A Asja Ambiente Italia spa, Torino, B CNR-IRSA,Roma, C Austep srl, Milano

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1 Anaerobic Digestion of olive and milk production byproducts Roberto Zocchi A, Davide Mescia A, Luigi Rutigliano A, Mauro Crippa B,Alessandro Massone B, Giuseppe Mininni C, Camilla Braguglia C, Andrea Gianico C A Asja Ambiente Italia spa, Torino, B CNR-IRSA,Roma, C Austep srl, Milano Summary In this work, the main waste products of the agricultural and food sector in Puglia were investigated. Through careful evaluation of the chemical composition of wastes that are difficult to dispose, we identified anaerobic digestion as the best choice to process olive mill wastewaters (OMW) and milk whey. Pre-treatment grinding and homogenization increase the contact surface area; whereas ultrasound treatment was studied to further the dissolved amount of the organic fraction of the pomace. Anaerobic digestion batch and continuous tests have allowed us to define the best plans for feeding, as well as the optimal running conditions to obtain a good yield of biogas. Employing milk whey seems to balance the C/N ratio and the polyphenol level in the biomass, which improves efficiency of energy generation process. 1.Introduction Italy is the world s second largest producer of olive oil with around 600,00 tons commercialized, of which, around 36 % are produced exclusively in the region of Puglia. The treatment and disposal of olive oil refuse represent the principle problems for Mediterranean oil producers. Olive pressing is realized either at traditional plants called, a pressa, which operate discontinuously, or at continuous plants differentiated by whether they produce two or three phases. Taken together, the systems produce 3 phases: (i) olive oil, (ii) husks, which is the solid residue, and (iii) olive mill wastewaters (OMW). Those three phases respectively represent 20, 30, and 50 % of the total weight of the olives used for pressing. [1] The evolution of continuous systems that utilize centrifugal methods of separation has brought an improved efficiency to the process, but also, problems with managing olive refuse. The two phase process produces husk with an elevated water content, meanwhile the three phase process produces, in addition to a less humid husk, a notable quantity of OMW. Currently, the only solution adopted for OMW is spreading over agricultural terrain, and is regulated by law 524/96, which puts quantitative limits by unit of surface area and constrains the spreading by the distance from inhabited centers, the height of groundwater, and the type of culture being cultivated. Laws of ever increasing restrictiveness on the agricultural use of oil from the husks and the increase of the cost of energy, have reduced the commercial value of olive husks. For husks with a water content higher than 50%, like that produced from 2 phase extraction plant, the value has been reduced entirely to zero. The project, financed by the Puglia region (POR ), to develop the technical and scientific competencies to realize plants that generate electric energy fed by biogas produced by anaerobic digestion of olive husks and wastewaters, arises from the objective of resolving all the problems of management and disposal of olive refuse. Anaerobic digestion confronts the question of disposal not in terms of management of olive refuse, but instead, in terms of exploiting a resource. The yield and quality of biogas produced depend on multiple factors that should be taken into consideration. Before all else, it was necessary to identify the organic substrates of interest at the regional level, both in quantitative and qualitative terms. From the substrates produced in Puglia, 1

