The anaerobic digestion (AD) process is a consolidated industrial approach to produce biogas from organic matrices. AD process can be performed either by using mono substrate digestion or co-digestion of different substrates. The latter strategy has some advantages over the mono substrate AD process, including macro/micro nutrition balance. However, the feedstock mixture generally affects the biogas production. Thus, preliminary tests dedicated to determine the best relative substrate amounts are typically carried out. The present study was performed to evaluate the employment of statistical approach to set up such preliminary tests. The design of experiment, response surface methodology (RSM) and specifically mixture design were used to define the experiments, whose aim was optimizing a response variable (biomethane production) related to the co-digestion of grape pomace, olive mill wastewater, and corn stover. The design points were set up according to the augmented simplex centroid design on the vertices, edges, and interior of the triangle representing the pure, binary and mixture of the three substrates, respectively. The batch assays were performed in triplicate under a thermophilic condition at 50?C using a methanogenic inoculum and the experiment lasted 76 days. The analysis was performed three times per week in order to measure the produced biogas. A statistically significant polynomial model was developed according to the experimental results obtained from the design points and the optimization was performed considering the obtained model.Finally, the obtained optimal point suggested being consistent with the experimental results, which proved the significance of the method and final model.

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APPLICATION OF STATISTICAL METHODS IN THE OPTIMIZATION OF FEEDSTOCK
MIXTURES FOR ANAEROBIC CO-DIGESTION PROCESSES
INTRODUCTION
Anaerobic digestion process can be fed by a single organic material/waste or by combining different organic substrates in a feedstock mixture (co-digestion process).
The latter one has some advantages over the mono substrate digestion as a result of different interactions among different kinds of substrates, such as:
• substrate nutrition balance
• higher availability of low-cost feedstock
• more efficient waste management
• Higher overall biomethane production (BMP)
However, in order to optimize the anaerobic co-digestion process, the composition of the feedstock mixture has to be determined in terms of relative amounts of
single substrates through preliminary test.
AIM OF THE STUDY
• Application of a statistical approach to determine the optimal relative amounts of 3 target substrates in order to maximize the process BMP
• The Design of Experiments and Surface Response Methodology, in particular Mixture Design, were used to design the experiment.
DESIGN OF EXPERIMENT
 Mixture Design
Based on a triangle whose vertices represent the pure component, edges the binary combination, and interior points combination of the three substrates.
 It helps to collect the appropriate data that can be analyzed statistically and prevents the waste of time and money in the case in which the experiment would
have been performed by randomly selected compositions.
 A mathematical polynomial model including high order terms can be developed according to the data collected from the design points.
 An optimization can be performed according to the obtained model and the goal of the process optimization.
 Historical data related to each process can be utilized by using this method.
FIXED PARAMETERS
 Thermophilic condition at 50 ◦C
 pH: was regularly adjusted under nitrogen
flux to the value between 7.5-7.6
 Agitation rate: 115 rpm
 Batch Reactors: 100 ml Pyrex bottles in
triplicates
 Duration: 76 days
 Experimental results:
The experimental results suggested that:
 The highest biomethane production (over fed VSs) was obtained by a mixture of the three
substrates and not by the digestion of a mono substrate or of a combination of two.
 In particular, The highest biomethane production was obtained by the assay with high
proportion of Corn Stover and low proportion of Olive Mill Wastewater.
RESULTS
 Different order polynomial models were developed according to the collected
experimental data.
 The statistical analysis were performed to choose the best model.
 A 3D response surface was plotted and by maximizing the response variable,
which was the methane production over fed VSs, the optimal point according to
the obtained model was calculated.
Lorenzo Bertin, Mohammadali Mozafarian, Giorgia Liberati, Gonzalo A. Martinez, Joana M. Bendada Domingos, Fabio Fava
Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna - DICAM, Italy
CONCLUSION
Co-Digestion of substrates was proved to result to higher methane production
Mixture design and statistical methods can fairly predict the behaviour of mixtures
Statistical methods can be used to optimize anaerobic co-digestion processes
VARIABLES
feedstock mixture containing Grape Pomace, Corn Stover,
and Olive Mill Wastewater
EXPERIMENTAL CONDITIONS
22 Experimental conditions (marked from A to
W) were set up in triplicate
Samples were prepared according to the statistical design
Gas volume
measurement by a glass
syringe
Biogas composition
measurement by gas
chromatography
ANALYSIS AND DATA COLLECTION ( 3 times per week)
Biogas production plots
EXPERIMENT SETUP
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