A scalable and sustainable wastewater treatment technology using a methanotroph-microalga co-culture

Abstract

Rapidly growing human population generates large volumes of industrial, municipal, and agricultural wastewater. Traditional wastewater processes face major drawbacks with high energy consumption, inefficient nutrient recovery, excess sludge disposal, and carbon emissions. However, microalgal-based technologies have received more focus in recent years as an alternative wastewater treatment process as they can address significant challenges faced in conventional multistep wastewater treatment process. Specifically, researchers have demonstrated the feasibility of using microalgae as an efficient and cost-effective recycling method for rapid nutrient recovery. Microalgae can also reduce carbon emission through biogas upgrading to biomethane while generating biomass that can be used as a sustainable feedstock for producing valuable products such as biofuels. Despite these advantages, microalgal technology for tertiary wastewater treatment has its own unique challenges. There exists a safety risk when oxygen gas produced during photosynthesis is mixed with methane in biogas. In addition, small-scale water resource recovery facilities (WRRFs) may not utilize the upgraded biogas for combined heat and power (CHP) as several clean up steps render the process uneconomical.

In this research, a highly promising methanotroph-microalga co-culture technology is presented for upgrading biogas while simultaneously bioremediating wastewater effluents using Methylococcus capsulatus and Chlorella sorokiniana as the model co-culture pair. A fast, online experimental-computational protocol was developed for frequent characterization of the co-culture without expensive equipment and time-consuming methods. The developed protocol allowed individual strain biomass estimations as well as O2 and CO2 gas consumption and production rates using yields coefficients. As compared to reducing pollutant levels with freshwater, cultivation of the co-culture on unsterilized municipal anaerobic liquid digestate diluted with secondary clarifier effluent (CLE) has demonstrated higher biomass production and faster recovery of nitrogen and phosphorus where all wastewater samples were acquired from the South Columbus Water Resource Recovery facility. Additionally, the co-culture technology grown on wastewater did not require micronutrient supplementation, as the co-culture biomass productivities between unsterilized AD diluted with CLE and the co-culture grown on defined ammonium mineral salts were not statistically different. Consequently, these results demonstrate the potential for significantly reducing the operating costs of the co-culture technology when the process is scaled up.

Furthermore, studies comparing the sequential C. sorokiniana and M. capsulatus single cultures to the co-culture has demonstrated that the co-culture has advantages over the single cultures. The metabolic coupling of the co-culture enables a significant increase in biomass production and nutrient recovery as compared to the sequential single cultures. Also, the co-culture can co-utilize both CH4 and CO2 in biogas through bioconversion into microbial biomass without external oxygen supply. Thus, process safety is enhanced as the in situ produced O2 is consumed by M. capsulatus before it can be mixed with CH4 in biogas. More importantly, the results suggest the co-culture can play a critical role in reducing air pollution as the co-culture simultaneously and completely captured both CH4 and CO2 in the vial experiments. Growth under an analogous amount of biogas substrate demonstrated the co-culture recovered up to 100% of the inorganic nutrient while the sequential single cultures recovered up to 55% and neither the sequential single cultures nor the co-culture was able to recover the organic nutrient fraction in the diluted AD effluent.

Using a bench-scale photobioreactor, the co-culture converted biogas into microbial biomass to achieve steady state co-culture biomass productivity of 0.818 g/L/day. Under continuous growth, steady state was reached due to irradiance and O2 limitations on the co-culture. Despite these limitations, the co-culture achieved good illuminated areal productivity of 22.8 g/m2/day under chemostat cultivation and the estimated CH4 and CO2 gas consumption rates were 0.634 and 0.658 mmol/g/h, respectively. Ammonia-nitrogen (NH3-N) and orthophosphate (PO43--P) was continuously recovered during continuous cultivation with the outflow residual NH3-N and PO43--P reaching as low as 0.038 mg/L and 3.1 mg/L, respectively. Overall, the results indicate that the co-culture platform can be an economical technology for upgrading biogas into microbial biomass and mitigating AD effluent pollution by recovering nutrients from unsterilized wastewater.