AU2015230864A1 - Method for measuring concentration of dissolved organic nitrogen of secondary effluent - Google Patents

Abstract

METHOD FOR MEASURING CONCENTRATION OF DISSOLVED ORGANIC NITROGEN OF SECONDARY EFFLUENT The invention discloses a method for measuring a concentration of DON of a secondary effluent, which belongs to the field of municipal sewage treatment. The method includes: charting the standard curve, establishing the mathematical model, and measuring the concentration of the DON in the secondary effluent. Based on establishment of the mathematical model among the concentration of the DON in the sewage and the contents of the protein and the HA therein, the concentration of the DON in the secondary effluent is calculated by the directly measured contents of the protein and the HA therein, and besides the constituents of the DON are provided, which reflects the treating effect of the wastewater treatment plant. The method of the invention features in accurate measurement, convenient operation, and strong applicability. ct) 'Co.~ cl ctf -o ct ot *~C 0 - o c - 0 - ~ I)~ * _ _ _ _ _ _ -ct --- J L -j - -- -----------------------------------e- ------

Description

METHOD FOR MEASURING CONCENTRATION OF DISSOLVED ORGANIC NITROGEN OF SECONDARY EFFLUENT FIELD OF THE INVENTION [0001] The invention relates to the field of municipal sewage treatment, and more particularly to a method for measuring a concentration of dissolved organic nitrogen (DON) of a secondary effluent. BACKGROUND OF THE INVENTION [0002] DON generally refers to organic nitrogen left in the water sample after filtration by the filter membrane having the pore size of 0.45 jim. The DON is widely distributed in the biochemically treated secondary effluent and is a part of the dissolved organic matters. With the continuous upgrading and development of the wastewater treatment plant, a large amount of the inorganic nitrogen can be removed by the nitration-denitrification in the sewage treatment process. By contrast, the DON is difficult to be removed in the sewage treatment process, which results in rising ratio of the DON to the total nitrogen in the secondary effluent. Thus, DON is an important part of the total nitrogen in the effluent and easily results in eutrophication of lakes and rivers. The DON may also result in the increase consumption of chlorine, membrane pollution, and production of disinfection byproducts. The wastewater treatment plant is the important places to degrade and discharge the DON, and therefore, the monitoring of the DON in the sewage is critical. [0003] The current measurement of the DON in the sewage is realized mainly based on calculating the difference value between the total dissolved nitrogen (TDN) and the dissolved inorganic nitrogen (DIN) including ammonia nitrogen (NH), nitrate nitrogen 1 (N03-), and nitrite nitrogen (NO2). For most of the secondary effluent from the wastewater treatment plant, the measurement is disturbed due to the existence of a certain amount of N0 3 _. Some researchers hold the opinion that when DIN/TDN>0.6, errors occur in the measurement of the DON. In order to tackle this problem many researchers propose to firstly remove the DIN and then measure the TDN so as to improve the accuracy of the measurement of the DON in the water. Lee et al. has adopted the dialysis to remove the DIN and the removal rate thereof 24 hrs later reaches 70% (Lee, W., Westerhoff, P., Dissolved organic nitrogen measurement using dialysis pretreatment, Environ. Sci. Technol., 2005, 39, 879-884). Currently, dialysis membrane (100 Daltons) has been adopted to remove the DIN, and this method has a recovery rate of the DON of the upland