# A Comprehensive Derivation and Application of Reference Values for Benchmarking the Energy Performance of Activated Sludge Wastewater Treatment

^{*}

## Abstract

**:**

_{0}for mechanical aerators. Fourteen Portuguese urban WWTPs (very diverse in size and inflows) were analyzed, and aeration (0.08–1.03 kWh/m

^{3}) represented 25–80% of total energy consumption (0.23–1.30 kWh/m

^{3}). The reference values for excellent performance were 0.23–0.39 kWh/m

^{3}(P25–P75) for AS systems with air diffusers and 0.33–0.80 kWh/m

^{3}for those with mechanical aerators. A comprehensive application in one WWTP (16–18 d solids retention time) showed the system’s ability at identifying which operating conditions to adjust (to F/M ratio lower than 0.09 d

^{−1}and decreasing aeration during the low season) to improve the energy performance/savings while maintaining the treatment’s effectiveness and reliability.

## 1. Introduction

## 2. Performance Indices of Energy Efficiency and Related Operating Conditions

#### 2.1. The General Framework Developed

_{0}, R

_{100}, R

_{200,}and R

_{300}.

_{100 min}and R

_{100 max}and is based on the recommended literature range; (ii) performance is null (R

_{0}) when it exceeds 25% tolerance (or other, customized) over the lower and the upper limits of the recommended range; and (iii) good performance (R

_{200 min}; R

_{200 max}) is based on a technical-economic balance, obtained from the literature (a broader range for R

_{100}and a narrower range for R

_{200}) and/or using a general criterion of cost-effectiveness illustrated in Figure 1:

- The R
_{200}range is on the upper side of the R_{100}range if the higher the state variable, the lower the cost associated (type 3a, Figure 1), e.g., for hydraulic loading; half of the broader range, (R_{100 min}+ R_{100 max})/2, is considered R_{200 min,}and R_{200 max}is obtained by applying a margin of tolerance to R_{100 max}; - The R
_{200}range is on the lower side of R_{100}range if the lower the state variable, the lower the cost associated (type 3b, Figure 1), e.g., for detention time; half of the broader range, (R_{100 min}+ R_{100 max})/2, is considered R_{200 max,}and R_{200 min}is obtained by applying a margin of tolerance to R_{100 min}.

_{300}depends on the specific operating conditions of each treatment facility. Thus, a WWTP wishing to extend these indices to the 200–300 range (good–excellent performance) must determine the optimal conditions of each treatment step, which represent the best balance between effectiveness (achievement of the target) and efficiency (minimum resource consumption).

#### 2.2. State-Variables Selected for Energy Performance of AS Systems

^{3}of treated wastewater, is the key state-variable in each use and is given by Equation (1):

- P = power (W);
- Q = treated wastewater flowrate (m
^{3}/h).

#### 2.3. Reference Values Derived for the State Variables of AS Energy Performance

#### 2.3.1. Aeration and Mixing

_{V O2}) is calculated based on the oxygen requirements for the biodegradation of carbonaceous material (R

_{O2}, Equation (2)) and on the oxygen transferred under field conditions (N). When nitrification is to take place, the oxygen requirements include the oxygen required for oxidizing ammonia and nitrite to nitrate [41,42], as expressed in Equation (2):

_{O2}= Q (S

_{0}− S) − 1.42 P

_{X,bio}+ 4.57 Q (NO

_{x}) − 2.86 Q (NOx − NO

_{3out})

- R
_{O2}= total oxygen required (g O_{2}/h); - S
_{0}, S = influent and effluent soluble BOD_{L}, ultimate carbonaceous BOD (mg O_{2}/L); - BOD = biochemical oxygen demand;
- 1.42 = stoichiometric ratio (g O
_{2}/g VSS); - VSS = volatile suspended solids (mg/L);
- P
_{X,bio}= biomass as volatile suspended solids (g VSS/h):P_{X,bio}= P_{X,VSS}− nbVSS Q - P
_{X,VSS}= the net waste activated sludge produced each day (g VSS/h):P_{X,VSS}= VX/24θ_{c} - V = reactor volume (m
^{3}); - X = mixed-liquor VSS (mg/L);
- ${\mathsf{\theta}}_{\mathrm{c}}$ = solids retention time (d);
- nbVSS = nonbiodegradable VSS in influent (mg/L);
- 4.33 and 2.86 = stoichiometric ratios (g O
_{2}/g N); - NO
_{x}= amount of NO_{3}-N produced from the nitrification of NH_{4}-N (mg N/L):NO_{x}= Nt_{in}− NH_{4out}− 0.12 P_{Xbio}/Q - Nt
_{in}= influent N concentration (mg N/L); - NH
_{4out}= effluent ammonia concentration (mg N/L); - 0.12 = stoichiometric ratio (g N/g VSS);
- NO
_{3out}= effluent nitrate concentration (mg N/L).

_{0}= 1.6 BOD

_{5}(the concentration of total 5-d biochemical oxygen demand influent to the reactor, in mg/L), and substituting Equations (3), (4) and (5) in Equation (2), one obtains:

_{O2}= Q 1.6 BOD

_{5}+ 1.71 Q (Nt

_{in}− NH

_{4out}) + 2.86 Q NO

_{3out}−1.625 (VX/24θ

_{c}− nvVSS Q)

_{O2}(g O

_{2}/h, Equation (6)) by the oxygen transferred under field conditions (N, kg O

_{2}/(kWh)) yields P (W), which is substituted in Equation (1) to obtain the unit energy consumption for aeration (Ev

_{O2}, Wh/m

^{3}):

- θ = hydraulic detention time in aeration tank (h);θ = V/Q

- N
_{m =}N for mechanical aerators (kg O_{2}/(kWh)); - N
_{0}= oxygen transferred to water at 20ºC and zero-dissolved oxygen (kg O_{2}/(kWh)), an equipment-specific value; - β = salinity-surface tension correction factor (typically 0.9-0.99 [9]);
- α = oxygen transfer correction factor for water (typically 0.4-0.8 [9]);
- C
_{L}= operating oxygen concentration (mg/L); - C
_{walt}= oxygen saturation concentration = C_{s}F_{a;} - C
_{s}= oxygen saturation concentration at sea level with temperature (T, °C); - F
_{a}= oxygen solubility correction factor for altitude (h, m) = 1 $-0.0001\mathrm{h};$

_{w}, kW):

- N
_{d}_{=}N for air diffusers (kg O_{2}/(kWh)); - AOR = actual oxygen transfer rate (under field conditions) (kg O
_{2}/h) (Equation (11) adapted from [9]); - SOR = standard oxygen transfer rate (under standard conditions, 20 °C, 1 atm, 0 mg O
_{2}/L) (kg O_{2}/h): - SOR = 835.2 w SOTE
- 835.2 is the conversion factor of w units, from kg air/s to kg O
_{2}/h (0.232 (kg O_{2}/ kg air) multiplied by 3600 (s/h)); - w = weight of air flow (kg air/s);
- SOTE = standard oxygen transfer efficiency (unitless), equipment/specific value
- P
_{w}= power requirements of each blower (kW) [42]:

- R = universal gas constant for air (8.314 J/(mol K));
- T = absolute inlet temperature (K);
- 8.199 = conversion factor (g/mol) = 28.97 n, with n = (k − 1)/k, where k is the specific heat ratio. For single-stage centrifugal blower power calculations, a value of 1.395 is used for k for dry air and n = 0.283;
- ${\mathrm{p}}_{1}$, ${\mathrm{p}}_{2}$ = absolute inlet and outlet pressure, respectively (kPa).

