Anaerobic co-digestion of olive mill solid waste and microalga Scenedesmus quadricauda: effect of different carbon to nitrogen ratios on process performance and kinetics

Anaerobic digestion (AD) is one of the most efficient processes for treating agri-food waste in order to obtain renewable energy. Olive mill solid waste (OMSW) is the main residue from the two-phase olive oil manufacturing process; it has a high organic content and high C/N ratio, which hinders its AD, giving low methane yield. In the present study, a microalga, Scenedesmus quadricauda, was used as co-substrate for the AD of OMSW to compensate for its nitrogen deficiency. The robustness and the high growth rate of S. quadricauda make this microalga a potential source of nitrogen to co-digest with carbon-rich substrates. Different co-digestion mixtures of OMSW-microalgae and the single substrates were tested. For all co-digestion mixtures, the alkalinity value at the end of the experiment remained below 4889 +/- 245 mg CaCO3 L-1 and pH in the range of 7.50-7.67 indicating stability and good process performance. Results showed the highest methane yield (461 mL CH4 STP g(-1) VS added) for the co-digestion mixture 75% OMSW-25% S. quadricauda (C/N = 25.3), which was 104 and 23% higher than that obtained from the single microalga (C/N = 5.6) and OMSW (C/N = 31.9), respectively. No ammonia inhibition was detected despite the high protein content of the microalgae. The transference function model allowed for adequately fitting the experimental results of methane production with time in the anaerobic experiments. The highest maximum methane production rate, R-m, among the different co-digestion mixtures assayed was obtained for the mixture 75% OMSW-25% S. quadricauda with a value of 89 mL CH4 g(-1) VS day(-1).