Lead pipe and lead-tin solder scale formation and structure: A conceptual model.

Lead-tin solder and lead service lines (LSLs) are important sources of lead, and after LSLs are removed, lead-tin solder will remain a major source of lead. A better understanding of the factors that control lead release from solder joints can help water utilities reduce lead. This paper reviews the reactions that take place at galvanic connections involving both lead-tin solder and lead pipe in contact with copper.

Effect of pore-blocking background compounds on the kinetics of trace organic contaminant desorption from activated carbon.

This study examined the effect of pore-blocking (PB) background organic matter, which is known to hinder adsorption kinetics, on the rate of trace contaminant desorption. Adsorption, displaced desorption (DD) and nondisplaced desorption (NDD) kinetic tests were performed using powdered activated carbon (PAC) that was preloaded with natural organic matter (NOM). Since the NOM contained both strongly competing (SC) and PB components, the proposed model separated the contributions of the SC and PB NOM to the overall diffusion coefficient of the target contaminant.

Effect of strongly competing background compounds on the kinetics of trace organic contaminant desorption from activated carbon.

Strongly competing (SC) compounds, naturally found in any drinking water source, are known to decrease the adsorption capacity of activated carbon for trace contaminants. While the effect of these substances on the capacity and adsorption kinetics of trace contaminants is fairly well studied, relatively little is known about their impact on desorption kinetics. The purpose of this study was to investigate the relationship between SC matter and trace compound desorption kinetics.

Effects of powdered activated carbon pore size distribution on the competitive adsorption of aqueous atrazine and natural organic matter.

The pore size distribution (PSD) of adsorbents has been found to be an important factor that affects adsorption capacity for organic compounds; consequently, it should influence competitive adsorption in multisolute systems. This research was conducted to show howthe PSD of activated carbon affects the competition between natural organic matter (NOM) and the trace organic contaminant atrazine, with a primary emphasis on quantifying the pore blocking mechanism of NOM competition. Isotherm tests were performed for both atrazine and NOM from a groundwater on five powdered activated carbons (PACs) with widely different PSDs.

Three-component adsorption modeling to evaluate and improve integrated sorption-membrane processes.

Integrated sorption-membrane (ISM) processes combining low-pressure membranes with adsorbents are increasingly popular because they cost-effectively expand low-pressure membrane treatment to include dissolved contaminant removal. However, contemporary ISM processes often exhibit antagonistic tradeoffs between adsorption and membrane performance that were investigated using state-of-the-art adsorption models that include both of the predominant competitive effects of natural organic matter: direct site competition and pore blockage. Two currently used ISM process configurations, powdered activated carbon-ultrafiltration (PAC-UF) and adsorptive floc blanket reactor-ultrafiltration (FBR-UF), were compared with a novel configuration, upflow adsorption-ultrafiltration (UA-UF), which consists of a moving-bed of granular activated carbon upstream of a membrane.

Application of a three-component competitive adsorption model to evaluate and optimize granular activated carbon systems.

A recently developed kinetic model for granular activated carbon (GAC) adsorbers (COMPSORB-GAC) that quantitatively describes the adsorption of trace organic contaminant in the presence of competing natural organic matter (NOM) was applied to evaluate the performance of different GAC system configurations: conventional fixed-bed adsorbers, layered upflow carbon adsorbers (LUCA), and moving-bed adsorbers (with few or many bed sections). COMPSORB-GAC separately tracks the adsorption of three components: a trace compound, a strongly competing NOM fraction that reduces trace compound equilibrium capacity, and a pore-blocking NOM fraction that reduces kinetics. Performance was simulated for various design criteria and with model parameters derived for two natural waters with significantly different NOM concentrations.

Three-component competitive adsorption model for fixed-bed and moving-bed granular activated carbon adsorbers. Part II. Model parameterization and verification.

COMPSORB-GAC is a 3-component competitive adsorption kinetic model for granular activated carbon (GAC) adsorbers that was developed in Part I of this study, including a proposed procedure for determining model parameters in natural water applications with background natural organic matter (NOM). Part II of this study demonstrates the proposed parameterization procedure and validates the modeling approach by comparing predictions with experimental breakthrough curves at multiple empty-bed contact times for both fixed-bed and moving-bed reactors. The parameterization procedure consists of a set of independent, short-term experimental tests with fresh and batch preloaded adsorbents and then data fitting using both classic and recently developed theoretical expressions.

Three-component competitive adsorption model for fixed-bed and moving-bed granular activated carbon adsorbers. Part I. Model development.

Heterogeneous natural organic matter (NOM) present in all natural waters impedes trace organic contaminant adsorption, and predictive modeling of granular activated carbon (GAC) adsorber performance is often compromised by inadequate accounting forthese competitive effects. Thus, a 3-component adsorption model, COMPSORB-GAC, is developed that separately tracks NOM adsorption and its competitive effects as a function of NOM surface loading. In this model, NOM is simplified into two fictive fractions with distinct competitive effects on trace compound adsorption: a smaller, strongly competing fraction that reduces equilibrium capacity and a larger pore-blocking fraction that reduces adsorption kinetics (both external film mass transfer and surface diffusion).

