For microalgae biofuel production, a major cost factor is the provision of water and nutrients (Davis and Aden 2011; Borowitzka and Moheimani 2013), which can both be provided by wastewater. Microalgae ponding systems were developed in the 1950s for municipal sewage treatment (see Oswald 2003 for a review of the early work), and this approach continues to serve as a starting point for the development of cost-efficient algae biomass for fuels production. At least 70 % of the cost of wastewater treatment can be attributed to secondary and tertiary treat­ment. Much of this is due to the energy costs of oxygen transfer in biological secondary treatment and chemical requirements in tertiary treatment.

Dr. William Oswald at the University of California-Berkeley and his colleagues over the following 50 years (Oswald 2003) developed the fundamental engineering design parameters and described the basic biological processes in bioremediation in high-rate ponds. Microscopic algae convert about 2 % of total solar energy to algal biomass. The photosynthetically generated oxygen is consumed by bacterial pop­ulations that decompose organic wastes to simple nutrients including CO2. Although algae-based secondary and tertiary treatment is economically feasible, at least in warm regions with ample land, few municipal algae ponds attempt to control species composition or even harvest the algal biomass (Benemann and Oswald 1996). Two of the persistent problems noted in the early years of inves­tigation were maintaining a stable algal population in the treatment ponds and harvesting the algae in an efficient and economic manner. Open pond cultures are subject to all the variations that occur in natural ponds and lakes. At any given time, there can be a major shift in algal dominance such as a transition from green algae to cyanobacteria (potentially toxic) or there can be a sudden collapse of the com­munity structure due to algae bloom crashes and predation by zooplankton. The sudden death of phytoplankton communities, or algal blooms, is thought to result from several factors including insufficient light for photosynthesis, limiting nutri­ents, phycoviruses (Brussaard 2004), the aging of the blooms, and perhaps by photoinhibition in gas-vaculate cyanobacteria.

Successful open-air algal monoculture is currently limited to a small number of species that can tolerate extremes of pH or salinity that preclude invasion by competing microbes and undesired predators conquering unenclosed outdoor ponds. Enclosed bioreactors mitigate some of the problems of maintaining mono­cultures and predation issues, but capital and labor costs limit their use to pro­duction of high-value products. Other major hurdles to economically feasible algae — based biofuels include the use of high oil strains adapted to the local environment, development of resource-specific production and management systems, and, at least in the short term, coupling algae culture with mitigation of environmental problems and co-production of high-value compounds.

Minimal nutritional requirements for algal growth can be estimated from the approximate molecular formula of algal biomass: C(048) H(183) N(011) P(0.0i) (Grobbelaar 2004). The chemical composition of municipal and dairy wastewaters typically has less N than P relative to algal biomass (Fulton 2009). Although CO2 limits algal growth in high-rate oxidation ponds (Benemann and Oswald 1996), when CO2 is supplemented, N typically limits algal growth on municipal (Bene — mann et al. 1977) and agricultural (Lundquist et al. 2011) wastewaters. N limitation has long been known to be a trigger for lipid synthesis in some algal species. In a variety of microalgae, as the nitrogen or phosphorus levels drop limiting growth, there is a rapid increase in oil (lipid) content (Takagi et al. 2000; Li et al. 2008). The mechanisms involved in the “N-trigger” have remained elusive. Although recently comparative proteomics and transcriptome analysis of the haptophyte Tisochrysis lutea (formerly known as Isochrysis galbana) have revealed a wide variety of proteins and transcripts involved in various pathways including lipid, carbohydrate, amino acid, energy, and pigment metabolisms, photosynthesis, protein translation, stress responses, and cell division in strains are subjected to N limitation (Garnier et al. 2014). Most of the oleaginous algae are non-motile. They are thought to produce lipids as a buoyancy compensator to position them in the ideal part of the water column during the light/dark cycle of photosynthesis. As a survival mecha­nism to counter the depletion of nutrient supplies, they may produce high levels of lipid to bring them to the surface where the wind may blow them to a more nutrient — rich environment. This insight favors the concept of two-phase semi-continuous cultures for wastewater treatment to promote lipid biosynthesis. As the rapidly growing algae take up all the available nitrogen and phosphorus, biosynthetic pathways for growth are nutrient limited and oil content rises.

