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Sewage reuse water quality index

by:BOQU     2023-03-23
When the sewage is reused as the supplementary water of the circulating cooling system, the water quality index of the recycled water should be considered in conjunction with the operation of the circulating cooling system. In the circulating cooling water system, due to the water quality of the supplementary water, the problems of scaling, corrosion and a large number of microbial reproduction usually occur. Corrosion and microbial reproduction are related, and the control of the water quality of the circulating cooling system is also solved. Start with these three questions. At present, the supplementary water of the circulating cooling system of various enterprises basically adopts clean surface water, ground water or tap water, and each has formed a relatively complete water quality stability control method. After the supplementary water is replaced with recycled sewage, problems that may occur during operation can be solved. It is obtained by analyzing the composition change of make-up water quality. In general, compared with other clean water sources, reclaimed sewage has the following characteristics: (1) Higher total dissolved solids; (2) High concentrations of COD and BOD5; (3) High concentration of ammonia nitrogen; (4) Large number of bacterial communities, The concentration of suspended solids is high. When the total dissolved solids are high, the corrosion tendency of the system will increase, and scaling may occur when the content of calcium and magnesium ions is high; A large number of multiplication in the circulation system will cause microbial slime. If the sticky scale adheres to the pipe wall or heat exchanger wall, local corrosion will occur; The reproduction of the bacteria provides a large number of inoculated bacteria, which creates conditions for the production of microbial slime. For this reason, the above indicators should be analyzed in the sewage reuse project. For the total dissolved solids in supplementary water, the control standards of each enterprise are different, the lowest is 500mg/L, and the highest is 1000mg/L. Petrochemical companies generally control it within a lower range. Studies [1] have also shown that when the total dissolved solids are in At around 850mg/L, the circulating cooling system can still operate stably. It is recommended that the upper limit of the total dissolved solids in the supplementary water of the circulating system be 1000mg/L. Desalination measures should be taken beyond this value. Regarding the COD standard, the American Water Pollution Control Association recommends a value of 75mg/L. Chinese researchers propose a standard of 40mg/L for the first class, 60mg/L for the second class, and some companies propose an index of 20mg/L. Relevant studies have shown that when the petrochemical secondary treatment sewage is reused in circulating water after advanced treatment (COD average 44mg/L), the growth and reproduction of microorganisms is similar to that of tap water, and there is no large-scale reproduction. The main reason is that the organic matter in the recycled water is not easy to be degraded by microorganisms, that is, it cannot be used as a carbon source for microbial metabolism. Therefore, there is no need to put too high requirements on the COD of the recycled water, and it is recommended to use 40mg/L. For BOD5, since it can be directly used as a microbial substrate, it is recommended to use a lower value of 5mg/L. Regarding the ammonia nitrogen index, there are two suggested values ​​at home and abroad, namely 3mg/L and 1mg/L, and 1mg/L is recommended. Studies have shown that for the reused water after advanced treatment, even if the number of heterotrophic bacteria in the supplemented water is large, compared with tap water as supplemented water, there is no large-scale proliferation of microorganisms, which can be completely controlled by using suitable fungicides, and the sewage backwater In the treatment, coagulation sedimentation + filtration is the most basic operation unit, which can remove a large number of bacteria while removing suspended solids, so there is no need to put forward special control indicators for the number of heterotrophic bacteria.
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