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1 Introduction

Oswald and Golueke (1960) first proposed the large-scale production of algae as a biofuel feedstock using high rate algal ponds (HRAP), with wastewater providing the make-up water and nutrients. HRAP have since been developed as a low-cost efficient wastewater treatment technology (Oswald 1988a; Craggs 2005). The concept of producing biofuel from algae harvested from wastewater treatment HRAP effluent is a niche opportunity that could be economical today, since the harvested algal biomass would essentially be a “free” by-product of wastewater treatment.

2 Wastewater Treatment Ponds

Wastewater treatment ponds rely on algal photosynthesis to harness sunlight energy and provide oxygen to drive aerobic bacterial degradation of organic compounds. The wastewater nutrients that are released are, in turn, assimilated into algal biomass (Oswald 1988a). However facultative ponds, the unmixed, ∼1 m deep ponds that are widely used throughout the world for wastewater treatment do not consistently provide a high level of nutrient removal and have very low algal biomass productivity. For example, in New Zealand, such ponds have an average annual production of little more than 10 tonne ha−1 year−1 (Davies-Colley et al. 1995; Craggs et al. 2003), well below that required for economical biofuel production. High Rate Algal Ponds (HRAP) have much higher treatment performance and algal productivity, and could provide sufficient algal biomass to be economically used as a biofuel feedstock.

2.1 High Rate Algal Ponds

HRAPs are paddlewheel mixed, shallow, open channel raceways (Fig. 9.1) that were developed in the late 1950s for wastewater treatment and resource recovery by Oswald and co-workers at the University of California at Berkeley (Oswald et al. 1957). Oswald applied HRAP technology to treat the wastewater of several northern California cities over the next 40 years. For example, St Helena (in 1967) (Fig. 9.2) and Delhi (in 1997), and both systems still operate today. HRAPs are used at wastewater treatment plants around the world and have been shown to be capable of treating a variety of agricultural and industrial wastes (Oswald 1988a; Craggs 2005).

Fig. 9.1
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Plan view and side elevation of a high rate algal pond with CO2 addition

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HRAPs are also used by the majority of algal farms that produce algae for high value health food supplements, pigments and chemicals (Borowitzka and Borowitzka 1988; Weissman et al. 1988). Earth-lined HRAPs have much lower capital costs than closed photobioreactors but at current fossil fuel prices it is unlikely that even the relatively simple HRAP could be economically used for algal biofuel production alone, without producing high value co-products or providing beneficial functions (such as wastewater treatment) (Oswald and Golueke 1960; Benemann and Oswald 1996; Benemann 2003).

2.2 Algal Production in HRAPs

New Zealand HRAPs treating domestic wastewater have been shown to have algal yields of about 0.2 tonne ML−1 of wastewater and productivities of almost 30 tonne ha−1⋅year, which is between two to three-fold that of facultative wastewater treatment ponds (Craggs et al. 2003). However, algal production in such ponds is severely carbon-limited due to the low C:N ratio of wastewaters (typically 4:1 for domestic wastewater) compared to algal biomass, which can range from about 10:1 to 5:1, depending, respectively, on whether N is limiting or not (Benemann 2003; van Harmelen and Oonk 2006; Lundquist 2008). Thus domestic wastewaters contain insufficient carbon to remove all of the nitrogen by assimilation into algal biomass. Carbon-limitation in wastewater treatment HRAPs is indicated by elevated daytime pond water pH, resulting from the use of bicarbonate ions as a CO2 source for algal photosynthesis, releasing hydroxide ions which can increase pond water pH to >10 (Oswald 1988a; Garcia et al. 2000; Craggs 2005; Kong et al. 2010; Park and Craggs 2010a).

The growth of many algal species is increasingly inhibited at a pond water pH greater than 8 (Kong et al. 2010). For example, the productivity of the alga Chlorella sp. declined by 22% when pH was raised from 8 to 9 (Weissman et al. 1988). Although, some algae are still capable of growing under carbon limited conditions, including many of the desmids, e.g. Ankistrodesmus sp. which grows well at pH 10 (Weissman et al. 1988).

