between 325 °C to 420 °C with saturated (16:0 and 18:0) and unsaturated (18:1) fatty acids unveiled thermal decomposition of FAME due to isomerization, hydrogenation, and pyrolysis (Shin et al., 2011). Therefore, several researchers have suggested high MeOH: oil molar ratio to reduce this extreme temperature. Thus, in SCM transesterification process 42:1 molar ratio of MeOH: oil is preferred to avoid the excessive rise in temperature. However, excess methanol is the major nuisance for the development of SCM process at industrial scale. Therefore, development of techno–
economical and eco–friendly SCM process is essential for the sustainable production of biodiesel.
after lipid extraction is a competent raw material for methane production which recovers more energy than energy obtain only from microalgae oil (Sialve et al., 2009).
A typical composition of microalgae biomass can be represented as CO0.48H1.83N0.11P0.01 (Chisti, 2007). However, this excessive amount of N and P do not add any benefits to environmental, economic and sustainable aspect towards biofuel production. Hence, if the anaerobic digestion is coupled to process algae waste generated from biodiesel production, it will not only reutilize nitrogen and phosphorous but also produces methane. The energetic balance of the microalgae biofuel process can be developed by the high calorific value of methane (Uggetti et al., 2017). Hence combining biodiesel production with biogas would definitely suffice the energy demand in an economically viable manner.
Nonetheless, in present time the microalgae anaerobic digestion (AD) is not a well–
established technology as it faces many challenges, most significantly low concentration of substrate (due to less biomass accumulation of microalgae), poor cell wall degradability and low carbon to nitrogen (C/N) ratio (Ward et al., 2014). Golueke et al., (1957) have first reported a methane yield of 170–320 mL/g VS from AD of microalgae Chlorella and Scenedesmus grown on wastewater. They observed the low methane yield up to 30 days of hydraulic retention time (HRT) due to less biomass concentration and poor digestibility of microalgae cell wall.
Further, disconcerting in microalgae AD is lower C/N ratio. A typical C/N ratio in microalgae ranges from 4.16 to 7.82, however, an optimal C/N ratio of 20–30 is a prerequisite for anaerobic microflora for efficient biogas production. The disproportion of C/N ratio below 20 leads to noxious ammonia (NH3) secretion in the digester that has the inhibitory effect on the methanogenic bacteria resulting in a discrepancy of volatile fatty acids (VFA) accumulation in the digester (Sreekrishnan et al., 2004; Wang
et al., 2012). Hence, to meet these challenges researchers have explored the anaerobic co–digestion (A–CoD) process, where microalgae have been co–digested with other waste streams or biomass to increase the substrate loading capacity and C/N ratio. The main idea of co–digestion is to mix different substrates and solve issues related to an AD on a single substrate (Table 2.6). The co–digestion of different environmental wastes showed that better methane generation potential can be achieved by using an optimum ratio of the substrates, than using a single organic material (Lee et al., 2013).
Yen and Brune (2007) have used paper waste to improve C/N ratio of the microalgae Scenedesmus and Chlorella from 6.7 to 36.4. They observed the best co–digestion with 1:1 ratio of paper and microalgae biomass with final C/N ratio of 18 resulting in 50 % increase in biogas production compared to digestion of only microalgae biomass. They also suggested that for better biogas production, the optimum C/N ratio must lie in between 20:1 to 25:1.
India, as one of the biggest agricultural country, produces millions of tons of rice residue each year. It is to be noted that for every ton of rice harvested, 1.35 tons of rice straw (RS) remain in field unutilized with high energy potential. Direct incorporation of rice residue into soil may lead to deteriorating the soil conditions and releases methane gas to the atmosphere which is 23 times more potent greenhouse gas than CO2. Hence, this connexion of microalgae biomass with agriculture residues such as rice straw, wheat straw (WS), rice husk (RH) etc. will boost the net biogas yield. This assemblage upsurges the loading capacity, HRT, balances the C/N ratio and also decreases the NH3 toxicity occurred in microalgae AD. Furthermore, the digested sludge can be reutilized for microalgae cultivation. Hence, the coupling of microalgae biodiesel with biogas generation will be a virtuous option for zero waste energy generation process. Co–digestion of microalgae biomass with agriculture residues
potentially paves a way not only for biogas production but also as microalgae biorefinery which involves utilization of microalgae as a raw material for various biofuel production (Cheng et al., 2014; Lee et al., 2013; Solé–Bundó et al., 2017).
Moreover, the organic matter composition has a profound effect on anaerobic digestion that delimits the relationship between the organic matter used and biomethane potential, which is based not only on its biodegradable fraction but also on the non–biodegradable fraction. There is an empirical relationship between the biochemical, organic and elemental composition of the substrate with the methane potential (Park and Li, 2012).
Therefore, based on the elemental composition the theoretical methane potential can give a clear insight into maximum methane production from the specific substrate.
Additionally, researchers have evolved various strategies of BMP co–digestion using mathematical models and expressions to save cost and experimental runtime.
Mathematically, the degradation rate of each group of compounds can be described by a differential kinetic equation. The knowledge of the biodegradation kinetics and methane production could be helpful for the methane prediction from a specific substrate and can provide information for optimum mixing to improve bio–methane potential (Cecchi et al., 1991).
Table 2.6: Microalgae species co–digested with various biomass for methane production.
Microalgae Co–
digestion
HRT (days)
BMP (mL/g
VS)
References
Spirulina platensis# – 30 290 Sumprasit et al., (2017)
Chlorella
sorokiniana# – 42 253 Ayala–Parra et al.,
(2017)
Scenedesmus residue – – 100–140 Uggetti et al., (2017) Chlorella vulgaris Cattle
manure 90 415 Mahdy et al., (2017) Scenedesmus
Scenedesmus# Scenedesmus*
Pig manure 70
163 192 102
Astals et al., (2015) Nannochloropsis Corn sialge 225 280 Schwede et al., (2013)
Chlorella WAS 45 262 Wang et al., (2012)
Taihu blue algae Corn straw 30 325 Zhong et al., (2012)
# defatted biomass, WAS waste activated sludge, RS rice straw, * protein extracted
From the above discussion, it is clear that huge efforts have been made for microalgae biodiesel production. All the major facets of culture condition optimization for increasing biomass and lipid productivity was investigated by several researchers. Most of the studies have been carried out in improvising the downstream processing steps and incorporation of the various novel and advanced process technologies to attain high biodiesel yield.
Moreover, it was proposed that an integration of phycoremediation with concurrent biodiesel production could be the only possible route for sustainable fuel synthesis.
Hence, the other biofuel potentials of microalgae biomass were appraised in literature incorporating the concept of microalgae biorefinery. All the above discrete reports
showed immense efforts towards establishing the sustainable microalgae biodiesel production. Though, with that great efforts the microalgae biodiesel is not yet marketed and lag behind the conventional biodiesel production processes. Most of the studies do not consider the economic aspect of microalgae biofuel production process. The comprehensive review of the literature suggests that very few measures have been taken for microalgae biofuel marketing considering techno–economical aspect.
Till today no complete report has been presented where all the aspects of enhancement of productivity, reduction of processing steps and utilization of waste in a biorefinery concept have been dealt. The consolidation of all the aforementioned exertions can be a good measure to develop a technical, economical and sustainable microalgae biodiesel. Hence, the present thesis put the effort and tried the best to cover all the major challenges of microalgae biodiesel commercialization.