Assessing Marimo Granules for Sustainable Wastewater Treatment: A Comparative Study with Microalgal-Bacterial Granular Sludge

There is growing concern over traditional wastewater treatment methods, such as the activated sludge process, which are energy-intensive and contribute to greenhouse gas emissions. In contrast, photo-driven biological treatment processes, leveraging the symbiotic relationships between microalgae and bacteria, offer a promising alternative. These processes not only reduce energy consumption but also fix carbon through natural photosynthesis, making them attractive for next-generation wastewater treatment technologies. Marimo, predominantly comprising the green algae Aegagropila linnaei, forms spherical aggregates in freshwater systems and is known for its water purification capabilities.  To this account, a new study published in Chemical Engineering Journal and conducted by Ms. Penghui Sun and Professor Bin Ji from the Wuhan University of Science and Technology, the authors assessed the viability and efficacy of marimo, a natural microalgal-bacterial granular consortium, for wastewater treatment compared to the well-studied system microalgal-bacterial granular sludge (MBGS) cultivated in a laboratory setting.

The research team prepared two stages of synthetic wastewater to simulate municipal wastewater, with varying compositions to mimic different pollutant loads and types encountered in real-world scenarios. The study used 12 sealed glass reactors with effective volumes of 50 mL each, half of which were dedicated to marimo and the other half to MBGS. Each reactor contained a single granule, ensuring consistency in the experimental conditions. The reactors were exposed to outdoor conditions, with natural day-night cycles, for a duration of 28 days. They conducted an indoor batch-mode experiment to study the effect of different granule sizes of marimo on pollutant removal efficiency under continuous lighting conditions, aiming to understand the impact of granule size on treatment efficacy. The authors employed a range of standard and advanced analytical techniques to assess the physical, chemical, and biological characteristics of the granules and to evaluate their performance in wastewater treatment including chemical analyses for measuring pollutants such as Chemical Oxygen Demand (COD), nitrogen, and phosphorus and physical measurements to assess granule size, density, and settling velocity. Moreover, micro computed tomography scans was performed to visualize the internal structure of the granules and metagenomics sequencing to elucidate the microbial community structure and diversity within the granules.

The authors showed that marimo exhibited a compact, homogeneous structure, differing significantly from the layered, looser structure of MBGS. This physical distinction was reflected in their microbial communities, with marimo primarily hosting Chlorophyta (Aegagropila) and MBGS dominated by Cyanobacteria (Leptolyngbya and Pantanalinema). Moreover, they found that both marimo and MBGS demonstrated the capacity to remove significant amounts of organics and nutrients across day-night cycles. Initially, marimo showed slightly lower removal efficiencies compared to MBGS, but as it adapted to the wastewater conditions, the differences in performance between the two systems diminished. Furthermore, the study revealed distinct microbial communities between marimo and MBGS, with specific algae and bacteria populations associated with each. Metabolic function analyses indicated that both systems shared similar metabolic pathways related to pollutant degradation, suggesting functional convergence over time. Additionally, the indoor experiments highlighted the influence of granule size on the pollutant removal efficiency, with smaller marimo granules exhibiting better performance. This suggests that optimizing granule size could enhance treatment efficacy. One of the study’s key contributions is the demonstration of the gradual similarity in the morphology, performance, and function between marimo and MBGS, suggesting a homologous relationship. This insight opens new avenues for the development of nature-derived solutions for wastewater treatment, aligning with the principles of environmental sustainability and the circular bioeconomy. By leveraging natural systems like marimo, which possess inherent advantages such as compact structure and high density, the study paves the way for innovative, efficient, and sustainable wastewater treatment technologies.

In conclusion, the research presented by Ms. Sun and Professor Ji demonstrated the potential of marimo as a viable and efficient system for wastewater treatment, comparable to MBGS under certain conditions. Indeed, the new study highlights the potential of marimo as a sustainable, efficient, and nature-based solution for wastewater treatment, urging further exploration into its cultivation, optimization, and application in real-world settings emphasizing the role of natural systems in sustainable environmental engineering solutions.