https://orcid.org/0009-0006-0005-078X
ABSTRACT
The main objective of this study was to evaluate the quality and reuse potential of treated effluents by constructed wetlands (CW) systems utilized for decentralized wastewater treatment in urban and rural scenarios in southern Brazil. The assessment, based on Brazilian discharge standards and wastewater reuse guidelines, revealed a close link between treatment efficiency and technological configurations, considering diverse inlet wastewater sources. Most CW treated effluents complied with regulatory standards for disposal, prompting the exploration of reuse scenarios in urban and agricultural settings. However, the study underscores the imperative for rigorous disinfection due to the limited microbiological removal capacity of CW. Overall, the research emphasizes the adaptability and resilience of CW in treating diverse wastewater compositions, highlighting their pivotal role in advancing circular economy principles within sanitation services. Importantly, the findings suggest significant potential for treated wastewater reuse in both urban and agricultural settings. Urban applications, including toilet flushing, floor washing, and garden irrigation, prove feasible for CW systems situated in residential and commercial buildings. Meanwhile, rural CW systems demonstrate suitability for reclaimed water directed toward crop fertigation, tailored to the specific needs and irrigation methods of various crops.
HIGHLIGHTS
Nine systems with constructed wetlands adopted as secondary treatment units were analyzed.
The treated wastewater meets the quality required for its final disposal.
The treated wastewater has the potential to be reused in urban and agricultural scenarios.
Different design parameters were applied, reinforcing the robustness of the ecotechnology.
For effective reuse, it is necessary to conduct risk analysis studies.
INTRODUCTION
Constructed wetland (CWs) ecotechnology has emerged as a global powerhouse, revolutionizing water management and wastewater treatment with its green and multifunctional prowess. Recognized for its environmental and economic advantages, CWs stand as pivotal nature-based solutions (NBSs) on the world stage (Stefanakis 2019; Gupta et al. 2024). These robust systems, characterized by ease of operation and maintenance, exhibit high potential for application in developing countries, effectively treating diverse wastewaters and outlasting conventional technologies when properly maintained (Dotro et al. 2017; Rodriguez-Dominguez et al. 2020).
In the realm of rural sanitation, CW systems offer a promising and sustainable solution, accommodating various wastewater sources from domestic and agricultural activities. In urban contexts, CW proves versatile as an onsite treatment option and a viable choice for small communities and clusters (Moreira & Dias 2020). This ecotechnology, often referred to as low-cost operational expenditure (OPEX), not only delivers refined quality treated wastewater but also produces biomass, contributing to its cost-effectiveness in terms of effluent management, resource recovery, environmental protection, and public health risk reduction.
The economic value of CW systems extends further with the potential use of ornamental plants, the prospect of treated wastewater reuse, and biomass utilization for animal feeding or energy generation. Particularly in Brazil, where climatic conditions favor pollutant removal, CW systems present an intriguing solution for decentralized wastewater treatment (Machado et al. 2017). However, for these systems to operate optimally, adherence to design conditions, such as hydraulic and organic loading rates, is crucial. Regarding the use of vertical flow CW, Platzer (1999) recommends a hydraulic load (HLR) of 100–120 mm/day in cold climate regions and a limit of 250 mm/day for hot climate regions and a recommended range of applied organic load of between 15 and 32 g BOD/m2·day. Hoffmann et al. (2011) indicate HLR values between 60 and 80 mm/day are suitable for horizontal flow CW in hot climate regions, such as Latin America.
Guidelines to regulate and control the reuse of treated wastewater have been developed in several countries, while nations lacking specific legislation have mostly adopted the water quality standards from the World Health Organization Guidelines, the Australian Guidelines for Water Recycling, or the United States Environmental Protection Agency (USEPA-EPA/600/R-12/618) as guiding criteria (Gonçalves et al. 2021).
