Partial denitrification is a second-generation TN removal process. It is the second and limiting step of the nitrification-partial denitrification-anammox (PDNA) process for TN removal. In nitrification-PDNA, ammonia is allowed to be nitrified fully to nitrate, then partially denitrified to nitrite at a condition that maximizes nitrite accumulation. The process (Figure 2-7) saves 45% oxygen (aeration energy) and 79% of the external carbon source (Ma et al., 2016). The nitrification-PDNA process is currently considered to be the most promising second-generation process for mainstream wastewater treatment. The process also can be very successful for nitrogen-rich industrial wastewater treatment such as chemical fertilizer wastewater, explosives wastewater and other production wastewater (Yang Zhang et al., 2019). A summary of previous
detailed studies can be found in Chapter 5, Table 5-1. However, some general information is highlighted here.
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Figure 2. 7 Partial denitrification-anammox (Ma et al., 2016)
In addition to S. Ge et al., 2012 studied the effect of COD/N ratio in a batch reactor for different types of electron donors (methanol, acetate, and glucose), on the denitrification rates from nitrate, with ratios of 1, 6, 10, 15, 20, and 25 (VSS was 1250 mg/L). They also studied specific nitrite accumulation rates (SNAR) during transformation as well as the denitrification rates from nitrite and concluded that the initial COD/NO3--N affected the level of (i) nitrate denitrification rate (initial, phase 1), (ii) nitrite denitrification (during nitrate denitrification), (iii) nitrate denitrification (phase 2) and (iv) nitrite denitrification (phase 2) and (v) nitrite accumulation during denitrification from nitrate. The highest SDNRs were 0.64 (glucose), 0.35 (acetate), 0.029 (methanol) and 0.04 mgN/mgVSS-d, respectively. Moreover, the highest accumulation occurred with glucose (22.32 mgN/L) at SNAR of 0.34 mgN/mgVSS-d. Ge et al. attributed nitrite accumulation to the competition between electron acceptors enzymes (Nir and Nar reductase). They also recommended the use of pH and ORP to control the denitrification process (real-time control). S. Ge et al., 2012 found two high accumulation points occurred with glucose of 22.32 and 20.01 mgN/L at COD/N of 15 and 10, respectively (table 2-12). Whereas the maximum nitrite accumulated with methanol was 19.34 mgN/L at COD/N of 25. Acetate was found to cause the lowest-highest accumulation among the three carbon sources with nitrite accumulation of 17 mgN/L at COD/N of 25. It can be concluded from Table 2-13 the highest nitrite accumulation occurred at the highest SNAR of 0.34 mgN/mgVSS-d.
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Table 2.12 Effects of carbon sources on the nitrite accumulation in the first phase (Ge et al., 2012). COD/NO3-N Carbon source Accumulation time, hr Nitrite acumulation, mgN/L SNAR-NO2, mgN/mgVSS- d SDNR-NO3, mgN/mgVSS- d 1 Acetate 2 1.86±0.94 0.013±0.005 0.052±0.01 Methanol 1 3.17±0.45 0.05±0.014 0.073±0.004 Glucose 1 2.1±0.05 0.025±0.002 0.096±0.008 6 Acetate 2 8.44±0.81 0.062±0.011 0.28±0.03 Methanol 4 10.82±1.52 0.043±0.002 0.13±0.04 Glucose 2 14.51±1.05 0.11±0.02 0.24±0.07 10 Acetate 2 12.03±0.47 0.095±0.004 0.32±0.03 Methanol 4 12.03±2.41 0.062±0.004 0.14±0.03 Glucose 1 20.01±1.44 0.31±0.05 0.55±0.12 15 Acetate 1 15.68±3.04 0.24±0.03 0.55±0.05 Methanol 4 14.68±2.16 0.057±0.016 0.16±0.07 Glucose 1 22.32±1.85 0.34±0.03 0.64±0.2 25 Acetate 1 17.19±2.08 0.22±0.01 0.6±0.09 Methanol 2 19.35±1.26 0.25±0.05 0.4±0.04 Glucose 4 10.43±0.23 0.14±0.03 0.04±0.04
Shi et al., 2019 studied the effect of pH (adjusted to 9) on achieving partial denitrification (PDN) in a 6 L anoxic-sequencing batch biofilm reactor (SBBR, sponge carrier media,), the cycles were varied started with 6 hr-cycle (116 days) and then reduced to 2hr-cycle. However, the authors use 3 reaction times (5 hrs and 3hrs-period 1, 1 hr-period 2, no settling), the nitrate feed and diluted MWW-COD (205 mgCOD/L before dilution) feed were 30mgN/L and 145 mgCOD/L, respectively, and the COD/N ranged between 4-6 and DO≤0.1 mg/L. The results showed a high nitrite accumulation of nitrite 23.5 and 25.2 at H of 7 and 9, respectively, implying that there is no effectct of COD/N ratio and pH on PDN. However, in batch tests with acetate carbon source Shi found a higher accumulation at pH 9 than pH 7 with nitrite accumulation percentage and concentrations of (90.3%, 22.5 mgN/L) and (91.8%, 22), respectively. In the same study, Shi found that the maximum nitrite accumulated reached after 60 minutes in day after day 126 while before that day the was reached after 180 minutes, implying long term operation improve the microbial structure to carry out nitrite accumulation in a shorter period. However, he concluded possible optimum COD/N ratio 6.2, which is higher than previously believed ratio of 3 (Cui et al., 2017). Shi finally found that thauera has the highest relative abundance (21.9%), followed by Flavobacterium (9.6%) and Thiobacillus (6.3%).
