RODRIGO MÁXIMO SÁNCHEZ ROMÁN DESINFECÇÃO SOLAR DE ESGOTO DOMÉSTICO PARA USO NA AGRICULTURA FAMILIAR

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1 RODRIGO MÁXIMO SÁNCHEZ ROMÁN DESINFECÇÃO SOLAR DE ESGOTO DOMÉSTICO PARA USO NA AGRICULTURA FAMILIAR Tese apresentada à Universidade Federal de Viçosa, como parte das exigências do Programa de Pós Graduação em Engenharia Agrícola, para obtenção do título de Doctor Scientiae. VIÇOSA MINAS GERAIS - BRASIL 2006

2 RODRIGO MÁXIMO SÁNCHEZ ROMÁN DESINFECÇÃO SOLAR DE ESGOTO DOMÉSTICO PARA USO NA AGRICULTURA FAMILIAR Tese apresentada à Universidade Federal de Viçosa, como parte das exigências do Programa de Pós Graduação em Engenharia Agrícola, para obtenção do título de Doctor Scientiae. APROVADA: 17 de abril de Prof. Gilberto Chohaku Sediyama Conselheiro Prof. Ann Honor Mounteer Prof. Rubens Alves de Oliveira Prof. Jorge Adilio Penna Prof. Antonio Alves Soares Orientador

3 Dichoso el que halla sabiduría, el que obtiene inteligencia; porque son más provechosas que la plata y rinden mayores beneficios que el oro. Proverbios 3:13-14 Desde que existe el Mundo hay una cosa cierta, unos hacen los muros y otros hacen las puertas. Carlos Varela ii

4 A nuestra Esperanza, quien vive en nosotros. A mi padre, permanente en mí. A mis hijos Rodrigo Javier, Gabriela y Alejandro. Por quienes lucho diariamente para dejarles un mundo mejor que el que encontré, por ser un mejor ser humano, un ejemplo a seguir, pero sobre todo, un buen padre. A Alba. Amiga, confidente, amante. Desde nuestro primer día hasta mi último suspiro. Siempre conmigo y en mí. Siempre. A mis hermanos Ricardo y Jaime. Compañeros incondicionales de todas mis jornadas. iii

5 AGRADECIMENTOS Ao Brasil. Ao maravilhoso povo brasileiro. À Universidade Federal de Viçosa. Ao Departamento de Engenharia Agrícola da Universidade Federal de Viçosa, pela oportunidade de realizar meus estudos de doutorado. Ao CNPq-PPG pela bolsa de estudos, e à Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG). Ao professor Antonio Alves Soares, pela orientação, pelo empenho em viabilizar a realização deste trabalho. Ao professor Antonio Teixeira de Matos (Conselheiro) pelas contribuições e sugestões oportunas. Ao professor Gilberto Chohaku Sediyama (Conselheiro), pela ajuda durante todo o desenvolvimento deste trabalho, pelo carinho e a boa vontade para comigo; sempre levarei boas lembranças e gratidão. Ao professor Og Francisco de Souza, pelo apoio durante a realização deste trabalho, pelas sugestões e pela disposição. iv

6 Aos professores Paulo Afonso Ferreira, Everardo Chartuni Mantovani, Márcio Mota Ramos e Mauro Aparecido Martinez, pelos ensinamentos e pela franca disposição com que sempre me receberam. Ao Gil (Gilberto E. dos Santos), pela competência em operar a unidade experimental, ajuda na coleta de dados de campo, pela dedicação, pelo constante bom-humor, pela criatividade, pela amizade e as aulas de português. Ao Simão, pela grande ajuda no Laboratório de Qualidade de Água, pelo profissionalismo, e pelas conversas bem-humoradas. Ao Mozart do Brasil, colega, bom amigo, sorrisos francos e trabalho profissional. Ao funcionário do Laboratório de Hidráulica, Chicão, pelo apoio durante a realização deste trabalho e pela simpatia com que sempre me recebeu. Aos funcionários do Departamento de Engenharia Agrícola, Edna, Galinári, Marcos e Zé Mauro, pela acolhedora atenção. Ao Chiquinho, pela ajuda na tarefa de comprar alguns dos materiais necessários para a construção do projeto. À Patrícia Vaz de Mello, amiga incondicional sempre que precisei de apoio durante toda esta jornada. Aos funcionários do Registro Escolar, em especial para Eloísa. Ao Jorge Xavier, funcionário de Relações Internacionais, sempre atencioso. Ao Gilcemir, na Pró - Reitoria de Pesquisa e Pós-Graduação, sempre bemhumorado, eficiente e amigável. Aos colegas Renata Rodrigues, Felizardo Rocha, Edio Costa, Marcelo Rossi, Suzana dos Santos, Denise Freitas e Ralini Ferreira de Melo. Aos amigos que encontramos em Viçosa. v

7 Al Dr. Edgar Meléndez, quien nos ayudó desde antes de nuestra llegada a Brasil, y durante toda nuestra estadía. Dina y Juan José, amigos llegados desde otras tierras, pero residentes en nuestros recuerdos. Franklin Valbuena, grande amigo, voz que iluminó horas difíciles y hombro fiel donde apoyarse. Adrián Molina y su esposa, Emilio Campos y familia, compatriotas que nos apoyaron desde los primeros días en Viçosa, y que estuvieron siempre presentes. A mi hijo Alejandro, que me ayudó con el portugués y con la división geográfica de Brasil prestándome su libro, de cuarta serie, cuando estaba escribiendo la tesis. Así como también con el sotaque, cuando estaba preparando la presentación del seminario. A mis hijos Rodrigo Javier y Gabriela por soportar la separación. A Alba, por compartir éste sueño y luchar junto a mí para alcanzarlo. A Dios, que siempre me ha protegido y guiado. De su mano camino en todo momento. vi