In the international scenario, CW associated with other technologies are well-used alternatives for the production of reclaimed water, with reuse applied in agricultural irrigation of vineyards (Milani et al. 2020), of food crops (Šereš et al. 2021), to flush toilets, and in the production of drinking water (Lakho et al. 2021). However, the use of wastewater treated by CW is not yet a conventional practice in Brazil and an important aspect for that limitation is the lack of a guarantee of effective disinfection, essential for reuse purposes (Colares et al. 2019). Therefore, the treatment system requires a post-treatment technology for removal of pathogenic microorganisms. Several studies developed projects in this context, evaluating the potential reuse of wastewater treated in different CW configurations and disinfected with different methodologies, such as solar disinfection (Santos et al. 2020), ozonation (Colares et al. 2019), and photocatalytic ozonation (Horn et al. 2014), but there is still a lack of case studies applied on a full scale.
By evaluating the onsite applicability of different CW systems for decentralized wastewater treatment in both urban and rural scenarios in southern Brazil, the research aims to shed light on the efficiency of these treatments and the untapped potential uses of the reclaimed water.
MATERIALS AND METHODS
Different CW systems applied for decentralized wastewater treatment were evaluated. The systems were grouped according to the different application scenarios (urban area, in multifamily and multiple commercial buildings, and rural area). It is noteworthy that the treatment plants covered in this study were chosen based on the different criteria associated with the following: (i) being implemented in Southern Brazil; (ii) have been operating for more than a year, and (iii) have an influent primary treatment system.
The evaluation of the performance of the CW was based on the monitoring of the operation and maintenance activities and analysis of the quality of the treated effluent. Moreover, the potential reuse of the reclaimed water was evaluated according to national reference documents that recommend quality standards for urban or agricultural purposes.
Wastewater treatment plant description
All treatment plants analyzed for the study are located in Southern Brazil, Santa Catarina state, between the latitudes 26°00'S and 30°00'S, and longitudes 48°30'W and 54°00'W. According to the Köeppen climate classification, part of the state is classified as a subtropical climate (Cfa) and part as a temperate climate (Cfb). The average annual temperature varies between 11 and 20 °C and the total annual rainfall varies between 1,100 and 2,900 mm (Pandolfo et al. 2002), being well distributed throughout the year. The altitude of the State varied between 0 and 1,753 m, with an average of 510 m. Most of the systems studied are at sea level, with an average annual temperature between 19 and 20 °C.
Summary image of the studied WC treatment systems.
All CW used are preceded by decant-digesters [septic tanks (ST) or anaerobic baffled reactors (ABR)] as primary treatment technologies. For class 1, a partially saturated vertical flow CW (VFCW-PS) was applied. In the case of class 2, all CW are horizontal modality (HFCW). Conversely, vertical flow constructed wetlands (VFCW), being aerated, were adopted in Class 3 systems. In the vertical systems, feeding occurred through pumping, which operated in pulses for 0.5 h, with the rest between 2.5 and 3.5 h, depending on the system. Furthermore, in the aerated VFCW, the oxygen is supplied through an air blower and distributed throughout the bottom of the reactor, while the influent is distributed at the top, with the air flow ascending and the influent flow descending. The blower runs for 20 h a day. More information about the systems is available in the supplementary material (Table SM1).
All CW units had sand and gravel as a filtering material and most systems adopt disinfection chlorination of the treated effluent while some systems located in commercial buildings added a phosphorus precipitation phase, prior to the disinfection of the treated effluent. It should be noted that the phosphorus precipitation phase was added to these treatment plants to comply with local legislation. The final destination of these treated effluents varied between disposal in the soil, into a river, or into the pluvial system, the latter being allowed in the state by Normative Instruction n° 05 of the Institute of the Environment of Santa Catarina (IMA 2019).
The final destination of the treated effluent from each system was linked to the terrain relief and the availability of surface water resources close to the system.
The population equivalent (PE) attended ranged from 31 to 300 and the design flow ranged from 5 to 231 m3/day, in which Class 1 had a design flow of 231 m3/day. However, during the research there was a contribution of only 4.5% of PE (100 inhabitants) and there was a variation from 150 to 200 PE among the Class 2 systems. In Class 3, there was a significative variation among the systems, with lower values in System 3a (PE of 31 inhabitants and design flow of 5 m3/day) and greater wastewater generation in System 3e (PE of 319 inhabitants and 35 m3/day). The longest running system is located in the rural inn System 2b implanted in 1994 and the most current in 2015 at the maritime terminal (System 3e). The design flows and organic loading rate (OLR), the applied values, and average hydraulic loading rate (HLR) are available in the supplementary material (Table SM2).
