The Role of Alternate Oxic–Anoxic Cycles in Full-Scale Sludge Stabilization for Energy Savings

Abstract

Sludge management constitutes a significant share of the operational costs in wastewater treatment. Given the financial and bureaucratic challenges associated with implementing new technologies, process optimization often represents the most feasible approach for existing facilities. This study presents the results of four full-scale batch stabilization tests conducted in the aerobic sludge stabilization unit of a wastewater treatment plant in northern Italy. The objective was to evaluate the potential of alternating oxic–anoxic cycle stabilization in terms of the energy consumption and sludge treatment performance. Operational parameters were monitored and evaluated. Stabilized and dewatered sludge samples, as well as the liquid fraction from the dewatering process, were collected and analyzed. Energy consumption was continuously monitored. Data were normalized and a comparative model was developed to evaluate performance against traditional continuous aeration, using results from previous tests. The findings indicate that alternating cycle stabilization achieved comparable stabilization efficiency to continuous aeration, with an energy demand of about one-third of that required for continuous aeration. Additional benefits of the alternating cycle strategy included improved nitrogen removal and enhanced sludge dewaterability. This experimental study demonstrates how full-scale functional testing in existing treatment units can support process optimization within a circular economy framework, contributing to reduced resource consumption and an improved sludge quality.

1. Introduction

Sewage sludge management represents a significant challenge and a critical issue in water cycle management. Wastewater treatment plants (WWTPs) generate sludge that must either be recovered for material and energy purposes or properly disposed of [1].

Sludge management challenges are closely linked to its quantity and quality, influencing treatment costs and, within a circular economy framework, its potential for reuse and market value [2]. In 2022, the European Union (EU-27) produced an estimated 9–10 million tons of sewage sludge [3]. In Italy, approximately 3.2 million tons of sludge were generated and 3 million tons managed; of this, 54% was disposed of, while 43% was recovered [4]. Despite sludge constituting only 1% of the total incoming water volume at WWTPs, its transportation and disposal can account for more than 30% of a plant’s total operating costs [5,6]. In Italy, recovery/disposal costs in 2021 ranged between 120 and 200 EUR/t [2,7]. Thus, interventions directly at a WWTP can be applied to minimize sludge production and associated management costs [8].

Biological and chemical stabilization are the most commonly employed sludge treatments in Europe [9]. Biological stabilization relies on biological processes to decrease the concentration of suspended solids in sludge, under either aerobic or anaerobic conditions. While aerobic stabilization is operationally simpler than anaerobic digestion and does not cause odor-related issues, it does not produce biogas and, despite its lower initial investment costs, entails higher operational expenses due to the electricity required for aeration [10,11]. Other sludge treatment methods employed to enhance sludge reduction and stabilization include thermal treatment, mechanical disintegration, and chemical oxidation, which can be applied using conventional or advanced techniques [9,10,11,12,13,14].

Energy consumption plays a key role in the selection of sludge treatment technologies, as it represents the second-largest operational expense of WWTPs after personnel costs [15]. Another crucial consideration, particularly significant in full-scale applications within existing WWTPs, is that modifying the plant configuration or implementing new technologies may be impractical, due to economic constraints, regulatory requirements, or space availability. Consequently, optimizing existing technologies and processes becomes a priority for conventional treatment plants with biological stabilization units [5,16]. Importantly, it aligns with the new regulatory framework aimed at achieving energy and climate neutrality [17].

Alternating cycle stabilization involves alternating oxic–anoxic phases within a reactor treating biological excess sludge through intermittent aeration, which is automatically regulated based on dissolved oxygen (DO) and oxidation-reduction potential (ORP) parameters [18]. Time limits are imposed on both phases to optimize performance. Time, DO, and ORP thresholds are adjusted according to the specific operational requirements of a given WWTP [19]. This process is well-established in the water treatment line, where the strategic alternation of aerobic and anaerobic conditions within a biological reactor influences microbial biomass growth [20,21,22]. In general, biomass growth decreases as environmental conditions shift from aerobic to anaerobic states, thereby reducing sludge production by limiting biomass proliferation [11]. Furthermore, the alternating cycles approach enhances nitrification and denitrification processes. The process is relatively simple and cost-effective and can be readily retrofitted into existing wastewater treatment plants or integrated into new system designs [8,11,18].

