Algal Biorefinery: routes for energy and nutrient recovery

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Research into using microalgae for the bioremediation of waste streams, for example those from tertiary waste water treatment, has been gaining momentum in the UK. However, if biomass is not suitable for the food chain, generating bioenergy via co-digestion with other feedstocks is a possible route. The University of Cambridge partners are currently involved in ecosystem service research that involves using by-products from drinking water purification to grow algae, and this project seeks to "close the loop" on nutrient retrieval. This project built on the 2014 AB-SIG award that provided evidence that methane could be produced from digested algae that were grown on waste water from one source. The tests confirm that the anaerobic digestion is a possible route of utilisation of algal biomass. Anaero have devised a scale-down version of an anaerobic digester, capable of running 15 experiments in parallel. We have been repeating the earlier studies, using more species and pasteurisation processes and using algae grown on wastewater from more than one source to look for variation in the process. In the context of this project, microalgae are playing a provisioning role, generating bioenergy and converting nutrients into a useable form, which will play a vital role in contributing to the bioeconomy market in the UK.

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Lab- scale evaluation of biogas potential from microalgae

While food and growth are priorities in some developing nations, the expansion of “high living standards” is seen as a pre-requisite for “sustainability” in more developed nations. In both cases, energy security is a fundamental factor for economic performance and environmental impact. The concerns of these two “worlds” come into apparent conflict as developed economies search for “sustainable” sources of energy in the form of energy crops where agriculture land is diverted to growing fuel rather than food.
The search for alternatives to energy crops (or “purpose grown crops”) for sustainable power generation includes the potential use of algae biomass as a carbon sink and energy source that could potentially avoid the use of good agricultural land to grow energy. The results achieved suggest that the processes such as algae-based wastewater treatment could be sources of biomass for energy generation in anaerobic digestion (AD) plants as well as capturing nutrients, which can be returned to agricultural soils in the form of digestate.
The Biomethane Potential equipment developed by Anaero Technology was used to evaluate three types of algae and maize, the most widespread energy crop in AD. The relative biogas yield over a period of 30 days achieved was Phaeodactylum tricornutum (719.9 l.kg-1 VS), Scenedesmus obliquus (401.27 l.kg-1 VS) and Tetraselmis suecica (372.7 l.kg-1 VS), while Maize produced 750l.kg-1VS.
The tests confirm that the anaerobic digestion of algae biomass is a possible route of utilisation of algae biomass. However, the high biomass yields required for energy-specific algae growth are still not being achieved even in warmer countries. Further, development work on efficient harvesting of microalgae is scarce, limiting the potential of algae for energy generation. Complementing environmental applications, such as use of algae for wastewater treatment, with energy generation and nutrient recovery through anaerobic digestion could be the best route forward while improvements on biomass yield and harvesting methods that make the process self-sustaining are developed. It is apparent, from the literature search on micro algae for AD, that biogas potentials and energy content are being evaluated in isolation from data on growth and harvesting rates for the algae species used. Future evaluations of energy and resource potential from algae should include data on rates of algae biomass production (t ds/ha.y) in order to enable consistent evaluation and comparison with established alternatives for land use, such as energy crops. This misrepresentation could be creating an “algae bubble” that could damage the long term potential of this technology.

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