Design regimes differ from the applied rates, in which a reduced load is observed especially in System 1, working with 7% of the designed flow, with an overload noted in System 3a, operating with an organic load almost four times higher than the designed. It is interesting to point out that in System 3d, despite the applied flow being lower than the designed value, the system received an organic load almost three times higher than the designed value. In addition, a varied applied HLR, between 11 and 434 mm/day, implies variation of oxygen available in the bed media.
Monitoring of the wastewater treatment performance
The systems performances were evaluated based on onsite flow measurements and collection of samples for laboratory analysis. The wastewater monitoring was executed in different periods and frequency, in which flow data were analyzed in Class 1 and 3 systems and physical–chemical and microbiological parameters were evaluated in all systems.
Samples were collected for the laboratory analysis at different points in the evaluated systems, such as (i) at the inlet and outlet of each CW in Class 2 systems and (ii) at the inlet of the primary treatment unit and the outlet of the CW unit in Class 1 and Class 3 systems (3a, 3b, 3c, 3d, and 3e). Regarding this, it is important to point out that the raw effluents were not collected for analysis in Class 2 systems, only the primary effluent post-septic tank treatments were submitted for analysis. The frequency of laboratory analysis of the influent and treated effluent varies from system to system, depending on the needs of the enterprise. Subsequently, the data were compiled into a spreadsheet and the treatment performance of each system was evaluated.
The evaluated variables were: pH, chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total suspended solids (TSS), turbidity, ammonium nitrogen (N-
), ortho-phosphate phosphorus (P-
), total coliform (TC), and E. coli (EC). The analysis of all parameters followed APHA (2005) except for the N-
, which followed Vogel (1981) recommendations.
Potential reuse of treated wastewater
The research proceeded to the evaluation of the attendance of the quality of the recovered water obtained with the treatment in the different modalities of CW in the studied systems according to the different contexts, regarding the parameters recommended for reuse by different Brazilian legislations. Although there is no official national guideline for reuse of treated wastewater, there are some instruments that recommend quality standards according to different uses, among them: (i) the ‘Interáguas’ Program (2018), developed by the Ministry of Cities of the Federal Government, a set of action plans created to guide a reuse policy in Brazil; (ii) the Brazilian Standard N. 16783/19 that established the criteria for reuse in buildings (ABNT 2019); (iii) a research by ‘PROSAB’ (Research Program in Sanitation) on standards for agricultural, urban, aquaculture, and toilet reuse (Bastos et al. 2008); (iv) a manual published by the National Water Agency in partnership with Industry Associations in the State of São Paulo on water conservation and reuse in buildings (Sautchuk et al. 2005); (v) Minas Gerais State Regulation that stands out nationally with its legislation covering several types of reuse, including fertigation (CERH-MG 2020).
In this sense, analyzing different classifications presented by the reference documents, the uses were defined for two purposes according to the contexts in which they are inserted: urban reuse or agriculture reuse.
RESULTS AND DISCUSSION
CW treatment performances
In general, the results indicate that the treatment efficiency is directly related to the different technological arrangements and CW configurations adopted in each system, considering the inlet wastewater, and respective organic and hydraulic loads applied.
Table 1 summarizes the treatment performance of CW units, exposing the average influent and effluent quality parameters and the removal efficiency of each parameter. In Class 2 systems, the treatment efficiencies were calculated over the inlet and outlet CW systems, disregarding the decant-digesters. Thus, the global efficiency of the systems is greater than those presented.
