Municipal wastewater discharge standards for ammonia nitrogen in Brazil: technical elements to guide decisions Open Access

With population increase and diversification of human activities, the use of water resources has intensified, resulting in more significant concerns regarding the preservation of the quantity and quality of water sources. In Brazilian metropolitan regions, there is a high demand for water; however, poor-quality water persists, polluted mainly by domestic sewage without adequate treatment. This scenario results in low water security in these regions (ANA 2019). In 2012, the Panorama of Surface Water Quality in Brazil, published by the National Agency for Water and Sanitation (ANA), showed that domestic wastewater represents the primary pressure on water resources in the country. This finding is due to the low population index served by both collection and treatment, and ineffective treatment of wastewater when performed (ANA 2012).

One way to control water pollution is to define, apply, and enforce effluent standards for wastewater discharge (Oliveira & von Sperling 2008a). There are two types of standards in Brazilian legislation: water bodies and discharge standards. Both standards are needed because of the difficulty in maintaining effective control of pollution sources based only on the quality of the receiving water body (von Sperling 2014).

In Brazil, the National Environment Council (CONAMA) Resolution n. 20/1986 established conditions for the disposal of effluents into water bodies at the national level. The standard for total ammonia nitrogen discharge was 5 mg/L (Brasil 1986). In 2005, CONAMA Resolution n. 357/2005 (Brasil 2005) revoked the previous rule and established a new disposal standard of 20 mg/L, which was later amended by CONAMA Resolution n. 430/2011 (Brasil 2011), which maintained the permissible limit of 20 mg/L. However, since 2005, the standard of ammonia nitrogen has no longer been required for sanitary sewage system discharge, only for other types of effluents.

There are 26 states in Brazil which must comply with federal law, but have the option of including more stringent effluent standards in the state legislation (von Sperling 2016). Morais & Santos (2019) identified eight Brazilian states that require ammonia nitrogen effluent discharge standards, according to their specific legislation. The limit value is currently between 0.5 and 20 mg/L, depending on the state. In three of these eight states, the limit was not applicable to sanitary sewage systems, consistent with federal legislation.

Minas Gerais is Brazil's fourth-largest state and has the second-largest population, with an area of 586,513 km² and approximately 19.6 million inhabitants, according to the Brazilian Institute of Geography and Statistics (IBGE 2010). Its area is greater than that of France, which comprises 549,087 km² (WORLD BANK 2022).

In Minas Gerais, the Normative Deliberation of the State Environmental Policy Council (COPAM) n. 10/1986 established norms and standards for water quality and effluent discharge, in which a limit value of 5 mg/L of ammonia nitrogen was required for effluent discharge (Minas Gerais 1986). The standard was changed to 20 mg/L when the legislation was updated by the Joint Normative Deliberation of COPAM and the State Water Resources Council (CERH), n. 01/2008, which is no longer applicable to sanitary sewage treatment systems (Minas Gerais 2008).

According to von Sperling (2016), the relaxation of discharge standards at the federal and Minas Gerais state levels aimed to allow the implementation of less efficient treatment systems, such as upflow anaerobic sludge blanket (UASB) reactors without post-treatment. Treatment plants based on these reactors cannot obtain their permits if they fail to comply with more stringent standards.

According to the Minas Gerais Institute of Water Management (IGAM 2018b), a large part of the surface water quality violations in the state are related to the inefficiency or nonexistence of wastewater treatment systems. Among the concerns identified in the report, ammonia nitrogen had a violation rate of 10%. According to (IGAM 2018b) violations occur due to the lack or inefficiency of sewage treatment systems in Minas Gerais and the absence of advanced technologies that remove nutrients from sewage treatment plants (STPs) (IGAM 2018b).

In Minas Gerais State, discussions are taking place between the environmental agency, the water resources management agency, the academy, and the sanitation regulatory agency to update state legislation without revisions since 2008. One of the topics is the possibility of adopting an ammonia nitrogen discharge standard for sanitary effluents, which is not currently required. Due to the difficulty of accessing monitoring data from STPs operating at full scale in the state and the scarcity of analysis and interpretation of the results, there is little technical basis for decision makers. Thus, the present study aimed to evaluate data from 49 full-scale STPs in Minas Gerais State to provide a guide for decisions based on the operational performance of the plants.

