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Article

Evaluating the Performance of Sewage Treatment Plants Containing Up-Flow Anaerobic Sludge Blanket Reactors Followed or Not by Post-Treatments

by
Juan Pablo Pereira Lima
and
André Aguiar
*
Institute of Natural Resources, Federal University of Itajubá, Itajubá 37500-903, MG, Brazil
*
Author to whom correspondence should be addressed.
Environments 2025, 12(5), 146; https://doi.org/10.3390/environments12050146
Submission received: 22 March 2025 / Revised: 23 April 2025 / Accepted: 24 April 2025 / Published: 1 May 2025
(This article belongs to the Special Issue Environmental Pollution Risk Assessment)
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Abstract

:
Sewage treatment is essential to prevent disease transmission and adverse environmental impacts. This study evaluated the performance of four Sewage Treatment Plants (STPs) in two cities in the state of Minas Gerais, Brazil. Two STPs (Santana and São José) that have Up-Flow Anaerobic Sludge Blanket (UASB) reactors as the sole biological treatment stage did not comply with the discharge standards in receiving water bodies, particularly for SetS, TSS and O&G parameters. This shows the need for improvements, such as the implementation of post-treatment. For the other plants that have UASB reactors followed by an activated sludge system (Industrial Complex STP) or an up-flow anaerobic filter (Carbonita STP) as post-treatment, only the O&G parameter was not met. With the exception of one of the STPs that lackes post-treatment (São José STP), the other three met the required minimum removals of 60% for BOD5 and 55% for COD. The Carbonita STP promoted the highest average removals of BOD5 and COD, at 90% and 86%, respectively. Despite the discharge of industrial wastewater into the sewage collection network of one of the cities in this study, the biodegradability of the raw sewage remained high (BOD5/COD ratio > 0.4). The wastewater treated by the STPs that have post-treatment showed greater potential for reuse in agricultural practices.

1. Introduction

The world’s population has been facing serious water-related problems, such its pollution and scarcity, which have been attributed to unbridled population growth, lack of basic sanitation and industrialization [1,2,3]. It is estimated that half of the world’s population will face water scarcity for at least one month a year and that the water demand will increase by 30% by 2050 [4]. This scenario highlights Sustainable Development Goal No. 6 of the UN 2030 Agenda [5], which aims to ensure universal access to safe drinking water and sustainable sanitation services.
Around 80% of water intended for human consumption returns to the environment as municipal sewage containing various pollutants such as organic matter and nutrients. According to the Brazilian National Sanitation Information System (SNIS), 6 billion m3 of wastewater were collected in Brazil in 2022. However, sewage collection serves only 56% of the Brazilian population, and just 52.2% of the collected wastewater was treated [6]. In 2020, the New Regulatory Framework for Basic Sanitation (Law 14.026/2020) was implemented, establishing the goals of expanding sewage collection and treatment to at least 90% of the Brazilian population by 2033. This law changed the concession and contracting regime for sanitation policies, encouraging both public measures and private initiatives [7]. Despite this important government initiative, recent studies have shown that sewage treatment in many Brazilian municipalities is still inadequate, as it has not met all the disposal standards established by environmental agencies [8,9,10]. This underscores the need for the implementation of improvements.
The construction and operation of STPs are essential to ensure the safe disposal of wastewater into the environment. Without adequate treatment, sewage can cause serious public health problems due to the presence of pathogenic organisms associated with infections, such as gastroenteritis, amoebiasis and salmonellosis [11]. In addition, its disposal into water bodies increases the concentration of organic matter, which can promote the excessive growth of microorganisms and the consequent reduction in dissolved oxygen levels, compromising the survival of aquatic fauna [11,12]. The presence of nitrogen and phosphorus in sewage is also harmful, as these nutrients can cause the eutrophication of water bodies [13,14].
Although the disposal of treated sewage in Brazil commonly occurs in water bodies, it is interesting to reuse treated wastewater in agricultural irrigation, which is a current trend and implies reducing the demand for drinking water, in addition to providing nutrients and residual organic matter to plants [15,16]. In the state of Minas Gerais, the use of treated wastewater from dairies [17] and slaughterhouses [18] has been common in fertigation practices. However, specific standards must be met for this form of disposal.
Anaerobic biological processes are an attractive technology for treating wastewater, and UASB reactors have been widely applied for sewage decontamination in tropical countries [19]. Chernicharo et al. [20] reported that 40% of STPs in Brazil had UASB reactors, and the percentage was higher in the state of Minas Gerais, above 70% [21]. In these reactors, the sewage flows upwards and comes into contact with sludge granules containing bacteria that anaerobically degrade organic matter and generate biogas, a mixture composed mainly of methane and carbon dioxide. The upper part of the reactor contains a three-phase separator that separates the biogas from the treated sewage, while the sludge tends to settle internally [22]. The advantages of this bioreactor include low operating costs (no aeration required), reduced space requirements and the ability to treat various types of wastewater [22,23]. However, it has limitations in removing nitrogen and phosphorus [24,25], and especially pathogens [8,26,27]. Unlike nutrients, the presence of pathogens renders the use of treated sewage in fertigation practices unfeasible [16].
Expanding sewage collection and treatment services to all Brazilian municipalities is crucial; however, it is equally important to verify that the operating STPs comply with the required environmental standards for wastewater discharge into receiving water bodies. Recent studies have found that STPs located in the Brazilian states of Ceará [8] and Minas Gerais [10], which use only the UASB reactor as the biological treatment stage, were not effective in removing pollutants. Therefore, it is important to not only evaluate the performance of other STPs containing only UASB reactors but also to evaluate and compare with the performance of STPs that have some type of post-treatment after this reactor. Therefore, this study aimed to evaluate the performance of STPs in two municipalities in Minas Gerais, containing UASB reactors followed or not by post-treatment. The city of Varginha has three STPs, two of which (Santana and São José) use only UASB reactors, while the third (Industrial Complex) contains an activated sludge system as post-treatment. The STP in the municipality of Carbonita (Carbonita STP) has UASB reactors followed by an up-flow anaerobic filter and surface runoff in the soil before discharge into a receiving water body. The performance analysis of the four STPs aimed to verify whether recent data on the characteristics of treated sewage (concentrations of organic matter, solids, O&G and pH) comply with the standards for discharge into water bodies established by Brazilian environmental agencies. The characteristics of the treated wastewater were also analyzed with respect to the regulatory requirements currently in force for water reuse.

