Anaerobic Digestion
INTRODUCTION
In the anaerobic treatment the organic matter is decomposed into biogas, methane (CH4) and carbon dioxide (CO2), process in which a different oxygen molecule acts as a terminal electron acceptor. Methane production occurs in different natural environments, such as swamps, lakes, rivers and sea sediments, as well as in the digestive organs of ruminant animals, where the redox potential is around -300mV. It is estimated that anaerobic digestion with methane formation is responsible for the complete mineralization of 5 to 10% of all the organic matter available on the earth.
Anaerobic digestion represents an accurately balanced ecological system, where different populations of microorganisms present specialized functions, and the breakdown of organic compounds is usually considered a two stage process. In the first stage, a group of facultative and anaerobic bacteria converts, by hydrolysis and
fermentation, the complex organic compounds (proteins, carbohydrates and fats) into simpler organic compounds, mainly volatile fatty acids (VFA), as well as carbon dioxide and hydrogen gases. In the second stage, the organic acids and hydrogen are converted into methane and carbon dioxide. This conversion is performed by a special group of microorganisms, namely methanogens, which are strictly anaerobic. These microorganisms have two main functions, produce methane which enables the removal of organic carbon and they keep the hydrogen (H2) partial pressure low enough to allow conditions in the medium for fermenting and acid producing bacteria to produce soluble compounds such as acetic acid.
Anaerobic Digestion
INTRODUCTION
In the anaerobic treatment the organic matter is decomposed into biogas, methane (CH4) and carbon dioxide (CO2), process in which a different oxygen molecule acts as a terminal electron acceptor. Methane production occurs in different natural environments, such as swamps, lakes, rivers and sea sediments, as well as in the digestive organs of ruminant animals, where the redox potential is around -300mV. It is estimated that anaerobic digestion with methane formation is responsible for the complete mineralization of 5 to 10% of all the organic matter available on the earth.
Anaerobic digestion represents an accurately balanced ecological system, where different populations of microorganisms present specialized functions, and the breakdown of organic compounds is usually considered a two stage process. In the first stage, a group of facultative and anaerobic bacteria converts, by hydrolysis and
fermentation, the complex organic compounds (proteins, carbohydrates and fats) into simpler organic compounds, mainly volatile fatty acids (VFA), as well as carbon dioxide and hydrogen gases. In the second stage, the organic acids and hydrogen are converted into methane and carbon dioxide. This conversion is performed by a special group of microorganisms, namely methanogens, which are strictly anaerobic. These microorganisms have two main functions, produce methane which enables the removal of organic carbon and they keep the hydrogen (H2) partial pressure low enough to allow conditions in the medium for fermenting and acid producing bacteria to produce soluble compounds such as acetic acid.
Anaerobic Digestion of Primary Sewage Effluent
SUMMARY
A hybrid system comprised an up-flow packed bed anaerobic reactor and a down-flow trickling-filter reactor connected in series was shown to effectively treat primary clarifier effluent. When a clarifier and sand filter were added to the system, the effluent water quality achieved values of BOD5 and TSS that were below the EPA’s water discharge limits of 30 mg/l and equivalent to highly efficient activated sludge systems. Best results were achieved at a hydraulic retention time of seven hours and with interna recycle applied to both the anaerobic and aerobic reactors. A scale-up evaluation of the system to treat three million gallons per day indicated total land use of approximately 0.6 acre, which is on the same scale currently used at the host wastewater treatment facility to treat primary clarifier effluent using activated sludge technology. An energy balanced showed that the tested system would utilize 48% of the energy currently used to operate the activated sludge system.
Anaerobic Digestion of Primary Sewage Effluent
SUMMARY
A hybrid system comprised an up-flow packed bed anaerobic reactor and a down-flow trickling-filter reactor connected in series was shown to effectively treat primary clarifier effluent. When a clarifier and sand filter were added to the system, the effluent water quality achieved values of BOD5 and TSS that were below the EPA’s water discharge limits of 30 mg/l and equivalent to highly efficient activated sludge systems. Best results were achieved at a hydraulic retention time of seven hours and with interna recycle applied to both the anaerobic and aerobic reactors. A scale-up evaluation of the system to treat three million gallons per day indicated total land use of approximately 0.6 acre, which is on the same scale currently used at the host wastewater treatment facility to treat primary clarifier effluent using activated sludge technology. An energy balanced showed that the tested system would utilize 48% of the energy currently used to operate the activated sludge system.
