Taming Biofilm: Evaluating Chemical Strategies for Biofilm Mitigation in the Catskill Aqueduct

Authors:

  • Marc Santos PE, Thomas McEnerney PE – Hazen and Sawyer
  • Vasyl Kravchyk, Jon Hoffman PE, Todd West PE – New York City Department of Environmental Protection

Recent assessments indicate that existing biofilm deposits contribute to a reduction of hydraulic capacity in the aqueduct which supplies 40% of New York City’s water demand. Prior analysis of biofilm samples collected from the aqueduct have detected relatively high concentrations of manganese and iron oxides and the presence of filamentous metals oxidizing bacteria. These filamentous bacteria are associated with hydraulic capacity effects which exceed those expected from consideration of biofilm thickness alone, and have been linked to significant hydraulic reduction in water transmission systems.

Pilot-scale test loop evaluations were performed to investigate chemical treatment alternatives for mitigating biofilm fouling of raw water conduits in order to reduce energy loss and restore hydraulic capacity. Treatment evaluations have been performed using small-scale, polyethylene test loops utilizing a continuous supply of untreated water drawn from the Ashokan Reservoir supply. Test loop size and flow rates were designed to replicate velocities observed in the Catskill aqueduct and produce friction factors sufficient to distinguish effects of chemical addition alternatives. Chemical treatment alternatives investigated in the study include sodium hypochlorite, chlorine dioxide, and chloramines dosed at various concentrations between 0.25-4.0 mg/L.

The results of this study demonstrated visible fouling and significant changes in test loop friction factors resulting from nine weeks of operation with untreated Ashokan Reservoir water. Chemical treatment strategies were performed to evaluate the efficacy of each chemical to (1) reduce of established biofilm deposits on fouled tubing and (2) mitigate new biofilm growth in clean tubing. Seasonal variations in water quality parameters were monitored to identify key characteristics influencing biofilm hydraulic effects. Routine monitoring was supplemented by periodic sampling of test loop biofilm and aqueduct biofilm for development of bacteriological profiles using genetic sequencing. All results were evaluated collectively in an attempt to associate changes in water quality, friction factor, and bacteriological characteristics of test loop biofilm.

This paper will discuss test loop data demonstrating hydraulic impacts of biofilm deposits and chemical treatment strategies employed to reduce biofilm induced hydraulic effects.

Bacteriological data will be presented to support hydraulic changes observed during biofilm growth and treatment phases. Outcomes of the study are relevant to water transmission systems and are important to system operators looking to increase or maintain hydraulic capacity of aging infrastructure.

For more information, please contact the author at msantos@hazenandsawyer.com.

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