^{3}m

^{3}for mechanical aerators and, in the case of diffusors, mixing rates of 10–15 m

^{3}air/(10

^{3}m

^{3}water.min) are generally used [41,42].

_{mix m}, Wh/m

^{3}) depend on the detention time (θ, h) in the aerobic zone:

- M
_{m}= specific power requirements in mechanical aeration (kW/10^{3}m^{3}).

_{d}, m

^{3}air/(10

^{3}m

^{3}water.min)) may be computed as:

- $\mathsf{\rho}$ = density of air (kg/m
^{3}) = 353.07/T, with T in K.

_{mix m}, Wh/m

^{3}):

_{O2}) and must provide oxic conditions throughout the reactor, expressed by the dissolved oxygen concentration, while ensuring perfect mixing conditions. The reference values proposed in Table 1 reflect this issue: R

_{300}is is the highest value between Ev

_{O2}and the typical minimum for Ev

_{mix}; R

_{100}is the highest value between 1.5 Ev

_{O2}(i.e., allowing a 50% tolerance) and the average value of the typical range for Ev

_{mix}; and R

_{0}is the highest value between a 100% tolerance to Ev

_{O2}and the typical maximum for Ev

_{mix}. Figure 3 summarizes the stepwise procedure developed to obtain the reference values (R

_{0}to R

_{300}) to build a performance function.

_{L}) and it is typically maintained at about 1–2 mg/L [41,42,43] or 0.5–2 mg/L [9]. The reference values shown in Table 2 were therefore established based on these ranges.

#### 2.3.2. Anoxic and/or Anaerobic Mixing

^{3}m

^{3}[41,42]. Similarly to aeration (Equation (15)), the unit energy requirements for mixing depend on the detention time (θ) in the anoxic and anaerobic zone(s) (Table 6).

#### 2.3.3. Recirculation

- Q
_{p}= pumping flowrate (m^{3}/h); - ∆H = pumping head (m);
- η = pump efficiency (unitless);
- γ = specific weight of secondary sludge (N/m
^{3});

_{p}, Wh/m

^{3}):

_{p}) corresponds to the return sludge flowrate (Q

_{r}, m

^{3}/h) and the return sludge pumping energy (Ev

_{R}, Wh/m

^{3}) depends on the recirculation ratio, the pumping head, and the pump efficiency:

- R = return sludge ratio = Q
_{r}/Q (unitless)

_{300}) performance, 30% for the minimum acceptable (R

_{100}) performance, and 20% for unsatisfactory performance (R

_{0}) (adapted from ERSAR reference values [49]).

#### 2.3.4. Sludge Wasting

_{w}, m

^{3}/h)) may be given by:

_{p}is Q

_{w}given by Equation (22), the sludge-wasting unit pumping energy (Ev

_{w}, Wh/m

^{3}) is given by:

_{R}~ 1/(1+R)

_{c}and R proposed in Table 4 and Table 7, respectively, whose typical ranges are also AS-type-specific.

_{R.}Thus, Equation (23) is simplified (there is no Q

_{w}dependence on R) and the reference values change accordingly (Table 11).

## 3. iEQTA WWTPs Analyzed

^{3}/d) and two treatment sequences: (i) activated sludge after primary sedimentation, designed for conventional aeration (CAS); and (ii) activated sludge without primary sedimentation, designed for extended aeration (EA). The five-year (2015–2019) data of these WWTPs are presented in Silva and Rosa [51], where the plant annual reliability for biochemical oxygen demand (BOD

_{5}), chemical oxygen demand (COD), and total suspended solids (TSS) was discussed. During the energy-measurement campaigns that were carried out, the WWTPs studied were operated under the conditions summarized in Table 12.

## 4. Results and Discussion

#### 4.1. Sensitivity Analysis of Aeration Efficiency

_{d}) is higher than that of mechanical aerators (N

_{m}) and, under field conditions, it depends on many variables as expressed by Equations (14) and (9), respectively. Therefore, a sensitivity analysis was conducted to understand to what extent each parameter affects the oxygen transfer, considering the typical value for each variable except one, which was allowed to vary one at a time within its typical range.

_{d}when α increases from 0.4 to 0.7 and a 64% increase in N

_{d}when SOTE increases from 0.25 to 0.4 (Figure 4). When F increases from 0.65 to 0.9, i.e., when the fouling decreases, N

_{d}increases 36% (Figure 4). For mechanical aerators, the oxygen transfer mainly varies with α and N

_{0}, namely, N

_{m}increases 76% when α increases from 0.4 to 0.7 and N

_{m}increases 91% when N

_{0}increases from 1.1 to 2.1 kg O

_{2}/(kWh) (Figure 5).

_{d}and N

_{m}are β (Table 13 and Table 14) and temperature (Figure 4a and Figure 5a). Drewnowski et al. [53] also found that the influence of the temperature on the oxygen transfer rate is virtually unnoticeable since, on the one hand, the oxygen solubility drops as the temperature increases, while, on the other hand, it raises the diffusion rate. Our results show that for air diffusers, N

_{d}slightly decreases with temperature in the 5–30 °C range, namely, from 1.8 to 1.6 kg O

_{2}/(kWh) for C

_{L}= 1 mg/L (Figure 4a). For mechanical aerators, the temperature effect on N

_{m}is even lower and depends on C

_{L}, with a turning point at 1 mg/L. In the 5–30 °C range, for C

_{L}= 1 mg/L, N

_{m}does not vary with temperature; above 1 mg/L it slightly increases (e.g., 3% for 0.5 mg/L), and below 1 mg/L it slightly decreases (e.g., 7% for 2 mg/L) (Table 14, Figure 5a).

^{3}/(h.m

^{2})) reduction (e.g., by increasing the diffuser diameter or the number of diffusors), blower system retrofitting to modulate the air flow (e.g., introducing adjustable-frequency drives (AFDs) or most-open-valve (MOV) logic to minimize the system pressure), or diffuser-type replacement [9]; (ii) the cleaning of the diffusers, which decreases the fouling (increasing F) [52]; (iii) for mechanical aerators, the increase of N

_{0}by equipment replacement; (iv) for both aerator types, the increase of the α-value by a solids retention time increase or by including an anoxic selector, both increasing the water quality [52]; and (v) the adjustment of the dissolved oxygen set point (C

_{L}decrease) [52].

#### 4.2. Energy Performance of iEQTA WWTPs

^{3}(median 0.70 kWh/m

^{3}) (Figure 6a). Aeration was the major energy consumer, 83–1031 Wh/m

^{3}(measured values; Figure 6c), representing 25–80% of the total energy consumption (median 51%) (Figure 6b), values that are consistent with other studies [2,9,23,29,30,31,32,33]. Foladori et al. [36] studied five small WWTPs, and the aeration varied from 68 Wh/m

^{3}to 799 Wh/m

^{3}, with the lower consumption in the WWTP with intermittent aeration and the higher consumption in the WWTP with 4 mg/L of C

_{L}. The reference values computed for these 14 WWTPs were in the 25–75 percentile range (P25–P75) of 244–618 Wh/m

^{3}with a median of 373 Wh/m

^{3}for excellent performance (R

_{300}), and 366–926 Wh/m

^{3}(P25–P75) and 560 Wh/m

^{3}(median) for acceptable performance (R

_{100}) (Figure 6c). Clustering these results per type of aerator, the reference values for excellent performance are, for air diffusers, 232–385 Wh/m

^{3}(P25–P75), with a median of 324 Wh/m

^{3}, and, for mechanical aerators, 325–800 Wh/m

^{3}(P25–P75), with a median of 560 Wh/m

^{3}. These values highlight the fact that air diffusers are more energy-efficient than mechanical aerators, in the analyzed conditions (Table 12). Figure 6d shows the majority of WWTPs analyzed, with air diffusers or mechanical aerators, presented excellent-acceptable performance (PX median 300).

