Low concentrations of dissolved oxygen (DO) are usually found in biological anoxic pre-denitrification reactors, causing a reduction in nitrogen removal efficiency. Therefore, the reduction of DO in such reactors is fundamental for achieving good nutrient removal. The article shows the results of an experimental study carried out to evaluate the effect of the anoxic reactor hydrodynamic model on both residual DO concentration and nitrogen removal efficiency. In particular, two hydrodynamic models were considered: the single completely mixed reactor and a series of four reactors that resemble plug-flow behaviour. The latter prove to be more effective in oxygen consumption, allowing a lower residual DO concentration than the former. The series of reactors also achieves better specific denitrification rates and higher denitrification efficiency. Moreover, the denitrification food to microrganism (F:M) ratio (F:MDEN) demonstrates a relevant synergic action in both controlling residual DO and improving the denitrification performance.

INTRODUCTION

Denitrification is the biological process where nitrite (NO3–N) is converted into gaseous nitrogen (N2) under anoxic conditions. Considering the pre-denitrification process, the reactor design is usually based on the denitrification rate (rDEN), assuming a zero-order kinetics with respect to both NO3–N and organic substrate, and considering the effect of temperature (T). Typical values of rDEN at 20°C are in the range 2.9–3.0 gNO3–N kgMLVSS−1 h−1 (MLVSS = mixed-liquor volatile suspended solids) (Barnard 1975; Sutton et al. 1975; US-EPA 1975; De Fraja Frangipane et al. 1978; Ekama 1984; Vismara 1998; Ekama & Wentzel 1999; Ekama 2011; Collivignarelli & Bertanza 2012; Torretta et al. 2014).

Dissolved oxygen (DO) is an inhibiting factor for denitrification, and concentrations in the range 0.2–0.4 mgO2 L−1 significantly reduce rDEN. DO is even higher in small wastewater treatment plants that are characterized by strong changes of sewage flow and quality (Raboni et al. 2013). DO presence within anoxic reactors depends on two opposing factors: (i) DO content into the inflow (sewage and mixed-liquor recycle); and (ii) oxygen consumption determined by the heterotrophic bacteria.

The possible inhibitory effects of DO on the denitrification kinetics were postulated by US-EPA (1975) and were highlighted by other studies (Badstreet & Johnson 1994; Oh & Silverstein 1999; Plòsz et al. 2003). The presence of 0.2 mgO2 L−1 lowers rDEN up to 40% of the maximum values measured without DO (Dawson & Murphy 1972; Tchobanoglous et al. 2003). To take account of such an effect, inhibition factors were introduced in various rDEN models, as described elsewhere (US-EPA 2009, 2010; Luciano et al. 2012; Fajardo et al. 2012).

For practical calculation of the denitrification reactor (DEN) volume, a semi-empirical relationship, which correlates the specific denitrification rate at T = 20 °C (SDNR20 °C) to the sludge loading referred to DEN, has been proposed (Burdick et al. 1982; US-EPA 1993; Tchobanoglous et al. 2003): 
formula
1
where SDNR = (Q · ΔNO3–N)/(VDEN· X); Q is the sewage flow rate (m3 d−1); ΔNO3–N is the NO3–N removed into DEN (gNO3–N m−3); VDEN is the DEN volume (m3); X is the biomass concentration in DEN (gMLVSS m−3); F:MDEN is the sludge loading referred to DEN (gBOD5 applied to gMLVSS−1 d−1). Values of SDNR observed in full-scale plants range from 0.04 to 0.42 gNO3–N gMLVSS−1 d−1 (Burdick et al. 1982; Henze 1991; Tchobanoglous et al. 2003), while US-EPA (2010) reports a more restricted range (0.05–0.15 gNO3–N gMLVSS−1 d−1 at 20°C).
As the temperature strongly affects the denitrification kinetics, the well-known correlation has been proposed: 
formula
2
where SDNRT is the SDNR at a generic temperature T; θ = 1.026 for Tchobanoglous et al. (2003) and θ = 1.07 for US-EPA (2010).
More recently, Raboni et al. (2014) highlighted the strong dependence of pre-denitrification SDNR on both DO and food to microrganism (F:M) ratio. They proposed the following equation: 
formula
3
where KO = 0.18 mgO2 L−1 is the DO inhibition constant; DO0 and B0 are the DO and biochemical oxygen demand (BOD5) concentrations of the whole flow rate entering the DEN (mg L−1); K = 0.11–0.18 is a constant; ηBOD is the BOD removal efficiency, which depends on the F:MDEN (ηBOD = 0.90 for F:MDEN = 0.4 kgBOD5 kgMLVSS−1 d−1; ηBOD = 0.95 for F:MDEN = 0.2 kgBOD5 kgMLVSS−1 d−1). They also proposed a more general equation based on the experimental correlation between SDNR20 °C, DO and BOD5 detected in the total flow rate entering the DEN.

