Journal of Environmental Engineering

This study examines the rates of solids dislodgment of three synthetic solids with varying degrees of adhesive characteristics, for which a parameter characterizing the ease of solids breakoff from the surface of filter grains is proposed. This parameter is determined by a technique of curve best‐fitting onto the experimentally obtained solids profiles of the backwash water. The 50% probability values of the parameters for clay/alum, ferric chloride, and clay/polymer are 0.165, 0.120 and $0.085s^{-1}\text{,}$ respectively, suggesting that the order of ease of solids breakoff is clay/alum, ferric chloride, and clay/polymer. This order is also confirmed by the experimental data of bed cleaning efficiency. Furthermore, an attempt is made to formulate a model for predicting the solids profile of backwash water and the amount of solids removal from a bed cleaning operation. The model is developed based on an assumption that the rate of solids dislodgment at a given time is proportional to the specific deposit within the filter bed at that time and its proportionality constant is primarily affected by the property of the deposited solids.

The deposition of small colloids or macromolecules in membrane pores and the subsequent irreversible fouling of the membrane is considered using a simple model for membrane fouling. As a strategy for avoiding pore fouling, it is proposed that foulants capable of entering membrane pores be aggregated to produce particles that are rejected at the membrane surface. Considerations in filtering suspensions of particles on membranes are discussed with emphasis on particle transport mechanisms as a function of particle size. It is concluded that particles larger than $3\mu m$ should not contribute significantly to membrane fouling at permeate fluxes that are currently feasible. For many membrane configurations, particles between 0.1 and $1\mu m$ will be most likely to contribute to fouling on membrane surfaces. Batch flocculation and membrane filtration experiments were conducted to test the feasibility of manipulating flocculation conditions to control particle size and fouling. Conditions that produced particles with a zeta potential near zero minimized fouling by reducing foulant penetration into pores, increasing the porosity of the deposited cake, and increasing the transport of potential foulants away from the membrane.

Individual carbon particles were examined using a Quantimet‐720‐23A programmable and computerized image analyzer to determine their size and shape characteristics. Particles in a given sieve size fraction are well represented by the log‐normal distribution. The use of the Sauter mean of particle diameters from projected area measurements is recommended to account for the effects due to the variations in particle size and shape. The solid phase diffusion coefficient estimated from batch reactor analysis, is found to be invariant with this particle diameter. Power‐law correlations are developed to indicate the effect of batch adsorber parameters and particle size on the external mass transfer coefficient.

Sludge is an inevitable by‐product of wastewater treatment. Its abundance poses disposal problems that can be drastically reduced if sludge can be converted for economical uses in construction as substitute materials. Digested and dewatered sludge, after incineration at a high temperature, yields a hard, cellular, porous mass with low unit weight. This hardened mass of sludge ash can be crushed to smaller‐sized aggregates, which, when graded in suitable proportions, manifest the basic attributes required of lightweight aggregates. When used as aggregates in the production of lightweight concrete, experimental results show that the resulting concrete satisfies the physical requirements of a lightweight concrete in terms of unit weight, strength, heat‐insulating properties, and fire resistance, thus indicating that sludge ash could be a potential source of suitable lightweight aggregates.

The aerated fluidized bed biofilm reactor (FBBR) was evaluated for treatment of a corn starch wastewater at food‐to‐microorganism (F/M) ratios ranging from 0.42 to 1.61 g 5‐day biochemical oxygen demand $\left({\mathrm{BOD}}_5\right)/g$ total volatile solids (TVS)‐day. Laboratory results indicated that good combined carbon oxidation‐nitrification, with removal efficiencies greater than 90%, could be achieved providing that F/M ratio and mean cell residence time (MCRT) were maintained at less than $1.0g{\mathrm{BOD}}_5/g$ TVS‐day and longer than five days, respectively. More than 95% of reactor biomass in an aerated FBBR was immobilized on the fluidized media. Depending on the ${\mathrm{BOD}}_5$ loadings applied, between 25 to 75 mg TVS could be immobilized per gram of sand. Good oxygenation performance, with bulk‐liquid DO concentrations consistently greater than $3.0\mathrm{mg}/L\text{,}$ was achievable because the fluidization requirements of media controlled the air injection rate. Both ${\mathrm{BOD}}_5$ removal and nitrification kinetics exhibited a pattern suggesting that mass transfer resistances of substrates could be significant within the region of low bulk‐liquid concentrations.

