For design purposes, a basis of 50% weight by volume (w/v) of biomass is used as the minimum requirement from the dewatering stage. The design outlines the best dewatering configuration in terms of energy consumption and economics. It is assumed that the initial cultivation within the bioreactors will be staggered so that dewatering equipment costs can be reduced. The staggering of the cultivation is entirely dependent on the bioreactor configuration and the dilution rate employed.

Fig. 3 Energy consumption for stand-alone dewatering by centrifugation

TFF Disc stack Belt filter

■ HTR □ ELR

Fig. 5 Comparison of the energy consumption of dewatering units operating as a (a) stand-alone options and (b) options preceding flocculation

process by up to 98%. Flocculation is particularly critical when the culture volumes are extremely large. Heasman et al. [14] stated that a flocculation efficiency of 80% in a 24-h period is about the average standard requirement for a typical large-scale flocculation work. This study assumes no further processing requirement due to residual accumulation of flocculants, as shown by Lubian [18] that P. tricornutum achieved approximately 90% efficiency from diminutive flocculant dosages.

By conducting the audit on the entire process, we wish to quantify the GHG captur­ing benefits achievable from this process. The quantified GHG emission saving can be used to calculate the economic benefits from the process and also its environ­mental impact.

Standard culture medium for Arthrospira spp. cultivation is rich in bicarbonate and carbonate. This medium is alkaline due to the presence of these ions. At pH values below 6.4, carbon dioxide is the predominant form of carbon source. At pH values between 6.4 and 10.3, the predominant form is bicarbonate. Above pH 10.3, the predominant form is carbonate. Miller and Colman [65] reported that bicarbonate is the carbon form preferentially assimilated by cyanobacteria, which explains the decreasing of Spirulina biomass concentration in the pond at pH values above 10.2­

10.4 [49].

Considering ammonia from ammonium salts or urea, in the chemical equilib­rium established at pH 9.3, both ammonium ions and ammonia are present. At pH values below 7.0, the predominant form is ammonium ion and above pH 12.0, only the ammonia is present in culture medium.

Ammonia uptake by S. platensis is pH-dependent. In alkaline conditions, ammonia enters the cell by simple diffusion, driven by a pH gradient, and is intracellularly assimilated by the action of the enzyme glutamine synthetase [10].

It should also be mentioned that ammonia, depending on its concentration and pH of the medium, is toxic to most microorganisms [1], including S. platensis [6]. These authors found that at pH 7.0, S. platensis LB1475/a and Anabaena sp. were not affected by ammonia. However, at pH 10.0 with ammonia concentrations of 10 mM, only 50% of photosynthetic activity remained for the S. platensis LB1475/a, while for Anabaena sp. photosynthetic activity has ceased. Thus, since the use of ammo­nium salts or urea as nitrogen sources lead to a release of ammonia in the culture medium, it would be limited by process by which the cultivation is carried out.

Therefore, pH is a very important variable to be studied, since it may affect the form of the carbon reservoir, urea decomposition and how ammonia is presented in the culture medium (protonated or unprotonated). Sanchez-Luna et al. [87] studied the effect of pH on A. platensis cultivation using urea as nitrogen source and found that A. platensis grows well at pH 9.0-10.0, with an optimum pH of 9.5. This pH range is in agreement with Belay [ 5] , who suggests maintaining the pH of the medium above 9.5 in outdoor cultivations aiming to avoid contamination by other microalgae.

The antiviral family is one of the widest families of bioactive compounds isolated from marine sources, or at least one of the most studied. In this group, there are compounds like polysaccharides, terpenoids, proteins, sulfated flavones, and fatty acids. When measuring the antiviral activity, the general trend is to treat well-known mammal cells with the extract and then monitor the viral infection with the micro­scope. Huheihel et al. [63] used green monkey kidney cells (vero cells) treated with polysaccharides extracted from Porphyridium sp., the cell culture was treated with herpes simplex viruses. Each day, the cultures were examined for evidence of the cytopathic effect, defined as areas of complete destruction of cells or of morpho­logically modified cells and expressed as the percentage of damaged cells in the inspected fi elds.

But similar test can be done in vivo, Huheihel et al. [63] applying locally (eyes and mouth) Porphyridium extracts in rabbits and rats; later, the animals were exposed to the virus. Inflammatory effects, illness, and weight changes were recorded over a period of 4 weeks posttreatment.

The primary treatment of biogas includes cooling, drying, and almost always remov­ing of hydrogen sulfide. More advanced applications of biogas require upgrading it to biomethane or removing carbon dioxide. The following methods are used for the removal of carbon dioxide from biogas: pressure swing absorption on zeolites, selective membrane separation, cryogenic separation, and biological or chemical fixation [104-106]. The typical technologies for biogas cleaning include scrubbing by solvents or an aqueous alkaline solution, absorption, and oxidation on solid sor­bents, chelation, precipitation in the form of poorly soluble metal sulfides, and bio­logical removal [105, 107, 108].

