Sewage sludge management is one of the main challenges pointed out by the European and local authorities, together with plant managers, in the past years. Its annual production in the European countries is over the 7.5 million Mg in dry basis, which must be treated to minimize or avoid its negative environmental impacts. Sewage sludge composting and biodrying processes are two similar treatment processes seeking different objectives. On one hand, sewage sludge composting aims at reducing both the volume and the biological activity of the material to obtain a product that can be used as fertilizer or soil amendment. On the other hand, sewage sludge biodrying seeks to remove moisture by using both the metabolic heat generated during the biodegradation of organic matter and high aeration rates to obtain a final product that can be used as biofuel. However, both technologies present unavoidable environmental and social impacts among which gaseous and odorous emissions derived from the biological degradation of organic matter stand out. Among all the gaseous compounds that can be present in these emissions, special mention is deserved for ammonia, hydrogen sulphide and volatile organic compounds (VOCs) as the main odour contributors and for methane and nitrous oxide as the main greenhouse gases. In this thesis, the study of the gaseous and odorous emissions generated during the treatment of sewage sludge has been focused on a full-scale sewage sludge composting process and on a bench-scale sewage sludge biodrying process. Firstly, the influence of the composting time on the gaseous and odorous emissions and on the biological stability achieved was assessed in a full-scale sewage sludge composting plant. The composting process was divided into two main phases, a first organic matter biodegradation phase performed in dynamic windrows with a retention time of 4 days, followed by a curing step performed in trapezoidal piles lasting about 30 days. By increasing the dynamic windrow retention time to 14 days, the organic matter biodegradation was enhanced, while the gaseous and odorous emissions generated during the curing phase were reduced. In addition, a full VOCs characterization was done, showing that organosulphur compounds, ketones and terpenes were the main VOC groups present in the gaseous emissions generated during the composting process. Finally, a bench-scale bioreactor for the sewage sludge biodrying was operated to monitor and fully characterize the gaseous and odorous emissions derived from this process, providing the first inventory of ammonia, hydrogen sulphide, total and specific VOCs, odour, methane and nitrous oxide emissions for this sewage sludge treatment technology. To put the obtained results into context, the gaseous and odorous emissions generated during the sewage sludge biodrying process performed were compared with gaseous and odorous emissions from sewage sludge composting literature, showing that, in general, lower gaseous and odorous emissions were generated during the biodrying process. In addition to the scientific interest of this thesis, it helps to increase the GICOM, GENOCOV and BETA research groups expertise on the sampling and analytical methodologies for gaseous and odorous emissions generated in bench and full-scale sewage sludge treatment facilities.
Study on the gaseous emissions during the biological treatment of sewage sludge. Characterization of VOCs and odour emissions
GONZALEZ ALE, D. (Author). 31 Jan 2020
Student thesis: Doctoral thesis
Student thesis: Doctoral thesis