Comprehensive analysis of the reasons for COD exceeding the standard

my country currently chooses COD as the main assessment method in urban sewage treatment plants. It has been nearly 20 years since the 'Pollutant Discharge Standards for Urban Sewage Treatment Plants' promulgated in 2002, and new discharge standards are coming soon. In 2015, the revision draft of the standard sparked heated discussions after it was published. In the revised consultation draft, the special emission limit of COD was lowered from 50mg/L to 30mg/L, which was questioned as being too strict and causing dystocia, and has not yet been officially promulgated. If the standard is passed, it will be a new gluttonous feast for interest groups and a nightmare for environmental protection operators. After all, the current indicators cannot be effectively controlled. There are three common reasons for the abnormal COD of the effluent. 1. Abnormal content of reducing inorganic substances To clarify this problem, we must first understand what COD is. For this concept, professional books explain: COD (Chemical Oxygen Demand) refers to the amount of oxidant consumed when a certain strong oxidant is used to treat water samples 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? Why does the concept refer to reducing substances? This is a very critical issue. The reason is that in general, the reducing substances in sewage are mainly organic matter, so in the field of water treatment, the two are regarded as the same thing. In fact, the measured value of COD also includes inorganic substances such as nitrite, sulfide, and ferrous salt, which are greater than the amount of real 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 intermediate section is abnormal, resulting in inaccurate COD measurement, while the actual situation of organic substances does not fluctuate. This situation often occurs in the commissioning stage of high ammonia nitrogen wastewater, because the high ammonia nitrogen wastewater will stay for a period of short-range nitrification in the initial stage, and nitrite will accumulate, resulting in abnormal COD! 2. Problems in the biochemical system, except for a few cases of abnormal organic matter content, the most common is the abnormal biochemical system. The 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 C, N, and P ratios, the sludge activity decreases, and the decomposition of organic matter is affected; (3) The dissolved oxygen in the water fluctuates, and the sludge activity is affected, which makes the sewage COD treatment efficiency abnormal; (4) The biochemical system enters toxic substances, sludge is poisoned, and affects the treatment efficiency; (5) Excessive salt enters the biochemical system, microbial activity decreases, metabolism is abnormal, and effluent COD increases; (6) Sludge aging, biochemical system Degradation performance decreases, effluent COD increases; (7) The hydrolysis effect of the anaerobic pool in the front section becomes poor, the degradability of the influent water in the aerobic pool section becomes poor, and the effluent water is abnormal; Particles, causing COD to rise. 3. Influent impact load In addition to the above situation, the fluctuation of influent water quality and water volume will also lead to abnormal COD of effluent: (1) The influent water volume increases, and the adjustment tank does not effectively buffer the impact, resulting in shortened residence time in the biochemical section, and organic matter does not (2) The quality of influent water fluctuates, and the biochemical system does not enter a stable state in an emergency, or the system is abnormal, affecting the degradation of organic matter, and the effluent water quality is abnormal.
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