the multi-stage activated sludge denitrification process separately removes bod5, nitrification and denitrification in several stages of reactor. first bod5 was removed by oxidation, then nitrification, and finally denitrification. each section has its own reaction tank and sedimentation tank, a separate reflux sludge and bacteria, single function, easy to adjust to the best operating conditions, to obtain the highest reaction rate. however, because of its many structures and large investment, the carbon source in sewage can not be used for denitrification, the consumption of medicine, energy consumption and operation cost are high, so it is gradually replaced by single-stage activated sludge process for denitrification.
the formulas for calculating methanol dosage in multi-stage activated sludge process are as follows:
this formula has the following limitations:
the coefficients 2.47 and 1.53 in formula (1) are calculated based on the theory of denitrification reaction formula (cod is 3.7 and 2.3). in the actual sewage treatment process, the situation is much more complicated. not only are there many beneficial and unfavorable substances for denitrification in sewage, but also the process is limited by the engineering environment conditions, and it is difficult to achieve the ideal reaction conditions. if the difference between theory and engineering practice is not taken into account, it will result in great errors.
(2) formula (1) is derived from the multi-stage activated sludge denitrification process, while the current mainstream process is single-stage activated sludge denitrification process. its ecosystem is more complex and its influencing factors are more. if the data of the multi-stage activated sludge denitrification process are manufactured by hand, obvious errors will also occur.
(3) formula (1) contains two parameters, nitrate nitrogen concentration [no3-n] and nitrite nitrogen concentration [no2-n], which need denitrification. for multi-stage activated sludge denitrification process, the parameters can be obtained by monitoring no3-n and no2-n in the influent of denitrification tank. however, for single-stage activated sludge denitrification system, the situation is much more complex. [if n o3-n and no2-n in the influent of the pre-anoxic tank are measured, the substitution formula (1) not only obtains the external carbon source, but also contains the internal carbon source in the original sewage, which is rather cumbersome to calculate and difficult to operate.
the limitation of formula (1) makes it difficult to apply to the practical engineering of single-stage activated sludge denitrification process. [for this reason, referring to german atv standard, the limitation of revised formula (1), a simple calculation method of additional carbon source suitable for single-stage activated sludge denitrification process is recommended in this paper.
a simple method for calculating the addition of external carbon sources
the unified formula is:
the main amendments to formula (4) are as follows:
(1) the amount of methanol in formula (1) is expressed by cod instead, which is beneficial to the calculation of various external carbon sources. besides methanol, the external carbon sources currently used are acetic acid, sodium acetate, glucose, etc. methanol is the most economical, but it is a flammable and explosive dangerous chemical. it is suitable for long-term use and large-dosage sewage treatment plants, and other safer ones are suitable for occasional use or small dosage. carbon source of
(2) the coefficient value of formula (1) is revised to 2.47 (cod is 3.7), and the theoretical value is revised to the value of actual engineering test. german atv standard is a guiding document for the design of single-stage activated sludge wastewater treatment plant. it stipulates that 1 kg no-3-n of denitrification should be added with external carbon source (cod) of 5 kg (equivalent to 3.33 kg of methanol). this is an empirical value derived from a large number of engineering practices and should be closer to the actual situation.
(3) all denitrification nitrogen is calculated according to nitrate nitrogen, neglecting the accumulation of nitrite nitrogen, thus simplifying the calculation. when the biological denitrification process is running steadily, nitrite accumulation will not occur in the system. in general, the nitrite concentration in the reaction tank is very low, which can be neglected. [only under special circumstances, when the system operates according to short-cut nitrification and denitrification, nitrite accumulation needs to be considered, which is not considered in general cases.]
(4) the dissolved oxygen in denitrification tank is very low, and the amount of carbon needed is very small, which can be neglected to simplify the calculation. for example, the a cell of a/o process usually controls do < 0.5mg/l, and the required additional carbon source is 0.5 x 0.87 x 1.5 = 0.65 [(cod) mg/l], which is only equivalent to the amount of additional carbon source required for 0.13mg/l nitrogen. the error is smaller than that of detection and calculation, and this has little effect on the results.
calculation of nitrogen removal by denitrification with external carbon source
1. wastewater treatment plants already in operation
2. sewage treatment plant in design
the nitrogen balance in fig. 1 regards the whole system as a closed system. the nitrogen entering the system is equal to the nitrogen discharged from the system, and the mass is conserved. the internal reflux in the system is not considered because it does not involve the in-out system, so the calculation is simplified. nitrogen assimilated into sludge is calculated as 5% of bod5 removal, i.e. 0.05 (si-se), and si, se are bod5 concentrations in influent and effluent, respectively.
the total nitrogen in influent and effluent includes various forms of nitrogen. the influent total nitrogen is mainly ammonia nitrogen and organic nitrogen. the effluent total nitrogen is mainly nitrate nitrogen and organic nitrogen denitrification. the removal of nitrogen is related to the size of anoxic tank in denitrification process and the concentration of bod5 in influent.
the concept of denitrification design parameters was proposed by atv-a131e in germany, which was defined as the ratio of denitrification nitrate nitrogen concentration to influent bod5 concentration, expressed as kde (kgno-3-n/kgbod5). from this, the denitrification removal nitrate nitrogen [no-3-n]=kdesi] could be calculated. in theory, consumption of 2.86 kgbod5 could denitrify 1 kg nitrate nitrogen, that is, kde=1/2.86=0.35 (kgno-3-n/kgbod5), but in practical engineering, it can be calculated as follows: bod5 entering the anoxic pool can not be utilized by denitrifying bacteria, only a fraction of which can be utilized. the actual kde is much less than 0.35.
the atv-a131e standard puts forward a table of denitrification design parameters which can be applied in practice by summarizing the data of practical projects. see table 2.
by substituting the above items into the nitrogen balance equation in fig. 1, it can be expressed as follows:
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