Common causes and solutions of excessive COD

Pay more attention to the public account: environmental protection water treatment

The quality of the influent water causes the COD of the effluent to exceed the standard

The influent water quality mainly includes factors such as influent pH, low water temperature, organic matter concentration, suspended solids, and the presence of refractory or inhibitory components.

1. Inlet water pH

Too high or too low the pH of the influent water will affect the biochemical system, resulting in the normal operation of the biochemical system or even the collapse of the system, and the microorganisms and denitrifying bacteria have no suitable living environment, which will inevitably lead to the decline of the system's ability to treat water quality, the deterioration of the treated water quality, and the increase of various indicators of the effluent. Therefore, when the pH of the influent water of the sewage treatment plant is too high or too low, the following measures should be taken in time:

Wastewater is neutralized in the pre-treatment or primary treatment stage, pH is detected along the sewage network, and abnormal pipeline segments are neutralized at the same time.

In the pretreatment and primary treatment stages, the wastewater is continuously circulated internally to prevent incomplete neutralization, and the influent water is slowly restored after the neutralization adjustment is completed.

If it is judged that the wastewater with abnormal pH is about to affect the biochemical system, the return flow can be increased, which is equivalent to using the wastewater from the sedimentation tank to dilute the pH and reduce its impact on the biochemical stage.

2. The water temperature is too low

Too low water temperature will greatly reduce the activity of various microorganisms, and the pollutant indicators led by ammonia nitrogen will bear the brunt of the rising concentration trend, followed by total nitrogen, COD, etc.

Therefore, in order to minimize the impact of water temperature and ensure that the effluent water quality meets the standard, the following measures can be taken:

Starting from around mid-November each year, the sludge concentration can be slowly increased by gradually reducing the amount of sludge discharge, and the treatment effect of the biochemical treatment stage can be ensured by increasing the number of bacteria in the activated sludge.

When the water temperature is too low, the water inflow of the biochemical system can also be appropriately reduced, the reflux ratio can be reduced, and the residence time of wastewater in the biochemical stage can be increased.

3. Concentration of organic matter

The influent water quality changes and the organic matter concentration is too high, which in turn has a great impact on the activated sludge. When encountering high load, it will be found that the white foam of the biochemical tank increases, and the value of the on-line COD detection instrument of effluent increases, and when the sludge sedimentation ratio is made, it will be found that the sludge sedimentation performance decreases and the supernatant is turbid.

The removal of organic matter is reduced, the dissolved oxygen in the aerobic zone is reduced, and the protozoa are increased when the laboratory personnel observe the biomicroscopy. At this time, the water inlet of the biochemical system should be greatly reduced in time, and the water inlet can be stopped if conditions permit, the reflux ratio should be reduced, the aeration volume should be increased, and the system should be restored by suffocating the system.

4. There are refractory (or inhibitive) components in the influent

It is found that the COD of the effluent is increased, and some peers will do a stuffy exposure test: take about 50L of the mixture of the biochemical pool, first take a small amount of mixed liquid to precipitate, take the supernatant to filter and test the COD concentration without aeration test, and then through the laboratory small aerator has been suffocating, simulate increasing the residence time of the biochemical system, and take a small amount of mixed solution to precipitate and test the COD concentration every 4 hours.

Under the influence of organic load, the COD removal rate of the supernatant was low within 24-48 hours, and after 48 hours, the COD removal rate of the supernatant could reach 50% and continue to decrease steadily. However, if there is no change in the COD removal rate at 72 hours, it is necessary to suspect whether the system has entered the inhibitory substances that are refractory to degradation or have high concentrations. In most cases, we will find that the proportion of BOD is relatively low, even B/C is less than 0.20, and some refractory organic compounds have a certain inhibitory effect on activated sludge, which also has an impact on the sludge-water separation of activated sludge, which is manifested as turbidity of supernatant.

