(620b) Nutrient Recovery from Municipal Wastewater Using a Methanotroph-Microalgae Co-Culture

In recent times, human activity has increased the amount of nitrogen fixation in the world causing negative consequences for our environment (greenhouse effect, reducing the protective ozone layer, contaminating drinking water), the human health and the economy [1]. Thus, effective treatment of wastewater (rich in nutrient nitrogen) plays a key role in managing the nitrogen cycle and improving our ecosystems and public health.

At water resource recovery facilities (WRRFs), anaerobic digestion (AD) is the most widely employed method to reduce the amount of sludge from both primary and secondary treatment, as well as reducing pathogens and odor [2]. The nitrogen and phosphorus content in the AD effluent are usually removed by secondary treatment; however, secondary treatment consumes energy and does not always remove the nutrients to the desired degree, which often cause toxic algae bloom in water ways and costal area. In addition, the impurities (e.g., CO₂, H₂S, etc.) in biogas resulting from the AD are difficult to remove economically; thus, the value of biogas is low. As a result, 80% of WRRFs either flare the produced biogas or use it for heating, while only 20% of them use biogas for power generation or driving machinery [3].

In this work, we aim to develop a new wastewater treatment platform where we use a coculture of methanotroph-microalgae to augment the AD treatment. In the proposed framework, the methanotroph-microalgae coculture recovers nutrient from the AD effluent as well as both methane and carbon dioxide from biogas without requiring external oxygen supply. To realize the potential industrial application, the robustness of the co-culture must be determined considering the inhibitory compounds in wastewater. Such robustness will determine the level of required pretreatment of wastewater, which could significantly reduce the economic feasibility of the new platform.

In our previous work, we have assembled several synthetic methanotroph-photoautotroph cocultures that exhibit stable growth under various substrate delivery and illumination regimes [4]. In this work, using Methylococcus capsulatus - Chlorella sorokiniana as the model coculture system, we investigated the co-culture’s potential in combined nutrient recovery and biogas co-utilization on pretreated AD effluent. The performance of the co-culture technology was assessed on filtered AD effluent diluted with clarifier water; where both AD effluent and clarifier water were obtained from the South Columbus Water Resources Facility. Our results show that co-culture can be maintained completely on wastewater effluents with good nutrient removal. AD effluent pretreated by filtering has shown to maintain better growth than on AD effluent pretreated by a combination of filtering and autoclaving. We have also been able to support the co-culture on AD effluent after allowing the sludge to settle. In addition, the co-culture enabled ~85% recovery of total nitrogen and up to ~99% recovery of total phosphorus in the diluted AD effluent. Our results show that the methanotroph-microalgae co-culture is a promising platform for wastewater treatment through simultaneous nutrient recovery from AD effluent and biogas conversion with the potential of producing value added products from wastes.

References

[1] Driscoll, C., Whitall, D., Aber, J., Boyer, E., Castro, M., Cronan, C., ... Ollinger, S. (2003). Nitrogen Pollution: Sources and Consequences in the U.S. Northeast. Environment: Science and Policy for Sustainable Development, 45(7), 8–22.

[2] Cushman-Roisin, B; Anaerobic Digestion of Wastewater Sludge; Introduction to Environmental Engineering; Dartmouth College; New Hampshire. 2018

[3] Water Environmental Federation. Biogas production and use at water resource recovery facilities in the United States. Alexandria, VA: Water Environment Federation; 2013.

[4] Roberts, N.; Hilliard, M.; Bahr, K.; He, Q. P.; Wang, J. Coculture of Methanotrophs and Microalgae – a Flexible Platform for Biological CH4/CO2 Co-Utilization, 2017 AIChE Annual Conference, Oct. 28 – Nov. 3, 2017, Minneapolis, MN. This work won 2017 AIChE Session’s Best Paper Award.