Some studies investigated the performance of applying alternating oxic–anoxic phases to aerobic sludge digestion, demonstrating volatile suspended solids (VSS) reductions comparable to those achieved in continuously aerated digestion [10]. Hashimoto et al. (1982) [23] reported that nitrogen loads recirculated to wastewater treatment units were substantially reduced, while other studies [24,25,26] have documented nitrogen reduction rates ranging between 25% and 41% when employing alternating cycles. Although some research explored potential energy savings associated with alternating cycles [27,28,29], these aspects have not been thoroughly examined, particularly in full-scale applications.

Based on these considerations, this study aimed to evaluate the feasibility of implementing alternating oxic–anoxic aeration cycles within the aerobic stabilization unit of the sludge line of an existing WWTP at full scale. The performance of this approach will be assessed in terms of sludge stabilization, sludge reduction, nutrients removal, and energy consumption. Data from the tests will also be compared with previous results obtained from a study with continuous aeration in the same unit of the same plant [16]. This assessment, comparing different air supply strategies, will provide valuable insights into the potential benefits of applying alternating oxic–anoxic cycles to sludge stabilization units at full-scale for enhancing the energy and overall process efficiency.

2. Materials and Methods

2.1. The Study Site

This study was carried out at a WWTP in Lombardy, northern Italy. The WWTP has a nominal capacity of 90,000 population equivalents (PE) and operates using an activated sludge process in the water line and an anaerobic stabilization process in the sludge line.

The aerobic stabilization system consists of two adjacent, identical tanks (A and B), each with a volume of 874 m³ and a surface area of 165 m². The tanks are aerated by a SSI-ECD 350 fine bubble disk diffusers system and have dedicated KAESER FBS 660 blowers. The plant currently employs a continuous aeration strategy. Each tank is also equipped with Flygt SR 4530 mixers and probes connected to a programmable logic controller (PLC) to monitor turbidity, total suspended solids (TSS), ORP, DO, and temperature.

Following stabilization, the sludge undergoes post-thickening and mechanical dewatering by a mobile centrifuge from a third company, before being transferred to an external company for either recovery or disposal (Figure 1). Operational data from the plant, covering the period from January 2021 to August 2024, were retrieved from the plant managing company and analyzed using Microsoft Excel (2010). The removal efficiencies of main pollutants, sludge load, and age were calculated on a seasonal basis.

2.2. Experimental Tests

Four full-scale batch tests (AC1 to AC4) were conducted in tank A of the WWTP, using alternate oxic–anoxic cycles, while tank B continued working under standard conditions regularly. The PLC logic was set with a minimum aerobic phase duration of 15 min and a maximum of 60 min, with an ORP threshold of 50 mV. The anaerobic phase was set to a minimum of 10 min and a maximum of 90 min, with an ORP threshold of −180 mV. The tests were conducted at different times of the year and according to the plant operational needs. Alternate cycles were running continuously for 22 days, except for test AC1, during which the aeration was switched off overnight. The blower frequency was set to 30 Hz, its minimum operational level. However, in AC1 and AC2, the frequency was temporarily increased to 50 Hz to support the start of the process. After consultation with the blower manufacturer regarding the feasibility of further frequency reduction, the setting was adjusted to 27 Hz in tests AC3 and AC4.

For all experimental tests, the following standardized procedure was followed. Initially, tank A was emptied and filled with sludge. The mixers were then switched off, allowing the sludge to settle for approximately 5 h. Following sedimentation, the supernatant was removed, and the tank was refilled. An initial dewatering process was carried out with a mechanical centrifuge POLAT S570 (at 2400 rpm, for 5 h) with the addition of a 3.5% diluted polyelectrolyte solution (HIDROFLOC CL 1978 RC9) on day 0. The volume of sludge processed corresponded to the capacity of a single container, approximately 170–210 m³. Then, the alternating aeration cycles commenced and continued for 22 days. On the final day of the experiment, a second dewatering step was performed. Throughout the tests, the ORP was continuously monitored using the probe Endress+Hauser CPS12D installed in the tank connected to the PLC. In AC1 and AC2, before the installation, the automatic probe (Endress+Hauser Oxymas COS61D), DO, and sludge temperature (T) were measured manually using a Hach HQ40 portable probe three times per day at five different locations, with each measurement taken at a depth of approximately 4 m. Air temperature data were obtained from the nearest meteorological station in Sarnico (BG), and retrieved from the Regional Agency for Environmental Protection (ARPA) website (https://www.arpalombardia.it, accessed on 30 September 2024). During weekdays, sludge samples from stabilization tank were collected daily and analyzed at the company’s laboratory to determine TSS and volatile suspended solids (VSS) concentrations. During the dewatering, five samples of dewatered sludge were collected, one for each hour, and analyzed in the laboratory to determine TSS and VSS. A 1 L composite sample of the liquid fraction from dewatering, consisting of three sub-samples collected each hour, was analyzed at the company’s laboratory for total nitrogen (TN), ammoniacal nitrogen (NH₄⁺-N), biochemical oxygen demand (BOD), total chemical oxygen demand (COD), phosphorus (P), TSS, and VSS.