When affected by this type of wastewater, it is necessary to strengthen the operation and management of the anaerobic biochemical treatment process. In addition, we can adsorb such organic matter by adding activated carbon.

5. The suspended solids are too high

It is worth mentioning that this problem is not necessarily caused by external factors, and it may also be that the sludge precipitated in the primary treatment section is too much sludge and the sludge is not treated in time, and the mud layer is too high and enters the biochemical system with the wastewater.

Process control factors, resulting in excessive COD in effluent

The influencing factors of wastewater treatment process control mainly include dissolved oxygen, reflux ratio, sludge concentration, etc.

1. Dissolved oxygen

The AAO process is generally controlled below 0.2mg/L in the anaerobic section, 0.5mg/L in the anoxic section (the internal reflux ratio is controlled), and the aerobic section is controlled at 2-3mg/L.

In the usual work, we occasionally have an increase in the amount of influent water or an increase in the concentration of COD in the influent water, and the operators are busy with on-site affairs and do not find it in time or do not make adjustments for a long time, resulting in the dissolved oxygen in the aerobic zone is too low, even lower than 0.5mg/L, and finally the ammonia nitrogen, COD and other indicators of the effluent exceed the standard.

This requires our operators to have a certain sense of responsibility and business skills, as far as possible to eliminate the occurrence of such phenomena; once this situation occurs, we should increase the frequency of the fan or the amount of oxygen in time, and appropriately reduce the amount of water inflow, if this phenomenon is maintained for a long time, it is necessary to sample and analyze the secondary sedimentation tank or subsequent sections, and the water quality exceeds the standard and the water needs to stop the inlet and return the wastewater to treatment.

2. Reflux ratio

The reflux of sludge ensures the sludge concentration of the biochemical system, which also ensures the balance of microbial flora, and when the water quality is abnormal, the reflux ratio is controlled to maximize the residence time of wastewater in the system, and the wastewater is degraded more fully by using microorganisms.

Under normal circumstances, the sludge reflux ratio is generally controlled by 40%~70%, the reflux ratio is reduced, the residence time of sludge at the bottom of the secondary sedimentation tank is increased, and the concentration of reflux sludge is higher, and the sludge activity becomes larger, which increases the ability to degrade and adsorb organic matter;

It is advisable to control 200% of the reflux in the nitrifying solution to ensure that the dissolved oxygen in the anoxic zone is less than 0.5mg/L, so as to ensure the release of phosphorus in the anaerobic zone and the denitrification and denitrification in the anoxic zone.

3. Sludge concentration

The appropriate sludge concentration is the guarantee for the stability of the sewage treatment system. According to the influent concentration and seasonal variation, it is generally believed that the MLSS control range is between 3000-5000mg/L, which is sufficient to cope with the treatment of daily municipal wastewater.

Industrial wastewater needs to determine the appropriate sludge concentration according to the water quality of the sewage, generally not more than 10000mg/L, because the higher the sludge concentration, the greater the corresponding energy consumption ratio.

During the day-to-day operation, the wastewater treatment operators pay more attention to the MLVSS value. Because MLVSS can more intuitively reflect the amount of activated sludge, the results have excluded the influence of inorganic substances in activated sludge, and it generally accounts for about 0.6-0.7 of MLSS.

If the sludge concentration is increased due to suspended particles, as mentioned above, the proportion of MLVSS will be less than 50%, and the activity is poor, and the operator will carry out a large amount of sludge discharge according to the data of excessive sludge concentration, which will overwhelm the system and lead to water quality accidents that exceed the water quality standard.

Therefore, our daily operation management personnel should keep abreast of the MLVSS value, and be familiar with the comparison value with MLSS, generally there will be a relatively stable ratio, and there will be large fluctuations, especially when it is reduced to below 0.5, it is necessary to pay enough attention, because the inorganic components in the sludge are too high, and the reason for this phenomenon needs to be analyzed.