Anaerobic Digestion of Olive Mill Wastewater: Focusing on the Effect of Nitrogen Source

A comparative kinetic study was carried out on the anaerobic digestion of olive mill wastewater (OMW) and OMW that was previously fermented with Geotrichum candidum, Azotobacter chroococcum and Aspergillus terreus. The reactors used were continuously fed and contained sepiolite as support for the mediating bacteria. A kinetic model for multicomponent substrate removal by anaerobic digestion has been used. The model is based on the linear removal concept which is a special case of the broader Monod equation. The second-order kinetic constant, k2(s), was found to be influenced by the pretreatment carried out, and was 4.2, 4.0 and 2.5 times higher for Aspergillus, Azotobacter and Geotrichum-pretreated OMWs than that obtained in the anaerobic digestion of untreated OMW. This was significant at 95% confidence level. This behaviour is believed to be due to the lower levels of phenolic compounds and biotoxicity present in the pretreated OMWs. In fact, the kinetic constant increased when the phenolic compound content and biotoxicity of the pretreated OMWs decreased. In addition, the macroenergetic parameters of the anaerobic digestion of OMW, i.e. the specific rate of substrate uptake for cell maintenance, m, and the yield coefficient for the biomass, Y, decreased by a factor of 2.4, 3.6 and 5.1 and increased by a factor of 1.9, 2.2 and 2.4 respectively, for the OMWs previously treated with Geotrichum candidum, Azotobacter chroococcum and Aspergillus terreus in relation to the observed values for the untreated OMW.

A comparative kinetic study was carried out on the anaerobic digestion of olive mill wastewater (OMW) and OMW that was previously fermented with Aspergillus terreus. Two completely-mixed bioreactors were used for the study. The results obtained were evaluated using the Chen-Hashimoto methane production model to obtain values for the maximum specific growth rate (μmax) and kinetic constant (K) of the process for each case studied. The kinetic parameters were found to be influenced by the pretreatment carried out, and were 4.5 and 3.8 times higher for pretreated OMW than that obtained in the anaerobic digestion of untreated OMW. This was significant at 95 % confidence level. This behaviour is believed to be due to the lower levels of phenolic compounds and biotoxicity present in the pretreated OMW, as compared to untreated OMW, resulting in an improved process performance and stability. Finally, the experimental values of methane production were reproduced with deviations equal to or less than 10% in every case.

Kinetic study of anaerobic digestion of olive mill wastewater previously fermented with Aspergillus terreus

Anaerobic digestion of olive mill wastewater pretreated with Azotobacter chroococcum

Influence of different aerobic pretreatments on the kinetics of anaerobic digestion of olive mill wastewater

Effects of agitation and pretreatment on the batch anaerobic digestion of olive mil

A kinetic model to describe the anaerobic digestion of olive mill wastewater (OMW) and condensation water from thermally‐concentrated OMW has been developed. Two complementary kinetic descriptions, for substrate utilization and methane production, were derived. The model equations accounted for extracellular hydrolysis of complex substrates (OMW) and considered the hydrolysed products as the limiting substrates for cell growth and product formation according to Monod kinetics. Predicted relationships agreed well with experimental data. When extracellular hydrolysis was required and rate limiting, as in the case of the OMW, the equations were reduced to a Contois‐type form. However, if no extracellular hydrolysis of the substrate was involved, as in the case of the condensation water from thermally‐concentrated OMW, the equations were in the form of a Monod‐type relationship. Thus, the equations represented a generalized kinetic expression for anaerobic fermentation relating the Monod and Contois equations, which held in the two extreme cases.

The current research work attempted to investigate, for the first time, the impact of biochar addition, on anaerobic digestion of olive mill wastewater with different initial chemical oxygen demand loads in batch cultures (10 g/L, 15 g/L, and 20 g/L). Methane yields were compared by applying one-way analysis of variance (ANOVA) followed by post-hoc Tukey’s analysis. The results demonstrated that adding at 5 g/L biochar to olive mill wastewater with an initial chemical oxygen demand load of 20 g/L increased methane yield by 97.8% and mitigated volatile fatty acid accumulation compared to the control batch. According to the results of microbial community succession revealed by the Illumina amplicon sequencing, biochar supplementation significantly increased diversity of the microbial community and improved the abundance of potential genera involved in direct interspecies electron transfer, including Methanothrix and Methanosarcina. Consequently, biochar can be a promising alternative in terms of the recovery of metabolic activity during anaerobic digestion of olive mill wastewater at a large scale.

