Is the effluent failing to meet standards? I’ve compiled all the influencing factors!

Is the effluent failing to meet standards? I’ve compiled all the influencing factors!

During wastewater treatment, a variety of effluent‑quality issues may arise, such as non‑compliant levels of COD, ammonia nitrogen, total nitrogen (TN), and suspended solids (SS). Since the underlying principles of biochemical treatment are essentially the same, this paper takes domestic wastewater as the case study to summarize the common problems encountered during operation that lead to effluent non‑compliance.

I. Excessive Levels of Organic Compounds

The primary function of the conventional activated sludge process is to remove organic pollutants from municipal wastewater; when properly designed and operated, it can achieve effluent BOD5 and suspended solids concentrations both below 20 mg/L.

The main factors affecting the efficiency of organic matter treatment are:

1 , nutrients

In typical municipal wastewater, nutrients such as nitrogen and phosphorus generally suffice to meet the requirements of microorganisms, often in substantial excess. However, when industrial wastewater constitutes a significant proportion, it is important to verify that the carbon–nitrogen–phosphorus ratio complies with the 100:5:1 guideline. If the wastewater is deficient in nitrogen, ammonium salts are typically added; if it lacks phosphorus, phosphoric acid or phosphate compounds are usually introduced.

2 , pH

The pH of municipal wastewater is neutral, typically ranging from 6.5 to 7.5. A slight decrease in pH may result from anaerobic fermentation within the sewer system. During the rainy season, more pronounced pH declines are often attributable to urban acid rain, a phenomenon that is particularly pronounced in combined sewer systems. Sudden, large‑scale pH fluctuations—whether upward or downward—are usually caused by the substantial discharge of industrial effluents. To adjust the pH of wastewater, sodium hydroxide or sulfuric acid is commonly added; however, this significantly increases wastewater treatment costs.

3 , grease

When the oil content in wastewater is high, it reduces the aeration efficiency of aeration equipment; if the aeration rate is not increased, treatment performance will decline. However, increasing aeration inevitably raises wastewater treatment costs. Furthermore, elevated oil and grease levels can impair the settling characteristics of activated sludge and, in severe cases, trigger sludge bulking, resulting in effluent suspended solids (SS) exceeding regulatory limits. For influent with high oil and grease concentrations, an oil‑removal unit should be installed in the pretreatment stage.

4 , temperature

Temperature has a broad impact on the activated sludge process. First, it affects the metabolic activity of microorganisms in the sludge; during winter, if no corrective measures are implemented, treatment performance can deteriorate. Second, temperature influences the settling efficiency of the secondary clarifier: for example, temperature fluctuations may induce density currents, leading to short-circuiting; lower temperatures increase sludge viscosity, thereby reducing its settling characteristics. Moreover, temperature changes affect the performance of the aeration system: in summer, as the saturation concentration of dissolved oxygen declines, oxygen transfer becomes more challenging, resulting in reduced aeration efficiency, while air density decreases. To maintain a constant air supply, the aeration rate must be increased.

II. Excessive Ammonia Nitrogen Levels

The removal of ammonia nitrogen from wastewater is primarily achieved by incorporating a nitrification process into the conventional activated sludge system, specifically through extended aeration to reduce the system’s organic loading.

The causes of excessive ammonia nitrogen in the effluent are multifaceted, primarily including:

1 , Sludge Loading and Sludge Age

Biological nitrification is a low‑load process, with an F/M ratio typically ranging from 0.05 to 0.15 kg BOD/kg MLVSS·d. The lower the organic loading, the more complete the nitrification, and the higher the efficiency of converting NH3‑N to NO3‑‑N. Correspondingly, biological nitrification systems generally maintain a relatively long sludge retention time (SRT), as nitrifying bacteria have a long generation cycle. If the SRT is too short—resulting in a low biomass concentration—the nitrifying bacteria cannot be adequately cultivated, and nitrification will not be achieved. The optimal SRT depends on factors such as temperature; for biological systems whose primary objective is nitrogen removal, an SRT of 11–23 days is commonly adopted.

