Full Length ArticleDevelopment of a high-rate anaerobic thermophilic upflow packed bed reactor for efficient bioconversion of diluted three-phase olive mill wastewater into methane
Graphical abstract
Introduction
The main by-product derived from the three-phase olive mills during the olive oil extraction, is the olive mill wastewater (OMW). Around 750,000,000 productive olive trees are located worldwide, while 98% of the aforementioned are grown in the Mediterranean region [1]. Specifically, Greece, Italy and Spain (which dominate the world olive production), face hazardous environmental problems related to the huge disposals of the OMW produced every year [2].
The traditional olive oil extraction method (discontinuous extraction system) and the three-phase decanter process (continuous centrifugal extraction) result in olive oil at approximately 20%, and two waste streams or by-products, namely the solid (30%) and the liquid stream (50%) [2]. The highest amount of the aforementioned wastes is OMW, with an annual production reaching up to 30 million m3 and an average volume of 1200 kg/1000 kg of olives [3], [4]. Even though the two-phase decanter process (continuous centrifugal extraction) is extensively applied in the Mediterranean countries, a high percent of olive mills located in Greece (82%) are still using the three-phase system [5]. The specific characteristics of OMW may vary according to various factors. The most common ones are usually the method of the extraction process (two-phase and three-phase), the region of origin, the type and ripeness of olives, the application of fertilizers, and harvesting methods [6]. However, the composition of OMW typically consists of 83–96% moisture and 3.5–15% organic constituents while the remaining 0.5–2% comprises of mineral salts [7]. Typically, two-phase OMW is a viscous residue, characterized by higher solid content and thus higher chemical oxygen demand (COD) values compared to three-phase OMW, while the latter is known as a liquid residue, which is easier to manage.
Wastewater arising from olive processing has been characterized as one of the most hazardous industrial effluents due to its high COD values reaching up to 220 g/L and corresponding biochemical oxygen demand (BOD5) values of up to 100 g/L [8]. Besides its composition in organic compounds (BOD5, COD), OMW has a strong odor nuisance, while its high content of long-chain fatty acids and phenolics (1–7.5 g/L) can be harmful not only for the plants, but also for microorganisms [9]. In addition, the surface and groundwater pollution and the changes in soil quality have associated OMW to hazardous environment pollutants [10].
Targeting on waste management, various systems have been proposed and examined for the treatment/valorization of OMW employing usually biological, physicochemical, and advanced oxidation methods [11]. Such methods include aerobic [12] and anaerobic [13] bioprocesses, coagulation, adsorption [14], use of membrane systems [15], chemical and electrochemical treatment [16], [17] and recovery, such as added-value compounds for plant disease control in view of reducing the chemical/synthetic pesticides [18]. However, due to the high contained amounts of organic compounds in OMW, research interest has also been focused on the development of bioreactors targeting on organic load valorization through the production of gaseous biofuels [19], [20]. The produced gaseous mixture (CH4 and CO2) during the biological degradation of the organic material can be used for energy recovery [21] as a fuel for electricity production or as high quality fuel after its upgrade by reducing its CO2 content [22].
In a valorization point of view, the co-digestion of OMW is usually proposed [23], [24] in order to overcome limitations such as C/N ratio, nutrients imbalance, high concentrations of inhibitory compounds (heavy metals, phenolics, long chain fatty acids etc.) or unfavorable pH values [25], [26]. However, during the co-digestion process antagonistic phenomena could be exhibited due to substrates mixture failure [26], while economic factors such as the transportation costs for the substrates should be taken into account [27]. Concerning the applied temperature, mesophilic conditions are typically implemented due to the higher stability of the anaerobic systems. However, the thermophilic range could favor the overall process due to the expected pathogens reduction or extinction, higher enzymatic activity, increased hydrolysis rate and efficiency [28], [29].
High-rate processes usually aim to minimize the anaerobic reactor size and thus capital costs, while greater exploitation of the working volume is attempted by the microorganisms granulation or immobilization on biomass carriers [30], [31]. Among the high-rate processes developed during the past few years, the upflow packed bed reactor (UPBR) with recycling stream is characterized as one of the most successful ones referring to wastewater treatment [32]. Microorganisms are forming dense biofilm layers on the filling material into the UPBR reactor, succeeding increased biomass concentration with high bioactivity. Furthermore, the recycling stream ensures gentle and uniform mixing into the reactor. Another benefit of recirculation is the toxic constituents’ dilution, such as phenolics or produced volatile fatty acids (VFAs), and reduction of substrate inhibition [33]. Nevertheless, most of the designed systems depended on empirical parameters aiming at achieving a biomass self-control form and operational stability.
OMW anaerobic digestion has been widely studied, while various systems and operation conditions have been tested under mesophilic or thermophilic conditions [23], [34]. Concerning especially the thermophilic range, not only anaerobic digestion using OMW as mono-substrate, but also co-digestion (for example with olive mill solid wastes) have been examined with sufficient results [35], [36]. Additionally, the operation of high-rate systems, such as Upflow Anaerobic Sludge Blanket (UASB) or packed bed biofilm reactors under mesophilic conditions and their efficient yields [37], [38], paved the way for further research. The present study examined the performance of a high-rate anaerobic UPBR, operating under thermophilic conditions (55 °C) for the bioconversion of OMW as single substrate, into biogas. Thermophilic conditions target to accelerate the anticipated biochemical reactions (at higher than ambient temperature), increase the efficiency towards organic matter degradation and destruct possible existing pathogenic organisms in contrast to conventional systems operation [39], [40].
Section snippets
Microorganisms’ acclimatization to thermophilic conditions
The effluents from a mesophilic (37 °C) UASB reactor, which was also successfully deployed in treating three-phase OMW, were centrifuged twice at 4,000 rpm for 5 min. Afterwards, the settled sludge was put into a batch reactor (total volume of 3.5 L and working volume of 3 L), filled with effluent liquid from the mesophilic UASB reactor, flushed with N2 gas, closed and maintained under constant thermophilic temperature (55 ± 0.5 °C). For the first two days the batch reactor was kept without any
Wastewater physicochemical characterization
During the system operation, three different batches of OMW (1, 2, 3) were treated. Specifically, after the homogenization of each batch, representative samples from raw and twice-centrifuged OMW were received and measured in triplicate. The characteristics of raw OMW and the resulting derivative are summarized extensively in Table 3. Both raw and centrifuged OMW are characterized by high organic content, d- phenolic compounds, TS and VS, while TKN concentration remained low. Comparing raw and
Conclusion
An active mixed culture of anaerobic microorganisms, originating from a mesophilic UASB reactor treating OMW, was acclimatized under thermophilic conditions. A high-rate UPBR reactor was tested utilizing the acclimated culture, towards its treatment efficiency of diluted OMW under thermophilic conditions. The reactor operated efficiently at gradually decreased HRTs reaching the minimum of 4.2 d. Optimum performance was exhibited for the HRT of 5.6 d, where the organic matter and the phenolic
Funding
This work has been co-financed by the European Union and Greek national funds through the Regional Operational Program “Western Greece 2014–2020”, under the Call “Regional research and innovation strategies for smart specialization (RIS3) in Agro-food” (project: DER6-0021057 entitled “Integrated Energy and Environmental Exploitation of Olive Oil Production by-products, OLIVENERGY”).
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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