Common 25 Wastewater Treatment Problems and Solutions

Purpose and Importance of Biological Wastewater Treatment

1. Purpose of Biological Wastewater Treatment
   The main objectives of biological wastewater treatment are as follows:
   ① Flocculation and removal of colloidal solids in wastewater that cannot naturally settle.
   ② Stabilization and removal of organic matter in wastewater.
   ③ Removal of nutrients such as nitrogen and phosphorus.
2. Importance of Biological Wastewater Treatment
   ① Over 60% of organic matter in urban sewage can only be removed effectively and economically through biological methods.
   ② The removal of nitrogen from wastewater generally depends on biological treatment.
   ③ More than 90% of the urban wastewater treatment plants globally use biological treatment methods.
   ④ Most industrial wastewater treatment plants are also primarily based on biological treatment.

Role of Microorganisms in Biological Wastewater Treatment

Microorganisms play three main roles in biological wastewater treatment:
① Removal of dissolved organic matter (measured by COD or BOD5) by converting it into CO₂ and H₂O, as well as the removal of other dissolved inorganic nutrients such as nitrogen (which is ultimately converted into N₂ gas) and phosphorus (which is removed as phosphorus-rich residual sludge).
② Flocculation and degradation of colloidal solids: Some difficult-to-degrade particles or colloidal organic matter can be precipitated by extracellular polymeric substances produced by microorganisms, which have flocculating effects. These solids are removed along with the residual sludge or are adsorbed and slowly degraded within the system.
③ Stabilizing organic matter: Some toxic and harmful organic compounds can be partially or fully degraded by microorganisms, thus reducing their toxicity or converting them into inorganic substances, leading to stabilization.

Overview of Microbial Metabolic Processes

1. Diagram of Microbial Metabolic Processes in Biological Wastewater Treatment
2. Basic Elements of Microbial Metabolism
   ① Energy source: Chemical energy or light energy — chemotrophic (chemical energy) or phototrophic (light energy) nutrition.
   ② Carbon source: Organic carbon or inorganic carbon — heterotrophic (organic carbon) or autotrophic (inorganic carbon) nutrition.
   ③ Inorganic nutrients:
   • Macronutrients such as N, P, S, K, Ca, Mg, etc. In the treatment of industrial wastewater, balancing the nutrients (especially N and P) with the organic pollutants is sometimes crucial, and additional N and P may need to be added.
   • Micronutrients such as Fe, Co, Ni, Mo, etc. Micronutrients are essential for the growth of certain bacteria, like methanogens, and should be considered when designing and operating anaerobic biological reactors.
     ④ Special organic nutrients (also called growth factors, such as vitamins and biotin): These are critical for the growth of specific bacteria, so they may need to be supplemented in some cases.
3. Microbial Metabolic Processes Involved in Biological Wastewater Treatment
   ① Chemotrophic heterotrophic metabolism: This is the most common metabolic form in biological wastewater treatment, primarily used to remove organic matter from wastewater. It involves aerobic bacteria and anaerobic bacteria.
   ② Chemotrophic autotrophic metabolism: Also common in wastewater treatment, this process includes nitrifying bacteria (which oxidize ammonia nitrogen to nitrites or further to nitrates), hydrogen bacteria (still under research for practical applications), and iron bacteria.
   ③ Phototrophic heterotrophic metabolism: Utilizes photosynthetic bacteria to produce microbial proteins from high-concentration organic wastewater.
   ④ Phototrophic autotrophic metabolism: Rarely used in biological wastewater treatment.

1. Bacteria in Biological Wastewater Treatment

Bacteria play a crucial role in biological wastewater treatment. They are primarily categorized into two groups:Bacteria in Biological Wastewater Treatment and Archaebacteria (Archaea)

1. Classification of Bacteria Based on Oxygen Requirements, Energy Source, and Growth Temperature:

Based on Oxygen Requirements:

• Aerobic Bacteria: These bacteria require oxygen to survive and decompose organic matter in wastewater. They primarily use oxygen to oxidize organic pollutants.
• Facultative Bacteria: These bacteria can survive in both oxygen-rich and oxygen-poor environments. They can switch between aerobic and anaerobic metabolic pathways depending on the availability of oxygen.
• Anaerobic Bacteria: These bacteria thrive in environments without oxygen. They rely on anaerobic metabolism to degrade organic matter and produce gases like methane.