Based on initial results observed in AC1, which indicated a potential improvement in nitrogen removal, a manual dewatering procedure was introduced in subsequent tests and applied to sludge samples collected twice per week. The manual dewatering process was performed using a self-made centrifuge, consisting of a plastic container with a drum lined with a cotton rag. A 1 L sludge sample was mixed with polyelectrolyte at a 1:10 ratio and centrifuged for 30 s to separate the liquid and solid fractions. The liquid fraction was then filtered using a nylon cloth. The same physicochemical parameters measured in the liquid fraction from mechanical dewatering were also analyzed in the manually processed samples. Additionally, a manual dewatering sample was prepared on the same day as the initial and final dewatering to allow for comparison of analytical results and validation of the findings obtained through mechanical dewatering. A summary of the test parameters applied to each test is provided in

Table 1. Date, duration, and main settings of the experimental tests under alternate oxic–anoxic cycle aeration.

Test	Period	Duration [Days]	Note
AC1	1 December 2022–22 December 2022 (winter)	22	Alternate cycle off during the night; blowers frequency: 30 Hz (50 Hz the first 3 days)
AC2	11 April 2024–2 May 2024 (spring)	22	Blowers frequency: 30 Hz (3 days), 50 Hz (7 days), 30 Hz (the remaining 12 days). Manual dewatering data collection
AC3	24 June 2024–15 July 2024 (summer)	22	Blowers frequency: 27 Hz. Manual dewatering data collection
AC4	2 September 2024–23 September 2024 (autumn)	22	Blowers frequency: 27 Hz. Manual dewatering data collection

.

Data on current intensity were collected daily from the PLC to calculate the energy consumption of the blower. The absorbed power of the blower, P_w [kW], was calculated with a one-minute time interval using Equation (1):

P_w = √3 · I · V · cos(φ)

(1)

where I is the current intensity in [A], V is the voltage expressed in [V], and cos(φ) is the power factor. The values of V and cos(φ) were obtained from the blower’s nameplate specifications, equal to 398 V and 0.9, respectively. The adsorbed power of mixers was calculated with Equation (1) using a value of cos(φ) equal to 0.73.

To evaluate the sludge’s dewaterability over time, the quantities of total sludge produced during each dewatering session were calculated and standardized per 1 m³ of sludge in the stabilization tank to facilitate comparative analysis, distinguishing between dry matter and water content. The total amount of dewatered sludge (sludge_dew) derived from 1 m³ of stabilization sludge and the corresponding water content (water_dew) were calculated as follows (Equations (2) and (3)):

$${sludge}_{dew}={\displaystyle \frac{{TSS}_{conc,stab} \times 1 {\mathrm{m}}^3}{{TSS}_{dew}}}$$

(2)

$${water}_{dew}={sludge}_{dew}-{TSS}_{stab}$$

(3)

where TSS_conc,stab is the concentration of TSS in the stabilized sludge [kg/m³], TSS_stab is the corresponding quantity [kg] in 1 m³, and TSS_dew is the average value of TSS from laboratory analysis of the five samples collected during each dewatering session.

Since mechanical dewatering reduces water content without effecting the dry matter, the TSS quantity in 1 m³ of stabilization sludge remained constant in the resulting dewatered sludge.

2.3. Model and Comparison in Homogeneous Conditions

Since the four tests were conducted at different times of the year and involved sludge with varying initial characteristics, a model was developed to enable a meaningful comparison of the results. This process involved:

• Modeling the stabilization kinetics;
• Defining homogeneous conditions and scenarios;
• Simulating performance under homogeneous conditions.

Moreover, as mentioned in Section 1 (Introduction), the model was similarly applied to results obtained from previous experimental tests conducted in the same plant with continuous aeration [16].