_{5,}the higher the energy consumption (with no linear correlation)

_{.}On the other hand, it shows that plants operating with BOD

_{5}reliability above 0.9 (the minimum reliability needed to comply with EU directive discharge requirements [51]) presented higher energy consumption for aeration than the less reliable WWTPs, which were earlier found to be the CAS WWTPs [51]. Moreover, the type of aerator should be also considered in this analysis since air diffusers (labelled as ‘d’ in Figure 7) are more efficient than the mechanical aerators (‘m’ in Figure 7). Namely, for high strength influent (450–480 mg/L BOD

_{5in}), CAS treatment with air diffusors (WWTP G) is more energy efficient than EA treatment with air diffusors (WWTP N), and this is more efficient than with mechanical aerators (WWTP K). In turn, for medium-high strength influent, a similar energy consumption allowed >0.9 reliable BOD

_{5}treatment of a higher influent BOD

_{5}concentration by air diffusors compared to mechanical aerators, namely, (i) 366 Wh/m

^{3}for 324 mg/L with air diffusors (i.e., 1.13 kWh/kg BOD

_{5}, WWTP P) vs. 381 Wh/m

^{3}for 271 mg/L with mechanical aerators (1.41 kWh/kg BOD

_{5}, WWTP O), both with a strong textile effluent input, and (ii) 258 Wh/m

^{3}for 390 mg/L with air diffusors (i.e., 0.66 kWh/kg BOD

_{5}, WWTP M) vs. 270 Wh/m

^{3}for 368 mg/L with mechanical aerators (0.73 kWh/kg BOD

_{5}, WWTP H), both with a typical urban inflow. All seven of the abovementioned plants presented significant nitrogen removal (Table 12). No effect was found of the treated volume on the unit energy consumption in aeration.

^{3}) of the total energy consumed in the WWTP and varied within 1–15% (min-max; Figure 8a) and 9–192 Wh/m

^{3}(measured values; Figure 8b). In the five WWTPs studied by Foladori et al. [36], energy for recirculation varied from 30 Wh/m

^{3}to 226 Wh/m

^{3}.

^{3}, with a median of 11.2 Wh/m

^{3}for excellent performance (R

_{300}), and of 52–94 Wh/m

^{3}(P25–P75) and 75 Wh/m

^{3}(median) for acceptable performance (Figure 8b). The WWTPs analyzed presented acceptable to good performance (PX median 266, Figure 8c) and a pump efficiency of 53% (median, Figure 8d).

^{3}(2.7 Wh/m

^{3}median). The sludge wasting in the five WWTPs studied by Foladori et al. [36] varied from 2 Wh/m

^{3}to 17 Wh/m

^{3}.

^{3}for R

_{300}and 1.8–24 Wh/m

^{3}for acceptable performance (R

_{100}). Even with a lower impact, the performance for this energy consumption was good (PX median 220).

#### 4.3. Energy Performance Diagnosis and Improvement Measures for the WWTP K

^{3}/d capacity; and was operated, on average, at 81% of its capacity (Table 12). 1.8-year data (March 2018–December 2019) were used. During this period, WWTP K operated with the following:

- Influent wastewater: 172–495 mg/L BOD
_{5}(median 341 mg/L, P25–P75 292–382 mg/L), 245–1611 mg/L COD (median 971 mg/L, P25–P75 769–1125 mg/L), 103–545 mg/L TSS (median 327 mg/L, P25–P75 270–382 mg/L), and 31–90 mg/L N-total (median 59 mg/L, P25–P75 49–66 mg/L); - Operating conditions: 2930–5380 mg/L MLSS (median 4140 mg/L, P25–P75 3875–4505 mg/L), 17.3–52.3 h θ (median 28.9 h, P25–P75 24.8–35.8 h), 16–18 d θ
_{c}(median 16.8 d, P25–P75 16.5–17.2 d), and 0.04–0.13 d^{−1}F/M (median 0.08 d^{−1}, P25–P75 0.07–0.10 d^{−1}); - Reliability: 0.99–1.00 for BOD
_{5}, 0.98–0.97 for COD, 0.94–0.93 for TSS, and 0.95–0.90 for N–total, i.e., always above 0.9 for all parameters, the cut–off for the compliance [51].

_{c}below the minimum acceptable for extended aeration, i.e., 16–18 d vs. 20–40 d, typically. Nevertheless, 16–18 d are in the nitrification range, which provides the conditions for high-water quality and subsequently a high α-value, one of the two variables with the highest positive impact on energy consumption. Actually, these retention times corresponded to good–excellent performance of the treated wastewater quality and to >0.90 reliability for BOD

_{5}, COD, TSS, and N-total.

_{300}), from 37 Wh/m

^{3}to 615 Wh/m

^{3}(the upper limit of the light green zone in Figure 10); (ii) for good performance, from 376 Wh/m

^{3}to 1220 Wh/m

^{3}(the upper limit of the green zone); and (iii) for the minimum acceptable performance, from 603 Wh/m

^{3}to 2087 Wh/m

^{3}(the upper limit of the yellow zone). The reference values for acceptable performance (R

_{100}) also consider the energy required for mixing, which depends on the detention time, as explained in Table 1.

_{300}) showed a linear relation with F/M ratio; the higher the F/M, the higher the oxygen requirements (Figure 11). Using the k-means method for the clustering analysis of the relation between R

_{300}and F/M (with standardized values since R

_{300}and F/M scales were very different), the turning point of F/M identified was 0.09 d

^{−1}. The ANOVA p-value and the homogeneity of variance of the two clusters of F/M were computed— the p-value was 6.2E

^{−13}(<0.05) and the F values < F critical values—and the statistical differences were verified. Thus, if the water utility decreases the F/M from 0.11 d

^{−1}to 0.07 d

^{−1}, the energy requirement will decrease from 416 Wh/m

^{3}to 252 Wh/m

^{3}, which, considering the average treated wastewater of 12,312 m

^{3}/d, represents a potential saving of 2019 kWh/d or 505 kg CO

_{2e}/d of indirect carbon emission, using the Portuguese energy emission factor of 2019 (0.25 kg CO

_{2e}/(kWh)). The F/M ratio is therefore a key variable of energy performance that is easy to monitor and control.

^{3}in the winter months to 560 Wh/m

^{3}in the summer months. Nevertheless, it may be further improved during the summer, when the detention time increases due to lower influent flowrates (low season, Figure 12a) and the reactor is being over-aerated. The gains from better adjusting the energy consumed to the energy required, i.e., levering all days to energy PX 300, translate into a potential energy savings of, on average, 141 Wh/m

^{3}. Yu et al. [54], using Bayesian semi-parametric quantile regression, identified the temperature and the total nitrogen-rich wastewater as the factors associated with the higher level of energy consumption.

^{3}to 30 Wh/m

^{3}.

