Family, today let's talk about a long-standing problem in the sewage treatment industry - activated sludge does not sink! When this situation occurs, the staff of the sewage treatment plant will really feel overwhelmed, like cooking and burning a pot, and they must quickly find out the cause and solve it. Don't worry, let's break and analyze together. Where exactly went wrong.
1、 Sludge swelling is one of the main culprits
The most common reason is sludge swelling. Simply put, it means that microorganisms in activated sludge become irritable, undergo changes in morphology and structure, and become less prone to settling. Sludge expansion can be divided into filamentous bacterial expansion and non filamentous bacterial expansion.
The expansion of filamentous bacteria is due to the overgrowth of filamentous bacteria in activated sludge. Under normal circumstances, filamentous bacteria and other microorganisms coexist harmoniously, and everyone works together to treat wastewater. But once the environmental conditions are not suitable, filamentous bacteria begin to grow wildly, entwining together like a tangled mess, enveloping other microorganisms and causing the volume and density of the sludge to increase, naturally making it unable to sink. For example, it's like a pile of tangled vines suddenly emerging from the originally orderly queue, entangling everyone and causing the queue to become disorganized and unable to move.
There are many reasons that can cause filamentous bacteria to expand. Firstly, there is the issue of water quality. If the carbon to nitrogen ratio in wastewater is imbalanced, such as having too many carbon sources and insufficient nitrogen and phosphorus sources, filamentous bacteria will grow wildly due to overeating. Just like eating big fish and meat, imbalanced nutrition can also lead to physical problems. Also, insufficient dissolved oxygen can give filamentous bacteria the upper hand. Because filamentous bacteria are more tolerant to low oxygen environments than other microorganisms, when there is not enough oxygen in the water, other microorganisms "don't have the energy to work", but filamentous bacteria can take advantage of the opportunity to reproduce in large numbers. In addition, inappropriate water temperature and pH value may also cause filamentous bacteria to expand, such as sudden increase in water temperature or significant fluctuations in pH value, which may disrupt the balance between microorganisms.
Non filamentous bacterial expansion is usually caused by the presence of large amounts of surface active substances or oils in wastewater. These things will form a "protective film" on the surface of sludge particles, preventing them from aggregating and settling together. Imagine that the sludge particles are like small beads, with surface active substances and oils like a layer of oil applied to the beads. The beads are slippery and cannot sink tightly together.
2、 Aeration issues, microbial 'hypoxia or oxygen poisoning'
If there is a problem with the aeration process, the activated sludge will also cause emotional distress. Excessive aeration, which means that too much air is introduced into the sewage, will keep the microorganisms in the activated sludge in a highly active state, accelerate metabolism, and produce a large number of small bubbles that adhere to the sludge particles. These small bubbles are like adding "little wings" to the sludge, making it light and difficult to settle. Moreover, excessive aeration may also lead to the disintegration of sludge flocs, disrupting the originally tight sludge structure and turning it into scattered small particles, further affecting sedimentation performance.
Conversely, insufficient aeration is equally troublesome. As mentioned earlier, microorganisms need oxygen to decompose pollutants in wastewater. If the oxygen supply is insufficient, they cannot function properly, and the treatment effect will deteriorate, leading to anaerobic reactions. Anaerobic reactions produce a large amount of gases, such as hydrogen sulfide and methane, which cause sludge to float and not sink. Just like how people may have difficulty breathing and being unable to do work in an oxygen deficient environment, activated sludge will also "strike" and not settle properly due to oxygen deficiency.
3、 Nutritional imbalance, microbial malnutrition or overnutrition
The growth and metabolism of microorganisms in activated sludge require suitable nutrients, mainly carbon, nitrogen, and phosphorus. It is generally believed that their ratio is ideal at BOD5: N: P=100:5:1. If this ratio is imbalanced, the growth of microorganisms will be affected.
When nutrients such as nitrogen and phosphorus are insufficient in sewage, microorganisms are unable to synthesize sufficient cellular material, resulting in slow growth, reduced activity, and poorer sedimentation performance of sludge. This is like farmers farming, where there is not enough fertilizer in the soil, the crops grow thin and weak, and the yield is not high. On the contrary, if there are too many nutrients in the sewage, microorganisms will rapidly multiply and the amount of sludge will increase. However, the quality of these sludge is poor, the structure is loose, and it is not easy to settle. It's like eating too much and gaining weight, but the body becomes bloated and has difficulty moving around.
4、 Toxic substance invasion, microbial 'poisoning strike'
If toxic and harmful substances are mixed into the sewage, it will be a "disaster" for the microorganisms in the activated sludge. Heavy metal ions (such as chromium, cadmium, mercury), chemical agents (such as phenols, aldehydes), insecticides, etc., can all cause damage to the cellular structure and physiological functions of microorganisms. Once microorganisms are 'poisoned', they lose their activity and cannot undergo normal metabolism, coagulation, and sedimentation.
This is like a good team, when suddenly a few "disruptors" come in and make everyone lose interest in work, the efficiency of the team naturally drops sharply. Moreover, these toxic substances may alter the properties of sludge, making it viscous or loose, further affecting its settling performance.
5、 Excessive sludge age leads to microbial 'aging'
Sludge age refers to the average residence time of activated sludge in a sewage treatment system. If the sludge age is too long, it means that the microorganisms in the activated sludge are too old and have entered the aging period. These aging microorganisms have reduced activity, decreased ability to decompose pollutants, and their cellular structure has become loose and easily broken. The broken sludge particles are smaller and more difficult to settle.
Just like a company full of elderly and energetic employees, work efficiency is definitely not high and various problems may arise. Moreover, excessive sludge age may lead to sludge decay, producing unpleasant odors, and releasing gases that cause the sludge to float up.
6、 Sudden changes in the quality and quantity of incoming water, causing microorganisms to be unable to adapt to the environment
If there is a sudden change in the inlet water quality and quantity of the sewage treatment plant, the microorganisms in the activated sludge will also be "incompatible with the environment". For example, if a large amount of industrial wastewater is suddenly mixed into domestic sewage, the composition and concentration of the water quality will change, making it difficult for microorganisms to adapt, which will affect their normal metabolism and coagulation and sedimentation ability.
Alternatively, if the inflow suddenly increases significantly and exceeds the capacity of the sewage treatment system, the residence time of the sewage in the treatment tank will become shorter, and microorganisms will not have enough time to fully decompose pollutants. The quality of the sludge will also decrease, leading to difficulties in settling. This is like a small restaurant that usually only serves a few dozen people, but suddenly hundreds of people flood in. The chefs and servers are simply too busy, and the quality of the food and service efficiency will be greatly reduced.
When encountering the situation where activated sludge does not sink, we need to carefully investigate from multiple aspects such as water quality, aeration, nutrition, toxic substances, sludge age, and inflow situation, find the root cause of the problem, and then prescribe the right medicine. Only in this way can activated sludge restore its normal settling performance, make the sewage treatment system "healthy" again, continue to treat sewage well for us, and protect our environment!