8 BIOGRAFIA RODRIGO MÁXIMO SÁNCHEZ ROMÁN é filho de Ana Esperanza Román Morales e Eustacio Máximo Sánchez Díaz. Nasceu na cidade de Manágua, capital da Nicarágua, no dia 20 de outubro de Em 1979, iniciou estudos de formação em Engenharia em Irrigação e Drenagem no Instituto Superior de Ciencias Agropecuarias de La Habana, República de Cuba, concluindo-o em Em 1988, iniciou estudos de pósgraduação para obter o Master of Engineering in Agricultural Engineering, na Texas A&M University, concluindo-o em Em Março de 2002, iniciou seus estudos no programa de pós-graduação em nível de Doutorado no Departamento de Engenharia Agrícola da Universidade Federal de Viçosa (UFV), Viçosa, Minas Gerais, Brasil. Concentrando seus estudos na Área de Recursos Hídricos e Ambientais, e submetendo-se à defesa da tese em abril de vii

9 CONTEÚDO RESUMO... x ABSTRACT... xiii INTRODUÇÃO GERAL... 1 DOMESTIC WASTEWATER DISINFECTION USING SOLAR RADIATION... 7 INTRODUCTION... 7 BACKGROUND MATERIAL AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES APPENDIX POTENCIAL DO USO DA DESINFECÇÃO SOLAR DE ESGOTO DOMÉSTICO NO BRASIL INTRODUÇÃO MATERIAL E MÉTODOS RESULTADOS E DISCUSSÃO CONCLUSÕES RECOMENDAÇÕES REFERÊNCIAS BIBLIOGRÁFICAS APÊNDICE AVALIAÇÃO FINANCEIRA DA IMPLANTAÇÃO DE UM SISTEMA DE TRATAMENTO DE ESGOTO DOMÉSTICO USANDO RADIAÇÃO SOLAR VISANDO O USO DA ÁGUA viii

10 TRATADA NA AGRICULTURA FAMILIAR INTRODUÇÃO MÉTODOLOGIA RESULTADOS CONCLUSÕES REFERÊNCIAS BIBLIOGRÁFICAS APÊNDICE CONCLUSÕES GERAIS ix

11 RESUMO SÁNCHEZ-ROMÁN, Rodrigo Máximo, D.S. Universidade Federal de Viçosa, abril de Desinfecção solar de esgoto doméstico para uso na agricultura familiar. Orientador: Antonio Alves Soares. Conselheiros: Gilberto Chohaku Sediyama e Antonio Teixeira de Matos. Um sistema piloto para desinfetar o esgoto doméstico usando a radiação solar (SODIS) foi construído na Estação Experimental para Tratamento de Águas Residuárias Domésticas para Uso na Agricultura (EETARD-UA) do Departamento de Engenharia Agrícola da Universidade Federal de Viçosa, estado de Minas Gerais, Brasil. A Estação Experimental encontra-se a 20º45 14 S, 42º52 53 W, na altitude de 648,74 m acima do nível do mar. No reator, construído com blocos de concreto, foi aplicada água residuária doméstica tratada (ARDT) a qual passou por: a) tratamento preliminar; e b) tratamento primário em tanque séptico, com tempo de detenção de aproximadamente 14 horas. No reator solar, o efluente do tanque séptico x

12 permaneceu durante oito horas, em sistemas de tratamento em batelada. Amostras da água residuária foram retiradas a cada duas horas. As análises nelas realizadas foram: oxigênio dissolvido, turbidez, demanda química de oxigênio (DQO), sólidos suspensos totais (SST), coliformes totais e Escherichia coli (E. coli). A temperatura da água no reator solar foi medida a cada hora. Uma estação meteorológica automática, instalada na área da EETARD-UA, foi usada para coletar os dados de radiação solar. Todos os dados foram analisados estatisticamente e foi proposto um modelo para estimar a população final de E. coli, a partir da população inicialmente presente na água residuária, da lâmina de ARDT a ser tratada no reator solar e da quantidade de radiação. Usando esse modelo e os valores de horas de insolação de 202 estações meteorológicas distribuídas por todo o Brasil, foi determinado o tempo de exposição necessário para desinfetar a ARDT, objetivando o seu uso posterior como fertirrigação em agricultura familiar, com garantias de que a exposição à radiação solar reduz a população de E. coli a níveis recomendados pela Organização Mundial da Saúde (OMS) para água a ser usada sem nenhuma restrição sanitária na irrigação de culturas agrícolas. Assim, foram elaborados 48 mapas da distribuição mensal dos tempos de exposição para quatro profundidades (0,05; 0,10; 0,15 e 0,20 m) de ARDT a serem tratadas no Brasil. A viabilidade financeira do projeto foi avaliada considerando-se um núcleo familiar de quatro pessoas. Foram destacados na análise: o investimento inicial, os custos (manutenção, sementes, implementos, o pagamento da dívida, etc.), e as entradas geradas pela produção de alimentos xi

13 na horta familiar. Foi detectado que o valor presente líquido (VPL), a taxa interna de retorno (TIR), a taxa de retorno do capital (TRC) e a relação custo/benefício (C/B) foram favoráveis à implantação do projeto, quando foi considerado como fonte de financiamento o Programa Nacional de Fortalecimento da Agricultura Familiar Mulher (PRONAF - Mulher), nos quatro cenários analisados. xii

