Microalgal cultivation in wastewater from the fermentation effluent in Riboflavin (B2) manufacturing for biodiesel production

Highlights

• This study has demonstrated the feasibility to cultivate microalgae in wastewater.

• The mutual symbiosis was an effective process to remove and converse the nutrients.

• Feeding gas with different concentrations of CO₂ to the cultivation was studied.

• The acclimated microalgae can survive in high salinity wastewater.

Abstract

In this work, the acclimation of Chlorella pyrenoidosa in diluted wastewater was studied to produce biomass and remove chemical oxygen demand (COD), ammonia-N and phosphorous. The results indicated that the optimal conditions (the volume ratio of wastewater, light intensity, culture temperature, CO₂ concentration in feeding gas) which could influence the wastewater treatment efficiency were 0.05, 250 photons m⁻² s⁻¹, 28 °C and 5%, respectively. Under these conditions, the removal efficiency of COD reached up to 89.2%, while the total nitrogen and total phosphorous decreased by 64.52% and 82.20%, respectively. With the second treatment, COD in the wastewater was further reduced to less than 100 mg/L while it was only reduced to 542.9 mg/L after the first treatment. The treated wastewater could be discharged directly or subjected to for further treatment for recycling. In addition, 1.25 g/L of the biomass and 38.27% (dry basis, w%) of lipid content were reached after microalgal cultivation.

Introduction

The use of fossil fuel has led to many environmental problems such as air pollution and global warming. Therefore, finding renewable energy resources including biofuel has captured worldwide attention. The cultivation of algae, particularly green unicellular microalgae, has been studied to produce biofuel (Chen et al., 2011). Compared to other ways of producing biofuel, microalgal cultivation has several advantages such as less consumption of fresh water, high biomass productivity, taking non-food resource as raw materials and helping to reduce CO₂ concentration in the atmosphere (Demirbas, 2011, Khan et al., 2009). Besides, microalgal cultivation can also be used for wastewater treatment because the microalgae can consume the nutrients in wastewater for its own growth and biomass production (Olguín, 2003). Extensive research has been conducted to explore the feasibility of using microalgae to treat wastewater, especially for the removal of nitrogen, phosphorus and chemical oxygen demand (COD) from effluents (Hameed, 2007, Hernandez et al., 2006). Levels of several contaminant heavy metals have also been reported to be reduced by the cultivation of microalgae (Muñoz and Guieysse, 2006). In addition, the use of microalgae is a unique technology for the purpose of CO₂ sequestration, helping alleviate the trend toward global warming (Ono and Cuello, 2001). In recent studies, there is considerable interest in the combination of biological wastewater treatment processes and bio-energy production. All research results suggest that dual-use microalgal cultivation for wastewater treatment coupling with biofuel generation is an attractive option in terms of reducing the energy cost, CO₂ emissions, and nutrient and freshwater resource costs (Cho et al., 2013, Farooq et al., 2013, Kong et al., 2010). Waste-to-Energy conversion technologies have several advantages over traditional processes because of lower its operation and maintenance costs, low land requirement, low sludge production, applicability to most types of high organic content substrates, operation at low temperature and pressure, and for the production of biofuels (Elmitwalli et al., 2000).

Algae-bacteria symbiosis in wastewater treatment has been observed extensively. By taking advantages of the mutual symbiosis which occurs between algae and bacteria due to their combined ecological relationships, an effective process could be developed for the removal and the conversion of nutrients in wastewater. In this process, CO₂, the metabolic product of bacterial, is an essential nutrient for algal growth. Meanwhile, algae supply oxygen as a product of photosynthesis to bacteria (González et al., 2008). It is a mixed culture for bacteria and algae at the initial stage. However, Chlorella pyrenoidosa gradually increased and became the dominant species in the culture at later stage. The number of bacteria decreased as a result of the algae competition for nutrients. We found out that microalgae in algal-bacterial mixed cultures with the wastewater as the only nutrients grew much better than that in SE medium or sterile wastewater.

Hubei Guangji Pharmaceutical Co. Ltd is one of the main riboflavin (Vitamin B2) producers in China. Wastewater from its daily production is around 1500 ton/day. The wastewater was characterized as having a considerable pollutant load, substantial amount of total solids (TS), organic and ammonia nitrogen compounds, and high biochemical and chemical oxygen demands (BOD and COD). Although flocculation, up-flow anaerobic sludge blanket (UASB), and aerobic Anoxic/Oxic (A/O) techniques have been suggested to reduce the pollution of the wastewater, these techniques cannot remove sufficient nutrients to make treated wastewater to meet discharge standards. Therefore, environmentally friendly and cost-affordable sustainable technologies for this wastewater remediation are needed.

This study was conducted with the aim to evaluate wastewater from fermentation effluent in B2 (Riboflavin) process as growth medium for algal cultivation and to demonstrate the technical feasibility of removing waste nutrients (mainly ammonia-N and organics). The application of algal-bacterial systems for the reclamation of fermentation effluent provides microalgal O₂ produced by photosynthesis for bacterial breakdown of the organics via nutrients removing, and transforms N and P into the algal-bacterial biomass. In addition, we attempt to find out optimized culture conditions to improve the oil content of biomass.