R. Scott Summers

Diffusion of humic acid in dilute aqueous solution

Aqueous-diffusion coefficients of macromolecules of natural and synthetic origin in dilute solution (10–20 mg/liter) were estimated by measuring external mass transfer coefficients in a short fixed-bed activated-carbon column. The macromolecular material of natural origin was a commercial humic acid (HA) which had been separated by ultrafiltration into five size fractions in the nominal molecular weight (MW) range 500 to 100,000. The synthetic macromolecules were polymers of known composition, polyethylene oxide (PEO) and polystyrene sulfonate (PSS) obtained in fractions with narrow molecular weight distributions in the ranges 200 to 40,000 and 1,800 to 60,000, respectively. The diffusivities of all of the polymers decreased with increasing molecular weight, but the dependence was weaker for HA (DL ∞ MW−0.2) than for PEO and PSS (DL ∞ MW−0.5); the anomalous behavior of HA was interpreted to be an artifact of the MW determination by ultrafiltration. The diffusivity of HA increased by a factor of 10 as the ionic strength was increased from 10−3 to 1.0 M, and increased by a factor of 3 as pH was decreased from 10 to 5 at low ionic strength; these changes were attributed to coiling of HA molecules. Similar evidence of coiling was observed with PSS but not with PEO, in both diffusivity and ultrafiltration experiments, in accord with expectations for charged and uncharged macromolecules. The effects of temperature and mixtures of HA fractions on diffusivity were also investigated.

A reactive species model for chlorine decay and THM formation under rechlorination conditions

Chlorine is typically used within drinking water distribution systems to maintain a disinfectant residual and minimize biological regrowth. Typical distribution system models describe the loss of disinfectant due to reactions within the water matrix as first order with respect to chlorine concentration, with the reactants in excess. Recent work, however, has investigated relatively simple dynamic models that include a second, hypothetical reactive species. This work extends these latter models to account for discontinuities associated with rechlorination events, such as those caused by booster chlorination and by mixing at distribution system junction nodes. Mathematical arguments show that the reactive species model will always represent chlorine decay better than, or as well as, a first-order model, under single dose or rechlorination conditions; this result is confirmed by experiments on five different natural waters, and is further shown that the reactive species model can be significantly better under some rechlorination conditions. Trihalomethane (THM) formation was also monitored, and results show that a linear relationship between total THM (TTHM) formation and chlorine demand is appropriate under both single dose and rechlorination conditions. This linear relationship was estimated using the modeled chlorine demand from a calibrated reactive species model, and using the measured chlorine demand, both of which adequately represented the TTHM formation.

Activated carbon adsorption of humic substances: II. Size exclusion and electrostatic interactions

Abstract The activated carbon adsorption isotherms for a wide range of humic substance molecular size (MS) fractions display an inverse dependence on MS, when expressed on an adsorbent mass basis. However, normalizing the amount adsorbed on the basis of available adsorbent surface area accounts for the size exclusion behavior displayed by the MS fractions, resulting in convergence of their isotherms. The available surface area was calculated by relating the hydrodynamic size of the macromolecules to the adsorbent pore size. The effects of adsorbent charge and solution ionic strength conform to adsorption electrostatic principles when the isotherms are expressed on an available surface area basis. Adsorbents with progressively more positively charged surfaces adsorb more of the negatively charged humic substances. Increasing solution ionic strength suppresses adsorption by a positively charged adsorbent and enhances the adsorption by a negatively charged adsorbent. The characteristic, two-segment shape of the MS fraction isotherms is interpreted based upon changes in adsorbed macromolecule orientation.

Activated carbon adsorption of humic substances: I. Heterodisperse mixtures and desorption

Abstract The activated carbon isotherms of heterodisperse humic substances are analyzed with a widely accepted model developed for well-characterized polymer adsorption. The four humic substances examined included both fulvic and humic acids isolated from aquatic and terrestrial sources. The adsorption of the humic substances exhibits a dependence on the ratio of adsorbent mass to solution volume, resulting in a set of isotherms. Modifying the adsorption isotherm to explicitly account for the adsorbent dose results in a unique isotherm that relates the adsorbed mass to the unadsorbed mass expressed on a per unit adsorbent basis. A modified Freundlich equation was found to represent the adsorption behavior over a wide range of initial concentrations and adsorbent doses and for different adsorbents, ionic strengths, and temperatures. Reduction in the solution concentration did not result in measurable desorption of humic substances from the adsorbent surface. This was attributed to multiple-site adsorption and to the heterogeneous nature of both humic substances and activated carbon. Desorption was found to occur only at elevated pH values.