- Decreasing the F/M range from 0.04–0.13 d
^{−1}to 0.04–0.09 d^{−1}, to decrease the energy requirements; - Better adjusting (decreasing) aeration during the summer period when the flowrate decreases, to avoid excessive aeration and better modulate the energy consumed to the energy required; this could be done by submergence adjustment, speed adjustment, and on-off operation [9];
- Reducing the return sludge ratio in the summer period (e.g., from above 2 to 1.5);
- Further studying the feasibility and benefits of reducing the number of treatment lines operating in parallel in the summer (low season), when the detention time increases due to lower influent flowrates.

## 5. Conclusions

- The uses related to flow pumping, namely, the return sludge and sludge wasting, depend on the pumping head, and the AS sludge wasting also depends on the detention time. For instance, the energy for return sludge in extended aeration systems, considering a pumping head of 10 m and an efficiency of 50%, varies from 28 Wh/m
^{3}when the recirculation is the minimum value of the typical range (0.5) to 112 Wh/m^{3}for the maximum R of the typical range (2); - The mixing depends on the detention time in the aerated, anoxic, and anaerobic reactors. For instance, the increase of θ in the A2O anaerobic zone, from 0.5 h to 1.5 h, increases the maximum energy requirement for mixing (R
_{100}) from 7 Wh/m^{3}to 20 Wh/m^{3}; - The aeration depends on the influent BOD
_{5}and ammonia, the biomass wasted (determined by MLSS, θ_{c,}and θ), and the amount of oxygen transferred under field conditions. The oxygen transfer varies according to the mechanical aerator type (N_{0}), the diffuser type (SOTE), the compressor efficiency, and many field parameters (temperature; dissolved oxygen; altitude; the oxygen transfer correction factor for waste (α); and, for air diffusers, also the fouling factor (F) and the outlet pressure).

_{0}for mechanical aerators. These are therefore the key variables the improvement measures should address, as exemplified. For instance, for WWTP G, if α increases from 0.52 to 0.65, the energy requirements decrease by 20%, from 385 Wh/m

^{3}to 306 Wh/m

^{3}.

^{3}) represented 25–80% of the total energy consumption (0.23–1.30 kWh/m

^{3}). The reference values for excellent performance were 0.23–0.39 kWh/m

^{3}(P25–P75) for the AS systems with air diffusers and 0.33–0.80 kWh/m

^{3}for those with mechanical aerators.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 3.**The stepwise procedure to obtain the reference values (R

_{0}to R

_{300}) of the performance function of energy consumption for aeration and mixing in AS systems.

**Figure 4.**Oxygen transfer by air diffuser systems vs. temperature (

**a**), operating oxygen concentration (

**b**), outlet pressure (

**c**), oxygen transfer correction factor α (

**d**), fouling factor F (

**e**), and SOTE (

**f**) (considering, for the other parameters, their typical values in Table 13.

**Figure 5.**The oxygen transfer by mechanical aerators vs. temperature (

**a**), the operating oxygen concentration (

**b**), the oxygen transfer correction factor α (

**c**), and N

_{0}(

**d**) (considering, for the other parameters, their typical values in Table 14).

**Figure 6.**The energy consumption in the iEQTA WWTPs (

**a**) and in aeration (

**b**,

**c**), the computed reference values (

**c**), and the performance indices for aeration (

**d**).

**Figure 7.**The energy consumption for aeration vs. influent BOD

_{5}in the iEQTA WWTPs with BOD

_{5}reliability lower and higher than 0.9 (daily values from the energy-measurement campaigns).

**Figure 8.**The energy consumption for recirculation in the iEQTA WWTPs (% (

**a**) and Wh/m

^{3}(

**b**)), the computed reference values (

**b**), the performance indices (

**c**), and the pumps efficiency (

**d**).

**Figure 9.**The performance indices of detention time (

**a**), MLSS (

**b**), and solids retention time (

**c**), in the AS reactor of WWTP K.

**Figure 10.**The reference values for energy consumption in aeration over the 1.8-year period analyzed in WWTP K.

**Figure 12.**The performance indices of energy for aeration and temperature (

**a**), and the performance indices of the return sludge and of the associated unit energy (

**b**), over the period analyzed in WWTP K.

Assumptions | Ev_{O2} Reference Values (Wh/m^{3}) | |
---|---|---|

Mechanical Aerators | ||

R_{300} ↔ Ev_{O2} (Equation (7)) or Ev_{mix m} (Equation (15)) with M _{m} = 20 W/m^{3} (the highest value) | R_{300} ● | $\frac{1.6{\mathrm{BOD}}_{5}+1.71\left({\mathrm{Nt}}_{\mathrm{in}}-{\mathrm{NH}}_{4\mathrm{out}}\right)+2.86{\mathrm{NO}}_{3\mathrm{out}}-1.625\left(\frac{\mathrm{X}\mathsf{\theta}}{24{\mathsf{\theta}}_{\mathrm{c}}}-\mathrm{nbVSS}\right)}{\mathrm{N}}$ or 20 θ, the highest |

R_{100} ↔ 1.5 Ev_{O2} or Ev_{mix m} (Equation (15)) with M _{m} = 30 W/m^{3} (the highest value) | R_{100} ● | $1.5{\mathrm{R}}_{300}\mathrm{or}30\mathsf{\theta}$, the highest |

R_{0} ↔ 2 Ev_{O2} or Ev_{mix m} (Equation (15)) with M _{m} = 40 W/m^{3} (the highest value) | R_{0} ● | $2{\mathrm{R}}_{300}\mathrm{or}40\mathsf{\theta}$, the highest |

Air diffusers | ||

R_{300} ↔ Ev_{O2} (Equation (7)) or Ev_{mix d} (Equation (18)) with M _{d} = 10 m^{3}/(10^{3} m^{3}.min) and e = 0.9 (the highest value) | R_{300} ● | $\frac{1.6{\mathrm{BOD}}_{5}+1.71\left({\mathrm{Nt}}_{\mathrm{in}}-{\mathrm{NH}}_{4\mathrm{out}}\right)+2.86{\mathrm{NO}}_{3\mathrm{out}}-1.625\left(\frac{\mathrm{X}\mathsf{\theta}}{24{\mathsf{\theta}}_{\mathrm{c}}}-\mathrm{nbVSS}\right)}{\mathrm{N}}\mathrm{or}66.25\mathsf{\theta}\left[{\left(\frac{{\mathrm{P}}_{2}}{{\mathrm{P}}_{1}}\right)}^{0.283}-1\right]$, the highest |

R_{100} ↔ 1.5 Ev_{O2} or Ev_{mix d} (Equation (18)) with M _{d} = 12.5 m^{3}/(10^{3} m^{3}.min) and e = 0.9 (the highest value) | R_{100} ● | $1.5{\mathrm{R}}_{300}\mathrm{or}82.81\mathsf{\theta}\left[{\left(\frac{{\mathrm{P}}_{2}}{{\mathrm{P}}_{1}}\right)}^{0.283}-1\right]$, the highest |

R_{0} ↔ 2 Ev_{O2} or Ev_{mix d} (Equation (18) with M _{d} = 15 m^{3} /(10^{3} m^{3}.min) and e = 0.7 (the highest value) | R_{0} ● | $2{\mathrm{R}}_{300}\mathrm{or}127.77\mathsf{\theta}\left[{\left(\frac{{\mathrm{P}}_{2}}{{\mathrm{P}}_{1}}\right)}^{0.283}-1\right]$, the highest |

