Abstract
Anaerobic sludge fermentation is one of the main technological steps for treating the wastewater resulting from urban consumption in the food industry. Anaerobic fermentation ensures the sludge mineralization treatment and the fermentation gas (biogas) resulting from the process can be used for the production of thermal or electric energy. The fermentation process can take place in closed reinforced concrete tanks (digesters) having different structural forms: cylinder, tapering, ovoid depending on the type of fermentation: mesophilic or thermophilic and on the performance of sludge mixing and homogenization equipment. Fermentation tanks with a capacity of 1000-4000 m3 were designed in Romania so far, the most important being those of Bucharest wastewater treatment plant made of prestressed reinforced concrete having an ovoid shape. The present paper aims at presenting the issues related to designing the fermentation tanks of the water waste plants from urban consumption in the food industry and zoo-technical farms in a new concept in which the main fermentation tank is coupled with the fermenter for the fermentation gas storage. Consequently, hereinafter structural and functional requirements for this kind of structures are presented, together with design principles and methods that must be applied in order to fulfil the performance exigencies related to strength, stability, tightness and durability.
Highlights
The efficiency of the anaerobic fermentation process of sludge depends mainly on the following factors: - content of organic substance in waste water; - sludge temperature: 350-400 in the case of mesophilic fermentation and 500 in case of thermophilic fermentation; - mixing and homogenization degree of sludge needed to provide bacteria in mass nourishment and constant temperature throughout the sludge mass
From the analysis of the factors that determine the efficiency of the fermentation process, the main structural and technological requirements taken into account in the conception and design of the structure result and the following must be considered: - the structural shape must be adapted to the hydraulic homogenizing and mixing spectrum and the equipment considered;
- structures must be thermal insulated to reduce heat losses and gas quantities necessary for sludge preheating. - besides the requirements for ensuring structural strength and stability, the fermentation tanks must ensure tightness to the sludge hydrostatic pressure and fermentation gases pressure. - the requirement of durability of minimum 50 years requires the limitation of the crack opening in terms of tightness and especially due to the corrosive attack of the fermenting gas mixed with the water vapour
Summary
The efficiency of the anaerobic fermentation process of sludge depends mainly on the following factors: - content of organic substance in waste water; - sludge temperature: 350-400 in the case of mesophilic fermentation and 500 in case of thermophilic fermentation; - mixing and homogenization degree of sludge needed to provide bacteria in mass nourishment and constant temperature throughout the sludge mass. Taking into account these factors, it is necessary that the tanks will be equipped with performing mixing and homogenization devices able to prevent gravitational and thermal stratification of sludge. In order to avoid corrosive attack, it is necessary to provide anticorrosive protection inside, taking into account the acid-sulphurous type corrosion that occurs in the upper part of the tank. - effort and deformation states must be determined taking into account both the actions modelled by force systems and the action of temperature variations taking into account sludge temperatures and maximum outdoor temperatures in the summer and winter season. - depending on the storage capacity, it is necessary to consider the need to introduce the prestressing of concrete in order to eliminate the risk of cracking and increase the durability
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