Amendments to the Safe Drinking Water Act (PL‐93‐523) highlight the continuing problem of waterborne disease by mandating the EPA to promulgate; (1) Criteria by which filtration will be required for surface water supplies; and (2) disinfection requirements for all water supplies in the United States. There is interest on the part of the EPA in applying the $C\cdot t$ concept for determining the inactivation of Giardia lamblia cysts as a key criterion for determining exceptions to the filtration rule. If a utility, in addition to meeting other requirements, can demonstrate that through effective disinfection, manifested by a sufficient $C\cdot t$ value, it can reduce Giardia levels by 99.9%, then it will be exempted from surface water filtration. In this paper, a model for calculation of $C\cdot t$ values based on animal infectivity data is developed. The model, based on first‐order kinetics, relates $C\cdot t$ values to chlorine concentration, pH, and temperature. The model predictions are compared to laboratory data.

This study investigates the activated carbon adsorption kinetics of TOC from a surface water. The uptake of TOC from water in a batch reactor, as well as in a fixed bed, has been successfully predicted using a single solute, irreversible adsorption model, in which the pore diffusion coefficient is assumed to be changing throughout the process duration. The diffusion rate of the humic substances naturally occurring in surface waters gradually decreases as smaller and smaller pores are penetrated. The slow adsorption phenomenon can be reduced using sufficiently small carbon particles. Column tests showed that a higher degree of saturation and a longer breakthrough time can be achieved after reducing the particle‐size fraction of the activated carbon.

The dynamics of adsorption for various adsorbate‐adsorbent systems have been predicted successfully using the homogeneous surface diffusion model (HSDM). External film mass transfer and surface diffusion coefficients, as well as adsorption equilibrium parameters must be defined in order to solve the mathematical model. Simple batch experiments are usually performed to determine the mass transfer coefficients; whereas bottle tests are employed to obtain the equilibrium data. The parameter search procedure presented in this paper uniquely determines these parameters for the HSDM. No limitations are imposed on this search due to the dominance of one mass transport mechanism over the other, such as cases limited by either external film or surface diffusion resistance. Simple residual plots are used to examine the reliability of the estimated parameters and to judge whether the HSDM is the appropriate model for describing specific adsorption data.

Vapor extraction systems have recently been used in a variety of environmental engineering applications, including controlling methane migration and remediating problems associated with spills of volatile and semi‐volatile organic compounds. The differential equations that govern pressure flow of gas and vapor in soil are nonlinear in that gas density depends upon gas pressure. However, if the maximum pressure difference between any two points in the flow field is less than approximately 0.5 atmospheres, the differential equations developed to model groundwater flow provide good approximations to gas transport. Vapor extraction systems generally operate under pressure differences on the order of 0.2 atmospheres. Analytical and numerical groundwater flow models can therefore be used to model vapor and gas transport if the proper set of input variables is defined. Analytical groundwater flow models are used to evaluate the results of a field gas extraction test. The results of these evaluations indicate groundwater models can provide an efficient and readily‐accessible tool to aid in designing vapor extraction systems.

In an attempt to simulate the mitigation of chemical contamination by microbial enrichment in existing unsaturated soil systems, two important characteristics, i.e., the influence of microorganisms on soil water flow and chemical removal rates, were incorporated into a mathematical model (BIOSOIL). Variablestep and variable‐order Gear's Method was employed as a numerical approximation to solve the set of four differential equations. The resulting mathematical model described the behavior of the biosoil system under varying input condititons. From this modeling study of chemical removal, the following conclusions were drawn: (1) The depth of the unsaturated soil zone, a significant consideration in waste disposal practices, seems to be less crucial under bioremediation than generally thought; and (2) microbial distribution in a vertical column of soil is as important as the total population size of soil microorganisms. Study results suggest a guideline for the development of vertically well distributed biosoil systems: apply a limiting substrate at high concentration, at fast rates, and in infrequent cycles.

Water quality data from 30 stations of the National Stream Quality Accounting Network (NASQAN) in the Ohio River basin were examined for spatial adequacy. A direct statistical test of differences between two sample means (for small sample size and unknown population variances) found that for over onehalf of the 53 water quality parameters, differences in water quality could not be distinguished for more than one‐third of the station pairs. An indirect statistical approach, correlating mean water quality to 136 possible explanatory basin characteristics, resulted in reasonable cause‐effect relationships for only 23 parameters. Based on a method developed to determine a spatially adequate sample size, it is recommended that the current bimonthly or quarterly sampling frequency be increased to monthly for all parameters and that the current annual reporting period be increased to four years. Other suggestions to make the system more cost‐effective are included.

