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Enhancing anaerobic treatment of wastewaters containing oleic acid
No full textINTRODUCTIONLipids are one of the major organic pollutants in municipal and industrial wastewaters. Although domestic sewage typically contains about 40-100 mg/I lipids (Forster, 1992; Quéméneur and Marty, 1994), it is industrial wastewaters that are of greater concern when considering the higher lipid concentrations in the discharged effluents. Typical industries that generate lipids-containing wastewaters are dairy, edible oil and fat refinery, slaughterhouse and meatprocessing, rendering and wool scouring.In anaerobic wastewater treatment, lipids are readily biodegradable. However, the practical problems arisen in anaerobic treatment of lipids mainly are due to (i) inhibition of the methanogens and acetogens by long-chain fatty acids (LCFA), and (ii) washout/flotation of the biomass. These two problems manifest themselves particularly in the high-rate treatment systems. Among these systems, unsatisfactory treatment results in full-scale upflow anaerobic sludge bed (UASB) reactors and in lab-scale expanded granular sludge bed (EGSB) reactors are frequently encountered.This thesis is directed to find solutions for the problems and, consequently, to guarantee the efficiency and reliability of the two above-mentioned high-rate reactor systems because the UASB presently are and the EGSB potentially will become the most widely applied anaerobic wastewater treatment processes. In this respect, the research in this thesis was focused on the cytotoxicity, biosorption and biodegradation of LCFA. In addition to the achievement of the up-to-date highest loading rate, several novel findings from our investigations relevant to the insight of the complicated relationships between toxicity, sorption and degradation may facilitate the future treatment of wastewaters contaminated with LCFA.Based on the experimental results obtained in this study, we recommend five methods as solutions for the practical problems and as the state-of-the-arttechniques for the ultimate treatment. The five methods that are to be described in this Chapter are listed as follows: Use of granular sludge as inoculum;Acclimation of sludge to long-chain fatty acids;Application of thermophilic conditions;Prevention of excessive sorption; andRecirculation of washed out biomass. USE OF GRANULAR SLUDGE AS INOCULUMComparative toxicity of oleate to seven anaerobic sludges from various origins was studied (Chapter 2). The oleate toxicity on methanogenic activity was found to be rather closely correlated to the specific surface area of sludge than to the sludge origin and methanogenic activity, even though acclimated sludge (preexposed to LCFA) was the more active and the less susceptible. The suspended and flocculent sludges are much more susceptible to the toxicity than are the granular sludges. The oleate IC 50 levels derived for the granular sludges were 3-13 times higher than those for the suspended sludges at 40°C (Chapter 2) and 2 times higher than that for the flocculent sludge at 55°C (Chapter 3). Since the recovery of methanogenic activity for granular sludge after the occurrence of inhibition will take time periods from one week to more than one month (Chapter 3), it is reasonable to expect that the time required for suspended/flocculent sludge will even be longer.Apart from the lower susceptibility to oleate found for granular sludge in the present research, the high biomass densities in the granules minimize the distances between bacteria and maximize interspecies transfer of acetate and hydrogen between syntrophic fatty acid degraders and methanogens (Pauss et al., 1990; Thiele, et al., 1990). This of course favors the syntrophic degradation of LCFA.Regarding the full-scale application, the availability of sufficient amount of granular sludge has to be taken into consideration. To date, over 930 full-scale UASB reactors have been built (Habets, 1997) and more are under construction. This means that granular sludge will become available in near future in increasing amount. The use of granular sludges as inocula for start-up of reactors treating LCFA-wastewaters is, therefore, an