Journal of Environmental Engineering

Dick published a classic paper in the mid-1970s illuminating popular final settling tank beliefs as folklore with little factual or logical support. Unfortunately, some of that folklore persists in an evolved form and new folklore developed along with technology development. Secondary clarifier folklore examined in this paper include: (1) scraper mechanisms outperform suction pickup mechanisms; (2) surface overflow rate accurately dimensions secondary clarifiers; (3) secondary clarifiers should not be used for sludge storage under any operating conditions; (4) solids flux theory and state point analysis does not predict secondary clarifier failure; (5) dye studies always indicate secondary clarifier deficiencies; (6) rectangular clarifiers outperform circular clarifiers; (7) flocculator center wells encourage density currents; and (8) the sludge volume index must be high to produce low effluent suspended solids.

An approach is presented for the numerical optimization of low-pressure membrane filtration processes. A multidimensional optimization of an ultrafiltration system is formulated for cost minimization and numerically solved for key optimal design and operating variables. Hollow-fiber ultrafiltration operation under steady-state conditions is assumed and optimized with respect to fiber radius, fiber length, crossflow velocity, transmembrane pressure, and system recovery. Optimizations are performed over variable raw water conditions using a sequential quadratic programming (SQP) algorithm. For typical small to moderately sized low-pressure membrane facilities (≈1 mgd), optimal fiber design and membrane system operation is predicted to be largely influenced by the characteristic dominance of capital costs over operating costs. Thus, total treatment costs tend to be optimal at values of decision variables where permeate fluxes are maximized, within the constraints prescribed by the system, and assuming a fixed membrane cost per unit area. For raw waters demonstrating apparent to significant membrane fouling, optimal membrane treatment is predicted to be achieved by using relatively narrow hollow fibers and relatively high crossflow velocities. For relatively clean raw waters demonstrating very high sustainable permeate fluxes, operating at low crossflow velocities—or perhaps even under the dead-end mode of operation—appears to provide the most cost-effective operation.

Although disinfection was one of the major public health advances in the last century and continues to be so in the twenty-first century, the disinfectants themselves may react with naturally occurring materials in treated water to form unintended by-products, which may themselves pose risks. This is of particular concern with regard to the use of chlorine. Generation of disinfection by-products (DBPs) has been shown to be a function of various factors including total organic carbon concentration, type of organic precursor, chlorination level, pH, temperature, reaction time, and UV-254 absorbance. Another factor affecting DBP formation is the presence and concentration of the bromide ion in the raw or finished water. Bromine substitutes for chlorine to produce bromine-containing homologues of the more familiar chlorine species. The current list of by-products targeted for regulation contains brominated and mixed bromine-chlorine species of total trihalomethanes and haloacetic acids. These are known to form in bromide-containing waters when chlorinated. To control chlorination DBPs therefore requires an understanding of the factors that influence their formation. This paper presents a model that can be used to predict the formation of chlorinated, brominated, and mixed species compounds based on initial chlorine concentration, chlorine consumption, bromide ion concentration, and pH. The model clearly shows that higher levels of bromide in the water favor the formation of brominated compounds. Brominated compounds also form faster than chlorinated compounds.

It is estimated that the Departments of Defense, Energy, and Agriculture will spend up to 300 billion federal dollars on environmental remediation during the next century. Phytoremediation, the use of vegetation for the remediation of contaminated sediments, soils, and ground water, is an emerging technology for treating several categories of persistent, toxic contaminants. Although effective, phytoremediation is still in a developmental stage and therefore is not a widely accepted technology by regulatory agencies and public groups. Research is currently being conducted to validate the process effectiveness as well as increase regulatory and community acceptance. This research will focus on the ability of plants to hydroponically treat water contaminated with cadmium, chromium, nickel, radionuclides (cesium and strontium), and/or phthalates. Specifically, the effectiveness of dwarf sunflowers (Helianthus annuus) and two strains of mustard seed (Brassica juncea) were investigated. The selectivity, on a mass basis, for the sunflowers and Indian mustard was Cd > Ni > Cr and Cr > Ni > Cd, respectively. This selectivity order did not change when cesium was present. For the individual phthalate treatment, the selectivity was dibutyl phthalate > benzylbutyl phthalate > dioctyl phthalate. When metals were present, the uptake of phthalates was severely limited.

Activated sludge sequencing batch reactors were used to study BTX biodegradation under anoxic (denitrifying), microaerobic, and aerobic conditions. Toluene and m-xylene were biodegraded under denitrifying conditions, and the loss of these compounds correlated with the activity of reducing enzymes that were capable of oxidizing methyl viologen. Although benzene, p-, and o-xylene were recalcitrant under anoxic treatment, all three were biodegraded under microaerobic [<0.2 mg/L dissolved oxygen (DO)] and nitrate or nitrite (NO_x)-supplemented microaerobic conditions. Methyl viologen oxidation potential decreased under all microaerobic conditions while catechol 1,2-dioxygenase (C12O) and catechol 2,3-dioxygenase (C23O) were induced, indicating that the aromatic hydrocarbons were metabolized by aerobic pathways, even in the presence of NO_x and in the absence of measurable DO. The degree of C12O and C23O expression under microaerobic conditions was comparable to levels found under aerobic (DO > 4 mg/L) conditions. Benzene, p-, and o-xylene were biodegraded twice as fast under NO_x-supplemented compared to NO_x-free microaerobic conditions, and specific biodegradation rates under aerobic and NO_x-supplemented microaerobic conditions were comparable. Oxidation reduction potential successfully differentiated between the various electron acceptor conditions and proved to be a sensitive indicator.

