Sloan and Koh [178] summarized (a) laboratory measurements and (b) models that provide estimates of the heat of hydrate formation and dissociation, in addi­tion to equilibrium conditions (P, T, and inhibitor concentrations) of various states for many hydrate systems. Clarke and Bishnoi [17] and Kim et al. [79] studied the kinetics of hydrate formation and measured the activation energy and intrinsic rate constant of methane hydrate decomposition. Modeling by Moridis et al. [126, 138] has shown that dissociation kinetics plays a limited-to-no role in gas produc­tion from hydrate at the reservoir scale, but may be important in short-term labo­ratory studies.

Handa and Stupin [61, 202, 203] and Lu and Matsumoto (2002) investigated the effect of the properties of porous media on the hydration characteristics, and reported significant deviations between the measured hydration temperatures and those pre­dicted from the known equilibrium curve of pure methane, i. e., temperature shifts varied from -12.3 to 8°C. The implication of these studies is that the medium prop­erties and texture may play a defining role in hydrate equilibrium. The subject has not been fully addressed, and it has not been represented in numerical simulators, thus increasing the uncertainty of their predictions.

5.2.2 Thermal Properties

Laboratory measurements of thermal properties of methane HBS have been made by a number of researchers [56 60, 69, 126, 138, 164, 200, 204, 206, 207] Two thermal properties are important: thermal conductivity and specific heat. The specific heat of a GH-bearing medium can be computed using a mixing model and the specific heats of the components present, and does not pose a challenge. The thermal conductivity of pure methane hydrate differs from that of water by less than 10%. Although it is possible to make coarse estimates of the thermal conductivity of a hydrate bearing medium that has water and hydrate in the pore space by consid­ering the medium to be water saturated only [165], laboratory studies have shown that GH-bearing media have higher thermal conductivity than water-saturated media [126, 138] The difference is substantial, and thus this approximation is not valid because of the paramount importance of heat transfer in hydrate dissociation.

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.

The special design of flat plate photobioreactors with small distances between their translucent rectangular covers allows for cultivation with small layer thick­ness in response to the limited light path length (Figs. 4 and 6). Flat plate reactors can be compared with bubble columns with regard to aeration and mixing.

Movement of gas bubbles through the reactor induces mixing mainly along the vertical axis. The main difference is given by the short thickness of flat plate reactors which generally amounts only a few centimeters (e. g., [42]). If biomass concentrations during operation, absorbance and incident light intensities can be reasonably estimated during the planning process, adjustment to the light path length can widely prevent the occurrence of dark volumes. The relatively simple geometry facilitates scale-up tremendously, e. g., when several reactor modules are placed in north-south oriented “fences” (Fig. 4).

Instead of using glass plates, in some applications plastic bags are fixed in a metallic frame (Green Wall Panel [45], see also Fig. 4). The replacement of glass by much cheaper, transparent, disposable plastic bags is particularly interesting for commercial application. In this case, the reaction vessel can be exchanged when fouling or contamination makes a further utilization unfavorable.

The company Solix Biofuels (Fort Collins, CO, USA), for example, cultivates algae in submerged flat plastic bags. In principle, the concept can be traced back to the basic design of flat plate reactors. The fundamental setup of the third generation reactor concept (3G) is depicted in Fig. 7. Major advantage of the submerged reac­tion compartments is the fact that additional temperature control is not necessary in the system because the surrounding water acts as temperature buffer. Moreover, construction costs for the reactor are reduced as there is no need for a special scaf­fold supporting the flat reaction compartments. The demonstration facility (south­western Colorado) utilizes wastewater from coal-bed methane production. The innovative gas sparger system is integrated in the seam of the plastic bags and dis­tributes CO2 enriched gas of a nearby amine plant. Therewith, the concept aims at an integrated environmental-friendly biomass production connected with CO2 captur­ing. According to the information given by the official website Solix produces 5,000-8,000 gallons of algal oil per acre, per year (circa 42-67 t/ha/a, assuming an

oil density of 900 kg/m3) [43]. A fourth generation reactor is currently under devel­opment. Investigations on the replacement of the spargers by an integrated mem­brane aeration system are undertaken by the company [7, 47, 48].