14 ABSTRACT SÁNCHEZ-ROMÁN, Rodrigo Máximo, D.S. Universidade Federal de Viçosa, April Domestic wastewater solar disinfection for reuse in family agriculture. Advisor: Antonio Alves Soares. Committee members: Gilberto Chohaku Sediyama and Antonio Teixeira de Matos. A pilot study to disinfect domestic wastewater using solar radiation was built in the Experimental Station for Domestic Wastewater Treatment for Use in Agriculture (ESDWT-UA) of the Departamento de Engenharia Agrícola at Universidade Federal de Viçosa, Minas Gerais, Brazil. The experimental station is located at 20º45'14" S, 42º52'53" W, altitude meters above sea level (masl). The effluent from a residential development was used as supply; this treated domestic wastewater (TDWW) passed through a pretreatment unit, and primary treatment in septic tank with detention time of 14 hours. After that, in the solar reactor, the septic tank s effluent was exposed to solar radiation for eight hours in a batch system treatment. Samples of water xiii

15 were collected every two hours. The samples were analyzed for: dissolved oxygen, turbidity, chemical oxygen demand (COD), total suspended solids (TSS), total coliforms and Escherichia coli (E. coli). Water temperature in the solar reactor was taken every hour. An automatic meteorological station, installed at ESDWT-UA, was used to collect solar radiation data. All data were statistically analyzed, and a model was proposed to estimate the final E. coli population given the initial E. coli population in the TDWW, the TDWW depth to be treated at the solar reactor and the dose accumulated. Using the model proposed in this work, and the insolation values for 202 meteorological stations distributed throughout Brazil, the time of exposure necessary to disinfect the TDWW was determined reducing the E. coli population to values recommended by World Health Organization (WHO) for irrigation water to be used without any sanitary restriction in agriculture. 48 maps with the exposure times for monthly distribution were elaborated for four depths (0.05, 0.10, 0.15, 0.20 m) of TDWW to be used in Brazil. The financial viability of this project was evaluated considering a family of four people. The analysis considering: the initial investment, costs (maintenance, seeds, implements, debt payments, etc.), and incomes generated by the production of food in the family vegetable garden. It was found that the present value (VPL), the capital rate of return (TRC), internal rate of return (TIR) and cost/benefit (C/B) relationship were all favorable to the implantation of the project when Programa Nacional de Fortalecimento da Agricultura Familiar Mulher xiv

16 (PRONAF - Mulher), for four scenarios analyzed was considered as source of financing. xv

17 INTRODUÇÃO GERAL A degradação do meio ambiente, em nível mundial tende a piorar nos próximos anos (Simon e Kahn, 1984; Postel, 2000; Rosegrant et al., 2002). Isto tem levado os diversos atores da sociedade a procurarem a preservação dos recursos hídricos por meio da conscientização da população e de alternativas de uso e reuso da água. Sabe-se que o ser humano precisa de certa quantidade de água diariamente para sobreviver e que uma parcela desta é retornada como afluente ao meio ambiente, na forma de água residuária. As excreções humanas são importante fonte de nutrientes para as plantas. A cada dia, o ser humano excreta ao redor de 30 g de carbono (90 g de matéria orgânica), g de nitrogênio, 2 g de fósforo e 3 g de potássio (Drangert, 1998). Em um ano, um ser humano gera aproximadamente 500 L de urina e 50 L de fezes; correspondente a 4,5 kg de nitrogênio, 0,6 kg de fósforo e 1,2 kg de potássio; o que seria suficiente para produzir 250 kg de 1

18 cereais no ano; em outras palavras poderia ajudar a produzir parte de sua própria alimentação (Strauss, 2000, Windlad et al., 1999). Antes de um uso indiscriminado desses resíduos em cultivos agrícolas, deve-se considerar que as fezes humanas contêm microrganismos que podem ser potencialmente danosos à saúde humana. Os riscos atuais à saúde pública, quando o esgoto é usado na fertirrigação de culturas utilizadas no consumo humano, podem ser divididos em três grandes categorias. Na primeira estarão aquelas pessoas que consomem os vegetais produzidos com o uso do esgoto, sendo esta a categoria de risco do consumidor. Na segunda categoria estarão aqueles trabalhadores agrícolas que estão expostos ao esgoto e formam a categoria de risco do produtor. Na terceira categoria estarão todas as pessoas que moram ao redor do local onde o esgoto é usado na produção de alimentos, formando a categoria da população de risco de vizinhança (Strauss, 2000). Ao redor de 99,9% das águas residuárias é constituída por água, e o restante são sólidos orgânicos e inorgânicos. Tanto os patógenos como os nutrientes são constituintes importantes no planejamento da aplicação das águas residuárias domésticas (ARD) no cultivo agrícola. Análises físicoquímicas da água são essenciais para se garantir que não produzirão concentrações de elementos que provoquem danos estruturais ao solo ou ao meio ambiente, ou toxicidade às plantas. Existem diferentes sistemas de tratamento de esgoto que quando utilizados geram efluentes que podem ser aplicados na fertirrigação das culturas cujos custos variam com o grau de tratamento desejado. A 2

19 desinfecção por radiação solar é uma opção promissora, tendo em vista sua natureza não poluente, permanente disponibilidade e custo zero, especialmente no Brasil; um País tropical que possui potencial de energia solar durante o ano todo. Os objetivos de este trabalho são desenvolver um sistema de tratamento e desinfecção das ARD para residências de baixa renda, utilizando a energia solar, de tal maneira que seja possível seu reaproveitamento sem restrições sanitárias; desenvolver um modelo matemático para facilitar a predição da desinfecção solar (SODIS) das ARDT considerando a carga contaminante, a dose aplicada e a lâmina de ARDT tratada; determinar o potencial do uso da radiação solar na desinfecção das ARDT no Brasil; e avaliar financeiramente a implantação do reuso das ARDT usando SODIS numa horta familiar. O trabalho apresentado aqui consta de três partes. Na primeira, um modelo que permite estimar a população de E. coli, num dado momento, quando conhecidas a população inicial, a radiação solar incidente acumulada (dose acumulada), e a profundidade da lâmina de água no reator solar. Na segunda parte, foi determinado o tempo de exposição necessário para desinfetar as águas residuárias domésticas (ARDT) para cada região do Brasil, objetivando o seu uso posterior na agricultura familiar. Foram desenvolvidos 48 mapas da distribuição dos tempos de exposição para quatro profundidades (0,05; 0,10; 0,15 e 0,20 m) de ARDT no reator. Na terceira parte apresenta uma análise financeira de um projeto de tratamento de esgoto doméstico usando radiação solar para seu uso posterior 3

