Methods for Handling Several Abnormal Issues in Wastewater Treatment

During the wastewater treatment process, various wastewater-related issues may arise. For example, indicators such as COD, ammonia nitrogen, and SS may fail to meet standards, sludge bulking, floating sludge, and the death of active microorganisms. Since the principles of wastewater treatment are generally the same, wastewater treatment research has primarily been based on domestic sewage as a model. Below, we summarize the problems encountered during operation with domestic sewage as the target.

Influent Flow and Water Quality

Influent Flow

In some developing countries, the phenomenon of insufficient influent flow to urban sewage treatment plants is widespread. The reasons for this shortage include the commonly mentioned issue of lagging sewage collection network construction, as well as the issue of overestimating design capacity. These two factors have led to many sewage treatment plants being unable to operate at full capacity, even after being completed for several years. Some plants even have to treat river water from around the plant site, which increases the difficulty of controlling the wastewater treatment process and raises the cost of the engineering investment. This results in the idle use of assets and waste, unnecessarily consuming already limited wastewater treatment funds. 

In contrast, some sewage treatment plants are operating in a state of long-term overload. For example, the first phase of a certain sewage treatment plant has a capacity of 400,000 m³/d, and the second phase has a capacity of 240,000 m³/d. However, due to a lack of funds, the second phase of construction has been delayed. As a result, the first phase is treating 520,000 m³/d, and the effluent water quality has declined. Therefore, it is essential to reasonably determine the construction scale and phased development of sewage treatment plants, efficiently use pollution control funds, and maximize sewage collection rates as the prerequisites for achieving wastewater reduction.

Influent Water Quality

The sewage collection network is not properly integrated, and combined sewer systems (where stormwater and sewage share the same pipeline) are relatively common. Inadequate management of the pipeline network results in a significant proportion of stormwater, river water, and industrial wastewater entering the urban sewage treatment plants.

The following influent water quality conditions are unfavorable for the normal operation of sewage treatment plants:

a. The BOD and COD content in the influent is lower than the design values, while nitrogen, phosphorus, and other indicators are equal to or higher than the design values, making it more difficult to meet the discharge standards for nitrogen and phosphorus removal.

b. The oil or toxic substances carried by industrial wastewater have a significant impact on the biological systems of urban sewage treatment plants. In extreme cases, these oils or toxic substances can paralyze the entire biological system, causing the death of microbial strains, forcing the entire sewage treatment plant to re-culture activated sludge.

c. The influent water quality is too high, and the oxygen supply and sludge dewatering equipment specifications cannot meet the sewage and sludge treatment requirements. The impact of introducing landfill leachate into the urban sewage treatment plant operation requires sufficient attention.

For the issue of misalignment between sewage collection and treatment capacity, relevant authorities need to incorporate the construction of urban drainage networks and sewage treatment plants into the overall long-term and short-term urban planning. This will ensure that the sewage collection system and sewage treatment plants are constructed either simultaneously or in advance. Additionally, a thorough investigation of the sewage water quality within the service area of newly constructed sewage treatment plants should be conducted to reasonably determine the design influent water quality.

Effluent Water Quality

In recent years, urban sewage treatment plants built in some developing countries generally aim to meet local relevant standards. Even existing projects are gradually being upgraded and renovated to improve wastewater reduction efficiency.

According to the specified wastewater treatment discharge standards, each city's sewage treatment plant adopts treatment technologies suitable for local influent water quality and other objective conditions, while also strengthening operational management. However, during the actual operation and management of sewage treatment plants, issues from various sources may still arise, leading to effluent water quality that fails to meet the required standards.

1. Excessive Organic Matter

The main function of the traditional activated sludge process is to remove organic pollutants from urban sewage. A well-designed and properly operated activated sludge process can achieve effluent BOD₅ and SS levels below 20 mg/L. 

