When it comes to the issue of bubbles appearing in the measuring cylinder during sedimentation ratio in aerobic tanks, many friends who work in sewage treatment have probably encountered it. Originally waiting quietly for the separation of mud and water, small bubbles suddenly popped up from the measuring cylinder, sometimes in a series, making people wonder: what's going on? Don't panic, there are quite a few tricks here, let's talk about them one by one.
First of all, let's talk about the most common situation - the microorganisms in the sludge are still "panting". Think about it, the sludge in the aerobic tank is soaked in oxygen rich water every day, and the aerobic bacteria inside have long been accustomed to the days of inhaling oxygen and releasing carbon dioxide. When we were taking samples, a bucket of mud and water entered the measuring cylinder. Although no oxygen was added to it, there was still some oxygen left in the sludge. In addition, bacteria couldn't stop the car for a while and were still busy decomposing organic matter, resulting in a significant amount of carbon dioxide to be discharged. These carbon dioxide have nowhere to run, so they can only gather into small bubbles in the measuring cylinder and slowly float upwards. Especially in the first few minutes of sampling, the bubbles may be quite active, but after a while, when the oxygen is exhausted and the bacterial "work efficiency" decreases, the bubbles will be much less.
Furthermore, there may be reasons related to the operational techniques. For example, when taking samples, if the action is too forceful and the water in the measuring cylinder rushes in, it is easy to roll in the air. These small bubbles that are rolled in are hidden in the gaps of the sludge, which may not be visible at first. As the sludge slowly settles, they will emerge one by one, looking like bubbles produced by the sludge itself. There is also a situation where the measuring cylinder is not cleaned thoroughly and the inner wall is stained with some residual chemicals or oil from the last experiment. These things may react slightly with the mud and water, or make it easier for air to adhere to it and form bubbles. So, when taking samples, be steady and brush the measuring cylinder clean, maybe it can save a lot of trouble.
Then we have to consider the state of the sludge itself. If the sludge in the aerobic tank is a bit "abnormal", such as slight acidification, or if the sludge is too old and the microbial species inside have changed, it may produce some other gases. For example, if the dissolved oxygen in the pool suddenly runs out, an anaerobic environment may occur locally, and some facultative bacteria may take the opportunity to "disrupt" and produce gases such as methane or hydrogen sulfide when decomposing organic matter. These gases may not have a chance to emerge from the pool and will slowly release into bubbles as they follow the sludge into the measuring cylinder. In this case, the bubbles may sometimes have a strange odor, such as hydrogen sulfide, which sounds like rotten eggs. At this time, it is necessary to pay attention to the operating parameters of the aerobic tank to see if the dissolved oxygen is not controlled properly or if the inlet water load is too high.
Another easily overlooked point is the change in water temperature. The water in an aerobic tank usually has a certain temperature. After sampling, it is tested at room temperature. If the room temperature is lower than the water temperature in the tank, the solubility of dissolved gases in the water will increase, and there may be no bubbles temporarily; But if the room temperature is higher or the surrounding environment temperature slowly rises, the amount of gas that can dissolve in the water will decrease, and the excess gas will run out in the form of bubbles. This type of bubble generated by temperature changes is generally more uniform and may last for a period of time, and will not concentrate in a certain layer of sludge like bubbles produced by microorganisms.
In addition, the quality of the incoming water may also be the "driving force behind it". If the incoming water contains some easily fermentable organic matter, such as starch and carbohydrates, these substances may not be completely decomposed in the aerobic tank. After entering the measuring cylinder with the sludge, at an appropriate temperature, they may be further decomposed by microorganisms in the sludge, producing additional gases. There is also a situation where the incoming water contains some surfactants, such as dishwashing detergent and laundry detergent that may mix into domestic sewage. These substances can reduce the surface tension of the water, making it easier for bubbles to form and more difficult to burst. A small, persistent layer of bubbles may float in the measuring cylinder.
Finally, let me mention a rather special situation where sludge undergoes denitrification. Although aerobic tanks are mainly used for aerobic environments, if there are nitrates in the tank and the sludge settles in an anaerobic state in the measuring cylinder, some denitrifying bacteria will use nitrates as electron acceptors to decompose organic matter, which will produce nitrogen gas. Nitrogen is insoluble in water and naturally forms bubbles. This type of bubble may be more pronounced in the later stages of sedimentation, as the denitrification process requires a certain amount of time. At this point, we need to think about whether the nitrate concentration in the aerobic tank is too high, whether it is carried over by the returning sludge, or whether there are too many nitrogen-containing compounds in the influent.
Overall, the bubbles in the graduated cylinder may seem simple, but they may actually involve several aspects such as the operation status of the aerobic tank, microbial activity, and operational details. In this situation, don't rush to draw conclusions. Observe the size, quantity, and duration of the bubbles, as well as whether there is any odor. Combined with the parameters of dissolved oxygen, water temperature, and sludge age in the aerobic tank, make a comprehensive judgment and gradually figure out what went wrong. After all, sewage treatment is about finding patterns in these details and gradually adjusting the water, right?
