R1: "Foam" must be translated as foam. The original text: When it comes to dry sewage treatment, who hasn’t battled with sludge bulking? The secondary sedimentation tank, which was functioning perfectly, suddenly becomes difficult to separate sludge from water. The supernatant turns turbid, sludge floats like a pile of rotten mud on the surface, and the reflux pump draws nothing but foam, causing the effluent SS to skyrocket—no need to ask, it’s almost certainly sludge bulking at work. Translation:.
Sludge bulking sounds terrifying, but it's actually just a disruption in the "bacterial circle" within the biochemical tank. Simply put, it's a territorial dispute between filamentous bacteria and flocculent bacteria. Filamentous bacteria, leveraging their slender, space-efficient growth and ability to survive in low-oxygen, low-nutrient conditions, directly squeeze flocculent bacteria to the point of collapse. With fewer flocculent bacteria, sludge flocs disperse, leading to a sharp decline in settling performance, which naturally causes the secondary sedimentation tank to "collapse.".
To address sludge bulking, don't resort to haphazard solutions. First, identify the type of bulking, then apply targeted measures to ensure effectiveness.
Step 1: First determine whether it is filamentous bulking or non-filamentous bulking?
The two are brothers, but their temperaments are completely different, and their approaches to handling things are worlds apart. Getting them mixed up would only make matters worse.
Filamentous bulking is the most common type, accounting for over 90% of cases. How to diagnose it? Under a microscope, the sludge flocs are filled with slender filamentous bacteria, tangled like a mess of hemp. Then check the operational parameters—most likely, it's due to low DO, imbalanced influent load, or deficiencies in essential nutrients.
Filamentous bulking is much rarer, typically caused by excessively high influent load or sudden spikes in water temperature. Under the microscope, few filamentous bacteria are observed, while the sludge flocs appear particularly loose, resembling thin rice soup. The settling ratio (SV30) remains persistently high, and the sludge volume index (SVI) can surge above 300.
Step 2: Tailor the remedy to the ailment—different expansions, different strategies
1. Tackling Filamentous Sludge Bulking: Cut off its path, elevate flocculent bacteria to dominance
Filamentous bacteria can be so嚣张, but it's nothing more than taking advantage of the environment. Let's start by disrupting its "comfort zone.".
1. Sudden DO increase, suffocating filamentous bacteria
Filamentous bacteria thrive in low-oxygen environments. When the DO in the tank drops below 1mg/L, they start multiplying rapidly. We simply run the blowers at full capacity to raise the DO in the biochemical tank to 2~3mg/L, ensuring uniform aeration with no dead zones. If the aerators become clogged, clean them immediately; if the blower airflow is insufficient, temporarily adding more blowers is worth it—once the DO levels rise, the reproduction rate of filamentous bacteria drops instantly.
2. Adjust the load to prevent microbial flocculation from going hungry
Either the influent load is too low, leaving the flocculants starved and allowing filamentous bacteria to compete for food; or the load is too high, causing the biochemical tank to "indigest" and also prone to filamentous bacterial growth.
If the load is low, appropriately increase the influent COD concentration or reduce sludge discharge to raise the sludge concentration; if the load is high, limit the flow and divert part of the wastewater to the equalization tank to "reduce the burden" on the biochemical tank. The core is to stabilize the influent load within the range the flocculent bacteria can tolerate, typically 0.2~0.5 kgBOD₅/(kgMLSS·d).
3. Supplement nutrients to prevent microbial flocculation from lacking nutrition
Many industrial wastewater samples contain only carbon but are deficient in nitrogen and phosphorus, preventing floc bacteria from synthesizing their own substances. Filamentous bacteria then take advantage of this vulnerability. By supplementing with a C:N:P ratio of 100:5:1—adding urea or ammonium sulfate for nitrogen deficiency and potassium dihydrogen phosphate for phosphorus deficiency—you'll notice the floc bacteria gradually strengthening.
4. Emergency Measure: Add chemical agents to rapidly suppress expansion
If the effluent exceeds the standard, there's no time for slow adjustment—chemical agents must be used as an emergency measure. Adding an appropriate amount of sodium hypochlorite or hydrogen peroxide can directly inhibit the growth of filamentous bacteria; or adding polyaluminum chloride (PAC) and polyacrylamide (PAM) can aid sludge flocculation and sedimentation. However, remember that these agents are only for emergency use and should not be applied long-term, as they may also eliminate beneficial bacteria.
II. Addressing Non-Filamentous Sludge Bulking: Control the Load, Stabilize the Environment, Avoid Disturbances
Non-filamentous bulking is mostly "pushed out" and easier to handle, with the core approach being to reduce load and stabilize conditions.
Reduce the load immediately and stop overfeeding
Reduce the flow of high-concentration wastewater by partially closing the inlet valve, or even stop the water intake and allow the biochemical tank to undergo闷曝 for half a day to a full day, enabling the microorganisms to consume the excess organic matter.
Increase sludge discharge and reduce sludge concentration
When filamentous bulking occurs, the sludge concentration tends to be high. We then increase the sludge discharge to remove aged sludge, reduce the MLSS concentration, and allow the microorganisms to "start fresh.".
3. Maintain stable water temperature and avoid drastic fluctuations
When the water temperature exceeds 35°C, the activity of floc bacteria decreases, which can easily lead to bulging. In summer, a sunshade can be installed over the biochemical tank, or cold water can be sprayed for cooling to maintain the optimal temperature range of 20~30°C.
Step 3: Closing Work - Steady Operation, Don't Let Inflation Come Back Again
After the treatment of sludge expansion, the biggest fear is recurrence, so the key is the later operation and management.
1. Do a good job in water inlet pretreatment: do not slack off on grilles, sedimentation tanks, and regulating tanks, remove large particles of impurities and oils, balance the water quality and quantity of the inlet water, and avoid impact loads.
2. Fine monitoring: Measure SV30, SVI, DO, COD, and ammonia nitrogen every day. Once SVI exceeds 150, adjust the parameters as soon as possible to prevent expansion and explosion.
3. Do not randomly adjust the operating parameters: sludge age, aeration rate, reflux ratio, and other parameters. Once set, do not change them casually. If you want to adjust them, it should be done gradually and observed for a few days before continuing.
Finally, let me say a word: sludge swelling is not a terminal illness, but I'm afraid you might accidentally add medicine. As long as the type is identified first and then adjusted accordingly, the sludge will settle obediently within a week, and the secondary sedimentation tank will be clean and refreshing again.