Another commercially applied advancement of classic flat plate reactors was realized by Proviron (Hemiksem, Belgium) (Fig. 8). Their major focus was set on development of an efficient low-cost reactor suitable for large-scale outdoor appli­cations. Their approach comprises the incorporation of flat growth compartments (less than 1 cm thick) within water-filled plastic bags without any rigid structure. The major part of the setup is represented by water-filled chambers that are sepa­rated from reaction compartments. Water diffuses the impinging solar radiation, which should result in an equalized light distribution within the water-filled chamber. At the same time, temperature is regulated without any additional energy input. Moreover, the water-filled chambers themselves constitute the scaffold of the reac­tor. In the future, the low auxiliary energy demand of 20 kW/ha should be further reduced with control strategies that aim at adaption of aeration to light. According to the company’s outlook investment cost is expected to drop from currently 200,000€/ha to 100,000€/ha [31].

A straightforward approach to improve flat plate reactor productivity and enforce beneficial light/dark cycles was implemented in the flat-panel-airlift reactor [14] , a concept that was further improved for large-scale outdoor applications by the company Subitec (Fig. 9) . This reactor works according to the airlift principle. Compressed air is injected in the riser which induces an upward flow of liquid. Specific flow regimes are induced by the elaborate arrangement of interconnected chambers that are separated by baffles alternatively located at the front — and back­side of the reactor. Therewith radial mixing is substantially improved and cells circu­late between darker and more illuminated regions of the reactor. The specific design with indentations (acting as baffles) provides additional surfaces for light capturing and certainly contributes to light availability within the culture. After reaching the top of the flat plate reactor, the liquid volume descends in a downcomer with small diameter, so that the culture circulates repeatedly through the compartments.

According to information given by the company, the energy input ranges in between 100 and 200 W/m3. Total energy consumed in the process referred to bio­mass produced is specified to be below 20 MJ/kg dry mass and thus below the aver­age energy content of algae biomass (see above). Efforts are undertaken to further decrease this value [35].

To sum up, the flat-panel-airlift reactor concept integrates three beneficial char­acteristics: a short light path length, efficient mixing, and utilization of the intermit­tent light effect.

Two most common cultivation systems for microalgae are open ponds and closed photobioreactors. Open ponds come in many different shapes and forms, each hav­ing certain advantages and drawbacks. The types of ponds that are currently used in research and industry include raceway ponds, shallow big ponds, circular ponds, tanks and closed ponds. The location of the pond is a critical factor in determining the type of pond, microalgal strain and intensity of available light for photosynthe­sis. Due to the lack of control associated with open systems, the pond efficiency is a function of the local climate [19] . They are limited by key growth parameters

including light intensity, temperature, pH and dissolved oxygen concentration. Contamination by predators is another issue involved with open ponds. Local cli­mate and contamination can limit the cultivation system to unwanted algal strains which grow under severe conditions [13]. The cultivation system cost is a vital fac­tor when comparing open and closed cultivation systems. It is well known that the cost involved with cultivation ponds are significantly less than that of closed sys­tems. The construction, operating and maintenance costs of cultivation ponds are lower than photobioreactor options, the design of open ponds is technically less challenging, and they are more scalable [15]. Although cultivation ponds result in a relatively lower biomass concentration, the aforementioned features of the pond system make it a competitive cultivation option.

Tubular reactors are considered to be more appropriate for outdoor cultivation. The large illumination surface of the reactor which is created by translucent tubing is the main factor behind its outdoor suitability. The tubing can be arranged in vari­ous configurations and the appropriateness of the configuration depends on the specifications of the system. Common configurations include straight line and coiled tubing [35]. The geometry of the reactor is also important, as tubular reactors can be configured in a vertical, horizontal or inclined plane. Harun et al. [13] states that the major difference between the configurations is that the vertical design allows greater mass transfer and a decrease in energy usage, while the horizontal reactor is more scalable, but requires a large area of land. Tubular reactors make use of either airlift or air pump aeration for culture mixing. The airlift system is more preferred, espe­cially for scale-up purposes. Previous studies have shown that the scale-up of a tubular photobioreactor can play havoc with the mass transfer of the culture [22]. Large build-up of dissolved oxygen may occur within the tubing during scale-up and this can inhibit cell growth. Figure 1 shows a schematic diagram for open ponds, horizontal tubular reactor (HTR) and external loop tubular reactor (ELR).

This design study uses a potassium hydroxide (KOH) catalyst and methanol to syn­thesise FAME based on Sakai et al.’s [29] biodiesel production model, which states that 100 parts of oil and 40 parts of methanol with a KOH catalyst will produce 92 parts of FAME and 21.5 parts of crude glycerol. The study makes use of partially recycled methanol feedstock whose composition is 24 parts of recycled methanol and 16 parts of fresh methanol. Figure 8 shows the transesterification model. Applying this model to the oil yields from the solvent extraction gives a daily biod­iesel production of ~7,158 kg/day and an annual production of ~2,360 tonnes. The yield of glycerol as a byproduct is ~1,673 kg/day.