C_{L} Reference Values (mg/L) | Typical Values (mg/L) | |
---|---|---|

R_{200} (min; max) ● | 0.8; 1 | 0.5–2 [9]; 1–2 [41,42,43]; 2–3 [44] |

R_{100} (min; max) ● | 0.5; 2 | |

R_{0} (min; max) ● | 0.3; 2.5 |

Activated Sludge | MLSS Reference Values (mg/L) | Typical Values (mg/L) | ||
---|---|---|---|---|

AS-Type | R_{200} (min; max) ● | R_{100} (min; max) ● | R_{0} (min; max) ● | |

Complete mix | 3000; 4000 | 1500; 6000 | 1200; 7000 | 3000–6000 [44,45] 2000–3000 [43] 1500–4000 [41,42] |

Conventional plug flow | 1500; 2500 | 1000; 3000 | 800; 3600 | 1500–3000 [44,45] 2000–3000 [43] 1000–3000 [41,42,46] |

Extended aeration | 3000; 5000 | 2000; 6000 | 1600; 7000 | 3000–6000 [44,45] 2000–5000 [41,42] 3000–5000 [46] 2000–6000 [43] |

Oxidation ditch (C removal) | 3500; 5000 | 3000; 6000 | 2400; 7000 | 3000–6000 [44] 3000–5000 [41,42] 2000–6000 [43] |

Oxidation ditch (C+N removal) | 2500; 3500 | 2000; 4000 | 1600; 4800 | 2000–4000 [41,42,47] 2000–6000 [43] |

Anoxic/Aerobic (MLE) | 3200; 3800 | 3000; 4000 | 2400; 4800 | 3000–4000 [41,42,47] |

Bardenpho (4-stage) | 3200; 3800 | 3000; 4000 | 2400; 4800 | 3000–4000 [41,42,47] |

Bardenpho (5-stage) | 3000; 4000 | 2000; 5000 | 1600; 6000 | 3000–4000 [41,42,47] 2000–5000 [44] |

A/O (Anaerobic/Aerobic) | 3200; 3800 | 3000; 4000 | 2400; 4800 | 3000–4000 [41,42,47] |

A2/O | 3000; 3500 | 2000; 4000 | 1600; 4800 | 2000–4000 [44] 3000–4000 [41,42,47] |

UCT | 3000; 4000 | 2000; 5000 | 1600; 6000 | 2000–5000 [44] 3000–4000 [41,42,47] |

VIP | 2000; 3000 | 1500; 4000 | 1200; 4800 | 1500–3000 [44] 2000–4000 [41,42,47] |

Activated Sludge | θ_{c} Reference Values (d) | Typical Values (d) | ||
---|---|---|---|---|

AS-Type | R_{200} (min; max) ● | R_{100} (min; max) ● | R_{0} (min; max) ● | |

Complete mix | 5; 8 | 3; 15 | 2.5; 18 | 5–15 [44] 3–15 [41,42] 3–10 [43] 4–15 [45] |

Conventional plug flow | 5; 8 | 3; 15 | 2.5; 18 | 5–15 [44,47] 3–15 [41,42] 3–10 [43] 4–15 [45] |

Extended aeration | 22; 30 | 20; 40 | 15; 50 | 20–30 [43,44,45,47] 20–40 [41,42] |

Oxidation ditch (C removal) | 20; 25 | 15; 30 | 12; 35 | 20–30 [43,44,48] 15–30 [41,42] 20 [47] |

Oxidation ditch (C+N removal) | 22; 25 | 20; 30 | 15; 35 | 20–30 [41,42,47] |

Anoxic/Aerobic (MLE) | 9; 15 | 8; 20 | 7; 25 | 7–20 [41,42,47] |

Bardenpho (4-stage) | 11; 15 | 10; 20 | 8; 25 | 10–20 [41,42,47] |

Bardenpho (5-stage) | 12; 20 | 10; 30 | 8; 40 | 10–20 [41,42,47] 10–40 [44] |

A/O (Anaerobic/Aerobic) | 2.2; 4 | 2; 5 | 1.8; 6 | 2–5 [41,42,47] |

A2/O | 8; 20 | 4; 27 | 3; 34 | 5–25 [41,42,47] 4–27 [44] |

UCT | 12.5; 25 | 10; 30 | 7.5; 35 | 10–25 [41,42,47] 10–30 [44] |

VIP | 8; 9 | 5; 10 | 4; 12 | 5–10 [41,42,44,47] |

Activated Sludge | θ Reference Values (h) | Typical Values (h) | |||
---|---|---|---|---|---|

AS-Type | R_{200} (min; max) ● | R_{100} (min; max) ● | R_{0} (min; max) ● | ||

Complete mix | 3.2; 4 | 3; 5 | 2.5; 6 | 3–5 [41,42,44,45] 5–14 [43] | |

Conventional plug flow | 5; 6 | 4; 8 | 3; 10 | 4–8 [41,42,44,45] 5–14 [43] | |

Extended aeration | 20; 27 | 18; 36 | 14; 45 | 20–30 [41,42,43] 18–24 [45] 18–36 [44] 24 [46] | |

Oxidation ditch (C removal) | 18; 27 | 15; 36 | 12; 45 | 18–36 [44] 15–30 [41,42] 20–30 [43] 18 [47] | |

Oxidation ditch (C+N removal) | 20; 27 | 18; 36 | 14; 45 | 18–30 [41,42,47] 20–30 [43] | |

MLE | anoxic zone | 1.3; 2.2 | 1; 3 | 0.7; 3.8 | 1–3 [41,42,47] |

aerobic zone | 5; 9 | 4; 12 | 3; 15 | 4–12 [41,42,47] | |

Bardenpho 4 | 1st anoxic zone | 1.3; 2.2 | 1; 3 | 0.7; 3.8 | 1–3 [41,42,47] |

1st aerobic zone | 5; 9 | 4; 12 | 3; 15 | 4–12 [41,42,47] | |

2nd anoxic zone | 2.3; 3 | 2; 4 | 1.7; 5 | 2–4 [41,42,47] | |

2nd aerobic zone | 0.6; 0.8 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [41,42,47] | |

Bardenpho 5 | anaerobic zone | 1; 1.5 | 0.5; 2 | 0.4; 2.5 | 0.5–1.5 [41,42,47] 1–2 [44] |

1st anoxic zone | 2; 3 | 1; 4 | 0.7; 5 | 1–3 [41,42,47] 2–4 [44] | |

1st aerobic zone | 5; 9 | 4; 12 | 3; 15 | 4–12 [41,42,44,47] | |

2nd anoxic zone | 2.3; 3 | 2; 4 | 1.7; 5 | 2–4 [41,42,44,47] | |

2nd aerobic zone | 0.6; 0.8 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [41,42,44,47] | |