A simplified mathematical modeling methodology is proposed for the assessment of water quality in small tidal inlet systems. Towards this objective, a quasi‐steady, tidal exchange model is developed for predicting the periodic tidal variations in the concentration of conservative and decaying constituents in small, completely mixed embayments subject to steady loading and steady freshwater inflow. This model provides an inexpensive alternative to conventional multidimensional schemes and provides insight into the physical nature of the water quality response of these systems to the external forcing effects of ocean tides, freshwater inflow, and constituent loading. It is demonstrated in both theoretical arguments and by sample applications of the model that the low‐pass, hydraulic filtering response of tidal inlet systems influences water quality. The model demonstrates that tidal exchange processes are less efficient in hydraulically constricted systems, which have relatively short Helmholtz periods and large inlet impedances, than in systems with long natural periods. Consequently, the model reveals a greater tendency for pollutant accumulation in systems with short Helmholtz periods. Thus, the model provides both a conceptual framework and a screening tool for assessing the susceptibility of small tidal systems to pollutant accumulation.

Stormwater models, which are used extensively for stormwater drainage design and management, require calibration. The parameters usually estimated by calibration include infiltration and roughness. Parameters that can be determined by measurement include catchment area, slope, and length of flow path. The dependency of parameter values on the level of model discretization was investigated. Experiments were performed on a small urban catchment, and results of different levels of discretization were compared to observed hydrograph volume and peak runoff. The only factor found to require adjustment based on the level of discretization and the size of subcatchment was the overland flow time. Small‐scale discretization required a longer flow path or a higher Manning roughness coefficient than coarse discretization to predict lag times correctly.

Prior to the imposition of effective source‐control measures in 1970, large quantities of DDT were discharged to the Los Angeles County municipal sewer system and, subsequently, to the Pacific Ocean. A fraction of this material accumulated among sediments of the Palos Verdes shelf. While the bulk of the DDT lies 10–40 cm below the sediment surface, its fate may be affected by future wastewater‐treatment decisions at the Joint Water Pollution Control Plant (JWPCP), Los Angeles County's 385‐mgd treatment plant. To assess the impact of impending JWPCP secondary treatment requirements on shelf sediment quality (processes that potentially affect the distribution of chemical tracers among those sediments, including background sedimentation rate), contributions from effluent‐related solids, sediment mixing, and diffusive transport were incorporated in mathematical models. Model projections of surface‐sediment quality at the most heavily contaminated sites are sensitive to projected effluent solids concentrations, and thus treatmentlevel decisions at JWPCP. Full secondary treatment projections of surface‐sediment DDT concentration are 75% higher than those corresponding to continuation of present treatment practice.

On‐site sewage disposal systems (commonly septic tank/soil absorption systems) provide a viable and permanent management alternative for the treatment and disposal of wastewater in rural, unsewered communities. Population expansion in rural and semirural areas has presented challenges to develop economical, efficient on‐site sewage disposal systems in severly limited areas. Building sewage disposal systems in fill can be used to overcome site limitations such as high groundwater and shallow depth to ledge. An efficient design balances the requirements of hydraulic and renovation capacity, which depend strongly on the compacted, in‐situ permeability of the emplaced fill specified in the design phase. This paper reviews the procedures used to design subsurface sewage disposal systems in emplaced fill, to illustrate the importance of fill permeability and its relationship to system performance and economy. Procedures are suggested for identifying and verifying suitable material and for placing fill to meet design specifications. These procedures are illustrated by a case study of a site in Ashford, Connecticut.

The application of the errors in variables method (EVM) to environmental engineering practice using, as an example, the determination of kinetic parameters from a laboratory biodegradability study is presented. Model parameters must often be estimated from field or laboratory observations to describe biological treatability studies, adsorption, and water quality modeling. Such models consist of one or more linear or nonlinear equations. Usually, a function of the observed variables is designated as the ``dependent.'' The model parameter values minimizing the sum of squares of deviations between the observed and predicted ``dependent'' variable is accepted as ``best.'' Even for linear models, this method is biased when the independent variables are measured with error. Forced linearization of instrinsically nonlinear models may produce biased point estimates or interval estimates of parameter. Nonlinear parameter estimation methods when some or all of the variables are measured with error, designated as EVM, recently have been developed and utilized in chemical engineering for the reduction of thermodynamic data.