Journal of Environmental Engineering

Following the grounding of the Exxon Valdez in March 1989, many beaches in the Alaskan waters were fouled with oil. One of these was Kittiwake Beach of Knight Island in Prince William Sound. The U.S. Environmental Protection Agency initiated a research project to evaluate the application of nutrients to the surface of the beach to stimulate biodegradation of the surficial and infiltrated oil by indigenous microbial populations. Nutrients were dissolved in seawater and applied by sprinklers to study plots along the beach. As part of this project, a preliminary, simplified model of the beach hydraulics is developed to describe the advective fate of the applied nutrients. The retention time of nutrients in the beach is estimated to be on the order of 12 h (one tidal cycle) for the application method used. Simulations indicate that application of nutrients through a trench or subsurface drain placed above the high‐tide level, rather than directly on the beach by sprinklers, would result in significantly longer retention times. Design guidance for future remediation is provided to ensure nutrient coverage of the entire oiled profile of a permeable beach.

A statistical formulation for estimating the average time of detention within a pond for a captured runoff volume is presented. For a conservative estimator, it is assumed that mixing takes place during an event and that settling occurs over a period to empty the captured volume or the time between successive events, whichever is smaller. This analytically determined detention time is used in conjunction with a pollutant‐settling efficiency‐detention time curve to estimate the settling efficiency. This curve is generated from Stormwater Management Model (SWMM) program simulations and shown to be independent of runoff statistics, pond configuration, and arbitrary but constant influent concentration under complete mixing. The analytical detention time estimate, in combination with the settling efficiency curve provides a valuable desk top method for the planning‐level design of detention basins for pollutant removal. The method performs quite well compared to the results obtained from long‐term SWMM simulation runs.

This paper focuses on the competitive adsorption isotherms pertinent to an activated carbon adsorber treating contaminated drinking water. The water under consideration is a mixture of μg/L levels of 1,1,2‐trichloroethane (TCEA), used as a model toxic compound, and the natural organic matter (NOM) from river water. The presence of NOM greatly reduces the adsorption capacity of TCEA. The TCEA isotherms were greatly affected by the initial concentration of both solutes; the adsorption capacity of the NOM was only reduced by a maximum of 5–10% because of the TCEA. This is possibly the result of adsorption, which involves competition in only a fraction of adsorption sites. A new model, based on such an assumption, described the data fairly successfully; existing predictive models simulated the data poorly.

Bioparticles of widely varying size and density were partially intermixed and segregated according to size, along the length of a solid‐liquid biological fluidized bed. The biomass concentration was not very sensitive to the growth of bioparticles up to about 100 mg of attached biomass/g sand; then, it decreased rather rapidly as more biomass accumulated on the bioparticles. Although bed expansion characteristics with respect to upflow velocity could be described by the classic Richardson and Zaki equation, the expansion index could not be correlated with the terminal Reynolds number of a characteristic bioparticle with the Sauter‐mean diameter. The total bioparticle volume in the reactor estimated from the Sauter‐mean diameter was about 25% more than that calculated from the actual bioparticle size distribution.

The Leslie matrix model is applied to the analysis of the hydraulic age distribution of perfectly mixed, non‐steady‐state flow reactors. The relationship between the hydraulic age distribution and performance of these reactors is discussed, and concepts and algorithms developed in the paper are applied to a case study. Steady‐state flow and batch reactors are particular cases of the non‐steady‐state flow reactor. The percentage of fresh effluents (PFE) within the reactor is a better descriptor of the reactor performance than the mean residence time. When a reactor is strictly under steady‐state flow conditions, the ratio PFE/mean residence time is constant, and only then can the PFE be substituted by the mean residence time. High rate reactions (such as fecal coliforms removal in sewage treatment systems) correlate better with PFE of a few days (1–5 days in the discussed example), and low rate reactions [such as biochemical oxygen demand (BOD) removal] correlate better with the PFE of more days (30 days in the example). Computer algorithms for the calculation of the PFE and mean residence time are provided.

Equations to calculate the hydraulic residence time (HRT) of complete‐mix reactors (CSTR), with or without recirculation, under unsteady‐state condition are developed in this study. Recirculation does not affect the hydraulic residence time of a CSTR‐type reactor no matter which state (steady or unsteady) condition the tank is operated under. Response of a CSTR tank to a step change in hydraulic loading is evaluated by using the developed equations. Hydraulic residence time of a CSTR under a periodic influent flow rate is studied, and related normalized curves are proposed to assist design, operation, and evaluation of CSTR reactors. CSTRs can dampen hydraulic fluctuations, especially when the cycle length of influent fluctuations is shorter than one hydraulic residence time (time‐weighted average value) of the tank. The cycle length of the fluctuations of influent is found to be a key parameter in determining variations of hydraulic residence time. Shorter the normalized cycle length, fewer the variations; i.e., an increase of effective volume of the tank will improve its capability in handling influent fluctuations. An example of using the proposed normalized curve in the design of an aerated lagoon, for a given characteristic wastewater cited from the literature, is provided.