A/O | anaerobic zone | 0.7; 1.1 | 0.5; 1.5 | 0.3; 1.9 | 0.5–1.5 [41,42,47] |

aerobic zone | 1.3; 2.2 | 1; 3 | 0.7; 3.8 | 1–3 [41,42,47] | |

A2/O | anaerobic zone | 0.7; 1.1 | 0.5; 1.5 | 0.3; 1.9 | 0.5–1.5 [41,42,44,47] |

anoxic zone | 0.6; 0.8 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [41,42,44,47] | |

aerobic zone | 4; 6 | 3.5; 8 | 3; 10 | 3.5–6 [44] 4–8 [41,42,47] | |

UCT | anaerobic zone | 1.2; 1.5 | 1; 2 | 0.8; 2.5 | 1–2 [41,42,44,47] |

anoxic zone | 2.3; 3 | 2; 4 | 1.7; 5 | 2–4 [41,42,44,47] | |

aerobic zone | 5; 9 | 4; 12 | 3; 15 | 4–12 [41,42,44,47] | |

VIP | anaerobic zone | 1.2; 1.5 | 1; 2 | 0.8; 2.5 | 1–2 [41,42,44,47] |

anoxic zone | 1.2; 1.5 | 1; 2 | 0.8; 2.5 | 1–2 [41,42,44,47] | |

aerobic zone | 3; 4 | 2.5; 6 | 2; 8 | 2.5–4 [44] 4–6 [41,42,47] |

**Table 6.**The reference values of energy consumption for mixing in the AS anoxic and anaerobic zones.

Assumptions | Ev_{mix m} Reference Values (Wh/m^{3}), (θ in h) | |
---|---|---|

R_{300} ↔ Ev_{mix m} (Equation (15)) with M = 8 W/m^{3} | R_{300} ● | 8 θ |

R_{100} ↔ Ev_{mix m} (Equation (15)) with M = 13 W/m^{3} | R_{100} ● | 13 θ |

R_{0} ↔ Ev_{mix m} (Equation (15)) with M = 1.25 × 13 W/m^{3} | R_{0} ● | 16.3 θ |

Recirculation | R reference Values (unitless) | Typical Values | ||
---|---|---|---|---|

AS-Type | R_{200} (min; max) ● | R_{100} (min; max) ● | R_{0} (min; max) ● | |

Complete mix | 0.3; 0.8 | 0.25; 1 | 0.2; 1.2 | 0.25–1 [41,42,44,45] |

Conventional plug flow | 0.3; 0.5 | 0.25; 0.75 | 0.2; 0.9 | 0.25–0.5 [44,45] 0.25–0.75 [41,42] |

Extended aeration | 0.75; 1.5 | 0.5; 2 | 0.4; 2.4 | 0.25–2 [44] 0.25–1.5 [41,42,46] 0.75–1.5 [45] |

Oxidation ditch (C removal) | 0.75; 1.5 | 0.5; 2 | 0.4; 2.4 | 0.5–2 [44] 0.75–1.5 [41,42] |

Oxidation ditch (C+N removal) | 0.6; 0.8 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [41,42,47] |

Anoxic/Aerobic (MLE) | 0.6; 0.8 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [41,42,47] |

Bardenpho (4-stage) | 0.6; 0.8 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [41,42] |

Bardenpho (5-stage) | 0.8; 0.9 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [41,42] 0.8–1 [44] |

A/O | 0.3; 0.8 | 0.25; 1 | 0.2; 1.2 | 0.5–1 [41,42,47] |

A2/O | 0.25; 0.5 | 0.2; 1 | 0.16; 1.2 | 0.2–0.5 [44] 0.25–1 [41,42,47] |

UCT | 0.82; 0.9 | 0.8; 1 | 0.7; 1.2 | 0.8–1 [41,42,44,47] |

VIP | 0.8; 0.9 | 0.5; 1 | 0.4; 1.2 | 0.5–1 [44] 0.8–1 [41,42,47] |

Assumptions | Ev_{R} Reference Values (Wh/m^{3}) | |||
---|---|---|---|---|

R_{300} ↔ Ev_{R} (Equation (21)) with R = R_{100} min of R (Table 7) and η = 0.5R _{100} ↔ Ev_{R} (Equation (21)) with R = R_{100} max of R (Table 7) and η = 0.3R _{0} ↔ Ev_{R} (Equation (21)) with R = R_{0} max of R (Table 7) and η = 0.2 | AS-type | R_{300} ● | R_{100} ● | R_{0} ● |

Conv. plug flow | $1.4\Delta \mathrm{H}$ | $7.0\Delta \mathrm{H}$ | $12.6\Delta \mathrm{H}$ | |

Complete mix | $9.3\Delta \mathrm{H}$ | $16.8\Delta \mathrm{H}$ | ||

A/O | ||||

A2/O | $1.1\Delta \mathrm{H}$ | |||

OD (C + N) | $2.8\Delta \mathrm{H}$ | |||

MLE | ||||

Bardenpho 4 | ||||

Bardenpho 5 | ||||

VIP | ||||

UCT | $4.5\Delta \mathrm{H}$ | |||

OD (C) | $2.8\Delta \mathrm{H}$ | $18.7\Delta \mathrm{H}$ | $33.6\Delta \mathrm{H}$ | |

Ext. aeration |

Internal Recirculation | R_{i} Reference Values (unitless) | Typical Values | |||
---|---|---|---|---|---|

AS-Type | R_{200} (min; max) ● | R_{100} (min; max) ● | R_{0} (min; max) ● | ||

Anoxic/Aerobic (MLE) | 1.2; 1.6 | 1; 2 | 0.8; 2.4 | 1–2 [41,42,47] | |

Bardenpho 4 | 2.4; 3.2 | 2; 4 | 1.6; 4.8 | 2–4 [41,42] | |

Bardenpho 5 | 3.5; 4.5 | 2; 6 | 1.6; 7.2 | 2–4 [41,42] 4–6 [44] | |

A2/O | 1.2; 3 | 1; 4 | 0.8; 4.8 | 1–3 [44] 1–4 [41,42,47] | |

UCT | from anoxic zone | 2; 4 | 1; 6 | 0.8; 7 | 1–6 [44] 2–4 [41,42,47] |

from aerobic zone | 0.8; 1.2 | 0.5; 3 | 0.4; 3.6 | 0.5–1 [44] 1–3 [41,42,47] | |

VIP | from anoxic zone | 1.5; 2.5 | 1; 4 | 0.8; 4.8 | 2–4 [44] 1–2 [41,42,47] |

from aerobic zone | 1.2; 2.4 | 1; 3 | 0.8; 3.6 | 1–3 [41,42,47] |

**Table 10.**The reference values of the energy consumption for the internal recirculation in AS systems.

Internal Recirculation | E_{V} Reference Values (Wh/m^{3}) | ||
---|---|---|---|

AS-Type | R_{200} ● | R_{100} ● | R_{0} ● |

UCT aerobic | $2.8\Delta \mathrm{H}$ | $28\Delta \mathrm{H}$ | $50\Delta \mathrm{H}$ |

VIP aerobic | $5.6\Delta \mathrm{H}$ | ||

MLE | $19\Delta \mathrm{H}$ | $34\Delta \mathrm{H}$ | |

VIP anoxic | $37\Delta \mathrm{H}$ | $67\Delta \mathrm{H}$ | |

A2O | |||

Bardenpho 4 | $11.2\Delta \mathrm{H}$ | ||

Bardenpho 5 | $56\Delta \mathrm{H}$ | $98\Delta \mathrm{H}$ | |

UCT anoxic | $5.6\Delta \mathrm{H}$ |

Assumptions | ${\mathbf{Ev}}_{\mathbf{w}}\mathbf{Reference}\mathbf{Values}(\mathbf{Wh}/{\mathbf{m}}^{3}),(\mathsf{\theta}$$\mathbf{in}\mathbf{h},\mathbf{\Delta}\mathbf{H}\mathbf{in}\mathbf{m})$ | ||||||
---|---|---|---|---|---|---|---|

