Dual role of microalgae: Phycoremediation of domestic wastewater and biomass production for sustainable biofuels production

Applied Energy 88 (2011) 3411–3424

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Dual role of microalgae: Phycoremediation of domestic wastewater and biomass production for sustainable biofuels production I. Rawat, R. Ranjith Kumar, T. Mutanda, F. Bux ⇑ Institute for Water and Wastewater Technology, Durban University of Technology, 19 Steve Biko Road, Durban 4000, South Africa

a r t i c l e

i n f o

Article history: Received 15 September 2010 Accepted 18 November 2010 Available online 18 December 2010 Keywords: Microalgae Phycoremediation Biomass Biorefinery Biofuel

a b s t r a c t Global threats of fuel shortages in the near future and climate change due to green-house gas emissions are posing serious challenges and hence and it is imperative to explore means for sustainable ways of averting the consequences. The dual application of microalgae for phycoremediation and biomass production for sustainable biofuels production is a feasible option. The use of high rate algal ponds (HRAPs) for nutrient removal has been in existence for some decades though the technology has not been fully harnessed for wastewater treatment. Therefore this paper discusses current knowledge regarding wastewater treatment using HRAPs and microalgal biomass production techniques using wastewater streams. The biomass harvesting methods and lipid extraction protocols are discussed in detail. Finally the paper discusses biodiesel production via transesterification of the lipids and other biofuels such as biomethane and bioethanol which are described using the biorefinery approach. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Microalgae are one of the most important bioresources that are currently receiving a lot of attention due to a multiplicity of reasons. The world is faced with energy challenges in the near future and it is reported that fossil fuel reserves will be depleted in half a century [1]. This will be an unprecedented vicissitude that will impact negatively on all anthropogenic activities most importantly agriculture, industry and commerce. With this in mind, it is crucial to explore renewable and cost-effective sources of energy for the future. It has been estimated that biomass could provide about 25% of global energy requirements and can also be a source of valuable chemicals, pharmaceuticals and food additives [2]. With the depletion and increase in prices of petrochemical fuels, the advent of innovative ways of generating biofuels using microalgae has the potential of off-setting these pertinent challenges [3]. In addition, the growing of urban population poses a serious threat to the environment due to the release of copious amounts of domestic municipal wastewater [4,5]. The use of microalgae is desirable since they are able to serve a dual role of bioremediation of wastewater as well as generating biomass for biofuel production with concomitant carbon dioxide sequestration [6,7]. In addition, wastewater remediation by microalgae is an eco-friendly process with no secondary pollution as long as the biomass produced is reused and allows efficient nutrient recycling [6,8,9]. ⇑ Corresponding author. Address: PO Box 1334, Durban 4000, South Africa. Tel.: +27 31 373 2346; fax: +27 31 373 2777. E-mail address: [email protected] (F. Bux). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.11.025

The release of industrial and municipal wastewater poses serious environmental challenges to the receiving water bodies [4,5]. The major effect of releasing wastewater rich in organic compounds and inorganic chemicals such as phosphates and nitrates is mainly eutrophication [5–8,10]. This is a global problem that can be solved by the use of microalgae whereby the wastewater is used as feed for microalgal growth. The advantage is that while the microalgae will be removing excess nutrients in the wastewater, there will be concomitant accumulation of biomass for downstream processing [8,9,11]. The use of a wide range of microalgae such as Chlorella, Scenedesmus, Phormidium, Botryococcus, Chlamydomonas and Spirulina for treating domestic wastewater has been reported and efficacy of this method is promising [7,12–14]. Research conducted by Chinnasamy et al. [12] demonstrated that a consortium of 15 native algal isolates showed >96% nutrient removal in treated wastewater. Biomass production potential and lipid content of this consortium cultivated in treated wastewater were 9.2–17.8 tons ha 1 year 1 and 6.82%, respectively. About 63.9% of algal oil obtained from the consortium could be converted into biodiesel [12]. There was a rapid decrease in the levels of metals, nitrates and phosphates after exposing the wastewater to microalgal treatment for short cultivation periods [14]. This clearly shows that microalgae are efficient at removing metals and nutrients from the wastewater to meet the stringent requirements according international standards. Domestic wastewater streams have been frequently used as a readily available and cost-effective substrate for microalgal growth for biomass production and nutrient removal [13–15]. The advantages of using microalgae for biodiesel production cannot be overemphasized [16]. Biodiesel can be generated from

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crops such as sugar cane, soybean, canola, rapeseed, maize, olive oil, non-edible jatropha, inter alia [2]. However the use of food crops for biofuels has generated much debate involving food security concerns. The main advantages of using microalgae as a source of biomass for biodiesel production are: high growth rates and short generation times, minimal land requirements, high lipid content, use of wastewater stream as nutrient feed with no need for chemicals such as herbicides and pesticides. However the main drawback of using oil producing microalgae for biomass production is that they are generally unicellular and are in suspension therefore very difficult to harvest [15]. In addition, lipid extraction procedures are complex and are at a developmental stage. Basically there are two main commercial cultivation systems for microalgae, open raceway ponds and closed photobioreactors [1,9,12].Microalgae growth in open raceway ponds is cheap and is also amenable to nutrient removal in domestic wastewater. Although they are associated with high volumetric productivities, the use of photobioreactors for phycoremediation is not feasible at large scale due to attendant problems of economics of scale [12].There is a wide array of harvesting methods that can be employed to harvest microalgal biomass such as centrifugation, flocculation, sedimentation and micro-filtration and any combination of these [9,16–18]. The use of sedimentation in combination with flocculation is reported to be cost effective due to minimal power consumption and use of gravity for biomass settling [9,18]. The downstream processing of the resultant biomass involves lipid extraction and transesterification of the oil into biodiesel and glycerol as the by-product. The biorefinery approach has introduced new vistas in the field of applied phycology in the sense that in addition to biofuel production, other value added products such as Docosahexaenoic acid (DHA) and carotenoids can also be produced from the biomass [16,19] The microalgal biomass can be used to produce a number of biofuels such as biomethane, bioethanol, biohydrogen, biobutanol among others [2,9,19].However, evaluation of various cultivation systems to grow an algal consortium in wastewater for phycoremediation and biofuel/bioenergy applications has not received much attention [12,15].Therefore the aim of this paper is to give an in-depth analysis and discussion on the current trends in terms of the dual role of the biotechnological application of microalgae for bioremediation of domestic municipal wastewaters and biomass production for biofuel production. Microalgal growth conditions, harvesting and downstream processing of the biomass as well as the biorefinery approach are discussed in detail.