The high pH inhibition of algal growth could in part be due to high free ammonia concentrations at high pH (Azov and Goldman 1982; Azov et al. 1982; Konig et al. 1987). For example, growth of the alga Scenedesmus obliquus at free ammonia concentrations of 34 and 51 g m−3 (pH 9.5 and 20–25 °C) was reduced by 50 and 90% respectively (Azov and Goldman 1982). Intense photosynthesis in HRAP also increases daytime pond water dissolved oxygen levels to >200% saturation. High oxygen levels have also been shown to reduce algal productivity (Weissman et al. 1988).

High pond water pH also inhibits the growth of aerobic heterotrophic bacteria that oxidize organic matter to CO2 in wastewater treatment HRAP (Weissman and Goebel 1987; Oswald 1988a; Craggs 2005). These bacteria typically have an optimum pH of ∼8.3, above which bacterial activity is increasingly inhibited (Craggs 2005).

Addition of CO2 to wastewater treatment HRAPs would therefore augment carbon availability and avoid pH inhibition to enhance algal production and nutrient removal by assimilation into algal biomass. This can be simply achieved through control of the HRAP water maximum pH to below 7.5–8.0 by CO2 addition. There is little published information on CO2 addition to wastewater treatment HRAPs, however, CO2 addition has been shown to more than double the productivity of algal cultures (Benemann et al. 1980; Azov et al. 1982; Benemann 2003; Lundquist 2008; Park and Craggs 2010a) and is practiced at all commercial algal farms (van Harmelen and Oonk 2006). The first small-scale experiments on CO2 addition to wastewater HRAP were conducted at Richmond, California (Benemann et al. 1980). Initial trials of CO2 addition to HRAP treating agricultural drainage waters were conducted by Gerhardt et al. (1991) and a large-scale trial was later successfully operated over several years.

Fig. 9.2
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Wastewater treatment HRAPs operating at St Helena (a) and Hilmer (b) in California

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For wastewater treatment HRAP the source of CO2 could be the flue gas from the power generated using the biogas produced by anaerobic digestion of solids removed from the wastewater, both as settled raw sewage sludge (during “primary treatment”) and the algal biomass harvested from the HRAPs (Benemann 2003). Further sources of CO2 could be from the digestion of the biomass residues resulting from the conversion of algal biomass to other biofuels, such as ethanol or biodiesel; the digestion of other organic waste sources such as household or garden waste; or directly using waste gaseous emissions, such as power plant flue gas (van Harmelen and Oonk 2006; Chisti 2008; Lardon et al. 2009). The use of HRAP to purify biogas (scrub CO2 and H2S) using cost-effective apparatus for mixing the gas into pond water has been demonstrated (Conde et al. 1993; Mandeno et al. 2005). Recent pilot-scale research during New Zealand summer conditions has shown that CO2 addition to wastewater HRAP can increase algal production by 30–100% (Yield: up to 0.3 tonne ML−1; projected productivity: as high as 60 tonne ha−1⋅year) (Heubeck et al. 2007; Park and Craggs 2010a) (Fig. 9.3).

Fig. 9.3
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One of the four wastewater treatment HRAP (1.25 ha) with CO2 addition at Christchurch, New Zealand

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A major disadvantage of wastewater treatment HRAPs is the relatively large land requirement compared with electromechanical treatment systems (e.g. activated sludge), however HRAPs have a smaller footprint than conventional facultative wastewater pond systems. The algal biomass production potential from wastewater treatment HRAP is limited by daily insolation and temperature, and hence the area necessary for effective year-round wastewater treatment increases with increasing latitude (Bouterfas et al. 2002; Jeon et al. 2005; Voltolina et al. 2005). For example a HRAP area of ∼1.7 ha is required per ML⋅day of wastewater flow under suitable Southern Californian climatic conditions, increasing to ∼2.7 ha ML−1⋅day under the limiting climatic conditions of New Zealand. Algal productivity increases are needed to improve the economics of algal biofuels, and could be achievable through research on HRAP design and operation, and selection of algal strains that thrive in the HRAP environment (high sunlight, high daytime pH, temperature and super-saturated dissolved oxygen).

