Comparison of startup and anaerobic wastewater treatment in UASB, hybrid and baffled reactorAbstractAn experimental study was carried out to compare the performance of selected anaerobic high rate reactors operated simultaneously at 37 °C. The three reactors,namely up¯ow anaerobic sludge bed reactor (UASB), hybrid of UASB reactor and anaerobic ®lter (anaerobic hybrid reactor ± AHR) and anaerobic baf¯ed
Comparison of startup and anaerobic wastewater treatment in UASB, hybrid and baffled reactor
Abstract
An experimental study was carried out to compare the performance of selected anaerobic high rate reactors operated simultaneously at 37 °C. The three reactors,namely up¯ow anaerobic sludge bed reactor (UASB), hybrid of UASB reactor and anaerobic ®lter (anaerobic hybrid reactor ± AHR) and anaerobic baf¯ed
reactor (ABR), were inoculated with the anaerobic digested sludge from municipal wastewater treatment plant and tested with synthetic wastewater. This wastewater contained sodium acetate and glucose with balanced nutrients and trace elements (COD 6000 mg á l)1). Organic loading rate (Bv) was increased gradually from an initial 0.5 kg á m)3 á d)1 to 15 kg á m)3 á d)1 in all the reactors. From the comparison of the reactors’ performance,the lowest biomass wash-out resulted from ABR.In the UASB, signi®cant biomass wash-out was observed at the Bv 6 kg á m)3 á d)1, and in the AHR at the Bv 12 kg á m)3 á d)1. The demand of sodium bicarbonate for pH maintenance in ABR was two times higher as for UASB and AHR. The ef®ciency of COD removal was comparable for all three reactors ± 80±90%. A faster biomass granulation was observed in the ABR than in the other two reactors. This fact is explained by the kinetic selection of ®lamentous bacteria of the Methanotrix sp.under a high (over 1.5 g á l)1) acetate concentration.
1
Introduction
Up¯ow Anaerobic Sludge Bed (UASB) reactor, Anaerobic Hybrid Reactor (AHR) and Anaerobic Baf¯ed Reactor (ABR) belong to the group of high-rate anaerobic reactors with a sludge bed. Granular biomass with high methanogenic activity and excellent settling properties can be cultivated in these reactors. The UASB reactor (in sequel as UASBR) consists of a sludge bed in the lower part and a three phase separator (g-l-s separator) in the upper part of the reactor [1]. The AHR is a combination of the UASBR and the anaerobic ®lter [2]. A layer of biomass carrier is situated in the upper part of the AHR instead of the g-l-s
separator. This layer separates the bubbles of the biogas from the biomass and acts as a support material for the biomass growth as well. The layer even has a notable ef- ®ciency as a suspended solids (SS) separator [3]. The ABR (®rst described by Bachmann [4]) contains a mixed culture of anaerobic microorganisms which is divided into compartments by baf¯es (mostly vertical baf¯es are used).A sequence of sludge blanket reactors is created in this simple way. An ABR performed in a propriet mode allows high hydraulic loadings connected with low biomass washout [4, 5]. The general principles of the anaerobic sludge
granulation were described by Hulshoff Pol [6].
In our previous works, we dealt with the performance of the above mentioned reactors one by one [3, 5, 7]. Even if there were some differences in the substrates used as a feed for these reactors, the similarity of them was high enough to make possible the comparison of the reactors performance (treatment ef®ciency, biomass wash-out,biomass granulation). In all of the cases, the substrate consisted of saccharides with lower molecular weight (glucose and sucrose) and the Volatile Fatty Acids (VFA),total COD was 4000±6000 mg á l)1.
Hence this work is dedicated to a detailed comparison of the performance of three different types of anaerobic high rate reactors performed under the same conditions with the same feed.
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3
Results and discussion
3.1
Hydraulic characteristics of the reactors
As already mentioned in the ``Experimental’’, the hydraulic characteristics were measured in reactors without biomass.The obtained distributions of the residence times in the reactors (so-called C-curves) are shown in Figs. 2±4.
