Abstract

Partial nitritation and anaerobic ammonium oxidation (PNA) is a useful process for the treatment of nitrogen-rich centrate from the dewatering of anaerobically digested sludge. A one-stage PNA moving bed biofilm reactor (MBBR) was started up without inoculum at Klagshamn wastewater treatment plant, southern Sweden. The reactor was designed to treat up to 200 kgN d−1, and heated dilution water was used during start-up. The nitrogen removal was >80% after 111 days of operation, and the nitrogen removal rate reached 1.8 gN m−2 d1 at 35 °C. The start-up period of the reactor was comparable to that of inoculated full-scale systems. The operating conditions of the system were found to be important, and online control of the free ammonia concentration played a crucial role. Ex situ batch activity tests were performed to evaluate process performance.

INTRODUCTION

The centrate from the dewatering of anaerobically digested sludge accounts for 15–20% of the total nitrogen load in a wastewater treatment plant (WWTP), with a hydraulic load of ∼1% of the incoming flow (Fux & Siegrist 2004). Due to the high energy demand for conventional nitrification-denitrification (∼50% of the total energy required at a WWTP), dedicated side-stream treatment processes are designed to reduce this nitrogen load in the main wastewater treatment line (Gao et al. 2014). Partial nitritation anammox (PNA) of nitrogen-rich centrate with a low soluble carbon content (sCOD/NH4+-N < 2) has evolved as a cost-effective and efficient alternative for side-stream nitrogen removal. This increase in popularity has led to over 100 full-scale applications worldwide (Lackner et al. 2014).

PNA is achieved through fully autotrophic nitrogen removal by combining partial nitritation and anammox (Lackner et al. 2014). Aerobic ammonium-oxidizing bacteria (AOB) oxidize 57% of the influent NH4+-N into NO2-N under aerobic oxidation. Anaerobic ammonium-oxidizing bacteria (AMX) oxidize the remaining NH4+-N into N2, with NO2-N as an electron acceptor. According to anammox stoichiometry, ∼11% of the oxidized nitrogen is produced as NO3-N (Strous et al. 1998). Hence, the oxygen demand and the need for an organic carbon source are reduced by 57% and 86% (Fux & Siegrist 2004; Wett 2007).

The PNA process can take place in suspended-growth or biofilm reactors, configured as one- or two-stage systems (Lackner et al. 2014). In a moving bed biofilm reactor (MBBR), the ammonium-oxidizing PNA biofilm is attached to plastic carriers. Operation at dissolved oxygen (DO) concentrations between 0.5 and 1.5 mg L−1 allows partial nitritation and anammox to co-occur in a one-stage reactor. The AOB are found in the outer aerobic biofilm layer, while the AMX thrive in the deeper anoxic layer (Christensson et al. 2013). The AMX compete with the co-existing NOB and fast-growing heterotrophic bacteria (HB) for NO2-N.

The start-up of a PNA system is challenging, and is influenced by several factors: (i) the slow growth rate of AMX, (ii) the choice of a suitable inoculum, (iii) effective AMX retention and NOB repression, and (iv) the inhibition of microbial groups as an effect of operational conditions (Rikkmann et al. 2018; Cai et al. 2020). Reported doubling times vary from 2–4 days (Lotti et al. 2015; Zhang et al. 2017) to 11 days (Strous et al. 1998), depending on the process configuration and operating conditions. Long start-up periods of 8–10 months are common when the influent centrate is the only source of AMX (Plaza et al. 2011; Rikkmann et al. 2018). Hence, researchers have focused on developing start-up strategies that shorten this period. It has been reported that the start-up period can be reduced to 5 months by using activated sludge (Mehrdad et al. 2014) and 2–4 months when using AMX sludge or pre-colonized carriers (Wett 2007; Christensson et al. 2013; Klaus et al. 2017). Start-up with AMX-inoculated carriers require considerable quantities of seeding carriers, between 3 and 15% of the design media fill (Christensson et al. 2013), which might not always be available. Kanders et al. (2016) recently reported the un-inoculated start-up of a one-stage PNA MBBR, with a pre-colonized nitritation biofilm on the carriers, in 120 days. Rapid establishment of an initial nitritation biofilm on the virgin carriers, prior to inoculation with AMX, indicated the theoretical potential to reduce the start-up period of a one-stage MBBR system to 56 days (Kowalski et al. 2019a). In addition, Kanders et al. (2018) claimed that the centrate from mesophilic digesters fed with high-sludge-age nitrifying bacteria is a reliable and adequate sole source of AMX for rapid PNA start-up. As the development of a PNA biofilm depends on the reactor conditions and a sufficient source of bacteria, the MBBR technique, with high and efficient biomass retention, should be suitable for rapid start-up with indigenous AMX.