2 milk whey was of great interest. Due both to a high concentration of organic substances, and the nearly year-round availability, it would help resolve the problem of olive refuse s seasonal availability. Once the biomass was identified, various preliminary treatments were implemented to improve the speed and efficiency of the transformation of organic matter, in such as a way as to optimize the biogas yield. The polyphenols present in the olive refuse, because of their anti-oxidant and anti-microbial characteristics, strongly limit degradation in addition to being responsible for the inhibition of germination, growth and development of diverse herbaceous plants. [2] The removal of these aromatic substances should be done in a selective manner, without reducing other organic substances that contribute to the production of biogas. There are multiple advantages to be found in combining an upstream anaerobic process with a downstream aerobic process. The digestate is semi-stabilized with consistently reduced levels of pathogens and emissions of odorous substances; furthermore, the heat generated from the anaerobic process can be exploited to meet energy demands from the aerobic process, thus permitting a reduction in emitted CO 2. 2.Report The residual biomass derived from agricultural and industrial activity typical in Puglia are defined in table 1: Biomass Quantity (t/year) Agricultural Residue 169,000 Milk Whey Waste 121,000 Wine Industry 201,450 Olive Mills 620,000 Meat Industry 7,890 Table 1 Residual biomass most relevant to Puglia Considering the quantities produced, the low economic value, and the problems of disposal, the wastes originating from the production of olive oil are the best choice by far. The wine sector actively contributes in terms of production of waste but they are generally sent to distilleries or employed for agricultural use. The wastes of agricultural sector are also important, but are difficult to recover or are used in agriculture or as combustibles. The residual material from dairy production (milk whey, dairy waste waters and by-products from cheese farms) are produced in inferior quantities with respect to the other sectors and have low commercial value, but are characterized by elevated concentrations of soluble COD (about three times higher than OMW). 2.1 Characterization of the biomasses Values on the concentrations of constituents of olive husk, OMW, and milk whey are presented in table 2. The work was performed by CNR-IRSA in ROME and by AUSTEP s.r.l., who are partners on the project. 2

3 Parameter OMW Olive Husk Milk Whey ph Total Solids (mg/l) ,000 19,200 Volatile Solids (mg/l) ,000 12,000 Volatile/total solids (%) 65% 98% 68% Soluble COD (mg/l) (50 % dilution) Total COD (mg/l) ,000 31,800 solcod/totcod (%) 87% 3.6% 65% Total N (mg/l) < Total P (mg/l) Polyphenols (mg/l Table 2 Chemical characterization of the biomasses examined The husk and OMW are recovered from a continuous three phase plant that is characterized by a husk with a polyphenol content around 1000 ppm; while the humid husks from the 2 phase plant have a concentration of around 8000 ppm. The husks have a very high potential level of total COD, but the amount of soluble COD, that which effectively contributes to the production of biogas, is only 3.6 % when the ratio between husk and distilled water is 1:1. Mixing OMW and milk whey contributes to increased solubility of organic substance present in the husk; in this way it is possible to adequately dilute the husk without adding water from the water supply. A feeding plan balanced in this way not only solves the problem of the disposal of milk and olive refuse, but makes a further contribution in terms of dissolved organic substance dissolved and biogas production. The utilization of milk whey as biomass in the entrance optimally results in the balancing of the feed mixture (25% OMW, 25 % husk, and 50% milk whey), because it contributes to an optimal C/N ratio (20-30) for anaerobic digestion. Furthermore the low content of polyphenols in milk whey, with respect to those contained in the OMW and in the husk, reduces their inhibitory effect on microbiotic growth, as it is directly proportional to the concentration of polyphenols present in the digested biomass. Analysis on the chemical composition of the mix of husk and OMW has demonstrated that the value of total COD diminishes over time, even when stored at low temperature (4 C). Therefore, the storage of such a mix of biomass should take those aspects into consideration. 2.2 Preliminary Treatments All the tests of fermentation support treatments of grinding and homogenization of the entering mixture before anaerobic digestion, in order to increase the contact area between biodegradable substances and microorganisms. The focus of these studies was the further pretreatments necessary to degrade phenol substances present in the feed mixture. The first pretreatment studied was sonication. The two objectives of sonication are, (1) degrade polyphenols and, (2) increase the solubility of the organic substance that is present in elevated concentrations in the solid phase of olive husk. Tests of increasing solubility through ultrasound were performed on all the mixes proposed. In addition, different energies of sonication were employed on the most likely feed mixture. The results of a sonication 3

