Improving COD Degradation Efficiency in Wastewater Treatment: Key Environmental Factors

Always having trouble with COD issues!? Inefficient biological pool treatment efficiency!?

 

Adjustment small tips you shouldn't miss~Effectively enhance COD degradation capability!

 

In many factories or industrial areas, various production processes can be found with biological treatment pools installed in the end wastewater treatment system, primarily aimed at reducing the COD in discharged water. COD emission standards vary from place to place, with direct discharge requiring levels below 100mg/L. In industrial areas, factories typically have large integrated biological treatment pools, allowing for higher COD emission concentrations, mostly between 500-600mg/L. Among the entire treatment process, only the biological treatment pool usually has the function of treating COD. This means that if the biological treatment pool fails, the COD treatment efficiency will drop sharply, highlighting the importance of this biological treatment unit.

 

The difficulty lies in the fact that biological treatment requires adjustment of more parameters compared to other chemical treatment methods, and there is also a wide range of variables, thereby increasing the complexity of this treatment unit. The following is a brief introduction to biological treatment, along with variations and tips to be aware of in operation. It is hoped that this will help managers who are troubled by improper operation of their own biological treatment pools or by low COD treatment efficiency.

 

▲Improper operation of the biological treatment pool can lead to unsatisfactory COD treatment efficiency.

 

1.Understanding the Biological Treatment Form (also known as Secondary Biological Treatment)

 

Wastewater treatment technologies can be classified into primary, secondary, and tertiary treatment. Secondary treatment refers specifically to biological treatment. Biological treatment methods include aerobic and anaerobic biological treatment (as shown in the diagram below). The biological treatment of wastewater relies on naturally occurring microorganisms that metabolize toxic organic substances in the environment into stable inorganic substances. By employing specific techniques, a conducive environment for microbial survival and reproduction is created, accelerating microbial growth and metabolic functions. This facilitates the removal of organic pollutants from wastewater, achieving effective degradation of COD or even recycling to realize the vision of zero discharge and zero pollution.

 

▲Determining the design type of the biological pool first helps guide subsequent improvements.

 

2.Sometimes, situations or things worsen before they improve. Domesticating activated sludge is no exception...

 

When deciding to use a biological treatment process to effectively degrade COD and address organic pollutants in wastewater, it is helpful to understand the approximate biodegradability of the wastewater. In simple terms, you can observe the concentration ratio of BOD/COD. If the ratio is above 25%, it indicates that the wastewater is biodegradable and you can consider initiating the domestication of activated sludge.

 

The most common biological treatment unit usually consists of four main parts: the aeration tank, sedimentation tank, sludge recirculation pipeline, and excess sludge disposal pipeline. Typically, organic wastewater undergoes pretreatment (such as screening, aeration, or vortex sand settling tank, and wastewater equalization tank) and primary settling tank/chemical coagulation sedimentation tank treatment before flowing into the aeration tank of the activated sludge system for further oxidation/degradation of residual colloidal and dissolved organic matter. When wastewater containing such residual organic matter flows into the aeration tank and under conditions of sufficient oxygen supply, nutrients, and absence of harmful substances, organic matter is first adsorbed by aerobic microorganisms, leading to the degradation of COD and BOD, forming stable substances (CO₂, H₂O, NH₃), and proliferating new microorganisms. The mixed sludge then flows into the sedimentation tank for solid-liquid separation. Most of the sedimentation sludge is recycled to the aeration tank (referred to as return sludge) to maintain an appropriate sludge volume in the aeration tank. Only a small portion of excess sludge needs to be disposed of (referred to as waste sludge) to maintain the proper sludge age. The disposed sludge requires separate sludge treatment/final disposal.

 

3. What are the environmental factors to control in a biological treatment pool?

 

How can we enhance the ability of a biological pool to degrade COD? The key to COD degradation in the biological pool lies in the quality of sludge domestication. In the initial stages of sludge domestication, the following situations may occur: 1. The concentration of COD does not decrease but increases. 2. Suspended solids in the water increase. 3. The water quality appears worse than before. At this point, it is important to examine each environmental factor affecting sludge domestication to determine the cause of these situations.

 

• Dissolved Oxygen: In aerobic activated sludge processes, insufficient oxygen supply can lead to anaerobic conditions, hindering normal metabolic processes and promoting filamentous bacteria growth. Generally, a dissolved oxygen concentration of around 2mg/L is recommended.
• Nutrients: Microbial metabolism requires certain proportions of nutrients. Besides the carbon source indicated by COD, nitrogen, phosphorus, and other trace elements are also needed. The recommended addition ratio for nitrogen and phosphorus is approximately C:N:P = 100:5:1.
• pH Value: For aerobic activated sludge processes, a pH range of 6.5-8.5 is optimal. pH values below 6.5 may lead to fungal competition with bacteria, with fungi gaining dominance below pH 4.5, severely affecting sedimentation. pH values above 9.0 may also impact microbial metabolic rates.
• Temperature: Water temperature is a critical factor influencing microbial growth activities. Generally, temperatures between 20-30°C are considered optimal for growth. Microbial metabolic processes start to be affected above 35°C and below 10°C.
• Toxic Substances: There are many substances toxic to microorganisms, including heavy metals, cyanides, phenols, and other organic compounds. These substances can damage microbial physiological structures or inhibit metabolic processes, resulting in poor COD degradation efficiency.

 

In summary:

 

If the biological pool has been operating for some time, and the odor gradually disappears, the sludge begins to loosen and float on the water surface, it is highly likely the first step of an effective response. Although it may temporarily increase TSS and ammonia nitrogen, with continued stable domestication, a significant improvement in COD degradation efficiency is imminent.

 

The primary purpose of wastewater treatment is to transform harmful substances in water into harmless ones. However, there isn't a single operation method or condition that can effectively handle complex organic compounds and degrade COD in industrial wastewater. Choosing appropriate methods and conditions to address complex and challenging industrial wastewater issues is a crucial aspect. Only through comprehensive understanding and experimentation can one find the most effective biological treatment conditions suitable for their own factory, achieving the expected COD treatment efficiency.

 

▲An oxidation tank that is not properly adjusted is prone to generating a large amount of suspended solids and unbearable odors, resulting in poor COD treatment effectiveness.

 

▲A well-adjusted oxidation tank results in reduced suspended solids, disappearance of odors, and high COD degradation efficiency.