OPTIMIZATION OF MUNICIPAL WASTEWATER TREATMENT SYSTEM WITH UASB PROCESS

Abstract

The application of upflow anaerobic sludge blanket (UASB) reactor to municipal wastewater treatment has gained momentum all over the world ever since the installation of world's first full scale plant at Kanpur (India) in 1989. Conventional sewage treatment technology requires high investments in terms of capital and OMR costs along with considerable skill in their installation, operation and maintenance. Under the complex socio-economic environment of developing countries, the choice of technology for municipal wastewater treatment is a crucial decision in view of the availability of limited funds and, for such purposes, the designer is called upon to select a suitable method of wastewater treatment, which could be capable of meeting the environmental quality objectives and giving the desired level of treatment before the wastewater is discharged to water bodies or on land. In a tropical country like India, anaerobic processes have a promising future in municipal wastewater treatment. During past three decades, much of the research work on UASB reactor has been carried out towards its start up and treatment potential as applied to different types of wastewaters. The optimization of various wastewater treatment systems has attracted the attention of researchers for a long time. However, with a poorly understood performance of UASB reactor in municipal wastewater treatment, its cost optimization is-not available in the literature so far. In the present study, municipal wastewater treatment system with UASB process along with a post-treatment unit, viz., a facultative waste stabilization pond or, alternatively, a facultative aerated lagoon has been considered for the purpose of the optimization work. Before taking up the cost optimization of total system function, the performance of UASB reactor has been studied for a wide spectrum of data and it has been revealed that the performance relationships available in the literature are characterized by their limited potential for application. The functioi*fform of one of these equations is also found inappropriate. In view of the same, an in-depth performance evaluation of UASB reactor in municipal wastewater treatment has been carried out. The outcome ofthis study has revealed that the performance of UASB reactor follows a first order kinetics. In this regard, the range of first order kinetic coefficient has been worked out. In the cost optimization, the optimum cost for the municipal wastewater treatment has been obtained by adopting two different approaches. The sizing parameters of constituents units in terms of L / B ratio and side water depth have been obtained for UASB reactor as well as post-treatment units corresponding to minimum cost. By adopting sizing parameters as design variables, the minimum cost of the system has been worked out for varying L/Bratio and side water depth in the selected range. On the basis of minimum cost worked out for different design volumes, a cost equation relating minimum cost per volume to design volume has been developed for circular and rectangular UASB reactors with respect to their situation on or below the ground along with facultative waste stabilization pond and facultative aerated lagoon. In preliminary optimum design, efficiency based cost computation has been carried out using the respective cost equations of the constituent units. As the cost equations are based on best- fit curve method, and the approximations involved in the curve fitting are also likely to come into picture, a comprehensive optimum design has also been carried out. It is revealed that the optimum costs obtained by the two approaches lie in good agreement with each other. Asensitivity analysis with respect to some of the design parameters has also been presented. It is seen that UASB systems along with post-treatment units tend to minimize total area of land 111 required for municipal wastewater treatment system. Acomparative presentation of the cost ofthe municipal wastewater treatment system with UASB process has been done with respect to some conventional wastewater treatment systems and it is revealed that the UASB system stands as the cheapest ofthe all the available options. It is also revealed that, in context of the cost structure of the system, UASB reactor is found to be the main treatment unit in the system. A comparison of the actual cost of some existing municipal wastewater treatment plants with UASB process against the optimum cost worked out by the design algorithm followed in this study indicates that UASB system in India are not necessarily close to optimum solution. It is revealed that a significant amount of saving can be achieved by employing the optimum design of UASB systems in municipal wastewater treatment. In addition, as the optimally designed UASB systems along with post-treatment unit tend to minimize the land area to a great extent and are also not dependent upon external power, there is an enormous scope of their application in municipal wastewater treatment in India Thus, it is expected that the present study will yield a cost-effective design of UASB systems for municipal wastewater treatment. IV