appropriate strategy. ACCLIMATION OF SLUDGE TO LONG-CHAIN FATTY ACIDSThe expected synergistic toxic effect that should be exerted by the LCFA-mixture (50% oleate, 35% palmitate and 15% stearate) was not observed for the granular sludge well acclimated to slaughterhouse wastewater. On the contrary a longer lag period of methane production in the non-acclimated granular sludge occurred due to oleate inhibition (Chapter 4). Moreover, in a batch test the sludge pre-exposed to LCFA (82% oleate) showed 2 times higher oleate degradation rate than did the non-exposed sludge. In continuous flow experiments, the reactor inoculated with the non-exposed sludge fails already at the influent concentration of 500 mg LCFA- (82% oleate-) COD/l while the reactor with preexposed sludge can successfully treat 4000 mg LCFA- (82% oleate-) COD/l (Hwu et al., 1997).The requirement of acclimation may turn into a bottleneck of the LCFA-wastewater treatment because the sources of LCFA-/Iipids-adapted granular sludge are currently limited (Hulshoff Pol, 1997) and the growth rate of LCFA-degraders is as slow as 0.3 d -1(Angelidaki and Ahring, 1995). Hence, for fullscale treatment the introduction of LCFA- wastewaters to reactors should start at low concentrations and allow acclimation and retention (see below) of the bacteria capable of LCFA degradation. APPLICATION OF THERMOPHILIC CONDITIONSThe oleate degradation rate and IC 50 level of an LCFA-exposed granular sludge were compared at thermophilic (55°C) and mesophilic (30°C) conditions (Chapter 3). Although oleate IC 50 levels obtained at mesophilic (2.8 mM) was 4-fold higher than that at thermophilic conditions (0.7 mM), the oleate degradation rate at thermophilic (124 mg oleate- COD/g VS·d) was also 4--fold higher than that at mesophilic conditions (33 mg oleate- COD/g VS·d). This means the thermophiles are more susceptible to oleate toxicity but, on the other hand, their growth rate is much higher and thus would rapidly recover the lost activity due to LCFA-inhibition by a rapid increase of bacterial population.When exposed to a relatively high sludge loading, i.e., 1.2 g oleate-COD/g VS, the thermophilic sludge took a 5-day time period to revive the methanogenic activity similar to the control while the mesophilic sludge took 28 days to achieve the same activity (Chapter 3). These results provide an important implication for the LCFA-wastewater treatment in practices, viz., thermophilic reactors can sooner be re-operated after shutdown by shock loadings. PREVENTION OF EXCESSIVE SORPTIONSorption of LCFA onto the surface of sludge granules leads to sludge flotation and inhibition (Chapter 4). The higher LCFA concentrations result in the higher sorption rates as well as larger sorption amounts, therefore induces the more serious flotation and inhibition. In batch tests, we estimated that an increase of 45 mg LCFA/g TS in adsorption amount can cause a 10% decrease in methane production rate. In a UASB reactor, the flotation of granular sludge started at the sludge loading rates exceeding 0.09 g LCFA- COD/g VSS.d, while the complete flotation occurred at the loading rates exceeding 0.2 g LCFA-COD/g VSS.*d.Flotation of both sludge granules and LCFA clusters occurred when LCFA were treated as the sole substrate in EGSB reactors operated at hydraulic retention times (HRT) < 6 h and liquid superficial upflow velocities (V up )>3 m/h (Chapter 5). Although a COD removal efficiency of 73% could be attained, the highest methane recovery achieved was below 15% (corresponding to a low conversion rate of 1.6 mg LCFA-COD/g VS·d). When the HRT was prolonged to 24 h (V up = 1, 4 or 7 m/h) and glucose or butyrate was supplied as cosubstrate (Chapters 5 and 6), the flotation became insignificant and LCFA degradation increased significantly. Herein, the most satisfactory results obtained have been 95% COD removal and 87% methane recovery (corresponding to the conversion rate of 93 mg LCFA-COD/g VS·d). in a reactor operated at HRT = 24 h and V up = 1 m/h (Chapter 6).It therefore is clear that the prevention of excessive (negative) LCFA adsorption onto granules' surface requires enhancement of LCFA biodegradation. This can be achieved by applying a longer substrate-biomass