The sanitation service provider, who requested confidentiality, provided the monitoring data from 49 STPs operating in Minas Gerais. The size of each plant was defined based on the Normative Deliberation COPAM n. 217/2017 (Minas Gerais 2017), which considers STPs that treat an average flow of 0.5–50 L/s, 50–100 L/s, and >100 L/s as small, medium, and large, respectively.

Table 1 presents the STPs' treatment processes, number and percentage of plants in each treatment technology, and minimum and maximum monthly inflow rates. Table 2 presents the treatment process and size of each STP in detail.

The period covered by the monitoring data ranges from 2006 to 2019. The data refer to the concentrations of total ammonia nitrogen sampled in the final effluent of the STPs and in the receiving water bodies, upstream and downstream of the disposals. Monitoring in the period took place on a biannual frequency at all sampling sites.

The violation percentage of the effluent discharge standard was calculated by considering the concentration of ammonia nitrogen in the final effluents of the STPs. The standard value of 20 mg/L was considered for analysis, established in the Joint Normative Deliberation COPAM/CERH-MG n. 01/2008, only for effluents not coming from sewage treatment systems. In other words, although this standard does not apply to domestic wastewater treatment systems, the violation percentages were calculated using this limit to determine the condition of the systems to meet a possible establishment of domestic wastewater regulations.

Effluent loads of ammonia nitrogen were also calculated for each STP. The effluent load represents the effluent concentration multiplied by the flow rate and is expressed as kg/d of ammonia nitrogen.

Then, data from STPs that adopted the same technology were gathered, and the effluent concentrations were statistically compared between the treatment technologies. The non-parametric Kruskal-Wallis statistical test was used, followed by Dunn's test, at a significance level of 5%. Statistical tests were conducted using Statistica 10.0 software, and graphs were built using the R programming language.

Niku et al. (1979) developed a methodology to assess the reliability of STPs on a probability basis, and several studies applied this method to STPs in Brazil; see Oliveira & von Sperling (2008b) and Alderson et al. (2015) for the complete calculus of the reliability analysis. In this study, it was calculated the expected percentage of compliance with the discharge standard from the actual effluent concentrations and the coefficient of variation of ammonia nitrogen for each STP. The expected compliance percentage for each discharge standard was obtained for a 95% level of reliability. This step was conducted because of the difficulty STP had in achieving the 20 mg/L standard value.

The Mann-Whitney non-parametric test was applied at a 5% significance level to compare the ammonia nitrogen concentrations in receiving water bodies upstream and downstream of the discharges and to verify the possible impacts resulting from each STP.

In Brazil, inland water bodies are categorized into classes, each with target values for water quality parameters that must be achieved to maintain their condition, compatible with locally predominant water uses. Therefore, the violation percentage of the surface water quality standards was calculated for each receiving water body, both upstream and downstream of the discharge, considering its rating class. This step was conducted to verify whether there was an increase in the violation percentage downstream of disposals. The permissible limits of ammonia nitrogen are listed in Table 3.

Table 4 presents the violation percentages of the discharge standards, considering a limit of 20 mg/L. The box-plot graph in Figure 1 shows the effluent concentrations and the considered discharge limit to better visualize the results. Figure 2 shows the effluent load for each STP.

Of the 49 STPs, 20 did not meet the discharge standard established for non-sanitary effluents in any of the samples evaluated, and 44 (approximately 90%) violated this limit in at least half of the evaluated period (Table 4). Importantly, anaerobic technologies (especially those using UASB reactors) presented the worst performance for this parameter, corroborating results from other studies on the inefficiency of these technologies for ammonia removal (Oliveira & von Sperling 2011; von Sperling 2014; Dantas et al. 2021).

Effluent loads were higher in STPs 12, 27, 28, and 37 (Figure 2), which are large-size STPs (Table 2) and are among the systems with the highest inflow rate. STP 43 (large UASB+CAS) is also among the STPs with the highest inflow rate; however, due to lower effluent concentrations of ammonia nitrogen (Figure 3), STP 43 was not among the highest loads.

The results showed variability in the performance of treatment processes, which leads to water pollution. Thus, despite the existence and operation of STPs contributing to the improvement of water security, the flexibilization of discharge standards allows for a more significant input of pollutants into water bodies via effluents from STPs. Furthermore, because these water bodies are often used for public supply, there is an increased risk to human health.

Violation percentages of the discharge standards calculated for each treatment process are presented in Table 5.

