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Comprehensive analysis of the reasons for COD exceeding the standard

by:BOQU     2022-10-08

My country currently chooses the COD-based assessment method in urban sewage treatment plants. It has been almost 20 years since the 'Pollutant Discharge Standard for Urban Sewage Treatment Plants' was promulgated in 2002, and a new discharge standard is about to come out. In 2015, the draft revision of the standard caused heated discussions. In the revised draft for comments, the special COD emission limit was reduced from 50mg/L to 30mg/L. If the standard is passed, it will be a new feast for interest groups and a nightmare for environmental operators~ After all, the current indicators cannot be effectively controlled. There are three common reasons for abnormal COD in effluent.

1. Abnormal content of reducing inorganic substances

To clarify this problem, we must first understand what COD is. In response to this concept, the explanation of professional books is CODChemicalOxygenDemand. It refers to the amount of oxidant consumed when a certain strong oxidant is used to treat a water sample under certain conditions. It is an indicator of the amount of reducing substances in water.

The concept is accurate, but the question is, isn't COD used to represent organic pollutants in sewage? How does the concept say reducing substances? This is a very critical issue, the reason is: that under normal circumstances, the reducing substances in sewage are mainly organic substances, so the two are regarded as the same thing in the field of water treatment. In fact, the measured value of COD also includes inorganic substances such as nitrite, sulfide, and ferrous salt, which is greater than the real amount of reducing organic substances in sewage.

This link includes a reason for the abnormal COD of the effluent, that is, the content of reducing inorganic substances in the influent or the middle section is abnormal, resulting in an inaccurate measurement of COD, while the actual situation of organic substances does not fluctuate. This situation often occurs in the debugging stage of high ammonia nitrogen wastewater, because high ammonia nitrogen wastewater will stay for a period of time in the initial stage of short-range nitrification, resulting in accumulation of nitrite, resulting in abnormal COD!

2. There is a problem with the biochemical system

Except for a few cases of abnormal organic matter content, the most common is an abnormal biochemical system. Common factors are:

1. Temperature fluctuations cause changes in sludge activity, which in turn affects the decomposition of COD;

2. The ratio of nutrients is unbalanced, such as the long-term imbalance of the C.N.P ratio, the decrease of sludge activity, and the impact of organic matter decomposition;

3. The dissolved oxygen in the water fluctuates and the sludge activity is affected, which makes the COD treatment efficiency of the sewage abnormal;

4. The biochemical system enters the toxic substances, and the sludge is poisoned, which affects the treatment efficiency;

5. Excessive salt enters the biochemical system, the microbial activity is reduced, the metabolism is abnormal, and the COD of the effluent is increased;

6. The sludge is aging, the degradation performance of the biochemical system decreases, and the COD of the effluent increases;

7. The hydrolysis effect of the anaerobic tank in the front section becomes poor, the degradability of the influent water in the aerobic tank section becomes poor, and the effluent is abnormal;

8. The aeration is too intense, the bacterial micelles are broken, and tiny particles are released, causing the COD to rise.


In addition to the above conditions, fluctuations in the quality and quantity of the influent water will also cause abnormal COD in the effluent:

1. The amount of water inflow increases and the adjustment tank does not effectively buffer the impact, resulting in shortened residence time in the biochemical section, the organic matter is not effectively removed, and the effluent is abnormal;

2. The water quality of the influent fluctuates, the biochemical system fails to enter a stable state of an emergency, or the system is abnormal, which affects the degradation of organic matter, and the water quality of the effluent is abnormal.

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