Aerobic wastewater treatment plants occasionally suffer from sludge bulking consisting of a low density that causes sedimentation problems in both continuous systems and SBR systems.
In many cases this bulking is due to the accelerated growth of filamentous bacteria that, when they form long threads of bacteria, do not allow the floccules to have good sedimentation as they do not compact well.
This behavior of bacteria occurs mostly in the change from winter to spring or summer to autumn which is when the temperature changes and the composition of bacteria in the mixed liquor has to adapt to the new water conditions. Filamentous bacteria are often more adaptable than the bacteria that form floccules and it is what causes the problem.
Evolutionarily filamentous bacteria are best prepared to capture nutrients as they expose a larger surface area of their cell membrane to the environment, allowing them to more easily capture nutrients and oxygen that is dissolved in water. When there is little oxygen or few nutrients in the water, the growth of filaments is favored as they are more adapted than the floccules to capture these nutrients and grow faster.
Thus, when there is little nutrients or little oxygen it is normal for the population of filamentous bacteria to grow at a rate that is not desired within the treatment plant.
To avoid such problems, it is a good idea to use the anoxic selectors that take advantage of the characteristics of each type of bacteria to discourage the growth of filamentous bacteria and promote the growth of those that form good floccules.
In an anoxic selector raw water and the sludge recirculation from the clarifier are received and mixed in an environment with little oxygen so that the filamentous bacteria cannot feed because although there is enough food there is not oxygen to degrade the organic matter but the bacteria that form floccules can store the food (what the filaments do not do) so that under the conditions of the selector they store the food and degrade it when pass to the aerobic reactor which favors the growth of floccule forming bacteria and discourages the growth of filamentous bacteria.
Fernando Manzaneque
fmanzaneque@texasbioengineering.com