Field-Scale Evaluation of Drinking Water Biofiltration

Abstract

Natural organic matter (NOM) is a complex mixture of organic material ubiquitous in natural waters. NOM can affect nearly all aspects of drinking water treatment. It can exert a demand on treatment chemicals, promote regrowth in distribution systems and can form genotoxic and/or carcinogenic disinfection by-products (DBPs) when exposed to disinfectant. Biofiltration is one treatment strategy that has potential to provide additional removal of NOM following coagulation. In biofiltration, bacteria indigenous to the source water form biofilms on filter media and use organic material as an energy source. This type of biological treatment within a filter has advantages over filtration with relatively biologically inert granular media because of its potential to provide additional NOM removal through biodegradation.

This thesis investigated conversion of full-scale anthracite-sand drinking water filters to biofilters through the removal of prechlorination. Results showed that filters operated in direct filtration mode could be converted in this way to reduce DBP formation in the plant effluent and distribution system without compromising water quality or filter performance. Biomass monitoring using adenosine triphosphate (ATP) showed that filter media biomass increased as a result of conversion. Further interpretation of the biomass data with a growth model demonstrated that consistency in biomass sampling within the context of the operational state of the filter or following significant process changes was critical information for long-term performance assessment. A concurrent pilot-scale investigation tested nutrient, oxidant and filter media enhancement strategies with the goal of improving NOM removal and further reducing DBP formation. Results showed that nutrient and oxidant addition could increase the filter biomass and alter the microbial community, but would not improve NOM removal or further reduce DBP formation potential.

Ultimately, despite reductions in DBP formation and increases in biofilter biomass, NOM removal across the biofilters remained unchanged with conversion and enhancements, posing a challenge for process monitoring. A novel method to measure oxygen demand was optimized for use in a drinking water matrix and used to evaluate NOM removal and transformation in the biofilters