2. Materials and Methods

2.1. Data Acquisition from STPs

This study analyzed the characterization data of raw and treated sewage samples from three STPs located in Varginha and one in Carbonita, both municipalities located in the state of Minas Gerais, Brazil. The data refer to the years 2020 and 2021 and were obtained from the Operational Inspection Reports that were published online by the Regulatory Agency for Water Supply and Sewage Services of the State of Minas Gerais (ARSAE). The values of the characterization parameters were extracted from reports No. 127/2021 for the STPs of Varginha (Santana, São José and Industrial Complex) [28] and No. 206/2021 for the STP of Carbonita [29]. The samples were collected monthly, although for some STPs, more than one sample was collected during the same month.

2.2. Assessment of Sewage Characterization Data

The determination of the parameters COD (chemical oxygen demand), BOD5 (biochemical oxygen demand after 5 days of incubation), pH, SetS (settleable solids), TSS (total suspended solids) and O&G (oils and greases) was performed according to APHA [30]. Only the mandatory parameters contained in the environmental licensing documents of each STP were monitored. The analysis of sewage sample characteristics was conducted in accordance with the discharge standards set by both federal and state environmental agencies. At the federal level, the assessment was based on the guidelines outlined in CONAMA Resolution No. 430/2011 [31]. For the state of Minas Gerais, the applicable environmental requirements are defined by the State Council for Environmental Policy (COPAM) and the State Council for Water Resources (CERH), as established in the Joint Normative Resolution No. 01/2008 [32]. The treated sewage samples from the four STPs were also evaluated for fertigation practices, through Normative Resolution CERH-MG No. 65/2020 [33], while the ABNT NBR 16783/2019 standard was used to evaluate the reuse of treated sewage in buildings [34]. Table 1 shows the disposal standards of the four aforementioned legislations.
The biodegradability of the sewage samples was assessed by calculating the BOD5/COD ratio. Using MATLAB 2024a, simple linear regression analyses were performed between different wastewater quality parameters to estimate a more complex laboratory analysis parameter based on another that is easier to quantify [35]. The same software was used for statistical analysis, including the Anderson–Darling test to assess the normality of the data.

3. Results and Discussion

3.1. Brief Description of STPs

Table 2 presents some aspects of the municipalities and their STPs. The Sanitation Company of the state of Minas Gerais (COPASA) is responsible for the sewage treatment in both cities. Varginha is located in the south of the state, where coffee is traditionally produced [36]. Varginha’s sewage collection network is 537,808 m long and serves 92.5% of its population [37]. Carbonita is a small municipality located in the northeast of the state of Minas Gerais, and the production of charcoal plays a significant role in its economy [38]. The STP of this municipality began operations in 2014 and was responsible for the decontamination of the Curralinho Stream [39].
The STPs in Varginha collectively treat approximately 98% of the sewage generated in the municipality. The Santana STP is responsible for treating about 70% of the total, while the São José STP treats the remainder [28]. The third STP treats the sewage generated in the municipality’s industrial district, which contains around 1500 employees [40]. Therefore, the sewage flow received at this plant is very low. The Santana STP receives a sewage flow that corresponds to one-third of its total capacity, while the other Varginha STPs receive well below one-third of their capacities [28]. The Carbonita STP treats approximately 60% of the sewage generated in the municipality [39] and operates at its maximum permitted flow [29].
The four STPs employ the same preliminary unit operations of screening and desanding, followed by more than one UASB reactor (in parallel) as the main treatment stage. In the Industrial Complex and Carbonita STPs, two other biological processes are used to complement the treatment, consisting of an activated sludge system and anaerobic filters, respectively. Regarding the anaerobic processes of treatment (UASB reactors and anaerobic filter), the organic matter is converted mainly into biogas.
At the Santana, São José and Carbonita STPs, a biogas burner is used to prevent emissions into the atmosphere, particularly due to the presence of methane, which is a powerful greenhouse gas [19]. The environmental licensing document of the Industrial Complex STP does not mention the existence of a biogas burner. Supposedly, this STP does not have a burner, likely due to the very low flow of sewage it treats. The treated sewage from the four STPs is discharged into water bodies.
The sludge generated in the UASB reactors of the three Varginha STPs is left in drying beds and later disposed of, together with the waste removed in the pretreatment, in the municipal sanitary landfill [28,40,41,42], while the sludge generated in the Carbonita STP is dried and disposed of in a landfill within the STP area [29]. Within the framework of the circular economy, sewage sludge has the potential to be utilized in agricultural applications, as long as its characteristics meet the chemical and microbiological criteria established for application in soil according to CONAMA Resolution No. 498 [44].
In addition to the aforementioned sustainable practices of using treated sewage for irrigation and biological sludge for soil fertilization, according to the study developed by Bressani-Ribeiro et al. [45], STPs that treat wastewater generated by a population of 100,000 or more inhabitants can be considered industries, as long as they add value to the resources generated. They proposed that the sludge could be burned to generate thermal energy, the waste separated in the desander should be used to make non-structural concrete and the biogas could be burned for thermal purposes or to generate electricity. The Santana STP would be the only one that fits into this category, as it treats the sewage of approximately 100,000 inhabitants, while the others are small-scale, but could also adopt sustainable practices.

