(215d) Valorisation of Three-Phase Olive Mill Wastewater with the Addition of High Protein Co-Substrates

Nowadays national and international legislation attempt to enforce reductions in greenhouse gas emissions, while minimization of dependency to fossil fuel gathers all-around agreement. At the same time optimization of waste management strategies towards zero waste generation is a major challenge for modern society. Olive Mill effluents (OME) is the effluent generated during the production of olive oil and accounts for the 60% of the overall weight of the olive fruit. OME is rich in carbon and phenols while its nitrogen content together with the pH are very low rendering common treatment methods ineffective to tackle it. A valorization process that can be used for the management of OME is Anaerobic Digestion (AD). A key attribute of energy efficiency schemes has been the utilization of negative value substrates for biogas generation in anaerobic digesters. Anaerobic digestion (AD) is a biological method for the treatment and valorization of organic material. Its product can be recovered and utilized for heat and/or electricity generation. The efficiency of the process is affected by the operational conditions selected for the digesters and it depends highly on the type of the organic substrate used as feedstock.

Common organic substrates for AD systems include municipal solid waste, agroindustial byproducts and waste including slaughterhouse and dairy industry waste. Several types of AD feedstock are rich in fat, oil and grease (FOG). This is a feature that is highly desirable, as fat has the highest biomethane potential of all natural macromolecules. However, high fat content can inhibit the digesters through flotation, accumulation, pH reduction, and a number of intracellular inhibition processes. Furthermore, agroindustial wastewater, including olive mill effluents (OME), is characterized by high concentration of phenols, low nitrogen availability and low pH. All of the above are known to inhibit the digestion process especially in overloaded or unstable systems.

Aim of this work was to investigate the effect of external addition of nitrogen into digesters operating with OME as single substrate. The co-substrates evaluated were low-value byproducts of animal and plant origin, namely bone and meat meal, blood meal, feather meal, mixture of feather and blood meal, fish meal, soy been meal and maize gluten.

Material and methods

OME was collected from a three-phase olive mill. The inoculum used was collected from four, six-liter digesters operating under steady state conditions with cattle manure as substrate and an OLR of approximately 3kg/VSm3-d. Protein-rich substrates were collected from a pet food manufacturer. All substrates were stored in a freezer at -20^oC after collection until further utilization.

In this work 250mL Duran bottles were used as batch digesters and for assessing the biomethane potential and biodegradability of the different substrates. In order to evaluate the batch process and generate comparable results two loading rates (10 and 20 kgVS/m³) were evaluated. The I/S ratio was set at 2 and 1 respectively. In the second stage of the experiment, four identical stainless steel reactors were used. The reactors were heated through the recirculation of hot water within the water jacket, feeding and removal of the substrates was taking place manually, while the mixing of the reactors was achieved with the application of immersed propellers and direct current mixing motors.

Results and discussion

Mono digestion of the substrates

The highest biogas production of the substrates under mono-digestion conditions was obtained using bone and meat meal with a value of 511 mLCH₄/gVS_added. This was approximately 16% higher compared to the production of biogas using blood meal and feather meal mixture with a production of 427 mLCH₄/gVS_added. Related to the plant-derived substrates the specific methane production of gluten and soy bean meal amounted to 495 mLCH₄/gVS_added and 460mLCH₄/gVS_added respectively.

Co-digestion of OME with high nitrogen substrates

In the second stage of this study OME was co-digested with high nitrogen substrates. The highest biogas production was attained using a combination of OME and fishmeal with a yield of 482 mLCH4/gVS_added corresponding to the 77% of the ultimate production for this mixture. Digestion of the other mixtures gave also very positive results with the mixture of OME & blood meal and of OME & soybean meal offering yields of 454 and 446 mLCH4/gVS_added corresponding to the 74 and 72% of the ultimate production for the two mixtures respectively. The lowest biogas production registered was the one obtained from a mixture containing OME & bone and meat meal with a yield of 418 mLCH4/gVS_added, equal to the 68% of the theoretical value.

Validation of the results in CSTR systems

In order to validate the results of the previous experimental stages, the biogas yield using four substrate mixtures composed of olive mill wastewater (OMWW) and protein-rich substrates were assessed in semi-continuous digesters. Feeding was provided daily throughout the week including the weekends. The four substrates chosen for analysis in the semi continuous systems were:

(a) bone and meat meal as it is the co-substrate with the highest market availability and the lowest price.

(b) blood meal; this substrate was selected due to its negative effects recorded when high loading rate was used in the batch digestion.

(c) fishmeal; this substrate offered the highest bio-methane production of all substrate mixtures assessed in the batch digestion stage.

(d) The last of the substrates assessed was gluten. This was the only plant-derived protein-rich substrate assessed; its selection was based on its higher protein content over the second candidate substrate, namely soybean meal.

In this experimentation stage the highest biogas production was achieved by the mixture containing blood meal with an average production equal to 449 mLCH4/gVS added, followed by biogas production from bone and meat meal with 443 mLCH4/gVS added. The lowest production was achieved when using the mixture containing FM with an average yield of 416 mLCH4/gVS added. Our results indicate that the type of protein added is insignificant; in all cases the yields of the four different mixtures were within 7% of each other ranging between 449 and 416 mLCH4/gVS added. In all cases VFA concentrations were very low and no inhibition was recorded in any of the reactors treating the different substrates.

Conclusion

OMWW, a heavily polluted effluent can be valorized through the anaerobic digestion process into biogas and digestate. The addition of nitrogen-rich substrates offers the opportunity to achieve a steady process with high bioconversion efficiency. Furthermore, the type of the co-substrate seems to be insignificant as no substantial differences in biogas yield were registered when fish, animal or plant derived proteins were used as nitrogen-carrying additives. This work offers a framework for the possibility to valorize the OMWW in treatment plants in the vicinity of the olive mills without the requirement of manure addition as carrier substrate. This development would minimize the risk of bacterial contamination from transportation of manure near olive mills. According to our results one ton of nitrogen-rich substrates can be used for the treatment of more than 50m³ of OMWW while at the end of the process the recovered effluent has significantly better characteristics as soil improver when compared to raw OMWW.