water and the drinking water exceeds 95%, thereby being effectively applicable for analysis of the DON in those upland water and drinking water with DIN/TDN>O.6. However, such the method causes 10% DON loss of the sewage and features in high cost in the apparatus and the dialysis membrane and long time (at least 24 hrs) consumption. [0004] In order to shorten pre-treatment time, some researchers conduct the removal of the DIN in the sewage by electrodialysis which costs a duration of 0.5 hr, the removal rate of the DIN reaches >90%, and the loss of the DON is 6% (Chon, K., et al., Quantification and characterization of dissolved organic nitrogen in wastewater effluents by electrodialysis treatment followed by size-exclusion chromatography with nitrogen detection. Water Res, 2013, 47(14): 5381-5391). It is known from the patent search that Chinese Patent Application No. 201010022653.1 has disclosed a method for measuring the concentration of the DON in water. Such the method realizes the measurement of the DON using the nanofiltration membrane technology; in the pretreatment process, the selective nanofiltration membrane having the molecular weight cutoff of 150-500 Dalton is adopted to perform the dead-end flow filtration or the cross flow filtration to enable the concentration enrichment of the DON to reach 6-15 folds. Although the electrodialysis 2 and the nanofiltration saves the time for the pretreatment, the DON can only be calculated based on the four indicators including the TN, NH N03-, and NO2; besides, the whole measuring process is troublesome and time-consuming, high-priced in the pretreatment, and the operators are highly required. Thus, a fast, simple, and economic measuring method is required applicable for sewage in need of multiple times and a large amount of measurements. SUMMARY OF THE INVENTION [0005] In view of the above problems, such as the high production cost, time-consuming, and troublesome operation process, existing in the measurement of the DON in the prior art, it is one objective of the invention to provide a method for measuring a concentration of DON of a secondary effluent from a wastewater treatment plant. The method is able to measure the content of the DON in the secondary effluent in the stably operated process fast, economically, and simply and to provide technical support for the daily monitoring and the evaluation of the treatment effect of the DON from the wastewater treatment plant. [0006] Technical scheme of the invention is as follows: [0007] A method for measuring a concentration of dissolved organic nitrogen (DON) of a secondary effluent from a wastewater treatment plant, the method comprises the following steps: [0008] a) charting a standard curve representing relation between a concentration of a protein/a humic acid (HA) and an absorbance value: preparing standard samples of the protein/HA of different concentrations, measuring the absorbance values thereof, and establishing the standard curve representing relation between the absorbance value and the concentration; 3 [0009] b) establishing a mathematical model: establishing a linear mathematical model among the concentrations of the protein, the HA, and the DON of the secondary effluent from the wastewater treatment plant using a Minitab response surface method (RSM), which comprises: [0010] 1) filtrating an aerobic sludge and the secondary effluent from the wastewater treatment plant by a filter membrane to remove a suspended matter, mixing the aerobic sludge and the secondary effluent after the filtration according to different ratios, and preparing at least 6 groups of samples of different DON concentrations; [0011] 2) dissolving Na 2