Based on Energy Source and Carbon Source Utilization:

• Photosynthetic Bacteria:
  • Photoautotrophs: These bacteria use light energy to fix carbon dioxide into organic compounds.
  • Photoheterotrophs: These bacteria also use light energy but require organic carbon sources for growth.
• Non-photosynthetic Bacteria:
  • Chemoautotrophs: These bacteria utilize chemical energy (e.g., hydrogen, sulfur compounds) to fix carbon dioxide and synthesize organic matter.
  • Chemoheterotrophs: These bacteria obtain both energy and carbon from organic compounds.

Based on Growth Temperature:

• Psychrophiles (Low-temperature bacteria): These bacteria grow best in temperatures between 10ºC and 15ºC, suitable for cold wastewater environments.
• Mesophiles (Moderate-temperature bacteria): These bacteria thrive in temperatures between 15ºC and 45ºC, which is the most common temperature range in wastewater treatment.
• Thermophiles (High-temperature bacteria): These bacteria grow at temperatures above 45ºC, typically used in the treatment of high-temperature industrial wastewater.

2. Fungi: Characteristics and Applications in Wastewater Treatment

Main Characteristics of Fungi:

1. Can Grow in Low Temperatures and Low pH: Fungi are resilient and can thrive in harsher environmental conditions compared to many bacteria.
2. Lower Nitrogen Requirements: Fungi generally require less nitrogen for growth, about half of what typical bacteria need.
3. Can Degrade Cellulose: Fungi are capable of breaking down cellulose, which is significant for treating wastewaters containing complex organic materials like paper or agricultural residues.

Applications of Fungi in Wastewater Treatment:

• Treatment of Specific Industrial Wastewater: Fungi can be used to treat wastewater from industries that produce lignocellulosic or complex organic compounds, such as paper mills.
• Composting of Solid Waste: Fungi play a key role in the biodegradation of solid organic waste during composting, improving the overall waste treatment process.

3. Protozoa and Metazoan Animals: Role in Wastewater Treatment

Protozoa:

• Primarily Feed on Bacteria: Protozoa are microorganisms that feed on bacteria and can help control bacterial populations in wastewater treatment systems.
• Can Serve as Indicator Organisms: The presence, type, and abundance of protozoa can indicate the quality of the treated effluent, as they are sensitive to changes in water quality.

Metazoan Animals:

• Feed on Protozoa: Metazoans (e.g., rotifers) primarily feed on protozoa, helping further control microbial populations in wastewater treatment systems.
• Can Serve as Indicator Organisms: Similar to protozoa, the presence and abundance of metazoans can reflect the overall performance of the wastewater treatment process.

4. What is Biological Wastewater Treatment and Why Is It Important?

What is Biological Wastewater Treatment?

Biological wastewater treatment is one of the most critical processes in wastewater treatment systems. It involves the use of microorganisms’ metabolic activities to remove soluble organic substances and some insoluble organic matter from wastewater. This process purifies the water by breaking down harmful contaminants.

In nature, ecosystems contain food chains where large fish eat smaller fish, which in turn eat shrimp, worms, and microorganisms, which degrade organic matter (like industrial wastewater, pesticides, fertilizers, etc.) in the water, ultimately converting it into inorganic substances. Without these microorganisms, rivers and lakes would quickly become polluted and unpleasant, with bad odors and little ecological balance.

Biological wastewater treatment systems replicate this natural process in artificial conditions. Microorganisms are concentrated in a treatment tank, and the environment is optimized (e.g., temperature, pH, oxygen, nutrients) to support microbial growth. Wastewater is then pumped into the tank, where microorganisms break down the organic matter, purifying the water.