20 numa horta familiar, considerando a fonte de ARDT gerada por uma família rural constituída por quatro membros. Os dados resultantes desta avaliação possibilitaram que se concluísse que o projeto é viável, sob o ponto de vista financeiro, com resultados promissórios de Valor Presente Líquido (VPL), Taxa Interna de Retorno (TIR), Taxa de Retorno do Capital (TRC) e relação Custo - Beneficio (C/B). Finalmente, se apresenta um capitulo com as conclusões gerais que poderão ser tiradas deste trabalho. 4

21 REFERÊNCIAS BIBLIOGRÁFICAS American Public Health Association (APHA), American Water Works Association (AWWA) and Water Environment Federation (WEF). Standard methods for the examination of water and wastewater. 19 th Edition. Washington: Am. Public Health Assoc., 1193 pp Drangert, J. O. Fighting the Urine Blindness to Provide more sanitation options. Water South Africa, Vol. 24(2) Fernandes, A. R. Dinâmica Operacional e Sustentabilidade de Empreendimentos Agroindustriais de Pequeno Porte. Tese de Doctor Scientiae em Ciência e Tecnologia de Alimentos: Universidade Federal de Viçosa. Viçosa, MG, Brasil. 307 pp Postel, S. Redesigning Irrigated Agriculture. In: National Irrigation Symposium. Proceedings of the 4 th Decennial Symposium. ASAE, St. Joseph, Michigan Rosegrant, M. W., Cai, X., Cline, S. A. World Water and Food to Dealing with Scarcity. International Food Policy Research Institute, Washington. 338 pp Simon, J. L.; Kahn, H. Resourceful Earth: A Response to Global Blackwell Publishers, New York Strauss, M Human Waste (Excreta and Wastewater) Reuse. Water and Sanitation in Developing Countries. Acessado em 30/07/2003: 5

22 Windlad, U., Esrey, S., Gough, J., Rapaport, D., Sawyer, R., Simpson-Hébert, M., Vargas, J. Saneamento Ecológico. Editado por: U. Windlad. Fundación Friedrich Ebert, Agencia de Cooperación Internacional para el Desarrollo. 101 pp

23 DOMESTIC WASTEWATER DISINFECTION USING SOLAR RADIATION Rodrigo M. Sánchez-Román, Antonio A. Soares, Antonio T. de Matos, Gilberto C. Sediyama, Og DeSouza The authors are: Rodrigo Máximo Sánchez-Román, Ph.D. Candidate in the Agricultural Engineering Department (DEA), Universidade Federal de Viçosa (UFV), Minas Gerais (MG), Brazil, CNPq fellowship, Antonio Alves Soares, Professor, DEA, UFV, MG, Brazil, Antonio Teixeira de Matos, Associate Professor, DEA, UFV, MG, Brazil, Gilberto Chohaku Sediyama, Professor, DEA, UFV, MG, Brazil, Og de Souza, Associate Professor, Animal Biology Department, UFV, MG, Brazil. Corresponding author: Rodrigo M. Sánchez-Román; Universidade Federal de Viçosa, Depto. de Engenharia Agrícola, Av. P. H. Rolfs s/n, Viçosa, MG , Brasil; phone: ; e- mail: ABSTRACT A pilot study was performed in Viçosa, Minas Gerais, Brazil, to determine the feasibility of using solar radiation to disinfect reclaimed domestic wastewater (TDWW). The effluent from a residential development was used as supply; this wastewater passed through a pre-treatment unit and a septic tank. After passing through the septic tank, the TDWW has mean values for COD, TSS and turbidity of 286 mg L -1, 54 mg L -1, and 65 NTU, respectively. A square cement block reactor was constructed and located at 20 o S, 42 o W to contain the reclaimed TDWW. Fixed depths of water (0.05; 0.10; 0.15; 0.20 m) were tested considering different time of exposure from November 2005 to May Solar UV disinfection (SODIS) was effective in reducing mean concentration of indicator microorganisms, such as Total Coliform (TC) and E. coli after an eight hours exposure to direct sunlight. Statistical analysis showed that the removal rates of TC and E. coli did not significantly vary due to turbidity or dissolved oxygen. A model was developed to estimate the surviving E. coli population after being exposed to SODIS knowing its initial population, depth of water being treated and solar energy received. Keywords: domestic wastewater disinfection, SODIS, water reuse, E. coli. INTRODUCTION Lack of water, water sanitation, and health care; as well as hunger and poverty affect billions of human beings. These problems tend to become worse in the next decades (Postel, 2000; Rosegrant et al., 2002). It is known 7

24 that the humans need a certain amount of water daily to survive, and that a considerable portion of it is returned as an effluent to the environment; in the form of domestic wastewater (DWW). The use of DWW in agriculture deals with important aspects such as water demand and supply to agriculture, increasing fertilizer prices, crop nutrient demand, environmental protection, and wastewater disposal. Even more, it relates to food production and supply, especially for low-income third world households. Throughout centuries humanity has gained a lot of experience reclaiming DWW to produce agricultural and aquicultural goods. According to Strauss (2000), around 10% of the world s sewer water are used for fertigation of agricultural crops and according to Cooper (1991) cited by Scott et al. (2000), this use may reaches 80% in developing countries. Human excrement is an important source of nutrients for plants. Every day, a person excretes around 30 g of carbon (90 g of organic matter), 10 to 12 g of nitrogen, 2 g of phosphorus and 3 g of potassium. In one year, one person generates, approximately, 500 L of urine and 50 L of feces; equivalent to fertilizer sufficient to produce 250 kg of cereals (Windland, 1999; Drangert, 1998). The nutrients excreted yearly by a human being could help to produce an important part of his food requirements. An indiscriminate use of DWW in agricultural is not recommended. It should be considered that human feces contain microorganisms that can be potentially harmful to human health. The majority of these pathogens dies after excreted from the human body due to adverse temperature and humidity 8

