Algal Biorefinery: A Synergetic Sustainable Solution to Wastewater Treatment and Biofuel Production

Abstract

In the recent years, due to heavy surge in the price of petrochemical products, researchers are getting interest towards renewable bioenergy resources such as algal-based biomass. In order to meet a world energy demand, current bioeconomy challenges and to produce valuable products, intensive and integrated research on algal biorefinery is highly required. Even though several research carried out study for the conversion of algae biomass to biofuel, but none of these proved economically viable. Hence, range of value added product (biodiesel, biochar, fertilizer, etc.) must be produced subsequently from algae. The utilization of microalgae for biomass production is better than agricultural crops as microalgae do not required fresh water for its growth, it can readily grow on wastewater throughout the year. Generation of wastewater is severe concern throughout the world and discharge of wastewater without proper treatment in to water bodies causes water pollution. Microalgae bear vast potential in significantly deescalating pollutant load (nitrate, TDS, ammonium, phosphate, organic load) from wastewater. The harvested algal biomass after remediation has significance role in producing biofuels and by-products in a sustainable way. In this chapter, emphasis would be given on role of algae in wastewater treatment and its biorefinary approach for sustainable energy development.

1. Introduction

Globally, the severe problem humanity is facing today is the availability of fresh water, wastewater generation and energy supply. As the continuous use of fossil fuel are depleting day by day the natural stock of fossil fuels. This natural energy reserve may end in next 45–50 years. This depletion is posing stress to continue the various anthropogenic activities i.e. industry, agriculture, production of precious chemicals for food, and pharmaceutical which directly affects the global economics due to shortage of energy sources. In these circumstances, this is the high time to identify and develop alternative, cost-effective, efficient renewable energy resources for enhancing the sustainability of anthropogenic activities. Algal biomass could be utilized to generate extensively energy support as algae bear high productivity of biomass [1]. The more concentration for diversification of agro-ecosystem from food to fuel is also fulfilled by the algal biofuel as these living being are also not required agricultural land due to their aquatic nature. As per the report of the Central Pollution Control Board of India [2], 71853 MLD [million liters per day] wastewater (considering both sewage and industrial discharge) is discharged into the water bodies of India and out of which only 37% get treated. In these days, pollution of natural resources especially water is also at alarming point and the climate change making it more serious. Thus, minimum contamination/waste of water and reuse of the contaminated/waste/used water is also the highly required. The presently available technologies of wastewater treatment are not only costly but also generate huge amount of sludge. The generated sludge after wastewater treatment essentially need to be treated and disposed, these two requirements further increases the financial effectiveness of the any technology [3, 4]. The algae mediated wastewater treatment is an environmentally sustainable and efficient approach and can be integrated with secondary wastewater treatment process. Algae are the small, mostly aquatic, photosynthetic (converts sunlight into the oil form stored energy) organisms, currently, getting more attention due to their capabilities to address the different environmental issues including energy. Microalgae have been noted for their enormous potential to remediate waste water i.e. Phyco-remediation. Phycoremediation is the utilization of alga culture to remove/biotransformation of pollutants, nutrients, xenobiotic from waste water. Phycoremediation can handle more than one environmental problem such as pH correction, BOD, COD and TDS removal simultaneously over the chemical methods. Phycoremediation consider highly eco-friendly as did not cause secondary pollution. Presently, biodiesel production utilizing microalga is not economically sound due to its cost. Thus, algal biorefinery concept can serve an important option to minimize the microalgal biofuel cost. Algal biorefinery is the analogous concept to present petroleum refinery as petroleum refinery produces multiples products including fuels from petroleum. Algal biorefinery is having potential to increase the values of the products obtained from the biomass feed stocks. Algal biorefinery can be integrated among biomass conversion process, fuels (low value, but high volume), intermediate compounds (low volume, but high value) and value added chemicals along with electricity generation through advanced technologies such as combined heat and power (CHP) technology. Microalgae are having high capacity to convert the solar energy to chemical energy per unit land than terrestrial phototrophs due to their high productive rate. Thus, microalgae can address the increasing energy demands as well as growing environmental issues such as climate change. Beside this, microalgae having some advantages as feedstock for value added product generation. The microalgae are capable of synthesize huge quantity of lipids (20–50% dry cell weight). The growth of algae is very fast compared to terrestrial plants (double the biomass within 20–25 days), so can be used for bioremediation [4]. Algae do not require arable land and fresh water for the growth. Algal biomass can also contribute significantly to reduce the enhanced atmospheric carbon. Keeping this view, the present chapter is focused towards the utilization of algae in wastewater treatment, biofuel, biofertilizer production, CO₂ sequestration, bioremediation and challenges with future perspective through algal biorefinery interventions.

