What you need to know about sludge bulking: influencing factors, formation conditions and solutions

Factors Leading to Sludge Bulking: 1. Temperature Low temperatures favor the growth of filamentous bacteria. Daigger et al. found that 10℃ easily leads to filamentous bulking, while raising the wastewater temperature to 22℃ makes sludge bulking less likely; 2. pH The suitable pH range for activated sludge microorganisms is 6.5~8.5. When the pH is less than 6, the activity of flocs weakens, and growth is inhibited, but filamentous bacteria can proliferate, replacing flocs as the dominant population, and the settling performance of sludge deteriorates significantly, resulting in sludge bulking. When the pH is below 4.5, fungi completely dominate. 3. DO Low DO is one of the main causes of filamentous sludge bulking. If DO becomes a limiting factor, floc growth is inhibited, while filamentous bacteria, due to their large specific surface area, more easily obtain dissolved oxygen for growth and reproduction, thus gaining a competitive advantage. Filamentous bacteria with low Ks have a higher specific growth rate than flocs at low substrate concentrations, which can explain the sludge bulking phenomenon caused by substrate limitation, dissolved oxygen limitation, and nutrient limitation. As long as dissolved oxygen is limiting, sludge bulking will occur under any load. In wastewater treatment, DO is controlled around 2; too high or too low can easily cause sludge bulking. 4. F/M Under low loads, due to the large specific surface area and low Ks of filamentous bacteria, they have a strong affinity for carbon sources and preferentially utilize them, creating a competitive advantage. Low F/M often occurs in completely mixed aeration tanks, oxidation ditches with high recirculation ratios (such as the Carrousel oxidation ditch), and aeration tanks with distributed influent along the length; low loads easily lead to filamentous sludge bulking, while high loads easily lead to sludge viscous bulking. Uneven load distribution, with the aerobic zone always operating under low load, easily causes the proliferation of filamentous bacteria. Li et al.'s research on the influence of sludge load parameters in membrane bioreactors showed that when F/M < 0.2 kg/kg·d, sludge bulking is easily triggered. An and Su et al. passed wastewater through an aerobic selector into a membrane bioreactor, adjusting F/M to 0.4 kg/kg·d, effectively controlling sludge bulking; while Laitinen and Luonis et al. used an anoxic selector to enhance denitrification and phosphorus removal, effectively solving sludge bulking. Under high organic loads, the reactor has abundant substrates. In this case, flocs have a stronger ability to adsorb and store nutrients than filamentous bacteria, allowing them to fully utilize high concentrations of substrates to rapidly proliferate, exhibiting a higher specific growth rate and inhibiting the growth of filamentous bacteria. However, if the DO concentration is insufficient, below 0.5 mg/L, floc proliferation is inhibited under low dissolved oxygen conditions, while filamentous bacteria, due to their larger specific surface area, can obtain oxygen even under low dissolved oxygen conditions, and their proliferation rate is significantly higher than that of flocs, resulting in sludge bulking under high load and low DO. Under low loads, due to the long-term lack of sufficient nutrients, floc growth is inhibited, while filamentous bacteria have a larger specific surface area, and their hyphae extend from the flocs to increase their nutrient uptake surface area. Due to the different research backgrounds and conditions of researchers, the research results are also inconsistent. In particular, the statements about the relationship between organic load and sludge bulking are quite confusing. Both high and low organic loads can cause sludge bulking. Ford and Eckenfeilder et al. found that sludge bulking can occur under both high and low loads. Palm et al. believe that depending on the load, bulking can occur under any DO concentration. Chudoba et al. believe that even for plug-flow aeration tanks, although there is a high concentration gradient along the length, bulking can also occur under high loads. 