2.3.1. Modeling the Stabilization Kinetics

The reduction of VSS was assumed to follow a first-order reaction (Equation (4)):

[VSS]_t = [VSS]₀ ^−k

(4)

where [VSS]_t and [VSS]₀ were VSS concentrations at time t and time t = 0, respectively, and k the reaction rate. For each experimental test, the VSS values measured from all samples on the day the sludge temperature stabilized were used in the model. The Excel exponential interpolation function was applied to determine k and the corresponding coefficient of determination (R²) (Equation (5)).

y = a · e^{−k x}

(5)

The value of k was normalized to standard homogeneous temperature conditions using the simplified Van’t Hoff–Arrhenius equation (Equation (6)):

k_(20°) = k · θ ^(T_(20°)^−T)

(6)

where k_(20°) was the reaction rate at the temperature of 20 °C; the coefficient θ was assumed to be equal to 1.04.

2.3.2. Definition of Homogeneous Conditions and Scenarios

The parameters considered during the simulations included VSS reduction rate, variations in dewaterability, specific energy consumption, and changes in the TN concentration in the recirculated liquid fraction.

The representative k₂₀ value for alternating cycles was calculated as the average of the k₂₀ values obtained from AC2 and AC3. Data from AC1 and AC4 were excluded because, in AC1, some operational parameters differed from those of the other alternating cycle stabilization tests, and in AC4, an unexpected decrease in sludge temperature was observed at day 18, possibly indicating a malfunction during the test.

For continuous aeration, the representative k₂₀ value was determined as the average of the values obtained from tests P1 (0.0126 d⁻¹) and P2 (k₂₀ = 0.0138 d⁻¹), resulting as equal to 0.013 d⁻¹.

The percentage of TSS in the dewatered sludge over time was calculated for each test using Equation (7):

TSS(t) = TSS₀ + m_TSS · t

(7)

where m_TSS represents the slope of the trendline identified through the linear interpolation of the TSS percentage values measured during the dewatering process. Similar to k₂₀, the representative m_TSS value for alternating cycles was taken as the average of m_TSS obtained from AC2 and AC3. For continuous aeration, the representative m_TSS value was assumed to be the same as that obtained from P1 (−0.045 d⁻¹). TSS₀ was calculated as the average value of results at time 0 of all tests.

The representative daily energy consumption for the alternating cycles tests was assumed to be 250 kWh/d. This value was obtained by averaging the observed data during the experimental tests, excluding the initial days when the alternating cycles were not yet fully operational. The specific energy consumption was then calculated by dividing the daily energy consumption by the tank volume (709.5 m³). The representative specific energy consumption for continuous aeration was considered equal to that observed during test P1 (1.16 kWh/m³ d).

The concentration of TN in the liquid fraction was calculated using Equation (8):

TN (t) = TN₀ + m_TN · t

(8)

where TN₀ was the initial percentage of nitrogen, assumed to be 20%. TN (t) was the nitrogen percentage after a certain time t, and m_TN was the slope of the trendline identified through the linear interpolation of the experimental TN concentration values in the liquid fraction from mechanical dewatering. To determine the representative value of m_TN for alternating cycles, a reduction in the TN concentration from 100 mg/L to 20 mg/L over a period of five weeks was assumed. For continuous aeration, the representative value of m_TN was determined based on an assumed increase in total nitrogen concentration from 100 mg/L to 300 mg/L over a period of four weeks.

Initial conditions and process temperatures were determined based on experimental results, for three different scenarios: summer, winter, and autumn/spring. A schema of the scenarios and simulation carried out is reported in Figure S1.

2.3.3. The Performance Simulation Under Homogeneous Conditions

The simulations were run for stabilization periods of 1 up to 5 weeks, under the continuous and alternate cycle aeration strategies, to determine the amount of VSS and of TSS and the VSS/TSS ratio in 1 m³ of stabilization sludge, in batch, and with continuous sludge feed into the stabilization tank; the percentage of TSS in the dewatered sludge; the amount of water in the dewatered sludge produced from 1 m³ of stabilization sludge; the total amount of dewatered sludge produced from 1 m³ of stabilization sludge; the energy consumption per 1 m³ of stabilization sludge treated; the TN concentration in the liquid fraction from dewatering; and the percentage of the TN load recirculated to the head of the plant with the liquid fraction relative to the total influent load.