2.3 Algal Grazers and Pathogens

Herbivorous zooplankton such as rotifers and cladocerans graze on HRAP algae, and when present at high density, can reduce pond water algal concentrations to low levels within a few days (Benemann et al. 1980; Picot et al. 1991; Cauchie et al. 1995; Nurdogan and Oswald 1995; van Harmelen and Oonk 2006; Smith et al. 2009). For example, rotifers and cladocerans at densities greater than 100 per litre were found to reduce wastewater treatment HRAP algal concentration by 90% within 2 days (Oswald 1980), and several days of grazing by a population of the cladoceran, Daphnia sp., reduced the chlorophyll a concentration of a pond by 99% (Cauchie et al. 1995). Algae are also susceptible to fungal parasitism and bacterial or viral infection which can also reduce the pond algal population within a few days and can result in changes in algal cell structure, diversity and succession (Wommack and Colwell 2000; Short and Suttle 2002; Kagami et al. 2007).

Therefore, to maximize HRAP algal productivity, populations of zooplankton grazers, parasitic fungi, and infective bacteria and viruses must be controlled. Zooplankton grazer populations may be limited by application of chemicals or invertebrate hormone mimics, or by increasing pond water pH to 11, particularly if the pond water has a high ammoniacal-N concentration (O’Brien and De Noyelles 1972; Schluter and Groeneweg 1981; Oswald 1988b). There are no practical control methods for fungal paracitism or bacterial and viral infections, and further research is required to fully understand their influence on algal productivity in wastewater treatment HRAP.

3 Wastewater Treatment in HRAPs

HRAPs can be used to provide effective aerobic treatment (oxidation of organic matter) and assimilate soluble nutrients from many types of wastewater (e.g., anaerobic pond effluent, domestic wastewater pre-treated to the primary or secondary level, agricultural wastewaters, etc.).

3.1 Aerobic Treatment

HRAP aeration efficiency in terms of the power required for gentle paddlewheel mixing and photosynthetic oxygen production by the algae, varies between 0.04 and 0.15 kWhe kg−1 O2 produced depending on season, insolation and other factors (Benemann et al. 1980; Oswald 1988b; Green et al. 1995). This equates to 50–110 kWhe ML−1 of wastewater. In comparison, activated sludge requires from 230 to 960 kWhe ML−1 (based on 0.4–1.7 kWhe kg−1 O2) (Owen 1982; Metcalf and Eddy, Inc. 1991; Green et al. 1995). HRAP integrated wastewater treatment amortized capital and operation costs (∼US450,000 ML−1) are only 25–33% of those of secondary-level activated sludge treatment (Green et al. 1995; Downing et al. 2002).

3.2 Nutrient Removal

Nitrogen removal by nitrification-denitrification is a common electromechanical nutrient removal process, but it is costly and for a typical wastewater primary effluent nitrogen concentration of 30 g N m−3 requires additional aeration energy of ∼400–1,000 kWhe ML−1 of wastewater (Owen 1982). HRAPs with CO2 addition could provide energy efficient tertiary-level nutrient removal for little additional energy cost (Benemann et al. 1980; Nurdogan and Oswald 1995; Woertz 2007; Park and Craggs 2010a). Algal biomass can exhibit N:P ratios ranging from nearly 4:1 (under nitrogen limiting conditions) to almost 40:1. These N:P ratios correspond to algal biomass N and P content percentages typically of 10% N and 1% P and 4% N and 0.4% P (under nitrogen limiting conditions). Due to this large, almost ten-fold, range in microalgal N:P ratios, N removal is the key issue in HRAP tertiary-level treatment (nutrient removal), since efficient P removal does generally not require additional algal biomass production above that needed for N assimilation. Near-complete assimilation of both N and P into algal biomass from wastewater is therefore theoretically possible (Benemann 2003).

Nutrient assimilation rates can reach 24 kg N ha−1⋅day and 3 kg P ha−1⋅day, based on the typical algal nutrient composition and a maximum productivity of 30 g m−2⋅day algal biomass (dry weight). These removals are achieved at much lower capital and operation costs compared to conventional mechanical treatment technologies (Owen 1982; Craggs et al. 1999). A critical issue for tertiary-level nutrient removal is the maintenance of high algal productivity even when dissolved N has been reduced to low levels (e.g. <1 g m−3). This has been demonstrated in preliminary trials by supplying nutrients as required (Benemann 2003), but more research is needed.