CO

3 and NaOH in distilled water to yield a reagent A, dissolving CuSO 4 5H 2 0 and potassium antimonyl tartrate to the distilled water to yield a reagent B, combining the reagent A and the reagent B to yield a reagent C, and mixing a Folin-Ciocalteu reagent (FCR) and the distilled water with a volume ratio of 3:1 to prepare a diluted FCR; [0012] 3) adding 2.5 mL of the sample of each group prepared in step 1) to each of a first test tube and a second test tube; adding 2.5 mL of the reagent A prepared in step 2) to the first test tube and mixing in a whirl mixer, allowing the first test tube to stand at a room temperature for 10 min, adding 0.3 mL of the diluted FCR prepared in step 2) to the first test tube and immediately mixing to yield a resulting solution, allowing the first test tube to stand at the room temperature for 45 min, and measuring an absorbance value of the resulting solution at a wavelength of 750 nm; adding 2.5 mL of the reagent C prepared in step 2) to the second test tube and mixing in the whirl mixer, allowing the second test tube to stand at the room temperature for 10 min, adding 0.3 mL of the diluted FCR prepared in step 2) to the second test tube and immediately mixing to yield a resulting solution, placing the second test tube at the room temperature for 45 min, 4 and measuring an absorbance value of the resulting solution at the wavelength of 750 nm; and calculating concentrations of the protein and the HA in the sample of each group prepared in step 1) based upon the absorbance values and the standard curve charted in step a); [0013] 4) adding 25 mL of the sample of each group prepared in step 1) to each suspension type dialysis bag, immersing the suspension type dialysis bags to the distilled water for dialysis so as to remove dissolved inorganic nitrogen (DIN) from the sludge sample, measuring concentrations of total dissolved nitrogen (TDN), NH 4 *, N03-, and NO2, and calculating a difference value between the concentration of TDN and a total concentration of NH4, N03-, and NO2, whereby obtaining a concentration of DON; and [0014] 5) establishing the linear mathematical model among the concentrations of the protein and the HA calculated in step 3) and the concentrations of DON obtained in step 4), performing linear fit to obtain a regression equation DON(mg/L)= ki+m1*CProtein+m2*CHS, in which, ki, mi, and m 2 represent coefficients of the binary linear regression equation, and Cpmtein and CHS represent the concentrations of the protein and the HA, respectively; and [0015] c) filtrating the secondary effluent from the wastewater treatment plant by the filter membrane, measuring the concentrations of the protein and the HA of the secondary effluent after the filtration according to the method of step 3), and calculating the concentration of the DON of the secondary effluent based on the regression equation of step 5). [0016] Preferably, the secondary effluent from the wastewater treatment plant is a secondary effluent after sludge treatment. [0017] Preferably, a pore size of the filter membrane in step 1) and step c) is 0.45 jim. 5 [0018] Preferably, in step 2), the reagent A is prepared by 10 g of Na 2

