*4.2.1 Organic carbon*

Organic carbon present in municipal and industrial wastewater are carbohydrates, fats, volatile fatty acids (VFAs), soaps, synthetic detergents, lignin, proteins and their decomposition products; as well as various natural and synthetic organic chemicals. Wastewater treatment and concomitant algal biofuel production has received increasing attention in recent years owing to its diverse environmental and economic benefits [44]. Organic carbon is accessible through monosodium glutamate wastewater, cheese whey permeates, sodium acetate, fruit peel, glucose, fructose, glycerol, etc. via mixotrophic microalgal growth mode. Tan and coworkers [45] reported that productivities of *C. vulgaris* cultured in wastewaters containing glucose and sodium acetate were 63.5 and 55.2 mgL−1 day−1, respectively. This accounted for the leap of 2.61 and 2.27 times the productivities, respectively, achieved under autotrophic metabolic modes. Also, *Chlorella vulgaris* cultivated in sodium acetate and glucose wastewaters recorded productivities of lipid at 17.35mgL−1 day−1 and carbohydrate at 18.75mgL−1 day−1, respectively, indicating that sodium acetate and glucose wastewaters have the potential to boost microalgal lipid production, which in turn may serve as feedstock for the biorefinery [46].

#### *4.2.2 Hydrodynamic stress*

Biotransformation kinetics in microalgae are driven by two cultivation systems: (i) open cultivation systems, (OCS) and (ii) closed cultivation systems, (CCS). Open cultivation systems employ open ponds, tanks and raceway ponds while the closed cultivations system utilizes closed photobioreactors (PBR), such as bubble column reactors (BCR), airlift reactors (ALR), tubular reactors (TR), plastic bag (single-use) reactors (SUR), stirred tank reactors (STR), and plate reactors (PR). The OCS attracts minimal capital, operating cost, and lesser energy for culture mixing. However, OCS require large land-mass for scale-up operations as they are prone to contamination and adverse weather conditions wherein they suffer evaporation and temperature fluctuations [47]. The CCS on the other hand, are operated at highly controlled conditions and are more efficient in terms of quality. PBRs can be designed and optimized to cultivate a chosen microalgal strain; since they occupy minimal landmass with enhanced luminous exposure to the microalgae cells and encounter little or no contamination. However, PBRs do have bio-fouling issues, cleaning difficulties, benthic microalgal growth, and high build-up of dissolved oxygen (DO) leading to growth obstructions, and high capital cost [48, 49].

The microalgae culture mixing regimes may vary from one PBR to another, and since the purpose of mixing is to ensure adequate exposure of all the microalgae cells to the growth index, variables such as thermal energy, nutrients adequate illumination, gas exchange, and the quality of microalgal biomass churned from each PBR varies in terms of cell density and biomass composition [50].

PBRs using microalgae to treat wastewater and to produce biomolecules are based on five basic criteria: (i) full control of the reaction conditions, (ii) increased efficient use of light energy, (iii) an adequate mixing system, (iv) reduced hydrodynamic stress on the cells and (v) flexible scale-up operations [51].

#### *4.2.3 Luminous exposure*

Biomass productivity depends largely on the quantity and quality of light available to microalgae cells during exponential growth, especially in the autotrophic metabolic mode. Lighting has a great influence on the synthesis of co-products in microalgal biomass as the cells increase pigmentation. Large quantities of solar radiation storage are enhanced as biomass, which can be transformed into solid, liquid, or gaseous fuels [52]. On the other hand, the exposition of microalgae cells to excessive illumination can cause photoinhibition, a phenomenon which describes termination of photocatalytic activity in the presence of illumination. Both photoperiod and light intensity influence microalgal growth, pigment production, biomass, and lipid productivities. High biomass and lipid productivities have been reported for stepwise strategic light-intensity increases during mixotrophic cultivation of microalgae. Cheirsilp and Torpee [53] reported the influence of light intensity on the growth and lipid accumulation of marine *Chlorella sp. and Nannochloropsis sp.*; and observed that the growth of marine *Chlorella sp.* increased when the light intensity was increased from 2000 to 8000 lux. Increasing the light intensity to 10,000 lux registered a slight decrease in the lux indices, which could be due to photoinhibition.

#### **5. Conversion of biomass to bioenergy production**

Generation of bioenergy from biomass is achieved in various ways and may be classified into three main categories, namely, physio-chemical, bio-chemical, and

**439**

**Figure 6.**

*Biomass conversion techniques.*

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

treatment of lignocellulosic and algae biomass.

**5.1 Pre-treatment methods**

**5.2 Physico-chemical means**

thermo-chemical processes [54]. The following are common techniques utilized in the conversion of biomass into biofuels, i.e. mechanical extraction, transesterification, pyrolysis, anaerobic digestion, fermentation, gasification, liquefaction, and fuel cell systems as shown in **Figure 6**. This subsection gives an overview of the conversion process, the factors affecting each process and the main products derived.

Prior to the application of a specific technique of biofuel generation from biomass, various pre-treatment or pre-processing steps may be carried out to aid effective conversion. Two main pre-treatment methods broadly classified under physio-thermal and chemical methods are usually applied based on the lignocellulosic substrates as discussed in the latter sections on lignocellulosic and the related conversion techniques. Processes such as drying, sizing, crushing, powdering, pelletizing, torrefaction and heating are common physio-thermal methods for pre-

This involves the mechanical extraction of oil from lignocellulosic and algae biomass, where the oil produced is further esterified to produce biodiesel. Biodiesel is

thermo-chemical processes [54]. The following are common techniques utilized in the conversion of biomass into biofuels, i.e. mechanical extraction, transesterification, pyrolysis, anaerobic digestion, fermentation, gasification, liquefaction, and fuel cell systems as shown in **Figure 6**. This subsection gives an overview of the conversion process, the factors affecting each process and the main products derived.
