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

Abstract

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 (S. 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 carbonrich substrates. Different co-digestion mixtures of OMSW-microalgae and the single substrate were tested. For all co-digestion mixtures, the alkalinity value at the end of the experiment remained below 4889±245 mg CaCO 3 /L and pH in the range of 7.50-7.67 indicating stability and good process performance. The results showed the highest methane yield (461 mL CH 4 2 STP/g 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 CH 4 /(g VS d).

Enhancing methane production from the invasive macroalga Rugulopteryx okamurae through anaerobic co-digestion with olive mill solid waste: process performance and kinetic analysis

“…They reported a 75% of biodegradability of Laminaria digitata during anaerobic assays. Fernández-Rodríguez et al (2019b) also found an OMSW biodegradability of 56%, very similar to that obtained in this experiment. In addition, the mentioned study also found an improvement in the biodegradability of different microalgae-OMSW mixtures compared with those obtained for the substrates separately.…”

“…The mesophilic batch experiments (BMP tests) were performed with an inoculum to substrate ratio fixed at 2 (as volatile solid (VS)) as described elsewhere (Fernández-Rodríguez et al 2019a). A 10% micronutrient solution (Fernández-Rodríguez et al 2019b) and nitrogen gas was added at the beginning of the experiment in order to keep anaerobic conditions. The anaerobic reactors of 250 mL total volume were placed in a thermostatic water bath at a controlled temperature of 35 ± 2 °C.…”

Impact of soft hydrothermal pre-treatments on the olive mill solid waste characteristics and its subsequent anaerobic digestion

“…The methane yield obtained during AD of 100% OMSW was 341 ± 22 mL CH 4 /g VS added. This value of methane yield obtained for the BMP test of OMSW was in accordance with previous studies[3,41,58]. After 30 days of experiment, the maximum methane yield obtained was 383 ± 2 mL CH 4 /g VS added for A1 pre-treatment.…”

Anaerobic digestion inhibition indicators and control strategies in processes treating industrial wastewater and wastes

“…The adjustment of the C:N ratio to 30 using ammonium, increased the CH 4 production and reduced the lag phase of anaerobic digestion of swine manure (Wijesinghe et al, 2019). Fernández-Rodríguez et al (2019) observed the highest methane yield in the anaerobic co-digestion of olive mill solid waste and microalga Scenedesmus quadricauda when C:N ratio was adjusted to 31.9 using the microalga as a nitrogen source. However, none of the existing studies discusses the possible inhibiting effects of the added compounds.…”