Simplification of the IAST for activated carbon adsorption of trace organic compounds from natural water.

Recent studies have shown that the ideal adsorbed solution theory (IAST) coupled with the concept of equivalent background compound (EBC) can be simplified for describing trace organic compound adsorption from natural water, provided that the adsorbent surface loading is dominated by competing natural organic matter. The resulting simplified IAST has been used to reduce the complexity of kinetic models for various dynamic adsorption processes. In order to be correctly applied, however, the simplified IAST requires some additional clarification and a quantitative evaluation of the deviation caused by the simplifying assumption.

Competitive effects of natural organic matter: parametrization and verification of the three-component adsorption model COMPSORB.

Natural organic matter (NOM) hinders adsorption of trace organic compounds on powdered activated carbon (PAC) via two dominant mechanisms: direct site competition and pore blockage. COMPSORB, a three-component model that incorporates these two competitive mechanisms, was developed in a previous study to describe the removal of trace contaminants in continuous-flow hybrid PAC adsorption/membrane filtration systems. Synthetic solutions containing two model compounds as surrogates for NOM were used in the original study to elucidate competitive effects and to verify the model.

Pore blockage effect of NOM on atrazine adsorption kinetics of PAC: the roles of PAC pore size distribution and NOM molecular weight.

Natural organic matter (NOM) in natural water has been found to have negative effects on the adsorption of various trace organic compounds by activated carbon through two major mechanisms: direct competition for sites and pore blockage. In this study, the pore blockage effect of NOM on atrazine adsorption kinetics was investigated. Two types of powdered activated carbon (PAC) and three natural waters were tested to determine the roles of PAC pore size distribution and NOM molecular weight distribution in the pore blockage mechanism.

Three-component competitive adsorption model for flow-through PAC systems. 2. Model application to a PAC/membrane system.

A three-component competitive adsorption kinetic model, developed and validated in part 1 of this study, was applied to a continuous-flow PAC/membrane system to study the effects of various system and operating parameters on organic removal. The model quantitatively describes the two competitive adsorption mechanisms that occur during adsorption of trace organic compounds by powdered activated carbon (PAC) in flow-through systems where the PAC is retained in the system: pore blockage and direct competition for adsorption sites. Model simulations were conducted to investigate the effects of influent water composition, membrane cleaning water quality, PAC pore size distribution, and system operation conditions such as hydraulic retention time, membrane cleaning interval, and PAC dosing method on treatment efficiency.

Three-component competitive adsorption model for flow-through PAC systems. 1. Model development and verification with a PAC/membrane system.

Natural organic matter (NOM) interferes with the adsorption of trace organic compounds on porous adsorbents such as powdered activated carbon (PAC) by pore blockage and direct competition for adsorption sites. The competitive effect of NOM in flow-through systems in which the retention time of the PAC is greater than the hydraulic retention time of the system can be magnified because NOM from the influent water can continue to adsorb on the PAC retained in the system. As a result, the adsorption capacity and the diffusion coefficient of trace compounds can decrease as NOM from the influent water accumulates.

Elucidating competitive adsorption mechanisms of atrazine and NOM using model compounds.

Based on the relative adsorbability of natural organic matter (NOM) fractions with different molecular weights (MWs), two model compounds, poly(styrene sulfonate) (PSS) (nominal MW=1800 Dalton) and p-dichlorobenzene (DCB), were chosen to study the competitive effect of large and small NOM molecules on atrazine adsorption by two powdered activated carbons (PACs) with different pore size distributions. Both isotherm and kinetic tests of atrazine adsorption were conducted using fresh PAC and PAC preloaded with the model compounds. The model compounds were found to affect atrazine adsorption through two different mechanisms due to their size difference: direct competition for sites by p-DCB and pore constriction/blockage by PSS-1.

The sensitivity of fixed-bed biological perchlorate removal to changes in operating conditions and water quality characteristics.

Flow rate, electron donor addition, and biomass control were evaluated in order to optimize perchlorate (ClO4-) removal from drinking water using biologically active carbon (BAC) filtration. Influent dissolved oxygen (DO) was lowered from ambient conditions to approximately 2.5 mg/L for all experiments using a nitrogen sparge.

Displacement effect of NOM on atrazine adsorption by PACs with different pore size distributions.

This study investigated displacement of atrazine by the strongly competing fraction of natural organic matter (NOM) in batch and continuous-flow powdered activated carbon/PAC) adsorption systems. Due to the displacement effect, atrazine adsorption capacity in a continuous flow PAC/microfiltration (MF) system, where the carbon retention time is greater than the hydraulic retention time, decreased with time or NOM throughput. The capacity was lower than that measured in a batch reactor or predicted by the equivalent background compound-ideal adsorbed solution theory (EBC-IAST) method.