Showing papers in "Journal of Environmental Engineering in 1996"

Landfill Siting Using Geographic Information Systems: A Demonstration

Abstract: Regulations and public opposition can make siting municipal solid waste landfills difficult. In this paper, spatial-AHP is presented, a method that identifies and ranks potential landfill areas for preliminary site assessment. Spatial-AHP takes into account regulatory restrictions, area attributes, and site assessment criteria provided by experts and/or users. The method uses an analytic environment provided by geographic information systems (GIS) and a decision-making method provided by the analytic hierarchy process (AHP). The GIS is used to manipulate and present spatial data. The AHP is used to rank potential landfill areas based on a wide variety of criteria. Spatial-AHP is demonstrated by applying it to a landfill site selection study in Cleveland County, Oklahoma. The paper also analyzes the effects of varying the relative importance of various siting criteria, landfill size, and location restriction severity.

Phytoremediation: Plant Uptake of Atrazine and Role of Root Exudates

Abstract: Phytoremediation is an emerging technology for degradation of organic chemicals. The potential of phytoremediation in the cleanup of contaminated sites and prevention of non-point-source pollution was examined with the pesticide atrazine in two experimental systems. Uptake was determined in batch experiments with 14 C ring-labeled atrazine and hybrid poplar trees grown in two soil types. Mineralization was studied utilizing soil microcosms with the addition of root exudates. Results indicate that poplar cuttings were able to uptake the majority of applied atrazine that was not tightly sorbed to the organic fraction of the soil, with no detectable adverse effects to the trees. The addition of root exudate to microcosms showed slight stimulation and the addition of ground-up root biomass revealed large stimulation of mineralization to 14 CO 2 . From experimentally acquired data, a mathematical model was developed to obtain kinetic rate constants. This research indicates that vegetative uptake and degradatio...

Removal of arsenic from ground water by iron oxide-coated sand

Abstract: In a laboratory study, iron oxide-coated sand showed promise as a medium for use in small systems or home-treatment units in developing areas of the world, for removing arsenic(III) and arsenic(V) from ground water. A low-cost, simple, home arsenic removal unit (material and fabrication cost: Rs. 200, cost of medium: Rs. 80, and regeneration cost: Rs. 5; 1 U.S. dollar = Rs. 35), containing 6 kg (4 L) of iron oxide-coated sand, produced 625 and 610, and 780 and 760 L of water in two cycles of runs when the influent arsenic(III) or arsenic(V) concentration was 1.0 mg/L. A detailed study addressing the effects of some important factors (selectivity of arsenic(III) and arsenic(V) over one another for removal, water pH, concentration and type of competing anions, and cations) on the process is needed. The home arsenic removal unit should be subjected to field trial to assess long-term effects on performance.

Answers-2000: runoff and sediment transport model

Abstract: A non-point source pollution management model, ANSWERS-2000, was developed to simulate long-term average annual runoff and sediment yield from agricultural watersheds. The model is based on the event-based ANSWERS model and is intended for use without calibration. The physically based Green-Ampt infiltration equation was incorporated into ANSWERS-2000 to improve estimates of infiltration. An evapotran­ spiration submodel was added to permit long-term, continuous simulation. The model was validated without calibration using data from the field-sized P2 and P4 watersheds in Watkinsville, Ga. Additional validation with limited calibration was done on the Owl Run watershed in Virginia. Model predictions of cumulative sediment yield were within 12% and 68% of observed values. Predicted cumulative runoff volumes ranged from 3% to 35% of observed values. Predictions of sediment yield and runoff volume for individual storms were less accurate, but generally within 200% of observed values. In a practical application, the use of the model in agricultural non-point-source pollution control planning was demonstrated.

Flushing of a Pb(II) contaminated soil using HCl, EDTA, and CaCl2

Abstract: Flushing of a Pb(II) synthetically contaminated sandy loam using 0.1 N HCl, 0.01 M EDTA, and 1 M CaCl 2 was investigated in the continuous-flow (column) mode. Initial Pb soil concentrations ranged ...

