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Por: Julio Moscoso, CEPIS Adviser in Wastewater Reuse / Guillermo León, CEPIS
Adviser in
Wastewater Treatment
In the Latin American and Caribbean Region, where the sewerage service covers only 49% of the population, more than 40 million cubic meters of wastewater are collected and discharged into rivers, lakes and seas every day. If by the year 2000 this basic service were expanded to 90% of the population, more than 100 million cubic meters of sewerage would be produced, aggravating even more the contamination problem. Less than 10% of the waste-water collected receive any treatment before being discharged into surface waters or before using it for irrigation.
Wastewater use for irrigation of edible crops pose a risk to the health of the population. The endemic outbreak of diarrheal diseases, parasitism, typhoid fever and salmonellosis that prevail in our continent demonstrates this critical situation, to which cholera was added since 1991.
The explosive growth of mega-cities has limited the use of surface waters for human con-sumption and hydroelectric power. As a consequence, the agricultural sector located in the outskirts of these large cities has been seriously affected and use wastewater as their only alternative. More than 400 000 ha irrigated with raw sewage without previous treatment reflect this problem. Information available up to March 1992 reports the existence of 350 000 ha in Mexico and 4 000 ha in Peru irrigated with these waters.
This situation is only the tip of the iceberg, since a lot more of agricultural land is irrigated with waters from lakes and rivers that largely exceed the maximum level of 1000 fecal coliforms per 100 ml recom-mended by WHO standards for the irrigation of vegetables to be eaten raw. With these levels of contamination, the risk of consuming contaminated food is high.
A study carried out by CEPIS in Lima, Peru, made it possible to compare the sanitary quality of food products irrigated with water from a non-contaminated river, raw sewage and treated waste-water. The quality of vegetables sold in Lima markets were also evaluated.
The results of the sanitary evaluation of the agricultural products are summarized in Figure 1. Presence of entozoic parasites was detected in 91% of the vegetables irrigated with raw water, the percentage declined when irrigation was made with treated wastewater and uncon-taminated surface water. With regard to the presence of Salmonella and concentration of Escherichia coli above the permissible levels set by in-ternational guidelines, the risk is
high when raw wastewater is used. Similar risk levels were found with treated wastewater, however, it should be noted that in this case, excess of fecal coliforms were due to overload of the stabilization ponds evaluated. This situation made it evident that the design and construction of treatment plants is as important as its adequate operation. Overloads should be avoided, although they frequently occur because expansion and improvement programs for treatment systems are neglected. As expected, vegetables irrigated with uncontaminated surface water presented low health risk.
The Figure also shows that contamination levels of vegetables sold in markets, regardless the water quality used to irrigate them, are similar to vegetables irrigated with raw wastewater. The reason may be found in the use of contaminated wastewater, the sprinkling of vegetables with contaminated water before their transportation to the markets, and the lack of hygiene during the entire marketing route, from the moment vegetables are taken from crop lands up to their consumption at homes.
In developed countries, where a case of typhoid or parasitism are exceptional, the objective of wastewater treatment is the removal of organic matter and nutrients. On the other hand, in developing countries, the main objective of wastewater treatment should be the removal of parasites, bacteria and pathogenic viruses that cause endemic diseases. Stabilization ponds are the technological option through which the target of "not pathogens" is fully reached.
If the only objective were to decontaminate water resources, no project would have financial feasibility. However, if the excellent bacteriological quality of stabilization pond effluents is taken as an advantage, as well as the nutrients it contains, benefits are obtained for agriculture, livestock, horticulture, aquaculture and forestation. Besides, agricultural and livestock production would be near to consumption centers. Thus, the prompt recovery of our limited water resource in the Region would be a reality. The design of these systems will be adjusted according to the effluent quality required for every use.
The use of wastewater also makes it possible the efficient use of water, the provision of natural fertilizers and food, the creation of employment sources and economic income, and the expansion of agricultural frontiers in desert areas.
Countries with a long tradition of fish culture are incorporating raw wastewater to fish ponds. It is the case of Calcutta in India, where there are more than 10 000 reservoirs fed with raw sewage, posing high sanitary risks that still have not been evaluated. On the other hand, developed countries raise fish to improve organic matter removal, without considering the quality of the product since it is not used for direct human consumption.
Adopting an intermediate situation, CEPIS carried out an aquaculture project using treated effluents from stabilization ponds located in San Juan de Miraflores, Lima, Peru. Wastewater was treated till it reached the appropriate quality to obtain suitable fish for direct human consumption. Four experimental crops of Nile tilapia, Oreochromis niloticus, were developed during the warm and cold seasons of Lima climate.
