Investigation Of Microbial Yield Kinetics For Denitrifying Microorganisms Using Ethanol As An External Carbon Source
Open AccessThe purpose of this study was to investigate the feasibility of ethanol as an alternate substrate for biological denitrification by measuring various growth kinetic parameters. The growth yield coefficient is measured for endogenous nitrate removal while external carbon substrate is still available for denitrifying population. In this study, 3 SBR tests, 4 Medium F/M tests and 9 Low F/M tests were performed. Mixed liquor was collected from BNR stage at Piscataway WWTP, Maryland, USA. The SBR Test 1 was done for 57 days and remaining two (SBR Test 2 and 3) for 8 days in a 10 L reactor at 20°C. Besides the SBR Test 1, 13 individual anoxic batch tests were performed for 30 h in a semi batch reactor (2 L) with 100 mg/L of MLSS. The C/N ratio of 4.47±1.03 and 6.76±1.53 mg COD/mg NO3-N were observed for SBR Test 1 and associated anoxic batch tests. The observed ratio in MFM Test 1, 2, 3 and 4 was 6.63, 9.14, 4.77 and 4.74±0.27 mg COD/mg NO3-N, respectively. SBR Test 2 and 3 gave the C/N ratio as 5.24±0.30 and 4.94±0.50 mg COD/mg NO3-N, respectively. The initial SDNR reported for SBR Test 1 was 4.02 mg NO3-N/g VSS/hour. The final SDNR observed was 22.42 mg NO3-N/g VSS/hour. The range of SDNR from anoxic batch tests was 1.37 - 52.32 mg NO3-N/g VSS/hour. SBR Test 2 and 3 showed similar SDNR at the beginning and end of the test, which was 1.08, 1.85 and 37.67 and 46.6 mg NO3-N/g VSS/hour, respectively. Five analytical methods were used in this study to calculate the Y. Method 1 and 2 were based on pCOD and sCOD data. Method 3 and 4 included nitrate, nitrite, sCOD and ethanol data. Method 5 used nitrate and nitrite data with pCOD. In Method 3 for SBR Test 1, the computed Y value was 0.56±0.11 g COD/g COD. Comparing all data points from 13 anoxic batch tests, the range of Y was calculated as 0.22±0.10, 0.36±0.13, 0.62±0.10 and 0.47±0.14 g COD/g COD using Method 1, 2, 3 and 5, respectively. Based on all methods, the Y value for this test was 0.42±0.24 g COD/g COD. Test results confirmed that nitrite accumulation inhibited the microbial growth population in this SBR test. In MFM Test 4 the range of Y value was 0.35±0.10, 0.32±0.16, 0.40±0.03, 0.37±0.05 and 0.31±0.15 g COD/g COD using Method 1 to 5, respectively. The final Y value for this test was 0.35±0.04 g COD/g COD. There was no nitrite accumulation during the four MFM tests. For SBR Test 2 and 3, the range of Y value was determined as 0.42±0.07, 0.43±0.13, 0.48±0.04, 0.46±0.07, 0.45±0.09 and 0.45±0.06, 0.46±0.11, 0.45±0.03, 0.47±0.02, 0.45±0.06 g COD/g COD using Method 1 to 5, respectively. The final value of Y for this two test was 0.45±0.03 and 0.46±0.01 g COD/g COD. Using Method 3, the corrected Y was 0.51±0.04, 0.49±0.05 g COD/ g COD and using Method 4, it was found to be 0.51±0.05, 0.53±0.04 g COD/g COD in SBR Test 2 and 3, respectively. Growth yield coefficient, carbon to nitrogen ratio and specific denitrification rate are among the criteria that are important for selection of an external carbon source for any WWTP. Size of the anoxic zone and faster denitrification rate can reduce the construction cost of the facility. Such parameters are helpful to modify or upgrade the capacity of the plant with optimal design of denitrification system.
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