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Article

Evaluating Energy Efficiency Parameters of Municipal Wastewater Treatment Plants in Terms of Management Strategies and Carbon Footprint Reduction: Insights from Three Polish Facilities

by
Iwona Kłosok-Bazan
1,
Adam Rak
1,
Joanna Boguniewicz-Zabłocka
1,*,
Anna Kuczuk
1 and
Andrea G. Capodaglio
2
1
Department of Thermal Engineering and Industrial Facilities, Faculty of Mechanical Engineering, Opole University of Technology, 45-758 Opole, Poland
2
Department of Civil Engineering & Architecture, University of Pavia, 27100 Pavia, Italy
*
Author to whom correspondence should be addressed.
Energies 2024, 17(22), 5745; https://doi.org/10.3390/en17225745
Submission received: 6 October 2024 / Revised: 3 November 2024 / Accepted: 13 November 2024 / Published: 17 November 2024
(This article belongs to the Special Issue Advances in Wastewater Treatment 2024)
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Abstract

:
Water management in cities is a critical factor for sustainable growth and development. Satisfying the current needs with respect for the future is not possible without properly managed water and wastewater systems. An essential element of wastewater systems is the wastewater treatment plant (WWTP). The nexus between wastewater treatments and energy demand is a well-known problem. In times of energy crisis, effective energy management in this critical infrastructure is a key task. The purpose of this article is to analyze WWTPs’ energy consumption with regard to proposed management strategies for managers, designers and decision makers. A detailed analysis of WWTP operational parameters and a proposal of improvement actions will be useful for applicability and benchmarking studies. Estimating the carbon footprint (CF) of selected WWTPs considering the indirect emissions due to energy consumption is an important step for developing energy neutrality of WWTPs. Due to the desire to deepen research in the area of a complex phenomenon, which is the energy management system in WWTPs, the research undertaken herein is based on the case study method of three water and sewage companies operating southwestern Poland. Each urban area has different specificities, natural conditions and needs. The presented results of the analyses may be the basis for developing directions for changes in national policy, other benchmarking studies, and improving the energy management system in WWTPs.

