Kinetic Studies for an Aerobic Packed Bed Biofilm Reactor for Treatment of Organic Wastewater with and without Phenol ()
1. Introduction
Biofilm reactor is a popular method for biological treatment of wastewater to combat high organic strengths owing to enhanced mean cell residence time and economical oxygen supply [1-4]. However, adequate mixing in the biofilm reactor is important to ensure uniform distribution of substrate, sufficient contact between the microorganism and substrate and prevention of localized accumulation of toxic matter. In absence of recirculation, mixing can be achieved to some extent by uniform supply and distribution of oxygen in the reactor. Wastewater emanated from petrochemical, pharmaceutical, coke oven plant etc contains high phenolic compounds along with appreciable amount of COD. COD removal from wastewater can be obtained by either pure culture organism or by mixed culture system. As wastewater from industry contains mixed nature of organic matters and toxic compounds, and the maintenance of pure culture in industry for the treatment process of wastewater is difficult to achieve, so it is advisable to use mixed culture for treatment of wastewater having high COD and phenol concentration.
Phenol biodegradation by pure cultures of bacteria has been described by substrate inhibition models [4-7]. Otherwise, when mixed cultures are used there is no unique kinetic model for general agreement, but phenol is often considered as inhibitory at high concentrations [8-10]. At low substrate concentration, however, the inhibition is negligible; the model proposed by Monod may be used to describe the biodegradation process by either pure or mixed cultures. Furthermore, if the substrate concentration is much lower than the half-velocity constant (K), then a first-order kinetic model often applies.
Wastewater having high concentration of COD and phenolic compounds can be treated mainly by either physico-chemical method e.g. adsorption, chemical coagulation, membrane filtration process or by biological methods such as activated sludge or anaerobic cultures under acclimatized condition. Some researchers consider activated sludge more attractive due to its various advantages [11-13]. Activated sludge reactors have been widely used for COD and phenol removal from industrial wastewater [14-16], but immobilized cell reactors offer several advantages over suspended cell reactors. These include higher biomass concentrations, allowing higher loading rates, and resistance to shock loading of inhibitory compounds, then requiring less time to revert to normal operation. Fixed film reactors are receiving increasing interest in wastewater treatment. Most of the works on fixed bed reactor have been done in anaerobic condition [17-19]. Few works have been done on combined aerobic-anaerobic fixed film reactor system for COD removal. G. R. Moosavi et al. [20] investigated the COD removal for high strength organic wastewater in this reactor and found COD removal efficiency about 95% under organic loading of 0.8-7.6 kg CODm-3day-1. R. Del Pozo and V. Diez [21] have studied the COD removal for organic matter containing wastewater in aerobic-anaerobic packed bed reactor and they found the efficiency to be 92% at organic loading of 0.39 kg COD m-3day-1. They have also found that most COD removal occurred mainly by aerobic process. In another study, Lorenzo Bertin et al. [22] has shown that an aerobic reactor, fed with olive mill effluent from anaerobic GAC reactor, could be a stable process to remove 24% and 39% of organic load under 50.42 and 2.04 gL-1day-1 of COD and phenol loading rates, respectively. In the present work, the activated sludge was collected from a return line of a nearby sewage treatment plant. The microbial consortia was grown on glass beads forming a fixed biofilm and used for further experiment. The main purposes of the present investigation are as follows: 1) To understand the behavior of the reactor under different operating conditions like different hydraulic and organic loading and finding out the optimum operating conditions of it. 2) To find out the change in efficiency of the reactor to treat the carbohydrate containing wastewater in the presence and absence of phenol. 3) To use the experimental data for fitting in the kinetic model for COD removal.
2. Materials and Methods
2.1. Collection of Inoculum
Biomass from a return line of a sewage treatment plant operating as activated sludge process was collected as inoculum for the reactor starting up. The inoculum was first grown as batch mode with nutrient medium in the reactor and air was supplied from the bottom through a mini-compressor. The culture was replenished with fresh substrate medium time to time so that the bacterial culture can get sufficient nutrient for their growth. After 6 weeks of operation, a thin film coating was found to develop on the glass beads, and then the operation was changed to continuous mode. Microorganisms were first grown on easily degradable substrate (up to 1000 mgL-1 COD contributed) and then acclimatized to phenol. Acclimatization of the activated sludge sample grown in the trickle bed reactor was carried out to initiate the ability of the sludge to biodegrade phenol along with other organic matter.