Wastewater treatment HRAP nutrient removal processes that occur at high pond water pH such as ammonia volatilisation and phosphate precipitation with cations (Nurdogan and Oswald 1995; Garcia et al. 2000; Craggs et al. 2003; Heubeck et al. 2007) are greatly reduced by CO2 addition to the pond. However, this can be offset by the increased assimilation of nutrients into algal biomass with the CO2 enhanced algal production. For example recent laboratory-scale (Woertz et al. 2009) and pilot-scale (Park and Craggs 2010a, b) studies have shown near complete nutrient removal with CO2 addition to wastewater HRAP. Park and Craggs (2010b) demonstrated that daytime control of maximum pH to below 8 with CO2 addition reduced nitrogen loss by ammonia volatilization from 24% (in a control HRAP without CO2 addition) to ∼9%.

3.3 Disinfection

Disinfection by chlorination requires little energy (3 kWh ML−1 of wastewater treated) for mixing, however, 20–540 kWh ML−1 is required to generate the chlorine depending on the quality of the effluent being disinfected (Owen 1982). Dechlorination of the treated effluent to remove potentially carcinogenic chlorinated hydrocarbons further adds to disinfection costs. Ultraviolet (UV) disinfection uses less electricity (20–100 kWh ML−1) than chlorination, but requires a high level of pre-treatment so that the influent has low turbidity. Ozonation is also preferable to chlorination but uses slightly more electricity (100–200 kWh ML−1) than UV disinfection (Owen 1982). Gently mixed High Rate Algal Ponds promote sunlight-driven disinfection mechanisms without the need for chemicals or additional power requirement. Algal photosynthesis augments the sunlight-driven mechanisms by elevating daytime pond water pH (>10) and dissolved oxygen concentrations (>300 %) (Davies-Colley 2005).

Despite their high efficiency, HRAP still remain to be widely applied for wastewater treatment. This is due to a combination of: a lack of wide-spread HRAP knowledge and design skills in the engineering profession, the relatively large land area required compared with electromechanical treatment systems, and availability of low cost fossil fuel derived electricity to operate electromechanical treatment systems. Moreover, wastewater treatment HRAP were not initially optimised for efficient nutrient removal and cost-effective algal harvest and were only recognised as a secondary-level rather than tertiary-level treatment system.

4 Wastewater Treatment HRAP Design

Depending on climate, HRAPs are typically designed with an organic loading rate of between 100 and 150 kg BOD5 ha−1⋅day. HRAPs depths vary between 0.2 and 0.8 m depending upon climate and clarity of influent wastewater. A paddlewheel is used to circulate the wastewater around the HRAP raceway at a horizontal velocity of 0.15–0.30 m s−1 which is sufficient to keep the algal colonies in suspension. Algal concentrations of 100–400 g TSS m−3 are common depending upon season. At the higher concentrations almost all of the available sunlight (PAR) is absorbed within the top 0.20 m of the HRAP, leaving the rest of the pond depth in the dark. However, the paddlewheel, along with turbulent eddies formed when the water flows around the raceway corners promote vertical mixing through the pond depth ensuring that the algal biomass is intermittently exposed to sunlight.