25 and the presence of ultraviolet light (UV). The rate of pathogen mortality is a function of the intensity of these factors in the environment. It is possible to use DWW as a secure source of water supply for agriculture; however, some prophylactic actions are necessary. Therefore, it is important to interrupt at some point the contamination cycle created by the presence of pathogens in DWW, eliminating the sanitary risks due to its use. The system to eliminate pathogens should be of low maintenance and operation costs, capable of being sustained with local resources, and have appropriate technology to guarantee the viability of the system as a whole (Strauss, 2000). Sunlight has a germicidal action that has been recognized since 1877, when Downes and Blunt researched the effect of light on bacteria and other organisms. A given survival ratio of bacteria being killed by radiation will be a function of a given exposure; accordingly to the Bunsen-Roscoe reciprocity law and regardless of whether the exposure consists of a high intensity for a short time or a low intensity for a corresponding longer time (Koller, 1952). When photons of light reach water, they are absorbed or scattered (Kirk, 1994). When absorbed, they react with pathogens DNA strings or suspended or dissolved solids. If scattered, they could also be absorbed or returned to the atmosphere. Solar radiation is the main source of thermal, chemical and dynamic processes on this planet. The radiant energy originated at the Sun that reaches Earth s surface is called solar radiation; its main characteristic is being an 9

26 electromagnetic field. The solar radiation s magnitude is the amount of radiant energy in a wavelength interval that crosses a unit of area perpendicular to its direction, and per unit time (W m -2 ). Irradiation refers to radiation arriving at a surface, whether or not the origin is the sun. Radiance is the radiant flux emitted by a unit area of a source. When radiance is known, the radiant flux density can be determined; which when integrated over the whole spectrum represents the amount of radiant energy that passes through a plane per unit time and per unit area, known as irradiance. In other words, irradiance is the radiant flux incident on a receiving surface from all directions, per unit area of surface, in W m -2 (Liou, 1980; Iqbal, 1983). Fluence, or dose, refers to the total amount of energy applied per unit area (J m -2 ); it is the irradiance multiplied by the exposure time. Solar radiation varies in intensity and is unevenly distributed in our planet, depending on geographical location, cloudiness, time of day and season (Iqbal, 1983; Liou, 1980). According to Acra et al. (1984) solar disinfection (SODIS) is a practical solution in the regions between latitudes 35 o N and 35 o S, where the great majority of the developing countries are located; and the most favorable region for SODIS is between 15 o and 35 o either latitude North or South, and the second best is between 15 o N and 15 o S. The most favorable region for SODIS has limited cloud coverage and rainfall, and usually over 3,000 hours of sunshine per year. The second best region has high humidity; cloud coverage is frequent provoking a high 10

27 proportion of scattered radiation. Besides, this region has an average of 2,500 hours of sunshine per year, with solar intensity almost uniform throughout the year due to slightly seasonal variations. BACKGROUND Acra et al. (1984) compared the germicidal effects of different light wavelengths in bacteria and concluded that the most bactericidal widths, that reach the surface of the earth, are between 315 and 400 nm, which correspond to the UV-A rays band. They also demonstrated that E. coli is the best organism to be used as indicator of the presence of other populations of fecal coliforms since it is more resistant to the lethal effects of sunlight. Studies carried out by Acra et al. (1997) proved the bactericidal effect of sun s UV rays to disinfect water. Wegelin et al. (1994), Joyce et al. (1996), Solarte et al. (1997), Saitoh et al. (2002), and Caslake et al. (2004) evaluated the synergism between UV exposition, temperatures achieved with the exposure to the sun and turbidity in water samples to disinfect water for human consumption. Joyce et al. (1996) working with samples of water contaminated with 20 x 10 5 CFU per ml of E. coli and turbidity of 12 or 200 NTU (nephelometric turbidity units), in 2-liter uncolored transparent plastic soft drink bottles, found that after seven hours of exposition to solar UV rays in Kenya (1 o 29'S; 36 o 38'E), no E. coli organisms were detected. Wegelin et al. (1994) verified that the bacteria logarithmic inactivation curves are generally concave and they present an early delay. 11

28 This shoulder corresponds to the delay that light has in producing a lethal effect in bacteria due to the DNA repair mechanisms. After this period has elapsed, the curve drops rapidly. McGuigan et al. (1998) and Oates et al. (2003) stated that it is possible to have 100% E. coli inactivation with less than eight hours of solar exposition. Kehoe et al. (2001) and Burgess et al. (2002) analyzed the effect of dissolved oxygen and turbidity in water before its exposure to UV radiation and the effect of aluminum foil reflectors; they found that the presence of dissolved oxygen and a reflective surface improves the efficiency in removing bacteria. On the other hand, as turbidity increases the bacterial inactivation rates decreases. Salih (2003) developed a mathematical model to predict the efficiency of SODIS considering the exposure to solar UV and the initial contamination load as independent variables. He used 6 by 9 mm wells and 3.54 mm water depth samples prepared using sterilized stream raw water and a wild type of E. coli and found out that around 8.88 x 10 3 kj m -2 exposure was required to reduce five log cycles. Yoon et al. (2004) disinfected DWW effluent from a secondary level wastewater treatment plant using UV lamps and found that the use of treated DWW for agricultural purposes in Korea is an effective and feasible alternative. The effluent presented an average concentration of 3.4 mg L -1 TSS (total suspended solids), 5.9 mg L -1 BOD (biochemical oxygen demand), 12