2. Wastewater treatment methods An understanding of the nature of wastewater is essential in the design and operation of treatment processes. Water pollution has been in existence since time immemorial. Disposing of liquid and solid waste in rivers, streams, lakes and oceans seemed convenient for mankind. The quantities of wastewater at any point may ‘‘over load’’ the bio-system disrupting the natural recycling processes such as photosynthesis, respiration, nitrogen fixation, evaporation and precipitation. Wastewater treatment is an important initiative which has to be taken more seriously for the betterment of society and our future. Wastewater treatment is a process, where contaminants are removed from wastewater including domestic wastewater, to produce waste stream or solid waste suitable for discharge or reuse. Domestic wastewater is a combination of water and other wastes originating from homes, commercial and industrial facilities, and institutions. Untreated wastewater generally contains high levels of organic material, numerous pathogenic microorganisms, as well as nutrients and toxic compounds. Disposal of municipal solid wastes

(MSW) in sanitary landfills is usually associated with soil, surface water and groundwater contamination when the landfill is not properly constructed. At present, collection and treatment of landfill leachates is one of the most pressing issues surrounding the operation of landfill sites [20]. It thus entails environmental and health hazards, consequently, must immediately be conveyed away from its generation source(s) and treated appropriately before final disposal. The ultimate goal of wastewater management is the protection of the environment in a manner commensurate with public health and socio-economic concerns. Biological treatment is an important aspect of industrial and municipal wastewater treatment and reuse processes [21]. Table 1 shows the composition of untreated domestic wastewater levels. Wastewater treatment methods are broadly classified into three categories; there are physical, chemical and biological. Fig. 1 lists the unit operations included within each category. Among the first treatment methods used were physical unit operations, in which mechanical forces are applied to remove contaminants. Today, they still form the basis of most process flow systems for wastewater treatment. Chemical processes used in wastewater treatment are designed to bring about some form of change by means of chemical reactions. They are always used in conjunction with physical unit operations and biological processes. In general, chemical unit processes have an inherent disadvantage compared to physical operations in that they are additive processes, since there is usually a net increase in the dissolved constituents of the wastewater. This can be a significant factor if the wastewater is to be reused. Physical, chemical and biological methods are used to remove contaminants from wastewater. In order to achieve different levels of contaminant removal, individual wastewater treatment procedures are combined into a variety of systems, classified as primary, secondary, and tertiary wastewater treatment [22,23]. More rigorous treatment of wastewater includes the removal of specific contaminants as well as the removal and control of nutrients [24], natural systems are also used for the treatment of wastewater in land-based applications. Sludge resulting from wastewater treatment operations is treated by various methods, in order to reduce Table 1 Typical composition of untreated domestic wastewater. Source: Adapted from [21] Wastewater Engineering, 3rd edition. Contaminants

Total solids (TS) Total dissolved solids (TDS) Fixed Volatile Suspended solids Fixed Volatile Settleable solids BOD5, 20 °C TOC COD Nitrogen (total as N) Organic Free ammonia Nitrites Nitrates Phosphorus (total as P) Organic Inorganic Chlorides Sulfate Alkalinity (as CaCO3) Grease Total coliforms Volatile organic compounds

Unit

mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mL L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 mg L 1 No/100 mL lg/L

Concentration Weak

Medium

Strong

350 250 145 105 100 20 80 5 110 80 250 20 8 12 0 0 4 1 3 30 20 50 50 106–107 <100

720 500 300 200 220 55 165 10 220 160 500 40 15 25 0 0 8 3 5 50 30 100 100 107–108 100–400

1200 850 525 325 350 75 275 20 400 290 1000 85 35 50 0 0 15 5 10 100 50 200 150 107–109 >400

I. Rawat et al. / Applied Energy 88 (2011) 3411–3424

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Fig. 1. Common wastewater treatment processes.

its water and organic content and make it suitable for final disposal and reuse. Primary treatment consists of temporarily holding the sewage in a quiescent basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface [25]. The settled and floating materials are removed and the remaining liquid may be discharged or subjected to secondary treatment. Secondary treatment removes dissolved and suspended biological matter. Secondary treatment is typically performed by indigenous, water-borne microorganisms in a managed habitat. Secondary treatment may require a separation process to remove the microorganisms from the treated water prior to discharge or tertiary treatment [21,25]. Tertiary treatment is sometimes defined as anything more than primary and secondary treatment [26]. Treated water is sometimes disinfected chemically or physically (for example by chlorination or micro-filtration) prior to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the various irrigation purposes. If it is sufficiently clean, it can also be used for groundwater recharge or agricultural purposes. Biological treatment processes are considered the most environmentally compatible and the least expensive of wastewater treatment methods [27]. These processes use microorganisms to break down the chemicals present in municipal wastes, valorize the residues by the production of added-value compounds such as a diverse range of microbial-derived substances including biopolymers and biofuels. High organic loads and presence of some classes of antimicrobial or biostatic compounds such as phenols and lipids, low pH values, low water activity and unbalanced composition of nutrients represent barriers that should be overcome to achieve an optimal biological process [28].It has been appreciated for some years now that microalgae can be potentially utilized for low cost and environmentally friendly wastewater treatment compared to other more commonly used treatment processes [29]. The selection of microorganisms for use as alternative fuel sources require a sustainable growth medium such as domestic wastewater streams. The majority of wastewaters contain very high concentrations of nutrients, particularly total N and total P concentration as well as toxic metals, so there is no requirement for costly chemical-based treatments [30]. According

to de la Noue et al. [29] the concentration of total N and P can be found at values of 10–100 mg L 1 in municipal wastewater and >1000 mg L 1 in agricultural effluent. Microalgae have potential to treat wastewater by efficiently accumulating nutrients and metals from the wastewater. Sustainable low cost wastewater treatment has been strongly proven by using microalgae [5]. Microalgae grown on wastewater for energy production has been proposed for a long time [31–34]. However, in recent years, microalgae seem to be a favorite candidate for this purpose, due to their ease of cultivation and the favourable possibility of their use as an alternative biomass for bioenergy production. Increase in global warming, depletion of fossil fuel and the need for mitigation of green-house gas (GHS) emissions, make exploration of the feasibility of biological wastewater treatment by microalgae coupled to biofuel production, vital. 3. Wastewater characteristics – physico-chemical parameters Wastewater quality may be defined by its physical, chemical, and biological constituents of wastewater and their sources are listed in Fig. 2. 3.1. Physical characteristics Wastewater temperature is important as it affects the chemical and biological reactions of aquatic organisms. Abnormally high temperature can increase undesirable planktonic species and fungi. Temperature is also very important in determination of various parameters such as pH, conductivity, saturation level of gases and various forms of alkalinity, etc. The colour of domestic wastewater is usually indicative of its age. Domestic septic tanks take on a black appearance due to biological reactions of organic and inorganic materials. The appearance of industrial wastewater depends upon the nature of the product manufactured. Odours present in wastewaters, are mainly due to dissolved impurities, often of organic nature caused by living and decaying aquatic organisms and accumulation of gases. The most characteristic odour of septic wastewater is that of hydrogen

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Domestic and industrial wastes, natural decay of organic materials.

Domestic water supply,Domestic and industrial wastes, soil erosion, inflowinfiltration.

Decomposing wastes water, industrial wastes Odour

Solids Colour

Domestic and industrial wastes

Physical properties

Temperature

Organic: Carbohydrate - Domestic, commercial and industrial wastes Fats, oil & grease - Domestic, commercial and industrial wastes Pesticide – Agriculture wastes, Phenols – Industrial wastes Protein – Domestic & industrial wastes Surfactants- Domestic & industrial wastes, Others – Natural decay of organic materials.

Chemical constituents

Wastewater Characteristics and their resources

Inorganic: Alkanity – Domestic waste & water supply, groundwater filtration. Chlorides – Domestic waste & water supply, groundwater. Filtration, water softeners. Heavy metals & pH - Industrial wastes Phosphorus - Domestic & industrial waste, natural runoff. Sulfur – Domestic water supply, Domestic & industrial waste. Toxic compounds – Industrial wastes.

Gases: Hydrogen sulfide & Methane – Decomposition of domestic wastes. Oxygen – Domestic water supply, surface – water infiltration.

Open watercourses, treatment plants.

Biological constituents

Animals

Open watercourses, treatment plants.

