In most biological wastewater treatment systems, activated sludge is considered the "heart" that determines the efficiency of treating pollutants. A system with modern technology and full equipment, but an unstable activated sludge system, can still experience effluent COD or Ammonia exceeding standards, poor sludge settling, odor generation, or prolonged recovery times after an incident.
Practice shows that many operational issues do not stem from the treatment technology but from the microbial system not being maintained under appropriate conditions. Therefore, clearly understanding what activated sludge is, how it works, and what factors affect it will help businesses operate their systems more stably, reduce risks, and optimize costs in the long term.
Activated Sludge is a collection of living microorganisms aggregating into suspended flocs in an aerobic biological tank. This is where most of the organic matter decomposition, Ammonia conversion, and treatment of various pollutants take place before the water is transferred to the settling stage.
The composition of activated sludge includes not only bacteria but also protozoa, fungi, higher microorganisms, and extracellular polymeric substances (EPS). These components bind together to form sludge flocs capable of adsorbing pollutants, maintaining biomass, and settling well in the secondary clarifier.
Unlike microbial preparations added from the outside, activated sludge is the result of the microbial system adapting and developing right under actual operating conditions. When provided with enough oxygen, nutrients, and time to develop, microorganisms will multiply, form a stable population, and create the specific treatment capacity of each system.
In reality, many businesses often confuse adding microbial preparations with maintaining activated sludge. Preparations only serve to supplement initial microbial strains or aid recovery after an incident, whereas long-term treatment efficiency depends on the sludge system's ability to develop and adapt during operation.
This is also why new wastewater treatment systems usually require time to cultivate sludge before achieving design efficiency. During this phase, microbial density is low and the floc structure is incomplete, making the capacity to treat COD, BOD, or Ammonia unstable.
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Activated sludge in wastewater treatment
Activated sludge can be imagined as a miniature "biological treatment plant," where billions of microorganisms continuously absorb and decompose pollutants in the wastewater. When wastewater enters the biological tank and is supplied with sufficient oxygen, microorganisms use organic matter, Ammonia, and other nutrients as an energy source to sustain life and develop biomass.
During this process, organic compounds are decomposed into CO₂, water, and new biomass, contributing to a reduction in COD and BOD concentrations in the wastewater. Simultaneously, the nitrifying bacteria group will convert Ammonia into Nitrite and Nitrate, creating the premise for the denitrification process in subsequent treatment structures.
Parallel to the pollutant treatment process, microorganisms secrete extracellular polymeric substances (EPS) that help them bind together into larger sludge flocs. These flocs not only create conditions for microorganisms to develop stably but also help the settling process occur more efficiently in the secondary clarifier.
After settling, a portion of the sludge is recirculated back to the biological tank to maintain the necessary microbial density. The excess sludge will be removed for treatment to prevent over-accumulation, which would affect the sludge age and the operational efficiency of the entire system.
This cycle occurs continuously throughout the treatment process and is the foundation of most biological treatment technologies such as Aerotank, AO, AAO, SBR, or MBR.
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Operating principle of activated sludge
Activated sludge is not simply a gathering place for microorganisms but also determines the efficiency of the entire biological treatment structure.
First and foremost, a healthy sludge system rapidly decomposes organic compounds, thereby reducing effluent COD and BOD. Simultaneously, it is the environment for nitrifying bacteria to develop and convert Ammonia into Nitrate, contributing to improving Nitrogen treatment efficiency.
Beyond the ability to treat pollutants, activated sludge directly affects the sludge's settling ability in the secondary clarifier. When sludge flocs develop well, the settled water will be clearer, suspended solids will decrease, and biomass separation efficiency will be improved.
Conversely, if the sludge system loses balance, businesses may encounter multiple issues such as floating sludge, sludge bulking, turbid settled water, increased effluent COD and Ammonia, or prolonged recovery times after incidents.
In many cases, maintaining stable activated sludge also helps businesses optimize operating costs. A healthy microbial system will reduce the need for supplemental microbial preparations, limit chemical adjustments, and help the system quickly adapt when the wastewater load changes.
During operation, activated sludge does not always maintain a stable state. Changes in influent water quality, aeration regimes, or operating conditions can all cause the microbial system to decline. If abnormal signs are detected early, businesses can make timely adjustments before effluent water quality is affected.