1. Introduction

The water supply is one of the most important services that ensures the efficient functioning of a community. Water is a non-substitutable foodstuff and guarantees minimum standards of hygiene; as such, it is an indispensable resource for meeting basic human needs. Water supply systems are an infrastructure designed to collect, treat and supply water for people and industry. These systems have their own specifics, because they are complicated technical structures in which individual objects play different roles. What is more, water supply systems belong to the so-called “critical infrastructure”, which is so essential that their continued operation is required to ensure the security of a given nation, its economy and the public’s health and/or safety [1]. Cities devote significant resources and efforts to the implementation of such essential services that are also major energy consumers, as they are responsible for nearly 35% of the energy consumption of all municipal public facilities [2,3], and are important sources of global greenhouse gas emissions; on average, CO2eq emissions for water services are estimated to be 10.6 kg/m3 of the water consumed [4]. Due to current economic, social, and regulatory pressures, it is essential to assess and improve the energy efficiency of existing and new service facilities. Concerning only wastewater treatment, the specific electric energy consumption of a wastewater treatment plant (WWTP) depends on many factors, including inflow quality, the WWTP’s capacity, the technology used, and the climate. WWTP energy consumption generally ranges from 0.4 to 0.9 kWh/m3 treated wastewater [2,5,6]; their energy efficiency has become increasingly critical, both due to energy costs and GHG emission reduction factors. For continued sustainability of the water sector, the most energy-intensive processes need to be identified, on a case-by-case basis, and either optimized or substituted with more efficient technologies [7]. Wastewater treatment plants (WWTPs) could potentially be transformed into self-sufficient and resource recovery facilities [8]. Several significant challenges have recently accumulated along the line for municipal water services. EU member states (along with other nations) have abruptly discovered their degree of dependence on fossil fuel imports from other nations [9]; therefore, energy efficiency, savings and recovery are also becoming primary goals in the water sector [10]. Geopolitical situations in Europe have considerably impacted the global economy, not just in the energy sector, but also in the availability of essential commodities. For example, as indicated by an EU Parliament resolution, recovery of ammonia, an essential constituent of fertilizer, from wastewater could partly substitute imported industrial sources while avoiding considerable GHG emissions [11,12,13]. The optimization of WWTP energy balance and of the related carbon footprint is a key contemporary task for facility operators that can contribute to increasing the reliability of operation of such critical infrastructures. This paper deals with energy management issues, and related GHG emissions, in WWTPs, based on a case study analysis of three facilities operating in southwestern Poland. The results of this analysis could indicate solutions for better energy management systems in these and other similar WWTPs and constitute the basis for developing guidelines for national policy improvement. The analysis could also lead to the introduction of measures to minimize individual WWTPs’ impact on GHG emission reduction.
Energy is an important component of WWTPs’ overall operational cost; it is generally the second-largest factor after personnel costs. WWTPs’ energy requirements are influenced by a multitude of factors such as demand increases with flowrate and pollutant loads; however, in general, larger facilities (PE > 100,000) have lower specific (per m3 wastewater treated) consumption rates, although there is no clear linear relationship between capacity and energy cost, as this depends on implemented operational optimization strategies [13].
Technology is also important. Traditional facilities designed as conventional biological activated sludge processes may require over 50–60% of the overall energy requirements, for the mere operation of compressors for air supply; this fraction further increases with the introduction of more performant membrane bioreactor (MBR) processes, which need additional aeration for media scouring, and increased pumping requirements. Nitrogen removal (nitrification/denitrification) implies higher oxygen consumption (e.g., for complete nitrification) [2]. A sizable (up to 15%) contribution to energy consumption can also be due to internal sludge handling (mixers and recirculation pumps). Effluent disinfection might also be quite energy-intensive; UV lamps may require up to 0.11 kWh/m3 treated [14].
Energy efficiency is also strongly related to a facility’s correct design, i.e., the match between planned and actual operational conditions; a plant working according to its design capacity works more efficiently (lower specific energy consumption) than one (with similar technology) with over-estimated design capacity [15]. Regulations also affect a facility’s energy demand; increasingly stringent effluent quality limits generally imply increased energy input either by treatment extension or more energy-demanding processes (e.g., MBR, advanced oxidation processes—AOPs), although the latter may show higher performance in terms of specific energy per mass unit of removed pollutants [16].
Studies have shown that WWTP energy consumption could be reduced up to 80% by implementing systematic programs to monitor and manage its use [17], by optimizing process operation [18,19,20], or by adopting alternative process technologies, e.g., by substituting energy-intensive aerobic processes with anaerobic technologies which simultaneously eliminate energy requirements and allow its recovery in the form of methane-rich biogas [21]. Many studies emphasize that it is essential to assess and improve the energy efficiency (EE) of existing facilities due to social and administrative pressures in order to ensure the long-term sustainability of the sector [22].
As every WWTP is faced with the need to assess critical areas where the most energy is consumed, energy audits should be used to assess the energy intensity of unit processes [23], which will help to identify areas where energy savings are possible; energy management systems, i.e., tools to support energy efficiency improvement in industry, could then be applied. These consist of a set of procedures and practices that ensure systematic planning, analysis, control, monitoring and improvement of energy use [24]. Since 2011, ISO 50001 “Energy management systems—Requirements with guidance for use” has been the global standard for energy management systems and is a key aspect of energy efficiency policy in industry [25]. Its implementation is intended to help organizations and industries to improve energy consumption, protect the environment and save production costs, without restricting the continuation of their activities, while fulfilling their corporate social responsibility [26]. The methodology has been applied to water and wastewater systems, as well as to many industrial sectors [27,28].
The implementation of energy benchmarking at wastewater treatment plants in Australia, as a subset of ISO 50001, and the subsequent optimization of their energy efficiency has brought tangible benefits to public water companies [29]. Results of ISO 50001 implementation in industrial settings also confirm that this approach enables a reduction in electricity consumption in cement production by 25%, which translated into CO2 emission reduction. In a comprehensive study, McKane et al. [30] state that under ISO 50001 management, cumulative energy savings in the industrial and service sectors of around 105 EJ could be achieved by 2030, with economic savings of nearly USD 700 billion and 6500 million Mt of avoided CO2 emissions.
The use of renewable energy sources (RES) based on distributed power generation systems could help individual WWTPs or their units to become energy-self-sufficient; such sources are being developed all around the world, and wastewater has been indicated as one such resource [31]. The most commonly exploited source of self-produced energy at WWTPs is biogas, produced by fermentation of excess biological solids. When used in CHP generators, it is seen as a sustainable way to recover energy from wastewater treatment plants [32]. In a case study of a WWTP in Poland, the energy recovered from wastewater and sewage sludge covered about 83% of the plant’s demand [33]. In the US, the City of Gresham’s WWTP was reported as the first facility nationally to become net energy-positive through biogas generation and recovery, saving about USD 500,000/y in energy bills [34]. WWTP-produced biogas can also contribute to the biofuels for transport EU strategy [35]. Other solutions using alternative sources of WWTP-derived clean energy include heat pumps for thermal energy recovery from wastewater heat [36,37,38]. Wastewater-recovered thermal energy has also been used as energy exported to district heating systems [39,40]. In Germany, analysis of WWTPs’ potential to provide ancillary services to power grids has suggested promising developments [41,42,43].
The carbon footprint of a WWTP is a measure of the total amount of greenhouse gases (GHGs) emitted as a result of its operations and production processes. It is calculated using data on energy and fuel consumption, emissions from production processes and other factors that affect GHG generation by the WWTP. Onsite direct CO2 emissions from organic matter oxidation are usually considered climate neutral, due to wastewater organics’ biogenic origin; N2O and CH4 are also direct GHG emissions from treatment processes. The former is generated during nitrification/denitrification processes, and has a much higher global warming potential (GWP) than CO2 (≈300 CO2 equivalents). Several studies on N2O WWTP emissions found that these accounted for up to 55.6% of the total [44]. Methane, produced by anaerobic digestion, is also a possible GHG emission from WWTPs, with CO2 equivalent of 25. Approximately 75% of direct onsite CH4 emissions are assumed to occur from the sludge treatment line, from fugitive losses from digester effluent, sludge storage, and CHP units. Facilities with poor sludge management and/or older infrastructure have higher emissions.
Indirect GHG emissions from externally acquired electricity or thermal power consumed by the WWTP for pumping and treatment operations dominate WWTPs’ contributions, accounting for 65–75% of the total [45]. Other “process chain” emissions are caused by chemical use (including chemicals’ “embedded” emissions, i.e., emission caused by treatment process chemicals’ production and transport), transportation and off-site disposal of residuals and sludge. All these should be factored in according to the “GHG Protocol Corporate Accounting and Reporting Standard” (GHG-PCARS) [46].
The purpose of this article is to analyze WWTPs’ energy consumption with regard to proposed management strategies for managers, designers and decision makers. A detailed analysis of WWTP operational parameters and proposal of improvement actions will be useful for applicability for a reduction in carbon footprint. Indicators such as kWh per different unit per year (kWh unit−1 year−1) are useful for benchmarking. Comparing energy use against industry standards or similar facilities can highlight areas for improvement.