3.2. Analysis of Sewage Quality Parameters for Disposal into Water Bodies

The performance assessment of a STP is done by comparing the values of the quality parameters of its treated wastewater with the standards established by environmental legislation. Brazilian legislation establishes different assessment criteria for a given parameter, such as a maximum permitted concentration, a minimum removal percentage or a minimum annual average of this percentage. For the discharge to be considered compliant, it is necessary to meet at least one of these criteria mentioned [31,32]. Since the monitoring period was only 10 months, it was not possible to assess this last criterion, much less the effect of seasonality. Although the data from the Carbonita and Varginha STPs were from different periods, this did not compromise the analysis of the data.
Figure 1 shows the BOD5 concentrations measured in samples obtained from the four STPs, before and after treatment. It was found that the samples from Santana (377.6 ± 139.6 mg.L−1) and São José (289.8 ± 82.9 mg.L−1) STPs had average BOD5 values relatively close to the typical value of 300 mg.L−1 for municipal wastewater in Brazilian cities [35]. On the other hand, the average BOD5 values for the Industrial Complex (504.3 ± 121.6 mg.L−1) and Carbonita (653.6 ± 128.2 mg.L−1) STPs were much higher. All raw sewage samples showed BOD5 values well above the limits established by legislation, reinforcing the need for treatment.
Analysis of the BOD5 data for the treated sewage indicated that the Santana and São José STPs met the maximum values allowed by federal legislation [31]. However, more than half of the values exceeded the limit established by state legislation, which is more restrictive [32]. Although they have post-treatment, some samples with BOD5 in non-compliance with both legislations were found for the Industrial Complex and Carbonita STPs.
Figure 2 shows that the average COD values of the samples from the Santana (722.3 ± 293.5 mg.L−1) and São José (572.5 ± 175.6 mg.L−1) STPs were closer to the typical value of 600 mg.L−1 for Brazilian cities [35], while the average values of the Industrial Complex (934.6 ± 161.3 mg.L−1) and Carbonita (1456.2 ± 236.7 mg.L−1) STPs were significantly higher. The higher BOD5 and COD values for the last two STPs suggest that a more effective treatment should be adopted for them when compared to the first two that do not have post-treatment.
As previously mentioned, current legislation also establishes that STPs may be in compliance as long as there is a minimum percentage removal of BOD5 and COD. Figure 3 shows these data and reveals a greater removal of BOD5 compared to COD for the four STPs. Considering the minimum requirement of 60% removal of BOD5, according to the two legislations, it was found that this criterion was met by all STPs, except for one sample from the São José STP. Similarly, the analysis of the COD percentage removal data revealed compliance with state legislation, except for two values below the minimum required of 55% for the São José STP. With the exception of this last STP, the disposal standards were met concerning organic matter concentration parameters.
The pH, SetS, TSS and O&G parameters were reported only for the treated sewage samples. As shown in Figure 4, the pH values were around 7 and within the ranges established by the two environmental legislations. Therefore, their disposal would have little effect on the pH of the receiving water bodies.
Figure 5 shows the SetS data. This parameter corresponds to the concentration of suspended solids that settle in 1 h in an Imhoff cone [35]. The presence of suspended solids in high concentrations can settle at the bottom of water bodies, and this contributes to undesirable anaerobic conditions [35]. Higher SetS values were found for the samples from the Santana and São José STPs, including values twice the limit permitted for disposal. The STPs containing post-treatment showed compliance when considering this sewage quality parameter.
Regarding TSS, approximately 50% of the samples from the Santana and São José STPs presented values above the permitted limit for disposal (Figure 6). These results suggest a possible undesirable drag of flocs/sludge from the UASB reactor, which can occur when the ascension velocity is greater than the sedimentation velocity [22,25]. The presence of a secondary decanter, inherent to the activated sludge system at the Industrial Complex STP, and an anaerobic filter at the Carbonita STP, may have contributed to a greater reduction of this parameter, with values below the maximum permitted for disposal.
Regarding the O&G data shown in Figure 7, all treated sewage samples complied with the maximum limit set by federal legislation for discharge [31]. However, at least one sample from each STP exceeded the limit of state legislation, with all samples from the Industrial Complex STP presenting values above the limit, even though it had a post-treatment stage. In a recent study, concentrations below 10 mg.L−1 were found for this parameter in similarly treated samples from a STP in Colombia [46].
Unlike for other STPs, performance evaluation of a plant containing a UASB reactor followed by an anaerobic filter is scarce in the literature. Oliveira and von Sperling [47] verified excellent performance in removing TSS and BOD5 (above 80%) in one of the two STPs containing a UASB reactor followed by an up-flow anaerobic filter in Brazil; however, only these two parameters were monitored in their study. A study carried out by Chernicharo and Machado [48] evaluated three pilot-scale treatment units containing the same sequence of biological processes. When considering the BOD5, COD and TSS data, the treated samples would be in compliance for disposal according to Brazilian standards. However, these authors did not evaluate the O&G parameter, making a comparative analysis unfeasible.
Table 3 shows the statistical summary of the parameters evaluated for the treated sewage samples. The average values of some wastewater quality parameters differed greatly between the STPs studied, such as TSS, which ranged from 33 to 97 mg.L−1. On the other hand, the variation in COD was very low, between 200 and 210 mg.L−1. The STPs containing post-treatment promoted greater average removal of COD and BOD5 and confirmed that the São José STP was the least effective. Since many of the coefficient of variation (CV) values were higher than 30%, this indicates high variability between the data [49]. For the Santana STP, the maximum values of the SetS and TSS parameters were verified, which were well above the discharge standards, while for the Industrial Complex STP, the maximum values of BOD5 and COD were verified. For the STPs containing post-treatment, the CV values were generally higher. When using the Anderson–Darling test, it was verified that most of the parameters presented p-value ≥ 0.05. Thus, it is assumed that the hypothesis that the data are normal was not rejected, considering a 5% significance level, except for BOD5 and SetS for the samples from the STPs containing post-treatment.