2 , reflux ratio

The recycle ratio in biological nitrification systems is generally higher than that in conventional activated sludge processes, primarily because the mixed liquor in these systems already contains substantial amounts of nitrate. If the recycle ratio is too low, the activated sludge would remain in the secondary clarifier for an extended period, increasing the likelihood of denitrification and resulting in sludge bulking or floating. Typically, the recycle ratio is maintained within the range of 50% to 100%.

3 , Hydraulic Retention Time

The hydraulic retention time in a biological nitrification aeration tank is also longer than that of the activated sludge process, typically requiring at least 8 hours or more. This is primarily because the nitrification rate is much lower than the removal rate of organic pollutants, necessitating a longer reaction time.

4 , BOD5/TKN

TKN It refers to the sum of organic nitrogen and ammonium nitrogen in water. In influent wastewater, the BOD5/TKN ratio is an important factor influencing nitrification performance. The higher the BOD5/TKN ratio, the smaller the proportion of nitrifying bacteria in the activated sludge, resulting in a lower nitrification rate and reduced nitrification efficiency under identical operating conditions; conversely, a lower BOD5/TKN ratio leads to higher nitrification efficiency. Operational experience from many municipal wastewater treatment plants has shown that the optimal BOD5/TKN range is approximately 2 to 3.

5 , nitrification rate

A specific process parameter in biological nitrification systems is the nitrification rate, which refers to the amount of ammonium nitrogen converted per unit mass of activated sludge per day. The magnitude of the nitrification rate depends on factors such as the proportion of nitrifying bacteria in the activated sludge and temperature; a typical value is 0.02 g NH3‑N/g MLVSS·d.

6 , dissolved oxygen

Nitrifying bacteria are obligate aerobes; they cease all metabolic activity in the absence of oxygen. Moreover, their oxygen uptake rate is significantly lower than that of heterotrophic bacteria that degrade organic matter. If adequate oxygen levels are not maintained, nitrifying bacteria will be unable to secure the oxygen they require. Therefore, it is essential to keep the dissolved oxygen concentration in the aerobic zone of the biological reactor above 2 mg/L, and under special circumstances, this level should be further increased.

7 , temperature

Nitrifying bacteria are also highly sensitive to temperature changes: when wastewater temperatures fall below 15°C, the nitrification rate declines markedly, and at temperatures below 5°C, their physiological activity ceases entirely. Consequently, during winter, effluent ammonia‑nitrogen levels often exceed regulatory limits, particularly in wastewater treatment plants located in northern regions.

8 , pH

Nitrifying bacteria are highly sensitive to pH; their biological activity is greatest within the pH range of 8 to 9. When the pH falls below 6.0 or exceeds 9.6, nitrifying bacterial activity is inhibited and eventually ceases. Therefore, the mixed liquor pH in biological nitrification systems should be maintained above 7.0 whenever possible.

III. Total Nitrogen Exceedance

Wastewater denitrification builds upon the biological nitrification process by incorporating a biological denitrification stage. In this stage, nitrate in the wastewater is reduced to nitrogen gas through a biochemical reaction catalyzed by microorganisms under anoxic conditions.

The causes of total nitrogen exceeding the effluent standard are multifaceted, primarily including:

1 , Sludge Loading and Sludge Age

Since biological nitrification is a prerequisite for biological denitrification, only robust nitrification can ensure efficient and stable denitrification. Consequently, nitrogen‑removal systems must operate at low or ultra‑low organic loading rates and maintain a long sludge retention time.

2 , internal and external recirculation ratio

In biological denitrification systems, the external return flow is lower than in purely biological nitrification systems. This is primarily because most of the nitrogen in the influent has already been removed, resulting in relatively low NO3⁻‑N concentrations in the secondary clarifier. Consequently, the risk of sludge bulking due to denitrification in the secondary clarifier is significantly reduced. Moreover, denitrification sludge settles more rapidly; therefore, while maintaining the required return sludge concentration, the return ratio can be lowered, thereby extending the hydraulic retention time of wastewater in the aeration tank.

In a well‑operating wastewater treatment plant, the external reflux ratio can be maintained below 50%, while the internal reflux ratio is typically kept within the range of 300% to 500%.

3 , denitrification rate

The denitrification rate refers to the amount of nitrate reduced per unit mass of activated sludge per day. It is influenced by factors such as temperature, with typical values ranging from 0.06 to 0.07 g NO3⁻‑N/g MLVSS·d.

