How to address the floating of sludge in chemical flocculation tanks?

Technical measures to control sludge flotation: ① The most feasible and economical method for stabilizing the quality of water entering the aeration tank is backflow of treated water, which is used to dilute and adjust the concentration of organic matter in the water entering the aeration tank, keeping it within a certain range. A prerequisite for backflow of treated water is that the treatment capacity of the wastewater treatment plant must be greater than the actual volume of water entering it. ②Wastewater treatment plants should consider installing tanks with a large capacity for equalization, and ensure proper control of the liquid level in these equalization tanks. High liquid levels can reduce the water volume buffering capacity of the homogenization tank, or even cause it to be lost ； Operating at a low liquid level not only results in poor homogenization but also allows oil and impurities from the bottom of the homogenization tank to enter the aeration tank, causing the activated sludge to be disturbed and float to the surface. The liquid level should be maintained at 50%–70%. ③Apply nutrients appropriately. Due to the imbalanced nutrient ratios in industrial wastewater, where carbon sources are often abundant while nutrients such as nitrogen and phosphorus are scarce, these nutrients must be added separately during the treatment of industrial wastewater. Urea and phosphates are generally used as nitrogen and phosphorus sources, but the amounts added should not be excessive. ④At the inlet of the aeration tank, there is a neutralization tank as well as a pH automatic adjustment system composed of an alkali tank, an acid tank, a pH detector, and a pH automatic control valve, to keep the pH value of the water entering the aeration tank within the required range. ⑤Pure oxygen aeration is used. The pure oxygen aeration system imported from West Germany has seen no sludge floating since it was put into operation 5 years ago. ⑥For sludge flotation caused by sludge poisoning, the aeration rate can be increased, the influent volume reduced, and dead sludge removed. ⑦The microbial composition of activated sludge mainly depends on the wastewater composition, flow pattern, operating conditions, and appropriate design. Since it is almost impossible to control the composition of wastewater during actual treatment, it is crucial to optimize the operating conditions and reactor design.

Introduction During the operation of wastewater treatment using the activated sludge process, there are various reasons that can suppress the activity of the activated sludge in the aeration tank, leading to changes in the properties and groups of microorganisms as well as a decrease in the removal rate of organic substrates. The excessive growth of certain microorganisms (such as filaments) can result in the formation of foam or scum. Mechanical stress during operation, along with trapped air bubbles, can reduce the specific gravity of the activated sludge, causing it to float to the surface. This not only increases the amount of suspended solids in the effluent but also **reduces the activity and quantity of activated sludge in the biological reaction system**. Based on the review of a large number of domestic and international studies, this paper discusses the causes of reduced activity and floating of activated sludge, as well as the detection and analysis methods and control techniques. 1 Main factors causing the floating of activated sludge 1.1 Quality of influent water 1.1.1 Excessive surfactants and oily compounds These substances can affect the stability and permeability of the cell membrane, leading to the loss of certain essential components within the cells, which in turn results in the stagnation of microbial growth and their death. When the influent to the aeration tank contains large amounts of such substances, a great deal of foam (bubbles) is generated. These bubbles readily aggregate on the flocs, causing the specific gravity of the activated sludge to decrease and making it float to the surface. Additionally, when the oil and grease content in the influent is excessively high, after aeration and mixing, the oils and greases aggregate on the surface of the flocs, depriving the bacteria of oxygen and causing them to die. This results in a decrease in specific gravity, causing the flocs to float to the surface. 1.1.2 pH shock: Excessively high or low pH values can affect the catalytic activity of extracellular enzymes of activated sludge microorganisms, as well as enzymes present in the cytoplasm and cell walls; they can also influence the microorganisms’ absorption of nutrients. When the pH in a continuous-flow aeration tank is <4.0 or >11.0, in most cases the microbial activity in the activated sludge is suppressed, or it loses its activity and even dies, resulting in sludge floating to the surface. Experimental results of treating beer wastewater and chemical wastewater using the SBR method show that when the inlet pH is 2.5–5.0 and 10.0–12.0, the lower (or higher) the pH, the more severe the suppression of sludge activity, and the greater the amount of sludge that floats to the surface. Maintain the pH value constant during the reaction period with a low pH level (3.5–7.0); in both types of wastewater, sludge flotation begins to occur at a pH level of ≤5.5. On the other hand, as the pH value increases, the flocculation of activated sludge increases due to the increase in ionizable functional groups of the extracellular polymers (although the negative charge also increases); however, once the pH value exceeds a certain range, the flocculation effect declines. It can be seen that the increase in electrical repulsion at this time also leads to deflocculation of the activated sludge (suspension, non-flocculation, deflocculation, and floating). 