The formation process of granular sludge is the result of the combination of microbial cells interacting with each other relatively stably under physicochemical and biological conditions. Granular sludge can also be seen as a special case of biofilm development.
However, anaerobic granular sludge has some disadvantages such as long formation time of sludge granules, difficulty in controlling operational conditions, the need to maintain a reactor under high temperature conditions, ineffective treatment of wastewater with low organic matter content, and especially low efficiency in treating organic matter and nutrients.
As a result, another type of granular sludge has been researched to overcome the disadvantages of anaerobic granular sludge, known as aerobic granular sludge.
1. Composition and Components of Aerobic Granular Sludge
Single sludge granules are formed in the first week of the operational phase. Sludge granules develop from an integrated structure characterized by filamentous bacteria serving as a backbone for the attachment of surrounding granules and flocs.
When the sludge granules have formed a stable and dense structure, acid-forming bacteria, nitrate-forming bacteria, nitrate-reducing bacteria, and denitrifying bacteria exist inside the granules. Granular sludge has the ability to process high levels of phosphorus due to the inner layer of granules being anaerobic bacteria, and there are bacteria capable of accumulating polyphosphate.
Additionally, the composition and quantity of microbial species in sludge granules depend on various factors such as organic matter type, organic loading rate, N/C ratio (cell mass ratio on substrate), and operational model conditions.
Read more: Settling sludge and floating sludge - Treatment methods
2. Formation Process of Aerobic Granular Sludge
The stages are detailed as follows: Adaptation stage: This stage begins from the start of the operational phase until sludge granule formation. Sludge granule seeds mainly consist of filamentous bacteria adhering to each other.
Granule formation stage: This stage starts from the formation of sludge granule seeds until the sludge granule is formed. Sludge granules include various types of bacteria coexisting.
Maturity stage: In this stage, sludge granules will grow rapidly and strongly in size and density. After the growth period, sludge granules will stabilize and balance. To maintain granule size in the range of 2 – 5 mm, good control of operational conditions is required.
Formation and Development Process of Sludge Granules
Read more: Microbial groups in wastewater treatment
3. Advantages and Disadvantages of Aerobic Granular Sludge
Advantages
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High microbial density in the granular sludge structure.
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High treatment efficiency once the granules are formed.
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Able to withstand high organic loading rates.
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Large sludge granule size allows for quick settling and less washout.
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Microorganisms have the ability to self-immobilize without the presence of carriers or media.
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Granular sludge has a three-dimensional spatial structure, making it easy to contact with wastewater.
Disadvantages
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Difficult to control the state and size of sludge granules.
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Sludge granules are unstable and easily break apart with changes in environmental conditions.
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Slow adaptation and granule formation processes.
Aerobic granular sludge is a new type of sludge researched to overcome the disadvantages of traditional activated sludge processes while inheriting the superior characteristics of anaerobic granular sludge. In addition to the above advantages, aerobic granular sludge also has good settling properties, which improves solid-liquid separation in wastewater treatment processes.
Read more: Leachate Treatment Technologies
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