Comparative measurements of microbial activity in drinking water biofilters

Tetrazolium reduction assays, phospholipid analysis, and 16S rRNA (rDNA) sequence analysis were applied to assess the distribution, composition and activity of microbial communities developing in biofilters treating non-ozonated and ozonated drinking water. The response of media-attached biomass to both operating temperature (3 degrees C vs. > 12 degrees C) and ozone application point was assessed. As judged by 2-(p-iodo-phenyl)-3-(p-nitrophenyl)-s-phenyl tetrazolium chloride (INT) reduction, the dehydrogenase activity in biofilter systems that were operated with non-ozonated water was 55% lower than in identical filters operating with ozonated water. There was no significant difference between the microbiological activity measured in a biofilter series treating ozonated water and an identical series where ozonated water was introduced at an intermediate point. The biomass levels in biofilter systems that were operated with ozonated water were 47% higher on average than identical systems operated with non-ozonated water. Operating temperature had no significant impact on total biomass levels; however, specific dehydrogenase activity was 70% higher in systems operated at ambient temperatures (> 12 degrees C) than in systems held at 3 degrees C. Phospholipid and rDNA analysis suggests that there was a community structure response to ozone application and operating temperature, but no response to different ozone application points.

Removal of trace organic micropollutants by drinking water biological filters.

The long-term removal of 34 trace organic micropollutants (<1 μg L(-1)) was evaluated and modeled in drinking water biological filters with sand media from a full-scale plant. The micropollutants included pesticides, pharmaceuticals, and personal care products, some of which are endocrine disrupting chemicals, and represent a wide range of uses, chemical structures, adsorbabilities, and biodegradabilities. Micropollutant removal ranged from no measurable removal (<15%) for 13 compounds to removal below the detection limit and followed one of four trends over the one year study period: steady state removal throughout, increasing removal to steady state (acclimation), decreasing removal, or no removal (recalcitrant). Removals for all 19 nonrecalcitrant compounds followed first-order kinetics when at steady state with increased removal at longer empty bed contact times (EBCT). Rate constants were calculated, 0.02-0.37 min(-1), and used in a pseudo-first-order rate model with the EBCT to predict removals in laboratory biofilters at a different EBCT and influent conditions. Drinking water biofiltration has the potential to be an effective process for the control of many trace organic contaminants and a pseudo-first-order model can serve as an appropriate method for approximating performance.

Character and Chlorine Reactivity of Dissolved Organic Matter from a Mountain Pine Beetle Impacted Watershed

Lodgepole pine needle leachates from trees killed by the mountain pine beetle epidemic in Colorado were evaluated for dissolved organic matter (DOM) character, biodegradation, treatability by coagulation and disinfection byproduct (DBP) formation. An average of 8.0 (±0.62) mg-DOC/g-dry weight of litter was leached from three sets of needle samples representing different levels of forest floor degradation. Fluorescence analysis included collection of excitation and emission matrices, examination of peak intensities and development of a 4-component parallel factor (PARAFAC) analysis model. Peak intensity and PARAFAC analyses provided complementary results showing that fresh leachates were initially dominated by polyphenolic/protein-like components (60-70%) and humic-like fluorescence increased (40-70%) after biodegradation. Humic-like components were removed by coagulation (20-64%), while polyphenolic/protein-like components were not, which may create challenges for utilities required to meet OM removal regulations. DBP formation yields after 24 h chlorination were 20.5-26.4 μg/mg-DOC for trihalomethanes and 9.0-14.5 μg/mg-DOC for haloacetic acids for fresh leachates; increased after biodegradation to 19.2-64.2 and 7.1-30.9 μg/mg-DOC, respectively; and decreased after coagulation (fresh: 11.3-17.7;5.7-7.6 μg/mg-DOC, respectively; biodegraded: 12.0-27.3 and 2.9-7.2 μg/mg-DOC, respectively), reflective of changes in concentration of humic material. Humic-like PARAFAC components and peak intensities were positively correlated (R(2) ≥ 0.45) to DBP concentrations, while polyphenolic/protein-like components were not (R(2) ≤ 0.17).

