One of the biggest headaches for us in sewage treatment is definitely sludge expansion. When this sludge expands, the entire treatment system becomes chaotic. Today we will talk about the two major "troubles" in sludge expansion - non filamentous bacterial expansion and filamentous bacterial expansion. Let's talk about the differences between them, how to distinguish them, and how to deal with them.
1、 Appearance and settlement: the most intuitive clues for judgment
Let's start with the most easily observable aspects, which are the appearance and settling properties of the sludge. When you place the expanded sludge of filamentous bacteria in a measuring cylinder to settle, you can see obvious filamentous substances, like a messy pile of hair strands inside. And these filamentous bacteria will intertwine with each other, making the structure of the sludge very loose. The settling speed is slow, and the supernatant is also very turbid, with many small particles suspended inside.
The sludge expanded by non filamentous bacteria looks relatively uniform and does not have that obvious filamentous structure. Its poor settling ability is due to a change in the specific gravity of the sludge, not due to the disturbance of filamentous bacteria. When settling, the sludge may appear in a clumped state, but it will quickly disperse, and the suspended solids in the supernatant will be less compared to the expansion of filamentous bacteria, but the water quality is still not clear.
For example, the swollen sludge of filamentous bacteria is like a bowl of noodles that have been left for a long time. When the noodles (filamentous bacteria) are tangled together, the soup (supernatant) also becomes turbid; The sludge bulked by non filamentous bacteria is more like a bowl of boiled Congee. Although there is no obvious "foreign matter", the overall state is also abnormal. Water and rice (sludge) are mixed together and cannot distinguish each other.
2、 The world under the microscope: seeing the essential differences clearly
Just looking at the appearance is not enough. To make accurate judgments, one must also rely on the "miracle tool" of a microscope. When the sludge sample is placed under a microscope, the expansion of filamentous bacteria becomes clear at a glance, and a large number of slender filamentous bacteria can be seen extending from the sludge flocs. There are still many types of filamentous bacteria, such as jersey bacteria, sulfur producing bacteria, and so on. Different filamentous fungi also have different morphologies, some are straight and long, some are slightly curved, and there are also some branches.
On the other hand, when non filamentous bacteria expand, a large number of filamentous bacteria cannot be seen under a microscope, but rather some other abnormal conditions can be observed. For example, the structure of microbial flocs in sludge is disrupted, and the originally tightly packed microbial flocs become loose, with uneven distribution of microorganisms inside. It is also possible to see changes in the number and species of some protozoa, such as a decrease in the number of bell worms, and an increase in small flagellates and amoebas that have adapted to unfavorable environments. This is like in an ecosystem, where the original "dominant species" (such as normal fungal colonies and nematodes) lose their dominance, while some "weed type" species (such as small flagellates and amoebas) begin to proliferate in large numbers.
3、 Cause: Different 'troublemakers'
The causes of these two types of expansion are also very different. The expansion of filamentous bacteria is mainly due to the environmental conditions in the sewage treatment system, which provide more favorable growth conditions for filamentous bacteria than other microorganisms. For example, if the dissolved oxygen in water is too low, filamentous bacteria are better adapted to low dissolved oxygen environments than ordinary bacterial colonies. They can reach into the water to obtain oxygen, while bacterial colonies do not have this ability and gradually cannot compete with filamentous bacteria.
There is also a situation where the proportion of nutrients in water is imbalanced, resulting in an inappropriate ratio of carbon, nitrogen, and phosphorus. Generally speaking, the BOD5: N: P ratio in sewage should be maintained at around 100:5:1, which is more appropriate. If the nitrogen and phosphorus content is too low, filamentous bacteria will take advantage of the opportunity to reproduce in large numbers. In addition, water temperature also has an impact. Under medium temperature conditions (25-35 ℃), certain filamentous bacteria grow particularly fast.
The reason for the expansion of non filamentous bacteria is very different from that of filamentous bacteria. The most common is sudden changes in water quality, such as the presence of large amounts of toxic and harmful substances in the inflow, such as heavy metals, phenols, sulfides, and so on. These substances can inhibit the normal metabolism of microorganisms, disrupt the structure of microbial flocs, and cause sludge to swell. Another issue is the drastic fluctuation of pH value. If the pH value of the incoming water is too low or too high, exceeding the suitable range for microorganisms (usually 6.5-8.5), it can also cause non filamentous bacteria to expand. In addition, when the sludge load is too high, that is, there is too much organic matter in the influent, the microorganisms suddenly "eat up" and cannot metabolize normally, which can also cause sludge swelling.
4、 Response strategy: Targeted treatment is crucial
Since we know their differences and causes, how should we handle them? There are many ways to prevent filamentous bacteria from expanding. Firstly, you can try adjusting the dissolved oxygen by increasing the aeration rate to keep the dissolved oxygen in the water at a suitable level, generally controlled at 2-4mg/L. This can inhibit the growth of filamentous bacteria and give the bacteria in the bacterial colony a chance to regain their advantage.
If it is a problem of imbalanced nutrient ratios, then an appropriate amount of nitrogen and phosphorus nutrients need to be added to the sewage to adjust the carbon nitrogen phosphorus ratio to an appropriate range. However, when adding, you need to pay attention to the amount used, too much or too little won't work. Some special techniques can also be used, such as setting selectors in the aeration tank. Selectors can create an environment conducive to the growth of bacterial colonies, allowing them to gain an advantage in competition with filamentous bacteria. In the selector, by controlling the hydraulic retention time and dissolved oxygen concentration, the bacterial colonies can quickly adsorb and utilize organic matter in wastewater, while filamentous bacteria are not as flexible, thus achieving the goal of inhibiting filamentous bacterial expansion.
For non filamentous bacterial expansion, the treatment method also needs to be targeted at its causes. If it is due to excessive toxic and harmful substances in the water quality, it is necessary to add pre-treatment facilities at the front end, such as setting up neutralization tanks and sedimentation tanks, to remove or reduce the toxic and harmful substances to a concentration that microorganisms can tolerate. If there is a problem of pH fluctuation, a pH adjustment device needs to be installed to monitor the pH value of the incoming water in real time, and adjust the pH value to the appropriate range by adding acid or alkali in a timely manner. If the sludge load is too high, it is necessary to reduce the concentration of organic matter in the influent. This can be achieved by adjusting the influent flow rate or increasing the sludge return ratio, allowing microorganisms enough time and ability to decompose organic matter.
Although both non filamentous and filamentous bacterial expansion can cause sludge to swell and pose challenges to wastewater treatment, they have significant differences in appearance, microscopic characteristics, causes, and treatment methods. As long as we master this knowledge and carefully observe and analyze it in practical work, we can accurately determine which type of expansion it is, and then take the correct treatment measures to restore the normal operation of the sewage treatment system.