Influent and effluent average quality, standard deviation, and removal efficiency (% or log) in studied systems
| Parameters | pH | COD (mg/L) | BOD (mg/L) | Turbidity (NTU) | TSS (mg/L) | N- (mg/L) | P- (mg/L) | TC (MPN/100 mL) | E. coli (MPN/100 mL) |
|---|---|---|---|---|---|---|---|---|---|
| System 1 –anaerobic baffle reactor followed by vertical flow partially saturated constructed wetland | |||||||||
| Influent | 7.0 ± 0.3 (40) | 277 ± 121 (40) | 158 ± 45 (8) | 50 ± 43 (40) | 53 ± 16 (40) | 17 ± 9 (40) | 2.1 × 108 (14) | 3.2 × 107 (14) | |
| Effluent | 6.5 ± 0.4 (40) | 19 ± 10 (32) | 5 ± 5 (9) | 3 ± 3 (40) | 4 ± 2 (40) | 1 ± 1 (40) | 2.52 × 105 (14) | 2.19 × 104 (14) | |
| Removal | 93% | 97% | 94% | 93% | 92% | 1.4 log | 2.04 log | ||
| System 2a– septic tank followed by horizontal flow constructed wetland | |||||||||
| Influent | 6.5 ± 0,3 (8) | 421 ± 218 (8) | 282 ± 192 (8) | 110 ± 88 (8) | 77 ± 120 (8) | 29 ± 12 (8) | 1.9 × 106 (8) | 8.9 × 105 (8) | |
| Effluent | 6.2 ± 0.4 (8) | 93 ± 84 (8) | 59 ± 75 (8) | 38 ± 42 (8) | 35 ± 65 (8) | 17 ± 22 (8) | 2.34 × 105 (8) | 2.79 × 104 (8) | |
| Removal | 78% | 79% | 65% | 55% | 40% | 0.91 log | 1.50 log | ||
| System 2b– septic tank followed by horizontal flow constructed wetland | |||||||||
| Influent | 5.6 ± 0,9 (12) | 1,699 ± 944 (12) | 993 ± 402 (12) | 219 ± 90 (12) | 274 ± 205 (12) | 61 ± 31 (12) | 31 ± 12 (12) | 1.9 × 106 (12) | |
| Effluent | 6.0 ± 0.8 (12) | 30 ± 21 (12) | 31 ± 25 (12) | 187 ± 160 (12) | 45 ± 30 (12) | 18 ± 56 (12) | 7 ± 34 (12) | 2,.57 × 104 (12) | |
| Removal | 98% | 97% | 15% | 84% | 70% | 76% | 1.87 log | ||
| System 2c– septic tank followed by horizontal flow constructed wetland | |||||||||
| Influent | 6.3 ± 0,2 (13) | 676 ± 791 (13) | 326 ± 394 (13) | 104 ± 127 (13) | 451 ± 700 (13) | 28 ± 21 (13) | 4 ± 4 (13) | 5.9 × 106 (13) | |
| Effluent | 6.1 ± 0.2 (13) | 76 ± 48 (13) | 27 ± 25 (13) | 26 ± 14 (13) | 82 ± 33 (13) | 14 ± 9 (13) | 1 ± 2 (13) | 4.82 × 105 (13) | |
| Removal | 89% | 92% | 75% | 82% | 50% | 65% | 1.09 log | ||
| System 3a– septic tank followed by vertical flow constructed wetland | |||||||||
| Influent | 7.87 ± 0.6 (7) | 435 ± 199 (7) | 211 ± 118 (7) | – | 198 ± 61 (7) | 16 ± 6 (7) | |||
| Effluent | 7.20 (7) | 133 (7) | 47 (7) | 105 (7) | 11 (7) | ||||
| Removal | 69% | 78% | 47% | 31% | |||||
| System 3b– septic tank followed by vertical flow constructed wetland | |||||||||
| Influent | 7.58 ± 0.36 (7) | 495 ± 166 (7) | 187 ± 102 (7) | 184 ± 61 (7) | 13 ± 7 (7) | ||||
| Effluent | 6.93 (7) | 82 (7) | 26 (7) | 57 (7) | 8 (7) | ||||
| Removal | 83% | 86% | 69% | 38% | |||||
| System 3c– septic tank followed by vertical flow constructed wetland | |||||||||
| Influent | 153 ± 70 (64) | 50 ± 37 (64) | 11 ± 9a (64) | ||||||
| Effluent | 6.62 (64) | 49 (64) | 11 (64) | 4a (64) | |||||
| Removal | 68% | 78% | 64%a | ||||||
| System 3d– anaerobic baffle reactor followed by vertical flow constructed wetland | |||||||||
| Influent | 7.20 ± 0.69 (27) | 724 ± 388 (27) | 390 ± 174 (6) | 134 ± 124 (27) | 103 ± 36 (27) | 27 ± 11 (27) | 1.09 × 109 (11) | ||
| Effluent | 6.31 ± 0.93 (27) | 179 ± 161 (19) | 48 ± 14 (9) | 22 ± 17 (27) | 54 ± 32 (27) | 10 ± 7 (27) | 9.15 × 107 (11) | ||