5. N, P Nutrients It is generally believed that BOD5:N=100:5:1 is the suitable ratio for microorganisms in wastewater. Wastewater with unbalanced N and P content will cause filamentous and non-filamentous sludge bulking. Filamentous sludge bulking: R.E. Sheder et al. found that in the absence of N, due to the large specific surface area and low Ks of filamentous bacteria, they have a strong affinity for nutrients such as N and P, preferentially utilizing them and gaining a competitive advantage; Non-filamentous sludge bulking: When BOD5/N is 100:3, flocs do not have sufficient N to complete metabolism, so they store organic matter extracellularly in the form of highly hydrophilic polysaccharide extracellular polymeric substances (EPS). Therefore, the influent C/N ratio should be reduced. 6. Trace Elements Complete-mix activated sludge processes promote excessive growth of filamentous bacteria, which can be analyzed using the trace metal deficiency theory. Because filamentous bacteria have a larger specific surface area than flocs, they have a greater adsorption capacity for trace metals when the trace metal content is insufficient, thus inhibiting the growth of flocs. 7. Toxic Substances When toxic industrial wastewater enters a wastewater treatment plant, the microorganisms in the activated sludge will experience poisoning. Novak, in his research on non-filamentous sludge bulking, found that after flocs absorb toxic substances from wastewater, the secretion of viscous substances decreases, and physiological activities become abnormal, which may cause sludge bulking. 8. Other Causes of Sludge Bulking 1. It is generally believed that sludge with low suspended solids and high dissolved and readily biodegradable organic matter, especially those containing low-molecular-weight hydrocarbons, sugars, and organic acids, are prone to filamentous bulking, such as beer, food, dairy, and paper wastewater; filamentous bacteria have weak hydrolysis ability for high-molecular-weight substances and are difficult to absorb insoluble substances, but they can directly utilize low-molecular-weight organic matter as an energy source. The most representative filamentous bacteria are Sphaerotilus natans, which can directly utilize sugars such as glucose, sucrose, and lactose. When wastewater contains a lot of soluble organic matter, it is easy to induce filamentous bulking, while wastewater with insoluble organic matter as the removal target is not easy to cause sludge bulking. Van et al. found that low-molecular-weight soluble organic matter such as glucose and acetate easily cause sludge bulking, while high-molecular-weight starch does not easily cause sludge bulking; 2. Putrefied wastewater, and wastewater containing a large amount of hydrogen sulfide, stays for too long in sewage pipes and primary sedimentation tanks, undergoing early digestion, causing a decrease in pH and producing low-molecular-weight soluble organic matter and hydrogen sulfide that are beneficial for filamentous bacteria uptake, causing sulfur-metabolizing filamentous bacteria. However, most hydrogen sulfide is a byproduct of anaerobic fermentation, and anaerobic fermentation produces a large amount of small-molecule organic acids, which is the main cause of sludge bulking; 3. Some anaerobic devices, although the effluent contains a large amount of hydrogen sulfide, will not cause sludge bulking when the volatile organic acid concentration is very low. When the volatile organic acid reaches a certain concentration, the main low-molecular-weight organic acids (acetic acid, propionic acid) are easily degraded, thus increasing the oxygen consumption rate and causing DO-limited bulking. Solutions to Sludge Bulking 1. Emergency Measures (1) Add flocculants, such as diatomaceous earth, clay, anaerobic sludge, metal salts, and coagulants, such as ferrous salts (ferrous chloride 5~50 mg/L) and aluminum salts (alum 10~100 mg/L). (2) Use disinfectant oxidants, such as using chlorine in the return sludge, with a dosage of generally 2~10 kg Cl2/1000 kg dry sludge, which can control sludge bulking in the aeration tank and disinfect the secondary effluent, while minimizing the amount of chlorine needed to control sludge bulking. A copper ion concentration of 0.75 mg/L or a salt concentration of 4 g/L has a good effect on inhibiting filamentous sludge bulking. However, this method only treats the symptoms, not the root cause. 