2. Materials and Methods

This study analyzes the electric energy consumption of three purposely selected municipal WWTPs within a wide range of capacity and loading rates, located in southwestern Poland, in the rural and peri-urban areas of the Opolskie Voivodeship. To understand the drivers of increasing energy use in wastewater treatment plants, this paper focuses on total energy use rather than energy demand and energy costs.

2.1. Description of the Study Areas

The study area and plant locations are represented in Figure 1. The selected plants’ capacity ranges from 6000 to 130,000 P.E. and nominal flow capacity ranges between 600 and 18,000 m3/d. As such, the selected facilities represent the current Polish situation of small rural and urban facilities, adopting different treatment technologies and energy recovery options. The characteristics of the selected WWTPs are summarized in Table 1.
This wide range of facility capacity (representing a common situation in Poland) was deliberately selected to allow specific analyses and identification of optimal solutions within each plant category, established in the strategic national document “National Urban Wastewater Treatment Program” [47]. WWTP1 is a wastewater treatment facility located in an urban area, utilizing a three-phase activated sludge system that includes nitrification and pre-denitrification processes. This setup allows for effective biological organic matter and nitrogen removal, treating wastewater efficiently in line with modern standards. The plant’s sludge treatment line incorporates anaerobic digestion (AD). The AD process generates biogas, which is captured and used for energy recovery to power some of the plant’s operations, enhancing its sustainability. WWTP2 uses Sequencing Batch Reactor (SBR) technology, which treats wastewater in batches rather than a continuous flow, which allows for greater control over the treatment stages. SBR systems operate in cycles mainly for domestic wastewater. WWTP3 handles both domestic and industrial wastewater, and is equipped with two Complete Mix Module (CMM) 300 reactors. WWTP3 begins with a pretreatment phase that includes sand removal and a coagulation chamber to remove large particles and non-biodegradable materials, helping to protect the downstream treatment processes from clogging or damage. These three WWTPs demonstrate a range of technologies used to treat urban wastewater, each tailored to meet the unique demands of its setting.

