Main Factors Affecting COD Removal Performance in Wastewater Treatment
The main factors affecting COD removal performance include:
1. Nutrients
In general municipal wastewater, nutrients such as nitrogen and phosphorus are usually sufficient—and often excessive—for microbial growth. However, when industrial wastewater accounts for a large proportion, it is necessary to check whether the carbon, nitrogen, and phosphorus (C:N:P) ratio meets the typical requirement of 100:5:1.
If nitrogen is deficient, ammonium salts are commonly added; if phosphorus is deficient, phosphoric acid or phosphate salts are usually supplemented.
2. pH
Wastewater pH is typically near neutral, generally in the range of 6.5–7.5. A slight decrease in pH may result from anaerobic fermentation in wastewater conveyance pipelines. During the rainy season, a significant pH drop is often caused by urban acid rain, especially in combined sewer systems.
Sudden and large pH fluctuations—whether increases or decreases—are usually due to the discharge of large volumes of industrial wastewater. pH adjustment is commonly achieved by dosing sodium hydroxide or sulfuric acid, but this will significantly increase treatment costs.
3. Oil and Grease
High concentrations of oil and grease in wastewater can reduce the aeration efficiency of aeration equipment. Without increasing aeration, treatment performance will decline; however, increasing aeration inevitably raises operating costs.
In addition, high oil and grease content can impair the settling performance of activated sludge and, in severe cases, cause sludge bulking, leading to excessive effluent suspended solids (SS). For influent with high oil content, oil removal facilities should be added in the pretreatment stage.
4. Temperature
Temperature has a wide-ranging impact on the activated sludge process. First, it affects microbial activity in activated sludge; during winter, low temperatures can reduce treatment efficiency if no control measures are taken.
Second, temperature influences the separation performance of secondary clarifiers. For example, temperature changes may induce density currents in sedimentation tanks, causing short-circuiting. A decrease in temperature increases sludge viscosity, reducing settling performance.
Temperature variations also affect aeration system efficiency. In summer, higher temperatures reduce the saturation concentration of dissolved oxygen, making oxygen transfer more difficult and lowering aeration efficiency. At the same time, higher temperatures reduce air density; to maintain the same air supply rate, the airflow must be increased.