2.2. Nutrient Medium and Culture Condition
The required nutrients for the growth of the microorganisms were supplied by the medium with compositions as follows in 1L of solution: Glucose 0.5 g, peptone 0.2 g, beef extract 0.2 g, Yeast extract 0.2 g, K2HPO4 0.3 g, KH2PO4 0.15 g, NH4Cl 0.2 g, CaCl2 0.1 g, FeCl3 0.1 g, MgCl2 0.1 g. The glucose concentration was varied to vary the inlet COD to the reactor according to need of organic loading variation, while peptone and yeast extract concentration dropped to zero during the study of COD removal performance of the reactor.
2.3. Analytical Procedure
COD was measured by standard closed reflux method in HACH (USA) make reflux apparatus. The sample was oxidized with K2Cr2O7 in a strong acidic condition (H2SO4), followed by titration of the excess dichromate with Mohr salt solution. Phenol was determined by standard 4-amino antipyrene method.
2.4. Experimental Set Up
The experimental set up is shown in Figure 1. The bioreactor was 65 cm high with 2.5 cm internal diameter of cylindrical Borosil make glass column. Glass beads with average 3 mm diameter were used as inert support media for the biofilm growth. There was an in-built perforated support at the bottom to hold the glass beads. The treated water after filtration was taken as the effluent. Inlet and outlet for the wastewater were provided at the top and bottom of the bioreactor, respectively. There were two other sampling ports at 25 cm and 45 cm from the top of the reactor for collecting intermediate samples at times. The synthetic wastewater was prepared with glucose, as concentration ranging from 100-400 mgL-1 by diluting the glucose stock in distilled water along with necessary nutrients in proportionate amount as stated above. The synthetic wastewater was fed to the reactor from an aspirator
bottle in the down-flow mode. The effluent was collected from the bottom outlet of the reactor and filtered to remove the suspended solids. Then the samples are tested for residual COD. All the COD removal studies were done for a single pass of wastewater though the bioreactor without any recirculation. After finding out the optimum hydraulic and organic loading rates for the reactor, 50 mgL-1 phenol was fed to the reactor having glucose as the major carbon source other than phenol in the synthetic wastewater. In these set of experiments, residual COD was tested in the effluent.
2.5. Hydraulic Loading Rates (HLR)
The flow rates of the wastewater fed to the reactor were calculated in order to operate at low hydraulic loading rates. Although there is no general agreement about limits for operating in this regime, but first superficial velocities were selected in such a way that the corresponding hydraulic loadings ranged from 4.27-9.96 m3m-2day-1. The reactor performances in steady state conditions were evaluated at four hydraulic loading rates: 4.27, 6.4, 7.11 and 9.96 m3m-2day-1.
2.6. Organic Loading Rates (OLR)
For each of the hydraulic loading rates, initially, the bioreactor was fed with synthetic wastewater containing carbohydrate without phenol at different organic loadings. Total COD of the influent for each run is measured. The inlet COD concentrations range were 100-400 mgL-1 corresponding to organic loading rate varying from 0.72-4.8 Kgm-3day-1. Percentage removal of COD at each HLR and OLR were determined. The COD removal performances of the reactor at the optimum HLR were also examined for treatment of wastewater having 50 mgL-1 of phenol along with glucose as organic matter. The concentration of glucose in the influent wastewater was varied in such a way that the total organic loading fell within the range of OLR stated above even in presence of phenol in inlet wastewater.
3. Results and Discussion
3.1. Effect of Hydraulic Loading Rates on Organic Removal Efficiency
Figure 2 shows that the percentage COD removal vs. hydraulic loading at different organic loadings. It is seen from the above figure that for a particular HLR, the removal efficiency decreases as OLR increases. Organic matter was well degraded almost 100% corresponding to the hydraulic loading of 4.27 m3m-2day-1 and organic loading of 0.72-1.44 Kgm-3day-1. At OLR of 2.037 Kg m-3day-1, the percentage COD removal was 88.33, but a further increase in OLR, a sharp descend of COD removal to 60% is observed for the HLR at 4.27 m3m-2day-1. Further, when HLR is increased to 6.4 m3m-2day-1, then also 100% COD removal is achieved at OLR of 1.08 Kg m-3day-1. The COD removal was decreased to 83% when the applied OLR was doubled at 2.16 Kgm-3day-1, keeping the HLR constant at 6.4 m3m-2day-1. While studying the performance of the reactor at higher HLR i.e. at 7.11 m3m-2day-1, 100% removal was possible to achieve only up to an OLR of 1.2 Kgm-3day-1, beyond that, a further increase in OLR to 2.4 Kgm-3day-1, the percent removal was declined significantly to 66.5%. An Increase of HLR up to a value of 9.96 m3m-2day-1 exhibited poor removal efficiency.