Algal Research 5 (2014) 204–214 Contents lists available at ScienceDirect Algal Research journal homepage: www.elsevier.com/locate/algal Review article Anaerobic digestion of algae biomass: A review A.J. Ward a,⁎,D.M.Lewisa,F.B.Greenb a Microalgae Engineering Research Group, The University of Adelaide, School of Chemical Engineering, Adelaide, South Australia, Australia b Oswald Green Technologies, Inc., USA article info abstract Article history: The anaerobic digestion of microalgae is a prospective environmentally feasible option for creating a renewable Received 22 July 2013 source of energy for industrial and domestic needs. Microalgae anaerobic digestion is a key unit process that in- Received in revised form 29 January 2014 tegrates efficiency and beneficially into the production of microalgae derived biofuels. Anaerobic digestion culmi- Accepted 1 February 2014 nating in methane fermentation improves the economic viability of microalgae liquid biofuel production and Available online 22 February 2014 presents an opportunity for power generation from wastewater derived microalgae. However the anaerobic di- Keywords: gestion of microalgae biomass is not straight forward due to several technical restraints including low concentra- Microalgae tion of digestible biodegradable substrate, recalcitrant substrate constituents, cell wall degradability, low carbon Anaerobic digestion to nitrogen ratio, ammonia toxicity and effects from salinity and associated metal ions. Nutrient recovery Current production methods for liquid biofuel production from microalgae produce approximately 60–70% resid- Methane biogas ual biomass that is currently a byproduct. Anaerobic digestion provides biogas, but it can also provide essential Biofuel nutrient recovery from lipid extracted microalgae biomass. The biogas produced from the anaerobic digestion process can be used to generate onsite electrical power or thermal heat to offset biomass processing and extrac- tion processes. When both of these processes are integrated and operated simultaneously, the benefits to microalgae biofuel production and wastewater treatment derived energy production are increased significantly. To consider the integration of anaerobic digestion into a commercial-scale integrated microalgae production and biofuel refinery facility or wastewater treatment plant we present a review of the literature, the current state of the art and future directions for research. © 2014 Elsevier B.V. All rights reserved. Contents 1. Introduction.............................................................. 205 1.1. Algaebasedbiofuels....................................................... 205 1.2. Historicalperspectiveofanaerobicdigestion............................................ 205 2. Macroalgaeandanaerobicdigestion................................................... 205 3. Microalgaeandanaerobicdigestion................................................... 206 3.1. Historicalandcurrentperspectives................................................ 206 4. Problemswithanaerobicdigestionofmicroalgae............................................. 207 4.1. Lowconcentrationofdigestiblesubstrate.............................................. 207 4.2. Cellwalldegradabilityandpre-treatmentofmicroalgaebiomass................................... 208 4.3. Thecarbon/nitrogenratioassociatedwithmicroalgaebiomass.................................... 209 4.4. Lipidsandmicroalgae...................................................... 209 5. Theoreticalmethaneproduction..................................................... 210 6. Inhibitionofanaerobicdigestion..................................................... 210 6.1. Ammonia-nitrogentoxicity.................................................... 210 6.2. Salinemicroalgaeandtheeffectofsalinity............................................. 211 6.3. Sulfuranditsroleinanaerobicdigestion.............................................. 211 7. Bacterialconsortiumanditsroleinanaerobicdigestion.......................................... 212 8. Anaerobicdigestionandnutrientrecycling................................................ 212 ⁎ Corresponding author at: The University of Adelaide, School of Chemical Engineering Gate 5, Frome Road, Adelaide, South Australia 5005, Australia. Tel.: +61 8 8313 3150, +61 410389528 (mobile); fax: +61 8 83134373. E-mail address: [email protected] (A.J. Ward). http://dx.doi.org/10.1016/j.algal.2014.02.001 2211-9264/© 2014 Elsevier B.V. All rights reserved. A.J. Ward et al. / Algal Research 5 (2014) 204–214 205 9. Conclusions.............................................................. 212 Acknowledgements............................................................. 213 References................................................................. 