contact time, i.e., a prolonged HRT; provided that the present reactor system is in common use. RECIRCULATION OF WASHED OUT BIOMASSAlthough the conversion rate in continuous-flow reactors reached up to 93 mg LCFA- COD/g VS·d). (Chapter 6), this rate was still lower than that obtained in batch experiments, i.e., 124 mg oleate-COD/g VS·d). (Chapter 3). The difference between these two rates would be even more significant if the LCFA-acclimated sludge inoculated in the continuous- flow reactors (Chapter 6) also would have been used in the batch experiments (Chapter 3). Since almost all granular sludge was retained in the continuous reactor, the observed difference draws a question: should any other factors influenced by reactor hydrodynamic parameters contribute to the difference?Based on the observations in Chapter 5 that the higher V up results in the lower LCFA conversion, we first speculated that the breakage of granules found in a higher V up deteriorates the syntrophic degradation of LCFA. We therefore conducted experiments using two reactors operating at the V up of 1 and 8 m/h (Chapter 6). When the reactors were changed to closed systems, despite the diameter of granules became two times smaller at 8 m/h, any significant differences of oleate conversion were not found between the two distinct V up These results indicate that this speculation is inconclusive.The answer for the question remained unclear until batch tests were conducted by use of the granular (diameters = 1-3 mm) and washed out (diameters = 50-100 μm) sludges originating from the same reactor operated at HRT = 24 h and V up 1 m/h (Chapter 6). The LCFA conversion rate of the washed out sludge was 129 mg LCFA-COD/g VS·d). while that of the granular sludge was 84 mg LCFA- COD/g VS·d). It therefore is clear that the loss of the fine biomass bearing highly active LCFA degradability will impede the reactor performance.Subsequently, we verified the importance of in-reactor retention of the washed out biomass (Chapter 6). By recycling the washed out biomass to the reactor also operated at HRT = 24 h and V up = 1 m/h, we eventually obtained a COD removal efficiency of 97% and an LCFA conversion rate of 304 mg LCFA-COD/g VS·d). Moreover, in comparison with the reactor operated at the same hydrodynamic parameters but without returning the washed out biomass, the recycled reactor system achieved an 18% higher conversion rate based on the same sludge loading rates imposed in both reactor systems. The increased conversion rate can be attributed to the increase of LCFA-degraders concentration that is due mainly to the recycling. The best treatment performance achieved in this thesis is the up-to-date highest among those reported in anaerobic bioreactor systems treating LCFA- containing wastewaters. REFERENCESAngelidaki, I. and Ahring, B.K. (1995). Establishment and characterization of anaerobic thermophilic (55°C) enrichmentculture degrading long-chain fatty acids. Appl. Environ. Microbiol., 61, 2442-2445.Forster, C.F. (1992). Oils, fats and greases in wastewater treatment. J. Chem. Technol. Biotechnol., 55, 402-404.Habets, L.H.A. (1997). Personal communication. Paques BV, PO Box 52, 8560 AB Balk, The Netherlands.Hulshoff Pol, L.W. (1997). Personal communication. Department of Environmental Technology, Wageningen Agricultural University, PO Box 8129, 6700 EV Wageningen, The Netherlands.Hwu, C.-S., van Beek, B., van Lier, J.B. and Lettinga, G. (1997). Effects of cosubstrates and sludge acclimation on anaerobic degradation of oleic acid by granular sludge. in preparation.Pauss, A., Samson, R. and Guiot, S. (1990). Thermodynamic evidence of trophic microniches in methanogenic granular sludge bed reactors. Appl. Microbiol. Biotechnol., 33, 88- 92.Quéméneur, M. and Marty, Y. (1994). Fatty acids and sterols in domestic wastewaters. Water Res., 28, 1217-1226.Thiele, J.H., Wu, W.-M. and Jain, M.K. (1990). Ecoengineering high rate anaerobic digestion systems: analysis of improved syntrophic biomethanation catalysts. Biotechnol. Bioeng., 35, 990-999
Adsorption of Long-Chain Fatty Acid onto Granular Sludge and Its Effect on Upflow Anaerobic Sludge Bed Reactor Operation
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