentration was increased 100% with a shock period of 12 h (E) or 24 h (F) and then increased 200% with a shock period of 12 h (G) or 24 h (H). The reactor performance under the substrate shocks is plotted in Fig. 2. A sudden high substrate concentration led to nitrogen overload, which may exceed the reactor’s potential and inhibit the bio-reaction. With the shocks, the ANAMMOX reactor seemed to be sensitive to these short-term exposure experiments. An accumulation of TN in the effluent was observed for the four substrate shocks, which was accompanied by a fluctuation of the pH. Once the favorable environmental condition was resumed, the bacteria then regained their previous levels of conversion. The results of the reactor performance operating under the substrate shocks are listed in Table 2. As expected, the peak TN concentration was 708.9 mg l1 (379% increase from baseline), with a recovery time of 106 h, which was observed with shock H, revealing the severe environment inside the reactor. Shocks F and G both suffered the same equivalent nitrogen overload; however, under the former condition, the reactor took less time to return to the original level. Therefore, similar to the inflow rate shocks, when there was a long shock time, the reactor showed a large fluctuation. With the substrate shocks, the mass balances were also calculated, as listed in Table 3. The sequence of W levels was as follows: Wshock G > Wshock E > Wshock H > Wshock F > WPPS. The reactor performance was improved with the substrate shocks, similar to that of the inflow rate shocks. An overload by a substrate shock caused a disturbance in the reactor, resulting in specific responses. Under the inflow rate shocks, the substrate concentration was not as high as under the substrate shocks, which generated a high substrate concentration that hampered the ANAMMOX pathway directly. By comparing the peak substrate values and recovery times in Table 2, a conclusion can be drawn that the ANAMMOX reactor was unresponsive to hydraulic shocks, yet sensitive to substrate shocks when subjected to the same nitrogen overload.

3.3. The impact of transient changes in the pH on the ANAMMOX performance To operate successfully, the pH was found to be critical for a stable ANAMMOX pathway, and a range of 6.7–8.3 was reported to be suitable for manipulation (Strous et al., 1999). Nevertheless, an ANAMMOX reactor was also combined with the SHARON (single reactor system for high ammonium removal over nitrite) process for the released nitrite, and the low pH of the effluent from the SHARON process could interfere with the ANAMMOX pathway. Additionally, it was reported that a high pH was also capable of intensifying the inhibition of the ANAMMOX process by free ammonia (Fernández et al., 2012). Here, pH values of 4 and 10 were created in the reactor for 12 h to test the pH shocks on the ANAMMOX operation. When the low-pH wastewater was generated, the pH in the reactor decreased and then recovered to the normal level at the termination of the shock. During and after the shock period, the pH varied between 6.39 and 7.54, which was within the optimum pH range of 6.7–8.3. The effluent substrate concentration in the reactor increased only in the first hour; however, with the nitrogen removal efficiency consolidated by external stimulation, the TN concentration decreased to approximately 159.9 ± 8.0 mg l1 (Fig. 3I). Due to the H+ consumed by the ANAMMOX reaction, the pH in the reactor was higher than in the influent. With acidic wastewater, the influx neutralized the alkalinity in the reactor while providing an appropriate pH condition for the ANAMMOX pathway. The results showed an excellent property of the sludge for adaptation and biodegradation of wastewater at a low pH. An increase in the influent pH to 10 was maintained for 12 h, and the reactor’s response is illustrated in Fig. 3J. Unlike the low pH shock, the alkaline wastewater triggered a large disturbance and caused an increase in the pH from 7.84 to 9.17. The pH only returned to its normal level of 7.85 after 50 h of normal operation. During approximately the first 10 h of this shock, the reactor seemed insensitive to an exposure to wastewater with a high pH because the effluent TN concentration remained at 135.8 ± 8.4 mg l1, which was close to the original level. However, at 12 h, a fluctuation occurred. During the fluctuation period, the TN concentration reached a peak level of approximately 209.0 mg l1 (41.2% increase from baseline) at 22 h and then recovered to the normal level gradually. In the present UASB reactor, it was found that a recovery from high pH stress conditions was possible. The ammonia can easily penetrate the cell membrane, and the uncharged soluble ammonia molecules created a proton imbalance and specific enzyme inhibitions (Sprott and Patel, 1986). It was suggested that free ammonia caused the deterioration of the ANAMMOX reactors at 13–90 NH3–N mg l1 (Waki et al., 2007) or even as low as 1.7 mg NH3–N mg l1 (Jung et al., 2007). In addi-

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Fig. 2. The performance of the UASB under a 2.0-fold concentration shock for 12 h (E) and 24 h (F) and a 3.0-fold concentration shock for 12 h (G) and 24 h (H). h: total nitrogen, d: pH, : conductivity.