2.2. Determination of Basic Energy Consumption Indicator (I)

On the basis of operational data provided by the facilities’ operators, an assessment of their actual electricity consumption was carried out for three consecutive years (Y1, Y2, Y3). Electricity consumption was determined based on the readings of the main electric meters at the treatment plants (for indicator calculation total for the plant, in case of aeration only for aeration unit). A basic energy consumption indicator (I) is the ratio between the energy consumption and one relevant parameter in the plant. In the study, the basic energy consumption indicators (indices of unit electricity consumption) were determined in relation to the volume of treated wastewater (IV), the plant size expressed in PE (I P.E.) and the removed pollutant load (IBOD5, ICOD, IS), as presented in Table 2. The energy intensity of the treatment plant was analyzed on the basis of quarterly data.
The calculation of the correlation between pollutant load and electricity consumption was also evaluated. The pollutant load, calculated as the difference between the concentrations of raw and treated wastewater multiplied by the volume of wastewater treated in the period analyzed, was correlated with the average electricity consumption (monthly energy consumption) for the period under review.

2.3. Carbon Footprint Estimation

A WWTP’s carbon footprint is the sum of GHG emissions from energy use and the technological processes, primarily carbon dioxide (CO2), emitted directly or indirectly. The studied WWTPs represent facilities with different process configurations, levels of energy neutrality and options for sludge disposal. In this study, CF analysis is based on the analysis of Scope 2 emissions from historical operating data. This means that indirect emissions resulting from the electricity purchased by the WWTP, which is essential to power the equipment and processes in the facility, are analyzed. This electricity is generated off-site and, in Poland, predominantly comes from fossil fuels.
The carbon footprint was then calculated, taking into account that, in Poland, electricity comes entirely from coal combustion; therefore, according to year 2021 conversion factors, 1 kWh of energy implies emissions of 657.1 g CO2.

3. Results and Discussion

The electricity consumption of the considered WWTPs ranged from 0.6 to 1.5 kWh/m3, with an average value of electricity consumption rate of 1.26 kWh/m3; considering that the actual number of PE served was lower than the design assumptions, the effective electricity consumption varied from 22.10 to 112.3 kWh/PE-year. Considering that the optimal target for overall electricity consumption at WWTPs is indicated as 20 kWh/PE-year, with a guide value at 26 kWh/PE-year [2], the observed range ranges from within the norm to very high. Smaller WWTPs generally have higher electricity consumption per population equivalent PE due to reduced economies of scale and less efficient energy use relative to their capacity. Conversely, larger plants tend to achieve better energy efficiency, benefiting from more advanced technological solutions and optimized operational processes. However, even large plants can have higher energy consumption if operating conditions are not optimized, for example, due to inefficient aeration systems, obsolete equipment or increased energy requirements associated with certain sludge treatment technologies. Therefore, factors such as plant size, technological innovation and operational efficiency are critical in determining whether electricity consumption remains within recommended limits.

3.1. Description of WWTP Performance

The averages of daily operating parameter measurements for each individual quarter (Q1–Q4) in the considered period of three years are presented in Tables S1–S3. Throughout the first year (Y1) of analyzed data, WWTP 1 operated hydraulically underloaded, as shown by a comparison of the average volumes of wastewater incoming flow (10,400–13,800 m3/d) to design flow (18,000 m3/d); for most of the year, the facility operated at just 60% of the project’s design. Specifically, for about 1/4 of the year, inflow did not exceed 11,000 m3/d. An analysis of the hydraulic load highlights that the maximum flows occur in Q1 and Q4; this is due to incoming meltwater and rainwater in spring and winter. The parameters BOD5 and COD have been chosen as representative indicators of biodegradable and non-biodegradable matter, and their reduction is closely linked to energy consumption. Concentrations of biodegradable compounds (BOD5) varied in the range of 298–383 mg/L with an average of 356 mg/L. COD was in the range from 610 mg/L to 769 mg/L. Despite a very wide range of observed concentration values of the parameters, the average ratio of COD/BOD5 was consistently below 2 (e.g., in Y3, 1.78). This qualifies the influent wastewater as easily or moderately easily decomposed. A summary of the effluent monitoring of the treated wastewater indicates that parameters do not exceed permitted values.
A similar situation was observed in the case of WWTP2, with the facility operating at an actual load of about 55% of the design for most of the year. BOD5 in influent ranged from 258 mg/L to 782 mg/L, with an average of 530 mg/L. Similarly large fluctuations refer to COD, where values in raw wastewater ranged from 290 mg/L to 1472 mg/L, with an average of 324 mg/L. These large fluctuations of pollutants in raw sewage may be related to components other than domestic sewage; discharges from several industrial and service plants are directed into this locality’s sewage system, with much higher pollutant loads than domestic users. On the other hand, dilution is affected by rainwater collected by the combined sewage system; therefore, the average effluent BOD5 discharged into the river lies below 3 mg/L, as much as 80.0% lower than the applicable limit value (15.0 mg/L). The same ration applies to COD discharge, at 30 mg/L, much lower than the limit value (125.0 mg/L). Treated WWTP2 effluents indicate that monitored parameter values are always lower than those allowed in the water discharge permit.
A different situation was observed in WWTP3, which operated at close (90%) to the hydraulic design load, and in several periods, it was actually overloaded, i.e., incoming wastewater flow was greater than the design by around 10%. The observed BOD5 in the influent is usually around 400 mg/L. Fluctuations observed in autumn and winter measurements are higher than in the case of the mean value due to loads from storm water and snowmelt. Mean effluent concentrations did not exceed the limit value.