The variation of VSS concentration over time was calculated using Equation (4) for the three scenarios under the two different aeration strategies (continuous aeration and alternating cycles). The concentration of VSS with continuous sludge feed into the stabilization tank was calculated using Equation (9), applicable for first-order reactions in fully mixed reactors:

$${\theta }_H={\displaystyle \frac{{VSS}_i-{VSS}_u}{k\ast {VSS}_u}}$$

(9)

where θ_H is the sludge retention time, k the VSS abatement rate, and VSSi and VSSu are the VSS concentrations at the inlet and at the outlet of the tank, respectively.

The concentration of TSS over time was calculated as the difference between the initial TSS concentration and the concentration of VSS removed, for both batch process and continuous feed process. The percentage of the TN load in the inlet returned to the head of the plant (N_recirculated) was calculated using Equation (10):

$$N_{\mathit{recirculated}}={\displaystyle \frac{TN \left(t\right) \ast V_{i,ret}}{N_{i,med}}}$$

(10)

where N_i,med is the average TN load in the inlet [kg/d] calculated over the whole period analyzed and V_i,ret is the volume of the liquid fraction from the dewatering process returned to the head of the plant for each dewatering. V_i,ret is calculated as in Equation (11):

$$V_{i,ret}={\displaystyle \frac{V_{ret,year}}{365}}$$

(11)

which is the total quantity of liquid fraction from the dewatering process returned to the head of the plant in a year, divided by the number of days in a year (365 days). The V_ret,year was calculated as the difference between the sludge treated and dewatered in a year.

3. Results and Discussion

3.1. Plant Performance

The average flowrate of the WWTP in the studied period was 32,414 m³/d. The plant showed removal performances >93% for BOD, >91% for COD, >79% for TN, and >81% for P, with an exception for the last trimester (June–August 2024) corresponding to a period of intense precipitations that led to higher flowrates and, consequently, a dilution effect on incoming loads. The sludge load values remained relatively low, in the range 0.04–0.11 kg_BOD5/kg_TSS∙d. The sludge age varied between 7.0 and 19.8 days. The yearly sludge production corresponded to 5,458,220 kg in 2021, 5,787,060 kg in 2022, and 4,867,480 kg in 2023.

3.2. Experimental Tests

The air temperature (T_air) varied depending on the time of the year in which the experimental test was performed, with lower temperatures in December 2022 (AC1) and higher temperatures in June–July 2024 (AC3) and in September 2024 (AC4). The average temperatures in the sludge after stabilization (T_sludge) were lower in AC1 (16.3 °C), while showing values > 26 °C for the other tests (

Table 2. Main results from tests AC1, AC2, AC3, and AC4.

Test	Tair [°C] Min–Max	Average Tsludge [°C]	VSSi–VSSf [g/L] Δ (%)	(VSS/TSS)i–(VSS/TSS)f [%]	Δdis [%]	TSSi–TSSf [%]	TNi–TNf [mg/L]	Ec [kWh/m3]
AC1	1.3–8.5	16.3	9.3–7.0 (25)	75–71	15	21–22	65–38	5.3
AC2	6.3–17.9	26.9	13.4–9.1 (32)	69–59	28	21–23	117–49	12.5
AC3	19.3–28.7	33.5	14.6–11.2 (23)	62–56	24	24–27	93 –14	9.2
AC 4	25.0–31.4	28.5	9.4–8.0 (15)	65–61	17	18–20	130–20	7.1

).

In test AC1, the concentration of TSS decreased from 12.4 g/L to 9.9 g/L and the concentration of VSS from 9.3 g/L to 7.0 g/L, resulting in a reduction of 25% (Δ). The VSS/TSS ratio diminished from 75% to 71%. The quantity of dewatered sludge decreased from 56.1 kg at 20.9% TSS to 47.5 kg at 21.5% TSS, resulting in a 15% reduction (Δ_dis) in the volume of sludge generated and an enhancement in the dewaterability. In the liquid fraction from the initial and final dewatering stages, the concentration of TN decreased from 64.5 mg/L to 37.7 mg/L. The concentration of BOD and COD decreased from 1326 to 899 mg/L and 560 to 440 mg/L, respectively, while TP increased from 1.3 to 17.9 mg/L. Test AC1 was associated with an energy consumption (E_c) of 3909 kWh and a specific energy consumption of 5.3 kWh/m³.