2. Wastewater characteristics and their treatment

Due to industrialization, population expansion and modern life style the wastewater generation is increasing day by day. The discharged of wastewater without proper treatment in to waterbodies are continuously overloading the fresh water bodies and minimizing their self-cleaning capacity. This overloading of waste into the freshwater bodies, disturbing the nutrient recycling process along with the disturbance in biogeochemical cycles i.e. nitrogen, carbon and water cycles through physical (evaporation, precipitation etc.), ecological (eutrophication, bio-magnification etc.) and biological process (photosynthesis, nitrogen fixation, respiration etc.). Availability of fresh water/irrigation water for fulfillment of daily requirement of the human society is also decreasing in the present changing environment [5]. To meet out the current demand of water for various anthropogenic activities, this is very necessary to increase the water reuse potential. This can be achieved through proper treatment of the wastewater generated during different activities. Wastewater is generally composed of water and wastes originate from commercial, industrial, home and institution activities. Wastewater at the point of origin contains high organic load, colorants, pesticides, heavy metals, hydrocarbons, numerous pathogens, toxic compounds and nutrients. The minimum treatment of this generated wastewater is required and recommended before disposing into the environment. The mechanistic understanding of the environmental effects, influencing factors, controls and effective utilization of the treatment process is essential to design the treatment process and its operation. There are three methods which are used to treat wastewater. These methods are physical, chemical and biological methods. The treatment of waste water is general divided into three systems of treatment i.e. primary, secondary and tertiary based on the capacity to remove the different contaminants. The general wastewater treatment process details are depicted in Figure 1.

Figure 1.

General wastewater treatment process.

3. Algae mediated wastewater treatment

Algae are autotrophic organism however; there are some other algae, which are heterotrophy or mixotrophy in nature. The dominant mode of microalgae metabolism in wastewater is heterotrophic (approximately 50%) in nature. The heterotrophic microalgae metabolize organic components in wastes, and convert them into organic biomass along with inorganic components. Microalgae contribute approximately 50% of global primary production (GPP) i.e. most efficient convertor of solar energy to chemical energy and act as producer of aquatic food chain [6]. The quantity and quality of bioactive compounds of microalgae is based on the ambient environmental, ecological factors and taxonomic position. The removal of this generated algal biomass results in the purification of wastewater as their removal decreases the biological oxygen demand especially in case of their death to minimizing the chance of back release of nutrients in the ecosystem.

In 1960s Oswald and Gotta, [7] reported the potential role of microalgae for the removal of pollution load from the tertiary wastewater treatment by algae. Phycoremediation is the removal/biotransformation of pollutants such as nutrients, xenobiotics from wastewater and CO₂ from air. Thus, phycoremediation can be used for to extract nutrient from municipal wastewater/effluents which are rich in organic matter; to complete removal/transformation and degradation of xenobiotic compounds utilizing as biosorbent; to treatment of acidic wastewaters; to sequestrate CO₂; and to detect toxic compounds using algae-based biosensors. There are various studies which recommends the removal of nitrogen and phosphorous from wastewater to protect the waterbodies from eutrophication [5, 8, 9, 10, 11, 12]. The controlled growth of algae in wastewater leads to reduction of contamination load on natural resources and can also be enhance reuse efficiency. The utilization of algae in the treatment of different waste such as agro-based industrial wastes, sewage, industrial wastes (metal finishing, paper, and textile) and even landfill leachate [10, 12, 13, 14, 15]. Waste mitigation potential of an algal species entirely depends on the algal productivity, nutrient and pollutant removal efficiency, and cost of biomass harvest [16, 17, 18]. In addition to removal of pollutant load from wastewater, algae make available oxygen (O₂) to bacteria (heterotrophic aerobic) for mineralization of pollutants and CO₂ produces by bacterial catabolism is subsequently consumed by the photosynthetic activity of algae (Figure 2). The photosynthetic process of algae is reduces the pollutant volatilization through mechanical aeration and contribute to reduce the cost of operation directly. Thus, the dual purpose utilization of microalgae in biorefinery approaches is providing high sustainable solution to the long standing environmental concerns than any other equivalent approaches. The important products of the algal biorefinery are biomanure, biodiesel, ethanol, pharmaceutical, fish feed, biohydrogen and several other valuable products [19, 20, 21, 22]. The role of various microalgae in the remediation of wastewater is given in Table 1.

