2. Before we discuss DBP formation tests, we need to agree ...



C:\DAR\PUBL\MACNEILL\DBPFRAC.DOC

The Relationship Between Raw Water Quality and the Formation of Chlorination Byproducts

David A. Reckhow, Andrew MacNeill, Benedicte Mousset

Abstract

Introduction

Significance of this type of research

DBP formation and WQ modeling in watersheds.

|Fraction |Composition |

|Hydrophobic |Acids | |

| | Weak |Tannins; phenols; intermediate MW alkyl :monocarboxylic acids (C5-C8), dicarboxylic |

| | |acids (C8-C11) |

| | Strong |Fulvic acids; humic acids; high MW alkyl monocarboxylic acids (C9), and dicarboxylic |

| | |acids (C12); aromatic acids |

| |Bases |Amphoteric proteinaceous materials; high MW (C12) alkyl amines; alkyl pyridines; |

| | |aromatic amines |

| |Neutrals |Hydrocarbons; high MW (C6) methyl ketones; furans; most ethers; high MW (C5) alkyl |

| | |alcohols, and aldehydes; lactones; pyrrole |

|Hydrophilic |Acids |hydroxy acids; sugar acids; sulfonic acids; low MW U,alkyl monocarboxylic acids |

| | |(C1-C4), and ,dicarboxylic acids (C2-C7) |

| |Bases |low MW (C1-C11) alkyl amines; amino acids; purines; pyrimidines; pyridine; hydroxy |

| | |pyridines |

| |Neutrals |polysaccharides; Low MW (C1-C4) alkyl alcohols, aldehydes, and ketones; poly-ketones; |

| | |amides |

Based on: Leenheer and Noyes, 1984; Leenheer et al., 1982; and others

DBP Formation Tests

Before we discuss DBP formation tests, we need to agree on what it is we are talking about. I think its useful to divide these tests into 3 categories: (1) the high-dose formation potential (HDFP) tests; (2) the low-dose formation potential (LDFP) tests; and (3) the SDS tests. The HDFP is characterized by two attributes (a) it is not used to simulate any particular water system or water treatment scenario; and (b) it uses a sufficiently high chlorine dose so that the residual remains high and nearly constant with contact time, and independent of chlorine demand (e.g., we use a dose of 20 mg/L for samples with a chlorine demand of 8 mg/L or less, so that the residual is always between 12 and 20 mg/L). The HDFP is trying to be an unbias precursor test. This means it exhibits the same percent precursor recovery regardless of precursor concentration. Changing TOC concentrations in waters of high bromide does not present a complication with the HDFP, because the TOC "sees" the same oxidizing environment, regardless of what the actual TOC concentration is. This is not the case for the other tests. The LDFP is characterized also by two attributes (a) it is not used to simulate any particular water system or water treatment scenario; and (b) it uses a low chlorine dose so that the residual at the end of the contact time is close to what is common found in the taps of most US public water systems. The exact chlorine dose is adapted in some way to the sample's chlorine demand. This test has an inherent bias toward higher precursor recoveries for more highly colored waters. On the other hand, if carefully run, it can provide more accurate information for assessing the expected DBP concentrations at consumer's taps. The third test, the SDS, is characterized by a single attribute: it is designed to simulate the formation in a particular system on a particular day. This test uses a site-specific chlorine dose, pH, temperature and contact time. The values chosen are either based on an existing system on a particular day; or they are based on a very specific scenario intended to simulate a postulated system. I think the HDFP is most useful for the assessment of process performance. It can tell you what level of precursor removal is being achieved. I don't think it can tell you anything about mechanisms, nor can any of the other tests for that matter. The LDFP is most appropriate for comparing finished waters from parallel and alternative treatment trains. It is generally used when the precise disinfection scenario or distribution system characteristics are uncertain. It is not intended for assessing precursor removal across processes, just comparative precursor levels in the finished waters. Note that the Uniform Formation Test of Summers and Owen is a type of LDFP. The SDS is what should be used when the most accurate information about compliance and real-world concentrations are needed. The HDFP is the easiest test to run, and it is the most precise. This is because it is nearly independent of chlorine dose, so errors in dosing or excessive demands will introduce very little error. The LDFP and SDS tests are more labor-intensive, and they are prone to larger uncertainties. In summary, I use the following tests for different purposes as follows:

HDFP- for studies of isolated process performance; for understanding the behavior of complex treatment systems; for extrapolating findings at one utility to systems elsewhere in the country

LDFP- for comparisons of parallel treatment trains at a single pilot plant.