Factors affecting the effectiveness of organic matter removal include:

a. Nutrients

Generally, nitrogen, phosphorus, and other nutrients in urban sewage are sufficient to meet the needs of microorganisms, often in excess. However, when industrial wastewater makes up a significant portion of the influent, it is important to ensure the correct ratio of carbon, nitrogen, and phosphorus, typically 100:5:1. If there is a nitrogen deficiency in the sewage, ammonium salts are usually added. If there is a phosphorus deficiency, phosphoric acid or phosphates are typically added.

b. pH

The pH of urban sewage is typically neutral, generally between 6.5 and 7.5. A slight decrease in pH may be due to anaerobic fermentation in sewage pipelines. During the rainy season, a more significant decrease in pH is often caused by urban acid rain, which is particularly noticeable in combined sewer systems. Sudden and large pH fluctuations, whether an increase or decrease, are usually caused by the large discharge of industrial wastewater. To adjust the sewage pH, sodium hydroxide or sulfuric acid is typically added, though this will significantly increase treatment costs.

c. Grease

When the oil content in sewage is high, it can reduce the aeration efficiency of aeration equipment. If the aeration rate is not increased, treatment efficiency will drop, but increasing aeration will inevitably increase sewage treatment costs. Furthermore, high oil content in sewage can also reduce the settling performance of activated sludge. In severe cases, it can cause sludge bulking, leading to excessive SS in the effluent. For influent with high oil content, an oil removal device should be added in the pretreatment stage.

d. Temperature

Temperature has a broad impact on the activated sludge process. First, temperature affects the activity of microorganisms in activated sludge; during winter, when temperatures are lower, treatment efficiency will decrease unless temperature control measures are taken. Secondly, temperature affects the separation performance of secondary settling tanks. For example, temperature changes can cause stratification in the settling tank, leading to short-circuiting. A decrease in temperature can cause the viscosity of activated sludge to increase, reducing settling performance. Temperature fluctuations also affect the efficiency of the aeration system. In the summer, when the temperature rises, the saturation concentration of dissolved oxygen decreases, making oxygenation difficult and reducing aeration efficiency. This also lowers air density, and if the air supply remains constant, the air supply volume must be increased.

2. Excessive Ammonia Nitrogen

The removal of ammonia nitrogen in wastewater is mainly achieved by employing a nitrification process based on the traditional activated sludge method, which involves extended aeration to reduce system load.

There are several reasons for the excessive ammonia nitrogen in effluent, mainly including:

a. Sludge Load and Sludge Age

Biological nitrification is a low-load process, with F/M (food to microorganism ratio) typically ranging from 0.05 to 0.15 kg BOD/kg MLVSS·d. The lower the load, the more complete the nitrification process, and the higher the efficiency of NH₃-N conversion to NO₃^--N. In accordance with low load, the SRT (sludge retention time) of the biological nitrification system is generally longer, because nitrifying bacteria have a longer generation cycle. If the SRT is too short, meaning the sludge concentration is too low, nitrifying bacteria cannot be cultivated, and the nitrification effect will not be achieved. The appropriate SRT depends on factors such as temperature. For biological systems where nitrogen removal is the main goal, SRT typically ranges from 11 to 23 days.

b. Return Sludge Ratio

The return sludge ratio of a biological nitrification system is usually higher than that of the traditional activated sludge process. This is because the activated sludge in a biological nitrification system already contains a large amount of nitrate. If the return ratio is too low, the sludge in the secondary settling tank will have a longer retention time, leading to denitrification, causing sludge floatation. Typically, the return sludge ratio is controlled within 50% to 100%.

c. Hydraulic Retention Time

The hydraulic retention time in the biological nitrification aeration tank is also longer than in the activated sludge process, typically at least 8 hours. This is because the nitrification rate is much slower than the rate of organic pollutant removal, requiring a longer reaction time.

d. BOD₅/TKN

TKN refers to the sum of organic nitrogen and ammonia nitrogen in water. The BOD₅/TKN ratio in the influent wastewater is an important factor affecting the nitrification effect. The higher the BOD₅/TKN ratio, the smaller the proportion of nitrifying bacteria in the activated sludge, resulting in a slower nitrification rate and lower nitrification efficiency under the same operating conditions. Conversely, a smaller BOD₅/TKN ratio leads to higher nitrification efficiency. Operational practices in many urban wastewater treatment plants have shown that the optimal range for BOD₅/TKN is around 2 to 3.

