INVESTIGATING BIOLOGICAL ROOT CAUSES OF HYDRAULIC PERFORMANCE CHALLENGES IN DRINKING WATER BIOFILTERS

Abstract

United Nations SDG6 defines that providing safe drinking water to all (goal 6) and SDG12 mandates employing sustainable approaches for all. Utilizing indigenous microbial communities (IMC) to remove water contaminants provides a promising pathway to provide safe and affordable drinking water sustainably. Although drinking water biofiltration already achieves high productivity rates, researchers are pursuing further optimizing the system to improve the overall performance and lower maintenance. Intentional operation of biofilter mandates understanding the microbiome, which provides the foundation for developing educated monitoring, prediction, and optimization strategies. In this study, microbiome analysis was used to monitor the biofilter microbial community in regular and challenging filter events. Challenging filter events refer to events with lower hydraulic performance. In two years, regular operations and events such as diatom algae bloom and a media replacement were investigated. The diatom bloom event, representative of intense source water quality changes, resulted in a significantly lower filter runtime (24.9 h) than the average of 70 h. Beta diversity analysis showed a distinct difference between the fresh media and all other media. Besides to understanding the microbiome changes, the use of a clarification step to improve hydraulic performance was studied. It is known that a clarification step reduces the floc carryovers, however the impact of a clarification step prior to a biofilter has not been studied. Therefore, this study was designed and conducted on a pilot scale to investigate the impact of a sedimentation process prior to a biofilter. Results showed that sedimentation improved FRT up to 30% and reduced head-loss, and head-loss accumulation rate up to 29% and 35%, respectively. The NOM removal in effluent water did not change while the turbidity decreased significantly. Biological activity changed measured as EPS (reduced by 36%). The results of this study triggered the investigation of the biological root causes of the different EPS level observed. Thus, the two most biofilm-producing bacteria were isolated from full-scale biofilters. The isolated bacteria were then identified as Bacillus mycoides and Paenibacillus tundrae. The characterization step led to developing a reliable and high-throughput protocol for measuring bacterial growth curves and maximum growth rate. The protocol was developed addressing accuracy and repeatability issues, including (i) lid condensation, (ii) pathlength correction, (iii) inoculation method, and (iv) sampling time interval were investigated. The maximum growth rates were determined for Bacillus mycoides (0.99±0.03 h-1) and Paenibacillus tundrae (0.85 ± 0.025 h-1). It was hypothesized that the flocs buildup inhibits the nutrient diffusion to the bacteria. Subsequently, the impact of nutrient limitation, specifically P as an essential macronutrient, was studied under different P limited conditions for isolated bacteria. Carbohydrate and protein-EPS were increased when decreasing the available P in the medium while the ATP level decreased and reached a plateau. The results of this study showed that inhibiting nutrient diffusion due to the floc buildup could cause higher EPS production in the biofilter without a clarification step. In conclusion, this study provides a novel insight by combining detailed information about the IMC, traditional filter performance, and biological activity indicators allowing for a deeper understanding of the biological root causes of reduced hydraulic biofilter performance. A fundamental toolkit to predict and subsequently avoid challenging events, leading to a more sustainable and affordable drinking water biofiltration in large and small facilities was presented. The toolkit was examined in a full-scale water treatment facility providing the applicability aspect. Sedimentation as a clarification step was implemented for the first time and proved to be a successful optimization strategy improving water quality and hydraulic performances of the biofilters.