Abstract
The treatment of growing production of municipal sewage sludge has become a significant global problem. Drying of digested sewage sludge is a promising alternative to sludge disposal at dumping sites. The research objective of this study was to find the optimal heat source for a sludge drying plant in a large municipal sewage treatment plant (people equivalent: 250,000). Two boundary heat supply cases were analyzed in the paper: cogeneration of heat and power (CHP) units, internal combustion (IC) engines fired with natural gas, and plant supplied with a gas boiler. The aim of the research was to find the optimal size of the cogeneration unit cooperating with the gas boiler as heat sources for a given drying plant case with the maximum net present (NPV) value as the objective function. The results of the conducted optimization show higher profitability of cases with larger cogeneration unit. For the basic assumptions, the maximum NPV is obtained for the largest analyzed CHP unit: 1300 kW of thermal power output. Sensitivity analyses show that the varying gas and electricity prices can relocate the NPV maximum towards smaller CHP sizes. A supplementary energy analysis shows that implementing larger CHP units yields a higher energy efficiency of the system, up to 0.52.
Highlights
The handling of sewage sludge as the main by-product of wastewater treatment processes is one of the most significant challenges of sewage management [1]
The main goal of the analysis is to find the optimal share of cogeneration (CHP) implemented in conventional boiler-based system in terms of economic feasibility
On the basis of previous assumptions made on configuration modes of the heat source as well as data gained from a real project of constructing a municipal sewage sludge drying plant, the set of input data was prepared for the need of further investigation on net present value (NPV) indices
Summary
The handling of sewage sludge as the main by-product of wastewater treatment processes is one of the most significant challenges of sewage management [1]. According to [8], the increase in sewage sludge production has recently become a major environmental and economic issue due to the growing costs of pretreatment, transportation and disposal. To estimate the influence of these factors, an example of a typical large municipal sewage treatment plant has been examined In this plant, digested sludge is dewatered by means of a centrifuge to 20% of dry mass and removed to a dumping site or partly used in agriculture. Such a way of managing sediments requires modernization because of the aforementioned unfavourable legislative conditions as well as due to the limited access to agricultural areas, due to environmental risks and the lack of interest in the dewatered product by potential consumers.
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