Engineered Biofiltration for Improved Hydraulic and Water Treatment Performance

Water Research Foundation and the City of Arlington, 
Texas

Engineered Biofiltration for Improved Hydraulic and Water Treatment Performance

Water Research Foundation Tailored Collaboration 4215

Service Type: 
Project Highlights: 
  • The project included a 9-month pilot-scale study that identified two enhancement strategies that improved both water quality and hydraulic performance of a biofilter: nutrient- and peroxide-dosing.
  • These strategies controlled the formation of extracellular polymeric substances, a potential foulant of biological filters, while maintaining or increasing microbial activity.
  • Biofilter nutrient enhancement was found to decrease terminal headloss by ~15% relative to a control biofilter with no nutrient enhancement. Nutrient enhancement also sustainably decreased the breakthrough of 2-methylisoborneol (MIB), manganese, and dissolved organic carbon.
  • The peroxide enhancement strategy decreased terminal headloss up to ~60% relative to a control biofilter.

Introduction

Currently, drinking water biofiltration is largely operated as a passive process. Particle/turbidity removal and headloss drive the design and operation of biofilters as they do for conventional filters. Thus, biofilter design and operational parameters are typically limited to media configuration, backwash strategy, and loading rate. The biological removal of dissolved organic and inorganic contaminants is an anticipated benefit of biofiltration; however, common practice does not seek to enhance the biological activities associated with those removals. Indeed, in an effort to improve biofilter productivity and minimize headloss, many utilities use chlorinated or chloraminated water in the filter backwash or feed. This practice, however, can be detrimental to biological activity and might be ineffective in removing a potential foulant of biofilters – extracellular polymeric substances (EPS), which are produced by bacteria in the biofilter. EPS can occupy as much as 1,000 times the filter media void space occupied by bacteria, thereby playing a far more significant role than bacteria in media clogging, underdrain fouling, and filter headloss.

An alternative approach is to move the practice of biofiltration from a passive process, operated solely based on conventional filtration objectives, to a purposefully operated biological system, i.e. “Engineered Biofiltration.” The goal of Engineered Biofiltration is to target multiple water quality objectives while maintaining or improving hydraulic performance. The study presented here includes a pilot-scale evaluation of two engineered strategies to meet this goal: nutrient enhancement and peroxide enhancement.

Background

The City of Arlington, Texas (City) owns and operates two ozone/biofiltration drinking water treatment facilities, the Pierce-Burch South Water Treatment Plant (PBSWTP) and the John F. Kubala Water Treatment Plant (JKWTP). While the PBSWTP and JKWTP have performed well for many years, recently observed biofilter hydraulic and water treatment performance disruptions led the City to seek enhancement strategies for their biofiltration systems. Performance disruptions included underdrain cap clogging, increased decay rates of chloramine residuals (caused by organic carbon breakthrough), and decreased manganese (Mn) and 2-methylisoborneol (MIB) removals, resulting in color and odor complaints, respectively.

This study included ten months of biofiltration pilot-scale testing at the JKWTP to evaluate nutrient- and peroxide-enhancement for restoring and improving the performance of the City’s biofiltration process. This evaluation included characterizing changes in hydraulic performance, water treatment performance, and microbial activity with each biofilter enhancement strategy. The overall objective of this research was to identify strategies to enhance biofilter biological activity without compromising productivity or particulate removal. A broader goal of this work is to shift the industry-accepted biofiltration paradigm so that the design and operation of biofilters are driven not only by traditional filtration objectives (i.e., production and turbidity control) but also by biological treatment objectives.

Results and Discussion

Nutrient-Enhancement

Phosphoric acid was dosed at ~20 µg/L PO4-P to maintain a substrate limited condition based on a 100:10:1 C:N:P ratio. This strategy decreased biofilter terminal headloss by approximately 15 percent relative to a control, which translates to longer filter run times. This hydraulic performance improvement is illustrated Figure 1, which shows a typical parallel nutrient-enhanced and control biofilter headloss profiles obtained from a series of parallel biofilter runs. Microbial characterization data from this study suggest that nutrient enhancement can decrease EPS concentrations by >30 percent while increasing media activity levels (i.e., adenosine triphosphate, ATP) by >30 percent. The decrease in EPS is likely responsible for the observed decrease in biofilter headloss. These results suggest that the nutrient-enhancement strategy may mitigate underdrain cap clogging caused by EPS, thereby extending underdrain life. Phosphorus dosing also improved contaminant removal during biofiltration. The observed increase in media ATP concentration corresponded with higher removals of DOC, Mn, and MIB relative to the biofilter control. These results suggest that optimal contaminant removal may not be realized in many biofilters due to nutrient limitation. Furthermore, a conceptual cost evaluation indicated that these benefits can be achieved with little investment ($0.44/MG), and may provide a cost-benefit with improved unit filter run volumes.

WaterRF figure

Peroxide Enhancement

Hydrogen peroxide was dosed to a biofilter to augment the oxidative action and response of the biofiltration process. Preliminary testing of this strategy was conducted by dosing 1 mg/L of peroxide to a pilot filter for 10 days. The test filter demonstrated 15 percent removal of filter feed DOC (50 percent greater than that observed with the control) and complete removal of Mn and MIB. Peroxide-enhancement also decreased terminal headloss by >60 percent relative to the control biofilter. Peroxide supplementation did not impact ATP concentrations, suggesting that the decreased headloss trends are likely due to microbially-mediated peroxide oxidation of EPS rather than oxidation of active bacteria. Although further characterization of this strategy is necessary to identify specific mechanisms, the initial results suggest that this strategy deserves additional investigation as a potential tool for managing biofilter fouling.

Conclusions

This research sought to identify and characterize biofilter operational strategies that enhance biological activity without compromising productivity or particulate removal performance. Nutrient- and peroxide-enhancement improved biofilter hydraulics while maintaining or improving water treatment performance, making both operational strategies applicable to utilities with existing or planned biofiltration facilities.