used to cultivate the microalgae with the usage of natural water bodies like lakes,
ponds or lagoons, and artificial water supply systems. Use of closed system prevents
the contamination by other microbial species. Currently, many designs have been
used for closed systems based on PBR such as flat plate, column or tubular and are
classified on the basis of mode of operation and shape. Tubular and flat plate PBR
are the most commonly used closed system PBR. The closed system comes with
advantages but requires further detailed study on scale up, parameter control and
cost, and currently, it is not considered economically feasible at large scale.
Process intensification can be applied to PBR considering aspects like carbon
supply decoupling and mixing which will help significantly in ensuring proper supply
of carbon dioxide and removal of oxygen. It has been reported that using a hollow
fiber membrane can solve the problem of inefficient transfer up to an extent (Carvalho
et al. 2006). Obtaining the desired increase in internal surface area and application of
data on measurement, modeling, and control can also be a good process improvement
approach. In one of the studies, concentrated microalgae cultivation in continuous
mode was performed using resonant ultrasound field (RUF) which helped to enhance
medium replacement process also resulting into process intensification benefits. The
optimized process parameters reported were 1 MHz frequency and output intensity of
8 W/cm
2 with a circulating velocity of 2 mL/min leading to 93% collection of
microalgae in 2 h (Lee and Li 2016). Pfaffinger et al. (2016) investigated the use of flat
plate gas lift photobioreactor with continuous illumination using LEDs. The study
showed an increase in algal productivity by 113% and lipid productivity by 59%. The
design of the oscillatory baffled reactor was also utilized in developing the PBR which
gave increased gas transfer and reported to improve overall economics of microalgae
production (Abbott et al. 2015).
Other reactor configurations which have been also studied are rotating disk
biofilm reactor and biofilm reactor which were reported to give a yield of 3.2 and
3.64 g/m
2 /day, respectively, and also reported to help in overcoming the issues of
suspended cultures (Sebestyen et al. 2016; Choudhary et al. 2017).
2.1 Cultivation of Microalgae from Wastewater
High content of nitrogen and phosphorous in wastewater makes it one of the best
cultivation systems for microalgae. Total organic carbon of the wastewater can be
utilized by some of the microalgal species as food source (Wang et al. 2010).
Considering higher costs involved in microalgae production and wastewater
treatment, it can be a boon if microalgae can be produced using wastewater as the
cultivation medium. Algal ponds can be used for cultivation of microalgae using the
municipal, industrial, and agricultural wastewaters. Secondary-treated wastewater
contains nitrogen and phosphates in the range of 20–40 and 1–10 mg/L, respectively, which can help most microalgae strains to achieve high productivities
(Olguín 2012). Microalgae release oxygen which in turn can be used by other
microorganisms increasing the overall efficiency of aerobic degradation that can
4 Process Intensification of Biofuel Production …
63
ponds or lagoons, and artificial water supply systems. Use of closed system prevents
the contamination by other microbial species. Currently, many designs have been
used for closed systems based on PBR such as flat plate, column or tubular and are
classified on the basis of mode of operation and shape. Tubular and flat plate PBR
are the most commonly used closed system PBR. The closed system comes with
advantages but requires further detailed study on scale up, parameter control and
cost, and currently, it is not considered economically feasible at large scale.
Process intensification can be applied to PBR considering aspects like carbon
supply decoupling and mixing which will help significantly in ensuring proper supply
of carbon dioxide and removal of oxygen. It has been reported that using a hollow
fiber membrane can solve the problem of inefficient transfer up to an extent (Carvalho
et al. 2006). Obtaining the desired increase in internal surface area and application of
data on measurement, modeling, and control can also be a good process improvement
approach. In one of the studies, concentrated microalgae cultivation in continuous
mode was performed using resonant ultrasound field (RUF) which helped to enhance
medium replacement process also resulting into process intensification benefits. The
optimized process parameters reported were 1 MHz frequency and output intensity of
8 W/cm
2 with a circulating velocity of 2 mL/min leading to 93% collection of
microalgae in 2 h (Lee and Li 2016). Pfaffinger et al. (2016) investigated the use of flat
plate gas lift photobioreactor with continuous illumination using LEDs. The study
showed an increase in algal productivity by 113% and lipid productivity by 59%. The
design of the oscillatory baffled reactor was also utilized in developing the PBR which
gave increased gas transfer and reported to improve overall economics of microalgae
production (Abbott et al. 2015).
Other reactor configurations which have been also studied are rotating disk
biofilm reactor and biofilm reactor which were reported to give a yield of 3.2 and
3.64 g/m
2 /day, respectively, and also reported to help in overcoming the issues of
suspended cultures (Sebestyen et al. 2016; Choudhary et al. 2017).
2.1 Cultivation of Microalgae from Wastewater
High content of nitrogen and phosphorous in wastewater makes it one of the best
cultivation systems for microalgae. Total organic carbon of the wastewater can be
utilized by some of the microalgal species as food source (Wang et al. 2010).
Considering higher costs involved in microalgae production and wastewater
treatment, it can be a boon if microalgae can be produced using wastewater as the
cultivation medium. Algal ponds can be used for cultivation of microalgae using the
municipal, industrial, and agricultural wastewaters. Secondary-treated wastewater
contains nitrogen and phosphates in the range of 20–40 and 1–10 mg/L, respectively, which can help most microalgae strains to achieve high productivities
(Olguín 2012). Microalgae release oxygen which in turn can be used by other
microorganisms increasing the overall efficiency of aerobic degradation that can
4 Process Intensification of Biofuel Production …
63