Viruses

Plants

Domestic wastes Protista

Domestic wastes, treatment plants. Fig. 2. The physical, chemical, and biological characteristics of wastewater and their resources modified from [21].

sulfide, which produced by anaerobic microorganisms. The total solids content is denoted by various types of dissolved and suspended material that remains as residues in wastewater [35].

3.2. Chemical characteristics Organic material is normally a combination of carbon, hydrogen and oxygen and other important elements such as sulfur, phosphorous, irons and ammonia. Occurrence of ammonia in the wastewater can be accepted as the chemical evidence of organic pollution [36]. Sewage wastewater has large quantities of nitrogenous matter. The principal components of wastewater are proteins, carbohydrates, lipids, oils and urea and small quantities of several synthetic organic chemicals. The common inorganic compounds in wastewater are chloride, hydrogen, irons, nitrogen, phosphorus, sulfur and trace amounts of heavy metals [37].

3.3. Biological characteristics Biological parameters are perhaps of greatest importance to wastewater treatment. Naturally, wastewater contains large amounts of macro and microscopic organisms. The quantity of any species of micro and macro organism and aquatic animals in a receiving body of wastewater determines treatment effectiveness. Within treatment facilities, wastewater provides an ideal medium for potential microbial growth, irrespective of being anaerobic or aerobic wastewater treatment [38].

4. Phycoremediation Phycoremediation may be defined in a broad sense as the use of macroalgae or microalgae for the removal or biotransformation of pollutants, including nutrients and xenobiotics from wastewater

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and CO2 from waste air with concomitant biomass propagation [6,7,15,39].There are numerous processes of treating water, industrial effluents and solid wastes using microalgae aerobically as well as anaerobically. Remediation is generally subject to an array of regulatory requirements, and also can be based on assessments of human health and ecological risks where no legislative standards exist. The term phycoremediation was introduced by John [40] to refer to the remediation carried out by algae. The use of algae to treat wastewater has been in vogue for over 40 years, with one of the first descriptions of this application being reported by Oswald [41]. The use of microalgae for the treatment of municipal wastewater has been a subject of research and development for several decades [42,43]. Extensive work has been conducted to explore the feasibility of using microalgae for wastewater treatment, especially for the removal of nitrogen and phosphorus from effluents [33,44–47], which would otherwise result in eutrophication if dumped into lakes and rivers [48]. It is simply a matter of allowing the consumption of nitrogen and phosphorus by microalgae in a controlled manner that benefits rather than deteriorates the environment. Concentrations of several heavy metals have also been shown to be reduced by the cultivation of microalgae, which is a subject discussed extensively by [9]. Biological treatment enhances the removal of nutrients, heavy metals and pathogens and furnish O2 to heterotrophic aerobic bacteria to mineralize organic pollutants, using in turn the CO2 released from bacterial respiration (Fig. 3) [9]. Photosynthetic aeration is therefore especially interesting to reduce operation costs and limit the risks for pollutant volatilization under mechanical aeration and recent studies have shown that microalgae can indeed support the aerobic degradation of various hazardous contaminants [9,49]. The mechanisms involved in microalgae nutrient removal from industrial wastewaters are similar to that from domestic wastewaters treatment. Phycoremediation comprises several applications: (i) nutrient removal from municipal wastewater and effluents rich in organic matter; (ii) nutrient and xenobiotic compounds removal with the aid of algae-based biosorbents; (iii) treatment of acidic and metal wastewaters; (iv) CO2 sequestration; (v) transformation and degradation of xenobiotics; and (vi) detection of toxic compounds with the aid of algae-based biosensors. Nutrient removal with the aid of microalgae compares very favourably to other conventional technologies [50].

accumulation and conversion of wastewater nutrients to biomass and lipids. The capability of microalgae to degrade hazardous organic pollutants is well known. Chlorella, Ankistrodesmus and Scenedesmus species have been already successfully used for the treatment of olive oil, mill wastewaters and paper industry wastewaters [38,51–53]. One way to investigate the capability of algae to biodegrade organic pollutants in municipal waste is to encourage the cells to grow in the presence of the pollutants [54,55] and findings showed that both cyanobacteria (blue-green algae) and eukaryotic microalgae were capable of biotransforming naphthalene to four major metabolites, 1-naphthol, 4-hydrox-4-tetralone, cis-naphthalene dihydrodiol and trans-naphthalene dihydrodiol at concentrations which were non-toxic. The formation of cisnaphthalene dihydrodiol was the first demonstration in a eukaryotic cell Pinto [56] showed the capability of the two green algae, Ankistrodesmus braunii and Scenedesmus quadricauda, to remove over 50% of the low molecular weight phenols contained in olive mill waste when grown in dark condition. Lima et al. [57] reported p-nitrophenol removal of 50 mg L 1 d 1 by a consortium of Chlorella vulgaris and Chlorella pyrenoidosa under un-optimized conditions, which was close to the 100 mg L 1 d 1 achieved with Pseudomonas species by [58]. Examples given in Table 2 demonstrate that algae are indeed capable of contributing to the degradation of environmental pollutants, either by directly transforming the pollutant in question or by enhancing the degradation potential of the microbial community present. The biomass resulting from the treatment of wastewaters can be easily converted into added value products. Depending by the species used for this purpose, the resulting biomass can be applied for different aims, including the use as additives for animal feed, the extraction of added value products

Table 2 Microalgae contributing to the degradation of environmental pollutants. S. no

Microalgae

Aquatic microalgae

Types of wastewater

Reference

1

Prototheca zopfii

Fresh water

[106]

2

Chlamydomonas species Chlorella pyrenoidosa Chlorella sp.

Fresh water Fresh and brackish Fresh and marine water Fresh water

Degraded petroleum hydrocarbons found in Louisiana crude and motor oils waste Meta cleavage in wastewater Degradation of azo dyes wastewater Anaerobic digested dairy waste Olive oil mill wastewaters and paper industry wastewaters.

[38,51,52,101]

Domestic wastewater treatment

[110]

Wastewater treatment under aerobic dark heterotrophic conditions Secondarily treated sewage in batch and continuous cultures Removal of ammonia from anaerobic digestion effluent containing high levels of ammonium and alkalinity

[71]

3 4

4.1. Microalgae used in phycoremediation The growth of microalgae is indicative of water pollution since they respond typically to many ions and toxins. Blue-green algae are ideally suited to play a dual role of treating wastewater in the process of effective utilization of different constituents essential for growth leading to enhanced biomass production. The release of free oxygen is of major significance in organically enriched wastewater, promoting aerobic degradation processes by and other microorganisms. Secondly the role of microalgae is the

Fig. 3. Principle of photosynthetic oxygenation in BOD removal process [9].