Key and process parameters that should be controlled are the pH, temperature, and concentrations of DO, NO2-N and FA, especially during process initiation when conversion and growth rates are slow (Cai et al. 2020). Anthonisen et al. (1976) have described the dependence of the inhibitory effect of FA on pH, temperature, and NH4+-N concentration. In general, AOB and AMX tolerate higher concentrations of FA (10–150 mg L−1 and 2–150 mg L−1, respectively) than NOB (0.1–1 mg L−1) (Aktan et al. 2012; Jaroszynski et al. 2012). Furthermore, AMX are sensitive to NO2-N concentrations between 5 and 400 mg L−1 (Wett 2007; Lotti et al. 2012). Levels of FA <10 mg L−1 and NO2-N <40 mg L−1 have been found to be critical concentrations for successful start-up (Klaus et al. 2017; Rikkmann et al. 2018). The DO affinity of NOB is lower than that of AOB, thus low DO concentration can be used to suppress NOB activity while favoring the growth of AMX in the inner layer of the biofilm (Strous et al. 1997; Wett et al. 2013). Intermittent aeration with a low DO setpoint has been reported to be effective in achieving a suitable oxygen gradient in the biofilm during start-up (Lackner et al. 2014).

The rapid start-up of pilot or lab reactors without AMX inoculum has been widely discussed in the literature (Mehrdad et al. 2014; Kowalski et al. 2019a; Cai et al. 2020), but to the best of our knowledge, only Kanders et al. (2016) have reported such a start-up in a full-scale PNA MBBR. The objectives of this study were therefore, to demonstrate the rapid start-up of a full-scale PNA MBBR using only existing resources at the WWTP, and to present a detailed description of the start-up strategy.

METHODS

Klagshamn WWTP

Klagshamn WWTP is located in Malmö, southern Sweden, and is designed for 90,000 population equivalents, with an influent flow of 22,000 m3 d−1. Phosphorus is mainly removed by pre-precipitation using iron chloride as precipitant. Biological nitrogen removal is accomplished in an activated sludge process with simultaneous carbon removal and nitrification, followed by an MBBR process for post-denitrification with external carbon dosage. The final polishing step takes place in a dual-media filter. Primary and secondary sludge are mixed and co-digested in two mesophilic digesters operated in series. The centrate from the dewatering facility constitutes 2% of the total influent flow and 17–25% of the NH4+-N load in the WWTP. The ammonium mass load from the centrate varies between 100 and 150 kgNH4+-N d−1.

The PNA reactor

The centrate is pumped to the PNA reactor via a 400 m long pipe to a storage tank with a volume of 10 m3. This pipe is made of polypropylene to avoid a decrease in temperature and struvite precipitation. The insulated and covered PNA reactor is filled to 40% with carrier material, and a top-mounted mechanical mixer keeps the carriers in motion. The reactor design parameters are given in Table 1. The airflow is controlled and modulated by motor-actuated control valves using fixed DO control. After treatment, the effluent from the reactor is discharged to the inlet of the activated sludge process.