Nitric oxide (NO) emissions from soil amended with municipal wastewater biosolids or mineral fertilizer can adversely impact ozone levels in the lower troposphere and are studied here. A comprehensive laboratory protocol was developed and conducted to consider the influence of biosolids-amended (BSA) soil and mineral-fertilizer-amended (MFA) soil, as compared with unamended soil, on NO emissions with varying temperature, moisture content, and pH. NO plays an important role in the formation of tropospheric ozone. Minimization of NO flux from soil is advantageous, protecting air quality as well as conserving valued nitrogen fertilizers. BSA and MFA soils were found to produce NO emissions over five times greater than unamended soils. Distinctions between NO flux from MFA and BSA soil cannot be made. NO flux from soils at pH 6.25 and 6.75 were not found to be statistically different. Soils with higher water-filled pore space (WFPS) and temperature are generally shown to produce greater quantities of NO within the range of temperature (15°–28°C) and WFPS (3–40%) studied.

In this paper, an evaluation of four Gaussian dispersion models—GM, CALINE3, PAL2, and ISCST2—commonly used for vehicular pollution prediction is presented. The evaluation has been done using field data monitored at one of the metropolitan cities, Chennai (Madras), India. Various statistical parameters that indicate the model's accuracy in prediction have been calculated. The relative accuracies of the models are compared. Such an analysis will be of use to regulatory agencies in selection of an appropriate model with confidence. As per the analysis conducted in this study, the model GM proved to be the best among the four followed in order by CALINE3, ISCST2, and PAL2. The prediction by all four models was comparable when the wind-road angle was perpendicular with wind speed being 2–5 m/s and the atmospheric stability condition being neutral. The prediction was the most accurate for the case of ground level receptor-perpendicular wind direction combination. The model GM provided the best simulation for all conditions tried. The model CALINE3 was the next best but skewed toward underprediction, whereas the models ISCST2 and PAL2 were found to overpredict.

The resuspension of contaminated cohesive sediments can impact water quality by mobilizing sediment particles and adsorbed contaminants. Changes in psysicochemical and electrochemical environments, around resuspended sediment particles, may cause some contaminants to desorb into the water column. The contribution of contaminated sediments to degradation of water quality depends on an estimate of sediment resuspension rates. In this study, the resuspension of Georgia kaolinite sediments under varying pH conditions was investigated in laboratory flume experiments. Because the edge charge of kaolinite particles is pH dependent, kaolinite particles exhibit different modes of particle associations under varying pH conditions. The paper characterizes these particle associations and relates them to the resuspension of kaolinite sediments for varying pH values. Variations in sediment water content, changes from a stratified to a uniform sediment bed, changes in rheological properties, and variations in the electrophoretic mobility of kaolinite particles were all indicative of the changes in particle associations that resulted from changes in sediment pH. The critical shear stress and the erosion rate coefficient were evaluated for varying pH values and explained by particle associations. A rheometer was used to measure rheological properties of the settled sediment bed; the measured yield stresses had a direct correlation with critical shear stresses.

Clay, which is a natural fine-grained soil, was subjected to various levels of contamination by the heavy metal Zn, in a series of batch sorption experiments conducted at pHs of 3–7. Zinc speciation in the soil was subsequently analyzed using a sequential extraction technique, which fractionated the retained Zn into exchangeable, carbonate, reducible, organic, and residual fractions. The results showed that when Zn loading increased, the increase in Zn sorption by the clay was rather limited in acidic conditions. The Zn distribution between the dissolved phase and each fraction of the sorbed phases could be described with either the Freundlich or the Langmuir adsorption equations. If favorable pH existed, at high Zn loadings, precipitation was the most effective process of Zn removal from the dissolved phase. At low Zn loading levels, a relatively higher proportion of Zn was retained in the more persistent fractions. These findings indicated that the predominant mode of heavy metal retention in a soil can be significantly influenced by the level of soil contamination by the metal, in addition to geochemical factors and behaviors of the metal itself.

The use of in-situ ozone venting is an effective and economic technology to remediate soils contaminated with organic chemicals. A model was developed in this study to describe the transport of gaseous ozone in unsaturated soils. Mass balance equations for soil organic matter, phenanthrene, and ozone were incorporated into the model. The model was found to fit the experimental data obtained from one-dimensional columns, using the previously published rate expressions for the reactions of ozone with soil organic matter and phenanthrene. However, it was observed that the initial unsteady-state conditions for the gas pressure resulted in minor deviations between the simulated and experimental results. Based on the simulated results, the reactions of ozone with phenanthrene and organic matter can be modeled as either parallel or serial reactions. As the initial distribution of contaminant would be nonuniform and some contaminant would be sorbed or trapped in immobile regions, it is recommended that in situ ozonation be ceased no sooner than when the effluent ozone concentrations begins to stabilize.

This study compares various leachate management scenarios using a biologically reactive transport model, which is proposed in this study. The proposed model can be used to predict the contribution of biodegradation to contaminant attenuation and contaminant concentration in leachate over time. It can also be used to assess the extent of landfill stabilization in terms of local mass per bulk volume of remaining refuse available for transfer. A sensitivity analysis shows that landfill stabilization has significant sensitivity to most biokinetic parameters, the fluid-phase saturation constant, and the dissolution rate, in addition to the half-saturation constant and the retardation factor. The proposed model is applied to assess landfill stabilization under two control scenarios: leachate recycling versus continued input of clean water with no recirculation. The simulation results indicate that leachate recirculation provides more favorable conditions for development of an active anaerobic bacterial population and, hence, accelerates landfill stabilization.

The relative dissolution rates of several commercial hydrated limes were evaluated. Significant differences in dissolution rates were observed ranging from a time of <10 s for 90% dissolution to 1 min for a 5% slurry. A 2% slurry dissolved faster than a 5% slurry. The speed of lime dissolution may be important for some treatment processes.