e. Nitrification Rate

A specific process parameter for biological nitrification is the nitrification rate, which refers to the amount of ammonia nitrogen converted per unit weight of activated sludge per day. The nitrification rate depends on the proportion of nitrifying bacteria in the activated sludge and several factors such as temperature. The typical value is 0.02 g NH₃-N/g MLVSS·d.

f. Dissolved Oxygen

Nitrifying bacteria are obligate aerobes, meaning they cease their activity in the absence of oxygen. Additionally, their oxygen consumption rate is much lower than that of bacteria responsible for organic matter decomposition. If adequate oxygen levels are not maintained, nitrifying bacteria will not receive the oxygen they need. Therefore, the dissolved oxygen in the aerobic zone of the biological tank should be kept above 2 mg/L, and in special cases, the dissolved oxygen level should be further increased.

g. Temperature

Nitrifying bacteria are highly sensitive to temperature changes. When the temperature of the wastewater drops below 15°C, the nitrification rate significantly decreases. If the temperature falls below 5°C, the physiological activity of nitrifying bacteria will stop completely. Thus, in winter, especially in temperate regions, the phenomenon of excessive ammonia nitrogen in effluent from sewage treatment plants is more prominent.

h. pH

Nitrifying bacteria are sensitive to pH changes. Their biological activity is strongest within the pH range of 8 to 9. When the pH is lower than 6.0 or higher than 9.6, the biological activity of nitrifying bacteria is inhibited and tends to stop. Therefore, the pH of the mixed liquor in the biological nitrification system should be maintained above 7.0 as much as possible.

3. Excessive Total Nitrogen

Wastewater nitrogen removal is based on the biological nitrification process, with the addition of biological denitrification. Denitrification is a biochemical process in which nitrates in wastewater are reduced to nitrogen gas by microorganisms under anoxic conditions.

Several factors contribute to excessive total nitrogen in the effluent, including:

a. Sludge Load and Sludge Age

Since biological nitrification is the prerequisite for biological denitrification, effective nitrification is essential for achieving efficient and stable denitrification. Therefore, the nitrogen removal system must operate under low or ultra-low load conditions, using a high sludge age.

b. Internal and External Return Ratios

The external return ratio of the biological denitrification system is generally smaller than that of a simple biological nitrification system, mainly because most of the nitrogen in the influent wastewater has already been removed, and the concentration of NO₃^--N in the secondary settling tank is not high. Consequently, the risk of sludge floatation in the secondary settling tank due to denitrification is minimal. On the other hand, the sludge settling rate in the denitrification system is relatively fast, and the return ratio can be reduced, provided that the required return sludge concentration is maintained, allowing for extended retention time in the aeration tank.

In well-operated sewage treatment plants, the external return ratio can be controlled below 50%, while the internal return ratio is generally controlled between 300% and 500%.

c. Denitrification Rate

The denitrification rate refers to the amount of nitrate reduced by denitrification per unit weight of activated sludge per day. The denitrification rate is influenced by factors such as temperature, with a typical value ranging from 0.06 to 0.07 g NO₃^--N/g MLVSS·d.

d. Dissolved Oxygen in Anoxic Zone

For denitrification, it is ideal for the dissolved oxygen (DO) to be as low as possible, ideally zero, so that denitrifying bacteria can fully perform the denitrification process, improving nitrogen removal efficiency. However, in actual plant operations, it is difficult to control the DO in the anoxic zone below 0.5 mg/L, which affects the denitrification process and, in turn, the total nitrogen in the effluent.

e. BOD₅/TKN

Denitrifying bacteria carry out denitrification during the process of organic matter decomposition, so there must be sufficient organic matter in the influent entering the anoxic zone to ensure successful denitrification. Due to the lag in the construction of sewage collection networks at many wastewater treatment plants, the BOD₅ in the influent is lower than the design value, while nitrogen, phosphorus, and other indicators are equal to or higher than the design values. This results in an insufficient carbon source for denitrification, leading to frequent occurrences of excessive total nitrogen in the effluent.