H is a reduced (dimensionless) time:
where t is the real time and t is the mean (or theoretical) residence time.
The variable C expresses the ratio C . c=c0 where c is the tracer concentration in the ef¯uent and c0 is the concentration of the tracer that could be obtained if the amount of the tracer added to the reactor got dispersed throughout the whole reactor volume.
To characterise the residence time distribution, the mean value of distribution l is de®ned by the general
equation:
and the dispersion r2 x can be characterized as:
In this case of distribution .C . f .t.., the real value of the mean residence time is:
while the dispersion of the residence times can be calculated as:
Besides the C-curve, the system is characterized by the dispersion number (D/ul) and the theoretical number of ideally mixed reactors N (equal size reactors connected in series which as for performance correspond to the investigated system). The theoretical number of ideally mixed reactors can be calculated as the reciprocal value of the dispersion r2 x:
For hydraulically closed systems, the dispersion r2 x can be calculated from the relation:
where D is the longitudinal dispersion coef®cient, u the mean ¯ow velocity through the reactor, and L the length of the tank.
The obtained values of the dispersion coef®cient (D),dispersion .r2 x. and the theoretical number of ideally
mixed reactors (N) are given in Table 3.
It is interesting that the differences in the construction of the AHR and the UASBR (biomass carrier in the AHR and three phase separator in the UASBR) had no signi®-cant effect on the value of the dispersion number (D) and the dispersion .r2 x.. It can be assumed that the biogas production in the presence of biomass and wastewater will change the situation. The characteristics of the ABR (Table 3) are similar to those, obtained by Grobicki and Stuckey [11] for an ABR with the same number of compartments (4) and a similar hydraulic retention time (50±60 h).
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3.2
Performance of the reactors
The ®rst intention was to load the reactors equally and to gradually increase the organic loading (Bv). But the behaviour of the reactors was different even at low loadings.Thus the increase of Bv was adjusted according to the state and the performance of the individual reactors, that is treatment ef®ciency, concentration of VFA in the reactor and biomass wash-out. In the following period the loadings of the AHR and the UASBR were the same, while the loading of the ABR differed a little especially in the early
start-up phase. The time courses of Bv are represented in Figs. 5 (ABR) and 6 (AHR and UASBR).
The initial Bv of the reactors was 0.5 kg á m)3 á d)1. But already the ®rst increase of the Bv to 0.8 kg á m)3 á d)1 on the 7th day evoked a different response of the reactors.While the COD of the ABR ef¯uent remained almost constant (150 mg á l)1) and the difference between the COD of the ®ltered and the non ®ltered ef¯uent (in sequel as ``COD difference’’) was less than 50 mg á l)1, the COD of the AHR and UASBR ef¯uents reached 450 mg á l)1 and it took several days until it turned back to 250 mg á l)1. Time courses of VFA and COD in the ef¯uents from the ABR,AHR and UASBR are shown in Figs. 7±9, respectively.
The pH in the AHR and the UASBR was nearly constant,in the range of 7±7.3. On the contrary, the pH in the ABR rose from the ®rst to the last compartment. The average pH in the compartments were 6.3; 6.6; 6.7; 6.8. The pH of the ABR ef¯uent was 7. At a step change of the ABR loading to Bv = 3 kg á m)3 á d)1 a sharp drop of the pH occurred, especially in the ®rst compartment, below 6.Thus the NaHCO3 dosage to the ABR substrate was doubled ± up to 1 g NaHCO3 per g COD. Even if there were no more sharp decreases of the pH, its co-current increase in the compartments was observed during the whole
performance of the ABR.
