Abstract
This research evaluated the wastewater treatment system of the Punta Carnero sector, in relation to pollutant efficiency load removal, final effluent quality and impact on the ecosystem, and finally to determine if the final discharge can be reused for agricultural irrigation. The research was based on the affluent and effluent characterization of the system, carried out in three phases: i) Taking of simple samples, analyzed in an accredited water laboratory and analysis of the contaminant loads efficiency; ii) Review of results compared to the Table of “Discharge limits to a freshwater receiving body”; iii) Examination of results based on the “Water Quality Criteria for Agricultural Irrigation” Table of the Ecuadorian regulation TULSMA (2015)TULSMA. Libro VI de la calidad ambiental. Quito: Environment Ministry of Ecuador, 2015. p. 184.. BOD (62.42%), COD (62.41%) and FC (53.58%) removal efficiencies did not comply with current Ecuadorian regulations. The quality of the effluent with respect to the parameters evaluated for discharges to a freshwater receiving body denoted a non-optimal quality of final discharge, negatively impacting the ecosystem. Finally, the evaluation determined parameters that exceed the water quality criteria for agricultural irrigation allowed: Oils-Fats (5.65 mg/l), FC (62,900 NMP/100ml), Hg (0.00141 mg/l), OD (8.86 mg/l). After evaluating the wastewater treatment system, it was determined that the pollutant load removal efficiency and effluent quality is not optimal for discharge into a receiving water body, so it’s not suitable for reuse in agricultural irrigation.
Keywords:
affluent; effluent; water quality
Resumo
O objetivo da pesquisa foi avaliar o sistema de tratamento de águas residuais do setor de Punta Carnero, em relação à sua eficiência na remoção de cargas poluentes, a qualidade do efluente final e seu impacto no ecossistema e, finalmente, determinar se o descarte final pode ser reutilizado para irrigação agrícola. A pesquisa consistiu na caracterização da influência e do efluente do sistema, desenvolvido em três fases: i) Retirada de amostras simples, testadas em um laboratório de água credenciado e análise da eficiência das cargas poluentes; ii) Revisão dos resultados comparados com a Tabela de "Limites de descarga para um corpo receptor de água doce", iii) Exame dos resultados de acordo com a Tabela de "Critérios de qualidade da água para irrigação agrícola" da regulamentação equatoriana TULSMA (2015)TULSMA. Libro VI de la calidad ambiental. Quito: Environment Ministry of Ecuador, 2015. p. 184.. A eficiência de remoção da BDO (62,42%), COD (62,41%) e CF (53,58%) não estão em conformidade com os regulamentos atuais do Equador. A qualidade do efluente com respeito aos parâmetros avaliados para descargas em um corpo receptor de água doce, denotam uma qualidade não ideal de descarga final, afetando negativamente o ecossistema; finalmente, a avaliação determinou parâmetros que excedem os critérios de qualidade da água para irrigação agrícola: Óleos-Gorduras (5,65 mg/l), CF (62900 NMP/100ml), Hg (0,00141 mg/l), OD (8,86 mg/l). Após avaliação do sistema de tratamento de águas residuais, foi determinado que sua eficiência na remoção de cargas poluentes e sua qualidade de efluentes não é ótima para ser descarregada em um corpo de água receptor, portanto, não é adequada para reutilização na irrigação agrícola.
