Activated Sludge Process

The activated sludge process is a wastewater treatment method in which the carbonaceous organic matter of wastewater provides an energy source for the production of new cells for a mixed population of microorganisms in an aquatic aerobic environment. The microbes convert carbon into cell tissue and oxidize the dyes finally to carbon dioxide and water. In addition, a limited number of microorganisms may exist in activated sludge that obtains energy by oxidizing ammonia nitrogen to nitrate nitrogen in the process known as nitrification.

Aerobic granulation is a novel biotechnique for coloured wastewater treatment developed in SBR. Granulation is a process of microbial cell self-immobilization, resulting in a cell-structured shape characterized by dense biomass. Similar to anaerobic granules, aerobic granules have a number of advantages over conventional bioflocs, such as a round and compact structure, good settling ability, high biomass retention and ability to withstand high organic loading rate (Liu et al. 2006). Aerobic granulation by SBR has been demonstrated for the treatment of a wide variety of wastewaters, including industrial wastewater, nutrient-rich and toxic wastewaters (Fig. 12.1).

Bacteria constitute the majority of microorganisms present in the activated sludge. Bacteria that require organic compounds for their supply of carbon and energy (heterotrophic bacteria) predominate, but bacteria that use inorganic compounds for cell growth (autotrophic bacteria) also occur in proportion to concentrations of carbon and nitrogen. Both aerobic and anaerobic bacteria may exist in the activated sludge, but the preponderance of species are facultative, able to live either in the presence of or lack of dissolved oxygen.

Fungi, rotifers, and protozoans are also residents of activated sludge. The latter microorganisms are represented largely by ciliated species, but flagellated protozoan and amoebae may also be present. Protozoans serve as indicators of the activated sludge condition, and ciliated species are instrumental in removing Escherichia coli from sewage. Additionally, viruses of human origin may be found in the raw sewage influent, but a large percentage appears to be removed by the activated-sludge process.

The success of the activated-sludge process is dependent upon establishing a mixed community of microorganisms that will remove and consume organic waste material, that will aggregate and adhere in a process known as bio-flocculation, and that will settle in such a manner so as to produce a concentrated sludge (return activated sludge, or RAS) for recycling. Any of several types of activated sludge solids separations problems indicates an imbalance in the biological component of this process. In the ideal ''healthy'' system, filamentous organisms grow within a floc (a large aggregate of adherent, or floc-forming, microorganisms, such as bacteria) and give it strength, with few filaments protruding out into the surrounding bulk solution. In such a system, there is no interference with the compaction and settling rates of the activated sludge prior to its recycling.

Nevertheless, many researches have demonstrated partial or complete biodegradation of dyes by pure and mixed cultures of bacteria, fungi and algal species.

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