• Energy Research

There is an increasing interest in the production of bioplastics from biomass-based feedstocks to address the challenges associated with increasing global plastic consumption. Bioplastics are produced mainly from 1st generation feedstocks that compete with food production for agricultural resources. Recently, microalgae have gained interest as a feedstock for bioplastics production. Microalgae can be used in various ways to produce different types of bioplastics including various biodegradable and drop-in bioplastics. However, not much attention has been paid to different routes of bioplastics production from microalgae. This review examines the potential of using microalgae as a feedstock for bioplastics, with a focus on three key polymer synthesis routes: 1) use of natural polymers synthesised by microalgae, 2) chemical synthesis of polymers from microalgae-derived feedstocks and 3) microbial synthesis of polymers from microalgae-derived feedstocks. The technical and economic challenges associated with each route are analysed. The optimal route of using microalgae as a feedstock for bioplastics largely depends on the economics of the process. Conducting comparable feasibility studies for various routes is recommended to identify the most economically viable route for utilising microalgae to produce bioplastics. Microalgae has great potential for the bioplastic industry, however, to progress the research to commercialisation, future research emphasis should be placed on investigating various routes of utilising microalgae for bioplastics along with optimising the process for enhanced biomass productivity and polymer yield, characterising the produced polymers, investigating the co-production of bioplastics with other products, and integrating the production of bioplastics with the wastewater treatment.

Microalgae cultivation on wastewater is one of the most promising processes in perspective of green and circular economy. This study investigated the economics of integrating the microalgae cultivation with the wastewater treatment in perspective of biomass production and wastewater treatment. The cost of integrated process was evaluated for six cases: three cases for domestic wastewater at different stages of treatment including sewage, anaerobically digested domestic effluent, and centrate and three cases for industrial wastewater including agro-industrial wastewater, anaerobically digested piggery effluent, and anaerobically digested abattoir effluent. The cost of biomass production was found ranging from $ 0.39 to $ 0.92/kg with minimum for the anaerobically digested domestic effluent and centrate. The cost of wastewater treatment was found ranging from 0.18 to 1.69/m3 with minimum for the sewage. These costs did not include any credits generated from the biomass or the treated wastewater. The concentration of limiting nutrient, flowrate of wastewater, and the extent of nutrient removal are the major cost-influencing parameters for the integrated process.

The wastewater produced from the meat-processing industry is a rich source of nutrients which can be recovered using microalgae. This study assesses the potential of microalgae cultivation on abattoir wastewater based on its nutrient removal capacity from wastewater, biomass production and greenhouse gas (GHG) emission savings potential. Designing the treatment ponds at the recycling rate of almost 80% of treated water results in high-quality water containing less than 1 mg/L nitrogen and 12 mg/L phosphorus. At the same time, the process can produce valuable algal biomass (≈2 kg/m3 of abattoir wastewater) which can be further dewatered to make the process either economically self-sufficient or profit-making depending upon the use of algal biomass. It can finally avoid GHG emissions from 3.46 kg CO2-eq to 6.11 kg CO2-eq per m3 of wastewater treated depending upon the credit of the product displaced by the algal biomass.

Microalgae have tremendous potential for producing liquid renewable fuel. Many methods for converting microalgae to biofuel have been proposed; however, an economical and energetically feasible route for algal fuel production is yet to be found. This paper presents a review on the comparison of the most promising conversion pathways of microalgae to liquid fuel: hydrothermal liquefaction (HTL), wet extraction and non-destructive extraction. The comparison is based on important assessment parameters of product quality and yield, nutrient recovery, GHG emissions, energy and the cost associated with the production of fuel from microalgae, in order to better understand the pros and cons of each method. It was found that the HTL pathway produces more oil than the wet extraction pathway; however, higher concentrations of unwanted components are present in the HTL oil produced. Less nutrients (N and P) can be recovered in HTL compared to wet extraction. HTL consumes more fossil energy and generates higher GHG emissions than wet extraction, while the production cost of fuel from HTL pathway is lower than wet extraction pathway. There is considerable uncertainty in the comparison of the energy consumption and economics of the HTL pathway and the wet extraction pathway due to different scenarios analysed in the assessment studies. To be able to appropriately compare methodologies, the conversion methods should be analysed from growth to upgradation of oil utilising sufficiently similar assumptions and scenarios. Based on the data in available literature, wet oil extraction is the more appropriate system for biofuel production than HTL. However, the potential of alternative extraction/conversion technologies, such as, non-destructive extraction, need to be further assessed.