Cultivation of Aerobic Granules for Phenol Removal from Wastewater

The cultivation of aerobic phenol-degrading granules was first achieved by conditioning municipal activated sludge in batch culture for a period of two months by incubating with phenol which gradually increased in concentration from 50mgl-1 to 500 mg l-1 (Jiang et al., 2002). The acclimated activated sludge was then inoculated into a column-type SBR, which was fed with a synthetic wastewater with phenol as a sole carbon source. The reactor was operated sequentially in 4 h cycles (2 min fill, 205-230 min aeration, 5-30 min settling, and 3 min effluent withdrawal) with a volumetric exchange ratio of 50% to give a hydraulic residence time (HRT) of 8 h and a phenol loading rate was 1.5 g phenol l-1 day-1. A settling period of 30 min was initially imposed to avoid excessive washout of the acclimated biomass, and subsequently reduced to 10 min on day 20 and to

5 min from day 35 to the end of reactor operation on day 68. Granules first appeared on day 9 of reactor operation and eventually displaced the activated sludge flocs to become the dominant form of biomass within the reactor. At steady state, 80% by volume of the granules were between 0.35 and 0.60 mm in size. The step-wise decrease in settling periods selected for granules with improved settling characteristics and resulted in a concomitant increase in biomass concentration and nearly complete phenol removal in the reactor. Compared to a phenol concentration of 500 mg l-1 in the influent, the phenol concentration in the effluent decreased to below 0.2 mg l-1 beyond day 32 of reactor operation. To investigate the feasibility of using aerobically grown microbial granules for high-rate phenol biodegradation, the reactor operation was extended by imposing a higher loading rate of 2.5 kg phenol m-3 d-1 (Tay et al., 2004). Granules cultivated at this higher phenol loading did not diminish in their ability to remove phenol, and effluent phenol concentrations below 0.2 mg l-1 continued to be maintained.

The ability of granules to degrade phenol was evaluated by monitoring phenol disappearance at different phenol concentrations in batch experiments. The specific phenol degradation rate increased with phenol concentration from 0 to 500 mg phenol l-1, peaked at 1.4 g phenol g VSS-1 d-1, and declined with further increase in phenol concentration as substrate inhibition effects became important (Fig. 9.1). Akinetic analysis of the degradation data was performed based on Haldane's formula for self-inhibition as follows:

where V and Vmax are the specific and the theoretical maximum specific substrate degradation rates (g phenol g VSS-1 d-1), respectively, and S, Ks, and Ki are the substrate concentration, half-saturation constant, and inhibition constant (mg phenol l-1), respectively. There was a good fit between the degradation data and the Haldane equation, with calculated kinetic parameters of Vmax = 5.6 g phenol g VSS-1 d-1, Ks = 481 mg l-1, and Ki = 213 mg l-1 (Jiang et al., 2002).

The self-immobilization of microbial cells into granules allowed the cells inside the granules to tolerate high phenol concentrations and achieve good structural properties and biodegradation performance. The cell immobilization created a diffusion barrier and established a concentration

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Phenol concentration (mg l-1)

Fig. 9.1. Specific phenol degradation rates of aerobic granules at different phenol concentrations (Jiang et al., 2002).

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Phenol concentration (mg l-1)

Fig. 9.1. Specific phenol degradation rates of aerobic granules at different phenol concentrations (Jiang et al., 2002).

gradient that sheltered the microbial cells beneath the protective barrier by diminishing the toxic chemical below some threshold value to allow continued microbial activity and substrate utilization (Villaverde and Fernandez-Polanco, 1999). Although the specific phenol degradation rates gradually declined at phenol concentrations higher than 500 mg l-1, significant rates of phenol degradation could still be achieved up to a phenol concentration of 2000 mg l-1. The ability of the granules to degrade phenol was superior to activated sludge, where the highest specific phenol degradation rates were reported at phenol concentrations below 100 mg l-1 (Watanabe et al., 1996).

Such application of immobilized cells in wastewater treatment permits the degradation of higher concentrations of toxic pollutants than can be achieved with free cells. Immobilized cells are better protected against phenolic and other similarly inhibitory compounds. This has been documented for many microbial systems, including alginate-encapsulated cells that tolerated higher phenol concentrations than free cells (Bastos et al., 2001), and cells entrapped in hollow-fiber membranes that mitigated the effects of phenol inhibition (Loh et al., 2000). With the protection offered by immobilization, the need for the adaptation period normally required by free cells is eliminated, and the uptake of substrate is enhanced in comparison with free cells in the bulk liquid (Diaz et al., 2002; Moslemy et al., 2002). In addition to diffusion limitation, other possible mechanisms for tolerance of elevated phenol concentrations include binding of phenol by cells on biofilm exteriors that allowed internal bacterial cells to multiply without any inhibition, and modification in lipid composition of cell membranes to compensate for the increase in membrane fluidity induced by phenol (Keweloh et al., 1989; Yap et al., 1999). Although the underlying mechanism for phenol tolerance in the granules has not been pinpointed, and may be the result of several synergistic mechanisms, it is clear from this study that the formation of dense, compact granules can facilitate growth of microorganisms in the bioreactor.

While the granular structure serves an important function in protecting microorganism within the granules against chemical toxicity, this protection may also paradoxically create problems associated with slow diffusion of nutrients and oxygen into and waste metabolites out of the granules. The pulse feeding and high phenol loading regime created a situation of high phenol concentration in the bulk liquid during the initial part of each SBR cycle, and this probably enhanced the penetration of phenol substrate into the granule interior. Moreover, the limited solubility of oxygen vis-à-vis organic substrates such as phenol means that problems that may be caused by the onset of diffusion limitation are likely to be oxygen-related (Beun et al., 2002). Previous investigations into the microstructure and ecology of aerobic granules (Tay et al., 2002, 2003) support the view that oxygen diffusion may not be limiting for small granules, and that anaero-biosis and cell death from diffusion limitation might occur in the interiors of larger granules, although the slightly looser structure of these granules would have a compensatory effect and would have allowed them to have better access to oxygen and nutrients. Still, diffusion limitation can pose a serious problem in large granules, since not all the microorganisms can actively carry out the biodegradation of target substrates. In order to exploit aerobic granulation technology for efficient treatment of high-load wastewaters, operating controls should be imposed to limit the granule size and ensure that the granules consist entirely of actively biodegrading cells.

Was this article helpful?

0 0

Post a comment