SDS- for estimating whether a new or midified treatment system will achieve compliance.

In some respects we would like to achieve all of these goals at PVWC, however some prioritization must be made. Basically, I am in agreement that more LDFP data should be collected. But perhaps a careful assessment of the current objectives will lead us to SDS tests instead.

Objectives

Materials & Methods

Organic matter was extracted and fractionated according to the method of Leenheer and Noyes ( {Leenheer & Noyes 1984 #5790}). Material extracted for this work will be referred to as Natural Organic Matter or NOM, even though it is recognized that some of this organic matter may be a product of man’s activities.

All NOM fractions were analyzed for chlorine reactivity and DBP formation. Organic concentrates were diluted to about 5 mg/L DOC in a pH 7.00 buffer and dosed with chlorine for reaction times of 30 minutes to 3 days. In every case but on, a chlorine dose of 20 mg/L was used. This was to provide a relatively high and consistent chlorine residual for all samples. The chlorine demand were always lower than 10 mg/L, except fo the hydrophilic base fraction. This sample displayed a breakpoint -like behavior (Figure xx). As expected, analysis of this sample revealed a substantial amount of ammonia (0.21 mM in the diluted sample). Assuming a stoichiometry of 1.65 M-chlorine/M-nitrogen, one can trace an idealized breakpoint curve expected for this concentration of ammonia (dotted line in Figure xx). By comparison o fthis idealized curve and the actual measurements, it’s clear that the organic mattter consumed a substantial amount of hclorine. It also appears that some organic chloramine that is measureable by the DPD method persisted beyond the breakpoint. This amount to about 0.1 mole of (iodide-oxidizable) combined organic chlorine per mole of carbon.

Disinfection byproducts were measured over a rangeof contact times ofrom 30 minutes to 72 hours, depending on the fraction. A chlorine dose of 20 mg/L was used for all fractions except the hydrophilic bases. In this case it was necessary to appply 70 mg/L in order to obtain a significant free residual (about 4 mg/L) after 72 hours. Figure xx presents a comparison of the 72-hour specific totoal trihalomehtane formation potential (TTHMFP) for the NOM factions. These alues are expresssed as (g/-CHCl3/mg-DOC, rathan than as the total sum of hte four THMs in (g per mg DOC. It is done this way because the various fractions all had small, but varying amounts of bromide. This led to slsightly different distrubtions of the THMs. Consequently, the molar sum (converted to an equivalent mass) is the more appropriate measure for comparison purposes. As expected, the humic and fulvic acids show the highest specific THMFP. The observation by others ( {Babcock & Singer 1979 #120}; {Oliver & Thurman 1981 #290}; {Reckhow, Singer, et al. 1990 #4550}) that humic acids have higher specific THMFPs than their corresponding fulvic acids is supported by these data. One particularly interesting result was the high specific THMFP (59.7 (g/mg-C) observed fo rthe hydrophilic bases. These compounds probably contain a high density of amine nitrogen. Amines are not only capable of consuming chlorine in a breakpoint reaction, but they are activating groups as well. Thjere is both theoretical and empriical evidence that amines render their parent molecules far more reactive with chlorine, and lead to high levels of chlorine incorporation ( {Reckhow, Singer, et al. 1990 #4550}). Although most free amino acids do not produce substantial amounts of chloroform (exceptions: tyrosine and tryptophan), a synthetic eight-member polypeptide and a bovine thymus protein were shown to produce 51 (g/mg-C and 36 (g/mg-C, respetively ( {Croué, Hureiki, et al. 1991 #460}).