Table 1

Main design parameters of the full-scale reactor

ParameterValue
Volume (m3256 
Area × depth (m2 × m) 61 × 4.2 
Max load (kgN d−1200 
Type of carriers Anox™ K5 
Protected surface area (m2 m−3800 
Filling degree (%) 40 
Average flow (m3 h−110 
ParameterValue
Volume (m3256 
Area × depth (m2 × m) 61 × 4.2 
Max load (kgN d−1200 
Type of carriers Anox™ K5 
Protected surface area (m2 m−3800 
Filling degree (%) 40 
Average flow (m3 h−110 

Pre-precipitated and pre-settled dilution water (PSW) was used during start-up to avoid AMX inhibition due to FA and NO2-N. A heat exchanger was installed to pre-heat the dilution water to a temperature of ∼30 °C to improve the AMX growth rate. Two feed pumps, one for the centrate and one for the PSW, were controlled by variable frequency drives. The setpoint for the two flows varied depending on the process performance.

Temperature, pH, NH4+-N, NO2-N, NO3-N, sCOD, and DO were monitored with online sensors (WTW IQ SENSOR NET system from Xylem, Sweden). The FA concentration was monitored in the supervisory control and data acquisition system and used for manual control of the ammonium load. The characteristics of the influent to the PNA reactor are given in Table 2.

Table 2

Average properties of the influent

ParameterCentrateDilution water
NH4+-N (mg L−1580 ± 90 25 ± 5.0 
sCOD (mg L−1450 ± 60 50 ± 10 
Alkalinity HCO3 (mg L−13,700 ± 400 320 ± 50 
sCOD/NH4+-N 0.8 ± 0.32 2.3 ± 0.15 
TSS (mg L−1480 ± 390 50 ± 10 
ParameterCentrateDilution water
NH4+-N (mg L−1580 ± 90 25 ± 5.0 
sCOD (mg L−1450 ± 60 50 ± 10 
Alkalinity HCO3 (mg L−13,700 ± 400 320 ± 50 
sCOD/NH4+-N 0.8 ± 0.32 2.3 ± 0.15 
TSS (mg L−1480 ± 390 50 ± 10 
Free ammonia was calculated as described by Anthonisen et al. (1976):
formula
(1)

Start-up strategy

The start-up of the PNA process was divided into two phases, which were followed by a continuous operation phase.

Phase I

The first objective during start-up was to establish a biofilm on the virgin carriers. PSW (∼2 sCOD/NH4+-N) was available at the plant and proved to be a reliable source of HB for the rapid establishment of an initial biofilm. This biofilm matrix favors the attachment of AOB and AMX on the carrier material (Kowalski et al. 2019b). The second objective was to achieve stable nitritation by AOB, while avoiding AMX inhibition by restricting the NO2-N concentration to the range 30–50 mg L−1. The NOB activity was suppressed through intermittent aeration (DO setpoint of 0.6–0.8 mgO2 L−1) and by operation at inhibitory FA concentrations between 5 and 20 mg L−1. The supplies of PSW and centrate were controlled to ensure suitable NO2-N and FA concentrations, respectively.

Phase II

The first signs of AMX activity; that is, a discrepancy in the nitrogen balance over the reactor system and/or evidence of AMX activity during batch measurements, indicated the commencement of start-up phase II. The objective of phase II was further enrichment of the AMX bacteria culture by applying an NH4+-N load slightly exceeding the actual reduction capacity of the reactor. Care was taken not to overload the system. As the AOB community increased, the intermittent aeration became continuous to meet the increased oxygen demand.

Continuous operation

Transition to continuous operation was defined by treatment of all available centrate with a nitrogen reduction >80%.