The treatment system reduced total BOD levels from 112 to 68 mg/l. High production of algae ranged from 1 573 to 718 mg/l of chlorophyll A, according to the climate. Total ammonium ranged between 2.62 and 0.45 mg/l, which are tolerable values for Nile tilapia. The removal of fecal coliforms in the treatment process confirmed that the system was capable of reducing them up to 5 logarithms, making it possible to achieve an effluent with levels of 104. Since fish ponds work in batch, coliform concentration may be reduced in one logarithm to obtain the level of 103 recommended by WHO.
Under Lima climate conditions, it was possible to obtain 4 400 kg/ha of tilapia with an average weight of 250 g per unit at the end of the summer, without adding artificial food. Growth was reduced during winter time because temperature declined to 17° C. In conventional farms of the Amazon region, this production is obtained only when ponds are fertilized and concentrated food is provided. The abundant biomass of algae in treated wastewater replace artificial food, reducing production costs.
Fish quality was evaluated following the strict qualification proposed by Buras (1987). It accords a "very good" to fish with less than 10 bacteria per gram of muscle; "acceptable" to fish with 10 up to 50 bacteria; and "rejected" to fish with more than 50 bacteria per gram of muscle. It is important to note that fish sold in markets normally present larger number of bacteria than those mentioned in the qualification.
In three out of four experiments a "very good" qualification was achieved for 100% of the fish. In the third experiment, 6% of the fish was rejected due to a deliberate increase of fecal coliforms that surpassed 105. This experiment set up effluent quality limits used for tilapia culture. Once this limit is exceeded, the immunological system of the tilapia weakens and bacteria enter into the muscle. It was also observed that tilapia selfpurification capacity functions when coliform level is reduced over a minimum period of 30 days. This means that in the eventual case of an overload in the treatment system, the sanitary quality of the affected fish may be recovered.
Based on these results, a computerized model was prepared to calculate the layout of commercial farms in tropical and subtropical areas. Higher tropical temperature reduces raising period to seven months, obtaining up to three harvests per year. With this program it is easy to calculate, for example, that for achieving a production of 60 tons per year, 19 ha are required in subtropical climates, while tropical climates demand only 9 ha, thus reducing production cost.
This model also allows an economic evaluation. One may take the case of a tropical farm that produces 60 tons and requires an investment of US$76 000, and US$16 000 for operational costs per year. It determines a cost of US$0,31/kg compared to the price of US$1,00 to 3.00/kg. This low cost competes with commercial fishing and obtains 45% of internal rate of return, which indicates the high profitability of the project. This example does not consider land cost and assumes that the project is located in uncultivated areas, however, the model analyzes profit variation according to different costs of land and water treatment.
In 1991 the Ministry of Agriculture of Peru initiated a National Project on Irrigation Using Treated Wastewater to expand the agricultural frontier through irrigation of 18 000 ha with 20 m3 of sewerage produced in large cities of the Peruvian coast.
CEPIS gave technical assistance and focused on evaluating the capability of nutrients contained in treated wastewater to replace fertilizers. Different doses of fertilization normally applied to crops were valuated, including wastewater only (without fertilizers). Crops such as beans, string beans, broccoli, cabbage, and corn were tested.
The results obtained with "panamito" beans (figure 4), showed that production in all evaluated crops were similar, including the crop without fertilization. It was demonstrated that wastewater contributes all nutrients required by crops, saving fertilizers that often represent more than 50% of the production cost. Research carried out in Israel observed that certain fruit and grain crops were affected by high levels of nitrogen contained in treated wastewater, since it only favors the vegetative development of the plant. As a result, treatment systems are aimed at improving the removal of this nutrient. However, high concentration of nitrogen favors forage crops.
To diversify production and to improve efficiency while reducing investment risks, CEPIS is promoting treatment and use integrated models with agricultural, aquaculture and forest components.
As an example, we can mention a model designed for a tropical city of 50 000 inhabitants which generates 100 l/s of sewerage and requires a set of ponds covering 9 ha to irrigate 11 ha of vegetables, 30 ha of asparagus, 39 ha of cotton, and to feed 9 ha of fish ponds. Bearing in mind crop performance, the following variables were considered: tropical and subtropical climates, investment and operation costs, yield per year, net current value and internal rate of return. These agricultural and livestock modules may reach an internal rate of return of 71% and 42% in tropical and subtropical areas, respectively, which may be considered quite profitable.
CEPIS has proposed a revolving credit to finance the National Program on Irrigation Using Treated
Wastewater. An external financing of 13 million dollars, under soft conditions, would make it possible to develop 180 agricultural and livestock farms of 100 ha each one during a period of 20 years up to covering 18 000 ha.