5

6

Ankistrodesmus and Scenedesmus Scenedesmus quadricauda Spirulina platensis

Freshwater and brackish water Freshwater

7

Chlorella sorokiniana

8

Botryococcus braunii

Freshwater

9

Scenedesmus

Freshwater

[107] [108] [109]

[111,112]

[1,113]

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like carotenoids or other bio-molecules or the production of biofuel. In addition to the apparent benefit of combining microalgal biomass, and therefore biofuel, production and wastewater treatment, successful implementation of this strategy would allow the minimizing of the use of freshwater, another precious resource especially for dry or populous countries, for biofuel production. Algae collection worldwide contain thousands of different algae strains that, combined with recent advances in genetic engineering and material science, provide a good starting point for further development of microalgal biofuel production systems based on wastewaters treatment. Strain selection and characterization, as well as the breeding and adaption of strains with desirable phenotypes that allow the use of water resources of varying water quality, also play important roles in the perspective of using microalgae for biofuel production. Future algal strain improvement will utilize methodologies such as lipidomics, genomics, proteomics and metabolomics to screen for and develop new strains that exhibit high growth and lipid biosynthesis rates, broad environmental tolerances and the ability to produce high value by-products. 4.2. Advantages and challenges of phycoremediation Microalgae play an important role during the tertiary treatment of domestic wastewater in maturation ponds or the treatment of small to middle-scale domestic wastewater in facultative or aerobic ponds. Nitrogen uptake could be increased if the microalgae were preconditioned by starvation [29,38,59–61]. These hyper concentrated algal cultures, called ‘activated algae’ were shown to decrease the land and space requirements for microalgal treatment of wastewaters. This process removed nitrogen and phosphorus within very short period of time i.e., less than 1 h [50]. There is evidence that production of microalgae, given proper conditions, may be high enough even during colder periods to be of interest for wastewater treatment. However, this is to be verified under the actual local environmental conditions, since many strongly variable factors are involved when defining micro algal growth and species composition. Microalgae can be efficiently used to remove significant amount of nutrients because they require high amounts of nitrogen and phosphorus for protein (45–60% of microalgae dry weight), nucleic acid and phospholipid synthesis. Nutrient removal can also be further increased by NH3 stripping or NH3 precipitation due to the raise in the pH associated with photosynthesis [62–65]. This method is not appropriate for large scale wastewater treatment, therefore there is need to improve the technology.

They are shallow (0.3–0.6 m) in order to allow maximum light penetration. They can operate at short hydraulic retention time (HRT) in the range of 4–10 days depending on climatic conditions reducing the required surface area. Continuous mixing is provided to keep the cells in suspension and to expose them periodically to light. The most common design that has proven successful at large scale is the single loop paddlewheel mixed. Due to energy cost dependence on velocity, most ponds have been operated at velocities from l0 to 30 cm/s [67]. More recently, a special flow pattern was introduced to improve the efficiency of this type of ponds [68].HRAPs are by far the most cost-effective reactors available for liquid waste management and for efficient capture of solar energy [69]. The AIWPS system designed by Oswald LLC, consumes approximately 0–0.57 kW h/kg BOD. In contrast, mechanical aerated ponds consume a much higher amount of energy in the range of 0.80–6.41 kW h/kg BOD removed.

4.3.1. Advantages of high rate algal pond treatment HRAPs are the most cost-effective reactors for liquid waste management and capture of solar energy, and are used to treat waste from pig farms. In these systems, productivities of up to 50 t ha 1 y 1 are feasible. Algal biomass can be harvested from the HRAPs for animal feed, and can be seen as a component of integrated approaches to recycling of livestock wastes, in which algal wastewater treatment is a second step following an initial anaerobic treatment of high-strength organic wastewater [7,71]. Biofuel production in conjunction with wastewater treatment and fertiliser recycling is seen as a near-term application (5–10 years), since the algae are already used in wastewater treatment (Table 3) [72]. The HRAP showed remarkable performance and behaviour when compared to a series of three facultative ponds such as anaerobic, aerobic and alternative constructed facultative ponds on the same site by receiving the same sewage [73]; see also [74]. The HRAP is a photosynthetic reactor in which microscopic, photosynthetic algae live together with heterotrophic bacteria which degrade the sewage organic matter. This co-habitation was often called the HRAP symbiosis and represents the central idea of Oswald’s concept using the HRAP as a combined secondary/tertiary system for sewage treatment. Treatment of tannery effluent in a custom-designed high rate algal pond process, and its use as a carbon source in the generation

4.3. High rate algal pond (HRA) The three general types of maturation ponds employed in wastewater treatment are facultative ponds and anaerobic ponds the most common waste stabilisation ponds. Anaerobic ponds are several meters deep, free of dissolved oxygen and have high BOD removal rates [25]. The facultative ponds exhibit aerobic conditions on the surface due to photosynthetic oxygen production by algae and anaerobic conditions in the bottom layers and are the most common form of oxidation ponds. Aerobic ponds, also known as high-rate ponds, are shallow and completely oxygenated throughout [66]. Algal high-rate ponds (HRPs) were developed beginning in the 1950s as an alternative to unmixed oxidation ponds for BOD, suspended solids, and pathogen removal. HRPs have been designed to be shallow (30–100 cm) with a raceway shape and include a large paddle wheel vane pump to create a channel velocity sufficient for gentle mixing (Fig. 4). In contrast to facultative ponds, HRAPs are designed to promote algae growth.

Fig. 4. Schematic representation of the cascading raceway pond [70].

I. Rawat et al. / Applied Energy 88 (2011) 3411–3424 Table 3 Some of the microalgae dominant to treat wastewater and grow biomass in high rate algal pond (HRAP). HRA

Microalgae

References

Dominant microalgae in HRAP wastewater treatment

Macrocystis Pediastrum sp. Chlorophyta, Euglenophyta, Euglena, Chlamydomonas, Cyanophyta and Chrysophyta and Oscillatoria sp. Scenedesmus sp. and Micractinium sp. Pediastrum sp., Actinastrum sp. and Dictyosphaerium sp.

[114] [115,116] [116]

[117,61] [118]

and precipitation of metal sulfides, has been demonstrated through pilot scale to the implementation of a full-scale process [75]. The treatment of both mine drainage and zinc refinery wastewaters has been reported. A complementary role for microalgal production in the generation of alkalinity and bio-adsorptive removal of metals has been utilized and an Integrated ’’Algal Sulphate Reducing Ponding Process for the Treatment of Acidic and Metal Wastewaters’’ (ASPAM) has been described by Rose et al. [75]. 4.3.2. Limitation of high rate algal pond treatment Despite high-rate ponds providing promising wastewater treatment method by reducing bacteria, BOD, and nutrients; the effluent is rich in small microalgae (less than 10 lm) due to slow separation in the liquid phase. HRAP treatment faces problems like a dynamic equilibrium between algae net oxygen production, bacterial respiration due to low light penetration, rich microalgal biomass in HRAP, climate conditions and proper mixing [76]. According to Tilzer [54] HRA ponds require good amount of light, however light penetration is limited by increase in cell density. This limits the absorption of photosynthetic active radiation (PAR). PAR is affected by: (i) light density by PFD (Photo flux density) present at a given depth and (ii) capacity of algae to absorb photons. These drawbacks of HRAP by PFD impinge on the system besides diurnal effects to treat wastewater and increasing biomass productivity. Disadvantages of HRAP treatment are ever increasing solids handling, seasonality of algae growth, and a short track record for bioflocculation/settling of algae. Research has been done to counteract the effect of these disturbances in terms of stability and productivity of HRAP [77]. Another limitation of HRAP wastewater treatment is the presence of predatory zooplankton and protozoa that graze on beneficial microalgae and can reduce microalgal growth in HRAP within few days [72,78]. Fungal parasitism and viral infection can also significantly reduce the algal population within a few days in HRAP and trigger changes in algal cell structure, reduction of algal chlorophyll a, diversity and microalgal succession [79].HRAP treatment system is potentially viable method for wastewater treatment, however problems exist and it is difficult to predict clear solutions.