Analytical procedures

Grab samples were collected from the reactor and analyzed to evaluate the accuracy of the online sensors and to monitor the reactor nitrogen compounds. In addition, 24-h flow-proportional samples were collected three times a week from the reactor inlet and outlet, to calculate the surface specific load and mass balances, and the reduction rates of inorganic nitrogen (Inorg N) and FA. All samples were analyzed according to Swedish and ISO standard methods for the following parameters: NH4+-N, NO2-N, NOx-N (ISO 15923–1:2013), and chemical oxygen demand (COD) (SS 028142-2). Samples were filtered with an MGA filter (1.6 μm, Ahlstrom Munksjö) for the evaluation of sCOD.

Biofilm growth

The dry solids (DS) on two samples of carriers (five pieces each) was measured once a week, to evaluate biofilm growth. The carriers were dried at 105 °C for 2 h and weighed. The biomass was then removed by treating the carriers with 5M NaOH and washing them thoroughly with distilled water. The process of drying and weighing was then repeated, and the biomass on the carriers was determined as the difference between the initial and the final weight. The biomass was converted from gDS to gDS m−2 using a factor of 0.00242 m2/K5 carrier.

Activity tests

Endogenous respiration, and AOB and NOB activity were measured through the oxygen uptake rate (OUR), while the specific anammox activity (SAA) and the specific denitrification activity (SDA) were measured manometrically, as described below.

The OUR test

Eighty carriers, corresponding to a filling degree of 40%, were used to perform the OUR measurements. The activity of aerobic bacteria was determined by measuring the depletion of DO in the bulk solution over a limited time when different substrates were added, as initially described by Hagman & la Cour Jansen (2007).

When no substrates or inhibitors are added to the solution, the DO depletion is equal to the endogenous respiration. The combined OUR of AOB + NOB is measured by adding NH4+-N. Allylthiourea is then added to inhibit the AOB activity (Ginestet et al. 1998), and NO2-N, a substrate for the NOB is then added. The oxygen uptake rate is then defined as in Equation (2):
formula
(2)
where is the time derivate of the oxygen concentration in the solution, is the volume of the liquid phase, X is the number of carriers, and Ae [m2] is the effective area of one carrier.

The measurements were started when the solution was well saturated (7–8 mgO2 L−1). DO was measured (HACH LDO101) and recorded (HACH HQ40d) and the temperature was maintained at 28 °C.

Manometric tests

The SDA and SAA were determined by quantifying the amount of nitrogen gas produced. The measurements were performed by measuring the pressure increase in a closed reactor vessel, as described by Dapena-Mora et al. (2007). SDA and SAA are expressed as the nitrogen produced per unit time per unit biofilm area, as in Equation (3):
formula
(3)
where is the production rate of nitrogen gas, is the molar weight of nitrogen gas, X is the number of carriers used in the test, and is the effective area of each carrier. The factor of 60 × 24 was use to convert days to minutes.

NH4-N and NO2-N were used as substrates to determine SAA, and NO3-N and acetic acid to determine SDA.

The overpressure resulting from the production of nitrogen gas was logged using a GMH 5150 pressure meter, with a GMSD 350MR sensor and GSOFT3050 software (Greisinger electronic GmbH, Germany).

RESULTS AND DISCUSSION

Start-up and continuous operation

The one-stage PNA process was started at the end of December 2018, and the results from the first 200 days of operation are presented in this study (Figure 1). The vertical dashed line on day 79 indicates the transition from start-up phase I to phase II. After 111 days, all the available centrate could be treated and the nitrogen removal was >80%, which was the definition of continuous operation. The second vertical dashed line in Figure 1 indicates the transition to continuous operation mode.

Figure 1

Daily average values of influent flow (centrate and PSW), temperature, hydraulic retention time and DO concentration during the first 200 days of operation. Vertical lines on day 79 and day 111 indicate the termination of start-up phase I and phase II.

Figure 1

Daily average values of influent flow (centrate and PSW), temperature, hydraulic retention time and DO concentration during the first 200 days of operation. Vertical lines on day 79 and day 111 indicate the termination of start-up phase I and phase II.

