Optimization of the Anaerobic Digestion from Olive Oil Production's wastes

The aim of this thesis is the optimization of the anaerobic digestion of wastes derived from olive oil production, which represent one important economic sector of all the Mediterranean Countries. The main byproducts of this activity are the semi-solid Olive Pomace (OP), characterized by low pH, high content of organic matter and in particular of ligno-cellulosic materials, and a liquid one, the Olive Mill Waste Water (OMWW) which have a dark color, low pH and high content of polyphenolic substances. Because their features, these wastes are very polluting and not recommended with the traditional methods, which consists essentially to dispose on the soil with the consequent alteration of the chemical characteristics of the ground and the possibility to contaminate the groundwater. For these reasons anaerobic digestion represents the most sustainable method to treat these wastes, which permits the production of a gas rich in methane and a stabilized sludge poor in organic matter, the digestate usable as fertilizer. A first test, conducted in laboratory scale (about 100 ml) was made in order to evaluate the biogas production from a 10% w/w mixture composed by OP and OMWW. The biogas production was very low (0.22 NL/L) and consequentially the efficiency of process resulted of 0.3%. The high poly-phenols content in the OMWW are responsible of the inhibition of the methanogenic bacteria activity and owing the lignocelluloses nature of the largest organic structures determined the low biodegradability of the OP and OMWW. The first part of the thesis’ work consisted in the identification of the effects of chemical and physical pretreatments by AD fermentation tests in order to evaluate the biogas and methane production, abatement of the organic matter and reduction of the lag phase of 133 pretreated OP and OMWW to score the best one. In addition, considering that AD is aimed to produce energy, operational energy costs to perform the pretreatments were investigated, based on the consumption of direct energy (heat and electricity) as well as the indirect energy spent to produce the chemicals used. To this aim an Energy Sustainability Index (ESI) were introduced and applied to select the most efficient pretreatment method from an energy sustainability perspective. The simply dilution of OP with tap water (WP) has a beneficial effect in the anaerobic digestion, improving over 57 times the energy production respect to un-treated waste (NP). However considering the CH4 production, the best pretreatments resulted to be the CaCO3, FeCl3 addition and the ultrasonic pretreatment, which had an efficacy of 108, the highest among the tested pretreatment processes. Between the three best pretreatments the addition of 5g/L of CaCO3 to the reaction medium, resulted the only one able to give a positive energy balance between the energy obtained and that spent, due to the low energy required for the CaCO3 production. The CaCO3 pretreatment permits to pass from an efficiency of 0.3% to about 21%. After the identification of the CaCO3 addition as the best pretreatment, a test including the same OP- OMWW mixture with 5 g/L of CaCO3 has been conducted in a CSTR reactor having a volume of 2 Liters. Since the beginning of the prove, it was evident a problem with the mixing system due to a elevate value of the mixture’s viscosity, 160- 200 cP. The Rushton impeller resulted totally unable to guarantee an homogenous mixing of the reactor medium. In fact, the Rushton impeller can give a boost to the fluid only in a radial direction, resulting to be inefficient along the axial one and not able to guarante the circulation of the solid present in the feed, derived mainly from the breaking of the olive seeds during the olive oil production’s process. To improve the mixing of the reaction medium, four different impellers were tested:  Pelton Impeller;  Rushton impeller with six 45° inclined blades;  Marine Impeller with 3 blades;  Anchor Impeller. The best performances were obtained with the marine and the anchor impellers. The first is designed to guarantee mainly an axial movement to the fermenting fluid, which, at constant rpm, results to be superior than the Rushton with inclined blades. it guarantees also a radial movement to the reaction medium, which consents to involve in the mixing all 134 the reactor volume. Conversely, the optimal performances of the test conducted with the anchor impeller can be explained observing the presence of a well mixed region in the superior part of the reactor where anchor impeller was located, for the effect of the tangential movement given to the fluid, and a less agitated, but no stagnant, part in the inferior part of the reactor. This more quiet zone resulted ideal for the methanogenic bacteria’ conditions which do not tolerate intense mechanical stress derived from the impeller rotation. In this way their growth, reproduction and metabolism activity was favored with a consequent increasing of the methane production. An ulterior attempt to increase the efficiency of the AD process consisted in the simultaneous experimentation of the two best impellers (the marine and the anchor) with the two stage AD configuration, based on the separation into two interconnected steps, of the two distinctly different groups of bacteria (acidogens and methanogens), in order to maximize their growth by maintaining optimum conditions in each step for each particular group of bacteria. The two stage configuration resulted advantageous only for the AD conducted with the marine impeller using a reactor with a semi spherical bottom to improve the solids circulation. In this test the efficiency passed from 22.64% (mono-stage) to 30.24% (two stage configuration) with an improvement of rhe efficacy (η) of 1.34 times. Conversely, the two stage has worsened the efficiency of the AD with the anchor impeller whose efficiency dropped down from over 33% to 17.5%, which corresponds to η = 0.52. The marine impeller, in fact, gives an axial boots to the reaction medium which permits to the gas bubbles to leave the reaction volume, preventing the inhibition of the reactions involved in the AD. The semi-spherical bottom of the reactor also had a beneficial effect on the AD, permitting a better homogenization of the biomass and of the substrates into the reaction medium. Conversely, the anchor impeller which give a strong radial impulse and only a gentle axial movement to the reaction medium. This configuration gives a behavior, which can be similar to an UASB reactor, permits the growth, the reproduction and the activity of the methanogenic microorganisms which are not disturbed by strong mechanical stress transmitted by the mixing system. Although this good aspect, the quasi stagnant region does not consent the complete degassing of the hydrogen gas in the first acidogenic phase from the reaction medium. On the contrary the anchor impeller has not any impact on the mono-stage configuration where the biogas is formed essentially by methane, but has a strong inhibiting effect on the two stage configuration where the formed hydrogen is a reaction intermediate products. Lastly, a continuous test has been conducted using a pilot reactor having a working volume 135 of 1,800 liters. The olive oil production’s wastes were fed with milk whey, a byproduct form dairy activity. The codigestion was conducted in mesophilic conditions and had a duration of 75 days. The system reached a stationary condition with a biogas production of 1.4 L/L and a methane content of 70-75% v/v. This gas flow has an energetic annual potential of 55 GJ. Taking in account that Puglia’ s annual energetic consumption is of 375,000 GJ (Piano Energetico Ambientale Regionale – Regione Puglia, 2004), the anaerobic digestion of olive oil and dairy’s waste is able to cover the 0.015% of the regional energetic demand.