Difference between:
Aerobic & anaerobic digestion in sludge process
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ANAEROBIC
Anaerobic digestion is a biological wastewater treatment process that is used for treatment and reduction of organic wastes such as organic sludge or concentrated organic industrial waste which contain solids. As the quantity of organic solids are decreased in the sludge after this process, the treated sludge is easier to dispose due to less volume. Further, microbial action during the process also helps in biogas generation (methane) which can be used as a clean energy source.
At micro level, the anaerobic sludge digestion process is carried out by microbes under anaerobic conditions (i.e., oxygen is not used in the degradation process reactions). This makes it different from aerobic digestion wherein oxygen is used. Anaerobic digestion can also be more economical than aerobic treatment / aerobic septic system as aeration process is not required for the anaerobic digestion.
The process of anaerobic digestion takes place in three stages. The first stage involves liquefying of solid material in the sludge. This process is called hydrolysis. The second stage is rapid and involves digestion of the soluble solids that resulted from the previous stage. This process is carried out at molecular level by acid (primarily acetic, propionic and butyric acid etc.) producing anaerobic bacteria. The microbes involved in this stage are facultative anerobes, are heterotrophic and found in soil. They belong to diverse genera such as Escherichia, Flavobacterium, Alcaligenes, Aerobacter, Psuedomonas etc. and can function in a large pH range.
At the final stage which is called the gasification stage, the organic acid produced in the previous stage is used by certain microbes as substrate and methane and carbon dioxide gases are produced as a result. This stage is slower and is also called methanogenesis as it leads to production of methane. Microbes involved in this process are also anaerobic and belong to the Methanococcus, Methanobacterium, and Methanoscarcina genera.
The anaerobic digestion optimally occurs between 30 to 38 degree Celsius and can be done in single stage digestion system or two-stage digestion system. In the prior, all the reaction stages occur in the same anaerobic digestor. So care must be taken so as not to overfeed or underfeed the digestor with new sludge.
AEROBIC
The sewage or waste water/effluent from industrial operations contains organic impurities. Thus sewage or industrial waste water cannot be directly discharged to the environment. The organic impurities present will consume dissolved oxygen from the receiving bodies and make the receiving body unsafe for drinking use and survival of aquatic life. For safe discharge of sewage/industrial waste water to the environment or receiving body, it is very much essential to treat the sewage/industrial waste water.
For treating sewage/industrial effluent for their organic impurities generally aerobic biological treatment is used. The major process used worldwide is activated sludge process. It is an aerobic biological process that utilizes microorganisms such as bacteria, and protozoa for the decomposition of organic matter. This process has been named so since it involves the production of an activated mass of microorganisms which effectively stabilize the organic content of wastes in presence of oxygen and convert them to carbon dioxide and water which are safe to be disposed off in the environment. This is effectively achieved by a series of metabolic reactions carried out by the microorganisms.