Start-up phase I

To enhance biofilm development on the virgin carriers and washout of the suspended biomass, the hydraulic retention time was kept low (12 h) during the first 20 days of operation, and then continuously increased as the biofilm developed (Mehrdad et al. 2014).

Intermittent aeration cycles of 30 min were applied with aeration times varying between 20 and 25 min, depending on the influent load. The DO setpoint was ∼0.8–1.0 mg L−1, to promote the growth of AOB and to repress the growth of NOB during the early stage of start-up (Christensson et al. 2013; Wett et al. 2013). The stirring speed was adjusted to ensure low turbulence, encouraging the growth of a thicker biofilm. The pH was monitored and used in the determination of the FA concentration.

The average pH during the whole period (200 days) was relatively high, fluctuating between 7.5 and 8.5. Figure 2 shows the nitrogen fractions in the reactor, the inorganic nitrogen load, and the FA, obtained from the analysis of grab samples. At the beginning of start-up, when the biological activity is low, there is a risk of inhibition of AOB by FA if the ammonia loading exceeds the conversion rate (Lackner et al. 2014). The FA was therefore kept below 15 mg L−1 by the addition of dilution water at ratios between 1:10 and 1:15.

Figure 2

Concentrations of the nitrogen compounds (NH4+-N, NO3-N, NO2-N) in the bulk, inorganic nitrogen load (Inorg-N), and FA concentration, obtained from the analysis of grab samples.

Figure 2

Concentrations of the nitrogen compounds (NH4+-N, NO3-N, NO2-N) in the bulk, inorganic nitrogen load (Inorg-N), and FA concentration, obtained from the analysis of grab samples.

During this start-up, the COD-rich PWS used as dilution water served two purposes. Firstly, it prevented inhibitory conditions arising from high levels of FA and NO2-N (Mehrdad et al. 2014; Kanders et al. 2016). Secondly, the establishment of a biofilm containing HB on the carrier material will enhance the enrichment of AMX. A layer of HB biofilm used prior to inoculation with AMX biomass acts as an extracellular polymeric matrix that facilitates the rapid development of AMX (Kowalski et al. 2019b). In contrast to our study, Kanders et al. (2016) and Mehrdad et al. (2014) used dilution water (effluent and washing water, respectively) only to avoid substrate inhibition. Due to the strong dependence of the bacterial growth rate on temperature, the PSW was preheated to 30 °C.

The biomass on the carriers was examined for the first time on day 2. A thin biofilm, equivalent to 0.9 gDS m−2 (Figure 3) was detected. This biofilm was probably developed during the 7-day period before the reactor was brought into operation, when the centrate pump was brought into operation. The reactor was filled with PSW under 7 days and the aeration was on in 3 days, to soak and distribute the carriers. This procedure was recommended by the carrier supplier due to the hydrophobic characteristics of the carrier material. The first batch measurement, when the amount of biomass was insignificant, showed that bacterial activity was dominated by fast-growing endogenous HB. Biofilm development was rapid from the beginning of phase I, reaching 1.9 gDS m−2 on day 17, which is similar to 2.1 gDS m−2 previously reported after 22 days of operation (Kowalski et al. 2019b). Ammonium conversion activity was first observed on day 20, as an increase in NO3-N concentration in the reactor, to ∼40 mg L−1. Similar progress has been reported by Kowalski et al. (2019b), who also detected increased nitrification after two weeks.

Figure 3

(a) Biomass development on the carriers, expressed as dry solids (DS). (b) Endogen activity, AOB + NOB activity, and NOB activity obtained from batch OUR measurements. (c) Specific anammox activity and specific heterotrophic denitrification activity obtained from manometric batch measurements.

Figure 3

(a) Biomass development on the carriers, expressed as dry solids (DS). (b) Endogen activity, AOB + NOB activity, and NOB activity obtained from batch OUR measurements. (c) Specific anammox activity and specific heterotrophic denitrification activity obtained from manometric batch measurements.

