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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niclaoniu
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加载更多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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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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