Strategies to Optimize Microalgae Conversion to Biogas: Co-Digestion, Pretreatment and Hydraulic Retention Time

Abstract

This study aims at optimizing the anaerobic digestion (AD) of biomass in microalgal-based wastewater treatment systems. It comprises the co-digestion of microalgae with primary sludge, the thermal pretreatment (75 °C for 10 h) of microalgae and the role of the hydraulic retention time (HRT) in anaerobic digesters. Initially, a batch test comparing different microalgae (untreated and pretreated) and primary sludge proportions showed how the co-digestion improved the AD kinetics. The highest methane yield was observed by adding 75% of primary sludge to pretreated microalgae (339 mL CH4/g VS). This condition was then investigated in mesophilic lab-scale reactors. The average methane yield was 0.46 L CH4/g VS, which represented a 2.9-fold increase compared to pretreated microalgae mono-digestion. Conversely, microalgae showed a low methane yield despite the thermal pretreatment (0.16 L CH4/g VS). Indeed, microscopic analysis confirmed the presence of microalgae species with resistant cell walls (i.e., Stigioclonium sp. and diatoms). In order to improve their anaerobic biodegradability, the HRT was increased from 20 to 30 days, which led to a 50% methane yield increase. Overall, microalgae AD was substantially improved by the co-digestion with primary sludge, even without pretreatment, and increasing the HRT enhanced the AD of microalgae with resistant cell walls.

The effect of primary treatment of wastewater in high rate algal pond systems: Biomass and bioenergy recovery

“…This highlights the synergy of co-digesting microalgae with primary sludge, as also described in previous studies on co-digestion of microalgae and other C-rich substrates (Solé-Bundó et al, 2017b;Yen and Brune, 2007). The results are also in agreement with previous studies in which the co-digestion of microalgae and sewage sludge had a synergistic effect (Olsson et al, 2014;Solé-Bundó et al, 2018).…”

“…Please insert Table 5 The thermal pretreatment was applied in this study in order to increase microalgae biodegradability by breaking down their resistant cell wall, as suggested by previous studies (Solé-Bundó et al, 2018). Several studies on microalgae pretreatment for biogas production have been reported, including biological, chemical and physical pretreatments (Kendir and Ugurlu, 2018).…”

Combining Microwave Pretreatment with Iron Oxide Nanoparticles Enhanced Biogas and Hydrogen Yield from Green Algae

“…The addition of Fe 3 O 4 NPs showed an improvement of metals stabilization in the digestate resulted in an enhancement of biogas and methane production. Abdelsalam et al [28] examined the influence of Fe 3 O 4 NPs with different concentrations (5,10, and 20 mg/L) on biogas and methane yield from the AD of cattle manure (CM) slurry. Anaerobic fermentation of CM was carried out batch-wise at operating temperature and mixing rate of 37 ± 0.3 • C and 90 rpm for a hydraulic retention time (HRT) of 50 days.…”

Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion

“…Thus, an optimized OLR and HRT should be applied to achieve the expected specific methane yield. Possible solutions could be improving anaerobic digester configurations such as using membrane reactors or upflow anaerobic sludge blanket reactors to decouple the OLR and HRT [37,119] and on-line control of anaerobic digester operation [124]. These have not been applied for digesting algal biomass.…”