3.3. Additional Analyses on the Parameters BOD5 and COD

When comparing the methodologies to quantify the two organic matter concentration parameters, COD has the advantage of being commonly obtained in less than three hours, while BOD is measured after five days of sample incubation [50]. For this reason, it is convenient to correlate these two parameters through simple linear regression. BOD5 has been most commonly used in design equations, and determining this parameter as a function of COD would enable rapid adjustments in treatment processes, avoiding the discharge of biologically unstable wastewater [51,52]. Accordingly, the data for these two wastewater quality parameters were analyzed to identify possible linear correlations between them. A coefficient of determination (R2) greater than 0.8 is generally interpreted as indicating a strong correlation, suggesting that the model is reliable and suitable for application. For R2 values between 0.5 and 0.8 or below 0.5, the correlation is considered moderate and weak, respectively [53].
As shown in Figure 8, for samples of raw sewage from the Santana STP and treated sewage from the Industrial Complex STP, the R2 value was above 0.8. However, when excluding the most concentrated samples (outliers, in the upper right corner) from these two sample groups, their R2 values decreased to less than 0.4, suggesting that these correlations are not useful for estimating one parameter in function of the other. For the other sample groups, there was no need to exclude outliers and R2 values below 0.65 were found (Supplementary Materials; Figure S1), which are contraindicated for estimating one parameter in function of the other.
Numerous published works have examined the existence of linear correlations between other pairs of wastewater characterization parameters [17,51,52,54,55]. In the present work, additional linear regression analyses were performed when considering the other sewage quality parameters (Supplementary Materials; Table S1). Weak linear correlations were observed between organic matter concentration parameters (BOD5, COD) and solids (SetS, TSS) for the Varginha STP samples. For the Carbonita STP, moderate correlations were observed for TSS × COD, TSS × SetS and TSS × BOD5. In general, the various attempts to identify strong linear correlations were unsuccessful in the present study.
Still considering the BOD5 and COD parameters, the ratio between them was calculated as an indicator of the biodegradability of the sewage samples. Although von Sperling [35] suggests assesseing biodegradability through the COD/BOD5 ratio, the present study adopted the BOD5/COD ratio, as it facilitates a more straightforward interpretation, with values varying between 0 and 1. A BOD5/COD ratio exceeding 0.4 suggests that a significant portion of organic matter (>40%) is biodegradable, indicating that biological treatment processes are an appropriate option, as they are efficient and have low operating costs. Conversely, when the ratio is less than 0.15, physical–chemical processes are generally preferred, as the low biodegradability implies that most contaminants are resistant to biological degradation and may be classified as recalcitrant. For values between 0.15 and 0.40, a more detailed analysis of the wastewater composition is necessary to determine the most appropriate treatment process [35].
As shown in Figure 9, biodegradability was generally lower in the treated samples. This indicates that the biodegradable fraction was removed more in the treatment processes of the four STPs. When calculating the averages of the BOD5/COD ratios, the raw sewage samples presented a value higher than 0.46, which was expected according to the range from 0.42 to 0.59 reported by von Sperling [35]. After treatment, the average ratio of the treated sewage decreased to values between 0.32 and 0.40. Since these average values are generally in the intermediate biodegradability range, the implementation of another biological process may be an alternative to improve the performance of the STPs without post-treatment.
It is noteworthy that Brazilian industries are permitted to discharge both treated and untreated industrial or sanitary wastewater into the municipal sewage collection network. This practice can contribute to an increased pollution load of the wastewater to be treated in the STPs. Specifically, in the state of Minas Gerais, such discharges are allowed as long as they meet the criteria established by the Non-Domestic Effluent Receiving Program(PRECEND) of COPASA [56]. Table S2 (Supplementary Materials) shows some information from industries in different sectors that discharge their wastewater into the Varginha sewage collection network [57,58,59,60,61,62,63,64,65]. This information was obtained from environmental licensing documents from the State Secretariat for the Environment and Sustainable Development of Minas Gerais (SEMAD). Some industries in Varginha treat their wastewater or outsource this service for later disposal into the sewage collection network. The leachate generated in the municipal landfill is also disposed of in the sewage collection network but without any prior treatment [66]. Although rarely addressed in the literature, this survey of the disposal of wastewater generated by industries in the sewage collection network does not suggest that these were the main responsible for the non-compliance with some of the disposal standards by the STPs of Varginha. Although some industries have reported data on the discharge flow of their wastewater, the concentration of pollutants is unknown, and it is therefore impossible to assess the magnitude of their pollution load. For the Carbonita STP, which presented little data on non-compliance with discharge standards, no discharge of industrial wastewater into the sewage collection network of this municipality was found.
It is common to consider that very low BOD5/COD ratios may indicate the presence of toxic substances that inhibit microbial metabolism in biological treatment processes, as well as the contribution of industrial wastewater to the sewage collection network [35], since some industrial sectors such as the textile industry generate wastewater with low biodegradability [67]. Since all raw sewage samples showed high biodegradability, this suggests that industrial wastewater discharged into the Varginha sewage collection supposedly also possesses high biodegradability.

