Indirect regulation of temperature in raceway reactors by optimal management of culture depth

Process design and economic analysis for the production of microalgae from anaerobic digestates in open raceway ponds

A model based framework was established for large scale assessment of microalgae production using anaerobically digested effluent considering varied climatic parameters such as solar irradiance and air temperature. The aim of this research was to identify the optimum monthly average culture depth operation to minimize the cost of producing microalgae grown on anaerobic digestion effluents rich in ammoniacal nitrogen with concentration of 248 mg L-1. First, a productivity model combined with a thermal model was developed to simulate microalgae productivity in open raceway ponds as a function of climatic variables. Second, by combining the comprehensive open pond model with other harvesting equipment, the final techno economic model was developed to produce a microalgae product with 20 wt% biomass content and treated water with <1 mg L-1 ammoniacal nitrogen. The optimization approach on culture depth for outdoor open raceway ponds managed to reduce the cost of microalgae production grown in anaerobic digested wastewater up to 16 %, being a suitable solution for the production of low cost microalgae (1.7 AUD kg-1 dry weight) at possible scale of 1300 t dry weight microalgae yr-1.

Integrating microalgae growth in biomethane plants: Process design, modelling, and cost evaluation

The integration of microalgae cultivation in anaerobic digestion (AD) plants can take advantage of relevant nutrients (ammonium and ortho-phosphate) and CO2 loads. The proposed scheme of microalgae integration in existing biogas plants aims at producing approximately 250 t·y-1 of microalgal biomass, targeting the biostimulants market that is currently under rapid expansion. A full-scale biorefinery was designed to treat 50 kt·y-1 of raw liquid digestate from AD and 0.45 kt·y-1 of CO2 from biogas upgrading, and 0.40 kt·y-1 of sugar-rich solid by-products from a local confectionery industry. An innovative three-stage cultivation process was designed, modelled, and verified, including: i) microalgae inoculation in tubular PBRs to select the desired algal strains, ii) microalgae cultivation in raceway ponds under greenhouses, and iii) heterotrophic microalgae cultivation in fermenters. A detailed economic assessment of the proposed biorefinery allowed to compute a biomass production cost of 2.8 ± 0.3 €·kg DW-1, that is compatible with current downstream process costs to produce biostimulants, suggesting that the proposed nutrient recovery route is feasible from the technical and economic perspective. Based on the case study analysis, a discussion of process, bioproducts and policy barriers that currently hinder the development of microalgae-based biorefineries is presented.

Influence of pH and dissolved oxygen control strategies on the performance of pilot-scale microalgae raceways using fertilizer or wastewater as the nutrient source

Dissolved oxygen concentration and pH are controllable and cost-effective variables that determine the success of microalgae-related processes. The present study compares different control strategies for pH and dissolved oxygen in pilot-scale microalgae production systems. Two 80 m2 raceway reactors were used, one operated with freshwater plus fertilizer and the other with wastewater as the nutrient source. Both were in semi-continuous mode at a fixed dilution rate of 0.2 day-1. A comparison between the classical On-Off and more advanced pH control strategies, such as PI and Event-based control, was performed, focusing on biomass productivity and the influence of all the process parameters on microalgae growth; "No control" of pH was also assayed. The results show that Event-based control was the best algorithm when using freshwater plus fertilizer. In contrast, no significant differences were observed using the different control strategies when wastewater was the nutrient source. These experiments were performed through selective control strategy, prioritizing pH over dissolved oxygen; however, it was demonstrated that they did not allow to achieve satisfactory dissolved oxygen removal results, especially for the fertilizer system. After modifying the gas diffuser configuration and improving the mass transfer, independent on-off strategies have been developed, permitting effective control of both variables and increasing productivity by up to 20% in both systems. Concluding, a detailed analysis of the energy demand for each strategy implemented in terms of gas consumption and gas flow to biomass ratio is provided.