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4.4.1. Open raceway ponds Open raceway pond system is a cost-effective microalgal cultivation method for growing microalgae for nutrient removal in domestic wastewater for biomass production in excess of 0.5 g/L in some commercial raceway ponds [9,80,81]. However, the biomass concentration remains low because raceway ponds are poorly mixed and cannot sustain an optically dark zone [1].The operation of this outdoor culture open system for biomass production is easy and requires minimal maintenance [9,70,82]. The system has few operating costs, minimal power consumption and little overheads as compared to photobioreactors [1,83,84]. The advantage of this method is that readily available domestic municipal wastewater can be used as media for cultivation with the added benefit of bioremediation. In addition if the system is located near a power plant, cheaply available flue gas can be used to speed up the photosynthetic rates in the pond or pure carbon dioxide can be bubbled into the pond [85]. In order to avoid contamination from debris and rainfall, the raceway pond can be covered similar to the agricultural greenhouse concept. This allows maximum sunlight intensity. The depth of water in the pond should not exceed a maximum level of 30 cm to allow sufficient light penetration. The cascading system is more efficient than the single channel raceway pond due to prolonged mixing and extended retention times [9,70] Fig. 4. In order to access the physiological status of the microalgal cells, the following parameters must be closely monitored in the raceway pond: cell density, conductivity, pH, light intensity, temperature, salinity, evaporation rates, dissolved carbon dioxide, dissolved oxygen, TDS, ORP, nitrate and phosphate levels [16].Harvesting and dewatering of the microalgae can be performed by a micro-filtration system, settling after adding flocculants or by centrifugation which is however power intensive and therefore expensive at large scale [17,18,83]. After harvesting, drying of biomass can be achieved by cheaply using natural sunlight on drying beds. 4.4.2. Photobioreactors Photobioreactors (PBR) permit the monoculture growth of microalgae for extended periods as compared to open raceway ponds that are prone to contamination by other microflora and potential system failure [1,9,16,83]. The main advantage of photobioreactors is that they can produce large amounts of biomass [1,83]. The tubular PBR is the common design widely employed for microalgal cultivation since it is a continuous system. However, there are different designs and configurations of this PBR where the tubes can be arranged either vertically or horizontally for maximal solar capture [1,9]. It is important to stress that photobioreactors are not ideal for large scale phycoremediation due to the large volumes requiring remediation however they are important bioreactors for small scale microalgal cultivation [87]. At small scale operations, microalgae-based systems can significantly reduce both organic matter and nutrients present in piggery wastewaters at minimal energy cost in simple solar-powered photobioreactors [10]. 4.5. Microalgal growth conditions in High rate algal ponds (HRAP)

4.4. Bioreactor design for wastewater nutrient removal Successful nutrient removal and sufficient microalgal biomass accumulation require careful growth chamber design. There are a variety of bioreactors that can be used for the dual purpose of removing nutrients from wastewaters with simultaneous biomass propagation such as open raceway ponds, photobioreactors, open maturation/oxidation ponds, vertical tank reactors and polybags [9,12]. However the engineering of efficient microalgal growth chambers for nutrient removal and biomass production is an ongoing exercise [2].

High rate algal ponds (HRAPs) are operated for the treatment of wastewaters and production of microalgal biomass as a by-product [3,86]. The algal biomass produced and harvested from these wastewater treatment systems could be converted through various pathways to biofuels, for example anaerobic digestion to biogas, transesterification of lipids to biodiesel, fermentation of carbohydrate to bioethanol and high temperature conversion to bio-crude oil [3,9]. Phycoremediation using HRAPs may effectively replace the conventional tertiary treatment used for nutrient removal

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[10,15,86,87]. The advantage is that microalgae release oxygen during photosynthesis and no mechanical aeration of ponds is required (only paddle wheels for moderate mixing) for heterotrophic microbial organic matter degradation. In contrast, conventional tertiary treatment has a relative cost four times more than a conventional primary treatment [39]. HRAPs are shallow ponds in order to allow maximum sunlight penetration for microalgal photosynthesis and the capacity ranges from 1000 to 5000 m2 in large-scale applications [10,39,87]. It is desirable to identify growth conditions that induce high levels of biomass and neutral lipids (particularly triacylglycerol) such as nutrient limitation such as nitrogen and phosphorus stress [11]. 5. Biomass production – from wastewater effluent The mass production of algae has historically been for use as a food supplement or wastewater treatment [88]. The technology for production of biomass from wastewater has been present since the 1950s. Microalgae are efficient in the removal of nutrients from wastewater. Thus many microalgal species proliferate in wastewater due to the abundance of carbon, nitrogen and phosphorus that act as nutrients for the algae. Unicellular algae have shown great efficiency in the uptake of nutrients and have been found to show dominance in oxidation ponds [11]. Application of using wastewater for the production of biomass however, occurs only on a minor scale and generally in the form of waste stabilisation ponds or high rate algal ponds for the treatment of wastewater [11,88]. It has been suggested that growth of algae should focus on biomass productivity rather than lipid productivity as is the current thrust in the algal biofuels sector [89]. Large amounts of biomass produced improve the viability on conversion of biomass to alternate biofuels [11]. Consensus amongst researchers is that there is requirement of optimization of growth, lipid and harvesting at large scale. Production of algal biomass provides an efficient method of nutrient recycling not available via conventional wastewater treatment, biomass could be used as fertiliser or animal feed following the extraction of lipid [11]. Production of biomass from wastewater requires, similar production of biomass on artificial media, depends on a number of factors. However factors of heavy metal contamination require greater attention than in conventional production from media. Park et al. [3] has recorded the following to be desirable attributes of microalgal species for use in HRAPs, (1) High biomass productivity when grown on wastewater, (2) tolerances to seasonal and diurnal variation in outdoor conditions, (3) form aggregates to enhance ease of harvesting, (4) accumulation of high amounts of lipid or other valuable products. Algal biomass has the ability to accumulate heavy metals and may require desorption of metal before further processing after lipid extraction. Wastewater often contains nitrogen in the form of ammonia, which in high concentrations may be inhibitory to algal growth. Other considerations are pathogenic bacteria, predatory zooplankton and other grazers [3,11]. Laboratory studies have shown the potential for moderate lipid production (less than 10–30% DW) and high lipid productivity (up to 505 mg L 1 day 1) of algae grown on wastewater (Table 4). This suggests the potential of lowering the cost of algal biofuels production, which is currently not economically feasible [11,89]. 5.1. Harvesting strategies Effective wastewater treatment and the production of biomass for biofuels require separation of biomass from water. The selection of harvesting method is of great importance to the economics