3.4. General Assessment of Treatment Processes in STPs

Although most STPs were effective in removing BOD5 and COD, the two STPs without post-treatment, Santana and São José, did not meet the SetS, TSS and O&G parameters. This last parameter was also not met by the STPs with post-treatment. For optimal performance of the STPs, the preliminary treatment steps must avoid dragging inert material into the reactors, so as not to reduce their useful volume, as this would reduce the hydraulic retention time (HRT) in them [50]. For the three STPs in Varginha, increasing the HRT in the bioreactors is something possible to remove more pollutants, as they operate with an average flow rate well below their maximum capacity (Table 2). On the other hand, it is not feasible to increase the HRT in the UASB reactor at the Carbonita STP, since it already operates at its maximum flow limit. When evaluating different strategies to optimize sewage treatment by bench-scale UASB reactors, Rizvi et al. [23] found greater removal of COD and BOD5 for inocula with sludge ages between 120 and 150 days and temperatures between 25 and 30 °C (mesophilic). When evaluating different HRTs (3, 6, 9 and 12 h), these authors found a progressive increase in the removal of COD, BOD5, sulfate, O&G and TSS as a function of the increase in HRT. Although the removal rate (% removal/time) has been decreased, increasing the HRT allows for a longer lasting contact between the bacteria and the pollutants to be removed/degraded, and this can be suggested as an alternative for the UASB reactors at the Varginha STPs so that they remove more pollutants.
The adoption of new unit operations in STPs may be unfeasible for some of them due to space limitations, which requires alternatives that improve the performance of existing processes. In addition to increasing the HRT, modifications to the reactor configuration, such as the insertion of an anaerobic filter in the upper part of the UASB reactor itself, known as a hybrid anaerobic reactor (RAH), have been suggested as a way to improve the performance of some STPs in Brazil [10]. RAHs are known to provide high rates of organic load removal in the lower portion of the reactor and additionally remove volatile fatty acids and retain suspended solids in the upper portion of the filter [50]. However, this may not be an interesting alternative since two studies that compared these two reactors under similar conditions of sewage treatment have found a similar [68] or lower performance of the RAH [69].
Micronutrient supplementation can be an effective strategy to optimize sludge granulation in the UASB reactor and consequently improve its performance. By adding low concentrations of Ni (0.1 mg.L−1), Fe (10 mg.L−1) and Co (0.5 mg.L−1) together to treat distillery wastewater, Sharma and Singh [70] showed that COD removal efficiency increased from 67% to 76%. The addition of only 0.2 mg.L−1 of Ni promoted the highest COD removal, at 81%. Sludge granulation was assessed using the sludge volumetric index (SVI), which should be reduced to indicate improved settleability. The lowest SVI values were verified for the different treatment conditions that combined the micronutrients. Although there is an additional cost for the acquisition of these micronutrients, this strategy could also be evaluated to improve sewage treatment in this reactor.
The STPs in this study that have a biological process as post-treatment showed more satisfactory results. The Carbonita STP uses an up-flow anaerobic filter and surface runoff in the soil and presented only one sample with O&G above the maximum value allowed by state legislation, while the other parameters were in compliance. Anaerobic filters are cylindrical reactors with biomass adhered to a fixed support and operate with up-flow or down-flow [50]. Surface runoff polishing in the soil allows the removal of residual pollutants through different forms such as sedimentation, volatilization and adsorption [35]. Likewise, the Industrial Complex STP, which uses an activated sludge system as post-treatment, proved to be efficient, except for the O&G concentration above the maximum value allowed by state legislation in all samples. The activated sludge system consists of a tank containing aerobic and/or facultative microorganisms that aerobically degrade organic matter, followed by a decanter to retain the biological flocs produced. Part of the sludge that settles is returned to the aeration tank to maintain a high concentration of biocatalysts in the system and thus reduce the HRT [50].
In a study involving 18 STPs across various Brazilian municipalities, Oliveira and von Sperling [47] reported that treatment systems containing a UASB reactor followed by a post-treatment stage demonstrated superior performance in removing BOD5 and TSS, compared to those relying solely on a UASB reactor. Another example of the presence of more than one post-treatment stage can be observed at the STP in Pedralva, Brazil, where the treatment sequence includes a UASB reactor, a facultative stabilization pond and two sequential polishing ponds. The percentage removals of COD and BOD5 observed for this STP were 77.2% and 92.8%, respectively [9]. Therefore, if there is physical space and investment, it is possible to improve the performance of the Santana and São José STPs by adding post-treatment stages. Queiroz et al. [21] mentioned that new STPs in Minas Gerais that have UASB reactors will also have post-treatment stages.
Despite some data not complying with the disposal standards of the 2008 state legislation, a new state legislation recently came into force [71]. Among the changes established in the Joint Normative Resolution COPAM/CERH No. 08/2022, it is important to highlight the increase in the maximum concentration of O&G from 50 mg.L−1 to 100 mg.L−1, and the expansion of the pH range from 6–9 to 5–9. These new standards are now the same as those in federal legislation [31]. The other parameters analyzed in this study were not changed in this new state legislation. With the change in the maximum limit for O&G, all four STPs would be in compliance.
It is also important to highlight that this new state legislation established a limit of 20 mg.L−1 of total ammonia nitrogen (TAN), which has been required only for industrial wastewater. This new criterion raises concerns for STPs, as some studies have found an increase in the concentration of TAN in sewage treated by a UASB reactor [26,27], probably caused by ammonification [35]. The most common way to remove ammonia nitrogen from wastewater is through aerobic oxidation, carried out by nitrifying bacteria [50]. Since the Industrial Complex STP is the only one that has an aerobic biological process, it will supposedly have less difficulty in meeting this new criterion. As evidence of this, when using a similar treatment sequence (UASB followed by an activated sludge system) in a STP in the municipality of Betim (Brazil), Saliba and von Sperling [25] found that the average TAN concentration in sewage samples was reduced from 25.5 to 7.3 mg.L−1. In another study that evaluated two STPs in India containing this same sequence of biological treatment processes, TAN concentrations below 15 mg.L−1 were found [24]. Although nitrite and nitrate can also contribute to the eutrophication of water bodies, legislation in force does not establish maximum permitted concentrations for these nitrogen forms [31,32]. With regard to phosphorus removal, Brazilian environmental agencies only can set maximum permitted concentration in areas with a history of cyanobacteria blooms, mainly in regions where water is used for public supply [31].