Figure 2.

Principle of photosynthetic oxygenation in BOD removal.

S. No	Microalgae	Waste	Culture system	Biomass productivity	Reduction of pollutants	References
1.	Chlorella minutissima	Primary treated sewage wastewater	Race way ponds	0.44 ± 0.04 g/L	Reduction of TDS, P, NH4^+, NO3^−, BOD and COD by 94.3%, 67.4%, 48.2%, 88.8%, 93.2% and 80.5%, respectively	[13]
2.	Chlorella pyrenoidosa	Sewage treatment plant wastewater and synthetic wastewater	Bioreactor	—	SWW*: Reduction of Nitrate (99%), phosphate (77%), and COD (61%) from SWW; STPW: Reduction of nitrate (99%), phosphate (94%), and COD (87%)	[23]
3.	Chlorophyta, cyanobacteria, Acinetobacter, and Pseudomonas	Polyacrylamide (PAM)-containing wastewater	RAB reactors	—	PAM**, COD, TOC, and TN removed by 64.1 ± 2.0, 58 ± 1.5, 34.5 ± 1.5, and 85 ± 6.0%, respectively.	[9]
4.	C. minutissima, Scenedesmus spp N. muscorum and Consortium	Sewage wastewater	20 L capacity of; plastic bottles	0.4 g/L dry biomass of Chlorella	Chlorella reduces NH4^+-N (92%), NO3^−N (87%), PO4^3− -P (85%), and reduces TDS (96%), BOD (90%) and COD (81%). Scenedesmus spp removed 72% TDS and 92% NH4^+-N. Out of selected C. minutissima performed better	[14]
5.	Scenedesmus obliquus	Wastewater treatment plant	25 L glass containers	0.88 ± 0.04 g/L	Removal efficiencies of 71.2 ± 3.5% COD, 81.9 ± 3.8% NH4^+, ∼100.0% NO3^−, and 94.1 ± 4.7% PO4^3−.	[24]
6.	S. obliquus	Paddy-soaked wastewater	Polybags (PB), photobioreactors; (PBR) and race way ponds (RWP)	340 ± 2 mg/L/d	Reduction of Ammonical-N (96%), phosphates (97%)	[25]
7.	Scenedesmus sp. ISTGA1	Municipal sewage wastewater	Bioreactor	1.81 g/L	Reduction in BOD and COD by 86.74% and 88.82% respectively	[26]
8.	C. pyrenoidosa, Scenedesmus abundans and Anabaena ambigua	primary treated sewage wastewater	Bioreactor	—	52–88% reduction in the nutrient concentration	[27]
9.	C. minutissima	Primary treated sewage wastewater and tertiary treated Common Effluent Treatment; Plant (CETP)	5 L; capacity laboratory grade plastic tray	Sewage wastewater 0.75 g/L; while it was 0.43 g/L in CETP wastewater	Reduction from wastewater within 12 Days, TDS (90–98%), N (70–80%), P (60–70%), K (45–50%)	[10]
10.	Isochrysis	Modified f/2; medium with Palm Oil Mill Effluent (POME) and inorganic fertilizer	Photobioreactor (PBR) (1 L); versus outdoor cultures (glass aquarium, 9 L)	Biomass- PBR, 69 mg m − 2 day−1, Outdoor, 92 mg m − 2 day −1	NO3^−, PBR – 38.1%, Outdoor- 46.4%, PO4^3−, PBR – 86.6%, Outdoor- 83.3%	[28]
11.	Chlorella; minutissima, Scendesmus, Nostoc muscorum	Sewage wastewater	Poly house in plastic trays	0.79 ± 0.02 and 0.78 ± 0.01 g/Lin Scenedesmus and C. minutissima, respectively	≥ 90% reduction in TDS, BOD and Ammonium-N.	[8]

Table 1.