3.2. Determination of Basic Energy Consumption Coefficients and Energy Efficiency

There are various indicators to quantify specific energy use by WWTPs. As indicated before, one bulk indicator is kWh/PE served. Decreasing values were observed for specific energy consumption with the increasing PE served, due to economies of scale and more frequent process automation in large plants. Per capita energy consumption is more suitable for general technology benchmarking assessments than specific performance evaluations. A common practice is to express energy use in volumetric terms (kWh/m3 wastewater treated); however, this can also be misleading, since it considers only water pumped through a facility, and not pollutant mass removed. Therefore, plants subject to significant stormwater influx or parasite clean infiltration/inflow (e.g., from phreatic superficial aquifers) could erroneously appear more performant and energy-efficient. Figure 2 shows calculated volumetric indicators for the three considered facilities. For smaller volumes of wastewater treated, the energy efficiency indicator has the highest value due to the relatively fixed amount of energy associated with running the plant. Smaller plants often have to operate with the same basic infrastructure as larger plants, but with lower wastewater flows. This results in higher energy consumption per unit of wastewater treated, as the basic energy requirements of the plant remain constant regardless of the volume treated. In addition, smaller plants may not have the advanced technologies and optimization strategies available to larger plants, further contributing to lower overall energy efficiency. A more meaningful approach consists of comparing energy consumption in terms of mass of pollutants removed, for example, kWh/kg BOD, COD or TTS.
Calculated mass-based energy efficiency indicators for the considered facilities are summarized in Table 3. The table shows calculated energy efficiency indicators for the three plants, allowing a comparative analysis of their performance. Differences in energy efficiency can be attributed to various factors such as plant size, process configurations (BOD and COD removed) and the volume of wastewater treated. These results are consistent with values reported in the literature, which range from 1.1 to 6.09 kWh/kg BOD5 [46].
The calculated energy efficiency indicators (I) provide insights into the energy consumption characteristics of the three wastewater treatment plants (WWTPs) across different years. The Iv indicator accounts for the general energy demands of the facility, including pumping, aeration, and other processes that vary with the volume of wastewater. WWTP1 and WWTP2 have lower Iv values, ranging from 0.61 to 0.69 kWh/m3, suggesting efficient energy use per unit volume of wastewater. In contrast, WWTP3 shows significantly higher Iv values (1.07 to 1.15 kWh/m3), which may indicate either a higher energy demand due to older equipment or higher treatment standards for the influent wastewater. Also, the climate condition could impact those indicators; as mentioned in other studies, the electrical consumption increased from 0.36 kw/m3 to 0.51 kw/m3 when the rainfall intensity increased from 0.8 mm/min to 2.9 mm/min [48].
The IPE indicator allows for comparison between facilities of different sizes and helps assess energy use in relation to the plant’s capacity. All three plants maintain relatively low IPE values, with WWTP1 at a steady 0.11, WWTP2 between 0.14 and 0.15, and WWTP3 with a slight fluctuation between 0.15 and 0.21. The higher values in WWTP3 may indicate greater energy needs due to increased load or differences in treatment intensity. IBOD focuses specifically on the energy-intensive biological treatment processes. WWTP1 has the lowest values (1.85–1.91 kWh/kg BOD), followed by WWTP2 (2.50–2.58 kWh/kg BOD), with WWTP3 showing the highest values (2.93–3.56 kWh/kg BOD). This indicates that WWTP3 has less efficient biological treatment, which requires more energy per unit of BOD removed. ICOD covers both biological and some chemical oxidation processes. WWTP1 and WWTP2 maintain values under 1.0 kWh/kg COD, indicating a balanced energy use for COD reduction. WWTP3 has slightly higher values, ranging from 0.88 to 1.13 kWh/kg COD, which again point to less efficient treatment and higher initial COD loads in the influent. IS represents the energy associated with mechanical treatment processes. WWTP1 has the highest IS values (4.60–4.78 kWh/kg SS), suggesting high energy needs for solid removal. WWTP2 follows, with values around 3.14–3.15 kWh/kg SS. WWTP3, on the other hand, has significantly lower values, ranging from 1.55 to 2.38 kWh/kg SS, which could be related to having no need for pumping a large volume of wastewater to a sedimentation tank for solid removal.
Energy consumption in WWTPs is largely determined by location, quality of influent wastewater, process configurations, treatment capacity, required effluent quality, etc. As treatment standards become more stringent, the energy required to remove pollutants increases. In practice, energy demand decreases as plant treatment capacity increases, with a threshold of approximately 37,850 m3/d, beyond which further capacity increases have minimal impact on in-plant energy consumption [49]. The correlation between Biochemical Oxygen Demand (BOD) load and energy consumption is significant, especially in wastewater treatment processes. Higher BOD loads typically require more energy for treatment because more oxygen is needed to break down the organic matter. This increased energy consumption can be attributed to the need for more intensive aeration and other treatment processes to ensure that the effluent meets environmental standards [50]. Of the WWTPs analyzed, only WWTP 1 showed correlations between electricity consumption for the treatment process and the amount of load removed, expressed as BOD5. The results of the analyses are shown in Figure 3.
The trendline suggests a positive correlation between BOD5 load reduction and energy consumption. The positive correlation suggests that as WWTP 1 handles higher BOD5 loads, its energy requirements generally increase. This is consistent with expectations, as treating larger loads typically demands more energy. However, the relatively low R2 value indicates that BOD5 load reduction alone does not fully account for variations in energy consumption, implying that operational efficiency and equipment performance might also be significant.
For small wastewater treatment plants, the correlation between Biochemical Oxygen Demand (BOD) load and energy consumption can still be significant, but there are some unique factors to consider. Smaller plants often have higher specific energy consumption per unit of wastewater treated than larger plants. This is due to less efficient economies of scale and the need for more frequent maintenance and operational adjustments [51].
For example, smaller plants may experience greater variability in wastewater temperature, quality and dissolved oxygen levels, which can affect energy consumption. In addition, the type of treatment technology used can also influence energy consumption [52]. So, while the basic relationship between BOD load and energy consumption holds true, the specifics can be quite different for smaller plants. This can be seen in the other WWTPs analyzed in this study. Although the graphs shown (Figure 4 and Figure 5) indicate a lack of correlation, they may even indicate a high degree of flexibility in the operation of the wastewater treatment process. In Figure 5, most data points are clustered around the middle, where BOD5 load reduction is between 10,000 and 20,000 kg, and energy consumption ranges from about 55,000 to 65,000 kWh. This suggests that within this range, energy consumption and load reduction might be relatively stable or typical for this WWTP. With a very different composition of the wastewater entering WWTP 2 and WWTP 3, the effect of pollutant removal was satisfactory, so that the load removed was high. This is particularly true for the two points marked in red on the graph.