1.1.3 Effect of salt content: Adjusting the pH of the influent water cannot eliminate the effect of alkalinity on activated sludge. Adjusting the pH of alkaline influent neutralizes the alkaline substances, but produces salts. The osmotic pressure of salt solutions varies depending on their concentration, and osmotic pressure is one of the important factors affecting the survival of microorganisms. If the osmotic pressure of the solution in which the microorganism is found changes suddenly, it will lead to cell death. 1.1.4 Excessively high water temperature: The optimal temperature range for the microorganisms that make up activated sludge is generally 15–35°C. When the temperature exceeds 45°C, most of the microorganisms in the activated sludge die and float to the surface (except for microorganisms that have been long-term acclimated or are specially adapted). Additionally, Klaus Kriebitzsch et al. also found in experiments using the SBR process to determine the effect of temperature on intracellular enzyme activity that enzyme activity was relatively high at temperatures of 20, 30, and 40°C; however, above 50°C, the enzyme activity decreased significantly. 1.1.5 Toxic substrates Substrates that are toxic to aerobic activated sludge microorganisms include, primarily: excessive levels of COD, organic substances (phenols and their derivatives, alcohols, aldehydes, and certain organic acids), sulfides, heavy metals, and halides. High substrate concentrations can form stable compounds with the enzyme active sites in cells, preventing the matrix from approaching and being degraded, and may even cause the cells to die from poisoning. Once heavy metal ions enter cells, they mainly bind to the -SH groups on enzymes or proteins, thereby inactivating or denaturing them. Trace amounts of heavy metal ions can also continuously accumulate within cells, ultimately exerting a toxic effect on microorganisms (the oligodynamic effect). The most common halides are iodine and chlorine. Iodine irreversibly binds to tyrosine in bacterial proteins (or enzymes), forming diiodotyrosine, thereby inactivating the bacteria. Chlorine reacts with water to form hypochlorous acid, which decomposes to produce a strong oxidizing agent. Moreover, the mutation of organic substances in wastewater leads to a reduction or disappearance of the microorganisms that were previously domesticated and capable of degrading organic toxins. 1.2 Process Operation 1.2.1 Excessive aeration: Microorganisms enter a state of starvation, which leads to their own oxidation and progression into an aging phase; as a result, the dissolved oxygen concentration (DO) in the tank increases ； Or it may be due to poor sludge activity, an excessively high linear speed of the aeration impeller, resulting in excessive oxygen supply. In short, when DO increases, the sludge activity may be quite good in the short term, as metabolism speeds up and organic matter decomposes rapidly. However, over time, the sludge becomes light and fragmented (but without bubbles); it floats on the surface of the sedimentation tank like misty flower petals and is carried away by the water flow. This type of sludge has a light color, low activity, a reduced oxygen consumption rate, an increased sludge volume and sludge index, resulting in a significantly diminished treatment efficiency. 1.2.2 Sludge floating due to oxygen deficiency: The sludge is gray in color; if it remains without oxygen for too long, it turns black and often contains small bubbles. 1.2.3 Sludge flotation caused by denitrification: When the concentration of organic ammonia compounds or ammonia nitrogen in wastewater is high, it can be oxidized to NO3- by nitrate-oxidizing and nitrite-oxidizing bacteria under suitable conditions. If sludge accumulates in the secondary sedimentation tank or if the retention time is too long, the N2 produced by the reduction of NO3- will be adsorbed by the activated sludge flocs, causing the activated sludge to float to the surface. 1.2.4 Sludge flotation caused by excessive recirculation flow: A sudden increase in the recirculation flow can result in incomplete separation of air and water; bubbles from the aeration tank are carried into the sedimentation zone, causing the sludge to float. This type of sludge appears as granular particles with no change in color. The direction of its upward movement is turbulent, flowing directly from the wall of the flow-guiding area toward the wall of the sedimentation zone. 1.2.5 Sludge flotation caused by accumulation of sludge at the bottom of the secondary sedimentation tank. If the sludge at the bottom of the secondary sedimentation tank ferments, the generated CO2 and H2 will also adhere to the activated sludge, reducing its specific gravity and causing it to float. After the sludge decomposes to produce CH4 and H2S, it floats to the surface; first, small bubbles emerge from the water, followed by the black sludge rising to the surface. 