Sludge Wasting… | …From the R Line | …From the Aeration Tank | |||||

R_{300} ↔ Ev_{w} (Equation (23) with θ _{c} = R_{100 max} of θ_{c} (Table 4)R = R _{100 max} of R (Table 7)η = 0.5 R _{100} ↔ Ev_{w} (Equation (23) with θ _{c} = R_{100 min} of θ_{c} (Table 4)R = R _{100 min} of R (Table 7)η = 0.3 R _{0} ↔ Ev_{w} (Equation (23) withθ _{c} = R_{0 min} of θ_{c} (Table 4)R = R _{0 min} of R (Table 7)η = 0.2 The sludge wasting from the aeration tank does not depend on R | AS-type | R_{300} ● | R_{100} ● | R_{0} ● | R_{300} ● | R_{100} ● | R_{0} ● |

Extended aeration | $0.002\mathsf{\theta}\Delta \mathrm{H}$ | $0.013\mathsf{\theta}\Delta \mathrm{H}$ | $0.028\mathsf{\theta}\Delta \mathrm{H}$ | $0.006\mathsf{\theta}\Delta \mathrm{H}$ | $0.019\mathsf{\theta}\Delta \mathrm{H}$ | $0.039\mathsf{\theta}\Delta \mathrm{H}$ | |

OD (C+N) | $0.004\mathsf{\theta}\Delta \mathrm{H}$ | $0.008\mathsf{\theta}\Delta \mathrm{H}$ | |||||

OD (C) | $0.003\mathsf{\theta}\Delta \mathrm{H}$ | $0.017\mathsf{\theta}\Delta \mathrm{H}$ | $0.035\mathsf{\theta}\Delta \mathrm{H}$ | $0.026\mathsf{\theta}\Delta \mathrm{H}$ | $0.049\mathsf{\theta}\Delta \mathrm{H}$ | ||

UCT | $0.004\mathsf{\theta}\Delta \mathrm{H}$ | $0.022\mathsf{\theta}\Delta \mathrm{H}$ | $0.046\mathsf{\theta}\Delta \mathrm{H}$ | $0.039\mathsf{\theta}\Delta \mathrm{H}$ | $0.078\mathsf{\theta}\Delta \mathrm{H}$ | ||

Bardenpho 5 | $0.026\mathsf{\theta}\Delta \mathrm{H}$ | $0.052\mathsf{\theta}\Delta \mathrm{H}$ | $0.073\mathsf{\theta}\Delta \mathrm{H}$ | ||||

Bardenpho 4 | $0.006\mathsf{\theta}\Delta \mathrm{H}$ | $0.012\mathsf{\theta}\Delta \mathrm{H}$ | |||||

MLE | $0.032\mathsf{\theta}\Delta \mathrm{H}$ | $0.060\mathsf{\theta}\Delta \mathrm{H}$ | $0.049\mathsf{\theta}\Delta \mathrm{H}$ | $0.083\mathsf{\theta}\Delta \mathrm{H}$ | |||

A2/O | $0.004\mathsf{\theta}\Delta \mathrm{H}$ | $0.081\mathsf{\theta}\Delta \mathrm{H}$ | $0.168\mathsf{\theta}\Delta \mathrm{H}$ | $0.009\mathsf{\theta}\Delta \mathrm{H}$ | $0.097\mathsf{\theta}\Delta \mathrm{H}$ | $0.194\mathsf{\theta}\Delta \mathrm{H}$ | |

Complet mix | $0.008\mathsf{\theta}\Delta \mathrm{H}$ | $0.104\mathsf{\theta}\Delta \mathrm{H}$ | $0.194\mathsf{\theta}\Delta \mathrm{H}$ | $0.016\mathsf{\theta}\Delta \mathrm{H}$ | $0.130\mathsf{\theta}\Delta \mathrm{H}$ | $0.233\mathsf{\theta}\Delta \mathrm{H}$ | |

Conv. plug flow | $0.009\mathsf{\theta}\Delta \mathrm{H}$ | ||||||

VIP | $0.012\mathsf{\theta}\Delta \mathrm{H}$ | $0.052\mathsf{\theta}\Delta \mathrm{H}$ | $0.104\mathsf{\theta}\Delta \mathrm{H}$ | $0.023\mathsf{\theta}\Delta \mathrm{H}$ | $0.078\mathsf{\theta}\Delta \mathrm{H}$ | $0.146\mathsf{\theta}\Delta \mathrm{H}$ | |

A/O | $0.023\mathsf{\theta}\Delta \mathrm{H}$ | $0.156\mathsf{\theta}\Delta \mathrm{H}$ | $0.270\mathsf{\theta}\Delta \mathrm{H}$ | $0.047\mathsf{\theta}\Delta \mathrm{H}$ | $0.194\mathsf{\theta}\Delta \mathrm{H}$ | $0.324\mathsf{\theta}\Delta \mathrm{H}$ |

WWTPs (Labelled as in [51]) | B | D | E | F | G | H | I | J | K | M | N | O | P | P |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|

Type of treatment | CAS | CAS | CAS | CAS | CAS | EA | EA | EA | EA | EA | EA | EA | EA | EA |

Aeration type * | m | d | d | m | d | m | m | d | m | d | d | m | m | d |

Design flowrate (m ^{3}/d) | 4391 | 27,922 | 25,992 | 18,433 | 54,000 | 489 | 763 | 11,190 | 15,120 | 35,900 | 25,577 | 24,881 | 30,240 | 14,096 |

Q (m^{3}/d) | 4562 | 15,926 | 13,638 | 13,640 | 29,970 | 440 | 638 | 9300 | 12,238 | 18,370 | 22,062 | 27,733 | 29,421 | 12,071 |

Q_{w} (m^{3}/d) | 44 | 482 | 1017 | 876 | 1800 | 9 | 32 | 273 | 946 | 919 | 1999 | 1799 | 1800 | 568 |

R (%) | 77 | 118 | 493 | 180 | 129 | 30 | 175 | 72.8 | 144 | 183 | 123 | 88 | 103 | 115 |

BOD_{5in} (mg O_{2}/L) | 185 | 420 | 129 | 322 | 480 | 368 | 508 | 180 | 452.55 | 390 | 459 | 271 | 324 | 324 |

BOD_{5out} (mg O_{2}/L) | 20 | 58 | 7 | 24 | 20 | 8 | 10 | 8 | 10 | 6 | 15 | 5 | 16 | 16 |

X (mg VSS/L) | 3410 | 2920 | 1055 | 2333 | 4450 | 1758 | 4500 | 2285 | 3265 | 3145 | 3746 | 4405 | 3775 | 4440 |

MLSS (mg TSS/L) | 3680 | 3340 | 1138 | 2687 | 5245 | 2790 | 5020 | 2830 | 4090 | 3700 | 4460 | 5435 | 4620 | 5480 |

θ (h) | 9.2 | 23.7 | 7.5 | 11.5 | 12.6 | 40 | 37.5 | 14.7 | 29.6 | 20.1 | 33 | 20.5 | 24.7 | 30 |

θ_{c} (d) | 9.9 | 19.7 | 1.1 | 4.8 | 7.2 | 71 | 21.2 | 11.8 | 17.2 | 15.6 | 27.4 | 23 | 15.7 | 29 |