of biofuels production as harvesting can make up 20–30% of the total cost of production. Selection of the harvesting method depends strongly upon the characteristics of the culture grown [90]. Algae that are suitable for the remediation of wastewater and production of biofuels tend to be of unicellular forms of low density. This makes economical biomass harvesting difficult and the cost of biomass recovery significant [1,3,11,89,90]. Methods of biomass recovery include filtration, centrifugation, sedimentation and flocculation and floatation [16]. Continuous centrifugation is the preferred method for biomass separation as it is rapid, efficient and universal [16]. This is not economically feasible for large scale harvesting due to it process being highly energy intensive [11]. Gravity sedimentation is a common method of harvesting biomass. The process is rudimentary but works for various types of algae and is highly energy efficient. Filtration is another method commonly used. Both sedimentation and filtration may be used in conjunction with flocculation. Conventional flocculation works by the mechanism of dispersion of charge. Microorganisms are generally negatively charged. The addition of metal salts act to displace the charge and allow aggregation of the organisms thus allowing more efficient sedimentation or filtration to occur. Alum, as generally used in conventional wastewater treatment can suitably flocculate algal biomass, however it may impede oil extraction depending on the dominant algal strain being cultivated [91]. Flocculation may also be achieved by the by use of cationic polymers or the addition of alkali substances to increase the pH [90]. Use of cationic polymers is expensive and will likely adversely affect techno-economics. Autoflocculation is the spontaneous aggregation of particles, resulting in sedimentation of the microalagae. This may occur, or can be induced by limitation of carbon or certain abiotic factors [11]. Filtration is a method commonly used for solid–liquid separation. Vacuum filtration is effective in the recovery of larger algae (greater than 70 lm) when used in combination with a filter aid. Smaller algae however require the use of membrane micro-filtration or ultrafiltration for effective harvesting [90]. These methods tend to be costly, energy intensive and requires frequent membrane replacements (as a result of fouling) and pumping of the biomass [11]. Immobilization of the microalgal culture provides a ready to retrieve alternative for biomass retrieval. Immobilization is the artificial attachment or encapsulation in alginates or similar substances. This may only be suitable for the production of biomass and prevent the decrease in growth rate a high flow rate is required. This type of setup has been shown to take as much nutrients as that suspended algae. Immobilized biomass can be used for biofuel conversion by thermal or fermentative means. Lipid extraction efficiency and effects of the immobilization on lipid is yet to be determined [11]. Floatation of algal biomass has shown promise in terms of harvesting smaller and unicellular algae in laboratory scale trials. Ozonation-dispersed floatation and floatation employing a method of creating charged bubbles [92,93]. The mechanism of action is interaction with the negatively charged hydrophilic surfaces of algal cells. It is to be noted that the trial was carried out with washed cells [92]. Ozonation-dispersed flotation of cells grown in open pond culture may prove challenging due to contamination. Lipid content of C. vulgaris, harvested by ozonation-dispersed flotation should an increase from 31% to 55% in the flotation stage according to Cheng et al. [92]. An additional benefit of the use of ozone is its ability to cause cell lysis. Lysis of cells release biopolymers that act as coagulating agents thus enabling more effective separation. Algal cell lysis may also serve to enhance the extraction of lipid. A disadvantage of ozonation-dispersed flotation is an expensive process. The effectiveness of dissolved air flotation may be increased by reduction of the negative charge carried by the air bubbles. This

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I. Rawat et al. / Applied Energy 88 (2011) 3411–3424 Table 4 Comparison of biomass and lipid productivities grown on various wastewater [11]. Wastewater type

Microalgae species

Biomass (DW) productivity (mg L 1 day 1)

Lipid content (%DW)

Lipid productivity (mg L 1 day 1)

References

Municipal (primary treated) Municipal (centrate)

nd Chlamydomonas reinhardtii (biocoil-grown) Scenedesmus obliquus Botryococcus braunii Mix of Chlorella sp., Micractinium sp., Actinastrum sp. B. braunii Chlorella sp. Scenedesmus sp. Mix of Microspora willeana, Ulothrix zonata, Ulothrix aequalis Rhizoclonium hieroglyphicum, Oedogonium sp. R. hieroglyphicum R. hieroglyphicum

25a 2000

nd 25.25

nd 505

[119] [120]

26b 345.6c 270.7d

31.4i 17.85 9

8i 62 24.4

[121] [122] [123]

nd 9i 0.9i nd

69 230i mg m 0.54i nd

0.7i 1.2i

72i mg m 2 day 1 210i mg m 2 day 1

[127] [127]

81.4g 59h

13.6i 29

11i 17

[14,128] [123]

34 23 28 33 126.54

13.20 18.10 15.20 12.00 12.8

4.5 4.2 4.3 4.0 16.2

[12] [12] [12] [12] [129]

Municipal (secondary treated) Municipal (secondary treated) Municipal (primary treated + CO2) Agricultural Agricultural Agricultural Agricultural

(piggery manure with high NO3–N) (dairy manure with polystyrene foam support) (fermented swine urine) (anaerobically digested dairy manure)

Agricultural (swine effluent, maximum manure loading rate) Agricultural (daily effluent + CO2. maximum manure loading rate) Agricultural (digested dairy manure, 20 dilution) Agricultural (dairy wastewater, 25% dilution) Industrial (carpet mill, Industrial (carpet mill, Industrial (carpet mill, Industrial (carpet mill, Artificial wastewater

untreated) untreated) untreated) untreated)

Chlorella sp. Mix of Chlorella sp., Micractinium sp., Actinastrum sp. B. braunii Chlorella saccharophila Dunaliella tertiolecta Pleurochrysis carterae Scenedesmus sp.

700e 2.6 gm 6f 5.5 gm

2

day

1

2

day

1

10.7 gm 17.9 gm

2 2

day day

1 1

2

day

1

[124] [107] [125] [126]

nd – not determined; DW – dry weight. a Estimated from biomass value of  1000 mg L 1 after 40 days. b Estimated From biomass value of 1.1 mgL 1 h 1. c Estimated from biomass value of 14.4 mg L 1 h 1. d Estimated from biomass value of 812 mg L 1 after 3 days. e Estimated from biomass value of 7 g L 1 after 10 days. f Estimated from biomass value of 197 mg L 1 after 31 days. g Estimated from biomass value of 1.71 g L 1 after 21 days. h Estimated from lipid productivity and lipid content value. i Fatty acid content and productivity determined rather than total lipid.

may be achieved by the addition of a cationic surfactant or other chemicals to give a net positive charge [93]. 5.2. Lipid extraction methods Extraction of microalgal lipid is central to the production of biodiesel from microalgae. Lipid extraction is performed by chemical methods in the form of solvent extractions, physical methods or a combination of the two. Extraction methods used should be fast, effective and non-damaging to lipids extracted and easily scaledup [94]. Extraction by using a modified Bligh and Dyer [95] method is the most commonly used [16]. The choice of solvent for lipid extraction, as with harvesting, will depend on the type of microalgae grown. Other preferred characteristics of the solvents are that, they should be inexpensive, volatile, non-toxic and non-polar and poor extractors of other cellular components. Lipids have different types of associations which need to be disrupted for effective extraction. Hydrophobic interactions in non-polar/neutral lipids are disrupted by non-polar organic solvents; hydrogen bonding in polar lipids are disrupted by polar organic solvents such as alcohols. Change in pH towards more alkaline is useful for the disruption of stronger ionic forces that may be present [94]. Pre-treatment of samples may be required for oil extraction of certain types of biomass. This is generally not necessary for extraction from wet biomass, as solvents generally rupture the cells. When cell disruption is required, it may be accomplished by sonication, homogenisation, grinding, bead beating or freezing [94]. Other methods for cell disruption include autoclaving, osmotic shock, microwaving and freeze drying [16]. The selected method

of cell disruption will be determined by the type of biomass, state of biomass and scale that needs to be used. Direct esterification, simultaneous extraction and transesterification of microalgal fatty acids can be performed on both wet and dry biomass making it a versatile method of biofuels production. The process is multistep and requires a combination of solvent extraction, ultrasonication, heating at high pressure (3.5 atm), filtration, density separation of liquids and solvent and oil recovery by evaporation to dryness [96]. 5.3. Lipid identification methods Algae storage lipids differ from strain to strain and even within a single culture under different growth conditions. It is necessary to identify lipids as the lipid fraction will dictate the properties of the biodiesel produced. Lipid qualification and quantification can be carried out by several means including Nile red fluorescence microscopy, Nile red spectrofluorometry, Fourier transform infrared micro-spectroscopy (FTIR), Thin-layer chromatography (TLC), high pressure liquid chromatography (HPLC) or gas chromatography (GC) or any chromatography with mass spectrometry [16,94]. Nile red microcopy is used primarily to ascertain presence of lipid vesicles within cells as an initial screen for lipid accumulation and as a semi-quantitative method for lipid storage. Nile red spectrofluorometry can also be used as a semi-quantitative method of lipid content determination. These methods however give no indication as to the lipid fraction and type of lipid present. FTIR can be used for determination and quantification of lipid and carbohydrate storage and is an efficient tool for monitoring of lipid.