Palavras-chave:
afluente; efluente; qualidade da água
1. INTRODUCTION
In Latin America, the application of biological treatments to treat wastewater of domestic origin is more readily available (Vargas et al., 2020VARGAS, A.; CALDERÓN, J.; VELÁSQUEZ, D.; CASTRO, M., NÚÑEZ, D. Biological system analysis for domestic wastewater treatment in Colombia. Ingeniare: Chilean Journal of Engineering, v. 28, p. 315-322, 2020. https://dx.doi.org/10.4067/S0718-33052020000200315
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). This treatment is known as “conventional” because it’s common and widely used, which usually involves low costs (Roy and Saha, 2021ROY, M.; SAHA, R. 6 - Dyes and their removal technologies from wastewater: A critical review. In: BHATTACHARYYA, S.; MONDAL, N. K.; PLATOS, J.; SNÁŠEL, V.; KRÖMER, P. (eds). Intelligent Environmental Data Monitoring for Pollution Management. Elsevier, 2021. p. 127-160. ). The treatments are simple, efficient and cost-effective, and use microorganisms to degrade a large part of biodegradable waste from wastewater effluents (Al-Qodah et al., 2020AL-QODAH, Z.; AL-QUDAH, Y.; ASSIREY, E. Combined biological wastewater treatment with electrocoagulation as a post-polishing process: A review. Separation Science and Technology, v. 55, n. 13, p. 2334-2352, 2020. https://doi.org/10.1080/01496395.2019.1626891
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; Jung and Pauly, 2011JUNG, H.; PAULY, D. 4.19 - Water in the Pulp and Paper Industry. In: WILDERER, P. (ed.). Treatise on Water Science. Oxford: Elsevier, 2011. p. 667-683.). The main objectives of these treatments are the elimination of pathogenic microorganisms, suspended solids and the reduction of organic matter to an acceptable level (Grigorieva et al., 2013GRIGORIEVA, E. V.; BONDARENKO, N. V.; KHAILOV, E. N.; KOROBEINIKO, A. Analysis of optimal control problems for the process of wastewater biological treatment. Math Journal: Theory and Applications, v. 20, n. 2, p. 103-118, 2013.; Leite et al., 2005LEITE, V.; ATHAYDE, G.; SOUSA, J.; LOPES, W.; PRASAD, S.; SILVA, S. Tratamento de águas residuárias em lagoas de estabilização para aplicação na fertirrigação. Revista Brasileira de Engenharia Agrícola e Ambiental, v. 9, n. 1, p. 71-75, 2005. https://doi.org/10.1590/1807-1929/agriambi.v9nsupp71-75
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).
Stabilization lagoons are passive systems built to treat wastewater by biological processes (Florentino et al., 2019FLORENTINO, A.; COSTA, M.; NASCIMENTO, J.; FARES ABDALA, E.; MOTA, C.; DOS SANTOS, A. Identification of microalgae from waste stabilization ponds and evaluation of electroflotation by alternate current for simultaneous biomass separation and cell disruption. Engenharia Sanitaria e Ambiental, v. 24, n. 1, p. 177-186, 2019. https://doi.org/10.1590/S1413-41522019193972
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); they are used for the upgrading of liquid effluents from domestic, agricultural and industrial sources (Jimoh et al., 2019JIMOH, T. A.; KESHINRO, M. O.; COWAN, K. A. Microalgal-Bacterial Flocs and Extracellular Polymeric Substances: Two Essential and Valuable Products of Integrated Algal Pond Systems. Water, Air, & Soil Pollution, v. 230, n. 4, p. 95, 2019. https://doi.org/10.1007/s11270-019-4148-3
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; Nuñez and Fragoso, 2020NUÑEZ, J.; FRAGOSO, P. Use of aquatic macroinvertebrates as an evaluation system for stabilizing El Salguero lagoon (Colombia). Technological Information, v. 31, n. 3, p. 277-284, 2020. http://dx.doi.org/10.4067/S0718-07642020000300277
