Membrane Bioreactor (mbr) as an Advanced Wastewater Treatment Technology
Removal of Organic Matter and Suspended Solids
tải về 0.95 Mb. Chế độ xem pdf
|
TRICHLUCTHUADAT LIEN 1, TRICHLUCTHUADAT LIEN 1, TRICHLUCTHUADAT LIEN 1, TRICHLUCTHUADAT LIEN 1
| 5.3
Removal of Organic Matter and Suspended Solids Knowledge about COD removal mechanisms that occur when mixing an ac- tivated sludge with real wastewater is still scarce. The microbial response to dynamic conditions in a real wastewater treatment unit can be different from a simple increase in cell number (i.e., growth of microbial population), and include other substrate-removal mechanisms like sorption, accumula- tion, and storage [103]. There have been several investigations on treatment efficiencies of MBR and CAS processes operating under comparable con- ditions that have shown significantly improved performance of an MBR in terms of COD, NH 3 -N and SS removals [3, 30, 40, 51, 83, 104–111]. There are several factors that may contribute to the lower organic carbon content of MBR effluents as compared to CAS processes, like longer retention times, smaller floc sizes, etc. Côté et al. [100, 112] attributed the improved COD removal to the avoidance of biomass washout problems commonly encountered in activated sludge pro- cess, as well as to complete particulate retention by the membrane. Membrane rejection of a significant amount of soluble organic molecules and colloids makes their removal more effective due to a higher lyses activity in the re- actor induced by elevated concentrations of these compounds. Higher sludge ages that are achieved by long SRTs allow more complete mineralization of biodegradable raw water organics, but also an adaptation of microorgan- isms to less biodegradable compounds. Therefore, biomass can acclimatize to wastewater without being restricted to fast-growing and floc-forming mi- croorganisms. In a study of Al-Malack et al. [95], COD removal efficiency in immersed MBR was found to increase significantly with increase in MLSS concentration, however, the effect of SRT on permeate COD became insignif- icant for MLSS concentrations above of 3 g L –1 , which probably means that the organic loading rate was not high enough to show a significant differ- ence at higher biomass concentrations. Since typical sludge concentrations for immersed MBRs are between 15 and 25 g L –1 , elimination of organic matter and turbidity is almost independent on SRT, and average removals normally achieved for COD and SS are over 90 and nearly 100%, respectively [112]. Membrane Bioreactor (MBR) as an Advanced Wastewater Treatment Technology 63 Better performance of MBR operated at long SRTs can also be explained by the presence of dispersed bacteria that are advantageous in the overall pop- ulation competition when substrate concentration becomes very low, i.e., at low F/M ratio and high sludge age. Flocs in a bioreactor were found to be smaller (Fig. 13) [56], which can explain enhanced mass transfer for both oxygen and carbon, thus enabling a higher removal rate and more adaptabil- ity to changes in the influent quality and quantity [53, 113]. In another study it was demonstrated that the flocs were more active and displayed greater species diversity [104]. The overall capacity of biomass to degrade different carbon substrates does not change significantly at different SRTs, which confirms that MBR is capable of degrading a wide variety of carbon substrates in a similar fashion. This robustness of MBR treatment regarding turbidity and organic matter removals was confirmed in several studies [114]. Xing et al. [58] recorded high treatment efficiency regardless of the absolute level of sludge concentra- tion in the MBR, and unaffected by variations in SS and volatile suspended solids (VSS) influent concentrations. In another study, in spite of large fluctu- ations in the influent, COD effluent COD was always low and extremely stable, because upon the addition of organic substrates, biomass responded imme- diately with increased respiration activity [114]. It is assumed that there is an upper limit for organic loading rate in an MBR under which degradation performance is independent of biomass concentration and organic loading rate. Rosenberger et al. [44] found that for organic loading rates lower than 7 kg COD m –3 day –1 , COD removal in MBR was high and stable regardless of MLSS concentration and composition of microbial culture. Moreover, another study reported that the mineralization process was not impaired nor with the shifts in the morphological composition of microbial