Rapid industrialization and urbanization are generating enormous quantities of wastewater that must be treated before safe disposal. The wastewater (WW) associated with over population and industrialization are the major wastes, which pose an immense challenge to ensuring environmental sustainability around the world (Eze et al. 2018). The presence of excessive nutrients, such as nitrogen and phosphorous in the wastewater, can lead to the eutrophication in receiving streams and disturb the stability of the ecosystem (Cai et al. 2013). Hence, the removal of nutrients such as nitrogen and phosphorous from tertiary wastewater is vital to reduce the oxygen requirement of the receiving streams, save aquatic life, and prevent eutrophication in lakes and streams to protect human health. At present, a broad range of methods, including filtration, membrane technology, precipitation, the advanced oxidation process (AOP), and biological nutrient removal (BNR) using activated sludge are available for the removal of nutrients from tertiary wastewater. However, these techniques generally entail higher energy and maintenance costs, hazards associated with the disposal of chemicals, and the production of a large volume of waste sludge (Rajasulochana and Preethy 2016). In addition, the inability to eliminate nitrogen (N) and phosphorus (P) from tertiary wastewater simultaneously during the treatment process is a major drawback of these processes (Arbib et al. 2014).
Several studies have confirmed that microalgae can remove nitrogen and phosphorous during the treatment of tertiary municipal wastewater, which have gained much interest in recent years (Xin et al. 2010a; Khan and Yoshida 2008). The most widely applied cultures of microalgae for nutrient removal are based on species of Chlorella (Hernandez et al. 2006; Moreno Osorio et al. 2019), Scenedesmus (Shi et al. 2007), Botryococcus (Yu and Kim 2017) and spirulina (Olguín 2003). As the absence of one nutrient inhibits the removal of the other and vice versa, the nutrients N and P must be simultaneously present in culture media for microalgae growth. The concentration of nitrogen and phosphorous in wastewater varies, with nitrogen concentration varying between 15 and 90 ppm and phosphorous concentration varying between 4 and 20 ppm in municipal wastewater (Beuckels et al. 2015). As the main route of removal of nutrients by microalgae is through their uptake during growth, the microalgal growth rate directly influences the rate of removal of nutrients. Also, nitrogen and phosphorous can be concurrently consumed and removed effectively only if the nitrogen to phosphorous (NP) ratio of wastewater is in an appropriate range (Xin et al. 2010b).
Microalgae cultivation in wastewater through photosynthesis can overcome the aforesaid problems and offers many advantages, including the following: i) adds an economic value to tertiary wastewater effluents in terms of water and nutrient recovery (Arbib et al. 2014), ii) nitrogen and phosphorus are removed from wastewater simultaneously (Xin et al. 2010b), iii) microalgae grow about 10–50 times faster than terrestrial plants, resulting in the efficient conversion of CO2 into organic compounds (Y. Li et al. 2008; Razzak et al. 2017), iv) can generate a large volume of water suitable for recycling on-site and off-site or safe to discharge to surface water bodies, and v) can produce algae biomass suitable for bioenergy generation (Driver et al. 2014).
Previous studies have demonstrated that concentration of nutrients has a great impact on the rate of growth of microalgae and bio-fixation of CO2 (Razzak et al. 2017; Razzak et al. 2013). Chlorella Kessleri is a freshwater microalgae species, which is capable of removing nutrients from tertiary wastewater as well as capturing CO2 from the atmosphere (Arbib et al. 2014; Lee and Lee 2002). Li et al. (2012) have investigated the ability of Chlorella Kessleri to remove nutrients based on the intensity of light. Wang et al. (2012) have studied the heterotrophic cultivation of Chlorella Kessleri for the production of fatty acids with varying amounts of carbon and nitrogen supplements (Wang et al. 2014). Even though De Morais and Costa (2007) studied the growth kinetics of Chlorella kessleri under different CO2 concentrations, they did not conduct a detailed study of the bio-fixation capacity of CO2 (de Morais and Costa 2007). However, to the best of our understandings, a detailed study of the removal of nutrients by the microalgae Chlorella kessleri sp. as a function of the nitrogen to phosphorous ratio and the bio-fixation of CO2 under photoautotrophic condition s has not been conducted.
Finding the most appropriate nutrient ratio for the growth of microalgae is vital for the effective coupling of advanced wastewater treatment with bio-fixation of CO2. Hence, this study was conducted using a set of synthetic tertiary municipal wastewater samples based on the modified BBM with varying NP ratios. In this study, Chlorella kessleri sp. was cultivated in modified BBM to evaluate the growth, nutrient uptake, and CO2 bio-fixation at different nutrient ratios.