Why Is Biological Wastewater Treatment Important?

• Low Energy Consumption: Compared to other treatment methods, biological treatment is energy-efficient as it doesn’t require excessive energy input.
• No Chemical Additives: Biological processes do not require chemicals to be added to the water, reducing operating costs.
• Effective Treatment: Biological treatment is highly effective at removing organic pollutants, making it one of the most efficient wastewater treatment methods.
• Cost-Effective: It is more affordable than physical and chemical methods, making it ideal for large-scale wastewater treatment plants.

Biological treatment plays an essential role in wastewater purification and environmental protection by ensuring that polluted water is treated effectively and returned to the ecosystem without harm.

2. How do microorganisms break down and remove organic pollutants in wastewater?

In wastewater, there are organic substances like carbohydrates, fats, and proteins, which serve as food for microorganisms. During the metabolic process, part of the organic matter is degraded and incorporated into cellular material (biosynthesis), while another part is broken down into water and carbon dioxide (catabolism). Through this process, microorganisms degrade and remove the organic pollutants from the wastewater.

3. What factors affect microorganisms?

Microorganisms require not only nutrients but also suitable environmental factors to survive. These factors include temperature, pH, dissolved oxygen, osmotic pressure, etc. If the environmental conditions are not optimal, microbial activity can be inhibited, and they may even mutate or die.

4. What is the optimal temperature range for microorganisms to grow and reproduce?

In biological wastewater treatment, the optimal temperature range for microbial growth is typically between 16-30°C, with the highest temperature being 37-43°C. Below 10°C, microbial growth stops. Within the optimal temperature range, for every increase of 10°C, the microbial metabolic rate increases, and the COD (Chemical Oxygen Demand) removal rate improves by about 10%. Conversely, for every decrease of 10°C, the COD removal rate decreases by about 10%. Therefore, in winter, the biochemical COD removal rate is significantly lower than in other seasons.

5. What is the optimal pH range for microorganisms?

The life activities and metabolic processes of microorganisms are closely related to the pH level. Most microorganisms can tolerate a pH range between 4.5 and 9, but the optimal pH range is between 6.5 and 7.5. When the pH drops below 6.5, fungi start competing with bacteria. At a pH of 4.5, fungi become dominant in biological treatment tanks, severely affecting sludge settling. When the pH exceeds 9, microbial metabolic processes are hindered.

Different microorganisms have different pH tolerance ranges. In aerobic biological treatment, the pH can range between 6.5 and 8.5. In anaerobic biological treatment, microorganisms have stricter pH requirements, and the pH should be between 6.7 and 7.4.

6. What is dissolved oxygen (DO), and how does it relate to microorganisms?

Dissolved oxygen refers to the oxygen that is dissolved in water, which is essential for the survival of aerobic microorganisms and other aquatic life. Different types of microorganisms have varying requirements for dissolved oxygen:

• Aerobic microorganisms need sufficient dissolved oxygen to survive, typically around 3 mg/L, with a minimum of 2 mg/L.
• Facultative microorganisms can survive in both aerobic and anaerobic conditions, requiring dissolved oxygen levels between 0.2-2.0 mg/L.
• Anaerobic microorganisms require dissolved oxygen levels below 0.2 mg/L.

7. Why do high concentrations of saline wastewater have a significant impact on microorganisms?

To understand this, consider an experiment involving osmotic pressure: If a semipermeable membrane separates two solutions with different salt concentrations, water molecules from the lower concentration will pass through the membrane to the higher concentration. The pressure caused by the difference in liquid levels is known as osmotic pressure, and it will stop once it reaches a certain level.

In a similar manner, microorganisms are affected by osmotic pressure in saline solutions. Their cell walls act like semipermeable membranes. When the chloride concentration in the wastewater is below or equal to 2000 mg/L, the cell wall can withstand an osmotic pressure of 0.5-1.0 atm. However, when chloride concentrations exceed 5000 mg/L, the osmotic pressure increases to 10-30 atm. This causes water to move out of the microbial cells, leading to dehydration and possibly cell death.

