Elsevier

Water Research

Volume 34, Issue 1, 1 January 2000, Pages 31-42
Water Research

Effect of nitrate on acetate degradation in a sulfidogenic staged reactor

https://doi.org/10.1016/S0043-1354(99)00132-3Get rights and content

Abstract

The optimisation of acetate removal in sulfidogenic reactors by adding nitrate to the last compartment of a baffled reactor was evaluated. In a 5.4 l baffled reactor, containing three equal compartments, a VFA mixture (acetate:propionate:butyrate ratio 1:2:2 on COD basis; pH 8) was treated under mesophilic (30°C) and sulfidogenic (COD:SO2−4 ratio: 0.5) conditions. At a specific sludge loading rate of 1.1 g COD gVSS−1 d−1, a COD and sulfate removal of 80 and 35%, respectively, was obtained. In the baffled reactor, staging of the sulfidogenic VFA degradation occurred. Propionate and butyrate were mainly degraded in the first compartment. Their degradation was incomplete, resulting in elevated acetate concentrations in Compartment I. In the second and third compartment of the baffled reactor, a net degradation of acetate took place. Acetate was the sole substrate present in Compartment III and residual acetate concentrations of about 600 mg/l were present in the effluent at the highest specific sludge loading applied. In the three compartments, development of sludges with different maximum specific acetate degrading activities was observed. The maximal specific activity with acetate was three times higher in sludge growing in Compartment III as compared to sludge growing in Compartment I. Batch experiments showed that the addition of about 500 mg/l nitrate (COD/nitrate ratio of 2.5) increased the specific acetate degradation rate by 300%. Nitrate was denitrified (pH optimum of 7.0–8.0) with a temporary accumulation of the intermediate nitrite. When 2.3 g/l nitrate (COD/nitrate ratio of 0.5) was dosed in compartment III of the baffled reactor, the effluent acetate concentration could be decreased from 600 to 100 mg/l.

Introduction

In sulfidogenic reactors, organic matter is predominantly removed by sulfate reducing bacteria. These anaerobic reactor systems are applied when a maximum sulfate reduction is wanted, i.e. for the removal of sulfur and/or heavy metals from wastewaters (Hulshoff Pol et al., 1998). In high-rate sulfidogenic reactors supplied with organic matter as electron donor, acetate degradation is the rate limiting factor (Visser et al., 1993, Dries et al., 1998). The maximum specific acetate (C2) removal rates at which a complete acetate removal is achieved amounts to 0.4 gC2 Chemical Oxygen Demand (COD) per g volatile suspended solids (VSS) per day (Omil et al., 1996). Higher acetate loading rates result in residual effluent acetate levels of the sulfidogenic reactor. If the sulfidogenic reactor is followed by a biological sulfide removal step, this volatile fatty acid (VFA) induces the activity of elemental sulfur reducing bacteria, which reduce the elemental sulfur back to sulfide (Janssen et al., 1997). Thus, these VFA decrease the sulfide removal efficiency of the post treatment.

The acetate removal of high-rate sulfidogenic reactors could be optimised by increasing the activity of facultative (an)aerobic bacteria as acetate scavengers. This can be done by dosing alternative electron acceptors, e.g. oxygen or nitrate, into the reactor system (Lens et al., 1998b). Oxygen supply to anaerobic granular sludge reactors has been applied to promote granulation (Shen and Guiot, 1996), enhance the degradation of xenobiotics (Shen et al., 1996) or induce sulfur cycling activity (Takahashi and Kyosai, 1991).

Besides oxygen, nitrate could also be used for acetate removal from sulfidogenic reactors. Nitrate is much more soluble in water than oxygen (several g/l) and acetate is an excellent carbon source for denitrifying bacteria (Ganaye et al., 1996). In anaerobic process technology, direct integration of denitrification and methanogenesis in a single reactor has been suggested as an alternative method to the classical concept of anaerobic organic carbon removal followed by nitrification/denitrification for nitrogen removal (Akunna et al., 1992, Hendriksen and Ahring, 1996). In addition, the combination of denitrification and methanogenesis has also been considered in drinking water production. Both immobilised mixed cultures and bioreactors have been investigated to remove excess carbon source, mostly methanol, from denitrified effluents by methanogenesis (Lin and Chen, 1995).

The effect of oxygen or nitrate addition on the treatment performance of sulfidogenic reactors has, to the best of our knowledge, not yet been documented. The aim of this study was to determine whether denitrification is a feasible option to eliminate rest acetate levels from VFA fed sulfidogenic reactors. Nitrate was chosen because of its higher solubility in water than oxygen. The effect of nitrate on acetate degradation by sulfidogenic granular sludge was studied in batch experiments. Moreover, the potential to decrease the effluent acetate concentration of a sulfidogenic baffled reactor by the addition of nitrate to its last compartment was evaluated.

Section snippets

Experimental set-up

A baffled PVC reactor (Fig. 1) with three compartments of 1.8 l each was used for the experiments. The working volume of the reactor was determined by the position of the effluent outlet, located at 0.1 m from the bottom of the reactor. The reactor had one large lid on top, containing gas exits and openings for pH electrodes and sampling ports. The reactor design did not separate the gas phases of the different compartments, so that all three compartments contained the same gas phase. The

Performance of the baffled reactor

Throughout the experiment, both propionate and butyrate were hardly present in the effluent of the baffled reactor (Fig. 3). In contrast, acetate was the main contributor to the effluent COD (Fig. 3D). The COD removal efficiency of the baffled reactor gradually increased to about 100% during the first 40 days of operation at an average OLR up to 0.5 gCOD gVSS−1 d−1 (Fig. 2B). The sulfate removal increased from 20 to 60% during that period (Fig. 2B). Figure 3 shows that almost all propionate and

Reactor performance

This study showed that the addition of nitrate to the last compartment of a baffled (staged) reactor is an elegant effluent polishing method to remove unoxidised acetate by a sulfidogenic sludge reactor (Fig. 3, Fig. 8). The plug flow conditions (Grobicki and Stuckey, 1992) and concomitant substrate concentration profile in this reactor type (Fig. 3) result in optimal conditions for acetate removal, as it is the sole substrate present in the last compartment (Fig. 3D). Thus, acetate is used as

Conclusions

  • 1.

    A staged degradation of VFA occurs in the baffled reactor, leading to sludges with different metabolic properties. The acetate removal capacity of the sludge gradually increased in the subsequent compartments and was maximal (0.4 gCOD gVSS−1 d−1 at 30°C) in the last compartment.

  • 2.

    Nitrate reduction with the electron donor acetate proceeds via denitrification and not via ammonification in sulfidogenic granular sludge. Denitrification proceeds after a lag phase of 24 h at its optimal pH of 6.5–7.5.

  • 3.

Acknowledgements

We thank C. Bakker for help with the set-up of the reactor system and some initial experiments. We have grateful to Dr A. Klapwijk for stimulating discussions and remarks on this manuscript. This research was supported by a TMR Marie Curie grant (ERBFMBICT950250).

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