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Yang Zhang et al., 2019. also investigated partial denitrification (COD of 100mg/L, acetate) at COD/N of 3, 5 and 7 with cultured Acinetobacter Johnsonii , the maximum nitrite accumulation were 80.9%, 94.2 and 87.4%, respectively, and occurred after 13 hours from the experiment which indicating the optimum COD/N of 4-5 is the optimum. However, Zhang et al. illustrated that NaR is less competitive than NiR when COD/N is low (less than 3), and when COD/N increases the gene level of napA, which is located in the periplasm, increases and the denitrification from nitrate become much higher than nitrite. This imply that the nitrite and nitrate reductase enzymes availability as well as the COD/N ratio plays an importat role in nitrite and nitrate denitrification rates, and partial denitrification. He also found that there is no major pH effect on the PDN in the rang of 6-9.
Xiujie et al., 2019 studied partial denitrification with glucose in 3 SBRs (6 hrs cycle, 2cycle/d), the nitrite accumulated in SBR1 (anoxic) was 8.29 mgN/L , 9.94 mgN/L in SBR2 (aerobic-anoxic), 11.63 mgN/L in SBR3 (low DO) and at COD/N of 3 (30 mgNO3-N/L), and the denitrification rates from nitrate and nitrite were 17.24 mgN/L-hr and 3.38 mgN/L-hr in SBR1, 10.16 mgN/L-hr and 1.72 mgN/L-hr in SBR2 and 7.98 mgN/L-hr and 0.8 mgN/L-hr in SBR3, respectively, for the first 2.5 hr of the reaction and then reduced to 0.52 mgN/L-hr and 0.9 mgN/L-hr (2.5-8 hrs) for nitrate and nitrite, respectively. These results imply that nitrate reductase is more affected by carbon source deficiency than nitrite, whereas when carbon source available, the nitrate reductase activity is much higher than nitrite activity. Further explanation by Xiujie that the biomass in SBR1 has higher denitrification ability than SBR2 and SBR3 and nitrite reductase is more sensitive to DO than nitrate reductase, which causes nitrite accumulation. He also detected various phyla, and they are arranged from the highest to the lowest relative abundance Proteobacteria (44%) in SBR1, Bacteroidetes (19%) in SBR3, Acidobacteria(12.9%), Planctomycetes, Candidatus saccharibacteria, Chlorofexi). In the genus level Saccharibacteria, which is related to glucose, with a relative abundance of 5.35% in SBR1, 45.44% in SBR2, and 34.96% in SBR3. The relative abundance in SBR2 and SBR3 is related to the favourability of Saccharibacteria to DO. Other genera that was found and arranged according to relative abundance are Aridibacter, Pseudomonas, Thauera, Gemmobacter and Citrobacter, implying that the relative abundance of the genera are related to the carbon source (glucose).
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In a study by Du et al., 2017 in SBR followed UASB, the SDNRNO3-NO2 and SDNRNO2 with acetate were 0.47 and 0.42 mgN/mgVSS-d, respectively, whereas SDNRNO3-NO2 and SDNRNO2 with ethanol were 0.29 and 0.2 mgN/mgVSS-d. However, Le et al., 2019a and Le et al., 2019b were not able to accumulate nitrite with methanol, illustrating two elucidation for PDN which are 1- acetate and glycerol can enter directly to TCA cycle and stored in the cell in the form of poly‐3‐ hydroxybutyrate and then utilized as an electron donor during denitrification. A large pool of electrons is released within the cell during denitrification. While NaR has a higher electron capacity (2 e-/mole-nitrate) than NiR (e-/mole-nitrite), NaR could react with more electrons, which creates a much higher reduction rate than NiR reaction rate. The other explanation 2- when acetate or glycerol are used, NaR reacts with the electrons in the cytochrome b, which is located in the upstream region of the cytoplasm. At the same time, NiR accepts electrons from cytochrome c located in the downstream region. Acetate and glycerol donate electrons in the upstream region (cytoplasm b) which allow nitrite to accumulate, whereas methanol donate electrons in the down stream region which nitrite is not accumulated with methanol.
Moreover, Le et al., 2019a and Le et al., 2019b in the partial denitrification study (PDN-anammox) accumulated more than 80% nitrite at COD/N ratio of 2-3, and nitrite started to be denitrified when nitrate concentration was below 3 mgN/L. However, they did not consider denitrification from nitrite (partial nitrification/denitrification) to measure specific denitrification rates from nitrite in relation to COD/N ratios. To do so, an acclimatization step is required for denitrifies (denitrifying- bacteria and heterotrophs) with nitrite as an electron acceptor. Nitrite with a higher toxicity effect on the microbes than nitrate requires longer acclimatization time. Table 2-12 shows denitrification rates from accumulated nitrite with different carbon sources. It is clear that in the first phase of SDNR, acetate exhibited the highest denitrification rate from nitrate (0.35 d-1), while the denitrification rate from nitrite was highest with methanol in the second phase (0.041 d-1). In conclusion, several early-stage trials to investigate partial denitrification (PDN) (Table5-1, chapter 5) are in the research process; the study in chapter 5 shows PDN-trials through the control of COD/N, final nitrate concentration, high pH, and high DO parameters. However, until now, these parameters do not show they govern the PDN process since various researchers selected different COD/N with a wide gap. Controls such as increasing the pH will not make any difference
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since pH typically increases with alkalinity during denitrification; however, increasing DO will be very critical since nitrite may convert again to nitrate. The control through the effluent nitrate concentration was not proven because the effluent nitrate concentration varies with the carbon source and is also directly related to KNO3. Most of the recent researches combine PDN-anammox in one reactor, which did not analyze nitrite accumulation but anammox activity. Finally, none of the previous research studied the partial denitrification mechanism and compared nitrite and nitrate denitrifying biomass structure (type and biodiversity) and critically analyze the partial denitrification kinetics.