As seen in Figure 2 and Figure. 3, by increase of the retention time from 1.5 to 3 h, the nitrate reduction is obvious.
In the HRT of 5 h, acetic acid and NO
3 were reduced considerably, so that their concentrations were near zero in the effluent. It should be noted that like the pervious retention time, the average rate of acetic acid consumption was lower for the nitrate removal than stoichiometric constant; the obtained value for the mentioned constant was 2.07.
Discussion
The results considering effects of different HRTs on the nitrate removal showed the efficiency improved with increase of the hydraulic retention time. Increasing the hydraulic retention time give suficient time to microbes for reducing nitrogen and thereby removing it from wastewater. Addy et al, concluded that in beds with less hydraulic retention times, the nitrate removal (mass per volume) was significantly lower. This argues for the fact that bed designs incorporate sufficient time for nitrate removal
31. Similar results were found in the study of Christianson et al, who suggested that the minimum design retention times (7.5–79 h) should be increased to achive sufficient mass nitrogen reduction. However, they mentioned that caution should be considred in this regard, because by increase of the design retention times and enlargement of the corresponding bioreactors, the detrimental by-products may exacerbate under low flow conditions. According to Wang and Chu as well as Ovez et al., decrease of HRT to certain values increased the effluent nitrate concentrations and nitrite accumulation
32, 33.
Bed designers should optimize the system to address the expected flow rates and ensure the sufficient time for nitrate removal. Nowadays, many denitrifying bed designes incorporate with the hydraulic control components and adjust to the bypass flow in high flow events. Such design features provide the extended HRT and permit the flexibility, so that nitrate removal can be examined under different HRT strategies
34, 35 .
The statistical parameteric analysis showed a significant difference between the average removal percentage of the studied hydraulic retention times (p < 0.05). This shows that by increase the hydraulic retention time from 1.5 to 3 and 5 h, the nitrate removal efficiency increased. Furthermore, Wang et al. studied the efficiency of a laboratory-scale denitrification reactor packed with biodegradable snack ware in a low-temperature condition. They found that at a concentration of 50 mg/L for NO
3-N, 5 h of HRT was needed to complete the nitrate removal
36.
Meanwhile by increasing of the nitrate removal, the acetic acid consumption also augmented and the average acetic acid consumption was 1.91 ± 0.55 mg per each mg of the removed NO
3-N in HRT of 3 h. Some studies obtained the same results; for example, Sukias et al. found that the acetic acid requirement was 3.5 mg acetic acid per mg NO
3-N removal
37. Mohseni and Elliott reported that the acetic acid to nitrate nitrogen (A/N) ratio was in the range of 4.2 to 4.3
38.
So, we can conclude from these findings that another source of organic materials is probably available for heterotrophic bacteria in reactors such as the organic materials in influent (raw water) and the residue of died microorganisms. This source causes a decrease of acetic acid consumption and consequently lowers the constant of acetic acid consumption to the removed NO
3-N in comparison to the stoichiometry calculation.
Alkalinity is increased in heterotrophic
process, unlike the autotrophic denitrification. In autotrophic process, a part of the alkalinity is consumed as an inorganic carbon source
39, but in heterotrophic denitrification, the existing organic carbon is consumed and changed to inorganic carbon; so, the alkalinity is increased in the effluent
40. Zhao J. et al. found that the dominant denitrifiers, in a woodchip-based solid-phase denitrification
(W-SPD) bioreactor, were carbonaceous compound degrading bacteria and fermentative bacteria. Furthermore, this system was able to remove
92.5% – 96.4% of the nitrate
41.
Conclusion
The results show that, at HRTs of 1.5 h, 3 h, and 5 h 68.7%, 97%, and 98.5% nitrate removal were obtained, respectively. It is clear that by increase of the nitrate removal, the acetic acid consumption augmented. In the current study, we showed that constant rate of the acetic acid consumption to remove NO
3-N was lower than the stoichiometric with a proportion of 2:1 (acetic acid to NO
3-N). According to the results of this study, the autotrophic bacteria can remove nitrate from water under heterotrophic condition and high efficiency removal of nitrate decreased the consumption of organic matter (acetic acid).
Acknowledgement
The authors of the present study sincerely appreciate Shahid Beheshti University of Medical Sciences for funding this project.
Conflict of Interest
All authors declare they have no conflict of interest regarding the publication of this article.
Funding
This study was funded by Shahid Beheshti University of Medical Sciences, Tehran.