3.3. WWTP Footprint Calculations

The carbon footprint was calculated based on energy consumption data and emission factors, as presented in Table 4. The results for Scope 2 indicate total greenhouse gas emissions ranging from 145 to 1909 tons per year of CO2e, which reflects the emissions associated with purchased electricity during the analyzed period. Scope 2 emissions highlight the company’s dependence on energy suppliers and its energy efficiency. Reducing emissions in this area can be achieved by improving energy efficiency or purchasing energy from renewable sources.
Specific carbon footprints (scope 2) for WWTP1 and WWTP2 are similar, in the range of 0.4–0.45 kg CO2/m3y. WWTP3 had the largest carbon footprint, due to relatively high volume of WW treated (working also overloaded). The two plants with lower carbon footprints had in common that they were both partially self-sufficient in terms of heat and electricity. In general, energy consumption has the greatest impact on the carbon footprint.
Total CO2 emissions show a fluctuating trend, with emissions ranging from around 1.77 million kg in Y2 to almost 1.91 million kg in Y3 for WWTP1. WWTP3 consistently has the lowest total emissions among the plants. CO2 emissions per cubic meter of treated wastewater provide an indication of emission efficiency. The emissions are moderate for WWTP1 and WWTP2, ranging from 0.03980 kg/m3 to 0.4521 kg/m3. WWTP3 shows the highest emissions per m3 in all years, with values around 0.70–0.76 kg/m3, suggesting room for improvement. CO2 emissions per population equivalent (PE) provide a benchmark in relation to the population served by each plant. In the case of WWTP1, stable CO2 emissions around 0.073–0.074 kg indicate a consistent performance over the years. WWTP2 shows values around 0.093–0.098 kg per PE, slightly higher than WWTP1. WWTP3 has the highest CO2 emissions per PE, especially in Y2, with a value of 0.1370 kg, although it decreased to 0.1125 kg in Y3.
Recommendations to reduce the carbon footprint of each WWTP will be different. For plants with anaerobic digestion (such as WWTP1), increasing biogas production from sludge and using combined heat and power (CHP) systems can reduce reliance on grid electricity and thus reduce the carbon footprint. Integrating renewable energy (e.g., solar or wind) into the operation of WWTP2 and WWTP3 can directly reduce the CO2 emissions associated with fossil fuel-based electricity. As aeration is highly energy-intensive and a significant contributor to CO2 emissions, optimizing aeration processes can help reduce energy consumption and emissions at each WWTP. Older or poorly maintained equipment consumes more energy. Routine maintenance and replacing outdated components such as pumps and motors with energy-efficient models can result in significant energy savings [52]. Implementing these strategies can help reduce the carbon footprint of each plant by improving energy efficiency, increasing the use of renewable energy, and optimizing treatment processes.