High aerobic HB activity and NO3-N production, presumably by NOB, indicated that the aeration time could be reduced, which led to a daily average DO concentration of ∼0.1 mg L−1. The load to the reactor was maintained between 50 and 60 kgN d−1, corresponding to 0.5–0.7 gN m−2 d−1. As a result of the low bacterial activity, the reduction rate was low, 5–15 kgN d−1 or 0.1–0.2 gN m−2 d−1, resulting in a nitrogen removal rate of 5–10% (Figure 4).

Figure 4

(a) Inorganic nitrogen load and removal rate (kgN d−1), and effluent NO3-N (kgN d−1), NH4+-N, and inorganic nitrogen removal efficiencies (%). (b) Inorganic nitrogen load and removal rate (gN m2 d−1).

Figure 4

(a) Inorganic nitrogen load and removal rate (kgN d−1), and effluent NO3-N (kgN d−1), NH4+-N, and inorganic nitrogen removal efficiencies (%). (b) Inorganic nitrogen load and removal rate (gN m2 d−1).

Operational problems with the dilution water pump (day 23) led to elevated NO2-N concentrations in the reactor (170 mg L−1), exceeding the optimal concentration (<40 mg L−1) for the promotion of AMX growth (Klaus et al. 2017). The biofilm began to grow rapidly after 23 days, increasing from 1.9 to 12 gDS m−2. Kowalski et al. (2018) observed rapid biofilm growth simultaneously with high substrate load. The low removal rate in our study indicates also overloading. No significant change was observed in the sCOD/NH4+- N ratio, which varied between 1.1 ± 0.3 (at dilution ratios of 1:10–1:15) and 1.5 ± 0.3 (at dilution ratios of 1:2–1:3). The sCOD reduction and the ratio sCOD/ NH4+-N under the start-up period are available as supplementary material (Figures S1 and S2).

On day 50, the FA reached >20 mg L−1, which is higher than the setpoint in the start-up strategy. The reported ranges in which the FA concentration becomes inhibitory for AOB and AMX vary considerably (Aktan et al. 2012; Jaroszynski et al. 2012), and it was therefore necessary to determine the FA concentration tolerated in this particular system.

During sampling of the carriers for the determination of biomass on day 51, it was observed that the fluffy biofilm had become detached, leading to a significant loss of biomass, from 12 to 1 gDS m−2. A possible explanation of this sudden loss of biomass was a malfunctioning DO sensor, which caused an increased airflow rate; that is, high turbulence and shear stress. Ødegaard et al. (1994) have described various cases of biomass loss; that is, shear stress, predation by higher organisms, or changes in the reactor conditions. Kowalski et al. (2018) also reported that a loosely attached biofilm was easily detached from the carrier media when the mixing was intensified. It is not clear whether the increased FA (∼20 mg L−1), the slow increase in the sCOD/NH4+-N ratio, or the turbulence caused by the air flow alone, or all together caused the loss of biomass in this study.

The reactor was loaded with 60–80 kgN d−1 (0.5–1.0 gN m2 d−1) until day 60, and no significant reduction was observed (Figure 4); 5–10% of the nitrogen probably disappeared due to nitrogen assimilation by the biomass and some denitrification. After stabilizing the NO2-N concentration below 20 mg L−1 and the FA concentration in the range of 5–10 mg L−1, the inlet flow was reduced in order to reduce the load. This reduction led to a decreased NO2-N concentration <20 mg L−1 and NH4+-Nconverted/NO3-Nproduced increased to ∼12% (day 78). Although a discrepancy was observed in the nitrogen balance (22 kgN d−1), the activity measurements did not show any AMX activity. A new batch test was performed on day 81, when the first sign of AMX activity was confirmed (Figure 3). Kanders et al. (2018) reported reaching full capacity within 120 d of start-up without seeding, which confirms that AMX activity should appear about 3 months after start-up. In the present study, AMX activity was not observed or detected earlier, probably due to FA inhibition and high nitrogen load, but under favorable conditions the AMX started to consume NO2-N and the levels in the reactor decreased rapidly after day 79.