1.6 Process Economics

Whilst the production of biodiesel from microalgae has been shown to be technically feasible, the viability of microalgal biodiesel as a practical alternative will be ulti­mately determined by its ability to become cost competitive with the current fuels.

Andres Clarens and Lisa Colosi

Abstract Algae-derived bioenergy is being widely discussed as a promising alternative to bioenergy produced from terrestrial crops. Several life cycle assessment (LCA) studies have been published recently in an effort to anticipate the environmental impacts of large-scale algae-to-energy systems. LCA is a useful tool for understand­ing the environmental implications of technology, but it is very sensitive to model­ing assumptions and techniques. In this chapter, the methodological issues surrounding LCA of algae-to-energy systems are reviewed in the context of several of the recent papers with a particular focus on system boundaries, cultivation tech­niques, metrics, coproduct allocation, and uncertainty. The issues raised here are useful in two regards: (1) they enable an understanding of the differences between the published studies and allow LCA practitioners and others to more directly inter­pret the results and (2) they serve as a good starting point for future analysis of algae-to-energy technologies.

1 Introduction

The promise of using algae as a bountiful and renewable source of bioenergy has been attracting increasing attention over the last few decades [26] . This is because algae have a number of characteristics that make them appealing relative to other bioenergy sources. They are generally fast growing and produce more biomass per area of land than most terrestrial crops [19]. Certain species generate high concen­trations of lipids so they can be used to produce liquid fuels, such as biodiesel, using existing conversion technologies [18]. And since they are grown in water, they could

A. Clarens (*) • L. Colosi

Civil and Environmental Engineering, University of Virginia, Charlottesville, VA 22904, USA e-mail: aclarens@virginia. edu

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_32, 759

© Springer Science+Business Media New York 2013 also be cultivated in man-made ponds, which suggests their cultivation can be scaled up and operated in steady-state mode, greatly enhancing their potential for large — scale energy production. Over the past few years, interest in algae-to-energy tech­nologies has surged for a variety of reasons. Among them is the idea that algae could be used to sequester CO2 from fossil fuel burning sources, thereby reducing a major contributor to climate change [3] . Increasing petroleum prices, concerns about our dwindling fossil fuel reserves, and the perceived competition between food and fuel uses for crops that can be consumed as food have also contributed to interest in algae as a fuel source [23].

The heightened attention on algae-to-energy systems has resulted in a prolifera­tion of academic and industrial publications describing these technologies. A num­ber of these studies focus on quantifying the environmental impacts of algae-to-energy systems using life cycle assessment (LCA) techniques [8, 14, 24, 31]. LCA is a framework for assessing the environmental and energy implications of a process or product over its entire life cycle (LC), from resource extraction to final disposal. Over the past 10 years, LCA has emerged as a valuable tool for understanding the full environmental costs of complex engineering systems. It allows designers and engineers to avoid media shifting, whereby one environmental impact is avoided at the cost of some other, often hidden and worse, environmental burden [13]. LCA can also serve as a useful design tool that allows for a priori evaluation of different engineering decisions. By applying LCA in this way, it is possible that many tradi­tional sources of pollution can be avoided upstream rather than remediated after they are generated. Even though LCA has been widely practiced for over a decade, only recently have the techniques been applied to algae-to-energy processes.

The algae-related LCA studies appearing in the academic literature to date offer multiple perspectives on how large-scale algae-to-energy systems might be deployed. These studies are largely speculative because there is a lack of empirical data for long-term operation of full-scale commercial algae cultivation systems. In general, the results of algae LCA studies published to date are difficult to compare because of key modeling differences. The differences originate from several stages of the analyses. To begin with, the scope. e. g., system boundaries and functional unit of the studies, is different. Second, the data sources used in the studies, the way in which the studies report their results (i. e., metrics), and the manner in which they allocate burdens to different processes (e. g., coproducts) also vary quite a bit. This variability is to be expected given that there are, as yet, no norms for the industry that would suggest the most reasonable set of assumptions. Finally, the unsatisfac­tory way in which the studies handle uncertainty speaks to the lack of data in this fi eld. Table 1 highlights the array of different modeling assumptions that have been used in some of the LCA studies of algae-to-energy systems that have been pub­lished to date. It should be pointed out that each of these studies utilized a different functional unit and many use different modeling assumptions. Thus, it is no surprise that the results are difficult to compare.