| Removal | 75% | 88% | 84% | 48% | 62% | 1.07 log | |||
| System 3e– anaerobic baffle reactor followed by aerated vertical flow constructed wetland | |||||||||
| Influent | 7.17 ± 0.27 (3) | 1,159 ± 408 (3) | 428 ± 173 (3) | 34 ± 20a (3) | |||||
| Effluent | 6.00 (3) | 29 (3) | 5 (3) | 9a (3) | |||||
| Removal | 97% | 99% | 74%a | ||||||
| Parameters | pH | COD (mg/L) | BOD (mg/L) | Turbidity (NTU) | TSS (mg/L) | N- (mg/L) | P- (mg/L) | TC (MPN/100 mL) | E. coli (MPN/100 mL) |
|---|---|---|---|---|---|---|---|---|---|
| System 1 –anaerobic baffle reactor followed by vertical flow partially saturated constructed wetland | |||||||||
| Influent | 7.0 ± 0.3 (40) | 277 ± 121 (40) | 158 ± 45 (8) | 50 ± 43 (40) | 53 ± 16 (40) | 17 ± 9 (40) | 2.1 × 108 (14) | 3.2 × 107 (14) | |
| Effluent | 6.5 ± 0.4 (40) | 19 ± 10 (32) | 5 ± 5 (9) | 3 ± 3 (40) | 4 ± 2 (40) | 1 ± 1 (40) | 2.52 × 105 (14) | 2.19 × 104 (14) | |
| Removal | 93% | 97% | 94% | 93% | 92% | 1.4 log | 2.04 log | ||
| System 2a– septic tank followed by horizontal flow constructed wetland | |||||||||
| Influent | 6.5 ± 0,3 (8) | 421 ± 218 (8) | 282 ± 192 (8) | 110 ± 88 (8) | 77 ± 120 (8) | 29 ± 12 (8) | 1.9 × 106 (8) | 8.9 × 105 (8) | |
| Effluent | 6.2 ± 0.4 (8) | 93 ± 84 (8) | 59 ± 75 (8) | 38 ± 42 (8) | 35 ± 65 (8) | 17 ± 22 (8) | 2.34 × 105 (8) | 2.79 × 104 (8) | |
| Removal | 78% | 79% | 65% | 55% | 40% | 0.91 log | 1.50 log | ||
| System 2b– septic tank followed by horizontal flow constructed wetland | |||||||||
| Influent | 5.6 ± 0,9 (12) | 1,699 ± 944 (12) | 993 ± 402 (12) | 219 ± 90 (12) | 274 ± 205 (12) | 61 ± 31 (12) | 31 ± 12 (12) | 1.9 × 106 (12) | |
| Effluent | 6.0 ± 0.8 (12) | 30 ± 21 (12) | 31 ± 25 (12) | 187 ± 160 (12) | 45 ± 30 (12) | 18 ± 56 (12) | 7 ± 34 (12) | 2,.57 × 104 (12) | |
| Removal | 98% | 97% | 15% | 84% | 70% | 76% | 1.87 log | ||
| System 2c– septic tank followed by horizontal flow constructed wetland | |||||||||
| Influent | 6.3 ± 0,2 (13) | 676 ± 791 (13) | 326 ± 394 (13) | 104 ± 127 (13) | 451 ± 700 (13) | 28 ± 21 (13) | 4 ± 4 (13) | 5.9 × 106 (13) | |
| Effluent | 6.1 ± 0.2 (13) | 76 ± 48 (13) | 27 ± 25 (13) | 26 ± 14 (13) | 82 ± 33 (13) | 14 ± 9 (13) | 1 ± 2 (13) | 4.82 × 105 (13) | |
| Removal | 89% | 92% | 75% | 82% | 50% | 65% | 1.09 log | ||
| System 3a– septic tank followed by vertical flow constructed wetland | |||||||||
| Influent | 7.87 ± 0.6 (7) | 435 ± 199 (7) | 211 ± 118 (7) | – | 198 ± 61 (7) | 16 ± 6 (7) | |||
| Effluent | 7.20 (7) | 133 (7) | 47 (7) | 105 (7) | 11 (7) | ||||
| Removal | 69% | 78% | 47% | 31% | |||||
| System 3b– septic tank followed by vertical flow constructed wetland | |||||||||
| Influent | 7.58 ± 0.36 (7) | 495 ± 166 (7) | 187 ± 102 (7) | 184 ± 61 (7) | 13 ± 7 (7) | ||||
| Effluent | 6.93 (7) | 82 (7) | 26 (7) | 57 (7) | 8 (7) | ||||
| Removal | 83% | 86% | 69% | 38% | |||||
| System 3c– septic tank followed by vertical flow constructed wetland | |||||||||
| Influent | 153 ± 70 (64) | 50 ± 37 (64) | 11 ± 9a (64) | ||||||
| Effluent | 6.62 (64) | 49 (64) | 11 (64) | 4a (64) | |||||
| Removal | 68% | 78% | 64%a | ||||||
| System 3d– anaerobic baffle reactor followed by vertical flow constructed wetland | |||||||||
