**1. Introduction**

Water plays a vital role in all aspects of life, and the demand for wastewater treatment is currently a worldwide priority. Traditional sewage treatment consists of pre-treatment, physical, and chemical treatment of sewage water with high overhead expenses. The exploitation of natural resources worldwide to meet the energy demand has created a vast concern over environmental issues. Hence, wastewater treatment coupled with energy production and other valuable product generation. This method produces biomass that can be used further to produce valuable products and has a very low operational cost [1]. Recently, the use of microalgae for wastewater treatment has attracted much attention worldwide owing to its multifaceted benefits. Integrated biorefinery is a promising approach to microalgae cultivation for wastewater treatment with simultaneous production of high-value products and biofuel production [2, 3].

Furthermore, phycoremediation utilizes microalgae's ability to bio-sequester carbon dioxide, their high growth rates, high biomass production, high lipid productivities, and their ability to remove contaminants from wastewater and produce biofuels. Aside from bio-manure and biodiesel, algal biomass can also be used to produce bioethanol, hydrogen, and other valuable products [4]. Investigation into different microalgal species has established that they could bring down more than 98% of COD and BOD of sewage water. The impact of phycoremediation in treating sewage wastewater reduces greenhouse gas and sludge formation cost-effectively and energy-affluently.

Many studies have coded the usage of microalgae in various wastewater treatments like agricultural, municipal, dairy, piggery, and poultry wastewaters and industrial effluents [5]. Recently, some studies have reported on treating sewage wastewater and microalgae biomass generation for biofuel production [6]. *Chlorella vulgaris* is the most desired organism for simultaneous wastewater treatment and bioenergy production [7]. Pooja et al. [8] used *C. vulgaris* to simultaneously remove nutrients and pollutants from wastewater and recover biomass for biofertilizer application. The above study successfully proved the conversion of sewage to chemical fertilizer. Kumar et al. [9] used *C. vulgaris* for wastewater treatment and industrial flue gases for biomass production.

#### **2. Conventional sewage treatment process**

The main aim of sewage wastewater treatment is to remove the BOD, suspended solids, nutrients, pathogenic microbes, and toxicity. The typical sewage treatment process involves four to five steps, viz., preliminary, primary, secondary, tertiary, and disinfection. Preliminary sewage wastewater treatment removes large solid particles like rags, wood, heavy grit particles, and fecal matter. A well-designed sedimentation tank removes almost 70% of the settleable solids and 40% of BOD during the primary treatment of sewage [10]. A mixed population of heterotrophic bacteria further reduces the BOD in secondary sewage treatment. These bacteria facilitate the biological oxidation of BOD and can further remove almost 90% of pathogenic bacteria from sewage [11]. Tertiary sewage treatment removes all the organic ions, viz., ammonium, nitrate, and phosphate, either biologically or chemically [12].

In contrast, quaternary treatment aims to remove heavy metals, organic compounds, and soluble minerals [13]. Following tertiary treatment, disinfection kills all pathogenic microbes in the effluent. Disinfection can be achieved using a variety of physical and chemical methods. Ozone and UV light are the most preferred physical disinfection methods, while chlorine has been used extensively for disinfection [14].

#### **3. Microalgae for sewage wastewater treatment**

The use of microalgae for sewage wastewater treatment is cost-efficient, renewable source for biomass production and helps in carbon sequestration [15]. The microalgae can utilize the organic and inorganic carbon and inorganic nitrogen and phosphorus present in wastewater for their growth. Photosynthesis in microalgae helps heterotrophic bacteria degrade carbonaceous materials in wastewater treatment. Many studies have reported microalgae biomass production using nutrients removed from wastewater [16]. In addition, microalgae are efficient for carbon dioxide capture and nutrient removal from wastewater and are reported as a potential candidate for future energy production [17]. The microalgae can directly assimilate ammonia and phosphate from wastewater for their growth and metabolic functions [18]. Furthermore, the microalgae wastewater treatment process emits fewer greenhouse gases, as most of the nitrogen is being assimilated instead of converted to nitrogen oxide [19].

*Phyco-Remediation of Sewage Wastewater by Microalgae DOI: http://dx.doi.org/10.5772/intechopen.109257*


#### **Table 1.**

*Nutrient removal efficiency of microalgae from various wastewater.*

Microalgae have recently been extensively studied for their ability to treat wastewater effluents. The performance of various microalgal species for wastewater treatment varies with the range of wastewater types [20]. Prandini et al. [21] successfully demonstrated nutrient removal from piggery wastewater by *Scenedesmus obliquus*. Kothari et al. [22] have used *Chlorella pyrenoidosa* for dairy effluent treatment. Many studies have reported *C. vulgaris* as an ideal candidate for municipal wastewater effluent treatment [23]. Other microalgae used for wastewater treatment are *Chlamydomonas sp., Nanochloropsis sp., Dunaliella sp., Botryococcus sp*., etc. [24]. Microalgae at the secondary treatment phase or primary sewage wastewater to effluent standards are economical and eco-friendly approaches. The nutrient composition of primary sewage waste and secondary treatment effluent is almost the same but has different concentrations [25]. The concentration of nutrients, viz*.,* nitrogen and phosphorus, in primary sewage waste is higher than in secondary treatment effluent (**Table 1**).

