HDT 59: Wastewater Use
July Moscoso, CEPIS Adviser in Wastewater use. Guillermo León, CEPIS Adviser in Wastewater Treatment. September 1994.
Spanish version: HDT 59: Uso De Aguas Residuales
- Disposal and indiscriminate use of wastewater
- Treatment and sanitary use of wastewater
- Wastewater use in aquaculture
- Wastewater use in agriculture
- Integrated models of treatment and use
Disposal and indiscriminate use of wastewater
49% of the Region of Latin America and the Caribbean has sewer service; daily 40 million cubic meters of wastewater are collected and discharged into rivers, lakes and seas.
If in 2000 will be achieved the expantion of this basic service to 90% of the population, over 100 million cubic meters of wastewater will aggravate even more the contamination. Less than 10% of the volume collected by sewerage systems, is treated before being discharged into a body of surface water or prior to use for direct irrigation of agricultural products.
The use of wastewater for crops irrigation for human consumption increases risk factors for the population’s health. The endemic situations of diarrhea, parasites, typhoid fever and salmonellosis that prevail in our continent are no more than a reflection of this critical situationto which came to join cholera.
The imbalance between water resources and the explosive growth of large cities, has forced to prioritize the use of surface water for public supply and power generation. As a logical consequence, agricultural activity located on the outskirts of cities has been seriously affected and has opted for the use of wastewater as the only survival way. This is reflected in the existence of more than 400,000 agricultural hectareas which are directly irrigated with this water, most without prior treatment. In 1992 Mexico reported 350,000 hectares of farmland directly irrigated with wastewater, and on the coast of Peru there are more than 4000 hectareas irrigated with such water.
This is just the tip of the iceberg, since a higher amount of agricultural land is irrigated with surface water from rivers and canals that far exceed the maximum level of thousand fecal coliforms per 100 ml recommended by the World Health Organization (WHO) for irrigation of vegetables that are eaten raw. With these levels of contamination, the risk of consuming contaminated food is high.
A study by CEPIS in Lima, Peru, allowed to compare the sanitary quality of food products irrigated with unpolluted river waters and with raw and treated wastewater. Quality of food products normally sold in the markets of Lima was also evaluated.
The results of the health assessment of agricultural products are summarized in Figure 1. The 91% of products irrigated with raw wastewater showed the presence of intestinal parasites, the risk decreased in vegetables irrigated with treated wastewater and surface unpolluted water. With regard to the presence of Salmonella and and the concentrations of Escherichia coli above permissible levels according to international guidelines, the risk is high with the use of raw wastewater. Comparable levels of risk with the use of treated wastewater were found, howevershould be noted that excess in fecal coliforms was due to the overloading of the ponds stabilization system evaluated.
This situation indicates that as important as implementing a treatment plant is that this operates properly, avoiding overloads that frequently occur due to lack of expansion programs and the improvement of treatment systems. As expected, the products irrigated with unpolluted surface water presented a low risk.
On the same graph is shown that pollution levels of the products that are sold in markets, regardless of the quality of irrigation water, are comparable to those irrigated with untreated wastewater. These levels are due to: the use of contaminated wastewater, the cooling of products with polluted surface waters before their final destination in the large supply centers, and the lack of hygiene in food handling throughout the marketing route, from the moment that the product leave the farmland until reaching the households.
Treatment and sanitary use of wastewater
In developed countries the main goal of treatment is the removal of organic matter and nutrients because typhoid or a case of parasitism are exceptional. On the other hand, in developing countries, the primary objective of treating wastewater should be the removal of parasites, bacteria and viruses that cause endemic diseases. The technological option through which the goal of “nonpathogenic” is fully achieved, corresponds to the stabilization ponds.
