Montevideo's sewage is disposed into the sea by means of an outfall. Nevertheless as there are a lot of cities that are not on seashore, and most of these cities are discharging their effluents into rivers without proper treatment, a more strict sewage treatment must be done in order to achieve the discharge standards. For the 1,780,000 inhabitants the current sewage coverage is 27%. Nearly 80% of the collected effluents are treated in Imhoff tanks built many decades ago. Nowadays, since the population has increased, these systems are overloaded. The sludge coming out from these Imhoff tanks is disposed on land mostly without any previous treatment, except dewatering.
Several lagoons and an activated sludge plant are also in operation. Moreover, an UASB (Upflow Anaerobic Sludge Bed) has been built and an Imhoff tank modified to be used as an UASB, but at present time none of them are in operation.
As a consequence of this situation it is necessary to carry out studies in order to compare different systems that may solve the sewage treatment in Uruguay's innerland. These studies must consider the efficiency of the systems, the achievement of the discharge standards, the energy consumption, the biogas production, the simplicity of the operation and the sludge treatment and disposal. Also environmental and economical topics must be taken into account.
Municipal solid waste
In Uruguay 2500 tons of MSW are collected per day. Landfill is the only method used for its treatment and final disposal because of the high availability of land and the low costs. Nevertheless, most of the disposal sites are working as dumping ones, without leachate treatment or biogas collection.
Due to the refuse composition, 43% w/w of food waste and 28% paper and cardboard (Borzacconi et al. 1997), and the climatic conditions (annual average rainfall of about 1100 mm and evapotranspiration about 720 mm), biogas recovery can be an economical and environmental clean solution.