Evaluation of nanofiltration pretreatments for flux loss control

The loss of membrane flux due to fouling is a major impediment to the development of membrane processes for use in drinking water treatment. The objective of this work was to evaluate fouling in nanofiltration (NF) pilot systems fed conventionally-treated (coagulation/sedimentation/filtration) Ohio River water (CT-ORW) with various additional levels of pretreatment. The chosen additional pretreatments were intended to produce waters with varying biological-fouling potential. Five parallel membranes were fed CT-ORW, ozonated CT-ORW, ozonated/biofiltered CT-ORW, CT-ORW reduced to 7°C, and chloraminated CT-ORW. All systems showed significant flux decline indicating that methods beyond those needed for just biogrowth control are required for NF systems treating conventionally-treated surface waters. The NF systems fed ozonated, ozonated/biofiltered, and untreated CT-ORW had the least amount of flux decline over the course of the study; however, they had significant amounts of biological growth. Fouling in these systems was attributed to the deposition of extracellular material (polysaccharides) in the cake layer, either from the biogrowth on the membrane or carryover from the pretreatment. The low-temperature system had greater flux decline, but it had lower biogrowth than the ozonated, and ozonated/biofiltered and untreated CT-ORW systems. Although lower in biogrowth, the deposited organic material in the low-temperature system still showed a strong biological signature (polysaccharides and aminosugars). The chloraminated system had the greatest flux decline, but the least amount of biogrowth. The organic material deposited in the chloraminated system showed a high level of proteinaceous character.

Bacterial treatment effectiveness of point-of-use ceramic water filters.

Laboratory experiments were conducted on six point-of-use (POU) ceramic water filters that were manufactured in Nicaragua; two filters were used by families for ca. 4 years and the other filters had limited prior use in our lab. Water spiked with ca. 10(6)CFU/mL of Escherichia coli was dosed to the filters. Initial disinfection efficiencies ranged from 3 - 4.5 log, but the treatment efficiency decreased with subsequent batches of spiked water. Silver concentrations in the effluent water ranged from 0.04 - 1.75 ppb. Subsequent experiments that utilized feed water without a bacterial spike yielded 10(3)-10(5)CFU/mL bacteria in the effluent. Immediately after recoating four of the filters with a colloidal silver solution, the effluent silver concentrations increased to 36 - 45 ppb and bacterial disinfection efficiencies were 3.8-4.5 log. The treatment effectiveness decreased to 0.2 - 2.5 log after loading multiple batches of highly contaminated water. In subsequent loading of clean water, the effluent water contained <20-41 CFU/mL in two of the filters. This indicates that the silver had some benefit to reducing bacterial contamination by the filter. In general these POU filters were found to be effective, but showed loss of effectiveness with time and indicated a release of microbes into subsequent volumes of water passed through the system.

Critical analysis of commonly used fluorescence metrics to characterize dissolved organic matter.

The use of fluorescence spectroscopy for the analysis and characterization of dissolved organic matter (DOM) has gained widespread interest over the past decade, in part because of its ease of use and ability to provide bulk DOM chemical characteristics. However, the lack of standard approaches for analysis and data evaluation has complicated its use. This study utilized comparative statistics to systematically evaluate commonly used fluorescence metrics for DOM characterization to provide insight into the implications for data analysis and interpretation such as peak picking methods, carbon-normalized metrics and the fluorescence index (FI). The uncertainty associated with peak picking methods was evaluated, including the reporting of peak intensity and peak position. The linear relationship between fluorescence intensity and dissolved organic carbon (DOC) concentration was found to deviate from linearity at environmentally relevant concentrations and simultaneously across all peak regions. Comparative analysis suggests that the loss of linearity is composition specific and likely due to non-ideal intermolecular interactions of the DOM rather than the inner filter effects. For some DOM sources, Peak A deviated from linearity at optical densities a factor of 2 higher than that of Peak C. For carbon-normalized fluorescence intensities, the error associated with DOC measurements significantly decreases the ability to distinguish compositional differences. An in-depth analysis of FI determined that the metric is mostly driven by peak emission wavelength and less by emission spectra slope. This study also demonstrates that fluorescence intensity follows property balance principles, but the fluorescence index does not.