Figure 6.1 is a flow diagram of a hypothetical algae-based wastewater treatment systems with multiple benefits that take into account the drawbacks described in previous algae-culturing systems. The treatment process is based on a sequence of events designed to yield a high-quality effluent and a consistent supply of algae biomass:

1. Filtered primary wastewater is added to the first of three shallow raceways. There are three raceways: one is filling, one is in the algae growth stage, and the third raceway is in the harvesting stage. A fourth raceway may be needed to account for variations in productivity.

2. The filtered primary effluent is rich in biochemical oxygen demand (BOD), ammonium, and organically bound phosphorus. This effluent is mixed with a high concentration of a pure strain or strains of algae that are grown in a bank of photobioreactors. The algae in the photobioreactors are added to the pond when they are at or near the top of their log-phase growth. The dilution of the concentrated algae into the pond is adjusted to the lower end of early log-phase growth. These ideal strains were selected for their ability to grow rapidly on wastewater, adapted to local waters, high lipid content, and harvestability. In the daylight, the algae produce large quantities of oxygen that facilitate the oxidation of the various organic compounds that contribute to the BOD. During the dark cycle of photosynthesis, the algae can take up a variety of organic compounds including the organic compounds that were degraded during the light cycle of photosynthesis.

Possible Extraction of

Commercially Important

Feedstock for Polymer

be composted to produce a fertilizer and the M Pr°duction or ioplastics

high-nutrient liquid can be used

Fig. 6.1 Flow diagram of an algae-based wastewater treatment system. The boxes with the dashed border indicate an end product or benefit

3. Due to the log-phase growth, in 72-120 h, the algae biomass has increased fourfold to eightfold; the nutrients have been assimilated by the algae, and the BOD has dropped to the level associated with tertiary treatment. The short residence time in the open ponds lowers the likelihood of contamination by another strain of algae or predation by zooplankton mainly by competitive exclusion (Lang 1974; Hillebrand 2011).

4. The algae can be harvested by one of many physical separation processes (for more information of dewatering, see Chaps. 12—14), and the tertiary effluent can be disinfected and prepared for some form of water reuse. At this point, the algae can be ground up to yield a green crude or lysed in order to separate the lipid content from the aqueous and solid fractions.

5. The green crude can be processed into a variety of fuels in the same way the crude oil is processed. If the cells were lysed and just the lipid fraction was isolated, it would then be converted to biodiesel by one of a variety of physical, chemical, or enzymatic processes.

6. The algae biomass could potentially contain a number of valuable phyto­chemicals that can be extracted and purified for use in the chemical or personal care products industry. The biomass could also be used as a feedstock for the production of synthetic polymers.

7. If the previous option is not feasible, the cracked algae cells can be put into a digester to produce methane. Unlike activated sludge, this organic matter has a uniform composition, and since the cells are already lysed, the degradation of the organic matter and subsequent production of methane should be more efficient than digesters filled with activated sludge.

8. The energy demands on this process should be considerably less than tradi­tional secondary and tertiary treatment such that the energy produced by cogeneration could be sold on the grid.

9. The CO2 that is produced by burning the methane is usually released into the air. In order to raise algae in high concentrations, it is necessary to add CO2 to the water. By bubbling in CO2 into the rapidly growing algal cultures, it is possible to capture the majority of CO2 produced by the cogeneration system (for a recent review, see Raeesossadati et al. 2014 and Chap. 7).

10. In the event that carbon “cap and trade” rules are implemented, this process provides a way for substantially shrinking the carbon footprint of a wastewater treatment plant.

11. The solid residue from the digester can be composted to produce a high-quality fertilizer, and the liquid fraction of the digester (mixed liquor) can be sterilized and used as a nutrient source for the bank of photobioreactors.