3.5. Potential for Reuse of Treated Sewage

The practice of reusing treated sewage is a current trend and serves to reduce costs related to water collection and treatment, as well as to reduce the demand for advanced wastewater treatments [1,72,73]. Irrigation with wastewater for different types of crops is a relevant practice, especially in regions facing water scarcity [1,72]. However, for this type of destination, it is essential to apply treated sewage with caution in certain crops to minimize the risks of contamination of soil, groundwater and the vegetable itself [74,75]. An illustrative example of this practice is found in the municipality of Varginha, where a local slaughterhouse utilizes a portion of its treated wastewater for fertigation purposes [76].
To enable sewage reuse, specific legislation must be complied with. The ABNT NBR 16783/2019 standard outlines the guidelines for the characterization, design, implementation, operation and maintenance of non-potable water systems within buildings [34]. When analyzing the treated samples from the four STPs, it was found that the pH was within the range from 6.0 to 9.0. However, all BOD5 values were higher than the permitted limit of 20 mg·L−1, making this form of disposal unfeasible.
In the state of Minas Gerais, the direct reuse of non-potable water produced at STPs is specifically regulated by Normative Resolution CERH-MG No. 65/2020. This regulation permits its application for a range of purposes, including agroforestry, urban, environmental and industrial uses [33]. The pH data of the treated sewage samples were in accordance with this legislation. The presence of viable helminth eggs and thermotolerant coliforms are crucial indicators of pathogenicity but were not quantified according to the ARSAE reports.
Although the presence of nutrients (N, P) makes the UASB reactor effluent suitable for fertigation, this reactor is not very effective in removing helminth eggs and thermotolerant coliforms [26,27]. Chlorination may not be a good alternative to eliminate pathogens, since the UASB reactor effluent still has a considerable residual concentration of organic compounds, and the amount of organochlorine byproducts generated is a risk to be avoided [19]. When evaluating the UASB reactor followed by complementary biological processes, such as polishing ponds [15] and constructed wetlands [77], it was possible to verify almost complete removal of helminth eggs and coliforms. The STPs that contain post-treatment in the present study are likely able to remove more pathogens. As proof of this, studies have found that other STPs with the same biological treatment sequence as the Industrial Complex STP were able to remove 90% of fecal coliforms [24] and almost 100% of E. coli [25].
Although the legislation that allows reuse in irrigation systems does not define a maximum permitted concentration of TSS [33], there may be problems with obstruction in emitters when drip irrigation is used. A classification proposed by Bucks et al. [78] suggested that TSS concentrations below 50 mg.L−1 are considered risk-free, from 50 to 100 mg.L−1 are classified as moderate, and above 100 mg.L−1 indicate a high risk of obstruction. When considering the TSS data, the treated wastewater from the STPs without post-treatment (Santana and São José) would not be suitable for drip irrigation, since most samples presented concentrations above 100 mg.L−1. They would be more suitable for furrow irrigation systems [27]. On the other hand, the treated wastewater from the STPs containing post-treatment (Industrial Complex and Carbonita) would be suitable for both furrow and drip irrigation, since the TSS concentrations found varied between 10 mg.L−1 and 60 mg.L−1.