In general, it cannot be said that one particular study is more or less “correct” than any of the others. LCA challenges exist even for processes and products that

are well characterized and widely practiced. One widely cited, and related, example is the case of petroleum-based liquid fuels. Since the early 1990s, a substantial number of studies have been conducted describing the process of extracting the crude oil from the ground, transporting it, refining it, distributing and selling it, then burning it in cars and trucks [25]. Different studies resulted in very different esti­mates for the burdens of similar processes that are practiced in more or less the same manner around the world. To address these challenges, Argonne National Laboratory in the United States created the Greenhouse Gases and Regulated Emissions, and Energy Use in Transportation (GREET) model for estimating LC burdens associ­ated with petroleum-based transportation fuels in 1996 [33]. By synthesizing the results from various published LC models, and normalizing the system boundaries and allocation assumptions among analyzed cases, the creators of GREET produced a meta-model that is more representative of petroleum fuel production than any one given analysis. This occurs because the meta-model effectively neutralizes (i. e., washes out) some assumptions that can make any one particular study either over — or underestimate the true impacts of a given process. Since the algae-to-energy industry is currently undergoing such rapid development, it seems timely to con­sider standardization of LC methodology to improve the accuracy of LCA for algae — derived fuels.

This chapter is written for two primary audiences. The first is the algae-to-energy researchers wishing to model LC impacts of specific products or processes. For these uses, the material presented here should serve as a useful primer into the lan­guage of LCA as it relates to algae-to-energy processes. The second audience is the broader scientific and journalistic community. This community has occasionally misinterpreted the results of several recent algae LCAs. The material presented here should help educate the science-literate reader who has no background in LCA so that they can better understand the implications and conclusions of algae LCA studies. It is expected that successful engagement of both audiences should improve the quality of future algae LCA studies and contribute to discourse about the merits of algae-to-energy technologies.

Supercritical fluid extraction (SFE) represents an environmentally friendly alterna­tive to organic solvent extraction. Even though it was first introduced in 1879 by Hannay and Hogarth, the extraction method did not gain much scientific attention until around 1960 [18, 39]. In addition to avoiding the use of toxic solvent, SFE has many apparent advantages over organic solvent extraction. It produces extracts of higher purity, requires less processing steps and can be operated at moderate tem­peratures to minimize extract degradation [39, 42] . Supercritical fl uid generally has a high solvent power for non-polar components and a low affinity towards analytes with high molecular weights. The supercritical state is achieved when a substance is exposed to conditions exceeding its critical temperature (Tc) and pressure (Pc). In this state, the substance has a liquid-like density with a gas-like viscosity [6].

CO2 is the most commonly used fluid for SFE as it is cheap, non-flammable, inert, and readily available. Figure 5 shows the phase diagram for CO2 with its supercritical region. The critical temperature and pressure of CO2 are 304.1 K and 7.38 MPa respectively. Supercritical carbon dioxide (SCCO2) is often used in the extraction of thermolabile compounds as its low Tc enables complete extraction to occur without the application of excessive heating which may degrade analytes [29, 35]. SCCO2 is a highly effective extractant due to its high diffusivity and its easily manipulated solvent strength. The solvent power of SCCO2 towards a polar analyte can be improved by adding a polar modifier. The addition of methanol or water to SCCO2 allowed for successful extraction of polar compounds [35], while ethanol addition was found to increase the yield of lipids from Arthrospira maxima [36]. SCCO2 extraction has been applied in many fields including food industry, environ­mental science, and pharmaceuticals.

SFE can be classified as either an analytical or a preparative system. In the ana­lytical system, SFE apparatus is directly combined with a chromatographic device.

TEMPERATURE

Fig. 5 P-T phase diagram for carbon dioxide [35]

Even though this system enables rapid analytes examination, it cannot be used as a production system as any extracted analytes are immediately consumed during the chromatographic analysis. On the other hand, the preparative system has been used to produce pilot-scale quantities of various analytes from microorganisms, including chlorophyll from microalgae. Figure 6 shows a pilot-scale preparative SFE system. It consists of a solvent (in this case CO2) pump, a modifier pump, an extraction cell, valves, and two fractionation cells [17]. Detailed description of the SFE working

mechanism can be found elsewhere [17] . The microalgal biomass is placed in the extraction cell, while the supercritical fluid is depressurized in the fractionation cells equipped with temperature and pressure controllers. Upon depressurization, the supercritical fluid evaporates to the ambient as a gas, forcing analytes to precipitate in the fractionation cells. The dual-fractionation arrangement allows different ana­lytes to be precipitated in each cell based on their differential solubilities in the evaporating supercritical fluid.