4 Dissolved oxygen in the hypoxic zone

For denitrification, it is desirable to keep the dissolved oxygen (DO) as low as possible—ideally at zero—so that denitrifying bacteria can carry out denitrification “at full capacity,” thereby enhancing nitrogen removal efficiency. However, in actual plant operations, maintaining DO in the anoxic zone below 0.5 mg/L remains challenging, which in turn compromises the biological denitrification process and ultimately affects the effluent total nitrogen concentration.

5 , BOD5/TN

Because denitrifying bacteria carry out denitrification during the degradation of organic matter, sufficient organic carbon must be present in the wastewater entering the anoxic zone to ensure the smooth progression of denitrification. However, due to the lagging development of supporting pipeline networks at many wastewater treatment plants, the influent BOD5 often falls below design values, while nitrogen and phosphorus concentrations are equal to or exceed design levels. This mismatch results in an insufficient carbon source in the influent to meet the carbon requirements of denitrification, frequently leading to effluent total nitrogen exceeding regulatory limits.

6 , pH

Denitrifying bacteria are less sensitive to pH changes than nitrifying bacteria; they can maintain normal physiological metabolism across a pH range of 6 to 9. However, the optimal pH for biological denitrification lies between 6.5 and 8.0.

7 , temperature

Although denitrifying bacteria are less sensitive to temperature fluctuations than nitrifying bacteria, their denitrification performance still varies with temperature. Higher temperatures promote a faster denitrification rate, which reaches its peak at 30–35°C. When the temperature falls below 15°C, the denitrification rate declines markedly, and at 5°C, denitrification virtually ceases. Therefore, to maintain effective nitrogen removal during winter, it is necessary to increase the sludge retention time (SRT), raise the sludge concentration, or operate additional treatment units.

IV. Total Phosphorus Exceedance

Phosphorus removal in municipal wastewater treatment plants primarily relies on biological phosphorus removal: an anaerobic zone is added upstream of the aerobic zone, allowing polyphosphate‑accumulating organisms to alternate between anaerobic and aerobic conditions, thereby facilitating the release and uptake of phosphate. Phosphorus is then removed by discharging excess sludge. When biological phosphorus removal fails to meet effluent standards, chemical agents may be dosed to provide supplementary phosphorus removal. Chemical phosphorus removal typically involves coagulation, sedimentation, and filtration to convert phosphorus into insoluble solid precipitates, which are subsequently separated from the wastewater.

The reasons for total phosphorus exceeding the discharge standard in biological phosphorus removal systems are multifaceted, primarily including:

1 , Sludge Loading and Sludge Age

The anaerobic–aerobic biological phosphorus removal process is a high F/M, low SRT system. When the F/M ratio is high and the SRT is low, the amount of excess sludge discharged is correspondingly greater. Consequently, under conditions where the phosphorus content of the sludge remains constant, the amount of phosphorus removed increases, leading to improved phosphorus removal efficiency.

For biological systems whose primary objective is phosphorus removal, the F/M ratio typically ranges from 0.4 to 0.7 kg BOD5/kg MLSS·d, and the SRT is generally 3.5 to 7 days. However, the SRT should not be set too low; it must be maintained at a level that ensures effective BOD5 removal.

2 , BOD5/TP

To ensure effective phosphorus removal, the BOD5/TP ratio in the influent to the anaerobic zone should be maintained above 20. Since polyphosphate‑accumulating organisms belong to the genus *Bacillus* and exhibit relatively weak physiological activity, they can only assimilate the most readily degradable fractions of organic matter. Therefore, the influent must contain an adequate level of BOD5 to support the normal physiological metabolism of these microorganisms. However, in many municipal wastewater treatment plants, the actual influent often exhibits a low carbon source and elevated concentrations of nitrogen and phosphorus, resulting in a BOD5/TP ratio that fails to meet the requirements for biological phosphorus removal and thereby compromising its effectiveness.