In test AC2, the concentration of TSS decreased from 19.5 g/L to 15.3 g/L and the concentration of VSS from 13.4 g/L to 9.1 g/L (Δ = 32%); the VSS/TSS ratio diminished from 69% to 59%. The quantity of dewatered sludge decreased from 112 kg at 21% TSS to 82.3 kg at 22.8% TSS, resulting in a 27% reduction in the volume of sludge generated and an enhancement in the dewaterability. In the liquid fraction from the initial and final dewatering stages, the concentration of TN decreased from 117 mg/L to 49 mg/L. The concentration of BOD and COD decreased from 750 to 150 mg/L and 1730 to 447 mg/L, respectively, while TP decreased from 38.5 to 26.8 mg/L. Test AC2 was associated with an energy consumption of 8860 kWh and a specific energy consumption of 12.5 kWh/m³.

In test AC3, the concentration of TSS decreased from 23.6 g/L to 20 g/L, while the concentration of VSS decreased from 14.6 g/L to 11.2 g/L (Δ = 23%); the VSS/TSS ratio diminished from 62% to 56%. The quantity of dewatered sludge decreased from 121.7 kg at 24.1% TSS to 94.3 kg at 26.9% TSS, indicating a 23% reduction in the volume of sludge generated and an enhancement in the dewaterability. Additionally, the concentration of TN in the liquid fraction collected during the initial and final dewatering stages decreased from 90 mg/L to 35 mg/L. The concentration of BOD and COD decreased from 270 to 112 mg/L and 356 to 159 mg/L, respectively, while TP decreased from 37.1 to 29.3 mg/L. AC3 was associated with an energy consumption of 6520 kWh and a specific energy consumption of 9.2 kWh/m³.

AC4 showed a decrease in the concentration of TSS from 23.6 g/L to 20 g/L, while the concentration of VSS fell from 14.6 g/L to 11.2 g/L (Δ = 15%), resulting in a reduction of the VSS/TSS ratio from 65% to 61%. The quantity of dewatered sludge produced decreased from 94.4 kg at 18% TSS to 79.3 kg at 19.8% TSS, indicating a 16% reduction in the volume of sludge generated and an enhancement in the dewaterability. Additionally, the concentration of TN in the liquid fraction collected during the initial and final dewatering stages decreased from 121 mg/L to 19.3 mg/L. The concentration of BOD and COD decreased from 333 to 129 mg/L and 1728 to 672 mg/L, respectively, while TP decreased from 6.1 to 5.3 mg/L. AC4 was associated with an energy consumption of 5017 kWh and a specific energy consumption of 7.1 kWh/m³. The main results from the four experimental tests are summarized in Table 2.

All tests demonstrated improvements in dewaterability and nitrogen removal. The variability in the VSS abatement results was attributed to differences in the quality of the sludge treated in each test, as well as other factors such as the air and sludge temperature. The observed decrease in the TP concentration during the stabilization process observed in AC2, AC3, and AC4 was unexpected, as it was anticipated to increase due to the composition of the biomass [10]. This reduction could be attributed to the dewatering method, which, based on centrifugation and filtration, may have retained significant amounts of this pollutant in the solid fraction of the sludge.

Figure 2 reports the energy consumption measured for the blower, mixer, and the total across the tests. In all tests, energy consumption was primarily driven by blower C1. During the initial phase of the test, the energy consumption was higher compared to the later phase. This difference can be attributed to variations in the duration of the aerobic phases during the early cycles, as opposed to the extended anaerobic phases in the later cycles, where phase switching was mainly governed by the ORP setpoint. The peak in energy consumption observed in AC2 from days 4 to 10 corresponds to an increase in the blower frequency from 30 Hz to 50 Hz.

Notably, the lower total energy consumption observed in AC1 was due to the implementation of oxic–anoxic cycles exclusively during the day, further influenced by an initially higher blower frequency setting. Conversely, the higher energy consumption in AC2 was associated with the increased blower frequency on certain days. These variations, which could not be fully controlled or required adjustment due to the tests being conducted at full-scale in an operational WWTP, were accounted for when developing the model to compare the performance under standardized conditions.

3.3. Stabilization Kinetics and Comparison Parameters

The obtained values of k_P were equal to 0.0106 d⁻¹ (AC1), 0.0187 d⁻¹ (AC2), 0.0268 d⁻¹ (AC3), and 0.0153 d⁻¹ (AC4). The respective k₂₀ values resulted to be 0.0123 d⁻¹ (AC1), 0.0143 d⁻¹ (AC2), 0.0159 d⁻¹ (AC3), and 0.0109 d⁻¹ (AC4). The representative k₂₀ value for modeling alternating cycles was determined to be 0.015 d⁻¹.