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). Due to the low implementation cost, ease of operation, minimal energy consumption and high efficiency in the reduction of pathogenic organisms, this is the type of wastewater treatment most used in underdeveloped countries (Araújo et al., 2016ARAÚJO, G.; LIMA, I.; ARAÚJO, A.; SILVA, M. Avaliação experimental e modelagem matemática de filtros anaeróbios como alternativa de baixo custo para remoção de algas de efluentes de lagoas de estabilização. Engenharia Sanitária e Ambiental, v. 21, n. 4, p. 687-696, 2016. https://doi.org/10.1590/S1413-41522016134641
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; Li et al., 2018LI, M.; ZHANG, H.; LEMCKERT, C.; ROIKO, A.; STRATTON, H. On the hydrodynamics and treatment efficiency of waste stabilisation ponds: From a literature review to a strategic evaluation framework. Journal of Cleaner Production, v. 183, p. 495-514, 2018. https://doi.org/10.1016/j.jclepro.2018.01.199
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; Romero and Castillo, 2018ROMERO, T.; CASTILLO, Y. Updating of status of Mayabeque city stabilization lagoons. Hydraulic and Environmental Engineering, v. 39, n. 2, p. 72-85, 2018. ). There are generally three types of lagoon systems: anaerobic, facultative and maturation or aerobic, each with different design and treatment characteristics (Dos Santos and Van Haandel, 2021 DOS SANTOS, S. L.; VAN HAANDEL, A. Transformation of Waste Stabilization Ponds: Reengineering of an Obsolete Sewage Treatment System. Water 2021, v. 13, n. 9, p. 1193, 2021. https://doi.org/10.3390/w13091193
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; Matsumoto and Sánchez, 2016MATSUMOTO, T.; SÁNCHEZ, I. Performance of the Sewage Treatment Plant of São João de Iracema (Brazil). Ingeniería, v. 21, n. 2, p. 176-186, 2016. https://doi.org/10.14483/udistrital.jour.reving.2016.2.a04
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). Anaerobic lagoons operate in the absence of oxygen and have depths of 3 to 5 meters (Perú, 2007PERÚ. Comisión Nacional del Agua. Manual de Agua Potable, Alcantarillado y Saneamiento. Lima, 2007. 234p. ; Cortés et al., 2017CORTÉS, F.; TREVIÑO, A.; ESPINOZA, A.; SÁENZ, A.; ALCORTA, M.; GONZÁLEZ, J. et al. Optimization in the design of a wastewater treatment system integrated by three stabilization lagoon. Water Technology and Sciences, v. 8, n. 4, p. 139-155, 2017. https://doi.org/10.24850/j-tyca-2017-04-09
https://doi.org/10.24850/j-tyca-2017-04-...
); facultative lagoons decompose the Biochemical Oxygen Demand (BOD) or organic matter (Sánchez and Matsumoto, 2013SÁNCHEZ, I.; MATSUMOTO, T. Bathymetric survey and performance of waste stabilization ponds system. Journal of Agricultural Science, v. 30, n. 1, p. 65-78, 2013. ), by aerobic, anaerobic and facultative bacteria, with depths ranging from 1.5 to 2.5 meters (Sánchez et al., 2011SÁNCHEZ, R.; ROSA, E.; MORENO, M. Analysis of the reliability of operation of optional lagoons primaries in Villa Clara-Cuba. Chemical Technology, v. 31, n. 1, p. 23-38, 2011. ; Treviño and Cortés, 2016TREVIÑO, A.; CORTÉS, F. Reduced design method for stabilization lagoon. Mexican Journal of Agricultural Sciences, v. 7, n. 4, p. 729-742, 2016. ). Maturation lagoons are used at the end of treatment, and their depths vary from 0.5 to 1.5 meters (Cárdenas et al., 2005CÁRDENAS, C.; JAEGER, C.; VILLASMIL, H.; PERRUOLO, T.; YABROUDI, S.; LÓPEZ, F.; HERRERA, L.; CASTEJÓN, O. Evaluation of the units that conform the wastewater treatment plant south Maracaibo. Journal of the Engineering Technical Faculty of the Zulia's University, v. 28, n. 2, p. 97-109, 2005. ; Tilley, 2011TILLEY, E. et al. Compendium of sanitation systems and technologies. 2nd ed. Dübendorf: EAWAG, 2011. p. 161.). The efficiency of stabilization lagoons depends mainly on factors such as depth, hydraulic retention time, temperature, bacteria and algae (Bezerra et al., 2020BEZERRA, E.; DE SOUZA, I.; SAAVEDRA, N. Variability in phytoplankton community structure and influence on stabilization pond functioning. Revista Ambiente & Água, v. 15, n. 2, p. 1-13, 2020. https://doi.org/10.4136/ambi-agua.2507
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).