Protein Quality and Protein Digestibility of Vegetable Creams Reformulated with Microalgae Inclusion

Microalgae are considered a valuable source of proteins that are used to enhance the nutritional value of foods. In this study, a standard vegetable cream recipe was reformulated through the addition of single-cell ingredients from Arthrospira platensis (spirulina), Chlorella vulgaris, Tetraselmis chui, or Nannochloropsis oceanica at two levels of addition (1.5% and 3.0%). The impact of microalgae species and an addition level on the amino acid profile and protein in vitro digestibility of the vegetable creams was investigated. The addition of microalgae to vegetable creams improved the protein content and the amino acid nutritional profile of vegetable creams, whereas no significant differences were observed in protein digestibility, regardless of the species and level of addition, indicating a similar degree of protein digestibility in microalgae species despite differences in their protein content and amino acid profile. This study indicates that the incorporation of microalgae is a feasible strategy to increase the protein content and nutritional quality of foods.

Microalgae protein digestibility: How to crack open the black box?

Microalgae are booming as a sustainable protein source for human nutrition and animal feed. Nevertheless, certain strains were reported to have robust cell walls limiting protein digestibility. There are several disruption approaches to break down the cell integrity and increase digestive enzyme accessibility. This review's intent is to discuss the digestibility of microalgae proteins in intact cells and after their disruption. In intact single cells, the extent of protein digestibility is chiefly related to cell wall structural properties (differing among strains) as well as digestion method and when added to food or feed protein digestibility changes depending on the matrix's composition. The degree of effectiveness of the disruption method varies among studies, and it is complicated to compare them due to variabilities in digestibility models, strains, disruption method/conditions and their consequent impact on the microalgae cell structure. More exhaustive studies are still required to fill knowledge gaps on the structure of microalgal cell walls and to find efficient and cost-effective disruption technologies to increase proteins availability without hindering their quality.

A seasonal simulation approach for culture depth influence on the temperature for different characterized microalgae strains

Irradiance and temperature are among the most important variables that affect microalgae growth, being both difficult to control in outdoor raceway reactors utilized for large-scale production of microalgae biomass. They are mainly a function of the location of the reactors, thus, producing certain strains of microalgae in inappropriate places conduces to the failure of the systems. To be able to determine important parameters of any microalgae strains on the performance of the culture, such as the influence of irradiance and temperature, is a powerful tool in decision-making processes. In addition, whatever the strain and location, operation strategies must be defined for each specific case, such as the imposed dilution rate and culture depth, both influencing the light availability and temperature of the culture as major variables determining the biomass productivity. In this paper, a simulation-based methodology is proposed to establish the influence of season and culture depth on the 1-year age irradiance and temperature of the culture, and thus on the biomass productivity of different microalgae strains. Up to five of the most frequently produced strains, such as Spirulina platensis, Chlorella vulgaris, Nannochloropsis gaditana, Isochrysis galbana and Scenedesmus almeriensis have been considered. The challenge is to develop an easy-to-manage decision-making tool for the optimal design and operation of large-scale microalgae facilities. Especially, dates for microalgae production and culture depth at which the reactors must be operated will be provided, being valid for any microalgae strain. The proposed methodology will largely contribute to the risk of investment in this field, then to enlarge the relevance of the microalgae production industry. This article is protected by copyright. All rights reserved.

Annual assessment of the wastewater treatment capacity of the microalga Scenedesmus almeriensis and optimisation of operational conditions

The depth of the culture and the dilution rate have a striking effect on the biomass productivity and the nutrient recovery capacity of microalgal cultures. The combination of culture depth and dilution rate that allows to maximise the performance of the system depends on environmental conditions. In the current study, a response surface methodology was used to explore the relationship between the two most relevant operational conditions and the biomass productivity achieved in 8.3 m² pilot-scale raceways operated using urban wastewater. Four polynomial models were developed, one for each season of the year. The software predicted biomass productivities of 12.3, 25.6, 32.7, and 18.9 g·m^-2·day^-1 in winter, spring, summer, and autumn, respectively. The models were further validated at pilot-scale with R² values ranging within 0.81 and 0.91, depending on the season. Lower culture depths had the advantage of minimising nitrification and stripping but allow to process a lower volume of wastewater per surface area. Biomass productivity was higher at culture depths of 0.05 m, when compared to 0.12 and 0.20 m, while the optimal dilution rate was season-dependent. Results reported herein are useful for optimising the biomass productivity of raceway reactors located outdoors throughout the year.