4. Conclusions

STPs are designed to treat wastewater and consequently protect receiving water bodies, in addition to eventually enabling the reuse of treated wastewater. An analysis of data from four STPs containing UASB reactors revealed that, when used as the sole biological treatment step, STPs were not effective in meeting some of the discharge standards (SetS, TSS and O&G). Treatment improvements, such as increasing the HRT or implementing a post-treatment stage, are recommended, although this would require additional investment and more physical space. Regarding the results of STPs that use post-treatment, such as the Industrial Complex STP (activated sludge system) and Carbonita STP (up-flow anaerobic filter), it was observed that these presented greater compliance with wastewater discharge standards, except O&G. When considering the new state legislation for discharge into receiving water bodies, these two STPs would be in full compliance. However, all four STPs will probably need to adopt procedures to comply with the new maximum permitted concentration criterion of 20 mg.L−1 of total ammonia nitrogen, which commonly increases in sewage treated solely by UASB reactors. The raw sewage samples showed high biodegradability, even with a considerable contribution of industrial wastewater discharged into the sewage collection network of the municipality of Varginha. As expected, in all four STPs, biodegradability was reduced by the treatment processes. When considering reuse practices, the destination of wastewater treated by the UASB reactor with post-treatment is more suitable for fertigation. However, analyses regarding the presence of pathogens are necessary to ensure human and environmental safety. The New Regulatory Framework for Sanitation in Brazil must be able to expand sewage collection and treatment to almost its entire population, but it is also necessary that the existing STPs are effective in fully complying with the discharge standards for discharge into water bodies and that reuse practices be expanded to allow greater use of this water resource.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/environments12050146/s1, Figure S1: Simple linear regression analysis between BOD5 and COD data for samples of raw (●) and treated sewage (○) of the four STPs. Green symbols correspond to outliers; Table S1: Other correlations evaluated for the treated samples of the four STPs; Table S2: Generation of wastewater and sewage from industries in Varginha.

Author Contributions

Conceptualization, A.A.; methodology, A.A.; software, J.P.P.L.; formal analysis, J.P.P.L.; data curation, J.P.P.L.; writing—original draft preparation, J.P.P.L.; writing—review and editing, A.A.; visualization, A.A.; supervision, A.A.; project administration, A.A.; funding acquisition, A.A. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)—Grant Number 001. The authors also thank the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (Fapemig) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The English grammar review of this article was funded by the Natural Resources Institute of the Federal University of Itajubá.

Data Availability Statement

Data used in this work are duly referenced in the text.

Conflicts of Interest

The authors declare no conflicts of interest.

