Facilitating Wastewater Purification through Progressive Thawing by Microwave: Responses of Microbial Communities

Abstract

Freeze–thaw has been proved to be a simple, cost-effective, and highly efficient manner to purify wastewater. However, it remains unclear how microbial compositions and functions in meltwater differentiate over progressive thawing and how such differences affect the end product water quality. In this study, wastewater was frozen, progressively thawed via microwave and collected at five intervals: 5 min, 3 min, 3 min, 3 min, and 3 min (termed as T1~T5). It only took 8 min of microwave and 38.8% of total water to remove more than 75% of the dissolved salt and typical pathogenic microbes, and merely 11 min to reach a removal rate greater than 90%. The Shannon index indicated that the α diversity of bacterial and fungal communities significantly reduced from T1 to T5, and the NMDS dissimilarities also illustrated significantly different β diversity between T1 and T2 and T3, T4, and T5. The OTU-based bacterial and fungal co-occurrence networks developed from T1, T5, and CK were significantly different from each other and clustered in distinct modules. Microbial functional profiles further showed that the meltwater preferentially discharged at T1 selectively removed pathogenic and symbiotic fungi and bacterial subsets associated with endocrine diseases, carbohydrate metabolism, and aging. Yet, other microbial subsets tended to be selectively enriched in the end product at T5, such as saprotrophic fungi and bacterial subsets related to drug resistance, infectious diseases, cancers, and xenobiotics’ biodegradation and metabolism. Overall, the fast thawing facilitated by microwave and in turn the efficient removal of brines from ice crystals offered a new approach to overcome the prolonged time cost required by natural thawing. Selective discharge and enrichment of microbial subsets during progressive thawing alarmingly calls for in-depth investigations on the temporal fluxes of microbes when attempting to reuse wastewater in the regions suitable to apply freeze–thaw protocols.