The use SCCO. process to extract chlorophyll a from microalgal species has been reported [ 30 [ . Optimum extraction conditions were found to be 60°C and 400 bar for N. gaditana and 60°C and 500 bar for Synechococcus sp. Chlorophyll a yields for the two microalgae at these optimum conditions were, respectively, 2.24 and 0.72 mg chlorophyll/mg dry weight microalgae. Even though these yields were not comparable to those of traditional methanol extraction (18.5 mg chlorophyll/mg dry weight microalgae for N. gaditana and 4.10 mg chlorophyll/mg dry weight microalgae for Synechococcus sp.), SCCO2 extraction was found to be more selec­tive and the extracted chlorophyll a appeared to contain less impurities. An exten­sive evaluation comparing the two extraction systems for isolating microalgal chlorophyll is, unfortunately, not yet possible due to limited knowledge on the supercritical process. The commercial feasibility of using SCCO2 process to extract chlorophyll from microalgae is also a subject of further research.

Phenols are an important group of natural products with antioxidant and other bio­logical activities. These compounds play an important role in algal cell defense against abiotic and biotic stress. Several authors have recently published results regarding the total phenol content and antioxidant activity of algae [40] . Cinnamic acid esters (n-butyl 3,5-dimethoxy-4-hydroxycinnamate and isopropyl 3,5-dimethoxy-

4- hydroxycinnamate) and methyl 3,4,5-trihydroxybenzoate were studied using 1H and 13C NMR in brown algae Spatoglossum variabile [46]. Some of the first polyphenols found in algae (Fucus and Ascophyllum spp.) were phlorotannins. They are formed from the oligomeric structures of phloroglucinol (1,3,5-trihydroxyben — zene) [137]. Also, some flavanone glycosides have been found even in fresh water algae [86] .

The main bioactivity associated to phenolic compounds is antioxidant activity, which is also the main bioactivity of algal and microalgal phenolics [89]. Duan et al. [28] have demonstrated that antioxidant potency of crude extract from red algae (Polysiphoma urceoiata) correlated well with the total phenolic content. Strong cor­relation also existed between the polyphenol content and DPPH radical scavenging activity of a seaweed (H. fusiformis) extract [177], Using electron spin resonance spectrometry and comet assay, Heo et al. [51] found that phenolic content in sea­weeds could raise up to 1,352 mg/g on dry weight basis. The content and profile of phenolic substances in marine algae vary with the species. In marine brown algae, a group of polymers called phlorotannins comprises the major phenolic compounds [20], such as fucols, phlorethols, fucophlorethols, fuhalols, and halogenated and sulfited phlorotannins. Takamatsu et al. [186] showed that bromophenols isolated from several red marine algae exhibited antioxidant activities. These findings sug­gest that phlorotannins, the natural antioxidant compounds found in edible brown algae, can protect food products against oxidative degradation as well as prevent and/or treat free radical-related diseases [89] .

Some algal phenolic compounds have been associated with anti-inflammatory activity, such as rutin, hesperidin, morin, caffeic acid, catechol, catechin, and epi — gallocatechin gallate, whose have been identified in Porphyra genus. Kazlowska et al. [79] have studied recently the phenolic compounds in Porphyra dentata, they identified catechol, rutin, and hesperidin in crude extract using HPLC-DAD. They demonstrated that the crude extract and the phenolic compounds inhibited the production of nitric oxide in LPS-stimulated RAW 264.7 cells. Their results indicate that catechol and rutin, but not hesperidin, are primary bioactive phenolic compounds in the crude extract to suppress NO production in LPS-stimulated macrophages via NF-kB-dependent iNOS gene transcription. Data also explained the anti-inflammatory use and possible mechanism of P. dentata in iNOS — implicated diseases.