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Microalgal lipid profiling is generally done by gas chromatography with flame ionization detector (CG-FID) and is carried out using the methylated ester form of the lipid [16]. HPLC is a technique not commonly used, however as most fatty acids do not absorb UV at 254 nm (the wavelength commonly used in HPLC instruments) and requires conjugation of unsaturated fatty acids or acids containing aromatic moieties that allow easy detection [94]. 5.4. Transesterification Raw microalgal oil is high in viscosity, thus requiring conversion to lower molecular weight constituents in the form of fatty acid alkyl esters. Transesterfication is the process of converting raw microalgal lipid (triacyleglycerols/free fatty acids) to give renewable, non-toxic and biodegradable biodiesel (Fig. 5). The reaction is reversible and thus requires the supply of excess alcohol to maintain equilibrium shift towards the product and improve reaction rate [97]. Transesterification is a reaction of the parent oil with a short chain alcohol, usually methanol, in the presence of a catalyst. Products of the reaction are fatty acid methyl esters (FAME) and glycerol [16]. Catalysts that take part in the reaction are acids, bases or enzymes. Base catalysis is a faster reaction but is limited by the free fatty acids content. Free fatty acids contents in the region of 20–50% is responsible for saponification during base catalysed transesterification. Saponification is responsible for consumption of the base catalyst as well as making downstream recovery difficult [98]. Acid catalysis is suitable for transesterification of oils containing high levels of free fatty acids [99]. The reaction however is slow. Speeding up the acid catalysed reaction requires an increase in temperature and pressure making it prohibitively expensive at large scale [16]. The presence of moisture in the reaction is also responsible for saponification and reagents must therefore by dry [97]. Chemical catalysed transesterification does have its disadvantages in that the process is energy intensive, the catalyst needs to be removed from the product, alkaline water from washing requires remediation, water and free fatty acids result in loss of product due to saponification and recovery of glycerol is difficult [100]. Enzymatic catalysed or biocatalysed esterification is a viable method for parent oils containing high levels of free fatty acids as they can also be converted to alkyl esters (Fig. 6). Other benefits include moderate reaction conditions thus less energy intensive, lower alcohol to oil ratio requires for production and easier product recovery [100]. For large scale production this may not be economically viable due to high enzyme production costs and does not run the reaction to completeness [99,100].

Fig. 5. Transesterification of triacyleglycerol with alcohol in the presence of acid, base or enzyme catalyst to give Fatty acid methyl esters and glycerol [16,100].

Fig. 6. Flow diagram of enzyme mediated alcoholysis for FAME production [97].

Enzyme catalysis takes place in the form of immobilised lipase, whole cell catalyst and liquid lipase mediated. Immobilised lipase catalysis as the name suggests, employs suitable immobilised extracellular lipases as opposed to free lipases due to increased stability and the potential for repeated use without the requirement of complex separation. Glycerol produced during the alcoholysis is insoluble in oil and readily adheres to the immobilised lipase surface thus diminishing enzyme activity. Removal of glycerol can be a complex process and may impede the continuity of larger scale operations [100]. Whole cell biocatalysis utilizes intracellular lipases and is seen as a method of reducing lipase costs by negating the need for isolation, purification and immobilisation required in conventional immobilised extracellular lipases. Whole cells can be immobilised during cultivation of the organism and stabilised to allow prolonged use as a biocatalyst. Whole cell biocatalysts however, are prone to mass transfer resistance and may require pre-treatments to overcome this. Further research is required into the use of whole cell biocatalysts engineered to overproduce intracellular lipase for biodiesel. The reduction in cost given by this technology is a promising prospect for industrial biodiesel production [100]. Liquid lipase mediated catalysis is a promising prospect due to ease of preparation and thus lower cost. Liquid lipase mediated catalysis is conducted in a water containing system thus aiding recovery of the enzyme. Further research needs to be carried out on feedstocks used for production as well as enzyme recovery [100]. The use of oxide nanoparticles as catalysts for esterification of triacylglycerols from soybean oil has been tested for their efficiency as catalysts and have shown efficiency of up to 89% conversion. Metal oxide nanoparticles can have acidic or basic properties depending on the metal. This coupled with their large surface areas make them potentially suitable for use as heterogeneous catalysts for alcoholysis of long chained fatty acids. These particles are less toxic than conventional catalysts used. Much research is still required to determine the impact of this technology on the production of biodiesel [101]. 5.5. Thermal conversions of biomass to biofuels The thermal conversion of biomass to biofuels has come to the forefront strongly as an attractive feedstock for liquid fuel production as it is renewable, sustainable, environmentally friendly, is no threat to food security and has economic potential due to diminishing levels of fossil fuels [102]. Thermal decomposition of algal biomass can yield different types of energy fuels depending on temperature used for conversion as given in Fig. 7 [90]. Gasification produces syngas (mixture of combustible gases) by partial oxidation of biomass at high temperature (800–1000 °C). Biomass is reacted with oxygen and water in the form of steam to produce a mixture of carbon dioxide, hydrogen gas, nitrogen and methane [90]. Syngas can be burned directly or used as fuel for diesel or gas turbine engines [16]. Syngas produced by gasification is advantageous in that it can be produced from a wide variety of biomass [90]. Thermo-chemical liquefaction of biomass is used for conversion of wet biomass to bio-crude oil at low temperature (300–350 °C) and high pressure (50–200 atm) in the presence of a catalyst. Hydrothermal liquefaction occurs by decomposition of biomass into smaller molecules with high energy by utilizing high water activity in sub-critical conditions [103]. Liquefaction of microalgae have resulted in production of between 30% and 65% dry weight of oil depending on species used [16,90,103]. The major benefit of thermo-chemical liquefaction is the ability to use wet biomass. Reactors for however expensive due to the complexity [90]. Pyrolysis is the thermal conversion of one substance into another, in presence or absence of a catalyst and the absence of air

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Fig. 7. Potential energy products derived by thermal decomposition of microalgal biomass (adapted from [90]).

or oxygen. Pyrolysis of biomass produces charcoal, condensable organic liquids, acetic acid, acetone, methanol and non-condensable gaseous products by a simple, effective, wasteless and pollution free process [90,104,105]. Slow pyrolysis (350–700 °C) produces high charcoal content. Fast pyrolysis of microalgal biomass conventionally produces 60–75%wt. of liquid bio-oil, 15–25%wt. solid charcoal and 10–20%wt. non-condensable gases [16]. Flash pyrolysis uses moderate temperature (500 °C) with short vapour residence time for the conversion to liquid biofuels with conversion ratio of up to 95.5% [90]. Pyrolysis of algal biomass has given promising results and have been shown to produce higher quality bio-oil than lignocellulosic compounds [90]. Lipid containing biomass has been shown to produce higher heat balances and bio-oil yields [103]. The process of direct combustion is burning of biomass in the presence of air for the conversion of stored energy into hot gases usually in a furnace, boiler or steam turbine at temperatures above 800 °C. These gases cannot be stored and must be used immediately. The conversion efficiency of biomass to energy is more favourable than that of direct combustion of coal. A disadvantage of direct combustion is the requirement for biomass containing low amounts of water (<50%) giving the requirement for drying and other pre-treatments that may affect the energy balance. The cost of pre-treatment of biomass however makes it less viable than coal. The overall efficiency of the process may be improved by cocombustion of biomass with coal. There is currently little evidence of the viability of the combustion of algal biomass due to very limited data thus further research is required [90].