5 Harvest of Wastewater Treatment HRAP Algae

Effective and low-cost removal of algal biomass from HRAP effluent is imperative to achieve both a high effluent quality and economical production of harvested algal biomass (Benemann and Oswald 1996; Benemann 2003; Molina Grima et al. 2003; van Harmelen and Oonk 2006). Wastewater treatment HRAP algal harvest is challenging due to (1) low and varying cell concentration (typically <400 g m−3); (2) cell densities similar to water (1.08–1.13 g mL−1); (3) small cell size (5–20 μm); and (4) strong negative surface charge particularly during exponential growth phase (Moraine et al. 1979; Lavoie and de la Noue 1987). Various harvesting methods, including centrifugation (3 kWhe kg−1 algae), filtration or microstraining, sedimentation, and dissolved air flotation (0.6 kWhe kg−1 algae, in addition to the chemical flocculants required) can be used to remove algae from HRAP effluent (Shen et al. 2009; Tampier 2009; Brennan and Owende 2010; Mata et al. 2010). However, these processes are either not applicable (e.g. filtration, microstraining), are not economical for algal harvesting from wastewater treatment HRAPs, or increase parasitic energy losses, as indicated (Benemann 2003). Chemical flocculation forming large (1–5 mm) sized floccs (e.g. with metal salts or polyelectrolytes) is the process currently used to enable algal recovery from facultative oxidation pond effluents (Benemann and Oswald 1996). However, the chemical reactions are highly sensitive to pH and the high doses of flocculants required produce large amounts of sludge and may leave a residue in the treated effluent. The high energy requirement of centrifugation makes it only economically viable for secondary thickening of harvested algae (1–2 % solids) up to 30% solids (Tampier 2009).

The gentle paddlewheel mixing in wastewater treatment HRAPs selects for genera (including: Scenedesmus sp., Micrac-tinium sp., Actinastrum sp., Pediastrum sp., Dictyosphaerium sp., and Coelastrum sp.) that form large (50–200 μm) colonies (Oswald 1988a; Banat et al. 1990; Benemann 2003; Wells 2005; Heubeck et al. 2007; Park and Craggs 2010a). Algal colonies settle reasonably well under quiescent conditions (50–90 % removal) (Benemann et al. 1980; Green et al. 1996; Craggs et al. 2003). This settling phenomenon may be augmented by self-flocculation of the algal cells (bioflocculation) which seems to be promoted by stress conditions, such as nutrient (e.g. N) limitation and CO2 addition, and can produce a concentrated algal slurry (1–3 % DM) (Eisenberg et al. 1981; Benemann and Oswald 1996; Sheehan et al. 1998). Bioflocculation/aggregation of the algal and bacterial biomass and selection for easily settleable algal genera can be enhanced by recycling of a portion of the most easily settled algal/bacterial biomass back to the HRAP in a similar way to sludge recycle in the activated sludge process (Benemann et al. 1980; Tillett 1988; Benemann and Oswald 1996; Benemann 2003; Park and Craggs 2010a). However, further research is required to refine this low-cost harvest and algal selection process.

6 Economic and Environmental Benefits of Wastewater Treatment HRAP Algal Production

Wastewater treatment HRAP and commercial farm HRAP algal production are compared in Table 9.1. The main advantage of wastewater treatment HRAP is that the costs of algal production and harvest are essentially covered by the wastewater treatment plant capital and operation costs. Algal production as a by-product of wastewater treatment HRAP has fewer environmental impacts in terms of water footprint, energy use, fertiliser use, and blow-down water disposal (Borowitzka 1999, 2005; Benemann 2003; Tampier 2009; Clarens et al. 2010).

Table 9.1 Comparison between algal production HRAP and Wastewater treatment HRAP
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7 Wastewater Algal Biofuel Production

Biofuel conversion of harvested wastewater treatment HRAP algal biomass could involve one or a combination of four main pathways: (1) Anaerobic digestion to produce biogas (methane); (2) Extraction and transesterification of algal lipid triglycerides to produce biodiesel; (3) Fermentation of algal carbohydrates to ethanol or butanol and (4) Super critical water, gasification, pyrolysis or other thermochemical conversions of algal biomass to produce hydrocarbon gases and bio-crude oils (Craggs et al. 1999; USDoE 2005; Heubeck et al. 2007). Below we describe these four biofuel conversion pathways and potential GHG emission abatement from fossil fuel substitution; however the parasitic energy consumption and associated GHG emissions for conversion are not included.