The effluents from stabilization ponds for its bacteriological quality, can be used in any agricultural activity, from horticulture, agroindustrial crops and aquaculture even afforestation. The dimensioning of these systems will be linked to the quality of the effluents required for each type of
If the only objective was to decontaminate the water resource, all projects would be financially unviable. However, if the excellent bacteriological quality, and richness in nutrient that provide treated water through stabilization ponds are used, it is possible to obtain other benefits as agricultural production close to consumption centers. Thus, the early recovery of our limited water resources in the region would be a reality.
The use of wastewater also allows for other benefits, such as efficient water use, provision of natural fertilizers and food generation, employment and enconomic incomes, and the increase of the agricultural frontier in desert areas.
Wastewater use in aquaculture
In countries with great piscicultural tradition the wastewater is entering the farming ponds without any previous treatment. This is the case of Calcutta in India, where there are more than 10 000 ha of ponds are fed with raw sewage, causing a high health risk that has not yet been evaluated. In contrast, developed countries are using fish farming as a way to improve the removal of organic matter, regardless of the quality of the product because it is not intended for direct human consumption.
Adopting an intermediate situation, CEPIS ran the aquaculture project using treated wastewater of stabilization ponds located in San Juan, Lima, Peru. In this case, the wastewater is pretreated to reach the appropriate quality for a high production of fish suitable for human consumption. For two years four experimental crops of Nile tilapia, Oreochromis niloticus, were performed on a continuous basis during times of heat and cold climate of Lima.
The treatment system has reduced the levels of total BOD on ranges from 112 to 68 mg / l. The high production of algae was between the 1573 and 718 mg / l of chlorophyll A, according to the weather. The total ammonia fluctuated between 2.62 and 0.45 mg / l, tolerable values for Nile tilapia. The removal of fecal coliforms in the treatment process confirmed that the system is able to reduce up to 5 logarithms and enables an effluent with levels of 104. Because fish ponds work in “batch”, it is possible to reduce the concentration of coliforms in a logarithm and get the 103 level recommended by WHO.
Under the Lima conditions, you can get 4400 kg / ha of tilapia with an average weight of 250 g per unit at the end of the summer without adding artificial foods. The growth is very low during the winter because the temperature drops to 17 °C. In conventional farms in the Amazon, you can only get this level of production if ponds are fertilized and concentrated food is provided. The abundant biomass of algae in treated wastewater replace artificial feeding and therefore reduce production costs.
The quality of the fish was assessed according to strict qualification proposed by Buras (1987), which states as “very good” to the fish with less than 10 bacteria per gram of muscle; are “acceptable” those with 10-50 bacteria and are “rejected” the fish with more than 50 bacteria. It is important to note that fish sold in markets typically has higher bacterial load in muscle than those mentioned in the classification.
Three experiments were rated “very good” for 100% of the fish were “rejected” managed. Only in the third experiment, 6% of the fish were “rejected”, a situation that was caused by a deliberate increase of the fecal coliform level which exceeded 105 in the effluent. This allowed to establish, the limit effluent quality to be used in tilapia farming; exceeded the limit, the tilapia’s immune system is weakened and bacteria enters the muscle. Was also observed the self-purification capacity of these fishas long as the coliform level for a minimum period of 30 days is reduced. This means that in the unlikely event of an overload “accident” in the treatment system, the sanitary quality of the affected fish can be recovered.
The results have enabled the development of a computer model for sizing commercial farms in tropical and subtropical areas. The high temperature of the tropics can reduce the rearing period to seven monthsto give up to three crops a year.
With the program is easy to calculate, for example that to achieve a production of 60 tonnes per year are required 19 ha in places with subtropical climates, whereas in tropical climates only 9 ha are needed, a situation that also reduce the production cost.
This model also allows to perform an economic evaluation. You can take the case of a tropical farm that produces 60 tons and requires an investment of US $ 76,000 with annual operating costs of US $ 16,000, determining a cost of US $ 0.31 / kg compared to a price of US $ 1.00 to 3.00 / kg. The low cost allows compete with fishing and to obtain an internal return rate of 45% which indicates high profitability. This case has not considered land costs, in the assumption of exploit uncultivated areas, however, the model allows us to perform a sensitivity analysis to study the variation of the profitability with different land costs or water treatment.