The COD difference of the ef¯uents quantifying the amount of suspended solids (SS-COD), is shown in
Fig. 10. More intensive biomass wash-out from the sludge bed in the AHR and the UASBR was ®rst observed at a Bv of about 6 kg á m)3 á d)1, but it manifested itself only by an increased suspended solids content of the UASBR ef- ¯uent. The unchanged COD difference of the AHR ef¯uent was caused by an intensive separation of SS in the layer of biomass carriers. Higher COD difference in this reactor was observed only at a Bv of 12 kg á m)3 á d)1.The COD difference above the sludge bed in the ABR
increased already at Bv = 3 kg á m)3 á d)1. It was highest in the ®rst compartment and had a decreasing tendency along the reactor. However, no signi®cant biomass washout from the ABR occurred during the whole performance period. Initial biomass granulation in the ABR was observed after 35 days of performance at a Bv of 2.5±3 kg á m)3 á d)1 and after 60 days of performance and a Bv of about 6 kg á m)3 á d)1 in the AHR and the UASBR.The operation of the reactors was ®nished at a Bv of 15 kg á m)3 á d)1.
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The comparison of the reactors from the point of view of VFA concentration is interesting (Figs. 7±9). The concentration of VFA in the AHR and the UASBR was nearly the same. Only a minimal concentration gradient was noted along the reactors and the concentration of the individual VFA’s was less than 400 mg á l)1. The total concentration of VFA was always less than 840 (UASBR) an 760 (AHR) mg á l)1. Propionic and acetic acid were the prevailing VFA components. The situation was quite different
in the case of the ABR. The concentration gradient was very intensive along the reactor. The total concentration of VFA was highest in the ®rst compartment, never less than 1000 mg á l)1 (in most of the cases more than 1500 mg á l)1), and it decreased gradually towards the last compartment where it was 173 mg á l)1 in average,20±800 mg á l)1 in dependence on the loading. Acetic,propionic and L-lactic acid were the major VFA while in the ®rst compartment formic, butyric and valeric acid were present, too.
Figures 11A±C show the course of VFA concentration in the ef¯uents from the reactors after an increase of the loading from 9 to 12 kg á m)3 á d)1. Hydraulic retention time was 12 hours in this period. This increase was applied after 80 days of ABR and 94 days of AHR and UASBR performance. The response of the reactors was very similar and they regained their stable performance in two weeks.The step increase of the loading evoked an increase of the concentration of acetic and propionic acid in all reactors.The highest increase of the other VFA was observed in the UASBR. On the contrary, the AHR exhibited no increased
concentration of these VFA.
It seems to be that a high VFA concentration in the ®rst compartments of the ABR could contribute to the faster biomass granulation in the ABR than in the AHR and the UASBR. This idea is supported by the results published by Morvai et al. [12]. According to them, the modi®ed Haldane’s equation, which considers the substrate inhibition,better describes the kinetics of the fermentation of acetic acid than the Monod’s equation:
where q is the speci®c methane production, qmax is its maximal value in the case without inhibition, KS is a
constant (equal to the concentration where q . qmax=2., KI is the constant of the inhibition and n is the coef®cient of the sensitivity.
A comparison of methane production kinetics of a ``raw’’ sludge with no granule formation and a granulated sludge is represented in Fig. 12 applying the modi®ed Haldan’s equation according to Morvai [12]. The recommended concentration of acetate for biomass granulation is less than 0.2 g á l)1 [6]. As it is obvious from Fig. 7,conditions favouring the growth of granule forming microorganisms (mostly Methanotrix sp.) are not only acetate concentrations below 0.2 g á l)1, but even in the range of 1.5 g á l)1 and more. At an acetate concentrations over 1.5 g á l)1, the growth of non-®lamentous microorganisms (mostly Methanosarcina sp.) is much more limited by a substrate inhibition. According to this theory, conditions
in the ®rst compartments of the ABR could be more convenient for the growth of Methanotrix sp. bacteria than the conditions in the AHR and the UASBR.
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4
Conclusion
The performance of three different types of reactors ± UASB reactor, Anaerobic Hybrid Reactor and Anaerobic Baf¯ed Reactor was investigated in this work. Regarding the process of biomass granulation in these reactors,which were fed by a synthetic substrate on the basis of glucose and sodium acetate, the following conclusions can be drawn:
• The dispersion number obtained for the AHR and the UASBR was two times higher than that one obtained for the ABR.