The m_TSS values calculated for each test were 0.0295 d⁻¹ (AC1), 0.0829 d⁻¹ (AC2), 0.1362 d⁻¹ (AC3), and 0.0857 d⁻¹ (AC4). The representative m_TSS and m_TN values for modeling alternating cycles were calculated as 0.11 kg/(m³ d) and −2.29 g/(m³ d), respectively. TSS₀ resulted as equal to 21.32%.

Table 3. Parameters used for modeling performances under homogeneous conditions for alternate cycles.

Parameter	Scenarios
	Summer	Winter	Autumn/Spring
Sludge temperature [°C]	25	15	20
VSS/TSS [%]	65	75	70
TSS [kg/m3]	20	20	20
VSS [kg/m3]	13	15	14
Dewaterability [%]	21.32	21.32	21.32
TN [g/m3]	100	100	100
k20° [d−1]	0.15	0.15	0.15
mTSS [kg/(m3 d)]	0.1	0.1	0.1
mTN [g/(m3 d)]	−2.29	−2.29	−2.29
Daily specific energy consumption [kWh/(m3 d)]	1.16	1.16	1.16

summarizes the parameters derived from the tests, which were used to model the performance of the alternating cycle strategy under homogeneous conditions.

3.4. Performance Simulations Under Homogeneous Conditions

The results of the simulations comparing alternating cycle and continuous aeration strategies, in batch, reflected a strong similarity in the reduction yields of VSS, TSS, and of the VSS/TSS ratio, suggesting similar stabilization efficiencies. In the summer scenario, when considering a duration compatible with the plant’s operational requirements of 2 weeks, the difference in the VSS concentration was 4%, while the difference in the TSS concentration was 2%. After five weeks, these differences remained minimal, with an 8% difference in the VSS concentration and a 4% difference in the TSS concentration. Similarly, after 2 weeks the VSS/TSS ratio was 60% for continuous aeration and 59% for alternating cycles. After five weeks, the difference remained negligible (2%) (Figure 3,

Table 4. Differences [%] in performance observed between the two aeration strategies tested under the three scenarios assessed after 5 weeks, so the maximum time to which the model was run, and 2 weeks, which was assumed to be a compatible operating period for the plant object of this study.

Difference Between Continuous Aeration and Alternate Cycles (%)	Summer		Scenarios Winter		Autumn/Spring
	2 Weeks	5 Weeks	2 Weeks	5 Weeks	2 Weeks	5 Weeks
VSS abatement—batch	4	8	2	6	3	7
TSS abatement—batch	2	4	2	4	2	4
VSS abatement—continuous feeding	3	5	2	4	2	5
TSS abatement—continuous feeding	2	3	1	3	2	3
VSS/TSS	2	4	1	2	1	3
Dewaterability	11	24	11	23	11	24
TN recirculated	62	92	62	92	62	93
Energy consumption	70	70	70	70	70	70

). The performance under continuous sludge feeding conditions showed similar results. After 2 weeks, the difference in the VSS concentration was 3%, while the difference in the TSS concentration was 2%. After five weeks, the difference in the VSS concentration was 5%, while the difference in the TSS concentration was 3%. The reduction rates of VSS and TSS were slightly lower under continuous sludge feeding compared to those achieved when operating in batch mode (see Supplementary Materials, Figure S2). For the winter and autumn/spring scenarios, the results were similar, confirming a negligible influence of the aeration strategies adopted on the stabilization performances (Table 4, Figures S3 and S4). The negligible effect of the adoption of alternating oxic–anoxic cycles on VSS abatement was in line with what was reported in the literature [10].

An advantage of alternating cycle stabilization was the enhanced removal of TN from the sludge, driven by the alternation of aerobic and anaerobic conditions, which triggers a nitrification–denitrification mechanism (Figure 4, Table 4). Figure 3 illustrates the trends in the percentage of TN recirculated with the liquid fraction in simulations conducted under summer conditions, relative to the TN load entering the plant. Trends exhibited opposing behaviors between the two aeration strategies. Under continuous aeration, an increasing trend was observed: TN concentrations increased due to cell lysis that released intracellular nitrogen [10,30,31]. Under the alternating cycles strategy, the trend was decreasing; the alternating oxic–anoxic conditions allowed for biological nitrogen removal through the nitrification–denitrification pathway, leading to a reduction in TN in the liquid phase [32,33,34,35]. After two weeks, the reduction of TN was about 32% under alternate cycle aeration strategies, in line with other studies [32,33]; simulations conducted under winter and autumn/spring conditions showed a similar pattern (Figures S5 and S6).