3 , dissolved oxygen

The anaerobic zone must be maintained under strictly anaerobic conditions, with dissolved oxygen levels below 0.2 mg/L, so that polyphosphate‑accumulating organisms can effectively release phosphorus and ensure the efficiency of subsequent treatment. In contrast, the dissolved oxygen in the aerobic zone should be kept above 2.0 mg/L to enable these organisms to efficiently take up phosphorus. Consequently, inadequate control of dissolved oxygen in both the anaerobic and aerobic zones can significantly compromise biological phosphorus removal. Furthermore, some wastewater treatment plants receive influent from river channels, which often contains relatively high dissolved oxygen. If such influent is fed directly into the anaerobic zone, it can hinder the maintenance of anaerobic conditions and impair the phosphorus‑release capacity of polyphosphate‑accumulating bacteria.

4 , reflux ratio

The return ratio in an anaerobic–aerobic phosphorus removal system should not be too low; a sufficient return ratio must be maintained to promptly remove sludge from the secondary clarifier, thereby preventing polyphosphate‑accumulating organisms from releasing phosphorus under anaerobic conditions in the clarifier. While ensuring rapid sludge discharge, the return ratio should be minimized as much as possible to avoid shortening the actual hydraulic retention time of the sludge in the anaerobic zone, which could compromise phosphorus release.

In an anaerobic–aerobic phosphorus removal system, if the sludge exhibits good settling characteristics, a return ratio within the range of 50% to 70% is sufficient to ensure rapid sludge discharge.

5 , Hydraulic Retention Time

The hydraulic retention time of wastewater in the anaerobic zone typically ranges from 1.5 to 2.0 hours. If the retention time is too short, on the one hand, effective phosphorus release cannot be ensured; on the other hand, the facultative acidogenic bacteria in the sludge are unable to fully degrade the high-molecular-weight organic compounds in the wastewater into lower‑chain fatty acids for uptake by polyphosphate‑accumulating organisms, thereby also impairing phosphorus release.

The hydraulic retention time of wastewater in the aerobic zone is typically 4 to 6 hours, which ensures sufficient phosphorus removal.

6 , pH

Low pH favors phosphorus release, while high pH promotes phosphorus uptake; the overall phosphorus removal efficiency is the net result of these competing processes. Therefore, in biological phosphorus removal systems, it is advisable to maintain the mixed liquor pH within the range of 6.5 to 8.0.

As the requirements for total phosphorus in effluent continue to tighten, chemical phosphorus removal is being increasingly adopted alongside biological phosphorus removal. However, while chemical phosphorus removal enhances phosphorus removal efficiency, the addition of chemical agents also leads to a substantial increase in excess sludge production, thereby raising both the volume of sludge requiring treatment and the amount of dewatered sludge that must be disposed of.

In practice, the dosing point and dosage of chemical agents should be determined on the basis of experimental data and adjusted promptly to ensure that the effluent phosphorus concentration consistently meets regulatory standards while minimizing chemical consumption.

V. Excessive Suspended Solids

Whether the suspended solids in the effluent meet the standard largely depends on the quality of the biological system’s sludge, the settling performance of the secondary clarifier, and the adequacy of process control at the wastewater treatment plant. The main reasons for exceeding the suspended solids limit in the secondary clarifier effluent are as follows: the appropriateness of the design parameters for the secondary clarifier is a critical factor determining whether the suspended solids in the effluent will exceed the regulatory threshold. In many urban wastewater treatment plants, to reduce construction costs, designers often significantly shorten the hydraulic retention time and maximize the hydraulic surface loading. This practice frequently leads to sludge bulking during operation, resulting in elevated suspended solids levels in the effluent.

In addition, when certain wastewater treatment plants, due to operational adjustments, need to maintain a relatively high sludge concentration in the biological tanks, this can lead to an excessive solids surface loading in the secondary clarifiers, thereby compromising effluent quality. Consequently, it is generally recommended to allow substantial margins in the design and operation of these secondary clarifier parameters, facilitating process control and flexibility at the treatment plant.

In general, the primary process parameters that influence the settling performance of a sedimentation tank are hydraulic retention time, hydraulic surface loading, and sludge solids flux.

1 , Hydraulic Retention Time of the Secondary Sedimentation Tank

The hydraulic retention time of wastewater in the secondary sedimentation tank is a critical operational parameter. Only with an adequate retention time can effective flocculation be ensured and high settling efficiency achieved. Therefore, it is recommended to set the hydraulic retention time of the secondary sedimentation tank at approximately 3 to 4 hours.