Acceleration of landfill stabilization using leachate recycle

Abstract: A leachate recycle system was constructed and operated at an existing lined landfill in North-Central Florida to observe the effects of leachate recycle on landfill stabilization. Samples of leachate, landfill gas, and landfilled solid waste were collected and analyzed throughout a four-year period, before and after the start of leachate recycle. The settlement of landfilled waste was also measured in wetted and dry areas of the landfill. Leachate quality was not dramatically impacted by leachate recycle. Moisture content was significantly greater in the area of the landfill subjected to leachate recycle. Waste temperature and pH measurements indicated that conditions suitable for anaerobic decomposition were present in both the treated and untreated areas. Measurements of solid waste biochemical methane potential and subsidence showed that a greater degree of landfill stabilization had occurred in the leachate recycle area relative to the untreated area.

Multifactor spatial analysis for landfill siting

Abstract: Siting a landfill typically requires processing a significant amount of spatial data with respect to various siting rules, regulations, factors, and constraints. Manually performing such a spatial analysis with drawing tools is generally tedious. A modern geographical information system (GIS), although capable of manipulating spatial data to facilitate the analysis, lacks the ability to locate an optimal site when compactness and other factors are simultaneously considered. An appropriate siting model was therefore explored for use with a raster-based GIS. A mixed-integer programming model was developed to obtain a site with optimal compactness. A comparison was made between the model and two other previously proposed models in terms of their applicability and simplicity for raster-based data. The compactness model was further extended to include multiple siting factors with weights determined using map layer analysis functions provided by a GIS. This multifactor model was applied to analyze the effects of varied weights and factors on making a siting decision.

Simulating atrazine transport with hspf in an agricultural watershed

Abstract: Surface water pollution in rural areas is related to agricultural practices Field applications of pesticide can contribute to water degradation The model HSPF includes a module for simulating the degradation and transport of pesticides This model was tested using data from a 78 ha watershed in the province of Quebec, Canada Model parameters related to hydrologic and pesticide transport processes were calibrated The study period extended from June 1991 to November 1993 For the calibration period, correlations between observed and simulated streamflows were 073, 087, and 090 for daily, weekly, and monthly intervals, respectively; whereas for the verification period, values of 067, 091, and 093 were obtained Pesticide concentrations were measured at the outlet of the watershed from February to November 1993 This short period of time allowed calibration of the pesticide component but precluded verification Variations of atrazine concentration were well simulated by Hydrological Simulation Progr

Wettability of NAPL-Contaminated Sands

Abstract: The potential for changes in the wettability of a subsurface system following exposure of sands to a variety of non–aqueous-phase liquids (NAPLs) was assessed. Many of the NAPLs were collected from field sites with on-going NAPL recovery systems. Three methods were used to evaluate the wetting conditions: contact angle measurements, a qualitative bottle test, and capillary pressure-saturation curves. The results clearly show that a wide range of wetting conditions can be expected following spills of complex NAPL mixtures to the subsurface. NAPLs comprised of higher molecular weight constituents—such as creosote—or with added surfactants—such as gasoline—had a greater impact on the system wettability than the lower molecular weight NAPLs. Neat solvents did not have a significant impact on the wettability of quartz surfaces. Many of the petroleum products tested resulted in the formation of weakly water-wet surfaces. Under these conditions, the potential recovery of NAPL from the subsurface would be maximized. Much lower recoveries would be expected for the NAPL-wetting creosote and coal tar contaminants, however.