29 and turbidity of 1.1 NTU. The mean input concentration of total and fecal coliforms was 16 x 10 3 and 10 x 10 3 MPN/100 ml, respectively. For low income third world families some limitations of the studies that have been carried out seeking DWW disinfection using UV light stand out because: (a) they worked with UV lamps or with structures too complicated to be built and maintained when sunlight is the energy source used to treat DWW, (b) the objectives were mainly to disinfect water for human consumption, (c) lack of power supply in the shacks. This study seeks to develop a technology of DWW treatment that is simple and allows for its subsequent use in agriculture. The objective was not to produce sterile or potable water but to reduce the pathogen load to levels suggested by WHO (1989) for irrigation water without restrictions ( 1000 MPN fecal coliforms per 100 ml), and to develop a model to simulate pathogen mortality using the solar disinfection process. MATERIAL AND METHODS Pilot Study. A square cement block reactor was constructed at the Experimental Station for Reclamation of Domestic Wastewater for Agriculture Use (ESRDWW-AU) of the Department of Agricultural Engineering of the Universidade Federal de Viçosa, State of Minas Gerais, Brazil (Fig. 1). The reactor was located at 20 o S, 42 o W; and at an altitude of meters above sea level. The ESRDWW-AU can handle 2 L s -1 of DWW from a nearby located residential development. The DWW was pumped to a pre- 13

30 treatment unit and then passed through a concrete built septic tank (ABNT, 1993) with a residence time of approximately 14 hours and was designated as treated domestic wastewater (TDWW). The septic tank s effluent was diverted to the square concrete block reactor (SODIS unit) using m of 50 mm id PVC pipe. Fig. 1. Experimental Station for Reclamation of Domestic Wastewater for Agriculture Use of the Department of Agricultural Engineering at Universidade Federal de Viçosa scheme. Solar disinfection unit. The solar disinfection unit (SODIS unit; also, solar reactor) is a concrete block structure. Its base measures 1.5 by 1.5 m, with a height of 0.4 m. It has vertical walls and a northern orientation. Walls and bottom have raw concrete finish (Fig. 2). 14

31 Fig. 2. Solar disinfection unit used. Sample collection. Samples were collected according to Standard Method 1060B (APHA, 1995), and preserved according to Standard Method 1060C (APHA, 1995). TDWW was sampled on the southern part of the disinfection unit (Fig. 3). The first collection was done just after the desired water level was reached. Thereafter, samples were collected every two hours up to the end of each trial. Fig. 3. Sample collection. 15

32 A set of three samples were collected at each collection time, the first for microbiological analysis, the second for dissolved oxygen (DO), and the third for chemical oxygen demand (COD), total suspended solids (TSS), and turbidity, for a total of 106 sets collected. Trials were conducted in November 2004, March 2005, April 2005, and May 2005 under different cloudiness conditions, for a total of 20 days. Sample treatment. The septic tank s effluent was conveyed to the disinfection unit at 0.35 m above the bottom of the solar reactor (Fig. 4). Before each test, the inflow and the outflow valves were opened during 15 minutes to purge the solids that were deposited, during the night, at the bottom of the pipeline connecting the septic tank and the solar unit, having after that a representative effluent from the septic tank. Then, the valve at the bottom of the solar reactor was closed and the required TDWW height was set for each trial. Minimum and maximum air temperature ranged between 13.1 o C and 30.4 o C. Trials were performed starting at 8 a.m. and with eight, nine or ten hours of sunlight exposure, guaranteeing around 90% of the sunlight availability on every trial day. 16

33 Fig. 4. Treated domestic wastewater conveyed at the solar disinfection unit. Sample analysis. All trials analyses were performed according to (APHA, 1995): for DO (Standard Method 4500-O C), for turbidity (the instrument to measure turbidity was made by Companhia da Agua from Brazil, model TB1000; Standard Method 2130B), for COD (Standard Method 5220B), and for TSS (the filter paper was made by Machenery-Nagel from Germany, its characteristics are MN 85/70 BF Ø47mm, 0.6 µm; Standard Method 2540D). Water temperature was measured using a mercury thermometer (Standard Method 2550), with a range of -30 o C to 250 o C. Wastewater samples from the solar disinfection unit were tested for total coliforms and E. coli using the chromogenic substrate test (made by IDEXX, Quanti-Tray/2000; USA; Standard Method 9223B). The limit of detection is <1 coliform per 100 ml of water. Solar radiation measurement. An automatic weather station was installed by the experiment with a pyranometer sensor (LI-COR, LI-200SZ; 17

34 made in USA), with sensitivity of 80 µa per 1,000 W m -2. The spectral response curve is between 395 and 1,105 nm. Data were stored in intervals of half hour. The pyranometer was calibrated using a black & white pyranometer (The Eppley Laboratory Inc., Model 8-48; made in USA). Data analysis. Results from the water analysis were expressed according to Standard Method The bacteria population reduction in the TDWW exposed, due to SODIS, was analyzed using the Statistical Package R (The R Foundation for Statistical Computing, Version 2.1.0). Considering that the response variable is binary by nature, indicating the surviving microorganisms as a proportion of their initial population after exposition to SODIS, the logit regression model was considered during modeling and to fit the data. Therefore, the predicted values for the dependent variable will never be less than or equal to zero, nor greater than or equal to one; despite the value of the independent variables. The model used to fit the data is the S-shaped function, known as the two parameter logistic, as recommended by Ratskowsky (1989) and Crawley (2002): e = (1 + e ( a+ bx) y + ( a bx ) ) (1) Then, the equation used is: ( a0 + b1 x1 + b2 x b14 x14 ) e = ( a + b x + b x +... b ) (2) N / No + x 1+ e