The simulation results showed that the alternating cycle stabilization process improved the sludge dewaterability, leading to lower dewatered sludge production compared to the continuous aeration process. In the summer scenario, after two weeks of treatment, the amount of dewatered sludge produced by the alternating cycle model dropped from the initial value of 93.8 kg to 75.1 kg, while under continuous aeration, it decreased to 84.2 kg. After five weeks, the sludge produced by the alternating cycle model was 55.9 kg, while the sludge produced by the continuous aeration model was 73.3 kg. Similar results were obtained under the winter and spring/autumn scenarios (Tables S1–S3). The improved dewaterability under alternating cycles aeration may result from a combination of mechanisms contributing to more efficient water release during dewatering. The alternation between aerobic and anaerobic conditions promotes microbial stress and increased cell lysis, leading to a greater breakdown of organic matter [36]. This can reduce the amount of extracellular polymeric substances (EPS), which some studies reported to improve dewaterability [32,37,38,39]. Alternating redox conditions can lead to more compact and better-structured flocs, enhancing the water release during mechanical dewatering [40]. Finally, alternating cycles may create conditions for selecting microbial communities more efficient at reducing the bound water content [41,42] and helping suppress filamentous bacteria, which can impact the sludge structure, negatively affecting its dewaterability [43].

The major advantage of the alternating cycle strategy lies in energy saving [44]. The specific energy consumption remained unchanged across seasons, since its calculation was not influenced by the characteristics of the sludge quality or the temperature. The energy consumption after 2 weeks was 16.3 kWh for the continuous aeration strategy and 4.9 kWh for the alternate cycle strategy; after 5 weeks, the simulation reported a consumption of 40.7 kWh for the continuous aeration strategy and 12.3 kWh for the alternate cycle strategy. According to the simulation results, the alternating cycles strategy exhibited a 70% reduction in energy consumption compared to the continuous aeration strategy (Figure 5, Table 4). This reduction can be attributed to the decreased use of the blower, which remains inactive during anaerobic stabilization phases, and to the lowering of the blower frequency to its minimum while still maintaining a good performance. A reduction in energy consumption leads to lower energy purchase costs thanks to potential savings that can be allocated toward achieving energy neutrality. However, the frequent switching of blowers on and off may require more intensive maintenance or reduce the equipment’s lifespan, potentially impacting overall plant expenses. Energy savings of implementing an alternate oxic–anoxic cycle compared to aerobic stabilization in real plants were estimated in a few studies. Nardelli et al. [45] carried out an experimentation on three oxidation plants in the pre-alternate cycle (AC) and AC configurations. The specific energy consumption, on average, under the pre-AC configuration, of the three plants was of about 0.300 kWh/(PE d) and decreased to 0.120 kWh/(PE d) in the WWTP1, to 0.211 kWh/(PE d) for WWTP2, and 0.167 kWh/(PE d) for WWTP3, under AC conditions. The authors acknowledged that the energy savings (13–26%) were consistently associated with the duration of the anoxic phase, characterized by having the blowers off and mixers on. A study from Ranieri et al. [46] reported an average consumption of energy in the aerobic digestion of WWTP in two areas of Italy of about 1.02 kWh/m³, that is in line with our results for continuous aeration.

The results were similar under the three scenarios analyzed (Figures S7 and S8).

All the tests conducted in this study were performed in batch mode on a single plant, which inherently limits the conclusions that can be drawn. Future studies should focus on evaluating the effectiveness of alternating cycle stabilization under real conditions, with continuous sludge feeding. Additionally, to draw more generalizable conclusions, it will be essential to repeat the tests at other wastewater treatment plants.

4. Conclusions

Four full-scale batch experimental tests were conducted in the aerobic sludge stabilization unit of a WWTP to assess the performance of alternating oxic–anoxic cycles in terms of the stabilized sludge quality, reduction in dewatered sludge volume, and energy consumption. The normalized results were compared with those from the conventional continuous aeration strategy. The findings indicated that alternating aeration cycles in sludge stabilization led to a significant optimization of energy consumption due to the decreased use and optimized frequency of the blower, reducing it by approximately two-thirds. Additionally, alternating aeration cycles demonstrated advantages in nitrogen removal and improved sludge dewaterability, while not affecting the sludge stabilization performance. This study, based on data from a full-scale operational plant, provides evidence that adopting alternating aeration strategies can enhance the performance and reduce operational costs in existing plants, without requiring high capital investment.