| Influent | 7.20 ± 0.69 (27) | 724 ± 388 (27) | 390 ± 174 (6) | 134 ± 124 (27) | 103 ± 36 (27) | 27 ± 11 (27) | 1.09 × 109 (11) | ||
| Effluent | 6.31 ± 0.93 (27) | 179 ± 161 (19) | 48 ± 14 (9) | 22 ± 17 (27) | 54 ± 32 (27) | 10 ± 7 (27) | 9.15 × 107 (11) | ||
| Removal | 75% | 88% | 84% | 48% | 62% | 1.07 log | |||
| System 3e– anaerobic baffle reactor followed by aerated vertical flow constructed wetland | |||||||||
| Influent | 7.17 ± 0.27 (3) | 1,159 ± 408 (3) | 428 ± 173 (3) | 34 ± 20a (3) | |||||
| Effluent | 6.00 (3) | 29 (3) | 5 (3) | 9a (3) | |||||
| Removal | 97% | 99% | 74%a | ||||||
() number of valid samples.
aValues related to total phosphorus parameter.
Among the systems composed of HFCW (Class 2), the highest organic matter removal efficiency was obtained in System 2b, precisely the one that had the highest concentrations of COD and BOD, due to the mixture of domestic wastewater with the effluent generated in the food processing activities. System 2a, conversely, had the lowest efficiency and its effluent had the highest concentration of organic matter among non-commercial systems. Specifically, those systems reached between 78 and 98% for COD removal and 79–97% for BOD removal, very similarly to results reported by Machado et al. (2017) in an analysis of a series of HFCW performing in Brazil, with an average value of 76% COD removal and 81% BOD removal.
In turbidity removal, the highest efficiency was in System 2c, followed by 2a and 2b, with efficiencies of 75, 65, and 15%, respectively, and TSS removal was significant in Systems 2b (84%) and 2c (82%). Nutrient removal was less significative in systems composed of HFCW, with Systems 2a, 2b, and 2c showing efficiencies of 55, 70, and 50% for N-
and 40, 76, and 65% for P-
, respectively. Although similar or even lower nutrient removals in HFCW were reported in other studies, such as in Jácome et al. (2016) with 52% N-
removal and 66% TP removal, and in Licata et al. (2021) with 54% N-
removal and 41% TP removal.
System 1 achieved an 89% removal of COD and 94% removal of BOD, similarly to results reported by Bassani et al. (2021), with 92% COD removal and 91% BOD removal. On the removal of TSS and nutrients (N-
and P-
), the system obtained the highest efficiencies among the studied systems, with 89, 92, and 90%, respectively. The high ammonium–nitrogen removal efficiency may be associated with the partial saturation of the media, creating anaerobic conditions in the lower part and aerobic conditions in the top part, providing simultaneous nitrification and denitrification in the unit (Silveira et al. 2015).
Among the commercial systems (Class 3), all composed by VFCW, the best performance was achieved in System 3e, which is composed of an ABR followed by an aerated VFCW. This system received the wastewater with the highest concentrations of organic matter and achieved a removal efficiency of 97 and 99% of COD and BOD, respectively, fulfilling the disposal standard for the treated wastewater. As reported by Dotro et al. (2017), aerated VFCW are extremely efficient in organic matter and ammonium–nitrogen removal, with values close to 99% for BOD removal and 99% for N-
removal.