CO

3 , 2 g of NaOH, and 500 mL of the distilled water; and the reagent B is prepared by 0.781 g of CuSO 4 -5H 2 0, 1 g of potassium antimonyl tartrate, and 100 mL of the distilled water. [0019] Preferably, the reagent C in step 2) is prepared by mixing 49 mL of the reagent A and 2 mL of the reagent B. [0020] Preferably, the suspension type dialysis bag of step 4) has a molecular weight cutoff of 100 and is made of cellulose; and a dialysis time is 24 hrs. [0021] Preferably, in step 4) the concentrations of the TDN, NH4, N0 3 -, and NO2 are measured by ion chromatography, salicylate-hypochlorite spectrophotometry, the ion chromatography, and N-(1-naphthyl)-ethylenediamine spectrophotometry, respectively. [0022] Compared with the prior art, advantages of the invention are summarized as follows: [0023] 1) Based on the linear relation between the concentration of the DON and the concentrations of the protein and the HA in the secondary effluent treated by the activated sludge process, the mathematical model among the concentrations of the DON, the protein, and the HA in the sewage is established. That is, the contents of the protein and the HA in the secondary effluent can be directly measured to calculate the concentration of the DON. The method features accurate measurement, convenient operation, and strong applicability. [0024] 2) The method only requires a small amount of the water sample for conduct the measurement, thus, the measuring method is simple, and the time for analysis is short. [0025] 3) The method only requires the filter membrane having the pore size of 0.45 im to analyze the sample, and the complicate pretreatments, such as the dialysis, nanofiltration, or electrodialysis, are not required, thereby decreasing the operating requirements for the operators, the measurement cost, and the production cost. 6 [0026] 4) The contents of the protein and the HA in the secondary effluent are firstly measured by the method, and then the concentration of the DON of the secondary effluent is calculated by the mathematical model. Not only is the concentration of the DON calculated, but also the constituents of the DON of the secondary effluent are provided, thereby reflecting the treating effect of the wastewater treatment plant. [0027] 5) The linear mathematical model among the concentrations of the protein and the HA calculated in step 3) and the concentration of the DON obtained in step 4) are established by the Minitab software, and the linear fitting is performed to obtain the regression equation DON(mg/L)= ki+m1*CProtein+m2*CHS, in which, ki, mi, and m 2 represent the coefficients of the binary linear regression equation, and Cprotein and CHS represent the concentrations of the protein and the HA, respectively. Therefore, the concentration of the DON calculated by the equation fitted by the model has high reliability. [0028] 6) The absorbance value at 750 nm is measured, and the measuring result is accurate. BRIEF DESCRIPTION OF THE DRAWINGS [0029] FIG. 1 is a flow chart of a method for measuring a concentration of DON of a secondary effluent; [0030] FIG. 2 is a chart reflecting relation among the concentrations of DON, a protein, and an HA; and [0031] FIG. 3 is a chart reflecting relation between a measured concentration of DON and a concentration of DON calculated by a model. DETAILED DESCRIPTION OF THE EMBODIMENTS 7 [0032] For further illustrating the invention, experiments detailing a method for measuring a concentration of DON of a secondary effluent are described hereinbelow combined with the drawings. Example 1 [0033] A large amount of DON in the secondary effluent of the wastewater treatment plant exist in amino acids (basic unit of protein), products of microbes, and other large biological molecules, and such large molecules (like the protein) containing nitrogen are produced in the process of the biochemical treatment of the wastewater treatment plant. In addition, the HA in the feeding water of the wastewater treatment plant is also a part of the DON in the secondary effluent. For the active sludge process, the ratio of the type to the number of the microfloras in the sewage system is often stable once the process is stably operated. Thus, the ratio of the protein produced in the biochemical treatment and the organic matters of the large biological molecules containing nitrogen related to the microbes is stable. The DON of the secondary effluent can be expressed by the following equation: DON=K1*CHS+k*CProtein +k 2 *y 2 +k 3 *3 +k*yn.--------............. (1) [0034] in which, K 1 , ki, k 2 , ... k represent constants, y2, y3, ... y 1 n represent different types of large molecular organic matters related to the microbes, and Cprtein and CHS represent concentrations of the protein and the HA. [0035] Since the ratio of the proteins produced in the biochemical treatment process to the large molecular organic matters related to other microbes are stable, then DON=K1*CHS+k*CProtein + k 2 k 2 '* Cprotei +k 3 k 3 '* CPotein +knkn' *CProtein. (2) [0036] that is, 8 DON=K1*CHS + Cprotein1*(k+k 2 k 2 '+k 3 k 3 n +kk') -----............... (3) [0037] In which, ki', k 2 ', ... k,,' represent constants. [0038] The equation (3) can also be expressed as follows DON=K1*CHS +K2*CProtein.-----.............. (4) [0039] in which, K 1 and K 2 are constants, and K 2 = ki+k 2 k 2 '+k 3 k 3 '+ +knkn'. [0040] It is known from the equation (4) that the concentration of the DON is linearly related to the concentrations of the protein and the HA in the secondary effluent treated by the activated sludge process. The mathematical model among the concentrations of the DON, the protein, and the HA in the sewage is established, so that the contents of the protein and the HA in the secondary effluent can be directly measured to calculate the concentration of the DON via the mathematical model. [0041] As shown in FIG. 1, a method for method for measuring a concentration of DON of a secondary effluent was conducted as follows: [0042] 1) Preparation of water samples containing DON of different concentrations [0043] 200 mL of an aerobic sludge and a secondary effluent from a wastewater treatment plant (A/O process) were filtrated by filtration membranes having pore sizes of 0.45 ptm to remove suspended matters therefrom, respectively. 30 mL of the secondary effluent after the filtration was added to each of 6 test tubes, and then the aerobic sludge with volumes of 0, 1.5, 3.0, 6.0, 15.0, and 20.0 mL were respectively added to the 6 test tubes and mixed with the secondary effluent therein to yield six groups of samples of different DON concentrations. [0044] 2.5 mL of the sample of each group prepared in step 1) was added to each of a first test tube and a second test tube. 2.5 mL of a reagent A (prepared by dissolving 10 g of Na 2