Scaling bacterial filtration rates in different sized porous media

Abstract: Aquifer sediments contain a wide distribution of particle sizes, but only a single collector diameter (\id) can be used in a filtration equation. To establish a method for selecting a characteristic \id when media are composed of different sized particles, we measured bacterial retention in columns packed with either crushed quartz sand (separated into three different size ranges) or borosilicate glass beads. The best methods for choosing \id were those that produced nearly constant collision efficiencies (α’s). Characteristic diameters included: \id\d1\d0(10% of all particles were smaller), \id\d9\d0(90% of all particles were smaller), \id\i\da (arithmetic mean), and \id\i\dg (geometric mean), where all diameters were calculated using number, area, and volume size distributions. Bacterial α’s decreased in proportion to the distance traveled in the packed bed, and were scaled by the number of bacteria-sediment collisions using a dimensionless collision number (ξ). These comparisons indicated that characteristic diameters based on the smaller particles (\id\i\da and \id\i\dg using number distributions, and \id\d1\d0 using a volume distribution) most accurately described bacterial transport in the different-sized porous media.

Dry Deposition of Polycyclic Aromatic Hydrocarbons in Ambient Air

Abstract: Dry deposition and air sampling were undertaken, simultaneously, in the ambient air of an urban site and a petrochemical-industry (PCI) plant by using several dry deposition plates and PS-1 samplers from January to May 1994 in southern Taiwan. The dry deposition plate with a smooth surface was always pointed into the wind. Twenty-one polycyclic aromatic hydrocarbons (PAHs) were analyzed by a gas chromatography/mass spectrometer (GC/MSD). The dry deposition flux of total-PAHs in urban and PCI sites averaged 166 and 211 μg/m²·d, respectively. In general, the PAH dry deposition flux increased with increases in the PAH concentration in the ambient air. The PAH pattern of dry deposition flux in both urban and PCI sites were similar to the pattern measured by the filter of the PS-1 sampler and completely different from the PAH pattern in the gas phase. The higher molecular weight PAHs have higher dry deposition velocities. This is due to the fact that higher molecular weight PAHs primarily associated with the particle phase are deposited mostly by gravitational settling, while the gas phase PAHs are deposited mainly by diffusion. The dry deposition velocities due to gravitational settling are much higher than those due to diffusion. When the dry-deposition-velocity ranges of gas phase PAHs were between 0.001 and 0.010 cm/s, only the lower molecular-weight PAHs—Nap and AcPy—had a significant fraction of dry deposition flux contributed by the gas phase. All the remaining higher molecular-weight PAHs had more than 94.5% of their dry deposition flux resulting from the particle phase. This is due to the fact that higher molecular weight PAHs have a greater fraction in the particle phase and the dry deposition velocities of particulates are much higher than those of the gas phase.

Systematic evaluation of pollutant removal by urban wet detention ponds

Abstract: Three urban wet detention ponds in the Piedmont of North Carolina were monitored to investigate long-term pollutant removal as a function of surface to area ratios. Eleven storm events were monitored over a sampling period of 13 months. Urban runoff originating from the study area was characterized by event-mean concentrations for total suspended solids (135 mg/L), total Kjeldahl nitrogen (0.88 mg/L), total iron (6.11 mg/L), and total zinc (66 ug/L). Concentrations of copper and lead were consistently below the detection limits of 30 ug/L and 100 ug/L, respectively. The observed event-mean concentrations were generally lower than the national values reported by the Nationwide Urban Runoff Program. Particle sizes of sediment discharged in runoff were much finer than the national averages due to the predominant clayey soils in the region. This study demonstrates that surface to area ratio can be a useful predictor of wet pond performance. Utilizing 1--2% of the watershed area for the development of wet detention ponds at strategic locations could reduce pollutant loadings to meet targeted requirements of water quality improvement.