With CEPIS's technical assistance, two projects of treatment and wastewater reuse are under implementation at the Universidad Nacional de Ingeniería and the Universidad Agraria La Molina in Lima, Peru. An adequate management model will make it possible to demonstrate the technical and economic feasibility of these comprehensive systems.
With the same purpose, demonstrative units in several countries of the Region are being developed. In Mexico, the creation of a Regional Center for Wastewater Reuse has been proposed, which will include a demonstrative unit. A similar model for semi-arid areas of the Brazilian northeast is under consideration.
To encourage the promotion of these initiatives, CEPIS supports training in different countries of the Region. In the last two years, courses were held out in Costa Rica, Peru, Mexico and Venezuela.
Up to now, all efforts carried out by CEPIS in the field of sanitary use of wastewater aimed at improving sewage treatment through the generation of productive activities that absorb the cost of the treatment. We have the commitment to continue training and technical assistance programs to contri-bute to the development of technologies adequate to the reality of Latin America and the Caribbean.
(Documents available at CEPIS Library)
[1] Bartone, Carl R.; Castro de Esparza, María Luisa; Vargas de Mayo, Carmen; Rojas Chacón, Olga; Vitko, Tadeo G. (1985). San Juan Lagoons Supporting aquaculture; Integrated Recovery Project. Lima, The World Bank, Washington, D.C., CEPIS/PAHO.
[2] Buras, Netty; Duek, Lea; Niv, Sara; Hepher, Balfour; Sandbank, Enrico (1987). Microbiological aspects of fish grown in treated wastewater. Water Research, 21 (1):1-10.
[3] Castro de Esparza, María Luisa; León Suematsu, Guillermo (1992). Estudio Preliminar de la Remoción de Vibrio cholerae en Lagunas de Estabilización - San Juan de Miraflores, Lima -Perú. Informe Técnico 387, CEPIS, Lima.
[4] Castro de Esparza, María Luisa; Sáenz Forero, Rodolfo (1990). Evaluación de los Riesgos para la Salud por el Uso de las Aguas Residuales en Agricultura. CEPIS, Lima.
[5] León Suematsu, Guillermo; Moscoso Cavallini, Julio (1995). Estrategias para el Uso de Efluentes de Lagunas de Estabilización en América Latina - El Modelo de Acuicultura en Lima, Perú. Presentado a la Tercera Conferencia Internacional de Especialistas sobre Tecnología y Aplicaciones de Lagunas de Estabilización, organizada por la Asociación Internacional de Calidad del Agua (IAQW), João Pessoa, Brasil, 27-31 marzo.
[6] Instituto Mexicano de Tecnología del Agua (1993). Memoria del Taller Regional para las Américas sobre Aspectos de Salud, Agricultura y Ambiente Vinculados al Uso de Aguas Residuales, Jiutepec, Morelos, México, 8 al 12 de noviembre de 1993.
[7] OMS (1989). Directrices Sanitarias sobre el Uso de Aguas Residuales en Agricultura y Acuicultura. Ginebra, Serie de Informes Técnicos, 778.
[8] Moscoso Cavallini, Julio; Flórez Muñoz, Alberto (1991). Reuso en Acuicultura de las Aguas Residuales Tratadas en las Lagunas de Estabilización de San Juan, Sección I: Resumen Ejecutivo. CEPIS, Lima.
[9] Moscoso Cavallini, Julio; León Suematsu, Guillermo; Gil Merino, Elena (1991). Reuso en Acuicultura de las Aguas Residuales Tratadas en las Lagunas de Estabilización de San Juan, Sección II: Tratamiento de las Aguas Residuales y Aspectos Sanitarios. CEPIS, Lima.
[10] Moscoso Cavallini, Julio; Nava Cueto, Hugo (1991). Reuso en Acuicultura de las Aguas Residuales Tratadas en las Lagunas de Estabilización de San Juan, Sección III: Acuicultura. CEPIS, Lima.
[11] Moscoso Cavallini, Julio; Egocheaga, Luis (1991). Reuso en Acuicultura de las Aguas Residuales Tratadas en las Lagunas de Estabilización de San Juan, Sección IV: Factibilidad Técnica, Económica y Social. CEPIS, Lima.
[12] Roos, W. R. (1992). The Urban Pollution Problem in Latin America. Presentado en: Nagoya Seminar on Financing for the Environment, Nagoya, Japan.
[13] Yánez, Fabian. (1983). Indicator and Pathogen Organism Die-off in Ponds and Design under Tropical Conditions. Presentado en: 56th Annual Conference of the Water Pollution Control Federation, Atlanta, Georgia, 2-6 October.
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