The bioreactor was operated continuously over a period of three months with wastewater from the Wastewater Treatment Plant at the UNAM. Figure 2 shows the COD removal and organic load applied. It was observed that COD removal increases to 90% when the organic loading decreases to near 60 g COD/m2d.
Figure 2COD removal at different surface organic loading in BTR
In the first days of the operation in the biological reactor, there was an absence of oxygen in the bioreactor due to a poor suction in the Venturi tube. Since, 60 day tendency could be corrected by increasing the levels of oxygenation in the reactor. Figure 3 shows the dissolved oxygen in the reactor modules during the operation of the bioreactor. The poor COD removal from the reactor in the first few days may be related by the absence of oxygen, the
oxygenation in all the modules improved the removal of organic matter.
Figure 3. Dissolved oxygen in the reactor modules during BTR operation
In regard to the ammonium removal the bioreactor had an excellent performance. Figure 4 shows the removal of ammonium N-NH4 at different superficial N-NH4loading. When the N-NH4 loadings were around 100 g N-NH4 /m2 d the biological tubular reactor was unstable the removal of ammonium was near to 60% however, when the N-NH4 load decreases to 60 until 5 g N-NH4/m2 d the removal of ammonium was 100%.
Figure 4.Ammonium removal at different superficial N-NH4 loading in BTR
After 50 days of operation nitrite formation was observed in the bioreactor while the nitrates remained at levels close to 0 mg/L. As shown in Figure 5, this is a clear signal that nitrogen removal within the bioreactors due to the Anamox Process.
Figure 5 Nitrite and nitrates measurements in the bioreactor inlet and outlet.
Partial nitritation/anammox process, it is commonly used to remove nitrogen from rich ammonium wastewater. Anaerobic ammonium oxidizing bacteria (Anammox) are capable of oxidizing ammonium with nitrite as electron acceptor. Some works have demonstrated that bacterial competition is influenced in granular sludge by particle size. Nitrogen oxidizing
bacteria tend to grow in smaller particles due to a larger aerobic volume fraction whereas Anammox bacteria dominate in bigger granules due to smaller aerobic volume fraction (Winkler et al., 2012)
Figure 6 shows the TSS at the inlet and outlet of the system, it was observed that the reactor accumulates a considerable amount of SST. After day 58 was observed an excellent TSS removal obtaining an effluent near to 25 mg SST/L.
Figura 6 Total suspended solids in the bioreactor inlet and outlet.
Figure 7 on the left side shows a fraction of the support used for the microorganisms in it a considerable concentration of microorganisms within the polymer material can be observed.
When the SST was determined in the support, it had a concentration of 250 mg SST/cm2and the SST inside the reactor was estimated obtaining 392g of SST adhered to the support. And in the right side of Figure 7 shows the inlet and outlet of the reactor.
Figure 7Substratum with biofilm (left) Input and output of the bioreactor (right).
In recent works, a complete retention of the biological material within the system is an important requirement for a successful continuous operation of immobilized bioreactors, as the submerged membrane promotes the catalytic process (Chakraborty, 2012).
Moreover, the nonwoven fibrous support immobilized the biofilm in a fashion similar to traditional supports and it protected the cells of shear stress, microorganisms fixed to support generates an advantage and could solve some problems described in this type of
bioreactors(García-González and Durán-Moreno,2016). For example, in horizontal tubular bioreactors maintenance of stable suspended biomass concentration gradient along the bioreactor can be a problem due to biomass washout (Šantek, 2006).
Consequently, biofilm can offer favorable conditions for slow growing organisms such as nitrifying bacteria and anammox bacteria. As well known, aerobic ammonium-oxidizing bacteria (AerAOB) and anaerobic ammonium-oxidizing bacteria (AnAOB) with long generation periods perform two sequential reactions simultaneously under oxygen-limited condition in single stage anammox process. There is no doubt that biofilm-based processes are ideal candidates for AerAOB and AnAOB to co-exist in terms of aerobic region and anaerobic region along the biofilm depth (Liu et. al., 2017).
CONCLUSIONS
Tubular biological reactor showed to be a feasible technically option for the treatment of small-scale domestic wastewater due to the reactor provides an effluent free of ammonium nitrogen and high COD removal. When the biological tubular reactor was operated in continuous form and it was obtained with excellent results of N-NH4 removal, where the removal of nitrogen was 100% considering a range of N-NH4 loads of 0.5 to 20 g N-NH4/m2d. In terms of COD, the removals were from 50% to 98.43% with organic loads near to 60 g COD/m2d and it was unstable when the organic load was up 200 g COD/m2d.
Therefore, it is recommended to continue the study of these types of reactors to determine whether it is possible to carry out the scale of the technology.
Acknowledgements
The authors acknowledge the financial support for this research from the Dirección General de Asuntos del Personal Académico (DGAPA, PAPIIT-IT/102415) of the Universidad Nacional Autonóma de México (UNAM).
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