One of the common signs is that the water after the settling tank becomes turbid or contains many suspended solids, indicating that the sludge's flocculation and settling ability is declining. Cases of sludge floating on the surface of the clarifier or prolonged heavy foaming can also be related to a microbial imbalance or the excessive growth of filamentous bacteria.
Besides visual observation, changes in effluent indicators are also an important signal. When COD or Ammonia increases abnormally while wastewater flow and load do not change significantly, operators should immediately check the condition of the activated sludge instead of just adding chemicals or microbial preparations.
In reality, many treatment systems experience incidents not due to a lack of microorganisms but because operating conditions are no longer suitable. Therefore, comprehensively assessing the state of the sludge system is always more important than addressing individual symptoms.
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Activated sludge becoming unstable in a wastewater treatment system
Although activated sludge can self-adapt to operating conditions, treatment efficiency still depends on many different factors. If just one of these conditions changes too drastically, the activity of the microbial system can be affected, leading to a decline in COD and Ammonia treatment efficiency and the sludge's settling ability.
Dissolved oxygen is a crucial condition that helps aerobic microorganisms carry out organic decomposition and nitrification. If DO is too low, microorganisms will lack the oxygen needed to function, reducing treatment efficiency and potentially generating odors due to anaerobic decomposition. Conversely, maintaining too high a DO will increase electrical costs without necessarily bringing corresponding treatment efficiency.
Most microorganisms in activated sludge grow well within a neutral pH range and suitable temperatures. When pH or temperature changes suddenly, the microbial growth rate will decrease, especially for the nitrifying bacteria group, which is quite sensitive to environmental conditions.
Fluctuations in flow rate or influent pollutant concentration also directly affect activated sludge. When the load increases too quickly, the microbial system may not have enough time to adapt, leading to a decline in treatment efficiency. Therefore, maintaining a stable load is always one of the important principles when operating a wastewater treatment system.
Maintaining an appropriate amount of sludge ensures stable microbial density in the biological tank. If too much sludge is wasted, the system will lack the biomass to treat pollutants. Conversely, if sludge accumulates for too long, microbial activity may also decline and affect treatment efficiency.
Observing the color or morphology of sludge flocs only reflects a part of the microbial system's condition. For a more accurate assessment, operators should monitor the following indicators in combination:
| Indicator | Meaning | When Abnormal, Check |
| MLS | SReflects the amount of activated sludge maintained in the biological tank. | Sludge wasting and recirculation regimes. |
| MLVSS | Assesses the active organic biomass portion in the total sludge amount. | Sludge age and the development capacity of microorganisms. |
| SVI | Reflects the settling capacity of activated sludge through the sludge volume index.. | Early detection of sludge bulking or filamentous bacteria |
| SV30 | Quickly monitors sludge volume after 30 minutes of settling to assess sludge condition. | Rapidly assesses the flocculation and settling capacity of sludge. |
| Sludge Age (SRT) | Indicates the average time sludge is retained in the system and directly affects microbial development. | Adjust sludge wasting regime to maintain a stable microbial population. |
The condition of activated sludge should not be evaluated based solely on a single parameter. Combining multiple indicators with actual observation will help identify causes more accurately and formulate appropriate adjustment plans.
Note from operating experience: When COD and Ammonia both increase, many units often prioritize adding microbial preparations. However, before doing so, one should check DO, MLSS, SVI, sludge age, and influent load. In many cases, the cause is not a lack of microorganisms but operating conditions causing the activated sludge system to decline.
Activated sludge is the foundation of most biological treatment technologies today and plays a decisive role in the efficiency of organic and Ammonia treatment, as well as the stability of the entire wastewater treatment system. Maintaining a healthy sludge system depends not only on supplementing microorganisms but also requires tight control of operating conditions, periodic monitoring of key indicators, and timely adjustments when abnormal signs appear.
If your business is experiencing unstable activated sludge, or effluent COD or Ammonia frequently exceeds standards, Dai Nam's engineering team is ready to survey, evaluate the current situation, and propose optimal solutions tailored to each type of wastewater, helping the system operate stably and save costs in the long term.