35 where N is the E. coli population present in the TDWW after exposed to solar radiation (MPN/100 ml); No is the initial E. coli population present in the TDWW before been exposed to solar radiation (MPN/100 ml); x [1 14] are independent variables with their respective coefficients b [1 14]. These independent variables and their interactions are described on Table 1. Table 1. Variables and its interactions description used to fit the data Variable Variable symbol Units x 1 MJ m -2 x 2 m x 3 o C x 4 NTU x 5 mg L -1 x 6 mg L -1 x 7 mg L -1 Fluence Depth Temperature Turbidity Dissolved Oxygen Total Suspended Solid Chemical Oxygen Demand Interaction of variables Fluence : Temperature x 8 Fluence : Depth x 9 Fluence : Turbidity x 10 Depth : Temperature x 11 Depth : Turbidity x 12 Fluence : Depth : Temperature x 13 Fluence : Depth : Turbidity x 14 Data were analyzed using a linear mixed-effects model fit by maximum likelihood, with energy as fixed effect and each trial as random effects. Ten different statistical models have been built by combining the independent variables described above in several ways. The model that best described the data was the one presenting the lowest Akaike Information Criterion (AIC = -2 log Likelihood + 2 (number of parameters in the model + 1)) and the lowest 19

36 Bayesian Information Criterion (BIC = -2 log Likelihood + (number of parameters in the model x log sample size)). The Akaike Information Criterion, also named penalized log likelihood, is useful when comparing models, because it explicitly penalizes any superfluous parameters in the model. It is especially useful when non linear mixed effects models are being compared. AIC can be used to compare two models in the same way the analysis of variance does. According to Crawley (2002) the methods of model fitting and model simplification are exactly the same in mixed effects and analysis of variance. All that has changed is that we are interested in the variance explained by the random effects, rather than their effects sizes. RESULTS AND DISCUSSION The septic tank effluent contained as high as 96 x 10 6 and x 10 6 MPN/100 ml for total coliform and E. coli, respectively. The mean septic tank effluent values delivered to the SODIS unit were 13.6 x 10 6 and 3 x 10 6 MPN/100 ml for total coliform and E. coli, respectively. It can be noted that water quality parameters such as COD and DO improved during the wastewater treatment (Table 2); however no significant differences were found. 20

37 Table 2. Mean, maximum, minimum and standard deviation values for the collected data Parameter Units Mean Value Maximum Value Minimum Value Standard Deviation Depth = 0.05 m Dose MJ m TSS mg L Turbidity NTU TDWW Temperature o C DO mg L COD mg L Depth = 0.10 m Dose MJ m TSS mg L Turbidity NTU TDWW Temperature o C DO mg L COD mg L Depth = 0.15 m Dose MJ m TSS mg L Turbidity NTU TDWW Temperature o C DO mg L COD mg L Depth = 0.20 m Dose MJ m TSS mg L Turbidity NTU TDWW Temperature o C DO mg L COD mg L TSS total suspended solids; TDWW treated domestic wastewater; DO dissolved oxygen; COD chemical oxygen demand. 21

38 The energy required to reach low levels of N/No decreased as water depth decreased (Fig. 5). It is also clear that a lag time is required before the E. coli population starts to diminish creating the S-shaped behavior, showing a presence of a shoulder and two stages of inactivation, which agrees with Severin et al. (1983), Wegelin et al. (1994), Kehoe et al. (2001), Kowalski (2001), and Salih (2003). e a f c b c d d e f a b c Fluence (MJ m -2 ) Fig. 5. Proportion of initial population of E. coli, as a function of fluence and treated domestic wastewater depth when solar disinfection (SODIS) was used. The inactivation kinetics of microorganisms in its simplest form, the classical single-exponential decay law curve, known as Chick s law (Metcalf & Eddy, 1979) does not describe the trend of the data collected in this study. The multi-target survival model used by Salih (2003) takes into consideration 22

39 the presence of a shoulder and two stages of inactivation, as happened in this research. According to Wegelin et al. (1994), this survival model is common when pure bacterial strains are used under laboratory and sunlight irradiation, which is not the situation under evaluation. The two-stage survival model represents two separate populations of microorganisms that have different decay rates (Kowalski, 2001). The limitations of this model under the circumstances of this research are that the depth of water is not taken into consideration and the rates of population decay for each population are unknown. There was no great difference between model I, which uses dose and depth as independent variables, and model II which uses dose and temperature (Tables 3 and 4). According to the principle of parsimony, and since model I (eq. 3) presented the lowest values for AIC (504.71) and BIC (523.35) and that it is easier to work with water depth and energy received, it is recommended as the model to be used. Table 3. Measures of the fit of the models Model AIC BIC Model AIC BIC I II III IV V VI VII VIII IX X AIC Akaike Information Criterion; BIC Bayesian Information Criterion. 23

40 Table 4. Variables and coefficients for the models fitted Variables and Model coefficients I II III IV V VI VII VIII IX X a o x a 1 x F F F F F -1.2x10 13 F F F a 2 x d 17.27d 14.17d -4.33d -6.9x10 14 d -7.33d -7.67d a 3 x Tp Tp -2x10 14 Tp Tp Tp a 4 x tb 0.015tb 5.5x10 12 tb a 5 x 5 3.4x10 13 DO a 6 x 6 7.5x10 12 TSS a 7 x 7 6.8x10 11 COD a 8 x F Tp 0.042F Tp a 9 x F d 0.04F d 1.91F d 2.272F d 2.19F d a 10 x F tb F tb a 11 x d Tp a 12 x d tb a 13 x F d Tp a 14 x F d tb F fluence (MJ m -2 ); d depth of water (m); Tp temperature ( o C); tb turbidity (NTU); DO dissolved oxygen (mg L -1 ); TSS total suspended solids (mg L -1 ); COD chemical oxygen demand (mg L -1 ). 24

