(545am) Using Methanotroph-Microalgae Coculture for Wastewater Treatment

Currently, the amount of excess fixed nitrogen in the world is causing increasingly negative consequences for our ecosystems and the public health, including worsening of the greenhouse effect, reducing the protection ozone layer, adding to smog, contributing to acid rain, and contaminating drinking water^1,2. Consequently, effective treatment of wastewater (rich in nutrient nitrogen) plays a key role in managing the nitrogen cycle, and improving our ecosystems and the public health.

In the water resource recovery facilities (WRRFs), activated sludge is the most commonly applied approach to reduce biochemical oxygen demand (BOD) and remove nutrient (N and P). Anaerobic digestion (AD) is the most widely employed method to reduce the amount of sludge from both primary and secondary treatment, and to produce biogas, a renewable source of energy and feedstock to produce high-density fuels and chemicals. Besides, AD can mitigates pathogens and reduce odor. Because of these advantages, AD has been widely adopted by WRRFs in the US. It is estimated that 48% of total wastewater flows in the US is treated by AD, with at least 1238 WRRFs that currently process solids through AD. However, due to impurities (e.g., CO₂, H₂S, etc.) that are difficult to remove economically, the value of biogas is low. As a result, among 1238 WRRFs that has AD installed, about 80% (~975) of them flares the produced biogas or burn it for heating and cooking, while only 20% (~263) of them use biogas for power generation or driving machinery. The AD effluent is fed back to primary treatment to recover the nutrient.

In this work, we aim to develop a new wastewater treatment platform where we use coculture of methanotroph-microalgae to augment the AD treatment. In the proposed framework, the methanotroph-microalgae coculture not only recovers energy and carbon from biogas without requiring external oxygen supply, but also recover nutrient from AD effluent. In addition, the interdependent yet compartmentalized configuration of the coculture offers flexibility and more options for metabolic engineering for producing high-density fuels and commodity chemicals from various wastes. For any biotechnology to become potential industrial applications, the stability and robustness of the biocatalyst has to be established, in particular with respect to various contaminations contained in wastewater. Such robustness will determine the level of required pretreatment of wastewater, which could have a significant impact on 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 regimes³. In this work, using Methylosinus trichosporium - Chlorella sorokiniana as the model coculture system, we examine its potential in the combined biogas conversion and nutrient recovery from wastewater. Specifically, we first adapted both strains on diluted AD effluent using clarifier water (the water to be discharged from a WRRF), with all water samples provided from Columbus Water Works, and examined the performance of the coculture-based treatment. Our results show that cultivation of the coculture on AD effluent pretreated by filtering can support better growth than on AD effluent pretreated by a combination of filtering and autoclaving. In addition, we have shown that the coculture can be supported on AD effluent after allowing the sludge to settle.

Our results show that methanotroph-microalgae coculture offer a very promising route for simultaneous biogas conversion and nutrient recovery from AD effluent, while potentially producing value added products from wastes.

Reference

(1) Driscoll, C.; Whitall, D.; Aber, J.; Boyer, E.; Castro, M.; Cronan, C.; Goodale, C.; Groffman, P.; Hopkinson, C.; Lambert, K. Nitrogen Pollution: Sources and Consequences in the US Northeast. Environ. Sci. Policy Sustain. Dev. 2003, 45, 8.

(2) Galloway, J. N.; Dentener, F. J.; Capone, D. G.; Boyer, E. W.; Howarth, R. W.; Seitzinger, S. P.; Asner, G. P.; Cleveland, C. C.; Green, P. A.; Holland, E. A. Nitrogen Cycles: Past, Present, and Future. Biogeochemistry 2004, 70, 153.

(3) 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.