#### **4. Mechanism of nutrient removal by microalgae**

#### **4.1 Carbon**

As a primary carbon source, microalgae use CO2, whereas in aqueous conditions, CO2 splits into bicarbonate and carbonate ions depending on pH, temperature, and salinity [35]. Due to the low concentration of CO2 in the aquatic environment,

microalgae use a carbon concentration mechanism to minimize the loss of photosynthetic activity [36]. Microalgae convert inorganic carbon to organic carbon via the Calvin cycle, as it provides metabolic reactions to produce amino acids and lipids.

The carbon dioxide concentration in the wastewater is one of the essential factors that decide the growth of microalgae, i.e., low availability of inorganic carbon in wastewater limits microalgal growth. Hence, to improve microalgae growth, the wastewater is usually supplemented with carbon dioxide or bicarbonate salts [37].

Shen et al. [38] reported that *S. obliquus* at 5% CO2 concentration removes total nitrogen from the wastewater within 2 days. In contrast, the total nitrogen recovery is less even on the third day under ambient and higher concentrations. Many studies have reported that at elevated CO2 levels, biomass production and nutrient removal from wastewater via microalgae have improved [39, 40]. The microalgae tolerance to CO2 is strain specific and has few species acclimatized with CO2 concentrations up to 100% [41]. Microalgae metabolize organic carbon compounds from wastewater through photo-mixotrophy or strict heterotrophy [42]. Municipal wastewater is highly heterogeneous, with complex carbonaceous materials that limit its availability as an ideal carbon source for microalgae. Since municipal wastewater comprises majorly complex organic carbon compounds, their decomposition by heterotrophic microorganisms must be converted to viable carbon sources for microalgae [43]. It has been reported that supplementing inorganic carbon to wastewater enhances nutrient removal efficiency [42]. However, enriching the wastewater with organic carbon increases production costs.

#### **4.2 Nitrogen**

Microalgae can utilize nitrogen from various organic and inorganic sources. Ammonia is preferred among the various nitrogen sources as its assimilation and incorporation are more efficient. Ruiz-Marin et al. [30] reported that microalgae, viz., *S. obliquus* and *C. vulgaris* showed a preference for NH3 in wastewater compared to other nitrogen sources. Membrane transporter proteins easily assimilate ammonium, and once translocated, the ammonium is directly incorporated into amino acids required for growth and other functions. Whereas transport of NO3 and NO2 is an energy-dependent process, they must first be reduced to ammonium via enzymatic reaction, requiring reductant NADH and ferredoxin [44]. In the microalgae wastewater treatment process, nitrification decreases the ammonium, and nitrate production is not desired as microalgae do not eliminate it if ammonium is present. Many studies have reported in steady-state wastewater treatment, almost 80% of the NH3 is oxidized to NO3, with a maximum of 40% assimilated by microalgae [45].

In photoautotrophic microalgae, inorganic carbon is fixed by the Calvin cycle and enters the glycolytic pathway as glucose-3-phosphate. Once converted to acetyl CoA, pyruvate is transported to mitochondria and enters the TCA cycle. Acetyl Co-A is further metabolized to CO2, and ATP, reducing equivalents and carbon skeletons [46].

Organic carbon substrates are transported in the cytosol through the glycolytic or pentose phosphate pathways in heterotrophic mode. Glycerol can be used as an alternative carbon substrate, translocated across the membrane via passive diffusion into the cytosol of microalgae [42, 47].

#### **4.3 Phosphorus**

Phosphorus is an important element involved in many metabolic processes as well as structural component of microalgae [18]. In wastewater, inorganic P exists in many ionic forms and is mostly bioavailable than soluble organic P compounds for microalgae. Phosphorus is incorporated into organic compounds by phosphorylation of Adenosine diphosphate (ADP). It is an endergonic reaction that obtains energy either by oxidation of respiratory substrates or by photosynthetic electron transport chain [24]. If a wastewater in enriched with P, microalgae have the capacity to accumulate P beyond their metabolic needs and store as acid-insoluble polyphosphate granules via luxury uptake mechanism [48].

### **5. Factors affecting the microalgae wastewater treatment**

#### **5.1 Bacteria**

The use of microalgae in wastewater treatment has been extensively studied. It is impossible to avoid other organisms like bacteria and fungi in wastewater, and wastewater sterilization is not feasible due to the huge volumes to be processed. The common bacterial species dominated in sewage wastewater are from the classes Bacteroidia, Flavobacteria, Betaproteobacteria, and Gamma proteobacteria. In primary sewage wastewater, variations in bacterial community composition are noticed at different inoculation ratios of microalgae to sludge [49].