Having ascertained the considerable dependence of the denitrification rate on DO concentration, the paper verifies the effect of the DEN hydrodynamics on the residual concentration of DO and, consequently, on the process performance. A research experiment with two pilot plants, which differ from the hydrodynamic model of the anoxic reactor, has been developed. In the first plant, a series of four completely mixed reactors was implemented in order to simulate a hydrodynamic behaviour approaching the ‘plug-flow’, while in the second case, a single completely mixed reactor was chosen. Both solutions represent typical situations found in full-scale plants. The input of DO in the completely mixed reactor (mainly related to the mixed-liquor recycle) is immediately dispersed throughout the volume, and its consumption due to the heterotrophic bacteria is low. In contrast, the system of the four reactors in series allows a descending DO concentration from very high values in the first reactor to very small values in the last reactor. Such solutions are beneficial to denitrification efficiency because they increases DO consumption.

METHODS

Pilot plant description

The research was based on two parallel activated sludge pilot plants (Figure 1), consisting of a biological pre-denitrification stage (DEN; volume: 10 m3; liquid height: 1.8 m), an oxidation-nitrification stage (OX-NIT; volume: 20 m3; liquid height: 1.8 m; aeration system: micro-bubble), and a final sedimentation (SED; volume: 6 m3; diameter: 2 m). The plants differ in the DEN configuration:
  • Pilot plant 1 (Figure 1(a)) has a series of four identical completely mixed reactors (volume of a single reactor: 2.5 m3). Mixing is achieved by four slow vertical-axis mixers (power input: 11 W m−3).

  • Pilot plant 2 (Figure 1(b)) has a single completely mixed reactor. Mixing is achieved by one slow vertical-axis mixer (power input: 16 W m−3).

Figure 1

Layout of the two parallel pilot plants: pilot plant 1, having the denitrification stage characterized by a series of four reactors (a); pilot plant 2, having the denitrification stage characterized by a single completely mixed reactor (b).

Figure 1

Layout of the two parallel pilot plants: pilot plant 1, having the denitrification stage characterized by a series of four reactors (a); pilot plant 2, having the denitrification stage characterized by a single completely mixed reactor (b).

DEN water surfaces were covered with a 1 inch layer of floating plastic balls in order to reduce the oxygen mass transfer between atmosphere and water.

The instrumentation with continuous sampling for plant control consists of (Figure 2): 16 DO fixed probes (accuracy: 0.01 mg L−1; automatic calibration and temperature compensation); six pH fixed probes (accuracy: 0.05); four temperature fixed probes (accuracy: 0.05 °C); and four magnetic flow-meters (accuracy: 0.5% of the flow rate).

Figure 2

Scheme and instrumentation of the two pilot plants.

Figure 2

Scheme and instrumentation of the two pilot plants.

The pilot plants were fed by pre-treated (screening and aerated grit chamber) sewage from a 50,000 inhabitant town located in Northern Italy.

Pilot plant operating conditions and testing methods

The experimentation was carried out using the following steps.

  • Preliminary verification of the DEN hydrodynamic behaviour through reconstruction of the retention time distribution (RTD) curves (Tchobanoglous et al. 2003). A tracer solution of lithium chloride was used with a step dosage of 5 mgLi+ L−1. For the tests, tap water was fed to the plants.

  • Simultaneous conduction of the two pilot plants with the same operating conditions was used in order to check:

    • ○ the sewage quality and the overall treatment efficiency with respect to BOD5, chemical oxygen demand (COD), total nitrogen (TN) and suspended solids (SS);

    • ○ the DO concentration in the two types of DEN, and its influence on both SDNR and plant denitrification efficiency (ηDEN, %), taking into account the role of F:MDEN:

 
formula
4
where the subscripts ps and eff refer to the pre-treated sewage and the effluents, respectively.

The two pilot plants ran for a continuous period of 180 d, providing operating controls and analysis. In this period, the F:MDEN was set to:

  • 0.2 kgBOD5 d−1 kgMLVSS−1 for the first 60 d;

  • 0.3 kgBOD5 d−1 kgMLVSS−1 for the subsequent 60 d;

  • 0.4 kg BOD5 d−1 kgMLVSS−1, for the final 60 d.