CO

3 and 2 g of NaOH to 500 mL of distilled water) was added to the first test tube 9 and mixed with the sample in a whirl mixer, and then a resulting solution was allowed to stand at the room temperature for 10 min. After that, 0.3 mL of a diluted FCR ( prepared by diluting the FCR with the distilled water with a volume ratio of the FCR to the distilled water of 3:1) was added to the first tube and mixed with the solution in the tube with the same mode. A resulting solution was allowed to stand at the room temperature for 45 min and thereafter the absorbance value of the resulting solution was measured at a wavelength of 750 nm. 2.5 mL of a reagent C (prepared by mixing 49 mL of the reagent A and 2 mL of a reagent B, in which the reagent B is yielded by dissolving 0.781 g of CuSO 4 -5H 2 0 1 g of potassium antimonyl tartrate to 100 mL of the distilled water) was added to the second test tube and then treated with the similar means as described in the above so as to measure the absorbance value of a resulting solution at 750 nm. After that, a standard curve reflecting the relation between the absorbance value and the concentrations of the protein and the HA was established according to the protein and the HA of different concentrations. The concentrations of the protein and the HA in the sample were calculated based on the measured absorbance values of the six groups of mixed water samples. The concentrations of the protein were 3.05, 3.22 3.54 3.70 4.99, and 5.47 mg/L; and the concentrations of the HA were 35.55, 35.86, 35.65, 35.95, 35.53, and 35.62 mg/L. [0045] 3) Analysis of concentration data of DON by difference method [0046] 25 mL of the sample of each group prepared in step 1) was added to each suspension type dialysis bag (made of cellulose, MWCO 100), the suspension type dialysis bags was then immersed into the distilled water for performing dialysis for 24 hrs so as to remove the DIN from the sludge sample. Concentrations of the TDN, NH4, N03-, and NO2 were subsequently measured, specific data of which are listed as follows. Difference values between the concentration of TDN and a total concentration of NH4*, N0 3 -, and NO2 were 1.22, 1.26, 1.28, 1.36, 1.47, and 1.50 mg/L, respectively, which were equal to the concentrations of the DON of different group. 10 [0047] 4) Establishment of mathematical model [0048] The linear mathematical model among the concentrations of the protein and the HA calculated in step 2) and the concentration of the DON obtained in step 3) were established by the Minitab software, and the linear fitting was performed to obtain the regression equation DON(mg/L)= -3.23366+0.l20 8 8 9 *Cproten+0. 114845*CHS, in which, Corotein and CHS represented the concentrations of the protein and the HA, respectively. The probability P of the equation is smaller than a significance level of 0.05, which means that the regression effect is significant. The data fitting results are illustrated in FIG. 2 and specifically analyzed in the following table. Source Degree of freedom Seq SS Adj SS Adj MS F P Regression 2 0.065562 0.065562 0.032781 74.45 0.003 linearity 2 0.065562 0.065562 0.032781 74.45 0.003 Residual 3 0.001321 0.001321 0.000440 error total 5 0.066883 [0049] 5) Measurement of the DON of daily secondary effluent [0050] At least 5 mL of the secondary effluent from a wastewater treatment plant (A/O process) in Nanjing was collected respectively in April, August, September, and October and filtrated with the filtration membrane having the pore size of 0.45 jim. The concentrations of the protein and the HA in the secondary effluent were measured adopting the method of step 2), and the concentration of the DON in the secondary effluent was calculated based on the linear regression equation of step 4), i. e., 1.20, 1.42, 1.24, and 1.39 mg/L, respectively. In order to improve the accuracy and the stability of the measurement, the measurement was repeated for three times, and an average value thereof was calculated, results of which is illustrated in FIG. 3. It is known by the T-test that the p value was larger than the significance level 0.05, which means that the 11 measured concentration of the DON is not significantly different from the DON concentration calculated by the model. [0051] While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. 12

Claims (7)