7.1 Biogas Methane

Harvested algal biomass and algal residues remaining following lipid (oil) extraction or ethanol fermentation (see below) could be anaerobically digested to produce biogas. Oswald and Golueke (1960) found that algae could be digested to biogas (∼60 % methane) with an average yield of about 0.30 m3 (0.20 kg) CH4 kg−1 algal biomass with 50–60% volatile solids conversion. Lower yields have been attributed to both the relatively refractory nature of some algal cell walls and ammonium inhibition. Pretreatment (e.g. heating) of algal biomass has been shown to improve digestibility of algal biomass under mesophylic conditions (Chen and Oswald 1998). Inhibition of anaerobic digestion can occur at free ammonia concentrations of 4,000–6,000 g NH3-N m−3 (Siegrist et al. 2005). Algal biomass can contain over 50% protein (Becker 1988) of which up to 70 % may be released as ammonia during digestion (Golueke and Oswald 1959). Therefore, to avoid free ammonia toxicity of algal digestion, the algal biomass could be (1) concentrated to no more than 6–8% solids or (2) mixed with other organic waste substrates with a lower nitrogen content (e.g. sewage sludge) (Yen and Brune 2007). The largest and longest running (decades) demonstration of co-digestion of facultative wastewater pond algae with primary settled sewage sludge in a heated mesophylic mixed digester is at the Sunnyvale wastewater treatment plant, California, USA. Co-digestion of HRAP algae biomass with the wastewater solids removed by primary treatment could potentially double the overall methane production from HRAP integrated wastewater treatment (Benemann and Oswald 1996; Heubeck and Craggs 2007).

Cost-effective anaerobic digestion could be achieved using simple ambient temperature covered digester ponds, which could be fed with algal biomass harvested by bioflocculation (typically at ∼3% solids concentration), compared to the 5–10% solids required for conventional, and more expensive, mesophylic heated mixed digesters (Oswald 1988a). Moreover, biogas production rates from laboratory-scale ambient temperature covered digester ponds have been shown to be similar to those of heated mixed digesters; 0.21–0.28 m3 CH4 kg−1 algal volatile solids (VS) added (Sukias and Craggs 2011).

Biogas methane can be used directly for heating (33.8 MJ or 9.39 kWhheat m−3 CH4 at STP) or for electricity generation at 30% conversion efficiency (2.82 kWhelectricity m−3 CH4) and concomitant heat generation (4.70 kWhheat  m−3 CH4). Essentially ∼1 kWhe can be generated from the biogas produced from 1 kg algae (Oswald 1988a, b). Electricity generation can either be used to displace electricity requirements of the wastewater treatment plant; be exported to the national grid (requires a larger capital investment for transformers, line upgrade, etc.); or be used for peak load generation earning highest prices. Biogas may be cleaned (desulphurised, stripped of CO2, and dried) and compressed (>20 MPa) for export into a natural gas pipeline or use as transport fuel. Each cubic meter (0.67 kg) of biogas methane has an energy value (34 MJ) equivalent to ∼1 L of petrol.

7.2 Biodiesel

Biodiesel production from oils extracted from algae grown in HRAPs was the main research focus of the 30 year U.S. Dept. of Energy Aquatic Species Program (Weissman and Goebel.1987; Benemann and Oswald 1996; Sheehan et al. 1998). The program concluded that in suitable climates, algae have higher oil yields than most terrestrial crop plants due to their high productivity, with between 50 and 100 tonne algae dry matter ha−1⋅year and a 25–50 % oil (as triglycerides) content thought to be attainable, with the lower range being what is currently thought to be feasible and the higher values projecting what is thought to be possible in the future by applying the modern tools of molecular biology to algal mass cultivation (Benemann and Oswald 1996; Benemann 2003). Algal oil content and quality varies between species and even strains within a species, and with culture conditions, e.g. nitrogen limitation often greatly increasing oil content (Feinberg 1984; Benemann and Oswald 1996; Barclay et al. 1987; Coleman et al. 1987; Cooksey et al. 1987; Tillett 1988; Chelf 1990; Schenk et al. 2008; Guckert and Cooksey 1990; Brennan and Owende 2010). However, nitrogen limitation to stimulate lipid accumulation in algal cells may in turn reduce algal growth (Barclay et al. 1987; Coleman et al. 1987; Tillett 1988; Chelf 1990; Weyer et al. 2009), suggesting that the two conditions of high lipid and high algal productivity are likely to be mutually exclusive. A major issue is the economical extraction of the oil from the algae, if drying of the biomass is required this will add significantly to the overall costs, even for sun drying (the only plausible method). Algal oil contains a relatively low proportion (50%) of the mono-, di-, and triglycerides that are suitable for transesterification (Feinberg 1984), and the remaining long chain polyunsaturated fatty acids produce a viscous biodiesel that may polymerize over time into waxy solids, reducing engine efficiency and clogging filters and injectors (Feinberg 1984). In the case of algal biomass grown on wastewaters, maximizing oil content and yield would not be a priority, and, for the present, a yield of 0.12 L ­biodiesel kg–1 algae biomass would be a reasonable near-term goal for wastewater grown algae biomass.