f. pH

Denitrifying bacteria are less sensitive to pH changes than nitrifying bacteria. They can carry out normal physiological metabolism in a pH range of 6 to 9, but the optimal pH range for biological denitrification is between 6.5 and 8.0.

g. Temperature

Although denitrifying bacteria are not as sensitive to temperature changes as nitrifying bacteria, denitrification efficiency still varies with temperature. The higher the temperature, the higher the denitrification rate. Denitrification reaches its maximum rate at 30–35°C. When the temperature drops below 15°C, the denitrification rate significantly decreases, and at 5°C, denitrification almost stops. Therefore, in winter, to ensure effective nitrogen removal, it is necessary to increase SRT, raise the sludge concentration, or add more aeration tanks in operation.

4. Excessive Total Phosphorus

The main method for phosphorus removal in urban sewage treatment plants is biological phosphorus removal, which involves adding an anaerobic zone before the aerobic stage to allow phosphorus-accumulating organisms (PAOs) to alternate between anaerobic and aerobic conditions, enabling the release and uptake of phosphate. Phosphorus is removed through the discharge of excess sludge. On the other hand, when biological phosphorus removal is difficult to meet the standard, chemical agents can be added to assist in phosphorus removal. Chemical phosphorus removal primarily involves coagulation, sedimentation, and filtration methods to convert phosphorus into insoluble solid precipitates, which are then separated from the wastewater.

Several factors contribute to excessive total phosphorus in the effluent, including:

a. Sludge Load and Sludge Age

The anaerobic-aerobic biological phosphorus removal process is a high F/M (food to microorganism ratio), low SRT (sludge retention time) system. When F/M is high and SRT is low, more excess sludge is produced. Therefore, under conditions of a certain phosphorus content in the sludge, the phosphorus removal rate increases, and the phosphorus removal effect improves.

For biological systems where phosphorus removal is the primary goal, the typical F/M ratio is 0.4–0.7 kg BOD₅/kg MLSS·d, and SRT is 3.5–7 days. However, SRT should not be too low, and BOD₅ removal must be ensured as a prerequisite.

b. BOD₅/TP Ratio

To ensure effective phosphorus removal, the BOD₅/TP ratio in the influent entering the anaerobic zone should be greater than 20. Since phosphorus-accumulating organisms are relatively weak in their physiological activity and can only take up the easily decomposable portion of organic matter, sufficient BOD₅ must be present in the influent to support their normal metabolic activity. However, many urban sewage treatment plants experience low carbon sources and high concentrations of nitrogen, phosphorus, and other pollutants in the influent, leading to a BOD₅/TP ratio that is inadequate for biological phosphorus removal, which affects its effectiveness.

c. Dissolved Oxygen

The anaerobic zone should maintain strict anaerobic conditions, with dissolved oxygen (DO) levels below 0.2 mg/L, allowing phosphorus-accumulating organisms to release phosphorus effectively, ensuring subsequent treatment efficiency. In the aerobic zone, DO levels must be maintained above 2.0 mg/L for PAOs to effectively absorb phosphorus. Improper control of DO in both the anaerobic and aerobic zones will greatly affect the biological phosphorus removal effect. Additionally, some wastewater treatment plants receive influent from rivers with relatively high DO levels, which may make it difficult to maintain anaerobic conditions and negatively affect phosphorus release by PAOs.

d. Return Sludge Ratio

The return sludge ratio in anaerobic-aerobic phosphorus removal systems should not be too low. Sufficient return sludge is required to rapidly expel sludge from the secondary settling tank, preventing PAOs from encountering anaerobic conditions in the secondary settling tank and releasing phosphorus. While ensuring rapid sludge discharge, the return ratio should be minimized to avoid shortening the actual retention time of sludge in the anaerobic zone, which could affect phosphorus release.

