9.3.1 Consistency Period of Algal Growth in Industrial Wastewater

Survival in various environmental conditions requires the capability to quickly adapt and tolerate to stress. The wastewater environment can be toxic to microalgae. Microalgae can acclimatize to this environment, but the exact mechanisms that allow the microalgal strains to accept wastewater environments are indistinct. In addition, it is not clear whether the ability of microalgae to acclimate to wastewater is an inborn or species-specific trait. Large variations in the consistency period of microalgae acclimatization in IWW exist between various microalgal species, which is essential to identify the microalgal strains with a large consistency period for proper utilization of IWW as a culture medium.

Six different microalgae species such as Chlamydomonas debaryana, C. luteoviridis, C. vulgaris, Desmodesmus intermedius, Hindakia tetrachotoma and Parachlorella kessleri, which had never before been grown in wastewater environment, were acclimated over an eight-week period in secondary-treated MWW (Osundeko et al., 2014). Hydrothermal carbonization process wastewater was used to cultivate Chlorella sp. for 15 days. This microalga biomass concentration was increased by 644 mg L-' within ten days, and subsequently removed all the nutrients from the wastewater produced from hydrothermally treated sewage (McGaughy et al., 2019). Lau et al. (1996) found that the cells of C. vulgaris acclimated in wastewater for 14 days are physiologically more active than the non-acclimated cells. Because they took up a higher amount of nutrients from the wastewater for their development and metabolism, consequentially 86% of inorganic N and 70% of inorganic P was removed over the retention time of two days.

9.3.2 Optimization of Nutrients Concentration in Industrial Wastewater

Microalgae are better bioremediation agents than other microorganisms because they have the ability to covert the nutrients in IWW into biomass (Chinnasamy et al., 2010; Udaiyappana et al., 2017). The tiny size of microalgae provides a huge surface area that increases the nutrient uptake rate from IWW. However, the nutrient concentration of IWW influences the assimilation of nutrients for microalgae growth. The types and concentrations of pollution in IWW have major effects on the achievement of IWW management by integrated algal technologies. Therefore, prior to use, an appropriate pre-treatment is needed to convert complex organic compounds into simple ones so they can be efficiently utilized by the microalgae. Optimization by statistical methods is an efficient and potent approach for designing culture conditions to improve the microalgae as a growth medium in wastewater. In these methods, multiple variables can simultaneously be optimized to get information about the effects and interactions of variables. Moreover, a number of experiments to be performed also get reduced and prove salient in systematic investigations without compromising the accuracy of the representation of the environmental system (Ahmad et al., 2019).

The response surface methodology (RSM) for the multivariate optimization of dosage has been well studied (Krrolia et al., 2014); however, there are only a few studies on the use of RSM for optimization of nutrient concentration in IWW for microalgae cultivation. Ahmad et al. (2019) studied the effect of nutrient concentration in dairy industry wastewater (DIWW) on the growth of C. pyrenoidosa and biomass production, lipid productivity and FAME content using RSM. A maximum biomass concentration of 1.54 g L_l with a specific growth rate of 0.65 day1 was obtained with 50% DIWW, 100 mg L-1 of NO3- and 50 mg L 1 of POy3. Ultrasonic pre-treatment of leather industry effluent (LIE) was optimized by a model developed using RSM. Through the optimization of LIE, pre-treatment found that a low ultrasonic intensity of 0.35 W/mL could effectively promote effluent reduction for the growth of S. quadricauda KDPSC2 and lipid accumulation (Sarumathi and Dhandayuthapani, 2019).

9.3.3 Effect of Bacterial and Protozoan in Industrial Wastewater

Production of O, by algal photosynthesis reduces the requirement of external aeration, which is one of the advantages for the aerobic degradation of organic contaminants (Munoz and Guieysse, 2006), as mechanical aeration accounts for >50% of the total energy utilization of aerobic WWT (Tchobanoglous et al., 2003). Hence, photosynthetic microalgae can improve the energy efficiency of BOD removal from IWW by supplying aeration, through photosynthesis, to the heterotrophic aerobic bacteria (Munoz and Guieysse, 2006). This synergistic association can be used for the economical treatment of IWW (Brandi et al., 2000). This is especially beneficial in that many recalcitrant toxic compounds are to a great extent easier to biodegrade aerobically than anaerobically. For instance, in Russia, a microalgal and bacterial consortium was successfully used for black oil treatment and also detoxification of IWW (Safonova et al., 1999, 2004).

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