In everyday life, the principle of osmotic pressure is used when people pickle vegetables or preserve fish with salt, using the same mechanism to kill bacteria. In wastewater treatment, when chloride concentrations exceed 2000 mg/L, microbial activity is inhibited, and COD removal efficiency decreases. At concentrations above 8000 mg/L, microbial death can occur, and excessive foam generation in the sludge tank may be observed.

However, through long-term acclimatization, some microorganisms can adapt to high saline concentrations (above 10,000 mg/L). Yet, if the saline concentration in the wastewater fluctuates, these microorganisms can suffer from rapid osmotic imbalance, leading to cell swelling and death. Therefore, it is important to maintain consistent salt concentrations in wastewater treatment systems to avoid harm to the microbial community.

8. What is aerobic biological treatment? What is facultative biological treatment? What is the difference between the two?

Biological treatment can be divided into two major categories based on the oxygen requirements for microbial growth: aerobic biological treatment and anaerobic biological treatment. Anaerobic biological treatment can be further subdivided into facultative biological treatment and strict anaerobic biological treatment.

• Aerobic biological treatment: In this process, aerobic microorganisms require a large amount of oxygen to grow and reproduce, while degrading the organic matter in the wastewater. The process is typically performed under oxygen-rich conditions.
• Facultative biological treatment: In this process, facultative microorganisms can thrive in environments with low oxygen levels. They need only a small amount of oxygen to grow and degrade organic pollutants. If the oxygen concentration is too high, facultative microorganisms may not perform well, which can reduce their efficiency in treating organic substances.

The main differences between aerobic and facultative treatment are:

• Aerobic microorganisms typically handle wastewater with lower COD (Chemical Oxygen Demand) concentrations, generally in the range of 1000-1500 mg/L, and their COD removal efficiency is between 50-80%. The treatment time for aerobic processes is usually 12-24 hours.
• Facultative microorganisms are more adaptable to wastewater with higher COD concentrations, which can exceed 2000 mg/L, and their COD removal efficiency is also 50-80%. The treatment time for facultative treatment is similar to that of aerobic treatment, generally 12-24 hours.

One common approach is to combine facultative biological treatment and aerobic biological treatment. High COD wastewater is first treated with facultative treatment, and the effluent from the facultative tank is then used as the influent for the aerobic tank. This combined treatment can reduce the volume of the biological treatment tank, saving investment and operational costs.

9. What are the applications of biological treatment in wastewater treatment?

Biological treatment is widely used in wastewater treatment, with two main technologies:

1. Activated Sludge Process:
   • In this process, the biological treatment is carried out in an aerobic environment, where microorganisms grow and reproduce in suspension. These microorganisms form flocculated clusters (called activated sludge), which can adsorb and remove dissolved or colloidal pollutants from the wastewater.
   • During this process, the pollutants are absorbed by the microbial cells and oxidized in the presence of oxygen, releasing energy, CO₂, and H₂O.
   • The sludge concentration in the activated sludge process is typically around 4 g/L.
2. Biofilm Process:
   • In the biofilm process, microorganisms attach to a surface medium (such as a plastic carrier or other support materials), forming a biological film. This biofilm consists of a gel-like matrix and has a large surface area with numerous micropores, making it highly efficient for adsorption.
   • The biofilm microorganisms continue to degrade the organic pollutants as they come into contact with the wastewater, using the dissolved oxygen in the process. While degrading pollutants, the biofilm undergoes constant turnover, with older biofilm layers sloughing off and being carried away in the effluent.
   • The sludge concentration in the biofilm process is generally higher, around 6-8 g/L.

To improve sludge concentration and treatment efficiency, the activated sludge process and biofilm process can be combined. In this hybrid system, filler materials are added to the activated sludge tank to provide surfaces for the microorganisms to form biofilms. This type of system is known as a composite biological reactor, which can achieve higher sludge concentrations, typically around 14 g/L, and provides both suspended and attached microbial growth for more efficient treatment.