González and Chiroles (2011)GONZÁLEZ, M.; CHIROLES, S. Safe use and microbiological risks of wastewater for agriculture. Cuban Journal of Public Health, v. 37, n. 1, p. 61-73, 2011. argue that for every liter of wastewater, at least eight liters of freshwater are polluted, so it is important to consider that untreated or inadequately treated effluents are the main source of pollution to natural water bodies (Cedeño, 2020CEDEÑO, H. Analysis of the water quality parameters of the dead river effluent for possible reuse of Manta. Knowledge Pole: Scientific-Professional Journal, v. 5, n. 2, p. 579-604, 2020. http://dx.doi.org/10.23857/pc.v5i2.1299
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), changing their chemistry and seriously altering the ecosystems that depend on them (Lahera, 2010LAHERA, V. Sustainable Infrastructure: Wastewater Treatment Plants. Journal of Territorial Studies, v. 12, n. 2, p. 58-69, 2010. ). Wastewater treatment plants are built to reduce the impact of polluted effluents (Morera et al., 2017MORERA, S.; COROMINAS, L.; RIGOLA, M.; POCH, M., COMAS, J. Using a detailed inventory of a large wastewater treatment plant to estimate the relative importance of construction to the overall environmental impacts. Water Research, v. 122, p. 614-623, 2017. https://doi.org/10.1016/j.watres.2017.05.069
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); however, many facilities leave aside the external effects that may occur in the operation and maintenance phase in each of these production units (Hernández et al., 2017HERNÁNDEZ, A.; QUIMIS, A.; MOLINA, G.; MORENO, L. The treatment of wastewater in Portoviejo Canton and its potential environmental impact. UNESUM-Sciences: Multidisciplinary Scientific Journal, v. 1, n. 2, p. 47-58, 2017. https://doi.org/10.47230/unesum-ciencias.v1.n2.2017.17
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), so that the implementation of environmental regulations and standards are presented in order to define water quality criteria for discharge and ensure the non-contamination of water resources (Marçal and Silva, 2017MARÇAL, D.; SILVA, C. Avaliação do impacto do efluente da estação de tratamento de esgoto ETE-Pirajá sobre o Rio Parnaíba, Teresina (PI). Engenharia Sanitária e Ambiental, v. 22, n. 4, p. 761-772, 2017. https://doi.org/10.1590/S1413-41522017148242
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). In Ecuador, problems have arisen due to the absence of sufficient treatment and physical infrastructure (Montero et al., 2020MONTERO, F.; MOLINA, C.; PILLCO, B.; SARDUY, L.; DIÉGUEZ, K. Evaluation of the Environmental Impact of the Wastewater Treatment Plant Construction. Case Pindo Chico River, Puyo, Pastaza, Ecuador. Science, Environment and Climate, v. 3, n. 1, p. 23-39, 2020. https://doi.org/10.22206/cac.2020.v3i1.pp23-39
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), and it’s estimated that only 10% of wastewater is discharged into bodies of water with some type of effluent treatment (Sato et al., 2013SATO, T.; QADIR, M.; YAMAMOTO, S.; ENDO, T.; ZAHOOR, A. Global, regional, and country level need for data on wastewater generation, treatment, and use. Agricultural Water Management, v. 130, p. 1-13, 2013. https://doi.org/10.1016/j.agwat.2013.08.007
https://doi.org/10.1016/j.agwat.2013.08....