Selective examples of the microalgae for wastewater treatment along with system utilized biomass productivity and targeted pollutants.

*SWW-Sewage wastewater; **PAM-polyacrylamide.

4. Algal bio-refinery based production

Algal biorefinery approach aims to promote harvesting of several value-added products from the algae feedstock, towards economic and environmental effectivity of algal based technology. There are plenty of the research efforts has been made to harness the algal biomass to produce biofuel, biofertilizer, biodiesel, pharmaceutical products as well as for wastewater remediation. The industrial production of the biofuel through algal biomass utilizing different photobioreactors could be possible to burden off the current energy demand. The basic process of biodiesel production through bio-refinery is explained in Figure 3. Additionally, the nutrient qualities such as carbohydrates, proteins, nitrogen, phosphorous, potassium and other nutrients of the algal biomass can also be utilized as nutrient feed for animal as well as for fish etc. The algal dry biomass composition contains up to 46% Carbon (C), 10% Nitrogen (N) and 1% Phosphates (P) and 1 kilogram (kg) of dry algal biomass utilizes up to 1.7 kg carbon dioxide (CO2) (Hu et al., 2008). The algal biomass after remediation of wastewater having vast potential as biofertilizer. The N,P and K content in microalgae algae biomass varies from 7 to 9%, 1–2% and 0.1–1%, respectively [5, 13]. The algal biomass can also be used as biofertilizer in agro-ecosystems. The algal biomass not only provides essential nutrients to the agricultural crops but also significantly contribute to improve the soil carbon and soil fertility [5, 13]. Sharma et al. [5] in a study conducted on the impact of algae biomass as manure on the nitrate leaching reported that microalgal manure are slow releasing in nature and less leaching of nitrate as compared to chemical fertilizer was observed, so application of microalgae biomass also results in reduction of nitrate leaching from agricultural fields as compared to chemical fertilizer, hence prevent eutrophication of the water bodies. Thus based on the available literature and recent research it can be concluded and put forward that algal biorefinery is one of the most promising cutting-edge economic alternative of existing traditional technologies to cater the environment through direct reduction of the primary and/or secondary pollutants as well as sustainable solution for essential required developmental process.

Figure 3.

A conceptual process (Biorefinery based) for producing microalgae biofuels for better economy.

5. Microalgae as potential source of biofuels

Microalgae has a potential to deliver renewable energy resources such as biofuels. Due to the problem of global warming (burning of fossil fuels) and day-to-day surge in petroleum, prices the role of microalgae has been rethink by various domains for using as a source of clean energy. The reason behind microalgal biomass as suitable feedstock for biofuel as the algae has high biomass productivity, high lipid content and high photosynthetic efficiency than terrestrial plant. Algae is considered as third generation biofuel and have advantage over first and second generation biofuel in terms of readily available, ability to grow throughout the year, water consumption is very less, can grow on wastewater, ability to grow under harsh condition, and high biomass production. The oil content of algae compared to first and second generation biofuel is given in Table 2. In this section we will discussed the important product obtained from algae as biodiesel, biofertilizer and biochar.

5.1 Biodiesel

The recent research developments in microalgae reveals that microalgal biomass is one of the promising sources of biodiesel, which partially could met the demand of transportation sector. Using microalgae to produce biodiesel will not compromise production of food, fodder and other products derived from crops. The microalgae species such as Kirchneriella lunaris, Ankistrodesmus fusiformis, Chlamydocapsa bacillus, and Ankistrodesmus falcatus are prominent species for biodiesel production as they contain high polyunsaturated FAME [31]. The comparison of various oil yielding crops is given in Table 2. Thus, considering the potential of the algal based biodiesel production, it can be concluded that the biodiesel can be used to displace fossil diesel partially/completely. The oil percentage in various algae are in the range of 20–50% (Table 3) and increase in oil content can be achieved >80% by weight of dry biomass in microalgae. The oil productivity of microalgae is the mass of oil produced per unit volume of the microalgal broth per day, depends on the algal growth rate and the oil content of the biomass.