10. What are the similarities and differences between the biofilm process and the activated sludge process?

The biofilm process and the activated sludge process are both biological treatment methods, but they differ in reactor configurations. The key differences and similarities include:

Differences:

• Microorganism Attachment:
  • In the biofilm process, microorganisms are fixed on a substrate (carrier material), forming a biofilm.
  • In the activated sludge process, microorganisms are suspended in the liquid and not fixed to any surface.
• Sludge Formation:
  • In the biofilm process, the microorganisms form a stable ecological system on the carrier material, which results in less residual sludge because microorganisms’ energy consumption and waste generation are lower.
  • In the activated sludge process, the microorganisms grow in suspension and generate more residual sludge due to higher energy consumption.

Similarities:

• Mechanism: Both methods rely on aerobic microorganisms to degrade organic pollutants in the wastewater. The overall process of pollutant removal is similar in both systems, involving the oxidation of organic matter into carbon dioxide and water, aided by microbial metabolic activity.
• Microbial Composition: Both processes contain similar microbial communities, including bacteria, fungi, protozoa, and metazoans, which work together to break down organic matter.

In practice:

• Biofilm Process is often used in contact oxidation tanks, such as those in Shanghai Xinyi Baoluda Pharmaceutical Co. Ltd.
• Activated Sludge Process is often used in Sequencing Batch Reactors (SBR) for wastewater treatment.

11. What is activated sludge?

Activated sludge refers to the biological sludge in a biochemical treatment system that contains a diverse community of microorganisms. These microorganisms include bacteria, fungi, protozoa, and metazoans (such as rotifers, insect larvae, and worms). The microorganisms form a food chain, where bacteria and fungi break down complex organic compounds, obtain the necessary energy for their activities, and build cellular material. Protozoa feed on bacteria and fungi, and metazoans feed on protozoa, completing the food chain.

The activated sludge also contains inorganic substances and non-biodegradable organic matter that cannot be further broken down by the microorganisms. The moisture content of activated sludge is generally around 98-99%.

Because activated sludge has a large surface area (similar to flocculated particles like alum), it has a strong ability to adsorb and oxidize organic pollutants, making it effective in wastewater treatment.

12. How is the growth of activated sludge in the activated sludge and biofilm processes evaluated?

The evaluation of the growth of activated sludge in both processes differs:

• In the Biofilm Process:
  • The evaluation is mainly done by microscopic observation of the microbial community (biofilm).
• In the Activated Sludge Process:
  • In addition to microscopic observation, common indicators for evaluating activated sludge growth include:
    • Mixed Liquor Suspended Solids (MLSS): The total concentration of suspended solids in the mixed liquor.
    • Mixed Liquor Volatile Suspended Solids (MLVSS): The fraction of volatile solids in the mixed liquor, indicating the presence of microbial biomass.
    • Sludge Volume Index (SVI): A measure of the sludge’s ability to settle and compact.
    • Sludge Settleability (SV): The rate at which the sludge settles in the settling tank.

13. What type of microorganisms under microscopic observation indicates a good biological treatment effect?

The presence of micro metazoans (such as rotifers, nematodes, and other small metazoans) under microscopic observation indicates that the biological community in the activated sludge system is growing well. This is an indication of a stable ecological system in the activated sludge, which leads to optimal biological treatment performance.

The presence of these microorganisms is similar to how in a river with abundant fish, small fish and shrimp thrive. This suggests that the system’s ecological balance is maintained, and the biological treatment is effective, as the microorganisms are flourishing and effectively treating the organic pollutants in the wastewater.

14. What is Mixed Liquor Suspended Solids (MLSS)?

Mixed Liquor Suspended Solids (MLSS) refers to the concentration of dry sludge (including both organic and inorganic matter) in a unit volume of the mixed liquor in a biological treatment tank, typically measured in mg/L. It is used to characterize the concentration of activated sludge in the system. MLSS indicates the total biomass present in the system, including both active microorganisms and inert solids. In an SBR (Sequencing Batch Reactor) system, the MLSS is generally controlled within the range of 2000-4000 mg/L for optimal operation.