; Velasco et al., 2019VELASCO, G.; MONCAYO, J.; CHUQUER, D. Diagnosis of wastewater treatment system of Manta, Infoanalytic Journal, v. 7, n. 1, p. 27-39, 2019. https://doi.org/10.26807/ia.v7i1.93
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). Ecuadoran public institutions charged with regulating wastewater discharge include the Environmental Management Departments of Provincial Prefectures and Mayor’s Offices to the highest environmental authority in the country, which is the Ministry of the Environment (Peña et al., 2018PEÑA, S.; MAYORGA, J.; MONTOYA, R. Proposal for the treatment of wastewater from the city of Yaguachi (Ecuador). Science and Engineering, v. 39, n. 2, p. 161-167, 2018.). The reuse of wastewater for agricultural irrigation has advantages associated with the improvement of the fertility of agricultural soils by providing organic matter (Silva et al., 2018SILVA, J.; TORRES, P.; MADERA, C. Domestic wastewater reuse in agriculture. A review. Colombian Agronomy, v. 26, n. 2, p. 347-359, 2018. ), but the effluent discharged must be evaluated from the agronomic and bacteriological point of view (Cisneros and Saucedo, 2016CISNEROS, O.; SAUCEDO, H. Wastewater reuse in agriculture. Water Technology Institute Mexican, 2016. 170 p. ). Therefore, TULSMA (2015)TULSMA. Libro VI de la calidad ambiental. Quito: Environment Ministry of Ecuador, 2015. p. 184. states that the use of wastewater for irrigation is prohibited, except for that which has been previously treated and complies with the water quality levels for agricultural irrigation.
The scientific relevance of this research is characterized by exposing the high degree of contamination of the treated wastewater and potential risk to human contamination and environmental degradation through the analysis of Biochemical Oxygen Demand, Chemical Oxygen Demand, Total Nitrogen and Fecal Coliforms that can be harmful to the habitants, flora, and fauna of the study area. The wastewater treatment system using stabilization lagoons is located on Punta Carnero Road in Salinas Canton, Province of Santa Elena - Ecuador. It currently has a system of seven lagoons: three anaerobic, two facultative and two aerobic or maturation, which discharges its previously treated effluent into the Achayan River, which finally drains to Punta Carnero Beach towards the Pacific Ocean (Humanante, 2016HUMANANTE, J. Reduction of coliforms in effluent stabilization lagoon of Salinas - Libertad applying bacteria. 2016. Thesis (Master's Degree in Sanitary Engineering) - Guayaquil University, Guayaquil, 2016. ). This system is part of the eight lagoon systems in the Santa Elena Province under the responsibility of the company AGUAPEN-EP, which has been providing services to the province since 1998 (Suárez and Panchana, 2021SUÁREZ, J.; PANCHANA, R. Statistical evaluation of physical, chemical and bacteriological analysis parameters of the effluents of the wastewater treatment system located on the side of Punta Carnero road in Salinas Canton. 2021. Thesis (Civil Engineering) - Santa Elena Peninsula State University, La Libertad, 2021. ). This research evaluated the affluent and effluent of the wastewater treatment efficiency, the possible effects on the ecosystem produced by the quality of the final effluent, and finally determined if the final discharge is optimal for agricultural irrigation. The investigation was based on water quality analyses performed in an accredited water laboratory, through simple samples of wastewater taken at the inlet and outlet of the lagoon system on April 15, April 28 and July 6, 2021. The results of water quality tests were compared with the Unified Text of Secondary Environmental Legislation of Ecuador in relation to the General Standards Table for effluent discharge into freshwater bodies, which regulates the following parameters: Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Nitrogen (N) and Phosphorus (P). Finally, the results were also compared with water quality criteria for agricultural or irrigation water table, which regulates the following parameters: Oils-Fats, Aluminum, Arsenic, Beryllium, Boron, Cadmium, Zinc, Cobalt, Copper, Fecal Coliforms, Chromium, Fluoride, Iron, Parasite Eggs, Lithium, Floating Material, Mercury, Manganese, Molybdenum, Nickel, Nitrites, Dissolved Oxygen, pH, Lead, Selenium, Sulfates and Vanadium.
