Microalgae are unicellular microscopic organisms, like simple plants with no leaves and roots that grow through photosynthesis process. They capture carbon dioxide during photosynthesis and convert it into feed­stock that can be used as food, fertilizer, a source of med­icine and biodiesel (Chojnacka and Marquez-Rochaet,

2004) .

Growing algae in open pond system raise several con­cerns such as impossibility to control growth settings and contamination threats. Algal cells in open ponds are exposed to the environment, light deficiency, subject to risk of contamination, and heterogenous medium

depending upon the mixing mechanism, the shape of the ponds and the depth of the pond (Chojnacka and Marquez-Rochaet, 2004). On the other hand, closed ponds (photobioreactors) mitigate fluid culture contam­ination, and enhance full control over algal growth pa­rameters such as homogenous culture, pH, light penetration, and carbon dioxide input. They would use less space with high algal biomass yield. However, they are costly to build and maintain (Mulumba and Farag, 2012).

Design of tubular photobioreactor (TPBR) for algal cell growth was depicted in Figure 1.9. It has a main tank con­nected to two spiral tubes set in sequence. Both spiral parts were clear polyvinyl chloride (PVC) tubes of 1" external diameter and 3/4" internal diameter. The capac­ity of both spiral parts was 3.4 gallons. The main tank served as a feeding point of medium to the PVC tubes with a maximum capacity of 5 gallons (Chisti, 2007). Cul­ture medium was pumped into the tubings at a fixed flow rate. These tubes provided an area of 20 ft2 exposed to the fluorescence light. Air compressor supplies air to the
system for aeration and to serve as a source of carbon di­oxide (CO2). The air flow rate was set in the ranges of 190—210 gallons/h. In TPBR, selected algal strain was cultured using fresh medium with no modification. Algal growth and pH were measured over a period of time varying between 12 days and 14 days. A sample was taken every 2 days to quantify the turbidity using a spec­trophotometer at 682 nm and cell counts were performed using a microscope. The pH of culture was measured us­ing pH test strips.

The selected algal strain shows the typical growth curve of other microbes, which include lag, exponential or log, stationary and lytic phases. The length of each phase depends on light penetration, nutrients concen­tration, mixing mechanism, and the solubility of oxygen in medium. After reaching a stationary or lysis phase, algal culture was harvested by centrifugation followed by lyophilization to produce dry algal feedstock. Crude lipid from dried algal biomass was extracted using either modified Folch method (Cooksey et al., 1987) or Soxhlet extractor (Mulumba, 2010; Chojnacka and Marquez-Rochaet, 2004). In both methods, polar and nonpolar solvents such as methanol and chloroform/ hexane were used (Table 1.6). The combination of polar and nonpolar solvents enhances the extraction of both polar and nonpolar lipid.

BIOGAS

Biogas is obtained by anaerobic digestion (AD) of organic materials, which occurs inside the anaerobic bio­digester. Chemical composition of this biogas depends on several parameters, such as type of digester employed, the kind of organic material and the con­stancy of the feeding process of the biodigester. The most significant biogas components are methane (CH4), carbon dioxide (CO2) and sulfuric components (H2S). The composition of biogas is a crucial parameter,

because it allows identifying the suitable purification system, which aims to remove sulfuric gases and reduce the water volume, contributing to recover the combus­tion fuel conditions (Boe et al., 2007). Other important data collected from biogas analysis is referent to the low heat value, that combined to the efficiency and biogas consumption is important to estimate the electric generation potential. However, biogas production is much variable because it depends on several parame­ters, such as the kind of organic material (Liu et al.,

2004) . Biogas production involves three steps: fermenta­tion, which includes hydrolysis and acid genesis, acetone genesis and methane genesis. In the fermenta­tion process, during the hydrolysis the organic material is converted into smaller molecules and this material is transformed in soluble acids by acidogenese. Next step is acetanogenese process, transforming the products ob­tained in the first step into acetic acid, hydrogen and car­bon dioxide. The last step is referent to metanogenese process, producing methane gas through anaerobic bac­teria (Figure 1.10) (Seadi et al., 2008; Boe et al., 2007).