4. Conclusions

Municipal wastewater treatment plants (WWTPs) require significant amounts of energy to operate, with each plant having unique energy consumption characteristics that are influenced by technology, operational practices, and operating conditions such as the type of wastewater being treated, flow modifications, and so on. Based on data from three WWTPs in southwestern Poland, the research highlights the different energy needs and opportunities for optimization in different urban areas. Specific energy performance indicators and their correlation with greenhouse gas (GHG) reductions were identified, providing a basis for benchmarking energy efficiency across the sector. Of the indicators analyzed, the population equivalent (IPE) coefficient offers the greatest comparability, while the IBOD coefficient is the most reliable in reflecting the high energy demands of biological processes. Key factors influencing the energy performance of pretreatment include kWh per kilogram of TSS removed (ITSS). Optimization of the pre-treatment stage can also significantly improve biogas production and energy recovery. For example, the use of chemical pre-treatment (CEPT) or advanced filtration systems (fine screens or drum filters) instead of conventional sedimentation can increase organic matter capture for anaerobic digestion. Plants with higher energy efficiency tend to be those with advanced technologies, optimized processes and energy recovery systems, while those with lower efficiencies may suffer from inefficient aeration systems or outdated equipment. These findings highlight the importance of considering different factors to improve overall energy performance. Knowing the energy consumption indicators for wastewater treatment systems allows the selection of the most efficient technological variant when modernizing the system. A combined analysis of the energy requirements of the WWTP and the carbon footprint is essential, as energy consumption has the largest share in the total CF if the WWTP meets all of its energy requirements from non-renewable sources. Measuring the carbon footprint of a wastewater treatment plant is the first step in reducing it. For the time being, Poland is in a peculiar position; the vast majority of the country’s energy is produced on the basis of fossil fuels, that is, on the basis of raw materials whose energy use is particularly harmful to the environment. In response to strong consumption in energy, WWTPs, public agencies, local government and the wastewater industry need to explore and implement measures to ensure the achievement of the targets set out in the 2020 Climate and Energy Package. The best solution will be, on the one hand, to reduce energy consumption by modernizing the infrastructure, and on the other hand, to invest in renewable energy sources that can change this unfavorable trend.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en17225745/s1, Table S1: Operational parameters of WWTP1. Table S2: Operational parameters of WWTP2. Table S3: Operational parameters of WWTP3.

Author Contributions

Conceptualization, I.K.-B., A.K. and A.R.; methodology, I.K.-B., J.B.-Z. and A.G.C.; writing—original draft preparation I.K.-B., J.B.-Z. and A.G.C.; investigation, J.B.-Z., A.R. and I.K.-B.; resources writing—original draft preparation, A.K. and J.B.-Z.; writing—review and editing, A.G.C. and J.B.-Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Files.

Conflicts of Interest

The authors declare no conflicts of interest.