In anaerobic-aerobic phosphorus removal systems, if the sludge settles well, the return ratio should be in the range of 50–70% to ensure quick sludge discharge.

e. Hydraulic Retention Time

The hydraulic retention time in the anaerobic zone is typically between 1.5 and 2.0 hours. If the retention time is too short, phosphorus cannot be effectively released, and facultative acidogenic bacteria cannot fully decompose macromolecular organic matter in the wastewater into low molecular fatty acids, which are then available for PAOs to take up. This also affects phosphorus release.

The hydraulic retention time in the aerobic zone is typically between 4 and 6 hours, which ensures sufficient phosphorus uptake.

f. pH

Low pH is favorable for phosphorus release, while high pH facilitates phosphorus absorption. Phosphorus removal is a combination of release and absorption processes. Therefore, in biological phosphorus removal systems, the pH of the mixed liquor should be controlled between 6.5 and 8.0.

As the requirements for total phosphorus in effluent continue to increase, chemical phosphorus removal is being used more frequently in addition to biological phosphorus removal. However, while chemical phosphorus removal improves phosphorus removal efficiency, it also significantly increases the amount of excess sludge produced due to the addition of chemicals, which in turn increases sludge treatment and dewatering costs.

In practice, the dosing points and amounts of chemical agents should be determined based on experiments, and adjustments should be made in real time to ensure that effluent phosphorus levels remain stable and compliant, while minimizing chemical consumption.

5. Excessive Suspended Solids

The suspended solids levels in the effluent depend largely on the quality of the sludge in the biological system, the settling effect of the secondary settling tank, and the proper control of the sewage treatment plant’s process.

a. Process Parameter Selection

The selection of design parameters for the secondary settling tank is an important factor in determining whether the suspended solids in the effluent exceed the standard. Many urban sewage treatment plants, in order to reduce construction costs, have significantly shortened the hydraulic retention time and tried to increase the hydraulic surface load, leading to frequent sludge scouring in the secondary settling tank during operation, which results in excessive suspended solids in the effluent.

Additionally, some sewage treatment plants, due to operational needs, control the sludge concentration in the biological tank at higher levels, which also causes excessive surface load in the secondary settling tank, affecting effluent quality. Therefore, it is generally considered necessary to provide sufficient leeway for the design of these process parameters in the secondary settling tank to facilitate process control and adjustment.

In general, the primary process parameters affecting the settling effect in the sedimentation tank are hydraulic retention time, hydraulic surface load, and sludge flux.

b. Hydraulic Retention Time

The hydraulic retention time in the secondary settling tank is an important operational parameter. Sufficient retention time is required to ensure good flocculation and high sedimentation efficiency. Therefore, it is recommended to set the hydraulic retention time of the secondary settling tank at around 3–4 hours.

c. Hydraulic Surface Load

For a sedimentation tank, when the influent flow rate is constant, the size of particles it can remove is fixed. Among these particles, the smallest one will have a settling velocity equal to the hydraulic surface load of the tank. Therefore, the smaller the hydraulic surface load, the more particles can be removed, resulting in higher sedimentation efficiency and lower suspended solids in the effluent. Designing the secondary settling tank with a smaller hydraulic surface load is beneficial for the effective sedimentation of sludge and other suspended solids. It is generally recommended that the hydraulic surface load of the secondary settling tank be controlled within the range of 0.6–1.2 m³/m²·h.

d. Solids Surface Load

The size of the solids surface load in the secondary settling tank is also an important factor influencing sedimentation efficiency. The lower the solids surface load, the better the sludge concentration in the secondary settling tank. Conversely, if the solids surface load is too high, it will cause the sludge surface to be excessively high, and many sludge flocs will be unable to settle before flowing out with the wastewater, affecting the suspended solids levels in the effluent. Typically, the solids surface load in the secondary settling tank should not exceed 150 kg MLSS/m²·d.

e. Quality of Activated Sludge

The quality of activated sludge is a critical factor influencing whether suspended solids in the effluent exceed the standard. High-quality activated sludge is characterized by four main aspects: good adsorption performance, high biological activity, good settling performance, and good concentration performance.

