Bioenergy recovery from olive mill effluent in a hybrid reactor
Highlights
► Hybrid digester is a feasible alternative to maximize bioenergy recovery from OMW. ► Phenols concentration did not influence its conversion range. ► Reactor digested an acid influent (pH = 4.7) revealing a high buffering capacity. ► Microbial communities can be adapted to OMW by using a complementary substrate.
Introduction
Olive mill effluent (OME) or the so-called “black water” is a by-product from olive oil production. The utilization of this residual fraction as a source of energy, nutrients and irrigation water [1] emerges as an attractive solution for OME management.
The high theoretical energetic potential of this effluent is ascribed to the organic load, particularly to the oil content. Biogas production from complex oily wastewaters can be very profitable if operation problems are overtaken. Olive mill effluent toxicity toward microorganisms has been linked to lipidic and phenolic compounds [2], [3], [4], [5]. Several authors proposed different treatments before OME biodegradation to remove and/or degrade the OME toxic compounds [6], [7], [8]. However, the organic fraction is reduced by most of the pretreatments and consequently bioenergy recovery decreases. The addition of water and chemicals has been widely applied to enhance OME biodegradation (Table 1) [6], [8], [9], [10], [11], [12].
Co-digestion of OME with other agro-industrial by-products has been recently reported [13], [14], [15]. However, in a continuous operation, the highest OME volume fraction applied was 50% and chemicals were needed to correct the pH. Alternatively, the addition of a complementary wastewater stream to the olive mill effluent can be advantageously applied as it decreases the toxic compounds concentration and also provides the required pH, alkalinity and nutrients levels necessary for a successful anaerobic digestion. Consequently, higher proportions of OME can be treated [1], [16].
The reactor design is another important factor to achieve a good performance and an economic process. Different anaerobic reactor types have been investigated for the treatment of this effluent (Table 1). Up-flow Anaerobic Sludge Blanket (UASB) shows a tendency to wash the biomass from the system under an overload or when concentrated effluents are used [9], [17]. Anaerobic Filter (AF) is favored for the treatment of OME since it requires a shorter start-up time, and resists to high COD loadings and to high temporary overloads [18], [19]. Another up-flow packed bed digester is the Anaerobic Hybrid (AH) that gathers several positive aspects of both systems: tolerance to high loading rates and minimization of suspended solids washout which consequently enhances the reactor efficiency and provides better effluent quality [20], [21].
This work intends to investigate low-cost alternatives to maximize the energy recovery from OME, regarding mainly the feeding approach and the reactor design. The main objectives were to test and evaluate the performance of a hybrid reactor using (a) a complementary effluent stream in the feed mixture to avoid the use of chemicals and dilutions with water and (b) a packed bed length of only 1/3 of the digester height.
The complementary effluent was not only used to achieve process stability concerning the energetic valorization of both effluents but also to adapt the reactor to OME or to a great portion of it. The importance of using alternatively the hybrid reactor type and the feeding strategy was discussed regarding an anaerobic digestion plant applied to the energetic valorization of OME.
Section snippets
Experimental set-up
The anaerobic digestion experiments were performed in an up-flow anaerobic hybrid digester. The unit (Fig. 1) was built out of polyvinyl chloride (PVC) pipe with a total volume of about 2 dm3. A packed bed, selected in previous studies [1], was used to fill only 1/3 of reactor’s height. No device separator of solid/liquid/gas was installed and no substrate recycle was provided. It was semi-continuously fed by a time controlled peristaltic pump and maintained at 37 ± 1 °C using a water jacket. The
Reactor performance: biogas production and COD conversion
The reactor was operated at an OLR between 3.3 kg m−3 d−1 and 8.0 kg m−3 d−1, for 300 days (Table 3). Fig. 2 and Table 4 present the reactor performance results in terms of biogas production, methane content and COD conversion. A good biogas quality was detected throughout the experiment. The methane content in biogas decreased from a volume fraction between 73% and 79% (Period I) to 59% and 66% (Period VIII) with the increase of OLR. Conversely, a gradual enhancement of biogas productivities was
Conclusions
The hybrid digester, equipped with a packed bed of only 1/3 of its height, is a feasible alternative to maximize the bioenergy recovery from OME. It prevents the excessive loss of biomass and recovers easily over an accidental overload. It was demonstrated that microbial communities can be adapted to OME by using a feeding strategy based on effluents complementarity, avoiding dilution with water and chemicals.
It was concluded that only a phenolic fraction of 50–60% can be anaerobically
Acknowledgements
The authors acknowledge the financial support of Fundação para a Ciência e a Tecnologia (FCT/MCTES) and Fundo Social Europeu (FSE), through the project PTDC/ENR/69755/2006 and also through the grant awarded to Marta Gonçalves SFRH/BD/40746/2007.
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