2. Process Modification (1) Set up selectors. Selectors are high-organic-load zones (contact zones) set before or in the front section of the aeration tank, providing floc bacteria with high concentrations of absorbable dissolved substrates to improve their uptake and storage capacity, giving them an advantage in competition with filamentous bacteria. (2) Change the reactor form, such as changing from a completely mixed aeration tank to a plug-flow aeration tank, and from continuous influent to intermittent influent. Filamentous bacteria almost cannot absorb substrates in a completely anoxic environment, which allows functional bacteria that utilize substrates through denitrification and phosphorus removal to rapidly proliferate, so A/O and A/A/O systems can effectively control filamentous sludge bulking. In the A2/O process, the anaerobic and anoxic zones are not conducive to filamentous bacteria proliferation. If a part of the influent is bypassed to provide a carbon source in the aerobic section, filamentous bacteria will be at a disadvantage in the entire system. (3) Process operation control: For bulking caused by wastewater putrefaction, pre-aeration of digested wastewater can be performed, and sludge in the sedimentation tank should be removed promptly; for wastewater lacking N and P, urea, ammonium salts, fertilizers, or a mixture with domestic wastewater can be added promptly to make BOD5:N around 100:5:1; when nitrogen is deficient, high-nitrogen sludge supernatant can be added from the sludge digestion tank to the aeration tank; low dissolved oxygen can be addressed by increasing oxygen supply, using surface brush aeration oxidation ditches; to increase DO, the height of the effluent weir can be increased to increase the draft depth of the brush and enhance the aeration capacity of the brush; sludge bulking caused by low load can be addressed by appropriately increasing F/M; high-load sludge bulking can be addressed by using jet aeration to shear filamentous bacteria, and the high mass transfer efficiency of the jet provides sufficient dissolved oxygen. Increase hydraulic shear force: The strong hydraulic shear force generated during aeration causes the fluffy sludge to self-aggregate and compact, while also causing unstable filamentous bacteria on the surface of the flocs to detach. (4) In completely mixed aeration tanks, bulking occurs at loads of 0.1~0.5 kgBOD5/(kgMLSSd), while in plug-flow tanks, bulking occurs only when the sludge load is greater than 0.5 kgBOD5/(kgMLSS·d), and no bulking was found in intermittent reactors; changing hydraulic loads cause an increase in SVI, specifically analyzed as high loads and low dissolved oxygen stimulating the growth of filamentous bacteria, and the irreversibility of filamentous bacteria growth, causing sludge bulking, especially when the organic matter concentration increases sharply, it is very easy to cause sludge bulking; a sludge organic load of 0.5 kg/kg·d and DO at 2 mg/L can effectively control the growth of filamentous bacteria. (5) Recovery from sludge bulking caused by low load: Increase the sludge load, utilizing the fact that under high substrate concentration conditions, the storage capacity and maximum specific growth rate of flocs are higher than those of filamentous bacteria, creating an ecological environment in the reactor that is conducive to the growth and reproduction of flocs, allowing them to replace filamentous bacteria and gradually become the dominant bacterial species in the sludge, thus allowing the bulking sludge to gradually recover to normal. (6) Increasing the sludge return flow helps to improve the ability of floc bacteria to uptake organic matter and increase the competitiveness with filamentous bacteria, inhibiting filamentous bulking. The growth rate of filamentous bacteria is less than that of non-filamentous bacteria, and a long SRT is conducive to the growth of filamentous bacteria, so increasing the sludge discharge can effectively remove excessive filamentous bacteria in the tank. Moreover, under long sludge age conditions, sludge aging occurs, and the activity of aged sludge is insufficient, unable to compete with filamentous bacteria, which will make filamentous bacteria have a dominant position in the competition. 3. Explanation of the Natural Elimination of Sludge Bulking Sludge bulking leads to a large loss of sludge, reducing the MLSS concentration. As a result, when other conditions remain unchanged, the organic load increases, DO increases, and OUR decreases, all of which are conducive to inhibiting the proliferation of filamentous bacteria.