Wastewater use in agriculture
In 1991 the Ministry of Agriculture of Peru launched a National Irrigation Project with Treated Wastewater, intended to expand the agricultural frontier on the coast with 18,000 hectareas irrigated with with 20 m3 of wastewater produced in the major cities of the Peruvian coast.
CEPIS provided technical assistance and concentrated its efforts on assessing the level of fertilizer substitution by the nutrient supply of treated water. Different fertilization doses were evaluated from a control with only sewage (no fertilizer) to fertilization levels normally applied in commercial crops, besides wastewater irrigation similar to the control amount. Different commercial crops like frijol, beans, broccoli, cabbage, corn, etc. were tested.
As shown in the results obtained with the bean “panamito” (Figure 4) all crops tested showed similar production performance in all treatments, including the control without fertilization. Was demonstrated that sewage provide all the nutrients required by crops, which saves fertilizer costs that often represent over 50% of the production cost. Research conducted in Israel mentions that certain fruit and grain crops may be affected by high nitrogen levels existing in the treated wastewater, as it only favors the vegetative plant development. Therefore, their treatment systems are oriented to improve the removal of this nutrient. However, this high nitrogen concentration is favorable for forage crops, where it is desirable to promote the vegetative growth of the plant.
Integrated models of treatment and use
CEPIS is currently promoting integral units of treatment and use, where different agricultural, aquaculture and forestry components are combined in order to diversify production to improve efficiency and reduce investment risks.
We can mention as an example a dimensioned model in a tropical city of 50,000 inhabitants which generates 100 l/s of drains and requires a lagoons plant of 9 ha to irrigate 11 ha of vegetables, 30 ha of asparagus, 39 ha of cotton and feed 9 ha of fishponds. Considering the crops performance, have also been calculated variations of tropical and subtropical climates, investment and operating costs, annual income, net present value and internal rate of return. These agricultural modules can achieve a rate of return of 71 and 42% in tropical and subtropical areas, respectively, this can be considered quite profitable.
We have proposed that the National Program for Treated Wastewater Irrigation is financed using a rotative credit. An external $ 13 million financing under soft conditions, would develop 180 agribusinesses 180 of 100 ha each for a period of 20 years to cover 18,000 ha.
With technical assistance from CEPIS are being implemented two projects of treatment and reuse of wastewater at the University of Engineering and Agricultural University “La Molina” in Lima, Peru. A proper management model will demonstrate both technical and economic feasibility of these integrated systems.
For the same purpose, demonstration units are being developed in the countries of the region. Mexico has proposed the creation of a Regional Centre for Wastewater Reuse that would include a demonstration unit. Also is developing a similar model for the semiarid zone of the Brazilian northeast.
Consistent with this view of promoting, CEPIS supports training initiatives in different countries of the Region, standing out in the last two years the courses held in Costa Rica, Peru, Mexico and Venezuela.
So far, all efforts done by CEPIS in the field of sanitary use of wastewater have been aimed to improve sewage treatment by generating productive activities to absorb the cost of treatment. We are committed to continue making training programs and technical assistance to assist the development of treatment technologies and use of wastewater, appropriate to the reality of Latin American and the Carribbean.
(Documents available at the Library of the CEPIS )
- 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.
- 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.
- 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.
- 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.
- 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), Joþo Pessoa, Brasil, 27-31 marzo.
- 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.
- OMS (1989). Directrices Sanitarias sobre el Uso de Aguas Residuales en Agricultura y Acuicultura. Ginebra, Serie de Informes Técnicos, 778.
- 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.
- 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.
- 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.
- 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.
- Roos, W. R. (1992). The Urban Pollution Problem in Latin America. Presentado en: Nagoya Seminar on Financing for the Environment, Nagoya, Japan.
- 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.