The other commercial systems (3a, 3b, 3c, and 3d), all composed of VFCW, obtained values of 69, 83, 68, and 70% for COD, respectively, and 78, 86, 78, and 85% for BOD, respectively. Those results are greater than reported by Machado et al. (2017) regarding a series of VFCW applied in decentralized systems in Brazil, with an average of 61% COD removal and 67% BOD removal.
Only System 3d performed a TSS analysis with a removal of 78%. The removal of N-
was greater through System 3b with an efficiency of 69%, followed by Systems 3a and 3d, with 47 and 45%, respectively. Concerning phosphorus removal, Systems 3a, 3b, and 3d performed P-
analysis with removals of 31, 38, and 60%, respectively, while in Systems 3c and 3e, total phosphorus (TP) analyses were carried out, with efficiencies of 64 and 73%, respectively. Similar results were obtained in a study carried out using VFCW to treat municipal wastewater, with ammonium–nitrogen removal efficiencies between 31 and 70% and an average percentage phosphate removal of 62% (Abou-Elela & Hellal 2012). On the microbiological parameters, there was no significant removal of total coliforms and E. coli in the systems, varying between 1 and 2 log units of removal in non-commercial systems and with a reduction of 1.07 log unit for TC and 0.99 log unit for EC in System 3d, values consistent with those stated by Kadlec & Wallace (2008), reiterating that these pollutants are most significatively removed during the disinfection step.
Influent BOD (raw effluent – green; primary effluent – blue), treated effluent BOD (orange) and respective allowed BOD values for final disposal according to different destinations for (a) non-commercial (1 – multifamily residential building; 2 – rural area) and (b) commercial systems.
Influent BOD (raw effluent – green; primary effluent – blue), treated effluent BOD (orange) and respective allowed BOD values for final disposal according to different destinations for (a) non-commercial (1 – multifamily residential building; 2 – rural area) and (b) commercial systems.
System 1 is the only system that disposes of the treated wastewater into a small river. That kind of destination is supervised by the Santa Catarina State Council for the Environment (CONSEMA 2021), through Resolution n° 181/21. The parameters analyzed in System 1 (Table 1) are in accordance with the Resolution (BOD < 60 mg/L), meeting the legal requirements for proper release.
Systems 2a, 2b, 2c, and 3b dispose of the treated wastewater through soil infiltration, a destination that has no specific federal or state legislation. However, the neighbor state, with similar characteristics to the state of Santa Catarina, has a specific legislation for this matter—the Resolution 68/19 of the Rio Grande do Sul State Council for the Environment (CONSEMA 2019). Evaluating the CW effluent quality from Systems 2a, 2b, 2c, and 3b with that legislation, almost all legal requirements were fulfilled, but the parameter of coliforms, in which no system complied with the recommended one reinforcing the need to perform disinfection after the CW.
Finally, Systems 3a, 3c, 3d, and 3e release the treated effluent into the pluvial pipe system, whose release pattern is supervised by the Brazilian normative NBR 13969 of 1997 (ABNT 1997). Analyzing the treated effluent quality, all systems complied with the standards required by the normative, except System 3d, which did not meet the COD limits of a maximum of 150 mg/L.
Reuse potential
Classification of potential reuse of treated wastewater in the CW systems studied.
Classification of potential reuse of treated wastewater in the CW systems studied.
Looking critically at the results obtained and comparing with the values suggested by the Brazilian guideline instruments, it is noted that there is no national consensus on the analyzed parameters, reuse classes (restricted or unrestricted), and maximum values allowed. The guideline documents adopted in the research also present other types of parameters, such as electrical conductivity, true color, total residual chlorine, and helminth eggs, which were not evaluated in this study. By analyzing the results in their entirety, the systems can be classified for reuse as presented in Table 2, with some points to note.