Source	Oil (Liter/ hectare)
Algae	1,00,000
Oil Palm	1413
Coconut	2684
Jatropha	741
Rapeseed/Canola	1187
Peanut	1057
Sunflower	954
Safflower	776
Soybeans	636
Hemp	364
Corn	172

Table 2.

Comparison of algae with different crops for biofuel.

(Demirbas and Demirbas, [29]; Ahmad et al. [30]).

Microalgae	Oil content (%)	References
Botryococcus terribilis	49	[32]
Chlorella vulgaris	41–58	[33]
Chlorella emersonni	23–63	[33]
Chlamydomonas sp.	22.7	[33]
Desmodesmus sp.	6.5–9.1	[34]
Chlorella sp.	28–53	[34]
Scenedesmus sp.	17–24	[33]
Scenedesmus obliqus	30–50	[34]
Nannochloris sp.	31–68	[35]
Chlorella salina	11	[32]

Table 3.

Oil content of microalgae.

5.2 Bio-fertilizer

The microalgae can assimilate excess N&P from the wastewater and convert it into the valuable biomass which has potential as a manure for the agricultural crops. Various researches have reported that %N content in the dry algae biomass is significantly higher than the available organic manure (cow dung, farmyard manure etc.) [10, 13, 36]. The NPK content of dry algae biomass ranged from 3 to 7%, 0.5–2% and 0.4–0.8%, respectively [13, 14, 36, 37]. The algal based fertilizers are composed of high OC which support to increase the moisture retention capacity and nutrient bioavailability than chemical fertilizers and other organic inputs such as farm yard manure [38]. Algal bio-fertilizer being rich in carbohydrates, soluble protein contents and other important plant organic nutrients, ensure higher vegetative yield [39, 40]. The algal-biofertilizer input also enhance the microflora of the soils along with the availability of inorganic nutrients [13]. Renuka et al. [41] confirms that the microalgae-based biofertilizer decreases the nutrient losses as nutrients are slowly release into the soil and available to the crop in longer periods than the synthetic fertilizers. In a leaching experiment conducted by Sharma et al. [5] the application algae biomass (C. minutiisma) after harvesting from sewage wastewater results in reduction of nitrate leaching in spinach crops as compared to application of chemical fertilizer, hence prevent eutrophication in water bodies. The immobilization and mineralization of any fertilizer depends on its C:N ratio. If the C:N ratio of any fertilizer is more than 20, it promotes immobilization and therefore not advisable for application in soil. The C:N ratio of phycoremediated algae manure is around 9.16, hence its application promotes mineralization in the soil [13]. In addition, algae fertilizer also reported to reduce nitrate leaching from the agricultural fields than synthetic fertilizer [5, 21]. Therefore, it can be summarized that phycoremediation of sewage wastewater with biofertilizer production is a resource conservation approach and recycling of wastewater as well as nutrient for improvement in crop quality.

5.3 Biochar

Algae biomass is potential feedstock for various value added products. Since last decades, interest has been raised in production of biochar from microalgae biomass. As biochar is rich in organic carbon, so its application enhances carbon sequestration and improving the soil quality [42, 43, 44]. Generally, carbon, hydrogen, nitrogen and sulfur content in biochar is 48.45, 1.78, 1.47, and 0.78 (wt%) and it varies with the feedstock [45]. The microalgae derived biochar (Chlorella vulgaris FSP-E) is slightly alkaline in nature having carbon, hydrogen, nitrogen, oxygen and sulfur content (% dry wt) is 61.32,3.55, 9.76, 11.92, and 0.02% [46]. Similarly, Chaiwong et al. [47] reported volatile matter 16.8%, carbon 62.4%, and nitrogen 2.1% in spirogyra microalgae derived biochar. Generally, compared to lignocellulosic biochar, algae derived biochar have low organic carbon content and CEC, but having high nitrogen, P, K, Ca and Mg content [48]. Due to its high nutrient content and ion exchange capacity, algae biochar can be utilized for agricultural inputs and adsorbents in wastewater remediation [42]. Being an alkaline in nature, algae biochar could be used as amendment in acidic soil. Due to high biosorption capacity of associated with the large amount of functional group, microalgae biochar results enhancing the efficiency for the removal of organic contaminants [49]. Producing algae biochar also results in sequestration of atmospheric carbon dioxide, hence prevent global warming. Biochar is the carbon-enriched (coke) obtained after pyrolysis under temperatures (600–700°C) and under anaerobic conditions. The produce yield from pyrolysis is related to parameters, such as temperature, heating rate, and residence time [50]. The yield of biochar increased with decrease in pyrolysis temperature, and with increase in the duration. Chen et al. [45] showed that the yield of biochar algae in terrified microalgae residue at the temperature ranged from 200 to 300°C with a residence time of 15–60 min. Similarly, the yield of 50.8–95.7% in microalgae Chlamydomonas sp. JSC4 under the temperature of 200–300°C for 15–60 min [51]. Hence, it can be concluded that, production of algal biochar is expected to contribute to a further sustainable environment in the future.