7.3 Bioethanol

Bioethanol is produced from the carbohydrate portion of algal biomass by yeast fermentation followed by distillation to separate it from the other fermentation products. However, bioethanol production from algae is limited by the carbohydrate content of algae biomass (typically 20% of dry matter) and the portion of the carbohydrate that can be converted to fermentable sugars and then to ethanol (typically half to two thirds of the carbohydrate fraction). Thus the fermentable carbohydrate content of algae biomass (∼13% DM) is low compared with other bioethanol crops (e.g. ∼65% DM for maize) (Sheehan et al. 1998) and an average yield of 0.13 L bioethanol kg−1 algal biomass appears reasonable. As in the case of algal oil production, a higher content of fermentable starch or other carbohydrates can be induced by means of nitrogen (and other nutrient) limitation. However, this option has received relatively little attention, compared to oil production and is unlikely to be a high priority for algal biofuels production in conjunction with wastewater treatment.

7.4 Bio-Crude Oil

A novel technology for the conversion of algal biomass to biofuel is the super critical water reactor (SCWR) that mimics processes that may have produced fossil oil by using intense heat (∼374 °C) and pressure (∼22.1 MPa) to disassociate water and degrade organic compounds (Chandler et al. 1998; Yesodharan 2002; Matsumura et al. 2005). SCWR conversion has similar advantages to anaerobic digestion in that the algal biomass does not have to be dried (5–30% solids) and conversion is of the whole algal biomass rather than just the lipid or carbohydrate fraction. The SCWR produces a ‘bio-crude’ oil (with a conversion potential efficiency of ∼30%) from which a range of fuel products could be derived. An average yield of 0.4–0.5 L bio-crude oil kg−1 algal biomass might be achievable, but much more research is required to demonstrate the viability of this technology.

7.5 Other Algal Uses

7.5.1 Feeds

Algal biomass also has potential for use as high-protein feed supplements for aquaculture and livestock (chickens, pigs and ruminants) (Becker 1988). Microalgae can contain over 50% crude protein with a yield that is 25-fold higher than soy beans, the most widely cultivated protein crop (de la Noüe and de Pauw 1988). Therefore an average yield of 0.5 kg protein kg−1 algal biomass is reasonable.

7.5.2 High Value Products

High value products such as β-carotene, the polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), alloxanthin and pigments (chlorophylls and carotenoids) which all have much higher value (∼US$5,000 tonne−1 algae) than biofuel could potentially be extracted to increase the value to wastewater grown algal biomass (Apt and Behrens 1999; Wickfors and Ohno 2001).

8 Greenhouse Gas (GHG) Emission Abatement

Algal biofuel production from wastewater treatment HRAP abates greenhouse emissions by several mechanisms (Green et al. 1995; Benemann 2003):

8.1 Offset Equivalent Fossil Fuel Use GHG Emissions

Production of 1 tonne of algal biomass in wastewater treatment HRAP assimilates approximately 1.8 tonnes of CO2 (assuming an algal carbon content of 46 % dry weight) (Benemann 2003). Once converted to biofuel, this offsets the CO2 GHG emissions from equivalent fossil fuel use, which is dependent upon the type of fuel it replaces. For example, generation of electricity from biogas methane abates 0.4 kg CO2EQV kWh −1e from natural gas electricity generation compared to 1.0 kg CO2EQV kWh −1e from coal electricity generation. Actual substitution depends on the marginal source of power. GHG emission abatement from the substitution of diesel fuel and heavy bunker fuel with algal biodiesel and algal bio-crude oil are 2.68 kg CO2EQV L−1 and 2.99 kg CO2EQV L−1, respectively (NZMED 2007).