15. What is Mixed Liquor Volatile Suspended Solids (MLVSS)?

Mixed Liquor Volatile Suspended Solids (MLVSS) refers to the portion of the dry sludge in the mixed liquor that can be volatilized (combusted) at high temperatures. It is expressed in mg/L and provides a more accurate measure of the amount of active microorganisms in the system since it excludes inorganic solids. MLVSS is often used as an indicator of the microbial biomass present in the system, which plays a key role in the biodegradation of pollutants.

16. What is Sludge Volume (SV)?

Sludge Volume (SV) is the ratio of the volume of settled sludge to the total volume of mixed liquor in a 100 mL graduated cylinder after a 30-minute settling period, expressed as a percentage. This is sometimes referred to as SV30 (Sludge Volume after 30 minutes). SV is a key parameter for evaluating settling properties of the sludge. Generally, in biological treatment processes, SV values are expected to be between 20-40%. It is used to control the excess sludge discharge and to prevent abnormal phenomena such as sludge bulking.

17. What is Sludge Volume Index (SVI)?

Sludge Volume Index (SVI), also known as the Sludge Volume Index, measures the volume (in mL) occupied by 1 gram of dry sludge when in a wet state. The formula for calculating SVI is:SVI=SV×10MLSSSVI = \frac{{SV \times 10}}{{MLSS}}SVI=MLSSSV×10​

SVI removes the influence of sludge concentration (MLSS), giving a clearer indication of the settling properties and consolidation behavior of the activated sludge. Generally:

• When 60 < SVI < 100, the settling performance is good.
• When 100 < SVI < 200, the settling performance is average.
• When 200 < SVI < 300, the sludge shows signs of bulking.
• When SVI > 300, the sludge is considered bulked and problematic for efficient treatment.

18. What is Dissolved Oxygen (DO)?

Dissolved Oxygen (DO) represents the amount of oxygen dissolved in water, measured in mg/L. It is a crucial parameter for biological treatment processes since aerobic microorganisms require oxygen for the oxidation of organic pollutants. Different biological processes have different DO requirements:

• In anoxic (oxygen-limited) processes, DO is typically 0.2-2.0 mg/L.
• In aerobic (oxygen-rich) processes, such as SBR, DO is typically maintained in the range of 2.0-8.0 mg/L.
  • For processes like contact oxidation, the DO is controlled between 2.0-4.0 mg/L for optimal microbial activity.

19. What factors influence the dissolved oxygen concentration in wastewater?

The concentration of dissolved oxygen in wastewater depends on several factors, and it can be expressed using Henry’s Law:C=KH×PC = K_H \times PC=KH​×P

Where:

• C is the dissolved oxygen concentration in water.
• P is the partial pressure of oxygen in the air.
• K_H is the Henry’s Law constant, which is temperature-dependent.

In practice, the actual dissolved oxygen concentration is influenced by:

• Water temperature (higher temperatures lower the oxygen solubility).
• Effective water depth (affecting pressure and thus oxygen dissolution).
• Aeration rate (higher aeration leads to more oxygen being dissolved).
• Sludge concentration (higher sludge concentration can consume more oxygen).
• Salinity (higher salinity can reduce oxygen solubility).

20. Who provides the oxygen required by microorganisms during biochemical processes?

The oxygen required by microorganisms for biochemical processes is primarily supplied by Roots blowers (also known as blowers or aerators). These devices supply air (oxygen) to the bioreactor to support the metabolic processes of aerobic microorganisms that degrade organic contaminants.

21. Why is it necessary to regularly supplement wastewater with nutrients during biological treatment?

Microorganisms involved in biological treatment processes require nutrients for their metabolic processes, particularly for cell synthesis and energy production. These nutrients include carbon, nitrogen, phosphorus, and microelements. In certain types of industrial wastewater, such as from chemical production, the nutrient composition may be imbalanced. For example, a wastewater that only contains carbon and nitrogen but lacks phosphorus will not support optimal microbial growth. This deficiency must be addressed by adding phosphorus or other missing nutrients to maintain healthy microbial metabolism.