5.4 Carbon dioxide sequestration

Global climate is a challenging issue, and reason behind is increasing concentration of greenhouse gases in atmosphere. Currently, CO₂ concentration in the atmospheres is around 400 ppm and it may reach to 750 ppm by the end of century [52]. CO₂ is well known greenhouse gases contributing climate change and global warming. The industrialization, and population expansion is the main cause of greenhouse gases emission. Several technologies has develop for capturing CO₂, although biological capture of CO₂ is a potential and attraction alternative. The algae mediated CO₂ fixation coupled with wastewater treatment is gaining attention as compared to terrestrial plants [53]. The microalgae that are effective in CO₂ sequestration generally belongs to Chlorococcum, Chlorella, Scenedesmus and Euglena genus. The carbon dioxide sequestration potential of microalgae is around 10–50 times higher than terrestrial plants [54]. The nutrients content in wastewater (N & P) can be utilized by microalgae for source of food and resulting biomass could be utilized for biofuel, biofertilzer, biochar and value added products. Microalgae can be grown in photobioreactor by carbon dioxide from the point sources such as industry, cement kiln, thermal power plant etc. Tang et al. [55] conducted a study on the impact of CO₂ concentration on biomass productivity of algae Chlorella pyrenoidosa in a photobioreactor and found that at 10% CO2 concentration, biomass production was highest (1.8 g/L). However the process is cumbersome and faced problem in down streaming process (harvesting). Open pond system and closed PBR are generally suggested for the purpose of growing algae. Open pond system/raceway ponds are cost effective, but significant amount of CO₂ loss to the atmosphere as compared to closed PBR. The CO₂ sequestration with remediation of wastewater thorough algae is cost efficient, sustainable, and recycling approach.

6. Microalgal biomass production

6.1 Open ponds

Cultivation of algae in open pond is oldest and simple practice in which algae are cultivated in similar condition as external environment. This type of system was first introduced in 1950s [56, 57]. Open pond consists of close loop system for circulation which is around 0.3 m depth with a paddlewheel constructed as clay, or plastic-lined ponds. Paddlewheel is used for circulation of water and for proper aeration. Open pond system is still used for large scale production of algae in outdoor condition. With time various designs has emerged for open-pond systems, but three designs (race-way ponds, circular ponds, and unstirred ponds) succeeded for mass multiplication of algae (Figure 4).

Figure 4.

Open pond system for algae biomass production.

6.2 Closed photobiorector

As name suggest, growing of algae in closed system. Closed photobioreactor can produce higher biomass than closed system, but it is not cost effective. The most common cultivation technology in closed system is the photobioreactor (PBR) (Figure 5). Typically, closed reactors include tubular and flat bioreactors. The system consists of glass or plastic, although glass PBR is frequently used for large scale production [58]. In closed system, glass tubes are arranged normally in vertical, helical of in horizontal manner and mechanical pumps fixed to allow CO₂ and O₂ exchange. Closed PBR has advantage over open system as it harnesses more sunlight and hence enhance productivity (from 20 to 40 g/m /d) in short span of time, although it is costly due to complexity in structure [59]. The advantages and disadvantages between open pond and closed bioreactor is given in Table 4.

Figure 5.