4 test on a particular mixture (16.5 % husks, 16.5 % OMW, 67 % water from water supply) are shown in Figure 1: Soluble COD [mg/l] Mix Sonication Energy [kj/l] Figure 1 - Concentration of soluble COD as a function of sonication energy The effects of ultrasound on the degradation of polyphenols were negligible, while the effects on increasing solubility of the organic substance were not highly efficient. The unsatisfying results can be attributed to the presence of colloidal polysaccharides and polyphenols or to large organic macro-molecules that, different from the sludge that results, are difficult to dissolve in the liquid phase. In reality, we can attribute the small increase of soluble COD to thermal effects of the sonication. Further tests of thermal means to increase solubility will be performed within the coming months, in order to confirm their benefits on making COD more soluble. 2.3 Tests of Anaerobic Digestion Tests of batch anaerobic digestion under mesophilic conditions were performed to differentiate mixes with and without sonication. In table 3, the results of two different feed types are highlighted: mix A is 50% an anaerobic digestion inocolum made mud discharged from a civil waste purification plant and 50% from OMW, mix B is composed of 50 % the same anaerobic inocolum, 25% OMW, and 25% husks. The soluble COD in mix B increases over time, as shown in Table 3. The increase occurred despite the production of biogas, which is generated from oxidation of the dissolved substance and from consequent reduction of soluble COD, as is evidenced by results obtained from anaerobic treatment of mix A. Day Total Solids % Volatile Solids % TS/VS % Soluble COD [mg/l} VS degraded % Soluble COD degraded % Total polyphenols [mg/l] Produced Biogas [cm 3 ] A A A A B B N/A 50 B N/A 60 B Table 3 Parameters monitored during the process of anaerobic digestion 4

5 As said earlier, these anomalous results are due to the slow process of dissolving the organic fraction of the olive husk, which evidently occurs during the digestion phase and is the cause of the abnormal increase of COD observed during test B. This justifies the necessity of adopting pretreatment systems capable of dissolving the largest possible quantity of olive husk organic substance. In comparison, the organic substance in mix A starts in a dissolved form and infact, by the end of the test, registered a 90% degradation of soluble COD concentration and a cumulative production of cm 3. Continuous tests of anaerobic digestion were performed by AUSTEP. The feed plan was calculated to include husks, in addition to OMW, which was diluted in water in the ratios indicted in figure 2: Figure 2. Percentage of COD and VS converted into biogas over time The reactor reached steady state conditions after 2 weeks and it was clear that VS degradation was between 60-75%, while soluble COD was always at lower values. These results are caused by the retention time, which needs to be long in order to guarantee that the easily digestible organic substances are consistently dissolved to a great extent. This is a further confirmation of the problems related to dissolving that were noticed during the batch tests. In order to favor optimal microbial growth, potassium salt was added to maintain a C:N:P ratio of 350:5:1. In addition, after approximately 2 months it proved necessary to add sodium hydroxide to maintain a neutral ph. 5

6 Biogas Production [tons/day] Digestion Time [days] Figure 3. Daily production of biogas The percentage of methane in the biogas varied between 65-75%. 3.Conclusions The use of sonication for dissolving husk organic substance did not give the expected results. This is confirmed in anaerobic digestion tests, both batch and continuous, where the percentage of COD degraded is less that that observed for VS, justifying the need to develop new pretreatments in order to improve the efficiency of the process. The percentage of methane in the biogas is interesting, especially when compared with data in the literature regarding biogas produced from agricultural biomass. The critiques typical of the employment of olive refuse, such as the seasonal production, the chemical composition, the change of composition during storage and the high level of water consumption, can be surpassed by exploiting whey milk. Preliminary tests of anaerobic digestion substituting water with milk whey have given encouraging results. Works Cited [1] Università Politecnica delle Marche, Usi economicamente sostenibili degli scarti e dei sottoprodotti della lavorazione delle olive, [2] Brenes M., Garcia P., Romero C. and Garrido A. Ripe olives storage liquids reuse during the oxidation process. J. Food Sci. 1998; 63,