41 The statistical analysis of the data obtained during this research has shown that three parameters are important in SODIS of TDWW: initial E. coli population, depth of wastewater and fluence. DO, TSS, COD and turbidity were not statistically significant when the E. coli survival was modeled, neither were they when total coliform survival was modeled (Appendix). The results for model I are shown in Table 5 Table 5. Statistical parameters for Model I Linear mixed-effects model fit by maximum likelihood Data: NULL AIC BIC loglik Random effects: Formula: ~DOSE EXP Structure: General positive-definite, Log-Cholesky parametrization StdDev Corr (Intercept) 3.61E-07 (Intr) DOSE 2.18E-07 0 Residual 8.57E+02 Variance function: Structure: fixed weights Formula: ~invwt Fixed effects: (N/No) ~ DOSE + d Value Std.Error DF t-value p-value (Intercept) E-04 DOSE E+00 d E-03 Number of Observations: 106 Number of Groups: 20 The probability values for the coefficients of model I (eq. 3) are 7 x 10-4 ; 0; and 6 x 10-3 for the intercept, the variable Fluence (F), and the variable 25

42 Depth (d), respectively. The Standard deviation, when random effects were analyzed, presented values of 3.6 x 10-7 and 2.1 x 10-7 for the intercept and the variable Fluence, respectively. Therefore, Model I is expressed as: N No exp( F d ) ( 1+ exp( F d )) = (3) where N is the E. coli population present in the TDWW after exposed to solar radiation (MPN/100 ml); No is the initial E. coli population present in the TDWW before been exposed to solar radiation (MPN/100 ml); F is the fluence received by the TDWW (MJ m -2 ); and d is the TDWW depth in the solar reactor (m). Proportion of initial population (N/No) Experimental data Estimated data (eq. 3) Dose (MJ m -2 ) Fig. 6: Experimental and estimated data with Model I (equation 3) for a 0.05 m depth of treated domestic wastewater. 26

43 Fig. 6 shows the experimental data collected during the research and estimated data using Model I, as proposed on this paper, for 0.05 m depth of TDWW. Fig. 7 shows eq. 3 as a continuous surface containing the four depths studied, the range of fluence measured and the survival ratio of fecal coliform (E. coli). It is clear that shallower TDWW depths required less energy than deeper depths; not only because of the attenuation of light through the wastewater, but also because of the expected proportional increment of microorganisms that have to be hit by the same amount of photons of light to be inactivated throughout the column of wastewater. Fig. 7: Treated Domestic Wastewater disinfection using SODIS modeled with Equation 3. Ways to improve the surface reflectance of the internal walls and bottom of the reactor should be studied. In addition, vertical walls, which are simpler to be built, are not as efficient as sloped ones, which allow a better incidence of solar radiation. Improvements on these aspects should be studied. The recommendation is to build the walls of the solar reactor with a slope equal to 27

44 the latitude value. This way, the incidence of radiation will increase since shadows created by the solar reactor s walls will be diminished. Consideration should else be given to improve the reflectance of the walls to create an environment where not only direct radiation, but also indirect radiation is used. The amount of energy necessary to disinfect TDWW is not a value that can be established in hours of exposure, but rather as a dose applied (fluence accumulated). It depends mainly on the season of the year and the latitude. Therefore, no recommendation should be made in terms of hours. As an example, considering an initial E. coli population of 3.5 x 10 6 MPN/100 ml, a domestic wastewater depth of 0.05 m and a desired final population of E. coli equal to 1,000 MPN/100 ml, the total energy required will be of at least MJ m -2. If the water depth is increased to 0.20 m, the total energy demanded will be at least MJ m -2 for the same conditions. CONCLUSIONS The study presented here demonstrated that solar radiation can be used to disinfect TDWW to an adequate level to be used in fertigated agriculture. Levels of disinfection were obtained that fulfill the WHO s recommendations for unrestricted irrigation (category A water: <1000 MPN/100 ml). Reclamation of TDWW using solar radiation and a concrete structure have proven to be effective when the exposure time is adequate to reduce the microorganism population present in that effluent and the depth of treated domestic wastewater is less than 0.20 m. 28

45 A model to simulate and to design TDWW solar disinfection units was developed and presented. This model was developed to simulate the disinfection of TDWW using as dependent variables the initial E. coli population, the depth of wastewater and the dose accumulated; and it represents well real data. ACKNOWLEDGMENTS The authors thank CNPq and the Agricultural Engineering Department of the Universidade Federal de Viçosa for financial support of this work. REFERENCES ABNT (Associação Brasileira de Normas Técnicas) Projeto, construção e operação de tanques sépticos. NBR:7229, Rio de Janeiro, 15 pp. Acra, A.; Raffoul, Z.; Karahagopian, Y Solar disinfection of drinking water and oral rehydratation solutions: guidelines for household application in developing countries. Illustrated Publications, UNICEF, 52 pp. Acra, A.; Jurdi, M., Mu allem, H. Karahagopian, Y., Raffoni, Z Water Disinfection by Solar Radiation: assessment and application. Ottawa, IDRC, 23 pp. American Public Health Association (APHA), American Water Works Association (AWWA) and Water Environment Federation (WEF) Standard methods for the examination of water and wastewater (19 th Edition). Washington: Am. Public Health Assoc., 1193 pp. Burgess, S., Onyonge, C. Solar disinfection of water (A case study from Kenya) In: 28 th WEBC Conference Sustainable Environmental Sanitation and Water Services. Calcutta, India:

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