8.2 Reduced CO2 Emissions from Wastewater Treatment Through Lower Electricity Use

Low energy use HRAP wastewater treatment using sunlight energy through photosynthesis abates between 100 and 200 tonne of CO2 per ML treated compared to fossil energy that would have powered electromechanical treatment (e.g., activated sludge; Green et al. 1995; Benemann 2003). Nitrogen removal in HRAP would abate a further 100–200 tonne of CO2 per ML treated. HRAP, in addition to BOD5 reduction and nutrient removal, also promote solar disinfection decreasing the need for chemical or electromechanical disinfection (Davies-Colley 2005) and associated GHG emissions.

9 Fertiliser Recovery

As discussed, algal biomass is high in nitrogen (7% of dry matter for a non-limited culture), phosphorus (about 0.8% of dry matter) and micronutrients, thus 1 kg of algae biomass contains on average 70 g of N and 8 g of P. Algae harvested from wastewater HRAP effluent would allow the recycling of these nutrients, and reduce fossil-fuel consumption required for ammonia fertiliser synthesis and phosphate rock mining (Metting and Pyne 1986; Oswald 1988a). Typically, the manufacture of 1 kg of ammonia fertiliser requires the equivalent of 16 kWh and the processing of 1 kg of phosphate fertiliser requires the equivalent of 4.5 kWh of energy (Wood and Cowie 2004). Moreover, the manufacture of 1 tonne of nitrogen (N) fertiliser will release 3.15 tonne CO2EQV from natural gas and the mining of 1 tonne of phosphate fertiliser will release 1.39 tonne CO2EQV (West and Marland 2001; Wood and Cowie 2004). Therefore the use of 1 kg of algae (7%N, 0.8% P) as fertiliser would reduce CO2 emissions from inorganic fertiliser manufacture by 0.23 kg CO2EQV kg−1 Algae. Thus even at 7% N content, the energy savings, and greenhouse gas abatement from the use of algae biomass biofuel residues as fertiliser would be equivalent to those from the use of the algae biofuel.

10 Conclusions

Algal biofuel production in combination with wastewater treatment using HRAP provides a niche opportunity for community-level algal biofuel production that has several advantages over other approaches. Wastewater treatment HRAP provide energy efficient and effective tertiary-level wastewater treatment with significant cost savings over electromechanical wastewater treatment technologies. Wastewater is an excellent growth medium (water, nutrients and buffering) for the growth of naturally occurring algae, especially when augmented with CO2 addition from biogas produced and used in the treatment plant. Harvested algal biomass is a by-product of the HRAP wastewater treatment plant and the wastewater treatment function essentially funds the capital and operation costs of algal production. As wastewater treatment HRAPs naturally select for productive colonial algae, low-cost algal harvest can be achieved through gravity settling. Algal harvest can be improved by promoting biofloculation/aggregation of the algal-bacterial biomass. Several pathways are available to convert the harvested algal biomass to biofuel, however, those that use the whole algal biomass and require little or no dewatering of the harvested algae appear to be most appropriate for use in combination with wastewater treatment. In particular, anaerobic digestion of algal biomass along with the settled wastewater solids makes economic sense as the capital and operation costs of anaerobic digestion and biogas use infrastructure may also be funded by the wastewater treatment plant. Additional financial and environmental incentives for wastewater treatment HRAP are from fertilizer recovery and GHG abatement. Harvesting algae from wastewater treatment HRAP effluent enables recovery of wastewater nutrients that can be recycled as fertilizer after biofuel conversion. Wastewater treatment HRAP also provide GHG abatement from a combination of low energy wastewater treatment, renewable fuel production and fertiliser recovery.

Since wastewater treatment HRAP systems are already a viable technology for efficient tertiary-level wastewater treatment, they provide a ‘testing ground’ to develop and refine full-scale algal production, harvest and biofuel conversion technologies that may be implemented in the future when higher fossil fuel costs make stand alone HRAP systems for biofuel production economical.