6. Biorefinery approach and other biofuels The economics of biodiesel production can be significantly improved by using the biorefinery based production strategy where all the components of the biomass raw material are used to produce useful products [1,19]. Furthermore, it is recommended that a biorefinery approach is the best solution to combine and integrate various processes to maximize economic and environmental benefits, while minimizing waste and pollution [2,19]. Despite the salient drawbacks of biofuel production from microalgae, various processes can be used to convert biomass to energy. However, the spent biomass can be used as animal feed since it is rich in proteins, carbohydrates and other nutrients. Essentially, the biomass can be burned, transformed into a fuel gas through partial combustion, into a biogas through fermentation, into bioalcohol through

biochemical processes, into biodiesel, into a bio-oil or into a syngas from which chemicals and fuels can be synthesized [2]. 6.1. Bioethanol Generally two methods are employed for the production of bioethanol from microalgal biomass, namely fermentation (biochemical process) and gasification (thermo-chemical process) [19]. Microalgae are rich in carbohydrates and proteins which can be used as carbon sources for fermentation, therefore microalgae compete favourably with biomass derived from food crops such as sugar cane and maize. There is a moratorium on the use of food crops for the production of bioethanol in some countries due to issues pertaining to food security and agricultural land availability. Therefore microalgae are generating a lot of interest as biomass feedstock for bioethanol production [83]. Fermentation of the microalgal biomass is catalysd by microbes such as bacteria, yeast and fungi and the main by-products are CO2 and water. The spent biomass after fermentation is used for anaerobic digestion process for methane production so in essence all the organic matter is accounted for [19,83]. However research on bioethanol production from microalgal biomass is still in infancy and not yet commercialized, and to date work is ongoing to optimize conditions for improved bioethanol yield [83]. 6.2. Biomethane The interest in biomethane production emanates from the fact that biomethane fermentation technology produces valuable products such as biogas [19,83]. Biogas is a mixture of methane (55– 75%) and CO2 (25–45%) and is produced via anaerobic digestion of microalgal biomass by anaerobic microorganisms [19,83]. Biomethane can be used as fuel gas and can also be used to generate electricity while the spent biomass is used to make biofertilisers [19].The biogas production from this anaerobic digestion process is primarily affected by its organic loadings, temperatures, pH and retention time in reactors [83]. Although microalgae offer a good potential for biogas production, commercial production has still not been fully implemented [19]. 7. Conclusion Globally the principles of environmental sustainability and economic development are intertwined. One cannot ignore the fact

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that a holistic approach is required to achieve targets associated with management of natural resources. Decreased dependence on fossil based fuels has set the foundation for research on alternative cleaner energy sources. To this end hyper lipid producing microalgae as a resource has gained much attention. This review encompasses latest developments on exploiting microalgae for the treatment of domestic wastewater and biodiesel production. Microalgae have contributed to tertiary treatment in conventional wastewater treatment and more directly to BOD and nutrient removal in engineered systems such as high rate algal ponds. More recently research has focused on exploiting final wastewater streams that have residual nutrients such as nitrogen and phosphorus as a resource to harvest microalgae rather than a waste product. Providing optimum conditions promotes improved lipid production. Therefore the biomass provides dual benefits of wastewater treatment and oil production. The lipid is then transesterified in FAME which is used as biodiesel. However, currently much emphasis is also being placed on optimizing technology for harvesting of biomass and extraction of oil. The spent biomass can subsequently be used for the production of various other value added products such as bioethanol or biomethane. Alternatively the biomass can be subjected to thermal conversion for liquid fuel production. All these options have attracted much attention currently from researchers and scientist globally, as the benefits of a environmentally friendly approach has been in the forefront of sustainable development. References [1] Chisti Y. Biodiesel from microalgae. Biotechnol Adv 2007;25:249–306. [2] Briens C, Piskorz J, Berruti F. Biomass valorization for fuel and chemicals production – a review. Int J Chem React Eng 2008;6:1–49. [3] Park JBK, Craggs RJ, Shilton AN. Wastewater treatment high rate algal ponds for biofuel production. Bioresource Technol; in press. doi:10.1016/ j.biortech.2010.06.158. [4] Arora A, Saxena S. Cultivation of Azolla microphylla biomass on secondarytreated Delhi municipal effluents. Biomass Bioenergy 2005;29:60–4. [5] de-Bashan LE, Bashan Y. Immobilized microalgae for removing pollutants: review of practical aspects. Bioresource Technol 2010;101:1611–27. [6] Mulbry W, Kondrad S, Pizarro C, Kebede-Westhead E. Treatment of dairy manure effluent using freshwater algae: algal productivity and recovery of manure nutrients using pilot-scale algal turf scrubbers. Bioresource Technol 2008;99:8137–42. [7] Olguın EJ. Phycoremediation: key issues for cost-effective nutrient removal processes. Biotechnol Adv 2003;22:81–91. [8] Pizarro C, Mulbry W, Blersch D, Kangas P. An economic assessment of algal turf scrubber technology for treatment of dairy manure effluent. Ecol Eng 2006;26:321–7. [9] Munoz R, Guieysse B. Algal–bacterial processes for the treatment of hazardous contaminants: a review. Water Res 2008;40:2799–815. [10] Godos Id, Blanco S, García-Encina PA, Becares E, Munoz R. Long-term operation of high rate algal ponds for the bioremediation of piggery wastewaters at high loading rates. Bioresource Technol 2009;100:4332–9. [11] Pittman JK, Dean AP, Osundeko O. The potential of sustainable algal biofuel production using wastewater resources. Bioresource Technol; in press. doi:10.1016/j.biortech.2010.06.035. [12] Chinnasamy S, Bhatnagar A, Hunt RW, Das KC. Microalgae cultivation in a wastewater dominated by carpet mill effluents for biofuel applications. Bioresource Technol 2010;101:3097–105. [13] Kong Q-x, Li L, Martinez B, Chen P, Ruan R. Culture of microalgae Chlamydomonas reinhardtii in wastewater for biomass feedstock production. Appl Biochem Biotechnol 2010;160:9–18. [14] Wang L, Min M, Li Y, Chen P, Chen Y, Liu Y, et al. Cultivation of green algae Chlorella sp. in different wastewaters from municipal wastewater treatment plant. Appl Biochem Biotechnol; in press. doi:10.1007/s12010-009-8866-7. [15] Moreno-Garrido I. Microalgae immobilization: current techniques and uses. Bioresource Technol 2008;99:3949–64. [16] Mutanda T, Ramesh D, Karthikeyan S, Kumari S, Anandraj A, Bux F. Bioprospecting for hyper-lipid producing microalgal strains for sustainable biofuel production. Bioresource Technol; in press. doi:10.1016/ j.biortech.2010.06.077. [17] Danquah MK, Gladman B, Moheimani N, Forde GM. Microalgal growth characteristics and subsequent influence on dewatering efficiency. Chem Eng J 2009;151:73–8. [18] Grima EM, Belarbi EH, Fernandez FGA, Medina AR, Chisti Y. Recovery of microalgal biomass and metabolites: process options and economics. Biotechnol Adv 2003;20:491–515.

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