The first phase of start-up, when a few important events occur simultaneously; that is, the formation of the biofilm, the initiation of the nitritation and later the anammox at the same time as inhibition should be avoided, is the most critical for successful and rapid start-up.

Start-up phase II

As the activity of AMX increased (Figure 3), the load was gradually increased from 0.4 gN m−2 d−1 to 1.5 gN m−2 d−1, resulting in an increase in N reduction rate, from 0.3 gN m−2 d−1 to 1.2 gN m−2 d−1 (Figure 4(b)). On day 98, all the available centrate could be treated, which was one of the criteria for transition to continuous operation. However, due to operational problems with the centrate pump, this phase was extended to day 111 in an attempt to ensure stable operation. During phase II, the maximum nitrogen reduction was 1.64 gN m−2 d−1, corresponding to a reduction of 77%. The activity measurements showed a slight increase in NOB activity (Figure 3), which could explain the finding that the NO3-Nproduced corresponded to 10–16%, which is slightly higher than the theoretical value of 11%. The NOB could be suppressed by reducing the airflow. During this phase, the NH4+-N load was slightly higher than the actual capacity, in order to promote the growth of AMX and increase the SAA activity (Figure 3).

Continuous operation

The development of the biomass proceeded further, and the SAA activity increased from 1.7 to 6.4 gN m2 d−1. A mass balance showed that the NO3-Nproduced was 5.57 ± 1.2%, which is only about half of the theoretical value of 11%. Heterotrophic denitrification was confirmed by SDA measurements, showing a peak value of 1.1 gN m−2 d−1 for SDA (Figure 3). Although the sCOD /NH4+- Nin ratio was low (0.4 ± 0.2), the TSS mass balance indicated that some particulate COD was converted into sCOD, which could probably be utilized as a carbon source by the HB. As long as the denitrifying HB do not outcompete the AMX for NO2-N as substrate and the SAA increases, denitrification is not competitive and contributes to increased nitrogen removal in the reactor.

As the biofilm grew thicker with time, the DO setpoint in the bulk was increased to 1–1.5 mg L−1. New operational problems with the NH4+-N online sensor controlling the centrate flow to the reactor occurred on day 185. This led to a rapid increase in FA, to 53 mg L−1, and reversible inhibition of AMX and AOB was observed and confirmed by the batch activity measurements (Figure 3).

CONCLUSIONS

The start-up of a full-scale PNA MBBR process was successfully accomplished from virgin carriers without seeding within 111 days. Heated dilution water was a prerequisite for the rapid start-up during this study. The first sign of AMX activity was observed 79 days after the reactor was brought into operation. The centrate was found to be a reliable source for the seeding of AMX. If the process is started up without inoculum, it is of great importance to have access to dilution water to avoid inhibitory conditions during the initial phase. The maximum N removal rate achieved corresponded to 1.8 gN m−2 d−1, and >80% nitrogen reduction was achieved during continuous operation.

The results of this study show that FA concentrations >10 mg L−1 can disturb the process due to reversible inhibition. As the operation of the PNA process requires more attention than regular processes at a WWTP, an online control system for the FA concentration is recommended to ensure full capacity and stable operation.

This study highlights the importance of a stable biofilm development. Other parameters, including DO, FA, and temperature, must be balanced to ensure favorable conditions for the AMX bacteria. The retention of AMX is critical for the PNA process, and the MBBR is an effective solution.

ACKNOWLEDGEMENTS

The authors would like to thank Anders Johnsson, the manager of Klagshamn WWTP, for financial support and for making this project possible. Special thanks to Hillevi Gustavsson, Thomas Söderlindh, Jonas Nilsson, and the operational staff at the plant for their help and cooperation in constructing and operating the reactor. This research was partly financed by Sweden Water Research.

SUPPLEMENTARY MATERIAL

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.271.

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