1. A method for measuring a concentration of dissolved organic nitrogen (DON) of a secondary effluent from a wastewater treatment plant, the method comprising: a) charting a standard curve representing relation between a concentration of a protein/a humic acid (HA) and an absorbance value: preparing standard samples of the protein/HA of different concentrations, measuring the absorbance values thereof, and establishing the standard curve representing relation between the absorbance value and the concentration; b) establishing a mathematical model: establishing a linear mathematical model among the concentrations of the protein, the HA, and the DON of the secondary effluent from the wastewater treatment plant using a Minitab response surface method (RSM), the establishing process comprising: 1) filtrating an aerobic sludge and the secondary effluent from the wastewater treatment plant by a filter membrane to remove a suspended matter, mixing the aerobic sludge and the secondary effluent after the filtration according to different ratios, and preparing at least 6 groups of samples of different DON concentrations; 2) dissolving Na 2 CO 3 and NaOH in distilled water to yield a reagent A, dissolving CuSO 4 5H 2 0 and potassium antimonyl tartrate to the distilled water to yield a reagent B, combining the reagent A and the reagent B to yield a reagent C, and mixing a Folin-Ciocalteu reagent (FCR) and the distilled water with a volume ratio of 3:1 to prepare a 13 diluted FCR; 3) adding 2.5 mL of the sample of each group prepared in step 1) to each of a first test tube and a second test tube; adding 2.5 mL of the reagent A prepared in step 2) to the first test tube and mixing in a whirl mixer, allowing the first test tube to stand at a room temperature for 10 min, adding 0.3 mL of the diluted FCR prepared in step 2) to the first test tube and immediately mixing to yield a resulting solution, allowing the first test tube to stand at the room temperature for 45 min, and measuring an absorbance value of the resulting solution at a wavelength of 750 nm; adding 2.5 mL of the reagent C prepared in step 2) to the second test tube and mixing in the whirl mixer, allowing the second test tube to stand at the room temperature for 10 min, adding 0.3 mL of the diluted FCR prepared in step 2) to the second test tube and immediately mixing to yield a resulting solution, placing the second test tube at the room temperature for 45 min, and measuring an absorbance value of the resulting solution at the wavelength of 750 nm; and calculating concentrations of the protein and the HA in the sample of each group prepared in step 1) based upon the absorbance values and the standard curve charted in step a); 4) adding 25 mL of the sample of each group prepared in step 1) to each suspension type dialysis bag, immersing the suspension type dialysis bags to the distilled water for dialysis so as to remove dissolved inorganic nitrogen (DIN) from the sludge sample, measuring concentrations of total dissolved nitrogen (TDN), NH4, N03-, and NO2, and calculating a difference value between the concentration of TDN and a total concentration of NH4*, N0 3 -, and NO2, whereby obtaining a concentration of DON; and 14 5) establishing the linear mathematical model among the concentrations of the protein and the HA calculated in step 3) and the concentrations of DON obtained in step 4), performing linear fit to obtain a regression equation DON(mg/L)= ki+m1*CProteil+m2*CHS, in which, ki, mi, and m 2 represent coefficients of the binary linear regression equation, and Cprotem and CHS represent the concentrations of the protein and the HA, respectively; and c) filtrating the secondary effluent from the wastewater treatment plant by the filter membrane, measuring the concentrations of the protein and the HA of the secondary effluent after the filtration according to the method of step 3), and calculating the concentration of the DON of the secondary effluent based on the regression equation of step 5).

2. The method of claim 1, characterized in that the secondary effluent from the wastewater treatment plant is a secondary effluent after sludge treatment.

3. The method of claim 1, characterized in that a pore size of the filter membrane in step 1) and step c) is 0.45 ptm.

4. The method of claim 1, characterized in that in step 2), the reagent A is prepared by 10 g of Na 2 CO 3 , 2 g of NaOH, and 500 mL of the distilled water; and the reagent B is prepared by 0.781 g of CuSO 4 5H 2 0, 1 g of potassium antimonyl tartrate, and 100 mL of the distilled water.

5. The method of claim 1, characterized in that the reagent C in step 2) is prepared by mixing 49 mL of the reagent A and 2 mL of the reagent B. 15

6. The method of claim 1, characterized in that the suspension type dialysis bag of step 4) has a molecular weight cutoff of 100 and is made of cellulose; and a dialysis time is 24 hrs.

7. The method of claim 1, characterized in that in step 4) the concentrations of the TDN, NH4, N03-, and NO2 are measured by ion chromatography, salicylate-hypochlorite spectrophotometry, the ion chromatography, and N-(1-naphthyl)-ethylenediamine spectrophotometry, respectively. 16