Tubular photobioreactor with parallel tubes.

Method of cultivation	Advantages	Disadvantages
Open system	Cost effective, easy to maintain	Biomass production is low, low light use efficiency, high risk of contamination of other microorganism, not suitable for all sensitive microalgae species
Closed system	High biomass yield, high sunlight use efficiency, less space is required, can be suitable, highly suitable for monoculture and sensitive species	High cost in construction as well as in maintenance including cleaning of reactor

Table 4.

Comparison between open and closed photobioreactor system.

Yaakob et al. [60].

7. Challenges and future perspective

Despite of the importance of algae mediated wastewater treatment and further production of several bioproducts form harvested algae biomass such as (fuel, feed, food, ferilizer), some challenges are also associated with algae technology. Various microbial contaminants can also be act as inhibitors to algae growth. The pH and organic impurities such as i.e. lignin and tannins present in wastewater can affects the algal growth negatively and the concentration of heavy metals above the permissible levels can unfit the products for subsequent utilization of the pharmaceutical products. The microbe (bacteria, protozoa) present in wastewater may affect the growth of algae, there pre-treatment methods such as autoclaving, filtration is not feasible at large scale production. Therefore, advanced technologies are required for the removal of pathogens particularly for the commercial scale production. The major problem associated with conventional wastewater treatment process is generation of sludge. The algae mediated wastewater treatment process overcomes the problem of sludge generation as sludge contains only algae biomass [61]. Different types of wastewater has different composition in terms of pollution load like TDS, heavy metals contents, dissolved oxygen, so the selection of microalgae and its strain should be according to the source of the wastewater, resistibility to the pollution load, easily accessible and achieve the goal of preferred outcome. The harvesting methods of microalgae from wastewater are tedious, costly and laborious too, particularly for the unicellular microalgae. But with scientific development, biotechnological approach and emergence of advanced technology, the problem of microalgae harvesting would be elucidated. Genetic modifications of microalgae hold a great potential for biofuel production from commercialization point of view. However, there are certain challenges that need to be overcome for its large scale production. Hence, more and more studies are required to unfold the enzymatic pathway of lipid/ biofuel production to understand the mechanism involved in the process. To date, several metabolic engineering processes have been developed for enhanced production of algal biofuel, high carbohydrate and lipid content in algal biomass and improving the photosynthetic efficiency of algal species through the cellular expression or down regulation of various genes encoding a specific enzyme [62, 63, 64]. The complex nature of fatty acid biosynthetic pathway and lack of molecular transformation techniques for most of the oleaginous microalgae is cumbersome for genetic engineering process. Moreover, enhanced lipid production through genetic manipulation are not fully evolved and recent advancement in the genetic engineering methodology and techniques still promising to reach the desired goal. For the commercial purpose, mass multiplication of microalgae is required. The growth of microalgae is governed by the temperature, seasonal variations and climatic conditions. The laboratory facility with controlled condition of temperature, humidity and invariable seasonal variation is required for the mass multiplication. By viewing the importance of microalgae in the wastewater treatment, production of various valuable products and its combination with the other emerging technologies would definitely overcome the current challenges and cost in near future.

8. Conclusion

Algae are considered as a third generation biofuel, having high oil content than terrestrial crops. In the present scenario, biorefinery approach of microalgae is a promising approach towards reducing the cost of operation of decontamination as well as fuel production. Algae can easily grow on wastewater, which further preserving the resources (arable land and fresh water) for other purposes. In spite of producing various value added products from harvested algae biomass, it can act as a potential agent for wastewater remediation. Microalgae biomass production after wastewater remediation, could be a suitable fertilizer option. The microalgae biomass production reduces the organic load, and TDS, in wastewater which may further utilized as ferti-irrigation, hence reduces the burden on utilization of fresh water in a green circular economy. In addition, production of microalgae biochar which is rich in organic carbon, further enhances carbon sequestration and improving the soil quality and productivity. In this way, algae mediated wastewater treatment integrated with biochar, biodiesel and biofertilizer production from algae biomass is a recycling and resource conservation practice.

Acknowledgments

The authors are thankful to the ICAR-Indian Institute of Soil and Water Conservation, Dehradun for providing support.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this book chapter.

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