Electrophoretic Mobility of Cryptosporidium Oocysts and Giardia Cysts

Abstract: Electrophoretic mobilities were measured and zeta potentials calculated for oocysts of \ICryptosporidium\N and for cysts of both \IGiardia lamblia\N and \IG. muris\N. Measurements were made in both distilled water and in an unfiltered low-turbidity river water, and included measurements for both fresh and preserved organisms. Electrophoretic mobilities of oocysts and cysts were between -2.72 and -3.12 μm/s/V/cm corresponding to zeta potentials of -35 and -40 mV at neutral pH and decreased to less than -0.78 μm/s/V/cm (-10 mV) as pH was decreased to about 3.5. The electrophoretic mobilities and corresponding zeta potentials of oocysts and cysts were substantially modified in the presence of fulvic acid and of natural organics in unfiltered water. Common methods of oocyst and cyst preservation also altered electrophoretic mobilities and corresponding zeta potentials indicating that caution should be taken when conducting laboratory and pilot studies using preserved organisms.

Solution of the Advection-Dispersion Equation: Continuous Load of Finite Duration

Abstract: Field studies of solute fate and transport in streams and rivers often involve an experimental release of solutes at an upstream boundary for a finite period of time. A review of several standard references on surface-water-quality modeling indicates that the analytical solution to the constant-parameter advection-dispersion equation for this type of boundary condition has been generally overlooked. Here an exact analytical solution that considers a continuous load of finite duration is compared to an approximate analytical solution presented elsewhere. Results indicate that the exact analytical solution should be used for verification of numerical solutions and other solute-transport problems wherein a high level of accuracy is required.

Fate of organics during column studies of soil aquifer treatment

Abstract: Soil column experiments were performed to differentiate between abiotic and biological mechanisms for removal of residual organics from chlorinated municipal effluent during soil aquifer treatment (SAT) and to relate SAT efficiency to degree of wastewater treatment. Three water-quality parameters were examined: dissolved organic carbon (DOC), ultraviolet (UV) absorbance at 254 nm (UVA 254 ), and adsorbable organic halide (AOX). Secondary and tertiary-treated effluents were applied to 1-m unsaturated columns containing soils from the Sweetwater Underground Storage and Recovery Facility, Tucson, Ariz. DOC was removed primarily by biological activity during SAT. Through-column removal using chlorinated secondary effluent averaged 48%. UVA 254 reductions were dependent on biological and abiotic removal mechanisms. Aerobic biological activity did not contribute significantly to AOX removal. Through-column reductions for UVA 254 and AOX from secondary effluent averaged 30% and 17%, respectively. Forty-four perc...

Model for effective diffusivities in aerobic biofilms

Abstract: Reliable estimates of effective diffusivities in biofilms are required to calculate the combined diffusion and biotransformation of soluble substrates. For aerobic biofilms, effective diffusivities have been previously correlated empirically to the biofilm dry densities. Biofilms consist primarily of microbial cells, insoluble extracellular polymeric substances (EPS), and interstitial water. Biofilms are therefore a natural, heterogeneous, multiphasic material. Effective diffusive properties of such materials, both biological and nonbiological, have historically been correlated to the phase-volume fractions and diffusive properties of the individual phases. This paper presents a conceptually similar model for effective diffusivities in aerobic biofilms. The model predicts separate effects for increasing concentrations of microbial cells and insoluble EPS. Predicted effective diffusivities are expressed as a fraction of the corresponding dilute aqueous values.

Managerial Fuzzy Optimal Planning for Solid-Waste Management Systems

Abstract: The emphasis on waste reduction and recycling requirements prior to incineration and the promulgation of Good Combustion Practice (GCP) for emission control of trace organic compounds during incineration have created conflicting solid-waste management goals. The most critical questions in system planning include: to what extent are recycling and incineration compatible? And what are the subsequent economic impacts on the private and public sectors under specific management scenarios? However, the inherent complexity of composition, generation, and heat value of the waste streams as well as the stability of the secondary material market may result in additional difficulties in management decision making. This paper presents a nonlinear fuzzy goal programming approach for solving such questions. In particular, it demonstrates how fuzzy, or imprecise, objectives of the decision makers can be quantified through the use of specific membership functions in various types of management-planning scenarios.