tion, according to the FA equation provided by Anthonisen et al. (1976), a high pH and temperature lead to a high FA level, resulting in a loss of ANAMMOX activity. High pH shocks might also affect the availability of trace elements (Gadd and Griffiths, 1977), which caused an infertile condition for the bacteria. On the basis of Eq. (3), which represents the relationship between the pH and ANAMMOX activity by Zheng and Hu, 2001, the ANAMMOX bacterial activity under a high pH environmental stress was only 4.6– 77.6% of the level under normal conditions. Based on the reactor’s performance with the two pH shocks, the reactor appeared to be more susceptible to the alkaline wastewater (Table 4).

v¼

1:727 þ

½H  t þ 3:854 þ 158:450 ½Hþ 

ð3Þ

where v = ANAMMOX activity, mmol g VSS1 d1; t = regression coefficient; [H+] = the concentration of H+, mmol l1. 3.4. The impact of sodium chloride addition on the ANAMMOX performance Industrial wastewater from petroleum refining, textile processing, leather processing and food conservation contains large

amounts of ammonium and salts (Lefebvre and Moletta, 2006). Salinity is considered to be toxic to bacteria and capable of altering the microbial community (Jin et al., 2011), and 30 g l1 NaCl may be the threshold value for the ANAMMOX bacteria, which occurs to distinguish between slightly halophilic and moderately halophilic (Ma et al., 2012). In this trial, 30 g l1 NaCl was selected for addition into the influent, which lasted for 12 h, to identify the negative impacts of salinity on the ANAMMOX process. The variations of TN and pH in the effluent with the 30 g l1 NaCl shock were depicted in Fig. 3K. The transient NaCl addition was not capable of immediately degrading the reactor performance. Within the 12-h shock period, the TN concentration was stable at 131.1 ± 14.5 mg l1 with a pH of approximately 7.5 ± 0.1. When the normal fresh water condition was resumed after the shock, the TN removal efficiency dropped and the pH increased. The inhibition hysteresis emerged when the NaCl shock was removed. At 23 h, the TN concentration reached a peak level of 263.6 mg l1 (78.1% increase from baseline) and then descended afterward. Under the extreme stress, the Rs and Rp departed from the basic level, as shown in Fig. 4. In the ANAMMOX pathway, NO3–N was a product of the bacterial metabolism (Kartal et al., 2008) in which

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171

Fig. 3. The effect of shocks with a pH of 4 (I), a pH of 10 (J) and the addition of 30 g l1 NaCl (K) for 12 h on the reactor. h: total nitrogen, d: pH, : conductivity.

Table 4 The effects of the pH or salinity variation on the reactor performance. Shock No.

I

J

K

Original TN load(mg l1 d1) Shock duration (h) Inflow pH NaCl addition (g l1) Peak NH4+–N (mg l1) Peak NO2–N (mg l1) Peak NO3–N (mg l1) Recovery time(h)

560 12 4.0 —— 47.9 50.0 97.8 0

560 12 10.0 —— 69.9 91.4 78.7 59

560 12 7.54 30 92 96 95.6 108

approximately 4 mol of nitrite was oxidized to nitrate for each mole of fixed carbon. Here, the Rp was slightly low during the shock period, while it was high after the shock. It was shown that the growth of the ANAMMOX bacteria was vestigial under the environmental stress due to the nitrite accumulation that caused an uncoupling of growth from the bacterial activity. After the shock was removed, the ANAMMOX metabolism recovered and became superior to the initial state. Ma et al. (2012) found that the addition

Fig. 4. The stoichiometric ratio of the ANAMMOX pathway under the 30-g l1 NaCl condition. s: Rs, j: Rp.