2. MATERIAL AND METHODS
To carry out the investigation, the characteristics of the raw wastewater must first be known (Table 1). The research was developed in the wastewater treatment system of the Punta Carnero Sector in Salinas Canton, Santa Elena Province (Ecuador), located at UTM coordinates 509066 E, 9750151 N. Wastewater arriving to the system comes from the cantons of La Libertad and Salinas, which in turn is collected and transported through sanitary sewer networks to the wastewater treatment system. At present, the Stabilization lagoon system is composed of three anaerobic lagoons, two facultative and two aeration or maturation lagoons, which are inadequate for the current population. Biological treatment processes are carried out in the lagoons: anaerobic treatment in anaerobic lagoons and aerobic treatment in facultative and maturation lagoons, which are connected to each other by means of 400 mm diameter PVC pipes through distribution chambers; finally, the treated water passes through a chlorination labyrinth and its final effluent is discharged into a natural waterway (Humanante, 2016HUMANANTE, J. Reduction of coliforms in effluent stabilization lagoon of Salinas - Libertad applying bacteria. 2016. Thesis (Master's Degree in Sanitary Engineering) - Guayaquil University, Guayaquil, 2016. ). The research was carried out in three phases.
2.1. Pollutant load removal efficiency
The first phase involves determining the value of the pollutant load removal efficiency of the wastewater treatment system. First, two simple samples were taken in the lagoon system (Figure 1), simple samples representing the composition of the water for that specific time and location (Romero, 2004ROMERO, J. Tratamiento de aguas residuals. In: ROMERO ROJAS, J. A. Tratamiento de aguas residuales; teoria y principios de diseño. Bogotá: Escuela Colombiana de Ingenieria, 2004. p. 1248. ). The first simple sample was taken in the affluent of the system, water distribution chamber with outlet to anaerobic lagoons at UTM Coordinates 509203 E 9750365 N; the second simple sample was taken in the effluent of the water outlet system of the Parshall Flume with discharge to the Achayan River at UTM Coordinates 509042 E 9749661 N. Next, the samples taken in glass and plastic bottles of one liter capacity were refrigerated at a temperature of 4.3ºC, and one milliliter of sulfuric acid was added to the glass bottle. The samples were then transported to an accredited water testing laboratory, where they were analyzed and finally the test results were received approximately fifteen days after the samples were taken and left. Third, once the test results were obtained, the results were processed using the equation (Romero, 2004ROMERO, J. Tratamiento de aguas residuals. In: ROMERO ROJAS, J. A. Tratamiento de aguas residuales; teoria y principios de diseño. Bogotá: Escuela Colombiana de Ingenieria, 2004. p. 1248. ) to determine the pollutant load removal efficiency of the wastewater treatment system (Equation 1).
The equation shows: E (%) is the pollutant load removal efficiency in percent; S0 is the affluent pollutant load in milligrams per liter (mg/l) and S is the effluent pollutant load in milligrams per liter (mg/l).
Wastewater treatment system of the Punta Carnero sector, Salinas - Ecuador (Humanante, 2016HUMANANTE, J. Reduction of coliforms in effluent stabilization lagoon of Salinas - Libertad applying bacteria. 2016. Thesis (Master's Degree in Sanitary Engineering) - Guayaquil University, Guayaquil, 2016. ), simple sampling location.
2.2. Effluent water quality for discharge to a freshwater body
To determine the quality of effluent from the wastewater treatment system, in this second phase, simple samples were taken from the final effluent (discharge to the Achayan River) on three dates:
-
S1: Simple sample taken directly from the discharge, packaged in two plastic bottles and one glass bottle, both with a capacity of one liter (April 15, 2021).