Optimal Regional Scheduling of Solid Waste Systems. I: Model Development

Abstract: A deterministic linear programming model is presented that can be used to aid decision makers in the long-term scheduling of disposal and diversion options in a regional integrated solid waste management system. The model can be used to determine what types of integrated solid waste management programs to implement, and when to implement them, in order to minimize costs over a long planning period. The model is capable of handling multiple communities, landfills, and incinerators, and can incorporate the possible implementation of numerous collection and diversion options, such as recycling and composting programs. The model can incorporate mutually exclusive collection and facility options. The model also uses innovative approaches in determining the volume of waste land filled and energy content of wastes incinerated, given the effect of implemented diversion options on the waste stream. Alternative methods to handle costs in the objective function are discussed, including (1) unit costs; (2) mutually exclusive facility options, each with an associated unit cost; (3) fixed and operating costs; and (4) variable costs using the piecewise linear approximation method. In a companion paper, a hypothetical case study is used to explore the capabilities of the model.

Transport of Organic Compounds in Thermoplastic Geomembranes. I: Mathematical Model

Abstract: A quasi-two-dimensional partition-diffusion transport model was developed to determine the diffusion coefficient and partition coefficient for various types of geomembranes from measurements of aqueous organic compound concentrations in a confined, double-compartment apparatus with a geomembrane separating the two compartments. The geomembranes tested were high-density polyethylene (HDPE), very low-density polyethylene (VLDPE), and polyvinyl chloride (PVC) and the permeants were mixtures of methylene chloride, toluene, trichloroethylene (TCE), and \im-xylene at 10–100 mg/L. The diffusion coefficient increased exponentially with aqueous concentration and decreased exponentially with geomembrane thickness. The partition coefficient was unaffected by compound concentration and membrane thickness. As HDPE geomembranes were stretched by 5% of their original length, the partition coefficient increased by 0.15–0.6 times. VLDPE had 1.8–3.3 times greater partition coefficients and 1.6–2.8 times greater diffusion coefficients than HDPE, while PVC had 6.2–8.3 times greater partition coefficients and 1–1.8 times greater diffusion coefficients than HDPE.

Solid-Waste Management System Analysis with Noise Control and Traffic Congestion Limitations

Abstract: Conventional location/allocation models for solid-waste management usually focus on economic optimization with respect to mass balance, capacity limitation, operating, and financial constraints. But the siting of important facilities, such as landfills, incinerators, and tr!1?sfer stations, in a solid.wast~ ~an~gement s~stem still involves several explicit limitations that must be conSIdered through the use of ophmlzatlOn techmques. This paper presents a sustainable waste management strategy in which the decision makers and the environmen.tal analysts may put forward their views on the assimilati~e capacity ~f two major environmental. fact~rs: n~lse control and traffic congestion. A mixed integer programmmg model WIth the framework of dynamIC optImIzatIon still can be used efficiently to achieve this goal. The practical implementation is assessed by the case study of the Kaohsiung solid-waste management system in Taiwan. It shows that the incorporation of these two envi­ ronmental factors does alter the conventional waste-flow pattern in metropolitan solid-waste management systems.

Modeling Kinetics of Illuminated and Dark Advanced Oxidation Processes

Abstract: Advanced oxidation processes (AOPs) have great potential for the complete destruction of hazardous contaminants; however, the complex kinetics and mechanisms of the free-radical reactions involved often render process optimization difficult. In this paper, a kinetic model and associated rate expressions for AOPs are developed to facilitate evaluation and optimization of treatment performance. The kinetic model has been formulated based on key reactions known to occur in both illuminated and dark AOP systems in the aqueous phase. As a result of fast free-radical chain reactions, the hydroxyl radical (OHs˙), which is mostly responsible for decontamination, is expected to be at low steady-state concentrations, depending on treatment conditions. Thus, the OHs˙ concentration must be optimized with respect to treatment conditions to promote fast decontamination reactions and reduce wasteful scavenging reactions. A final rate expression is derived for the steady-state concentration of OHs˙ as a function of light...