213 1. Introduction illustrates the anaerobic digestion of biomass after cell wall disruption prior to conversion. The third pathway is the traditional biodiesel prac- 1.1. Algae based biofuels tice where lipid is extracted and residual algal biomass is converted to biogas by anaerobic digestion and methane fermentation. With the current increasing global population and the associated in- crease in fossil fuel use and demand, there has been an increased inter- 1.2. Historical perspective of anaerobic digestion est in renewable energy sources based on biomass transformation [1–3]. The use of agricultural derived biomass to produce biofuels has Historically anaerobic digestion has been exploited for the gained momentum [4,5]. This push for agricultural based biofuel pro- stabilisation of raw, domestic sewage sludge that is typically removed duction can lead to other less obvious problems for example eutrophica- from primary sedimentation basins [9]. However anaerobic digestion tion, resource depletion, reduced biodiversity due to current farming for bio-methane production has received renewed attention due to its practices and the direct competition with current food crops [5,6]. viability as an alternative and renewable fuel source [10,11]. Microalgae offer an interesting alternative feedstock for the produc- Municipal and Industrial anaerobic digestion of organic waste tion of biofuels. Microalgae have a high total yield and hence a lower streams is widely practiced and recognised as a mature technology for land use footprint and can utilise land areas that are unsuitable for producing biogas [12]. Anaerobic digestion has been included in the food production [1,7]. In addition microalgae production has the poten- “first generation of biofuels” that have been developed in recent years. tial to utilise CO2 emissions and offers the potential for a carbon neutral The “first generation biofuels” have focused largely on the production biofuel [8]. of biofuel from terrestrial plant crops [13]. In these systems solar energy Biofuel production from microalgal feedstock has several challenges is used to drive the photosynthetic fixation of carbon dioxide to organic to overcome before it can become a mainstream industry capable matter. The energy crop is harvested and then used directly as a com- of producing the quantity of biofuel required at a competitive price. bustible fuel or converted to another form such as ethanol, hydrogen Challenges faced by the industry include demand for fertiliser due to or methane [14–16].These“first generation biofuels” have been highly microalgae's significant utilisation of nutrients, high energy inputs criticised for their use of valuable food crops as feedstocks for fuel pro- required for harvesting and dewatering biomass and for the lipid ex- duction. This criticism is due to the utilisation of valuable agricultural traction and conversion processes. Anaerobic digestion can offer a path- land and scarce water resources for feedstock production. With the up- way to eliminate some of the overheads of the production cycle by ward pressure exerted on food prices, biofuel production from food producing biogas for utilisation in electricity production or thermal en- crops has been deemed unsustainable [6,16]. ergy production. This benefit is highly dependent on the biofuel plant process and location, and the costs associated with natural gas prices 2. Macroalgae and anaerobic digestion and electricity prices will determine the efficiency improvement resulting from the biogas utilisation. In addition, the recovery of valu- Interest in the cultivation of microalgae for the production of bio en- able nutrients from biomass via anaerobic digestion is essential for the ergy, as one form of solar energy, was born in the 1950s, but when the sustainability of the algae biofuel industry. It is anticipated that the in- global supply of oil was interrupted twice during the 1970s, interest in corporation of anaerobic digestion in microalgae biofuel production the cultivation of highly productive macroalgae for bio energy produc- and bio-refinery processes will increase the cost effectiveness of the tion was accelerated [11]. Macroalgae received a large amount of atten- production methods, helping it to become economically feasible and en- tion as a biofuel feedstock due to its prolific growth in eutrophic coastal vironmentally sustainable. Fig. 1 illustrates the conceptual implementa- water fouling beaches and coastal waterways. Anaerobic digestion has tion of anaerobic digestion into algal production processes. Three been used to dispose and process this material for the production of bio- pathways have been defined: pathway 1 shows the direct anaerobic di- gas [11,17]. Development of feedstocks