F/M (d^{−1}) | 0.14 | 0.12 | 0.39 | 0.3 | - | 0.13 | 0.07 | 0.13 | 0.1 | - | 0.09 | 0.07 | 0.08 | 0.06 |

nbVSS (mg/L) | 19 | 126 | 19 | 48 | 144 | 37 | 152 | 54 | 138 | 117 | 138 | 81 | 97 | 97 |

Nt_{in} (mg N/L) | 56 | 100 | 74 | 99 | 70 | 49 | 82 | 28 | 43 | 67 | 43 | 71 | 93 | 67 |

NH_{4out} (mg N/L) | 22 | 70 | 48 | 47 | 1.4 | 20 | 40 | 18 | 5 | 18 | 1.9 | 5 | 12 | 12 |

NO_{3out} (mg N/L) | 0.2 | 10 | 1 | 1 | 4.5 | 1 | 1 | 0.1 | 1.1 | 5.6 | 1.1 | 2.5 | 1.1 | 11 |

N_{0} (kg O_{2}/(kWh)) | 1.5 | - | - | 1.5 | - | 2.0 | 1.5 | - | 1.5 | - | - | 1.5 | 1.5 | - |

β | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.9 | 0.95 | 0.95 |

α | 0.69 | 0.64 | 0.83 | 0.69 | 0.52 | 0.68 | 0.53 | 0.68 | 0.59 | 0.62 | 0.57 | 0.60 | 0.56 | 0.50 |

C_{L} (mg/L) | 1 | 0.5 | 1.4 | 0.5 | 2 | 0.1 | 0.8 | 2.6 | 0.4 | 0.5 | 0.6 | 0.6 | 0.7 | 1.4 |

T in reactor (°C) | 22 | 24 | 28 | 23 | 25 | 23 | 23 | 12 | 28 | 23 | 31 | 27 | 23 | 27 |

C_{walt} (mg/L) | 8.8 | 8.43 | 7.87 | 8.72 | 8.15 | 8.72 | 8.72 | 10.63 | 7.9 | 8.76 | 7.18 | 8.06 | 8.6 | 8.03 |

Submergence (m) | 6 | 6.1 | 5 | 5 | 10 | 4 | 3.5 | 5.5 | - | 10 | 6 | - | - | 6 |

SOTE | - | 0.39 | 0.41 | - | 0.40 | - | - | 0.30 | - | 0.40 | 0.34 | - | - | 0.30 |

F | - | 0.8 | 0.8 | - | 0.8 | - | - | 0.8 | - | 0.8 | 0.7 | - | - | 0.7 |

e | - | 0.75 | 0.75 | - | 0.75 | - | - | 0.75 | - | 0.75 | 0.7 | - | - | 0.7 |

p_{2}/p_{1} | - | 1.65 | 1.61 | - | 1.66 | - | - | 1.61 | - | 1.66 | 1.50 | - | - | 1.50 |

ΔH return sludge (m) | - | 5 | - | - | - | 5 | 4 | 7 | 3.8 | - | 4 | - | 2.8 | - |

ΔH sludge wasting from R line (m) | - | 10 | 6 | - | - | 7 | - | - | - | - | - | - | ||

ΔH sludge wasting from reactor (m) | - | - | - | - | - | - | - | - | 2.8 | - | 10 | - | - | - |

N (kg O_{2}/(kW.h)) | 0.9 | 2.5 | 3.1 | 0.9 | 1.6 | 1.3 | 0.7 | 1.6 | 0.8 | 2.4 | 1.9 | 0.8 | 0.7 | 1.4 |

Parameter | Typical Value Considered | β Variation | T Variation | C_{L}Variation | P_{2}Variation | α Variation | F Variation | SOTE Variation |
---|---|---|---|---|---|---|---|---|

e (-) | 0.7 | 0.7 | 0.7 | 0.7 | 0.7 | 0.7 | 0.7 | 0.7 |

β (-) | 0.95 | 0.95 to 0.98 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 |

T (°C) | 20 | 20 | 5 to 30 | 20 | 20 | 20 | 20 | 20 |

h (m) | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 |

C_{s,20} (mg/L) | 9.08 | 9.08 | 9.08 | 9.08 | 9.08 | 9.08 | 9.08 | 9.08 |

C_{walt} (mg/L) | 9.25 | 9.25 | 9.25 | 9.25 | 9.25 | 9.25 | 9.25 | 9.25 |

C_{L} (mg/L) | 1 | 1 | 1 | 0.5 to 2 | 1 | 1 | 1 | 1 |

p_{1} (kPa) | 101 | 101 | 101 | 101 | 101 | 101 | 101 | 101 |

p_{2} (kPa) | 154.4 | 154.4 | 154.4 | 154.4 | 151 to 166 | 154.4 | 154.4 | 154.4 |

α (-) | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 | 0.4 to 0.7 | 0.5 | 0.5 |

F (-) | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.65 to 0.9 | 0.8 |

SOTE (-) | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.25 to 0.4 |

N_{d} (kg O_{2}/(kWh)) | 1.7 | 1.7 to 1.8 | 1.8 to 1.6 | 1.8 to 1.5 | 1.8 to 1.5 | 1.4 to 2.4 | 1.4 to 1.9 | 1.4 to 2.3 |

Parameter | Typical Value Considered | β Variation | T Variation | C_{L}Variation | α Variation | N_{0}Variation |
---|---|---|---|---|---|---|

β (-) | 0.95 | 0.95 to 0.98 | 0.95 | 0.95 | 0.95 | 0.95 |

T (°C) | 20 | 20 | 5 to 30 | 20 | 20 | 20 |

h (m) | 10 | 10 | 10 | 10 | 10 | 10 |

C_{s,20} (mg/L) | 9.08 | 9.08 | 9.08 | 9.08 | 9.08 | 9.08 |

C_{walt} (mg/L) | 9.25 | 9.25 | 9.25 | 9.25 | 9.25 | 9.25 |

C_{L} (mg/L) | 1 | 1 | 1 (0.5 and 2) | 0.5 to 2 | 1 | 1 |

α (-) | 0.5 | 0.5 | 0.5 | 0.5 | 0.4 to 0.7 | 0.5 |

N_{0} (kg O_{2}/kWh) | 1.5 | 0.3 | 0.3 | 0.3 | 0.3 | 1.1 to 2.1 |

N_{m} (kg O_{2}/kWh) | 0.64 | 0.64 to 0.67 | 0.64(0.67 to 0.69 for C _{L} = 0.5 mg/L)(0.58 to 0.54 for C _{L} = 2 mg/L) | 0.56 to 0.68 | 0.51 to 0.90 | 0.47 to 0.90 |

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**MDPI and ACS Style**

Silva, C.; Rosa, M.J.
A Comprehensive Derivation and Application of Reference Values for Benchmarking the Energy Performance of Activated Sludge Wastewater Treatment. *Water* **2022**, *14*, 1620.
https://doi.org/10.3390/w14101620

**AMA Style**

Silva C, Rosa MJ.
A Comprehensive Derivation and Application of Reference Values for Benchmarking the Energy Performance of Activated Sludge Wastewater Treatment. *Water*. 2022; 14(10):1620.
https://doi.org/10.3390/w14101620

**Chicago/Turabian Style**

Silva, Catarina, and Maria João Rosa.
2022. "A Comprehensive Derivation and Application of Reference Values for Benchmarking the Energy Performance of Activated Sludge Wastewater Treatment" *Water* 14, no. 10: 1620.
https://doi.org/10.3390/w14101620