The DO in OX-NIT was kept at 2.5 mgO2 L−1 on average in both the pilot plants, while the MLVSS concentration was maintained at 2.0 mg L−1. The operating conditions were as follows:

  • Average sewage flow rate Q = 2 m3 h−1;

  • Mixed-liquor recycle flow rate QML=3Q;

  • Sludge recycle flow rate, q = Q.

The monitored analytical parameters during the experiments were as follows:

  • BOD5, COD, TN and SS in the pre-treated sewage (daily average samplings);

  • TN and NO3–N in the total flow rate (sum of Q, QML and q) both ingoing and outgoing the DEN (daily average samplings);

  • BOD5, COD, total Kjeldahl nitrogen (TKN), NO3–N and SS in the pilot plant effluents (daily average samplings);

  • MLVSS and mixed-liquor suspended solids (MLSS) in DEN and OX-NIT (daily manual sampling);

  • temperature, DO and pH at the sampling points shown in Figure 2, were measured.

    Sampling and analysis were carried out in compliance with official standard methods of Italian legislation.

RESULTS AND DISCUSSION

Raw sewage and treated effluent mean quality

Table 1 shows the raw sewage and the treated effluent quality of the two pilot plants during the first 60 d (F:MDEN = 0.2 kgBOD5 d−1 kgMLVSS−1). During the experiments, the average temperature in DEN was 16.1 °C (range: 15.6–16.7 °C).

Table 1

Quality of the raw sewage and the treated effluent in both plants: mean (m) and standard deviation (sd) of daily samples (number of samples: 21 for sewage; 42 for the treated effluent)

   Treated effluent
 
  Raw sewage
 
Pilot plant 1
 
Pilot plant 2
 
Parameter Unit of measurement m sd m sd m sd 
COD mg L−1 288.0 59.9 85.5 15.3 88.3 16.1 
BOD5 mg L−1 127.9 40.0 12.1 2.5 12.6 2.6 
SS mg L−1 153.0 45.0 20.5 4.5 19.7 4.8 
TN mg L−1 27.6 5.1 5.9a 2.0a 7.8a 2.1a 
   Treated effluent
 
  Raw sewage
 
Pilot plant 1
 
Pilot plant 2
 
Parameter Unit of measurement m sd m sd m sd 
COD mg L−1 288.0 59.9 85.5 15.3 88.3 16.1 
BOD5 mg L−1 127.9 40.0 12.1 2.5 12.6 2.6 
SS mg L−1 153.0 45.0 20.5 4.5 19.7 4.8 
TN mg L−1 27.6 5.1 5.9a 2.0a 7.8a 2.1a 

aAll NO3–N because TKN in the effluent was always less than 0.5 mg L−1.

Data indicate a ‘low strength’ sewage. The average efficiency of the two pilot plants is quite similar (about: 70% for COD; 90% for BOD5; 87% for SS) except for TN, which shows some significant difference (78.6% for Plant 1; 71.7% for Plant 2).

RTD tests

Figure 3 shows the results of the tests carried out to verify the hydrodynamic behaviour of the two DENs.

Figure 3

RTD curves of the two DENs: comparison between experimental and theoretical results.

Figure 3

RTD curves of the two DENs: comparison between experimental and theoretical results.

The anoxic stage of Plant 1 proves to effectively behave as a series of four completely mixed reactors; in fact, the experimental curve overlaps almost perfectly (except in the first 50 min, maybe due to a by-pass effect). The theoretical curve represented by the formula: 
formula
5
where C is the tracer concentration in the last reactor effluent (mgLi+ L−1); Co is the tracer concentration in the first reactor influent (mgLi+ L−1); N is the number of reactors in series (N = 4 in the specific case); t is the time (h); tr (h) is the theoretical hydraulic retention time of the whole DEN volume (VDEN, m3): 
formula
6
Although not perfectly so, the experimental RTD curve relating to Plant 2 shows that DEN 2 has a behaviour very similar to a completely mixed reactor, which is described by the formula: 
formula
7

Pilot plants

Influence of the hydrodynamic models on DO

Figure 4 shows the DO concentration in the two DENs as a function of the denitrifications sludge loading (F:MDEN).

Figure 4

DO concentration in the DENs as a function of the sludge loading (F:MDEN). Pilot plant 1 data are represented for each reactor. The continuous lines and the shaded areas represent the mean and the 95% confidence interval, respectively.

Figure 4

DO concentration in the DENs as a function of the sludge loading (F:MDEN). Pilot plant 1 data are represented for each reactor. The continuous lines and the shaded areas represent the mean and the 95% confidence interval, respectively.

