The increasing adverse effects of global warming due to the current linear economic model urges the exploration and implementation of an alternative circular, clean, and sustainable economy. Such model involves waste recovery as an essential component. In this sense, municipal or industrial wastewater can be used as raw material to generate value-added products with the help of microorganisms such as microalgae and cyanobacteria. Such microorganisms have shown remarkable features, including high biomass growth rates, CO2 fixation capacity, and the production of biofuels and valuable by-products using wastewater as a substrate (Tiwari et al. 2019; Nilsson et al. 2020; You et al. 2022).
Recently, increasing consideration has been directed towards the development of sustainable biofuels from algal carbohydrates, such as bioethanol, biobutanol, and hydrogen gas, which represent an attractive alternative to reduce our dependence on fossil fuels. Some microalgae species can accumulate up to 65% of carbohydrates in terms of dry cell weight (dcw) in their cells, for which they are pondered as potential raw material for carbohydrate-derived biofuels (Xu et al. 2018; de Carvalho Silvello et al. 2022). Even the residual biomass from the carbohydrate extraction process (in the case of bioethanol production) could be considered as raw material for fertilizers under the concept of circular economy (Wuang et al. 2016; Ho et al. 2017).
However, there are still some drawbacks to be overcome, mainly i) the low accumulation of carbohydrates in the biomass, which leads to low fermentation yields, and ii) the energy-intensive harvesting process that represents high production costs (Nzayisenga et al. 2018; Arias et al. 2020; Wang et al. 2022). A high carbohydrate content in the biomass also carries additional benefits. For instance, Markou et al. ( 2012b) related the bio-flocculation efficiency of Arthrospira sp. to an increase of carbohydrate accumulation. Greater settling efficiency could avoid the need of a costly harvesting process.
Microalgae store carbohydrates as glycogen, starch, amylopectin, sucrose, or even extracellular cellulose as a response of the chloroplasts to hostile environments, such as unbalanced carbon to nitrogen or phosphorus ratios, N and P limitations or salinity changes (Arias et al. 2021). The utilization of wastewater as a source of nutrients to produce carbohydrate-rich biomass represents a promising approach to diminish production costs related to nutrients input (Gifuni et al. 2018; Arias et al. 2019). Recently, wastewaters of different sources such as domestic, urban and agricultural, have been used to cultivate microalgae and produce carbohydrates (Arcila and Buitrón 2016; Shayan et al. 2016; Arias et al. 2020). Notably, the use of industrial wastewater containing high C:N and C:P ratios resulted in high carbohydrate content without any additional carbon supply (Sánchez-Contreras et al. 2021). However, because of the high organic and nutrient loads of industrial wastewater, it is generally necessary to dilute the influent or operate at high hydraulic retention times to unbalance the C:nutrient ratios (de Farias Silva et al. 2020; Solís-Salinas et al. 2021). Recently, it was reported that the use of CO2 or bicarbonate also helped unbalance the C:nutrient ratio and increased the carbohydrate levels up to 70% dcw (Rueda et al. 2020).
Following that last approach, cooling tower wastewater (CWW) from recirculating cooling or evaporative cooling water systems represents a potential carbon source to produce carbohydrate-rich biomass. CWW contains high alkalinity and suspended and dissolved solids (i.e. calcium phosphate, calcium carbonate) (Calderón et al. 2018; CEMEX 2020). It is used to remove heat from towers in several industrial processes (Garrido Arias et al. 2021). Once the cooling process is finished, a significant fraction of CWW is recirculated to the towers, while another portion is poured into the environment (Li et al. 2020b; CONUEE 2021). It must be noticed that besides the alkali compounds, CWW also contains ion oxides, magnesium silicate, and silica, among other pollutants. Therefore, this water needs a treatment to remove organic and inorganic chemicals to be applied either for recirculation to the towers or poured into the environment (Saha et al. 2020b). Different technologies, especially physical and chemical alternatives, have been tested for CWW treatment, for instance, reverse osmosis, electrodialysis, electrochemical oxidation, nanofiltration, electrocoagulation, among others (Li et al. 2020b; Saha et al. 2020a; Soliman et al. 2022). However, most of these processes are expensive and/or energy extensive, limiting their large-scale applications.
Low-cost options such as constructed wetlands were also applied for this purpose in the study of (Saha et al. 2020b); however, this system required a supplementary treatment by electrochemical oxidation to eliminate macro- and micro-pollutants. Recently, photosynthetic-based approaches to treat CWW were proposed (Ortíz-Sánchez et al. 2022). It was demonstrated that CWW could naturally provide inorganic carbon and nutrient limitation to accumulate carbohydrates in batch cultivation; notwithstanding, the lack of macronutrients in CWW led to poor biomass growth and low productivity. Therefore, domestic wastewater must be included to promote biomass growth and carbohydrate content. In fact, combining both effluents could be an exciting alternative since CWW can be mixed with sewage from the same industry to be treated by low-cost biological options. Although the results were promising, it was unclear if this treatment could be applied mid- or long-term to grow biomass and accumulate carbohydrates simultaneously. Besides, the removal of micronutrients (i.e., heavy metals) present in CWW was not analyzed, nor was its impact on microbial population and settling properties.
In this context, this study aims at understanding the mechanisms of treatment, reuse, and valorization of real CWW from a cement processing industry mixed with domestic wastewater (DW) to produce carbohydrate-rich microalgal biomass with potential for synthesis of biofuels or other value-added products. A cyanobacteria-dominated consortium was inoculated in three semi-continuous reactors to investigate the influence of HRT on C:nutrients ratios, and their effect on microalgal growth, carbohydrate accumulation, and biomass settling properties. The impact of the wastewater on the content of micronutrients in the harvested biomass was discussed in detail.