Effect of Chemical Pre-treatment on Methane Production from Co-Digestion of Cow Manure and Pistachio Wastes

Iran is the highest producer of pistachio worldwide and has an annual production of about 500,000 tons of fresh pistachio by-product. Due to its high organic and phenolic content as well as high solids concentration, pistachio processing wastes create significant waste management problems unless they are properly managed. Anaerobic digestion is one of the acceptable environmental and economic technologies to control the pistachio wastes and produce energy and enrich fertilizer for soil. Good digestion for different wastes needs suitable mixture for co-substrates and pretreatments. Therefore, effect of different mixture of pistachio waste: cow manure and pretreatment factors of NaOH and H2O2 was studied in this research. Co-digestion was done in the 250 mL digesters for mesophilic condition using a combined D-optimal design. Their CH4 production were measured for 24 days. Then a model was developed among the CH4 production and amounts of pistachio waste, cow manure, NaOH and H2O2 to determine the effects of these parameters on the CH4 production. Model P-value was lower than 0.0001 and its lack of fit was 0.27, predicted R2 was 0.53 in reasonable agreement with adjusted R2 of 0.63. Adequate precision was 9.7 indicated an adequate signal for the model. Further, standard error of the model equaled to 0.4 which was very low, model R2 equaled to 0.69 and adjusted R2 was 0.63 which were high, indicating that the model was suitable for CH4 estimation. By illustrating the model in 3D graphs, it was shown that the use of NaOH had negative effect on the CH4 production while H2O2 had positive effect on the CH4 production. Higher amounts of cow manure improved the CH4 production while higher amounts of pistachio wastes decreased the CH4 production. These may happen because of the high amounts of phenolic and antioxidant compounds in the pistachio waste which are harmful for anaerobic microbes.