1.1.4 Biogas Utilization

The possible applications of biogas include:

• Heat or steam production via burning

• Electricity generation combined with heat and power production

• Usage as cooking gas instead of natural gas

• Usage as fuel for vehicles (upgrading to biomethane is necessary)

• Generation of electricity via fuel cells

• Production of chemicals

The C/N ratios for microalgae are in the range of 4-6. The addition of carbon rich cellulosic materials can balance the high nitrogen content. For example, addition of 25 and 50% of waste paper to a mixture of Scenedesmus spp. and Chlorella spp. resulted in a 1.59- and 2.05-fold increase in the methane yield (Fig. 17a) [410]. The optimal ratio between algal biomass (Scenedesmus spp. and Chlorella spp.) and waste paper was found to be 40% algae and 60% paper with corresponding C:N ratio equal to 22.6. The influence of the C/N ratio on the methane yield is shown in Fig. 17b. The authors also reported that paper addition stimulated cellulase activity in the anaerobic digester from 1.26 ± 0.14 mg/L-min (no paper added, C:N is equal to 6.7) to 3.02 ± 0.09 mg/L-min (50% paper, C:N is equal to 18).

Addition of A. maxima biomass to sewage sludge, peat extract, and spent sulfite liquor improved the VS reduction and methane yield (Fig. 18) [411]. Nutrient rich algal and cyanobacterium biomass can be added to nutrient limited waste products that cannot be digested as sole substrate.

Co-digestion of Macroalgae

The mix of Ulva and manure has a larger methane yield and production rate com­pared to pure Ulva biomass (Fig. 11c, d). Methane production from a mixture of alginate extraction residues and manure was lower compared to methane production from separate substrates [245]. Morand and coauthors speculated that co-digestion of different seaweeds can be problematic due to dissimilarity in digestion speeds [412]. But addition of Ulva to Sargassum tenerrimum (1-17 ratio) increased the methane yield and production rate [413].

From the previous discussion, it is obvious that hydrate deposits that are being con­sidered as production targets must have the following attributes:

• Confirmed presence of high hydrate saturation SH.

• Occurrence within sediments of sufficient reservoir quality.

• Site accessibility through proximity to existing infrastructure.

• Access to gas markets through pipeline availability.

Here we discuss the features and attributes of hydrate deposits that are likely targets for gas production, and we analyze the geologic and engineering factors that control their ultimate resource potential.

Many conventional laboratory investigations have been performed on laboratory — synthesized samples. These allow the flexibility of creating samples that have desired characteristics. Few laboratory-synthesized samples have been examined for uniformity, and this characteristic is often (perhaps hastily and undeservedly) assumed. Several methods are in use for making HBS, and each has its advantages and disadvantages.

Hydrate from ice. In this method, powdered ice is slowly melted in the presence of methane at the appropriate pressure and temperature [187]. As the ice melts, methane hydrate is formed. Sequential freezing and melting events can result in very high conversions to hydrate. The hydrate can then be chilled in liquid nitrogen, powdered, mixed with a selected chilled mineral medium, and compacted into a hydrate-bearing medium. Nanoscale examination of HBS formed this way by scan­ning electron microscope compares favorably to natural HBS from the Mallik site [186]. HBS formed this way will typically fill pores as well as be part of the frame of the medium.

Hydrate from partially water saturated media—Excess Gas. In this method, a pre­scribed amount of water is uniformly added to a mineral medium, compacted into a sample vessel, and the hydrate stability conditions are exceeded [61, 87, 208] . Hydrate formed using this technique typically cements mineral grains together forming a stiff sample [208].

Hydrate from partially water saturated media—Excess Water. Using a somewhat different approach, Priest et al. [154] formed methane hydrate by placing the quan­tity of gas needed to form a specific amount of hydrate in a porous sample, pressur­izing the sample with water, and then chilling the sample to bring it into the hydrate stability field. Their work suggests that hydrate interaction with the sediment is strongly dependent on hydrate morphology, with results indicating that hydrate formed this way is frame supporting. Additionally, natural GH may exhibit a differ­ent seismic signature depending on the environment in which it formed.

Hydrate from dissolved guest phase. In this method, water containing dissolved methane is flowed through a chilled porous medium where hydrate is formed. Although there have been some successes using this technique, it is difficult to con­trol and time consuming [184, 185].

Micromodel studies of gas hydrates. Several studies have been performed allowing direct microscopic examination of hydrate formation, aging, and dissociation in transparent micromodels [76-78, 191, 192]. These studies show formation of hydrate with and without a gas phase, formation of dendritic hydrate crystals that age over time into particulate hydrate crystals, and faceted hydrate crystals formed at low subcooling. The presence of a water film between the hydrate and the micro­model cell walls has been observed in some tests, but not in others leading to the conclusion that hydrate may cement grains together when formed with a low degree of subcooling.

In most autotrophic fermentations with R. eutropha reported in the literature, the initial gas mixture (typically 8:1:1 H2:CO2:O2) is within the explosive range for the H2 and O2 gas concentrations. The low aqueous solubility of both H2 and O2 presents challenges in making these gases bioavailable to R. eutropha cells [14] . As with many aerobic microbial biotransformations, the rate of gas mass transfer (dissolu­tion) from the gas to the liquid phase represents another potential rate-limiting step.

One strategy to reduce the explosion risk during autotrophic growth of R. eutro — pha is to keep the H2 and O2 gas streams physically separated. This can potentially be performed by the hollow fiber reactor setup discussed in Sect. 4.2 (Fig. 7). However, for initial screening, microfermenters (bioreactors with 1 mL or less working volume, discussed in [87-90]) can be used to optimize growth and produc­tion conditions prior to culture scale-up. Risk of explosion still exists in such sys­tems, but the small scale of the reactors would control the potential damage.