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S2: Simple sample taken directly from the discharge, packaged in two plastic bottles and one glass bottle, both with a capacity of one liter (April 28, 2021).
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S3: Simple sample taken directly from the discharge, packaged in two plastic bottles and one glass bottle, both with a capacity of one liter (July 6, 2021).
After the results of the tests of the simple samples taken at the accredited laboratory, “Grupo Quimico Marcos” were obtained; these data were processed in the MINITAB statistical software to determine the mean and standard deviation of the system final effluent. Once these indicators were obtained and represented graphically S1, S2 and S3, the quality of the water discharged into the Achayan River was determined. Once the quality of the discharged water was known, the general standard for effluent discharge to freshwater bodies was presented in relation to the parameters evaluated in the laboratory: Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Nitrogen (N) and Total Phosphorus (P), and the possible effects caused by poor wastewater treatment and its impact on the environment (Table 2).
2.3. Effluent water quality for agricultural irrigation water reuse
The third phase of the research was to determine if the water that had been treated and then discharged to the Achayan River (freshwater body) can be destined for water reuse for agricultural irrigation. For this purpose the guidelines established in the Unified Text of Secondary Environmental Legislation of Ecuador 2015 must be followed, where TULSMA (2015)TULSMA. Libro VI de la calidad ambiental. Quito: Environment Ministry of Ecuador, 2015. p. 184. states that water for agricultural use is understood as that used for crop irrigation and other related or complementary activities established by the competent bodies.
In this phase, the results obtained from the accredited water laboratory are compared with the water quality criteria for agricultural irrigation (Table 3); this table also mentions the environmental impacts caused by poor quality water reused for agricultural irrigation (Perú, 2006PERÚ. Ministerio de Salud. Dirección General de Salud Ambiental e Inocuidad Alimentaria. Riego de Vegetales y Bebederos de Animales. Lima, 2006. 134p.).
3. RESULTS AND DISCUSSION
The research shows the following results based on the evaluation carried out in three phases:
3.1. Pollutant load removal efficiency results
The pollutant load removal efficiency (Table 4) was determined based on the affluent, which is the incoming pollutant load (S0), and the effluent, which is the outgoing pollutant load (S) of the wastewater treatment system evaluated. A total of 28 parameters were evaluated, each of which was analyzed in the laboratory and the results obtained from the tests were processed in Excel software (Table 3), A graph of individual values is also shown, classifying the parameters that denoted an efficiency greater than 50%, less than 50%, no efficiency (0%) and negative values that indicate that the system is greater than that entering it (Figure 2).
Ecuadorian regulations only regulate three parameters for pollutant load removal efficiency. The removal of BOD and COD was 62.42% and 62.41%, respectively, which indicates that the efficiency of these parameters doesn’t reach the values established in current Ecuadorian regulations (INEN, 2012INEN. Normas para estudio y diseño de sistemas de Agua potable y disposición de aguas residuals PPara poblaciones mayores a 1000 habitantes. Quito, 2012. p. 420. ), where it’s established that there must be a 70-85% removal rate in order for the treatment in the lagoon system to be considered good. On the other hand, in the microbiological parameter of Fecal Coliforms, the removal efficiency was 53.58%, which doesn’t meet the requirements of the Environmental Secondary Legislation Text of 2015, where it provides for a removal of 99.9% of this pollutant, which relates that the treatment is not adequate.
The removal efficiency of COD and BOD of about 60% is relatively insufficient especially if the initial COD values in the raw water is high. This usually suggests the use of a pretreatment step before the biological step (Al-Qodah et al, 2019AL-QODAH, Z.; AL-QUDAH, Y.; OMAR, W. On the performance of electrocoagulation-assisted biological treatment processes: a review on the state of the art. Environmental Science and Pollution Research, v. 26, n. 28, p. 28689-28713, 2019. https://doi.org/10.1007/s11356-019-06053-6
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).