Considering the limit for non-sanitary effluents, a high percentage of violations of the ammonia nitrogen standard were observed for the treatment technologies with UASB reactors, reaching 100% for UASB+DAF and above 90% for UASB, UASB+TF, UASB+AF, and UASB+AF+MP (Table 5). The results show the limitation of UASB reactors in preventing the contamination of the receiving water bodies by ammonia nitrogen, even when post-treatment is applied.

Next, the ammonia nitrogen concentrations in the effluents of the 13 treatment technologies were compared using the Kruskal-Wallis test, which identified significant differences (significance level of 5%) among the technologies. Figure 3 shows the distribution of the data by technology.

The UASB process presented significantly higher ammonia nitrogen concentrations than those of the other seven treatment processes. Other systems that use UASB reactors followed by post-treatment (except for UASB+CAS) showed significantly higher concentrations than aerobic technologies, especially activated sludge and facultative/aerated ponds. The observed concentrations converged with the results obtained by Sato et al. (2006). For processes involving stabilization ponds, the effluent ammonia nitrogen concentrations showed greater variability than those reported by Espinosa et al. (2017).

The results may be related to the following aspects: inefficiency of UASB reactors in nitrogen removal, as reported by Almeida et al. (2018); the occurrence of nitrification in activated sludge systems operating at high temperatures in the Brazilian climate (von Sperling 2012); and ammonia volatilization (especially in maturation ponds) and nitrogen assimilation by algae in stabilization ponds (von Sperling 2002; Vera et al. 2013).

Significant differences were observed in the effluent quality from different treatment technologies. Aerobic technologies result in effluents with less potential for ammonia nitrogen contamination compared to STPs composed of UASB reactors. The violation percentages of the standards, presented in Tables 4 and 5, corroborate these findings.

Given the panorama of sewage treatment systems and the prevalence of anaerobic processes in Minas Gerais, there is a need for efficient control of these plants to avoid environmental damage arising from the poor performance of treatments or limitations inherent to the adopted processes.

Considering STPs have difficulties achieving the limit value of 20 mg/L, it was also investigated which percentage of compliance the treatment technologies would achieve if they maintained the same operating conditions using the methodology developed by Niku et al. (1979). The results obtained using different discharge standards (20 mg/L, 30 mg/L, and 40 mg/L) are presented in Table 6.

When a more relaxed standard of 40 mg/L was considered (twice what is currently required for industrial effluents), few STPs would obtain a high compliance percentage. For 40 mg/L, STP 18 (small UASB+TF), 45 (small FP), and 46 (medium ANP+FAP) achieved expected compliance percentages between 60 and 80%. Expected compliance percentages above 80% were achieved by STP 13 (large UASB), 34 (small EAAS), 35 (medium EAAS), 36 (small CAS), 37 (large CAS), 43 (large UASB+CAS), 44 (small FP), 47 (small ANP+FP), and 49 (medium FAP).

STPs that use aerobic technologies (activated sludge and facultative and aerated ponds) would achieve the highest compliance percentage. However, despite being the treatment type used in the greatest number of STPs under study (14 STPs of UASB and 14 of UASB+TF), with the 40 mg/L standard, only one STP of each type would achieve compliance above 60%. Oliveira & von Sperling (2008b) found low reliability of Brazilian STPs in terms of nutrients. According to the authors, these results were expected, because technologies have not been designed for nitrogen removal.

The Mann-Whitney test was carried out to compare water quality upstream and downstream of the discharges to verify whether STPs' effluent disposal caused negative impacts on the receiving bodies. This assessment did not consider the hydrodynamic conditions of the receiving water bodies (due to the absence of water flow data), the possible existence of irregular effluent discharges in the vicinity of the STP disposals, and land use and land cover in the regions. The STPs for which significant differences were identified are shown in Figure 4.

Significant changes were identified (significance level of 5%) in water quality downstream of the disposals. Significant differences in water quality were identified in more than 50% of STPs. In 100% of these cases, there was an increase in the ammonia nitrogen concentration downstream of the discharge (Figure 4). This finding indicates the need to control ammonia nitrogen concentration in the effluents of STPs. The results of studies by Barjoveanu et al. (2010), Waiser et al. (2010), and Dantas et al. (2021) corroborate the need to control the impacts related to this parameter.

As significant differences in ammonia nitrogen concentrations were identified between the upstream and downstream points in more than 50% of the STPs under study, it was necessary to verify whether this significant increase in concentrations also increased violations of water quality standards. Table 7 presents the violation percentages according to the rating class for each receiving water body.

