The current global target for biofuel feedstock crop production by 2030 would demand approximately 180 km3 of water, a demand that could be severely limiting given the overall worldwide depletion of freshwater sources. Because microalgae are a promising option for future biofuel production, finding ways to cultivate and harvest them with relatively little freshwater is both valuable and necessary. Chinnasamy et al. (2010) studied the feasibility of growing microalgae in wastewater consisting primarily of carpet mill effluents. This is especially appealing because it would not only greatly reduce the freshwater demand of microalgae cultivation, it would also serve to remove contaminants from the wastewater itself. The researchers found that a consortium of 15 native algal isolates could grow in treated wastewater with >96% nutrient removal, and 63.9% of their oil could be converted into biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–>. Chinnasamy et al. conclude that, while these results are promising, more research is needed to elucidate the mechanisms of anaerobic<!–[if supportFields]> XE “anaerobic” <![endif]–><!–[if supportFields]><![endif]–> digestion<!–[if supportFields]> XE “anaerobic digestion” <![endif]–><!–[if supportFields]><![endif]–> and thermochemical liquefaction in order for this process to be economically viable. —Karen de Wolski
Chinnasamy S., Bhatnagar A., Hunt R., Das KC., 2010. Microalgae cultivation in a wastewater dominated by carpet mill effluents for biofuel applications. Bioresource Technology 101, 3097–3105.
Chinnasamy et al. sought to determine the feasibility of microalgae cultivation in wastewater primarily consisting of carpet mill effluents. Annual worldwide domestic and industrial water consumption between 1987 and 2003 was estimated at 325 billion m3 and 665 billion m3 respectively. Approximately 247 million tons of algal biomass and 37 million tons of oil could be created if 50% of the wastewater from this consumed water was used for algae production. Due to the variation in wastewater composition, strains of microalgae able to grow in varying environments must be found if the technology is to advance. The wastewater from carpet mills is rich in phosphorous and nitrogen<!–[if supportFields]> XE “nitrogen” <![endif]–><!–[if supportFields]><![endif]–>, and the researchers therefore wanted to examine how microalgae could grow in the water and remove the contaminants. They additionally wanted to determine how consortium (multi-species) based technology functions for nutrient removal and biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production.
Wastewater consisting primarily of carpet mill effluent was collected from a utility company in Dalton, Georgia. In order to minimize temporal variation effects, wastewater was collected in large batches for all four seasons. Water was also collected from the treatment facility to enable the characterization of treated versus untreated wastewater. The algal taxa present and biovolume present in the samples were identified via standard protocol. The water samples were incubated in growth conditions to induce algal growth. The algae were then isolated by serial dilution and incubated on BG11 agar plates, from which individual colonies were selected and maintained. Thirteen microalgal strains and a consortium of wastewater isolates were identified and screened, and two freshwater and two marine forms and the consortium were selected for the timescale batch study.
Several different experiments were carried out within the study. First, biomass production and nutrient removal of the consortium was examined by growing the consortium in flasks of filtered and sterilized wastewater as a nutrient medium under two different levels of CO2 (ambient and 6%) and temperature (15 and 25 °C) conditions. Because the researchers also wanted to know the potential of cultivating the consortium in open ponds, the consortium was also cultivated in treated wastewater in four raceway ponds. The algae were harvested, dried, and the lipids were extracted for biomass analysis. Biodiesel was produced from crude microalgae oil via acid transesterification and base transesterification. The biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> was then analyzed with gas chromatography. Biomass of the harvested cells was quantified through filtration, and lipid content was measured gravimetrically with an automated extraction system. Additionally, total nitrogen<!–[if supportFields]> XE “nitrogen” <![endif]–><!–[if supportFields]><![endif]–> and phosphorous were determined via a persulfate method.
Because Chinnasamy et al. sought to examine temporal variation in nutrient concentration in wastewater, biochemical oxygen demand, chemical oxygen demand, total suspended solids, and several other parameters were measured in both treated and untreated water throughout the seasons. It was found that there are sufficient nutrients in both treated and untreated wastewater to support algae growth. About 27 species of green algae, 20 species of cyanobacteria<!–[if supportFields]> XE “cyanobacteria” <![endif]–><!–[if supportFields]><![endif]–>, and eight species of diatoms were found in the treated and untreated wastewater, with green algae and cyanobacteria dominating both water types in all seasons. The observed variations in responses of different species to different environmental conditions led the researchers to believe that parameters aside from nutrient availability are the most important in determining species composition.
Several different species, including the consortium, were grown in treated and untreated wastewater and standard growth medium. Several strains showed significant growth in both wastewater types. Marine forms were able to grow in treated and untreated water without any supplements, indicating themselves as having possibly the highest potential for growth in wastewater for biofuel in the future. The species with the highest growth in the preliminary screening were subjected to a time-scale study in treated and untreated carpet industry wastewater. Overall, the best performer was the consortium grown in treated wastewater, with the potential to generate 4060 L of oil ha–1 year–1. The researchers also calculated that it could produce ample biofuel (3860 L of oil ha–1 year–1) when cultivated in untreated wastewater. For the experiment examining consortium growth in different CO2 and temperature conditions, it was found that microalgae cultivated in 6% CO2 at 25 °C had the highest biomass productivity. The consortium’s performance was enhanced in treated wastewater. After 72 hours of incubation, nitrate and phosphate were almost completely removed from the growth medium, indicating high nutrient removal abilities. Biomass of the consortium algae grown in the raceway ponds was quantified and analyzed. Interestingly, the consortium had a high protein content (54.6%) and low lipid and carbohydrate content. When algal lipid content is lower than 40%, energetic cost of harvest can outweigh energetic added value of lipid recovery. Therefore, direct energy recovery may be necessary in algae with low levels of lipids.
To examine the viability of consortium algae-based biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production, crude algal oil was extracted from the biomass, chemically analyzed, and converted into biodiesel. The crude algal oil had a free fatty acid content of about 50%, a trait not conducive to biodiesel conversion. However, the total acid esterification showed a product yield of about 70.9%, with losses mainly resulting from oil impurities. While the biodiesel produced had a higher linolenic acid content (27.9%) than is normally acceptable (12%), the researchers speculate that the quality of the fuel could be improved by deriving some biomass from other non-food feedstocks.
The results of this study show that algal oil from mixed cultures of native algae is a feasible source of biostock for biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production. It will be necessary to find economical methods of crude oil refinement to minimize product impurities, and this remains a large obstacle. Despite the low lipid content of the consortium in this study, biomass recovery via thermochemical liquefaction could enhance recovery rates and reduce energy expenditure for algae cultures with low lipid content. Therefore, thermochemcial liquefaction should be studied and developed to make wastewater-cultivated consortium microalgae a commercially feasible process of biodiesel production.