Gas-phase removal of h2s and nh3 with dielectric barrier discharges

Abstract: Hydrogen sulfide and ammonia are two inorganic compounds that may cause severe odor problems. In this study, the effectiveness of applying dielectric barrier discharges (DBDs) to destroy and remove these two odor-causing compounds from gas streams via the generation of gas-phase radicals and high-energy electrons is experimentally evaluated with a bench-scale apparatus. Experimental results indicate that the removal efficiencies of both H{sub 2}S and NH{sub 3} with DBD plasmas depend on the gas composition, gas residence time, and applied voltage. Electronegative gases, such as O{sub 2} and H{sub 2}O{sub (g)}, tend to increase the electron attachment and therefore decrease removal efficiencies. Simultaneous removal of H{sub 2}S and NH{sub 3} from gas streams is accomplished with DBD technology. Results of this study indicate DBD is an effective technology for controlling inorganic odor-causing compounds.

Hazard ranking of landfills using fuzzy composite programming

Abstract: The environmental and health risks posed by unregulated landfills are concerns that must be addressed. These concerns have been highlighted with the recent reauthorization of the Resource Conservation and Recovery Act (RCRA) Subtitle D, which requires the closure of all unregulated landfills by October 1993. Most communities with unregulated landfills do not have the financial resources to conduct full-scale risk assessments. This paper proposes the use of a multicriteria assessment system as a tool for screening and prioritizing unregulated disposal sites according to their level of environmental and health hazard. This multicriteria assessment system uses a technique termed composite programming and allows for the use of imprecise information through fuzzy set theory. Using this methodology in landfill hazard assessment allows for the consideration of uncertainty associated with parameters that impact the hazard assessment. Additionally, the user can specify hazards that are most detrimental. The complexity of input parameters (first level indicators) were selected to minimize the time required to collect and/or analyze site-specific data. The result obtained in the assessment is a fuzzy number that indicates the most likely range of hazard and the largest likely range of hazard relative to the best and worst case scenarios. A case study, in which this method is applied to a small rural landfill, is presented to illustrate the methodology.

Stabilization/Solidification of Hazardous Wastes Using Fly Ash

Abstract: A simple and inexpensive method of stabilization/solidification (S/S) of hazardous wastes using C-grade fly ash as the only binding agent is presented. The process involves mixing the waste and fly ash and compacting the mixture for less than 3 s at 1.4–6.9 MPa to form a monolith. Toxicity characteristic leaching procedure (TCLP) results for a simulated mixed waste showed that the chromium concentration in the leachate is well below current Environmental Protection Agency (EPA) limits. For this particular waste, TCLP results were not significantly affected by the ratio of waste:fly ash in the block due to the limited range of waste concentration that can be processed. Furthermore, it is shown that the optimum operating conditions are at a waste pH of 9.2 and an applied pressure of 4.65 MPa.

Reliability-Based Design of Water-Distribution Systems

Abstract: A heuristic approach for reliability-based design of water-distribution systems (WDSs) is presented. The reliability is expressed by reliability factors, estimated through extended-period simulation using node-flow analysis that predicts the availability of water at different nodes during deficient conditions. The approach is iterative, considers pump and pipe failures only, and is based on the trade-off between reliability and cost of the WDSs. An initial WDS is selected and the ratio of marginal increase in reliability to marginal increase in cost (MIRMIC ratio) for each pipe by changing its size to next higher one is evaluated. Pipes with large MIRMIC ratios are selected for change to next higher size and the iterative procedure is continued until a WDS of desired reliability is obtained. The approach can also be applied to reliability-based upgrading of existing WDSs.