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Energies 17 05745 g001
Figure 1. Plant locations.
Figure 1. Plant locations.
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Figure 2. Selected energy performance indicators for the 3 considered facilities.
Figure 2. Selected energy performance indicators for the 3 considered facilities.
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Figure 3. Correlation between energy consumption and BOD5 load reduction in WWTP 1.
Figure 3. Correlation between energy consumption and BOD5 load reduction in WWTP 1.
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Figure 4. Correlation between energy consumption and BOD5 load reduction in WWTP 2.
Figure 4. Correlation between energy consumption and BOD5 load reduction in WWTP 2.
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Figure 5. Correlation between energy consumption and BOD5 load reduction in WWTP3.
Figure 5. Correlation between energy consumption and BOD5 load reduction in WWTP3.
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Table 1. Characteristics of analyzed WWTPs.
Table 1. Characteristics of analyzed WWTPs.
Facility IDDesign Flow [m3/d]Population Equivalent
[P.E.]		BOD5 Loading Rates [kg/d]COD Loading Rates [kg/d]Effluent Recipient
WTTP118,000134,40040298044Oder River
WTTP2300090004291129Gosciejowice Canal, Ścinawa River
WTTP36006000230686Mała Panew River
Detailed operational data of the facilities can be found in Tables S1–S3 in Supplementary Files.
Table 2. Specific energy consumption indicators.
Table 2. Specific energy consumption indicators.
ParameterUnitCalculationNote
IVkWh/m3Energy consumption kWh: Treated wastewater m3ratio between the daily energy consumption and the daily volume treated (annual average)
IPEkWh/PE −1 year −1Energy consumption kWh: Population Equivalent (PE)ratio between the annual energy consumption and the PE served in the plant
IBODkWh/kg BOD removedEnergy consumption: BOD removedratio between the annual energy consumption and the BOD removed, expressed in kg
ICODkWh/kg COD removedEnergy consumption: COD removedratio between the annual energy consumption and the BOD removed, expressed in kg
ISkWh/kg SS removedEnergy consumption: SSratio between the annual energy consumption and the BOD removed, expressed in kg
Table 3. Calculated energy efficiency indicators (I) for the three facilities.
Table 3. Calculated energy efficiency indicators (I) for the three facilities.
IWWTP1WWTP2WWTP3
Y1Y2Y3Y2Y3Y1Y2Y3
IV0.690.630.660.660.611.151.071.15
IPE0.110.110.110.150.140.150.210.17
IBOD1.891.911.852.582.502.513.562.93
ICOD0.950.970.951.110.890.931.130.88
IS4.634.604.783.143.151.552.381.78
Table 4. Carbon footprint calculations.
Table 4. Carbon footprint calculations.
CO2 Emission [kg]
Y1Y2Y3
WTTP11,811,0621,770,6671,909,318
WTTP2-248,996228,932
WTTP3145,688145,774163,502
CO2 Emission per m3 Wastewater [kg]
Y1Y2Y3
WTTP10.45210.41110.4345
WTTP2-0.43110.3980
WTTP30.75560.70230.7561
CO2 Emission per P.E.
Y1Y2Y3
WTTP10.07390.07430.0721
WTTP2-0.09820.0931
WTTP30.09630.13700.1125
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Kłosok-Bazan, I.; Rak, A.; Boguniewicz-Zabłocka, J.; Kuczuk, A.; Capodaglio, A.G. Evaluating Energy Efficiency Parameters of Municipal Wastewater Treatment Plants in Terms of Management Strategies and Carbon Footprint Reduction: Insights from Three Polish Facilities. Energies 2024, 17, 5745. https://doi.org/10.3390/en17225745

AMA Style

Kłosok-Bazan I, Rak A, Boguniewicz-Zabłocka J, Kuczuk A, Capodaglio AG. Evaluating Energy Efficiency Parameters of Municipal Wastewater Treatment Plants in Terms of Management Strategies and Carbon Footprint Reduction: Insights from Three Polish Facilities. Energies. 2024; 17(22):5745. https://doi.org/10.3390/en17225745

Chicago/Turabian Style

Kłosok-Bazan, Iwona, Adam Rak, Joanna Boguniewicz-Zabłocka, Anna Kuczuk, and Andrea G. Capodaglio. 2024. "Evaluating Energy Efficiency Parameters of Municipal Wastewater Treatment Plants in Terms of Management Strategies and Carbon Footprint Reduction: Insights from Three Polish Facilities" Energies 17, no. 22: 5745. https://doi.org/10.3390/en17225745

APA Style

Kłosok-Bazan, I., Rak, A., Boguniewicz-Zabłocka, J., Kuczuk, A., & Capodaglio, A. G. (2024). Evaluating Energy Efficiency Parameters of Municipal Wastewater Treatment Plants in Terms of Management Strategies and Carbon Footprint Reduction: Insights from Three Polish Facilities. Energies, 17(22), 5745. https://doi.org/10.3390/en17225745

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