Heavy metal removal efficiencies were obtained in the treatment system (Figure 2), in which: Al (99.82%), Cu (96.69%), Fe (96.54%), Zn (85.26%), Ni (69.25%), Cr (66.67%), BOD (62.42%), COD (62.41%) and CF (53.58%), had a pollutant load removal greater than 50%; F (43.56%), SO4 (43.03%), Oils and Fats (42.11%), Pb (40.00%), V (33.33%), Mn (25.00%), Li (10.00%) and B (4.60%), its pollutant load removal was below 50%; As, Cd, Co, Hg, Mo, NO2 and Se, no removal of pollutant load; finally, N (-5.66%), OD (-72.04) and Be (-100.00), presented an inefficient removal in their treatment since the final load increased in relation to their initial load.
3.2. Final effluent discharge quality results
It was determined that the effluent quality is not optimal for discharge with respect to BOD, COD and Total Nitrogen, while Total Phosphorus complies with present regulations.
The effluent quality of BOD (Figure 3), shows that it doesn’t comply with the maximum permissible limit of 100 milligrams of oxygen per liter, which means that exceeding the established limit would cause negative effects to the flora and fauna of the sector where the water is discharged. On the other hand, the effluent quality of COD (Figure 4), like BOD, doesn’t comply with the 200 milligrams of oxygen per liter established in the mentioned regulation, which denotes possible negative effects such as the absence of oxygen in aquatic microorganisms in the sector; Similarly, Total Nitrogen levels exceed the maximum permissible limit of 50 milligrams per liter (Figure 5), which indicates possible effects such as the vulnerability of plants and animals to high levels of N. However, the effluent quality of Total Phosphorus (Figure 6) complies with the maximum permissible limit of 10 milligrams per liter, which indicates that there are no problems in its discharge.
3.3. Results comparison obtained in the laboratory
Of the 27 parameters evaluated with the agricultural irrigation water quality criteria of the Unified Text of Secondary Environmental Legislation, the following comply with the requirements of the regulations Al (0.002 < 5.0), As (0.00215 < 0.1), Be (0.002 < 0.1), B (0.373 < 0.75), Cd (0.001 < 0.05), Cr (0.001 < 0.1), Co (0.00041 < 0.01), Cu (0.00116 < 0.2), Floating Matter (Absence), F (0.57 < 1), Fe (0.034 < 5), Pb (0.003 < 5), Li (0.009 < 2.5), Mn (0.066 < 0.2), , Mo (0.001 < 0.01), Ni (0.00123 < 0.2), NO2 (0.115 < 0.5), Parasite eggs (Absence), pH (7.88 < 6-9), Se (0.00306 < 0.02), Sulfates (94 < 250), V (0.002 < 0.1) and Zn (0.023 < 2), while OD (8.86 > 3), Fecal Coliforms (62900 > 1000), Hg (0.00141 > 0.001), and Oils and Fats (5.65 > Absence) exceed agricultural irrgiation water quality criteria.
4. CONCLUSIONS
The removal efficiency of the wastewater treatment system doesn’t meet the efficiencies established in existing regulations, so it is determined that the biological treatment is not optimal. It is proposed as an improvement to achieve these efficiencies that there should be more frequent control of the stabilization lagoons, as well as more frequent removal of the silt that is present in these lagoons, which doesn’t allow the water entering the system to be treated in an efficient manner.
According to the results of the study, the quality of the water discharged into the receiving body (Achayan River) is poor, which could have serious consequences in the future, since high concentrations of BOD, COD and N directly affect plants and animals in the sector, and indirectly affect people who bathe in the sea.
The treated wastewater is not suitable for reuse for agricultural irrigation because it doesn’t fully comply with the criteria established in Table 2 of the Ecuadorian Environmental Regulations.
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Publication Dates
-
Publication in this collection
06 May 2022 -
Date of issue
2022
History
-
Received
17 Dec 2021 -
Accepted
07 Mar 2022