Bacteria help the microalgae in wastewater treatment by providing CO2 via heterotrophic metabolism of organic matter and later mineralizing it to inorganic compounds that can be consumed directly by microalgae [50]. In return, microalgae produce oxygen via photosynthesis, which is required for heterotrophic bacterial growth during organic matter degradation [44]. The activated sludge treatment microalgae facilitate nitrification by generating a sufficient quantity of dissolved O2 [51]. The integration of a bacterial-microalgal approach for wastewater treatment is a promising approach as heterotrophic bacteria degrade the organic matter in the absence of aerated oxygen, as the microalgae provide O2, and similarly, the need for CO2 sparging is eliminated as bacterial respiration produces it [52].

#### **5.2 Industrial contaminants**

Microalgae can remove most industrial contaminants like heavy metals. Heavy metal contamination in wastewater is primarily due to industrial processing. The use of microalgae for wastewater treatment is termed phycoremediation, where algae uptake the nutrients, accumulate heavy metals, and degrade organic matter via symbiotic interaction with heterotrophic bacteria [53]. Microalgae has the potential to utilize waste as a nutritional source and reduce pollutants via enzymatic and metabolic processes. The microalgal metabolic pathways make them detoxify, transform and volatilize the heavy metal and xenobiotic pollution in wastewater [54]. Biosorption is the most commonly used mechanism by microalgae for either active or passive heavy metal uptake. Hence, biosorption is regarded as a cost-efficient way to eliminate heavy metals from industrial effluent [55]. The active algal biomass has a metal efflux metabolism-driven system for maintaining heavy metal concentration in intracellular space to avoid heavy metal toxicity. In microalgae, the heavy metal ions are distributed in cell vacuoles and organelles. In microalgae, the oxidation number of heavy metals is altered by various enzymatic reactions and makes them into less toxic forms. The microprecipitation of heavy metal removal in the form of phosphates and sulfates by active algal biomass is a practical approach to removing

heavy metals from wastewater [56]. The microalgal cell wall has an overall negative charge due to the presence of various functional groups; this makes the algal cell an entire binding site for heavy metal cations and involved in metal exchange via an ion-exchange mechanism [57].

#### **5.3 pH**

pH is an important abiotic parameter that decides the efficiency of wastewater treatment. The increased pH of wastewater leads to an adverse effect on bacterial activity. If the assimilation of inorganic carbon by microalgae is increased, the medium leads to an alkaline environment. Under the alkaline situation, the beneficial activity of aerobic and facultative bacteria in wastewater is impaired. Many studies have reported the inactivation of bacterial activity at higher pH [58]. At the pH of 8.5–9.5, wastewater bacterial community like coliforms and other pathogenic microbes has been drastically reduced [59]. Many mechanisms lead to the elimination of bacterial community in wastewater, i.e., conformational changes in bacterial membrane structure, respiratory chain damage, and increased susceptibility to exogenous factors like light, temperature, etc. [60]. Sutherland et al. [61] reported a reduction in nutrient removal efficiency at higher pH from primary sewage wastewater treatment via a microalgae consortium. Martinez et al. [62] reported the disruption of the cell wall of *S. obliquus* while treated with municipal sewage effluent at pH > 11.

#### **5.4 Temperature and light**

The indigenous microbial community in the wastewater will also compete for nutrients and microalgae. Hence, to promote microalgal growth, the factors like temperature and light intensity has to be considered [63]. The rate of photosynthesis by microalgae is directly proportional to the optimum light intensity, as, beyond optimum, photoinhibition will take place [64]. The illumination saturation point for microalgae varies between 200 and 400 μE m−2 s−1 [65]. The illumination period and amount of light intensity to microalgae-bacterial culture affect the nutrient removal efficiency from wastewater. The prolonged dark conditions during wastewater treatment via microalgae-bacteria consortium lower biomass recovery and chlorophyll. Gonzalez-Camejo et al. [66] reported in a bacterial-microalgal consortium for wastewater treatment, the lower light intensity of 40 μE m−2 s−1 favors the activity of nitrifying bacteria. Whereas, higher light intensities of 85–125 μE m−2 s−1 favor more microalgal growth over nitrifying bacteria.

The environmental temperature also plays a major role in nutrient removal efficiency from wastewater by microalgae. Ruiz-Martinez et al. [67] evaluated the ammonium removal efficiency of Scenedesmus sp. at different temperatures and found that the removal rate increases from 15 to 34°C. Similarly, Sforza et al. [68] assessed at a lower temperature of 15°C *Chlorella protothecoides* remove more NH4<sup>+</sup> N from effluent. The optimum temperature range of microalgae for wastewater treatment is between 10 and 30°C [65]. Usually, under normal conditions, higher temperature leads to a high growth rate and increased nutrient uptake by microalgae due to higher metabolic activity; these conditions are not always desired with wastewater treatment. Cultivating microalgae in wastewater at a lower temperature may also require less light intensity to minimize light saturation and photo-inhibition.