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of a small dose of NaCl stimulated the ANAMMOX activity. In this trial, the NaCl loading caused a disturbance in the ANAMMOX pathway, and when the normal operation commenced, the NaCl was gradually washed out by the fresh water. Therefore, the minor residual NaCl enhanced the activity and growth of the ANAMMOX biomass during this process. During 0–23 h, the Rs level revealed an abnormal phenomenon that accompanied the TN concentration fluctuation. Ma et al. (2012) subjected the ANAMMOX to shocks of 5–60 g l1 NaCl. Except for the 50 and 60 g l1 NaCl shocks under which there was almost no conversion of NH4+–N and NO2–N in reactor, the Rs levels (under 10–40 g l1 NaCl shocks) were all much higher than that in the PSS. Meanwhile, it was found that under external NaCl stress, the bacteria immediately released cellular constituents, which resulted in an increase in the soluble organic carbon and NH4+–N (Windey et al., 2005). Consequently, in the period of 12–23 h, during which there was a recuperative Rs, the high level of Rs might be due to the extra NH4+–N production. 3.5. Electrical conductivity as a robust control signal For a successful and steady reactor performance, the control strategy and operating regime are of paramount importance. A collapse of the reactor performance occurs if deteriorations are not detected in time. In an ANAMMOX reactor, the NO2–N and NH4+–N concentrations are important parameters and are destructive to the metabolism if the threshold is exceeded. It is necessary to monitor and regulate the substrate concentrations using a convenient and fast signal. Because an NO2–N online sensor is not currently available, the real-time NO2–N concentration is difficult to obtain. Joss et al. (2011) operated a Nitritation-ANAMMOX reactor that monitored the NH4+, NO3 and O2 concentrations using the corresponding probes, including the NO2 concentration, which was obtained by interpreting the online NH4+ signal. Control strategies based on the pH and oxidation–reduction potential have also been implemented successfully in other wastewater systems (Wett, 2007; Lackner and Horn, 2012). According to Eq. (1), in every cycle of the ANAMMOX pathway, 1 mol NH4+, 1.32 mol NO2 and 0.13 mol H+ are used, with 0.26 mol NO3 produced; hence, approximately 2.19 mol of free ions are removed. It is obvious that the electrical conductivity was the most intuitive description of the total free ions in water; therefore, it was governed by the conversion of NH4 + and NO2 ions to N2 molecules. In this study, shocks B, D, F, H, J and K were selected as representative cases, in which the EC was tested over all shocks (Figs. 1–

3). In the figures, the trends of EC were in agreement with the effluent TN concentration. Taking shock H, which had the highest substrate concentration, as an example, the EC reached a peak level of approximately 6578 ls cm1, which was higher than the original level of 3315 ls cm1. Meanwhile, the EC returned to a stable state as the deterioration weakened. During shock H, an abundance of NaCl was added, and the EC quickly increased because of the addition of free Na+ and Cl ions rather than the high residual free ions of the substrate in the reactor. Using the data from shocks B, D, F, H and J, it was possible to plot curves outlining the relationship between the TN and the EC, which showed good correlations (Fig. 5). In this trial, the EC signal seemed to be a good indicator of the ammonium and nitrite concentrations in the reactor. When the EC signal showed a large amplitude, it was an indicator of the increasing effluent concentrations of ammonium and nitrite. The load is rarely constant in full-scale wastewater plants in which the substrate concentrations, inflow rates and compositions of the wastewater are prone to changing frequently. Moreover, there is an order of magnitude difference between the lowest and highest loads during a given day. Inevitable disturbances by toxic matter, such as inorganic salts and heavy metals, are also created internal disturbances. When the EC probes were introduced, they acted as an alarm to help maintain the stable operation of the reactor and effluent quality, which allowed for unmanned operation during nights and weekends to save energy and other operational costs. The procedure of a sequencing batch reactor (SBR), which is one of the major configurations of an ANAMMOX reactor, contains the following four phases: feed, react, settle and discharge. To achieve the maximum reactor efficiency, a short cycle time is required. Therefore, the reacting and settling times are difficult to regulate. If a large amount of substrate remained in the settle phase, the ANAMMOX pathway could still proceed, with the generation of N2 disturbing the system process and resulting in the sludge washout. However, with the application of the EC probe, regulation of the system process can be performed online, and the control strategy can be based on the degree of substrate conversion related to the capacity of the reactor, which is more economical. To ensure safe operation in full-scale applications, further research should focus on extending the knowledge of parameter interactions between the EC and ANAMMOX process to gain a better understanding of the operating ranges and limitations. Other probes, in addition to the EC probe, should also be included in wastewater treatment systems to help diagnose failures and promote proper strategies. Current research is focused on defining other combinations of online probes that are suitable for operating controls.

Fig. 5. Correlation between the effluent nitrogen and the electrical conductivity under overloads (A) and a shock load with a pH of 10 (B).

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4. Conclusions These preliminary experimental results demonstrate that in the overloads, the nitrogen removal rate of ANAMMOX UASB was improved, and the shock period was an important factor influencing the reactor performance. When in the high pH shock, a deterioration of the reactor occurred with a decrease in the bacterial activity. Among those shocks, the NaCl addition caused the most serious deterioration, leading to a change in the stoichiometric ratio. The EC is considered to be a convenient and fast signal for controlling and monitoring the ANAMMOX process under transient-state conditions.

Acknowledgements This work was partially supported by the National Natural Science Foundation of China (No. 50808060 & No. 51078121), the Zhejiang Provincial Natural Science Foundation of China (No. Y5090072) the Program for Excellent Young Teachers in Hangzhou Normal University (HNUEYT) (JTAS 2011-01-020) and the Project of Zhejiang Key Scientific and Technological Innovation Team (2010R50039).

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