Scaling-Up the Anaerobic Digestion of Pretreated Microalgal Biomass within a Water Resource Recovery Facility

Abstract

Microalgae-based wastewater treatment plants are low-cost alternatives for recovering nutrients from contaminated effluents through microalgal biomass, which may be subsequently processed into valuable bioproducts and bioenergy. Anaerobic digestion for biogas and biomethane production is the most straightforward and applicable technology for bioenergy recovery. However, pretreatment techniques may be needed to enhance the anaerobic biodegradability of microalgae. To date, very few full-scale systems have been put through, due to acknowledged bottlenecks such as low biomass concentration after conventional harvesting and inefficient processing into valuable products. The aim of this study was to evaluate the anaerobic digestion of pretreated microalgal biomass in a demonstration-scale microalgae biorefinery, and to compare the results obtained with previous research conducted at lab-scale, in order to assess the scalability of this bioprocess. In the lab-scale experiments, real municipal wastewater was treated in high rate algal ponds (2 × 0.47 m3), and harvested microalgal biomass was thickened and digested to produce biogas. It was observed how the methane yield increased by 67% after implementing a thermal pretreatment step (at 75 °C for 10 h), and therefore the very same pretreatment was applied in the demonstration-scale study. In this case, agricultural runoff was treated in semi-closed tubular photobioreactors (3 × 11.7 m3), and harvested microalgal biomass was thickened and thermally pretreated before undergoing the anaerobic digestion to produce biogas. The results showed a VS removal of 70% in the reactor and a methane yield up to 0.24 L CH4/g VS, which were similar to the lab-scale results. Furthermore, photosynthetic biogas upgrading led to the production of biomethane, while the digestate was treated in a constructed wetland to obtain a biofertilizer. In this way, the demonstration-scale plant evidenced the feasibility of recovering resources (biomethane and biofertilizer) from agricultural runoff using microalgae-based systems coupled with anaerobic digestion of the microalgal biomass.

Sludge Treatment Wetland for Treating Microalgae Digestate Grown in Agricultural Runoff: A Technical, Economic, and Environmental Assessment

“…Briefly, the microalgae production system consisted of three semiclosed, horizontal tubular photobioreactors at a demonstrative scale (11.7 m 3 each), which were fed with a mixture of agricultural runoff (90% v/v) and partially treated domestic wastewater (10% v/v). More information on the demonstrative-scale plant can be found in [14,17,18]. The biomass produced in the photobioreactors was harvested using a lamella settling tank (700 L) and thickened in two gravity thickeners (200 L each) [19,20].…”

“…Approximately 11 kg of harvested biomass from the lamella settling tank creates a favorable environment for the initial plant development (Figure 2). be found in [14,17,18]. The biomass produced in the photobioreactors was harvested using a lamella settling tank (700 L) and thickened in two gravity thickeners (200 L each) [19,20].…”

Insights into removal of antibiotics by selected microalgae (Chlamydomonas reinhardtii, Chlorella sorokiniana, Dunaliella tertiolecta and Pseudokirchneriella subcapitata)

“…It is crucial for biorefineries to valorise the residual algal biomass used in wastewater treatment to guarantee the economic feasibility of the processes [79,80]. In this sense, anaerobic digestion is among the most straightforward and suitable technique for energy recovery [81]. Depending on the algae strain, the biomass will take up more or less contaminant, and algae with a higher degradation capacity and low adsorption capacity should be chosen for such processes.…”

Optimization and operation of a demonstrative full scale microalgae harvesting unit based on coagulation, flocculation and sedimentation

“…ECOTAN was switched for PAX (with a viscosity of 30 cP), which was used during the rest of the study, significantly improving the performance of the settler as it is shown in Figure S8 of the Supplementary Material. PAX-18 did not prevent the subsequent anaerobic digestion of the harvested biomass to perform successfully, achieving an average VS removal of 70% and a methane yield up to 0.24 L CH 4 /g VS [29]. In addition, the digestate was further stabilized and dewatered in sludge treatment wetland [24] which showed a satisfactory plant growth.…”