Graduation Year

2021

Document Type

Thesis

Degree

M.S.E.V.

Degree Name

MS in Environmental Engr. (M.S.E.V.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Katherine Alfredo, Ph.D.

Committee Member

Sarina Ergas, Ph.D.

Committee Member

James Mihelcic, Ph.D.

Keywords

chloramine, drinking water, EPA, nitrification, nitrite

Abstract

Point-of-use (POU) activated carbon (AC) filters are ubiquitous in many U.S. households. AC can reduce concentrations of lead, other heavy metals, and mitigate taste and odor issues. However, AC filters also remove residual disinfectants, thus allowing for the proliferation of microbes in the filter. In chloraminated systems, this can lead to localized, filter-induced nitrification. Most notably, high nitrite and nitrate in drinking water can cause methemoglobinemia (blue baby syndrome) in children under the age of three, raising public health concerns.

As a control measure for nitrification within distribution systems, utilities practice periodic, short-term secondary disinfectant switches from chloramine to free chlorine (chlorine conversion/ free-chlorine period (FClP)). This study investigates the impact of chlorine conversions on inline AC-POU filters and the occurrence of nitrification before, during, and after the conversion.

To test these impacts, a laboratory-based filter rig was constructed with three new, commercially available AC-POU filters. The City of Tampa piped water supplied within the laboratory was used as the influent to test impacts of the 7-28 August 2020 (FClP-1) and the 8-29 March (FClP-2) chlorine conversion. Filter 1 began operation 34 days before the FClP-1 Filter 2 started halfway through the FClP-1 and Filter 3 started once monochloramine concentrations stabilized in tap water samples post- FClP-1. Monitored influent and effluent water quality parameters included: total ATP, nitrite, nitrate, total chlorine, monochloramine, free-chlorine, pH, temperature, and dissolved oxygen.

Before the FClP-1, Filter 1 nitrite concentrations exceeded the EPA MCL after only 32 days of operation, implying filters can immediately nitrify after installation in a system expected to have biofilm and planktonic nitrifiers within the system (immediately before a conversion). During the free-chlorine periods (FClPs), effluent concentrations of nitrite, nitrate, and total-ATP decreased immediately, signifying microbial inactivation and nitrification reduction in the filters.

Post-conversion nitrification onset depended on filter age and whether the filters previously experienced nitrification. Nitrifiers were reactivated immediately in Filter 1 post-FClP-1, with observed nitrite and nitrate levels rapidly increasing, with nitrite exceeding 0.5 mg/L-N after only 6 days post chlorine conversion while Filter 2 & 3 delayed nitrification until 4 weeks. Previous incidences of nitrification within the filters had lasting water quality impacts. During FClP-2, effluent concentrations of nitrite, nitrate, and Total-ATP decreased immediately. However, post-FClP-2, all three filters re-nitrified immediately within a week, with filter 3 recording the highest concentrations (3.49 mg/L NO2 as N) and was the fastest to nitrify.

Increasing concentrations of nitrite, nitrate, and total ATP varied based on the filter operation condition. Though there was no statistically significant difference in overnight and weekend stagnation samples, stagnation, in general, resulted in greater concentrations of nitrite, nitrate, and total ATP counts while periodic flush samples recorded the lowest concentration below the USEPA MCLs. This research demonstrates that free chlorine conversions did little to mitigate nitrification in POU AC filters. Based on the results obtained in this study, AC POU in chloraminated water systems practicing periodic free chlorination raises possible public health concerns.

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