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Environments 12 00146 g001
Figure 1. BOD5 levels in raw and treated samples from the four STPs, along with the maximum concentrations allowed for disposal according to current legislation. Dashed red line: CONAMA No. 430/2011 [31]. Solid green line: COPAM/CERH-MG No. 01/2008 [32].
Figure 1. BOD5 levels in raw and treated samples from the four STPs, along with the maximum concentrations allowed for disposal according to current legislation. Dashed red line: CONAMA No. 430/2011 [31]. Solid green line: COPAM/CERH-MG No. 01/2008 [32].
Environments 12 00146 g001
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Figure 2. COD levels in raw and treated samples from the four STPs, along with the maximum concentration allowed for disposal (green line) according to current state legislation [32].
Figure 2. COD levels in raw and treated samples from the four STPs, along with the maximum concentration allowed for disposal (green line) according to current state legislation [32].
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Environments 12 00146 g003aEnvironments 12 00146 g003b
Figure 3. BOD5 and COD removal efficiency of the four STPs, along with the minimum removal values according to environmental legislation. Dashed red line: CONAMA nº 430/2011 [31]. Solid green line: COPAM/CERH-MG No. 01/2008 [32].
Figure 3. BOD5 and COD removal efficiency of the four STPs, along with the minimum removal values according to environmental legislation. Dashed red line: CONAMA nº 430/2011 [31]. Solid green line: COPAM/CERH-MG No. 01/2008 [32].
Environments 12 00146 g003aEnvironments 12 00146 g003b
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Figure 4. pH values of treated sewage samples, along with the ranges of values permitted for disposal. Dashed red lines: CONAMA No. 430/2011 [31]. Solid green lines: COPAM/CERH-MG No. 01/2008 [32].
Figure 4. pH values of treated sewage samples, along with the ranges of values permitted for disposal. Dashed red lines: CONAMA No. 430/2011 [31]. Solid green lines: COPAM/CERH-MG No. 01/2008 [32].
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Figure 5. SetS levels in treated samples from the four STPs, along with the maximum concentrations allowed for disposal according to current legislation. Dashed red line: CONAMA No. 430/2011 [31]. Solid green line: COPAM/CERH-MG No. 01/2008 [32].
Figure 5. SetS levels in treated samples from the four STPs, along with the maximum concentrations allowed for disposal according to current legislation. Dashed red line: CONAMA No. 430/2011 [31]. Solid green line: COPAM/CERH-MG No. 01/2008 [32].
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Figure 6. TSS levels in treated samples from the four STPs, along with the maximum concentration allowed for disposal (green line) according to current state legislation [32].
Figure 6. TSS levels in treated samples from the four STPs, along with the maximum concentration allowed for disposal (green line) according to current state legislation [32].
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Figure 7. O&G levels in treated samples from the four STPs, along with the maximum concentrations allowed for disposal according to current legislation. Dashed red line: CONAMA No. 430/2011 [31]; solid green line: COPAM/CERH-MG No. 01/2008 [32].
Figure 7. O&G levels in treated samples from the four STPs, along with the maximum concentrations allowed for disposal according to current legislation. Dashed red line: CONAMA No. 430/2011 [31]; solid green line: COPAM/CERH-MG No. 01/2008 [32].
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Figure 8. Simple linear regression analysis between BOD5 and COD data for samples of raw and treated sewage of two STPs. Green and blue circles—correlation for all data; blue circles—correlation after excluding outliers.
Figure 8. Simple linear regression analysis between BOD5 and COD data for samples of raw and treated sewage of two STPs. Green and blue circles—correlation for all data; blue circles—correlation after excluding outliers.
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Figure 9. BOD5/COD ratios for raw (●) and treated (○) sewage samples from the four STPs, alongside the biodegradability classification ranges proposed by von Sperling [35].
Figure 9. BOD5/COD ratios for raw (●) and treated (○) sewage samples from the four STPs, alongside the biodegradability classification ranges proposed by von Sperling [35].
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Table 1. Standards for sewage discharge into receiving water bodies and for reuse practices.
Table 1. Standards for sewage discharge into receiving water bodies and for reuse practices.
ParametersCONAMA 430/2011 [31]COPAM/CERH-MG 01/2008 [32]CERH-MG 65/2020 [33]ABNT NBR 16783/2019 [34]
BOD5 (mg.L−1)<120 or minimum removal of 60%<60 or minimum removal of 60%–<20
COD (mg.L−1)–<180 or minimum removal of 55%––
pH5.0–9.06.0–9.06.0–9.06.0–9.0
SetS (mL.L−1)<1<1––
TSS (mg.L−1)–<100––
O&G (mg.L−1)<100<50––
Thermotolerant coliforms or Escherichia coli (NMP.100 mL−1)––≤1 × 104 *; ≤ 1 × 106 **≤2 × 102
Viable helminth eggs (number of eggs.L−1)––≤1–
* Surface, localized or sprinkler fertigation. ** Superficial or localized fertigation without any contact with the edible product.
Table 2. Important aspects about the STPs evaluated in this study and the municipalities where they are located.
Table 2. Important aspects about the STPs evaluated in this study and the municipalities where they are located.
CityPopulation (Thousand
Inhabitants)		STP NameCurrent Average Operating Flow Rate
(L.s−1)		Maximum Allowed Flow Rate (L.s−1)Sewage Treatment ProcessesReceiving Water BodyReference
Varginha136.4Santana96.6280.0Screening, desander, 4 UASB reactorsVerde River[28,36,40,41,42]
São José66.3229.0Screening, desander, 4 UASB reactors
Industrial Complex0.493.3Screening, desander, 2 UASB reactors, 4 aeration tanks and 1 decanter (activated sludge system)
Carbonita8.5Carbonita13.4313.43Screening, desander, 2 UASB reactors, 2 up-flow anaerobic filters, surface runoff in the soilCurralinho Stream[29,38,43]
Table 3. Descriptive statistics of treated sewage data from the four STPs.
Table 3. Descriptive statistics of treated sewage data from the four STPs.
ParametersNumber of SamplesAverageStandard
Deviation		CV (%)MinimumMaximump-Value
Santana STP
BOD5 (mg.L−1)974.719.526.141.0107.00.674
COD (mg.L−1)9210.547.422.51392850.973
BOD5 removal (%)979.53.94.973.884.70.089
COD removal (%)968.88.312.158.184.40.663
SetS (mL.L−1)90.980.6061.20.12.00.624
SST (mg.L−1)2297.249.350.752560.038
O&G (mg.L−1)558.413.423.041.773.60.668
São José STP
BOD5 (mg.L−1)1567.417.626.1451100.345
COD (mg.L−1)22208.668.332.71193910.068
BOD5 removal (%)1072.78.912.255.083.20.352
COD removal (%)1061.512.119.635.480.20.564
SetS (mL.L−1)100.620.1829.00,11.80.105
SST (mg.L−1)2189.040.245.281440.261
O&G (mg.L−1)637.029.078.49,892.60.068
Industrial Complex STP
BOD5 (mg.L−1)985.549.758.1442040.005
COD (mg.L−1)9209.579.337.81193910.107
BOD5 removal (%)983.28.09.663.790.20.015
COD removal (%)977.95.77.365.983.60.058
SetS (mL.L−1)90.20.2100.00.10.6<0.005
SST (mg.L−1)1037.413.836.922640.305
O&G (mg.L−1)571.311.716.452.683.10.313
Carbonita STP
BOD5 (mg.L−1)1065.239.560.631104.60.014
COD (mg.L−1)10200.472.836.373257.20.527
BOD5 removal (%)1090.25.56.177.994.2<0.005
COD removal (%)1085.95.96.973.694.80.448
SetS (mL.L−1)80.320.1856.20.20.70.022
SST (mg.L−1)1032.823.170.44.266.70.414
O&G (mg.L−1)432.8730.091.33.971.80.617
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Lima, J.P.P.; Aguiar, A. Evaluating the Performance of Sewage Treatment Plants Containing Up-Flow Anaerobic Sludge Blanket Reactors Followed or Not by Post-Treatments. Environments 2025, 12, 146. https://doi.org/10.3390/environments12050146

AMA Style

Lima JPP, Aguiar A. Evaluating the Performance of Sewage Treatment Plants Containing Up-Flow Anaerobic Sludge Blanket Reactors Followed or Not by Post-Treatments. Environments. 2025; 12(5):146. https://doi.org/10.3390/environments12050146

Chicago/Turabian Style

Lima, Juan Pablo Pereira, and André Aguiar. 2025. "Evaluating the Performance of Sewage Treatment Plants Containing Up-Flow Anaerobic Sludge Blanket Reactors Followed or Not by Post-Treatments" Environments 12, no. 5: 146. https://doi.org/10.3390/environments12050146

APA Style

Lima, J. P. P., & Aguiar, A. (2025). Evaluating the Performance of Sewage Treatment Plants Containing Up-Flow Anaerobic Sludge Blanket Reactors Followed or Not by Post-Treatments. Environments, 12(5), 146. https://doi.org/10.3390/environments12050146

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