A kinetic study of anaerobic digestion of olive mill wastewater at mesophilic and thermophilic temperatures

Experimental trials of anaerobic digestion of olive mill wastewater (OMW) blended with other agro-industrial by-products were carried out to evaluate biogas production and sensitivity of the process to inhibiting compounds. Blends containing different percentages of OMW, digested liquid manure, and citrus peel were subjected to a batch anaerobic digestion process under both mesophilic and thermophilic conditions. The results showed that blends with percentages of OMW higher than 20% (v/v) had low methane yields due high concentrations of polyphenols (PPs) and/or volatile fatty acids (concentrations above 0.8 g kg–1 and 2.4 g L–1, respectively). The addition of other substrates such as citrus peel may have induced synergic inhibiting effects of PPs and essential oils (EO) on microbial growth. Thermophilic processes were more sensitive to these inhibiting compounds than mesophilic processes. The results of this study suggest that reducing PPs and EO concentrations in blends subject to anaerobic digestion below the inhibiting concentrations of 0.6 g L–1 and 0.5 g kg–1, respectively, is suitable. Additionally, it is advisable to maintain the volatile fatty acids content below 2 g L–1 to avoid its evident toxic effects on the growth of microorganisms in biochemical processes.

Biological treatments focused on stabilizing and detoxifying olive mill wastewater facilitate agronomic reuse for irrigation and fertilization. Anaerobic digestion is particularly attractive in view of energy recovery, but is severely hampered by the microbial toxicity of olive mill wastewater. In this work, the addition of biochar to the digestion mixture was studied to improve the stability and efficiency of the anaerobic process. Kinetics and yields of biogas production were evaluated in batch digestion tests with biochar concentrations ranging from 0 to 45 g L−1. The addition of biochar reduced sensibly the lag phase for methanogenesis and increased the maximum rate of biogas generation. Final yields of hydrogen and methane were not affected. Upon addition of biochar, soluble COD removal increased from 66% up to 84%, and phenolics removal increased from 50% up to 95%. Digestate phytotoxicity, as measured by seed germination tests, was reduced compared to raw wastewater. Addition of biochar further reduced phytotoxicity and, furthermore, a stimulatory effect was observed for a twenty-fold dilution. In conclusion, biochar addition enhances the anaerobic digestion of olive mill wastewaters by effectively reducing methanogenesis inhibition and digestate phytotoxicity, thus improving energy and biomass recovery.

Anaerobic acidogenic digestion of olive mill wastewaters in biofilm reactors packed with ceramic filters or granular activated carbon

Performances and microbial features of a granular activated carbon packed-bed biofilm reactor capable of an efficient anaerobic digestion of olive mill wastewaters

Anaerobic digestion of olive mill wastewaters is generally performed in anaerobic contact bioreactors where the removal of toxic phenols is often unsatisfactory. In the present work we show that a granular activated carbon packed-bed biofilm reactor can be successfully used to achieve effective and reproducible wastewater decontamination even at high organic loads. A comparison of 16S rRNA gene sequences of the inoculum and of biomass samples from different districts of the reactor revealed enrichment of specific microbial populations, probably minor members of the inoculum and/or of the olive mill wastewaters. They mainly consisted of the members of Proteobacteria, Flexibacter-Cytophaga-Bacteroides, and sulphate-reducing bacteria. The dominant sequence among Archaea (70% of clones) was closely related to Methanobacterium formicicum.

Tomatoes (Solanum lycopersicum) plant were provided with bio-fertilizers issued from anaerobic digestion of olive mill wastewater without and with 1%, 5% of phosphate residues in mesophilic conditions for 25 days. 1% of raw substrates (OMW raw; OMW + 1%PR raw; olive mill wastewater + 5%phosphate residues raw; and phosphate residues) and digestates (olive mill wastewater digestate, olive mill wastewater + 1%phosphate residues digestate and olive mill wastewater + 5%phosphate residues digestate) was provided fortnightly to the plants. Reclaimed water from a wastewater treatment plant located in the study site was used for automatically controlled irrigation. It contained a low level of chemical fertilizers to compare tomato plant growth, leaf analysis, steam water potential, production yield and fruit quality results to plants fed with bio-fertilizers. Generally, parameters and results were progressively increased during the growing and harvesting stage, which refer to the essential elements that cover the plant’s needs. Plants fed with bio-fertilizers showed the most extended plant height (olive mill wastewater + 5% phosphate residues raw), and the best accumulation of essential elements in leaves (olive mill wastewater + 1% phosphate residues digestate and olive mill wastewater + 5%phosphate residues digestate). The maximum average fruit weight per treatment (35.5 g) was obtained when applying the digestates mixture of olive mill wastewater raw and olive mill wastewater + 5% phosphate residues. The maximum yield production per plant was obtained when applying phosphates residues. Bio-fertilizers (digestates) showed good performances, high fruit quality and perfect tomato yield production compared to the control plants. Results obtained during this study are considered promising regarding environmental framework. However, this study was done in a laboratory scale and needs to be applied in a large scale to provide more data on the effectiveness of the digestates application. It is also recommended to apply these bio-fertilizers on different crops and various soils for a better evaluation.