Digestibility, or the amount of VS reduced (converted to biogas) during AD, is one of the most important characteristics of the feedstock. The amount of algal VS reduced varies in the range from 20 to 60% for most macro- and microalgae (Tables 16-19). Consequently, the conventional ADP is not able to convert all algal organic matter to biogas and a large fraction of energy is lost as low-value residues. Pretreatment of algal biomass is one of the strategies used for conditioning and increasing algal digest­ibility, methane yield, and degradation rate. Possible goals of pretreatment include:

• Disruption of cell wall

• Size reduction and increase of specific surface area of particulate biomass

• Crystallinity reduction of fiber materials (e. g., cellulose)

• Solubilization of recalcitrant and poorly biodegradable materials (e. g., hemicel — lulose, lignin)

• Partial hydrolysis of cell polymers

• Deactivation of toxic materials

The important requirements for pretreatment methods are to preserve the total organic matter content and to prevent the formation of inhibitory materials. Little is known about providing efficient solutions for increasing algal digestibility. A variety of pretreatment methods have been tested on waste-activated sludge (WAS), live­stock manure, pulp and paper residues, and lignocellulosic biomass [21, 189-193]. Similar methods can be potentially applied for algal biomass conditioning. Methods applied for biomass pretreatment can be classified into the following groups:

• Mechanical—grinding, milling, homogenization, ultrasonic treatment, liquid shear [194-205]

• Thermal—drying, steam pretreatment, hydrothermolysis [195, 206-209]

• Chemical—acid or alkali hydrolysis, ozonation, hydrogen peroxide treatment [198,208,210-213]

• Biological—temperature-phased AD enzymatic treatment [195, 198, 214, 215]

• Electrical—electro-Fenton [216,217]

• Irradiation—gamma-ray, electron-beam, microwave [218-222]

• Combination—thermochemical, wet oxidation [208, 211, 223-226]

Removal of 95% biochemical oxygen demand (BOD), 85% COD, 90% ammonia, >65% total nitrogen, and >99% of the pathogenic indicator microorganisms from municipal wastewater can be achieved in algal ponds [469]. The effluent quality is highly variable: BOD = 10-25 mg/L and COD = 50-85 mg/L [470]. Oswald and col­leagues developed and tested the Advanced Integrated Wastewater Pond System (AIWPS) [471-475]. This system consists of methane production in an advanced facultative pond, algal high rate pond, algal settling pond, and maturation pond.

This system provides a secondary effluent adequate for agricultural irrigation and has 4-5 times lower electrical power consumption per water flow compared to con­ventional activated sludge and extended aeration systems [476]. A conventional wastewater treatment system requires approximately one kWhr of electricity for aeration for the removal of 1 kg of BOD. In contrast, photosynthetic BOD oxidation does not require aeration but produces algal biomass that can be converted to roughly one kWh of electricity through ADP [477] .

Algal Cultivation in Anaerobic Digester Effluent

Recent developments allow ADP to be applied for the treatment of a wide range of wastewaters with organic contamination. But ADP has several drawbacks particu­larly high organic matter and ammonium concentrations in the AD effluent. Moreover, AD has low efficiency of phosphorus removal. A similar waste liquid is generated during AD of algae. This nutrient rich anaerobic effluent can serve as fertilizer for intensive algal production.

Green microalgae (Chlorella and Scenedesmus) cultivated in diluted dairy waste anaerobic digester effluent are able to switch from phototrophic to heterotrophic or mixotrophic growth, utilize native substrates present in effluent, and increase the biomass and triglyceride production rate [478]. Ammonia removal efficiency from anaerobic treated dairy wastewater reached 96% with a mixed green algae culture [479] and 99% with A. platensis [480]. Aragon reported the removal of 85% BOD, 75% COD, 80% ammonia, and >97% detergents during treatment of the anaerobic effluent from an urban wastewater treatment plant by two local algae species Scenedesmus acutus and C. vulgaris [481].

A “closed” system of methane generation from light energy via algal production and anaerobic digestion was described by Golueke and Oswald [109]. The liquid phase from the digester was used as culture media for algal growth. The average methane yield was 0.44 L/gVS, the maximum energy conversion efficiency from light to biomass was 3%, and the energy conversion efficiency for the entire unit was 2%. Ras and colleagues repeated the same experiment with C. vulgaris as the solar light capturing organism [482]. The methane yield was 0.24 L/gVS at an HRT of 28 days and an OLR of 0.7 gVS/L-day.

Ryther compared the productivity of G. tikvahiae and Ulva sp. in media enriched by AD effluent with the productivity in a conventional mineral enrichment medium [483]. Ulva sp. had a similar methane yield in both media, but G. tikvahiae had 50-75% lower productivity in an AD effluent-enriched medium compared to a control.