8 May 2024

Bulking of activated sludge: a challenge to purification efficiency

Bulking is a significant challenge to the efficiency and reliability of activated sludge biological purification systems. This swelling of the biomass inside the biological reactor, in fact, slows down the sedimentation speed of the sludge, which rising to the surface compromises the compaction of the secondary sedimentator, and reduces the concentration of activated sludge in the biological treatment stream.

Beyond certain limits, bulking can cause sludge to leak from the clarifier, and thus compromise the quality of the effluent and the entire pollutant removal process. Last but not least, it reduces the age of the sludge and its dehydration, making the management of purification plants more difficult

Types and causes of bulking

The causes of bulking can be different, but two types of bulking are commonly distinguished, each with their own:

• viscous bulking. Generally caused by an excess of nutrients or a high concentration of rapidly biodegradable substrates, it is resolved by balancing the nutrients, or by inserting bacteria with characteristics aimed at restoring the stability of the purification process, in addition to the correct sedimentation of the sludge.
• Stringy bulking. Due to the excessive proliferation of filamentous bacteria that create bridges between the sludge flakes, or compromise their sedimentation, it is linked for example to an excessive percentage of sugars in the wastewater. For this reason, to resolve filamentous bulking it is necessary to act on the recycling of sludge by adopting oxidizing products, or once again, by increasing the concentration of specific bacteria that favor correct settling of the sludge.

The causes of bulking, therefore, can be traced back to environmental factors, composition of the wastewater, hydraulic and organic load, as well as the management of the plant itself. The main ones include:

• excess nutrients. As we have seen, an imbalance of nutrients can cause the proliferation of some bacteria to the detriment of others: high levels of nitrogen and phosphorus, for example, can favor the excessive growth of filamentous bacteria.
• Fluctuations in the organic load, the variability of which influences microbial growth and the development of any filaments.
• Poor ventilation, which favors the growth of filamentous bacteria, once again to the detriment of aerobic ones.
• Inadequate shape or design of the biological reactor can ultimately favor the formation of dead or sedimentation zones, creating favorable conditions for bulking.

Solutions for the strategic management of bulking

To maintain the optimal performance of activated sludge purification plants, strategies aimed at restoring ideal nutritional and aerobic conditions are needed:

• Implement monitoring systems that detect any changes in the characteristics of the wastewater and biomass in real time.
• Optimize operating parameters, such as temperature, pH, hydraulic load and organic load, and the aeration capacity of the biological reactor, to promote aerobic conditions and reduce the growth of filamentous bacteria.
• Control and remove nutrients, to maintain balanced nitrogen and phosphorus levels through nitrification and denitrification processes.
• Carry out regular cleaning and preventative maintenance activities to ensure the correct functioning of the ventilation equipment and reduce the risk of biomass accumulation. In maintenance, chemical treatments such as flocculant polymers can also be used to encourage the sedimentation of solid sludge.

For your biological purification systems, including activated sludge, turn to Di Camillo’s experience and consultancy: for over fifty years, we have been creating and reselling systems and plants for the treatment and purification of waste water.