Both hydrodynamic models show that the increase of sludge loading allows a reduction of the DO concentration. Considering the configuration with the series of reactors (Plant 1), the first stage has a high DO concentration (0.61–0.73 mgO2 L−1, depending on the sludge loading) which is rapidly consumed: DO is almost halved in the second stage and it is below 0.1 mgO2 L−1 in the last reactor. The rapid initial drop is caused by the high concentration of both DO and BOD5, which influence the DO consumption rate as described by the following kinetic equation: 
formula
8
where rDO is the oxygen consumption rate (mgO2 L−1 h−1); rS is the substrate removal rate (mg L−1 h−1); S is the substrate concentration (mg L−1); KS and KDO are the half velocity constants referred to substrate and the DO, respectively (mg L−1); X is the VSS concentration in the mixed liquor (mg L−1); a, b and K are kinetic constants (dimensionless).

The average DO concentration in the single completely mixed reactor (Pilot plant 2) is less than halved (0.2–0.3 mgO2 L−1) compared to the initial concentration found in the four reactors in series and about three times higher than its final concentration.

Such results demonstrate the good performance in DO removal of the series of reactors compared to a single reactor.

Influence of the RTD models on both SDNR and denitrification efficiency

Figure 5 shows the SDNR of the two DEN hydrodynamic models as a function of the sludge loading in denitrification (F:MDEN).

Figure 5

SDNR as a function of the sludge loading in denitrification (F:MDEN) for the two reactor models. Average temperature: 16.1 °C (range: 15.6–16.7 °C).

Figure 5

SDNR as a function of the sludge loading in denitrification (F:MDEN) for the two reactor models. Average temperature: 16.1 °C (range: 15.6–16.7 °C).

The denitrification sludge loading significantly influences the SDNR: doubling the F:MDEN (from 0.2 to 0.4 kgBOD5 d−1 kgMLVSS−1), the average SDNR increase is about 54% in both reactor models. In addition, the hydraulic model affects the denitrification rate: the total SDNR of the series of four reactors (Plant 1) is 16.6% and 25% higher than the values measured with the single completely mixed reactor (Plant 2) at F:MDEN equal to 0.2 and 0.4 kgBOD5 d−1 kgMLVSS−1, respectively.

Figure 6 shows the denitrification performance of both pilot plants during the 180 operational days, expressed as the overall plant denitrification efficiency.

Figure 6

Denitrification efficiency (ηDEN) of the two pilot plants at three sludge loadings (F:MDEN).

Figure 6

Denitrification efficiency (ηDEN) of the two pilot plants at three sludge loadings (F:MDEN).

Data confirm the results of SDNR concerning the influence of the hydrodynamic model (Figure 5) on nitrogen removal. The denitrification efficiency with the series of reactors results are 6.7%, 9.7% and 12%, which are higher than for a single completely mixed reactor for F:MDEN equal to 0.2 kgBOD5 d−1 kgMLVSS−1, 0.3 kgBOD5 d−1 kgMLVSS−1 and 0.4 kgBOD5 d−1 kgMLVSS−1, respectively.

CONCLUSIONS

Dissolved oxygen in the biological DENs represents a serious limiting factor for the kinetics of the dissimilative reaction and, consequently, for the process efficiency. Nevertheless, small concentrations of DO are constantly present in biological pre-DENs, where they potentially cause adverse effects.

The experimental results obtained with a series of four reactors proved a capacity of oxygen consumption greater than a single completely mixed reactor (residual DO lower than 0.1 mgO2 L−1 compared to 0.18–0.30 mgO2 L−1). Such results are also positively influenced by high denitrification sludge loadings (F:MDEN). Tests carried out continuously on the two parallel pilot plants for 6 months evidenced that the SDNR raises both using four reactors in series instead of a single completely mixed reactor and increasing F:MDEN. Such an improvement amounts to +16.6% and +25% (in favour of the series of four reactors) with F:MDEN equal to 0.2 kgBOD5 d−1 kgMLVSS−1 and 0.4 kgBOD5 d−1 kgMLVSS−1, respectively. The tests demonstrated similar results considering denitrification removal efficiency (up to +12% with F:MDEN = 0.4 kgBOD5 d−1 kgMLVSS−1).

In conclusion, the experience highlights the importance of the hydrodynamic model of the anoxic reactor in conditioning the residual DO concentration and, consequently, the denitrification performance. For the same purpose, the right choice of the denitrification F:M ratio is of great importance.

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