Colloidal pollutants must first be adsorbed onto the activated sludge flocs, and then further adsorbed onto the surface of the bacteria to be decomposed and metabolized. Therefore, activated sludge with poor adsorption ability will be less effective at removing colloidal pollutants. The biological activity of activated sludge refers to the ability of microorganisms within the sludge flocs to decompose and metabolize organic pollutants. Poor biological activity leads to slower organic pollutant removal.

Only activated sludge with good settling performance can effectively separate sludge and water in the secondary settling tank. Conversely, if the settling performance of the sludge deteriorates, the separation effect is reduced, leading to turbid effluent, SS exceeding the standard, and in severe cases, a significant loss of activated sludge, which affects the biological treatment efficiency.

Only activated sludge with good concentration performance can achieve high sludge concentration in the secondary settling tank. On the other hand, if the concentration performance is poor and the sludge concentration decreases, a sufficient return sludge flow rate must be maintained. However, increasing the return sludge flow rate shortens the actual retention time in the aeration tank, reducing aeration time and affecting the treatment efficiency.

f. Influent SS/BOD₅

The ratio of MLVSS (mixed liquor volatile suspended solids) in the activated sludge system is closely related to the influent SS/BOD₅ ratio. When the influent SS/BOD₅ ratio is high, the proportion of MLVSS in the activated sludge is low, and vice versa. Operational experience shows that when the SS/BOD₅ ratio is below 1, the MLVSS proportion can be maintained above 50%, while when the SS/BOD₅ ratio is above 5, the MLVSS proportion drops to 20–30%. When the MLVSS proportion is low, in order to ensure nitrification, the system must maintain a high sludge age, which may lead to aging sludge and cause suspended solids to exceed the standard in the effluent.

g. Toxic Substances

Toxic substances such as strong acids, strong bases, or heavy metals in the influent can poison the activated sludge, rendering it ineffective. In severe cases, it can cause sludge disintegration, leading to failure of sedimentation and exceeding suspended solids in the effluent. The fundamental solution to this problem is to strengthen the management of upstream pollution sources.

h. Temperature

Temperature has a broad impact on the activated sludge process. First, temperature affects the activity of microorganisms in the activated sludge. In winter, when temperatures are low, treatment efficiency will decrease if no control measures are taken. Second, temperature affects the separation performance of the secondary settling tank. For example, temperature fluctuations can cause stratification in the secondary settling tank, leading to short-circuiting. A decrease in temperature increases the viscosity of activated sludge, reducing settling performance.

i. Sludge Cake Moisture Content

Currently, the primary indicator used to assess the sludge in urban sewage treatment plants is the moisture content of the sludge cake.

In many developing countries, urban sewage treatment plants currently in use or under construction generally use activated sludge processes, with relatively short sludge ages in the design and no sludge concentration or digestion facilities, resulting in a high volume of excess sludge with high organic content, making it difficult to dewater. Therefore, to control the moisture content of the sludge cake below 80%, it is necessary to increase the amount of PAM (polymer) added, which in turn increases sewage treatment costs.

To ensure effective sludge concentration and dewatering, the concentration of flocculants used in sludge dewatering should be controlled within the range of 0.1% to 0.5%. If the concentration is too low, large amounts of solution need to be added, increasing dosing frequency; if the concentration is too high, the viscosity of the flocculant increases, which can cause uneven mixing, increased resistance during dosing, and accelerated equipment wear and clogging of pipes. Additionally, the density of flocculants varies across different batches and models, requiring regular calibration and adjustments based on actual conditions to ensure proper dosage, optimal dewatering results, and reduced waste of chemicals. Furthermore, attention should be paid to preventing moisture and degradation of dry powder flocculants during storage and use.

As the demand for sludge moisture content continues to rise, sludge drying is gaining attention. Methods for sludge drying include sun drying, high-temperature drying, and adding lime as an additive. However, many places have prohibited the use of lime to reduce moisture content due to environmental concerns. Sun drying requires a large space and cannot prevent odor, while high-temperature drying consumes too much energy. Therefore, the focus is currently on improving chemical agents, dewatering machines, and processes, with more advanced techniques such as high-pressure diaphragm filtration and low-temperature drying being explored.