Treatability of s -Triazine Herbicide Metabolites Using Powdered Activated Carbon

Abstract: Widespread use of s-triazine herbicides such as atrazine, simazine, and cyanazine has led to the contamination of many ground-water and surface-water supplies with the parent compounds as well as by primary metabolites (e.g., deethylatrazine, deisopropylatrazine, and deethylcyanazine). Ozonation has been shown to produce many of the same byproducts. Activated carbon adsorption has been designated by the U.S. Environmental Protection Agency as the best available technology (BAT) for the treatment of herbicides in drinking water. Little data is available, however, on the applicability of activated carbon treatment for the control of the primary degradation products of herbicides. In this study, the adsorption of deethylatrazine, deisopro­ pylatrazine (deethylsimazine), and deethylcyanazine on powdered activated carbon (PAC) was examined includ­ ing development of Langmuir and Freundlich isotherm coefficients. It was found that deethylatrazine, deisopro­ pylatrazine and deethylcyanazine were readily treatable using powdered activated carbon (PAC). However, the PAC's adsorptive capacity for deethylatrazine, deisopropylatrazine, and deethylcyanazine was generally less than for atrazine resulting in higher estimated carbon costs.

Multiphase Distribution of Cohesive Sediments and Heavy Metals in Estuarine Systems

Abstract: Characterization of the types and concentrations of heavy metals in their various phases is needed to assess potential adverse effects on the aquatic environment and its ecosystems, and thereby to assure cost-effective mitigation and waste-allocation strategies. A two-dimensional vertically averaged finite-element model was extended to predict the spatial and temporal distribution of cohesive sediments and associated toxic heavy metals as a result of a pollutant input into an estuarine system. Constitutive relationships for cohesive sediment deposition and erosion, heavy metal adsorption and desorption, and the governing hydromechanical transport mechanisms were incorporated in an existing sediment transport model. Formulation and validation of a new relationship for deposition was based on data derived from experiments carried out by previous investigators. The fate and transport of cohesive sediments and associated nickel was simulated for south San Francisco Bay. The comparison of model performance against field observations indicated that the model is capable of representing important phenomena governing the fate of cohesive sediments and associated nickel.

From Cholera to Cancer to Cryptosporidiosis

Abstract: The introduction of piped water to cities in the mid-19th century led to the spread of cholera and typhoid in the United States and the other industrialized countries Filtration and then chlorination around the turn of the century virtually eliminated waterborne enteric disease in the industrialized world The development of synthetic organic chemicals following World War II and the recognition that chlorine, which is so important for disinfection, reacts with natural and other organic precursors in the water, producing carcinogenic byproducts, shifted the major emphasis in water quality in the industrial world away from infectious disease to a concern for control of trace chemical contaminants The recent emergence of waterborne giardiasis and cryptosporidiosis is shifting emphasis back to a concern for the control of enteric infectious disease Particularly troublesome is that water meeting current microbiological standards has been demonstrated to be responsible for diarrheal disease The principal conclusion to be drawn from these changes is the affirmation of the original principles for the protection of drinking water quality, that water intended for potable purposes should be drawn from the highest quality sources available, that the sources should be protected, and that the treatment must be appropriate and reliable

Mixing in Distribution System Storage Tanks: Its Effect on Water Quality

Abstract: Nearly all distribution systems in the United States include storage tanks and reservoirs. They are the most visible components of a water distribution system but are generally the least understood in terms of their impact on water quality. Long residence times in storage tanks can have negative impacts on water quality in drinking water distribution systems, including the loss of chlorine residuals and the regrowth of microorganisms. It is generally accepted that complete mixing maximizes water quality but little is understood about mixing regimes in general. This paper demonstrates the use of compartment models to characterize mixing in three tanks. It was found that the mixing regimes in these tanks were well characterized by compartment-type models and that these tanks were not completely mixed, contrary to conventional wisdom. It is clear that much more research is needed in order to truly understand mixing regimes in tanks and reservoirs and that this understanding, if properly applied, can have a positive impact on water quality.