1.3 Excessive growth of filamentous bacteria in activated sludge and sludge floating resulting from its control 1.3.1 Temperature and load The optimal growth conditions for Mocrothrix patricella are a temperature of 12–15°C and a sludge load of less than 0.1 kg/(kg·d). Its natural hydrophobicity results in poor dewaterability of activated sludge, with a maximum value of 490 mL/g. At temperatures above 20°C, M. parvicella does not proliferate even when the sludge load is 0.2 kg/(kg·d). It breaks into fragments of 30–80 μm in size, which float to the surface in the form of scum. 1.3.2 The filaments most frequently responsible for low-load swelling and sludge flotation due to the action of surfactants, lipids, and mechanical stress are: Microsporum, type 0092, and type 0041. An increase in the concentrations of surfactants and lipid compounds in the influent water, as well as inoculation and mechanical stress, can also promote the growth of Actinomycetes. Kappeleretal observed that mechanical stress (such as from centrifugal pumps) damages the compacted activated sludge flocs and leads to excessive growth of microfilaments. 1.3.3 Excessive addition of filamentous bacteria inhibitors: Hydrogen peroxide was added to the outlet of the aeration tank; after a few days, the filamentous bacteria disappeared, and the SVI decreased from 580 mL/g to 178 mL/g. Moreover, hydrogen peroxide also helps to maintain the DO level in the aeration tank and remove the H2S odor. However, adding too much will suppress the activity of the activated sludge and cause it to float to the surface. 2 Methods for detecting the inhibition of activated sludge activity and floating 2.1 Measuring the oxygen consumption rate (OUR) of sludge and ATP: By determining the oxygen consumption rate (OUR) of activated sludge, it is possible to determine whether any toxic substances have entered, what the load conditions are, and how the sludge discharge is balanced. If adenosine triphosphate (ATP) is measured simultaneously, the microbial biomass and activity can also be quantitatively analyzed from the perspective of functional capacity. According to the research by P.E. Jorgensen et al., measuring ATP content and OUR are reliable methods for detecting biomass activity. 2.2 Using indicator organisms to diagnose the status and performance of activated sludge: Microorganisms in activated sludge are examined under a microscope. Protozoa and metazoans (collectively referred to as microorganisms) are larger than bacterial cells, making them easier to observe, identify, and count under a microscope. They are also more sensitive to changes in external environmental conditions. As indicator organisms for diagnosing the status and performance of activated sludge, they are widely used in practical engineering applications. 3 Technical measures to control sludge floating ① The most feasible and economical method for stabilizing the quality of influent water in the aeration tank is the recirculation of treated water. This helps to dilute and regulate the concentration of organic matter in the influent, keeping it within a certain range. A prerequisite for this recirculation is that the treatment capacity of the wastewater treatment plant must be greater than the actual volume of incoming wastewater. ②Wastewater treatment plants should consider installing tanks with a large capacity for equalization, and ensure proper control of the liquid level in these equalization tanks. High liquid levels can reduce the water volume buffering capacity of the homogenization tank, or even cause it to be lost ； Operating at a low liquid level not only results in poor homogenization but also allows oil and impurities from the bottom of the homogenization tank to enter the aeration tank, causing the activated sludge to be disturbed and float to the surface. The liquid level should be maintained at 50%–70%. ③Apply nutrients appropriately. Due to the imbalanced nutrient ratios in industrial wastewater, where carbon sources are often abundant while nutrients such as nitrogen and phosphorus are scarce, these nutrients must be added separately during the treatment of industrial wastewater. Urea and phosphates are generally used as nitrogen and phosphorus sources, but the amounts added should not be excessive. ④At the inlet of the aeration tank, there is a neutralization tank as well as a pH automatic adjustment system composed of an alkali tank, an acid tank, a pH detector, and a pH automatic control valve, to keep the pH value of the water entering the aeration tank within the required range. ⑤Pure oxygen aeration is used. The pure oxygen aeration system imported from West Germany has seen no sludge floating since it was put into operation 5 years ago. ⑥For sludge flotation caused by sludge poisoning, the aeration rate can be increased, the influent volume reduced, and dead sludge removed. ⑦The microbial composition of activated sludge mainly depends on the wastewater composition, flow pattern, operating conditions, and appropriate design. Since it is almost impossible to control the composition of wastewater during actual treatment, it is crucial to optimize the operating conditions and reactor design.

If the sludge has a low specific gravity, then add weight to it

Add mud or other heavy substances to the water