Potential treated wastewater reuse according to different scenarios, applicability, and Brazilian guidelines
| Systems | Reuse | Document (class) | Observation |
|---|---|---|---|
| 1 | Unrestricted urban use (toilet flushing, floor washing, green areas irrigation) | ABNT NBR 16.783/19; Interáguas | Reduction of CT values after the disinfection stage. |
| 2a | Limited fertigation (surface or localized irrigation, avoiding any contact of reuse water with the food product) | CERH-MG | |
| 2b | Restricted agricultural use (Surface or aspersion irrigation; hydroponic cultivation of any crop not eaten raw, includes food and non-food crops, forages, pastures, and trees); limited fertigation. | PROSAB; CERH-MG | |
| 2c | Limited fertigation (surface or localized irrigation, avoiding any contact of reuse water with the food product) | CERH-MG | |
| 3b | Restricted urban use (landscape irrigation in restricted areas, clearing sewage systems, and civil construction) | Interáguas | Reduction of CT values after the disinfection stage. |
| 3c | Unrestricted urban use (toilet flushing, floor washing, green areas irrigation) | ABNT NBR 16.783/19; Interáguas | Reduction of CT values after the disinfection stage. |
| 3e | Unrestricted urban use (toilet flushing, floor washing, green areas irrigation) | ABNT NBR 16.783/19; Interáguas | Reduction of CT values after the disinfection stage. |
| Systems | Reuse | Document (class) | Observation |
|---|---|---|---|
| 1 | Unrestricted urban use (toilet flushing, floor washing, green areas irrigation) | ABNT NBR 16.783/19; Interáguas | Reduction of CT values after the disinfection stage. |
| 2a | Limited fertigation (surface or localized irrigation, avoiding any contact of reuse water with the food product) | CERH-MG | |
| 2b | Restricted agricultural use (Surface or aspersion irrigation; hydroponic cultivation of any crop not eaten raw, includes food and non-food crops, forages, pastures, and trees); limited fertigation. | PROSAB; CERH-MG | |
| 2c | Limited fertigation (surface or localized irrigation, avoiding any contact of reuse water with the food product) | CERH-MG | |
| 3b | Restricted urban use (landscape irrigation in restricted areas, clearing sewage systems, and civil construction) | Interáguas | Reduction of CT values after the disinfection stage. |
| 3c | Unrestricted urban use (toilet flushing, floor washing, green areas irrigation) | ABNT NBR 16.783/19; Interáguas | Reduction of CT values after the disinfection stage. |
| 3e | Unrestricted urban use (toilet flushing, floor washing, green areas irrigation) | ABNT NBR 16.783/19; Interáguas | Reduction of CT values after the disinfection stage. |
CONCLUSION
Nine decentralized domestic wastewater treatment plants in Southern Brazil were analyzed in this study, in which CW ecotechnology was adopted in secondary treatment units. The treated effluents were compared with the effluent discharge Brazilian standards and also with the national wastewater reuse guidelines. Four different constructed wetland modalities were designed in the studied systems, remaining operational even when different hydraulic and organic loads were applied, emphasizing the robustness of the ecotechnology. All systems produced a treated wastewater befitting the quality required for its final destination—that being the pluvial system, water body, or soil infiltration. Also, most systems showed themselves to be suitable for different reuse practices, reinforcing the important role of CW in the transition to a circular economy within sanitation services.
The wastewater treated by CW could be directed for urban or agricultural reuse, according to the scenario and applicability of the studied systems. Urban reuse is considered for those systems located in the residential building and in the multiple commercial buildings, with the application of the reclaimed water in activities like toilet flushing, floor washing, and garden irrigation. For the systems located in the rural inns, the agricultural reuse for crop irrigation and fertigation is more interesting and must be directed according to the needs of the different crops and irrigation methods.
The findings provide valuable insights for policymakers and practitioners, emphasizing the importance of effective disinfection measures, tailored guidelines, and the fostering of a national consensus. Moving forward, CW offer a promising nature-based solution for sustainable wastewater management, contributing to the transition to a circular economy in sanitation services, with the caveat that risk analysis studies are imperative for ensuring human and environmental safety in effluent reuse.
ACKNOWLEDGMENTS
The authors would like to thank the Research Support Foundation of the State of Santa Catarina – FAPESC (2021TR001812 and 2021TR750); the National Council for Scientific and Technological Development – CNPq (402114/2021-3 and 309572/2020-7); the Coordination for the Improvement of Higher Education Personnel – CAPES; Rotaria do Brasil Company.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.
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