2 , Hydraulic surface loading of the secondary sedimentation tank

For a sedimentation tank, when the inflow rate is constant, the size of the particles it can remove is also fixed. Among the particles that can be removed, the smallest particle has a settling velocity equal to the tank’s hydraulic surface loading. Consequently, the lower the hydraulic surface loading, the more particles can be removed, the higher the sedimentation efficiency, and the lower the suspended solids concentration in the effluent. Designing secondary clarifiers with a relatively low hydraulic surface loading promotes the effective settling of sludge and other suspended solids. It is generally recommended to maintain the hydraulic surface loading of secondary clarifiers within the range of 0.6 to 1.2 m³/m²·h.

3 , Solid surface loading of the secondary sedimentation tank

The solids surface loading of the secondary clarifier is another critical factor influencing its sedimentation performance. A lower solids surface loading results in more effective sludge thickening in the clarifier, whereas a higher loading leads to poorer thickening. Excessively high solids surface loading can cause the sludge blanket to rise excessively, allowing many sludge flocs to exit with the effluent before settling and thereby compromising the suspended solids concentration in the treated water. In general, the maximum allowable solids surface loading for a secondary clarifier should not exceed 150 kg MLSS/m²·d.

4 , Activated sludge quality

The quality of activated sludge is a key factor determining whether the effluent’s suspended solids exceed regulatory limits. High-quality activated sludge is characterized by four main attributes: excellent adsorption capacity, high biological activity, good settling performance, and effective thickening capability.

Colloidal pollutants must first be adsorbed onto the activated sludge flocs and then further adsorbed near the bacterial cell surfaces before they can be metabolized; consequently, activated sludge with poor adsorption capacity exhibits limited ability to remove colloidal contaminants. The biological activity of activated sludge refers to the capacity of microorganisms within the sludge flocs to degrade and metabolize organic pollutants; activated sludge with low biological activity will inevitably degrade organic pollutants at a slower rate.

Only activated sludge with good settling characteristics can achieve effective solid–liquid separation in the secondary clarifier. Conversely, if the sludge’s settling performance deteriorates, the separation efficiency will inevitably decline, resulting in turbid effluent from the secondary clarifier and elevated suspended solids (SS) levels; in severe cases, this may even lead to substantial loss of activated sludge, reducing the biomass within the system and, in turn, impairing the degradation and metabolic removal of organic pollutants. Furthermore, only activated sludge with strong thickening properties can attain a high sludge concentration in the secondary clarifier. If thickening performance is poor and the sludge concentration at discharge is low, it becomes necessary to maintain an adequate return sludge flow rate by increasing the reflux ratio. However, raising the reflux ratio shortens the actual hydraulic retention time of wastewater in the aeration tank, leading to insufficient aeration duration and compromising treatment efficiency.

5 , Influent SS/BOD5

The proportion of MLVSS in the activated sludge of a biological system is closely related to the influent SS/BOD5 ratio: when the influent SS/BOD5 is high, the MLVSS fraction in the sludge is low, and vice versa. Based on operational experience, when the SS/BOD ratio is below 1, the MLVSS fraction can be maintained at 50% or higher; however, when the SS/BOD5 ratio exceeds 5, the VSS fraction may drop to 20–30%. When the MLVSS fraction in the activated sludge is low, achieving adequate nitrification requires maintaining a relatively long sludge retention time, which can lead to pronounced sludge aging and result in effluent SS exceeding regulatory limits.

6 , toxic substances

When influent wastewater contains toxic substances such as strong acids, strong bases, or heavy metals, it can poison the activated sludge, impairing its treatment efficiency. In severe cases, this may lead to sludge disintegration, preventing proper settling and resulting in effluent suspended solids exceeding regulatory limits. The fundamental solution to activated‑sludge poisoning is to strengthen management of upstream pollution sources.

7 , temperature

Temperature has a broad impact on the activated sludge process. First, it affects the activity of microorganisms in the activated sludge; during winter, if no control measures are implemented, treatment efficiency will decline. Second, temperature influences the settling performance of the secondary clarifier. For example, temperature fluctuations can induce density currents in the clarifier, leading to short-circuiting; and as temperatures drop, the increased viscosity of the activated sludge can impair its settling characteristics.