Raymond M. Hozalski, William A. Arnold, Carrie R. Pearson, and Jeong-yub Lee

OBJECTIVES:

The specific objectives of this research were to

1. conduct a survey of water utilities to determine whether abiotic disinfection by-products (DBPs) degradation (i.e., reduction) is occurring in distribution systems and what conditions favor these reactions;

2. determine the kinetics and pathways of the degradation of a wide variety of DBPs in the presence of pipe solids collected from full-scale water distribution systems;

3. evaluate the effects of environmental conditions (dissolved oxygen) on abiotic DBP degradation rates in the presence of pipe solids; and

4. evaluate the degradation of DBPs in novel pipe reactors consisting of cast iron pipe sections.

BACKGROUND:

During the development of the Stage 2 Disinfectant/Disinfection By-Products Rule (D/DBP), a major weakness in modeling the formation and control of DBPs was a lack of understanding of the impact of distribution system phenomena. In particular, kinetic models of DBP degradation at the pipe wall for both regulated and emerging DBPs were needed. Improved quantitative models for predicting DBP reactivity will aid utilities in identifying DBP monitoring locations in distribution systems and in the development of treatment systems to remove these compounds from finished drinking water.

HIGHLIGHTS:

The results of this research indicated that many DBPs are degraded by iron metal and iron corrosion products. Dissolved oxygen does not inhibit the reaction of highly reactive (i.e., mass transfer limited) DBPs. Thus, abiotic reduction may be an important loss process for many DBPs in water distribution systems, especially halonitromethanes and bromine- and iodine-substituted DBPs. This was confirmed by observing the loss of tribromoacetic acid (TBAA) under aerobic conditions in reactors constructed from new and old cast iron pipes.

APPROACH:

A survey of water utilities in the United States and Canada was performed to assess (1) the occurrence of DBP degradation in distribution systems, (2) the contribution of abiotic DBP degradation to the overall degradation observed, and (3) the conditions that favor DBP degradation. Batch experiments were conducted in glass serum bottles to determine the kinetics and pathways of the degradation of a variety of DBPs in the presence of iron metal (Fe⁰) and cast iron pipe corrosion products obtained from drinking water distribution systems. Finally, DBP degradation experiments were performed in novel reactors constructed from 1 ft long sections of 6 inch diameter iron pipe to better mimic conditions in the distribution system.

RESULTS/FINDINGS:

DBP Degradation

The trichlorinated DBPs investigated in this work (trichloronitromethane [TCNM]; trichloroacetonitrile [TCAN]; 1,1,1-trichloropropanone [1,1,1-TCP]; trichloroacetic acid [TCAA]; and trichloromethane [TCM]) were all readily reduced by Fe⁰. Carbonate green rust, Fe^II/magnetite, and Fe^II/goethite were capable of reducing most of the DBPs studied except for TCAA (only reduced by green rust) and TCM (unreactive). Regardless of reductant, the reactivity trend was as follows: TCNM > TCAN > 1,1,1-TCP ~ trichloracetaldehyde (TCAh) >> TCAA >> TCM. DBPs that were degraded by iron minerals were also susceptible to degradation by iron corrosion products collected from water distribution system pipes. For example, the reduction of TCNM in the presence of iron corrosion products was rapid and was a function of the water soluble Fe^II in the solids. Hydrolysis was significant for 1,1,1-TCP, TCAh, and TCAN, and can lead to the formation of other DBPs (THMs and HAAs).

Effect of Oxygen on DBP Reactivity

The presence of dissolved oxygen does not affect the reaction kinetics of fast reacting DBPs (e.g., TCNM, TCAN) by Fe⁰ because these compounds react at mass transfer limited rates. Dissolved oxygen did slow the reaction of TCNM with iron corrosion products. For the slower reacting DBPs, dissolved oxygen inhibits reduction by all of the reductants studied.

DBP Degradation in Pipe Reactors

TBAA was degraded via reductive dehalogenation in reactors constructed from new and old cast iron pipes and operated at room temperature under aerobic conditions.

IMPACT:

The results of the experiments on the degradation of DBPs by iron metal, synthetic iron minerals, and iron corrosion products suggest that abiotic reductive transformation is likely to be an important process in controlling the fate of some DBPs in distribution systems, especially halonitromethanes, haloacetonitriles, brominated DBPs, and iodinated DBPs. This is significant because halonitromethanes and brominated and iodinated DBPs tend to be relatively toxic. In addition, the relatively rapid degradation of most DBPs in the presence of Fe⁰ suggests that a Fe⁰-based treatment system (e.g., packed-bed) may be effective at removing DBPs from water.

PARTICIPANTS:

St. Paul Regional Water Services (Saint Paul, Minn.) and Stevens Point Water Utility (Stevens Point, Wisc.) provided numerous samples of iron pipe from their distribution systems. Eighty-one utilities participated in the distribution system survey.