This is akin to the way humans need a balanced diet, including essential vitamins and minerals, along with carbohydrates, proteins, and fats for proper metabolic function. Without the necessary nutrients, microorganisms cannot carry out their degradation processes effectively, leading to reduced treatment efficiency.

22. What is the required ratio of nutrients (C:N:P) for microorganisms in wastewater?

Microorganisms, like plants and animals, require essential nutrients to grow and reproduce. The key nutrients needed for microbial growth in biological treatment processes are carbon (C), nitrogen (N), and phosphorus (P). In wastewater, these nutrients are typically required in a specific ratio for efficient biochemical treatment, especially for aerobic processes. The general nutrient ratio is C:N:P = 100:5:1 by weight. This ratio ensures that the microorganisms have adequate amounts of each nutrient for optimal growth and metabolic activity.

23. Why is there a generation of excess sludge?

During biological treatment, microorganisms in the activated sludge consume organic substances (mainly BOD, biochemical oxygen demand) present in the wastewater. These organic materials are used in two ways:

1. Oxidized to provide energy for the microorganisms’ life activities.
2. Utilized to synthesize new cell material, allowing the microorganisms to grow and reproduce.

As microorganisms grow and divide, some of the older microbial cells die, and this dead biomass contributes to the formation of excess sludge. This accumulation of biomass from both the death of older cells and the synthesis of new cells leads to the production of excess sludge.

24. How can the amount of excess sludge generation be estimated?

The amount of excess sludge generated is related to the BOD (Biochemical Oxygen Demand) removed from the wastewater. During microbial metabolism, part of the BOD is used to synthesize new cells, replacing the dead microbial biomass. Therefore, the excess sludge production is proportional to the amount of BOD removed.

In general, during engineering design, it is estimated that for every 1 kg of BOD5 treated, approximately 0.6-0.8 kg of excess sludge is generated. This is roughly equivalent to 3-4 kg of dry sludge with a 80% moisture content.

25. What is the Biological Activated Carbon Treatment (PACT)?

The Biological Activated Carbon Treatment (PACT) process is used to improve the removal of difficult-to-biodegrade pollutants in certain wastewater types, such as pharmaceutical effluents, where achieving the required COD (Chemical Oxygen Demand) removal to meet national standards (e.g., <100 mg/L) through biological treatment alone is challenging.

In the PACT process, powdered activated carbon (PAC) is added to the aeration tank along with the return activated sludge. The powdered activated carbon has a very large surface area and strong adsorption capacity, which enhances the adsorption ability of the sludge. This helps improve the removal efficiency of COD, particularly in aerobic conditions.

Key points about PACT:

• Higher Adsorption Capacity: The dynamic adsorption capacity of PAC in the PACT system can be 100-350% by weight. This means that 1 kg of PAC can adsorb 1.0-3.5 kg of COD.
• Effective for Difficult-to-Degrade Compounds: PACT is effective at removing toxic and hazardous organic pollutants that are hard to biodegrade.
• Cost-effective: Compared to granular activated carbon (GAC), PAC is more cost-effective because it avoids the issue of biofilm formation on the carbon particles, which can clog the system and reduce efficiency.
• Prevents Toxic Compound Build-up: The PAC helps in adsorbing toxic substances, keeping them at a low concentration and preventing them from interfering with the biological treatment process.

In practice, adding PAC periodically (every 15-30 days) in the SBR (Sequencing Batch Reactor) helps improve treatment efficiency, especially for toxic organic pollutants. The use of PAC in the biological treatment system also enhances COD removal by 10-40% depending on the wastewater characteristics and can help maintain a stable concentration of toxic substances at a low level, ensuring the normal operation of the biological system.

Additionally, PAC can help control ammonia nitrogen levels and prevent rebound of ammonia concentrations, ensuring that effluent standards for nitrogen removal are met.