population, or even with the occurrence of high numbers of filamentous bacteria [44]. Pollice et al. [65] tested a performance of MBR when its start-up was done without any sludge inoculum. Biodegradation of the influent COD as well as complete Fig. 13 Sludge particle size distributions at different SRTs [56] 64 J. Radjenovi´c et al. nitrification were consistently obtained already in the first days of operation, which demonstrated the high responsiveness of MBR. In aerated MBR, COD loss is also a result of the production of volatile compounds that are released from the system under aerated conditions [92]. Concerning turbidity removal, due to a complete retention of particulate matter by the membrane, there are no suspended solids found in the MBR ef- fluent, unlike the effluent of a conventional process. The UF/MF membrane can capture all SS in the reactor because of its fine pore size [115]. There- fore, non-biodegradable organic compounds are removed through filtration of particulates and discharged with the sludge. Gander et al. [116] reported that membrane contribution to the removal of organic matter was approxi- mately 30%, this roughly equating to the insoluble fraction that was removed via active biomass. In another study with an external membrane module, total COD removal was 97% on average, where 85% was removed by the bioreactor and only 12% resulted from membrane separation [58]. As far as HRT is concerned, results of Sun et al. [113] indicated a clear in- fluence of the operation time on biomass concentration (Fig. 14). Short HRT brings up a higher concentration of biomass because the volumetric organic loading is bigger (more food is supplied to microorganisms), although the ox- idation of organic matter might not be complete. On the other hand, Chaize et al. [39] recorded a complete nitrification and organics removal at HRT of only 2 h. Fig. 14 Growth of MLSS concentrations in submerged MBR at different HRTs [113] Membrane Bioreactor (MBR) as an Advanced Wastewater Treatment Technology 65 Aeration flow is also one of the main factors that affect the biochemical process of BOD and COD removals. The right amount of oxygen needs to be provided to the microorganisms in response to their three specific demands: 1. carbonaceous BOD (conversion of the carbonaceous organic matter in wastewater to cell tissue and various end products), 2. nitrogenous BOD (in the process of nitrification ammoniacal nitrogen is oxidized to the intermediate product nitrite, which is then converted to nitrate), 3. inorganic COD (oxidation of reduced inorganic compounds within the wastewater) [117]. Biomass characteristics such as SMP and EPS strongly influence the oxygen transfer, so therefore they will determine the rate of organics re- moval. These compounds are also widely recognized as the main membrane foulants [118]. SMP is soluble and thus is in the liquid phase and EPS is bounded to cells and makes a part of the solid phase. In order to reach the active sites on the bacterial cell membrane, the oxygen needs to penetrate the liquid film surrounding the flocs and then diffuse through the floc matrix (EPS) [117]. EPS amounts differ with changes in microbial state and operating conditions of the bioreactor. In the intermittently aerated MBR they increase in proportion to the non-aeration time [119]. Nevertheless, Ujang et al. [89] reported no significant difference in COD removal efficiency when varying aeration and non-aeration time, indicating that in intermittently aerated MBR organic matter can be degraded both under aerobic and anaerobic conditions. Also, over-aeration can bring about poor sludge characteristics such as bad floc structure and rather low sludge volume index (SVI), which can then be related to fouling [101]. In conclusion, immersed MBR is strongly capable of resisting shock- loadings, and variations in the inflow turbidity and organic matter content have no effect on their removal efficiencies. The removal of organic pollutants in terms of COD and SS has been proven to be very high and a good-quality effluent can be achieved during long-term operation. However, how to op- erate MBR systems efficiently remains a topic of argument because there is a lack of information on the development of microbial community structure in MBRs during nitrification [57]. It is important to distinguish among these different contributions to the overall COD removed, in order to better under- stand the dynamics of the process and to build-up a useful basis for process designing and modelling. tải về 0.95 Mb. Chia sẻ với bạn bè của bạn:
|