• The lowest biomass wash-out was observed from the ABR. Intensive biomass wash-out from the UASBR occurred at Bv = 6 kg á m)3 á d)1, while signi®cant biomass losses from the AHR were noticed, thanks to the separation effect of the carrier layer, only at Bv of 12 kg á m)3 á d)1.
• Due to an intense acidi®cation in the ®rst compartments of the ABR a doubled dosage of NaHCO3, as
compared to the dosage used in the other two reactors,was necessary to maintain the pH in the range suitable for the methanization.
• Negligible gradients of pH and VFA concentration were observed along the AHR and the UASBR. Exactly the contrary was the case of the ABR, where the differences along the reactor were notable. All four phases of the anaerobic process (hydrolysis, acidogenesis, acetogenesis,methanogenesis) proceed simultaneously in the AHR and the UASBR. Mixing with biogass prevents VFA accumulation at the bottom of the reactors and the acetic acid formed is immediately at the disposal of the acetotrophic methanogenic microorganisms. On the contrary, the compartmentalized design of the ABR is ideal to separate the phases of the anaerobic process.
• A faster biomass granulation was observed in the ABR than in the other two reactors. This can be explained by the kinetic selection of ®lamentous bacteria of the Methanotrix sp. under high (over 1, 5 g á l)1) acetate concentration, as decribed by Morvai et al. (1992).
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2
Experimental
All of the laboratory scale models used in this work (Fig. 1) were made of plexiglass. The size of the ABR was:43 cm length, 13 cm width and 30 cm height. The useful volume of the reactor (13.05 l) was divided into four compartments. A proper construction of the baf¯es allowed a wastewater ¯ow through the sludge bed from bottom to top. The water level was about 1 cm above the upper edges of the baf¯es between the ascending and descending sections. The AHR and the UASBR were made from a tube with an inside diameter of 7 cm, and the volume of each of them was 3.325 l. The upper part of the AHR was ®lled with a tubular plastic carrier. The characteristics of these carriers were: 15 mm inside diameter,19 mm outside diameter, 20 mm length and a speci®c surface of 544 m2 á m)3. The porosity of the carrier was 0.93, and 23.4% of the reactor volume was ®lled with the carriers.
The biogas from the reactors was bubbled through a 4 mol á l)1 solution of NaOH (entrapment of CO2 and H2S) and subsequently the amount of the CH4 produced was measured. Hydraulic characteristics, i.e. the distribution of the residence times in the reactors were measured using an impulse addition of a tracer to the reactors without biomass. KCl was used as a tracer and the reactors were fed with a distilled water. The concentration of KCl in the ef¯uent was measured conductometrically. The dependence of the conductivity of KCl solution on the concentration of KCl in distilled water was determined prior.
The reactors were inoculated with anaerobically stabilizedsludge from the central wastewater treatment plant Bratislava ± VrakunÏa. The amounts of the sludge used for the inoculation of the ABR, AHR and UASBR were 7.85 l,2 l and 2 l respectively (60% of the reactor volume). The average volatile suspended solids content of the sludge was 30.2 g á l)1. The synthetic wastewater used as a feed (Table 1) was enriched with trace elements adding 1 ml of trace elements stock solution (Table 2) to 1 l of synthetic
wastewater. The temperature of the reactors was held at 37 °C. All analyses were carried out according to Standard Methods [8]. Isotachophoresis was used for the measurement of VFA [9], while the titrimetric method of Kapp [10] was used to determine the total amount of VFA.
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我给你个建议，用软件翻译后，自己看词造句，加上专业名字从新组合后就是了
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我给你个建议，用软件翻译后，自己看词造句，加上专业名字从新组合后就是了
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伙计，这有些不是很困难，自己拿个词典边查边翻！人生都有第一次吗！呵呵
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就是啊
干吗不好好用词典呢
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