Other fractions were also important in the overall formation of THMs. Many of these are likely to be less amenable to removal in water treatment than the humic and fulvic acids. For this reason they may represent more important sources of THMs for the finished waters. In particular, the hydrophilic neutrals have nearly as high a speific THMFP (39.8 (g/mg-C) as the fulvic acid, and accounted for 24% of the total. The weak hydrophobic acids also had a moderate specific THMFP (29.6 (g/mg-C), but due to their low habundance, they were less iomportant in this water. As exp[ected, the hydrophilic acids showed a low specific THMFP (24.0 (g/mg-C). The hydrophobic neutrals and bases were lower still (17.7 and 5.0 (g/mg-C, respectively).

These data support the widespread assumption that humic substances are responsible for the majority of the THM formation in raw waters. Combined, the humic and fulvic acids accounted for nearly 60% of the composite THMFP in this moderately colored water (13% and 46%, respectively). This is at least a part of the reason why excellent correlations have been found between UV absorbance and THMFP (e.g {Singer, Barry, et al. 1981 #100}; {Edzwald, Becker, et al. 1985 #240}). The sum of all hydrophobic fractions constituted about 66% of the composite THMFP. This is in good agreement with the results of Hoehn and coworkers ( {Hoehn, Dixon, et al. 1984 #150}) and Collins and co-workers ( {Collins, Amy, et al. 1986 #7420}). However, in a less colored water it is likely htat hte humic substnaces would constitute a smaller fraction of the total. Also, it must be kept in mind that these fractions are operationally defined. Use of a larger k’ during separation of the fulvic acid would have probably resulted in more breakthrough into the hycrophilic acid faction, and a smaller contrubtuion from the fulvic acids. The THMFP for the humic acid fraction (68.6 (g/mg-C) was in good agreement with the values reported by Reckhow ( {Reckhow, Singer, et al. 1990 #4550}) for five aquatic hmics acids extracted with XAD-8 resin (average: 56.8 (g/mg-C, range: 47.2-68.2 (g/mg-C). These were determined under reaction conditions identical to those used in this study (i.e., 5 mg/L DOC, pH 7.0, 72 hours, 20 mg/L chlorine dose). The THMFP for the fulvic acid values (45.8 (g/mg-C) was also in good agreement with the fulvic acid values (average: 42.6 (g/mg-C, range: 30.8-50.8 (g/mg-C) reported by Reckhow ( {Reckhow, Singer, et al. 1990 #4550}). Many others have employed reaction conditions that differ slightly from those used here, and most of these studies hsave shown similar THM yeilds for extracted humic and fulvic acids (e.g., {Babcock & Singer 1979 #120}; {Oliver & Lawrence 1979 #200}; {Fleischacker & Randtke 1983 #130}; {Oliver & Thurman 1981 #290}).

Size fractionation of the fulvic acids and hydrophilic neutrals showed some important differences in specific THMFP (figure 36). In both organic extracts, the 1000-5000 AMW fraction gave the highest value, and the 10,000 AMW). Chlorination conditions were used ot simiulate the University of Iowa lime softening plant; pH 10.8, 6 mg/L chlorine dose, 10 hours at 3ºC. The relative THMFPs were low due to the low temperature and reaction time, and they are not directly comparable to the values reported here.

Oliver and Visser (Oliver and Visser, 1980) studies the THMFP

Results

Discussion

Allochthonous vs Autochthonous Precursors

The pyrole ring can readily form carbanions which render it susceptible to haloform formation ( {Morris & Baum 1977 #430}). These compounds can be found in chlorophyll, proteins (e.g., tryptophane), and other biological molecules. Morris and Baum found chlorophyll to produce large amounts of chloroform upon chlorination and recognized that algae must also be considered as a potentially-important source of THM precursors.

There is a considerable amount of evidence pointing to algal activity as a contributer to a water’s THM precursor content. Hoehn and co-workers observed strong positive correlations between chlorophyll-a concentrations and finished water THM concentrations in the Occoquan Reservoir ( {Hoehn, Barnes, et al. 1980 #200}). These authors also cultured several species of algae in the laboratory and conducted chlorination tests. They found that the extracellular products produced chloroform levels that were of the same order of magnitude as humic substances (on a carbon basis). There were indications that the density of precursors changed with changing growth phase, but this was inconclusive.

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Conclusions

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