There was an increase in violations downstream of disposals in 30 of the 49 STPs (61.2%). The Minas Gerais State legislation relaxed regulations in 2008 and allowed the discharge of treated STP effluents with more significant potential to change water quality in disagreement with the rating class of the receiving bodies (Soares & Silva 2018). Thus, the increase in violation percentages for ammonia nitrogen downstream of STPs indicates the need to control the discharge to avoid degrading the water quality.

Morais & Santos (2019) highlighted that measures that require industries to comply with the ammonia nitrogen standard but suppress the responsibility of sanitation service providers are contradictory in terms of environmental preservation. Furthermore, treated municipal wastewater is responsible for a high share of the polluting load released into water bodies.

According to von Sperling & Chernicharo (2002), effluent discharge standards exist for practical reasons, with the ultimate objective being water quality preservation. In addition to complying with the discharge standards, an effluent must also allow for compliance with the specific standards of the class of receiving water (von Sperling 2016). Thus, considering the increase in violations downstream of STPs, the need to make discharge and water quality standards compatible is evident. In this sense, the discharge standards are less restrictive in Minas Gerais than in other Brazilian states and countries, exacerbated by the flexibilization of standards for sewage treatment systems in 2008. It is important to note that fourteen years after COPAM/CERH-MG n. 01/2008 and 11 years after CONAMA n. 430/2011, insufficient performance was still observed for existing plants.

Therefore, increasing the efficiency of STPs and improving legislation regarding discharge standards are essential for preserving surface water quality in Minas Gerais. The adoption of progressive targets for sanitary effluent discharge standards is a viable option for sanitation service providers to have time to make the necessary adjustments to treatment processes to meet the standards. Another alternative is adopting more restrictive targets for larger STPs, as currently required in other Brazilian states (Morais & Santos 2019), because the largest disposals are a more significant point source of contamination to water bodies. Furthermore, as sewage treatment is adopted to preserve water bodies, the individual characteristics of the receiving watercourses should also be considered, which has been done in several countries in the establishment of standards (Preisner et al. 2020).

It is essential to highlight that this study focused on ammonia nitrogen because of the current discussions taking place in Minas Gerais about the possible adoption of a sanitary effluent discharge standard targeting this parameter. However, it is necessary to apply these same discussions to other variables of equivalent environmental relevance, which also do not have discharge standards in the current legislation. For example, total nitrogen is worth considering because nitrification in the effluent treatment process can remove ammonia nitrogen; ammonia and organic nitrogen from the raw sewage are converted to nitrite and nitrate in the treated effluent. However, without the denitrification step, the final effluent still contains nitrate concentrations that can harm the environment and cause eutrophication of water bodies. Another critical variable that contributes to eutrophication is phosphorus, which is not removed in Brazilian STPs. In addition, disinfection processes are not usually adopted in Brazilian STPs, which leads to high concentrations of microbiological indicators of fecal contamination in treated effluents, such as thermotolerant coliforms and Escherichia coli (Dantas et al. 2021). In particular, E. coli was the parameter with the highest number of water quality standard violations in Minas Gerais, with violation rates of approximately 50% between 2016 and 2019. In the same years, phosphorus was among the five variables with the highest violation percentages in the state (IGAM 2017, 2018a, 2019, 2021).

This study aimed to evaluate the performance of STPs in Minas Gerais State and the environmental impacts resulting from their effluent disposal into water bodies by evaluating the monitoring data from these plants. Low performance was detected in removing ammonia nitrogen for STPs that use anaerobic technologies. Despite being more efficient and presenting a lower violation percentage, STPs that employ aerobic technologies also caused water pollution, indicating the need for widespread improvement in wastewater treatment.

The water quality data analysis upstream and downstream of the discharges identified negative environmental impacts on the receiving water bodies due to increased ammonia nitrogen concentrations. This also led to an increase in the percentage of downstream violations. Adopting water quality standards alone has not been sufficient to ensure adequate control and treatment concerning ammonia nitrogen removal in STPs. Therefore, it is recommended to adopt sanitary effluent discharge standards, which can be gradually enforced, with progressive goals and different levels of requirements according to the STP sizes and characteristics of the receiving water bodies.

Future studies are recommended to develop numerical analysis or deployment of water quality modeling tools to further assess sewage impacts on water bodies.

The authors would like to thank the sanitation service provider for providing the monitoring data and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for their financial support during the course of this study.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.