Friends who deal with sewage treatment know that activated sludge is our "sewage treatment expert" - relying on the microorganisms inside to eat the pollutants in the sewage, the water quality can meet the standards. But sometimes this' little expert 'will suddenly' fall off the chain ': either the sludge turns black and smelly, or the settling property is poor, floating everywhere, and even the pollutant removal rate drops sharply. In fact, the poor properties of activated sludge are mostly due to maintenance problems. Today, let's talk in plain language about several common reasons to help you find the right problem.
1. If too much or too little food is given, the microorganisms will first become "hungry" or "stretched" and break down
The microorganisms in activated sludge rely entirely on the organic matter in the sewage to "eat". If the "appetite" is not controlled properly, the sludge will be the first to cause problems.
First, let's talk about "too much food" - that is, the concentration of organic matter in the sewage is too high (professionally called COD load too high). Microorganisms cannot finish eating so much 'rice' at once, and excess organic matter will accumulate in the sludge. This will not only make the sludge sticky and slow down sedimentation, but also breed some miscellaneous bacteria, such as filamentous bacteria (which will be mentioned later). In the end, the sludge will become a 'lump of sludge' that cannot settle and may carry a foul odor.
Let's take another look at 'less rice' - the concentration of organic matter in the sewage is too low. Microorganisms that have nothing to eat will start to "kill each other" or slowly die due to decreased vitality when hungry. At this point, the sludge will become particularly loose, with a lighter color (from normal yellow brown to light yellow). Although the settling property may improve, the amount of sludge will decrease, and the sewage treatment capacity will also decrease. The pollutants cannot be completely treated.
2. Insufficient or too much oxygen, causing microorganisms to "gasp for breath" or "suffer from oxygen poisoning"
When microorganisms treat wastewater, most of them are "aerobic bacteria" that rely on oxygen to work. Improper oxygen supply can immediately cause problems with the sludge's properties.
The most common is "insufficient oxygen" (inadequate aeration). Aerobic bacteria cannot decompose organic matter properly without oxygen, and instead allow anaerobic bacteria to multiply. Anaerobic bacteria decompose organic matter and produce hydrogen sulfide (stinky egg odor), so the sludge will emit a foul odor and its color will change from yellow brown to black. Moreover, lack of oxygen can cause filamentous bacteria to grow wildly, leading to sludge swelling - the sludge volume increases and floats on the water surface like cotton, unable to sink and cannot be discharged.
Some people also think that 'having more oxygen is always right', but in fact, it is not. If the aeration is too strong (too much oxygen), on the one hand, it will crush the activated sludge too much, causing the sludge particles to become smaller and the settling property to deteriorate; On the other hand, excessive oxygen can accelerate the metabolic rate of microorganisms, consume too much of their own substances, reduce the amount of sludge, and waste electricity, which is not worth the loss.
3. Water temperature and pH value "deviate", and microorganisms "live uncomfortably"
Microorganisms, like humans, are very picky about their living environment. Once the water temperature and pH value exceed their adaptation range, they will "strike".
Let's talk about the water temperature first. The most suitable water temperature for most aerobic bacteria is 20-30 ℃. If the water temperature is too low (such as below 10 ℃ in winter), the microbial activity will decrease, the speed of decomposing organic matter will slow down, the sludge treatment capacity will decrease, and even the sludge will become loose; If the water temperature is too high (such as exceeding 40 ℃ in summer), microorganisms will be "heat dead", the amount of sludge will decrease, and their characteristics will directly collapse.
Check the pH value again. Aerobic bacteria prefer neutral or weakly alkaline environments (pH 6.5-8.5). If there are a large amount of acidic substances in the sewage (such as sulfuric acid and hydrochloric acid in industrial wastewater), and the pH drops below 6, the microorganisms will be "acid dizzy" and their activity will decrease; If the pH value exceeds 9 and becomes strongly alkaline, microorganisms will also be damaged or even die. Moreover, abnormal pH values can also affect the flocculation of sludge - sludge particles cannot gather together, sedimentation deteriorates, and the sewage is filled with scattered sludge.
4. Toxic substances' sneak attack ', microorganisms' direct poisoning'
This is the most dangerous situation - if toxic substances such as heavy metals (mercury, chromium, lead), pesticides, organic solvents, or high concentrations of ammonia nitrogen and sulfides are mixed into the sewage, microorganisms will be directly "poisoned", with mild cases of decreased activity and severe cases of mass death.
The sludge after poisoning has obvious characteristics: firstly, the color becomes lighter, from yellow brown to gray white; Secondly, the settling property deteriorates, and the sludge acts like "loose sand" and cannot form large flocs; Finally, the processing effect plummeted, COD、 Ammonia nitrogen and other indicators suddenly increase, even exceeding the standard. Moreover, the impact of toxic substances is often irreversible. If the concentration is too high, the entire activated sludge system may need to be retrained, resulting in significant losses.
5. The age of the sludge is not well controlled, with too much "old sludge" or too little "new sludge"
The age of sludge refers to the time that activated sludge stays in the system, simply put, it is the "age of sludge". If the sludge is too old, there will be too much "old sludge" in the system - the microbial activity in these old sludge has decreased, and it is easy to breed filamentous bacteria, leading to sludge swelling; If the sludge age is too short, the "new sludge" will be discharged before it grows up, resulting in insufficient sludge volume, insufficient processing capacity, and poor flocculation and sedimentation of the new sludge.
Generally speaking, it is appropriate to control the sludge age of domestic sewage treatment within 5-15 days, while industrial wastewater needs to be adjusted according to water quality. If the sludge is not discharged regularly or the amount of sludge discharged is too much/too little, it will cause the sludge age to "deviate" and ultimately lead to a deterioration of the sludge properties.
In fact, activated sludge is like a "small ecosystem". As long as the "food" (organic matter), "air" (oxygen), "shelter" (water temperature, pH), and "age" (sludge age) are controlled well, and the "sneak attack" of toxic substances is monitored, it can continue to work well. If the characteristics of the sludge are found to be incorrect, referring to the above points for investigation, it is likely that the problem can be found - after all, raising "sludge" is like raising a pet, being careful is right!