Inhibition of respiration rates $\left({\mathrm{IC}}_{50}\right)$ of activated‐sludge (AS) microorganisms and a commercial surrogate culture, Polytox, were measured using the respirometric technique for 50 organic chemicals. The correlation between the ${\mathrm{IC}}_{50}$ values for the two cultures was found to be highly significant with $r^2=0.928$, the Polytox cultures being more sensitive than AS cultures. Using AS experimental inhibition data for a training set of 40 chemicals, a single‐variable quantitative structure activity relationship (QSAR) model was developed to estimate ${\mathrm{IC}}_{50}$ for AS. When this QSAR model was tested on a testing set of 10 “new” chemicals, the predicted ${\mathrm{IC}}_{50}$ values agreed well with the experimentally measured values, with $r^2=0.844$ at $p=0.0002$. Inhibition data reported in the literature for 32 additional chemicals were also compared with predictions made by the QSAR model, and the agreement was satisfactory, with an overall $r^2$ of 0.798, at $p=0.0001$.

A process for the treatment of industrial wastewater containing heavy metals, using fly ash adsorption and cement fixation of the metal‐laden adsorbent, was investigated. Results show that the fly ash can be an effective metal adsorbent, at least for Zn(II) and Cd(II) in dilute industrial wastewaters. Fly ash adsorption capacities for Zn(II) and Cd(II) were 0.27 mg/g and 0.05 mg/g, respectively. A mortar specimen prepared with 10% metal‐laden fly ash showed a 56‐day strength, about the same or even greater than that of cement alone. Leachates from the fixed metal‐laden fly ash, obtained by using both the ASTM and U.S. Environmental Protection Agency‐extraction procedure (USEPA‐EP) leaching tests, exhibit metal concentrations lower than the drinking water standards. Compressive strength and leaching test results suggest that metal‐laden fly ash can be considered for use in secondary construction materials.

Fluidized‐bed‐combustion (FBC) technology will become more common as coal‐combustion systems are retrofitted to meet air‐emissions standards mandated by the 1991 Clean Air Act. FBC ash will present new challenges for coal‐combustion by‐product management. Calcium‐based sorbents added to the combustion system of FBC make the properties of the resulting ash different from electric‐utility fly ash. Recycling options, used to divert electric‐utility ash from landfills, will have to be reevaluated for FBC ash. FBC ash from Central Soya Company's circulating FBC boiler, previously located in Chattanooga, Tennessee, was sampled over a 2‐year period. The ash was analyzed for chemical and physical properties and evaluated as a concrete admixture. Results show that concrete strength and long‐term durability are compromised by substituting FBC ash for Portland cement. The high pozzolanic activity of the ash and the decrease in leachability of some constituents with increasing ash content in concrete suggest that FBC ash has potential to be a good material for hazardous‐waste solidification and stabilization.

Solidification and/or stabilization with cementitious materials prior to burial is one option for disposal of liquid hazardous and radioactive wastes. A common design for disposal of cementitious waste forms is to pour the material into large belowground vaults. The leaching performance of partially degraded monolithic vaults is examined quantitatively for facilities located in humid to semiarid climates. Development of perched water on the vault roof leading to fracture flow through the structure is predicted for a wide range of climate and design conditions. Leaching controlled by diffusion in matrix blocks out to fractures is examined parametrically in relation to water flux rate and crack spacing. Depending upon the parameters examined, release rate may be controlled by water flux rate (advection controlled) or diffusion. Under some circumstances, contaminant release rates and exit concentrations are predicted to be inversely related. In this situation, minimization of release does not result in the lowest predicted groundwater concentrations below the vault.

An investigation was carried out using coupled electric‐hydraulic gradient to remove heavy metal contaminants from residual wastes. A mixture of spent foundry sand and millpond sludge containing high levels of Zn, Mn, and Pb was used as the contaminated source. An electric gradient was applied, in a specially constructed decontamination cell, to generate a low pH solution by electrode reactions. Simultaneously, the low pH solution was pumped through the residual waste under a constant hydraulic gradient. Experiments were carried out under 0, 100, 150, and 200 V, with a constant hydraulic gradient producing an average flow of about 1 cc/min. At 0 V with hydraulic gradient alone, negligible amount of metals were removed. It was found that higher voltages removed higher percentages of metals of interest from the solid medium. At 200 V both Mn and Zn removal was about 72%. The removal of Pb was slower compared to Mn and Zn. At 100 V a negligible amount of Pb was removed; at 200 V only 46% Pb was removed from the solid mixture. It was demonstrated that a coupled electric‐hydraulic gradient can be effectively utilized to remove heavy‐metal contaminants from a porous medium.

This paper presents results from laboratory investigations on utilizing ultrasonic and subsonic vibrations to facilitate removal of fine clay particles from the subsurface. A sonicator and a laboratory vibrator were used to vibrate mixtures of sand and pure clay minerals as well as a sample of natural soil from New Jersey. In column experiments, particle extraction resulted in an enhancement in the hydraulic conductivity. Effluent slurries from kaolinite mixtures contained more fine particles than those from montmorillonite mixtures. When subsonic vibrations were applied after ultrasonic treatment, additional quantities of fines were extracted. Abrasion and/or fracturing of sand‐ and silt‐sized grains occurred near the vibrating source. The formation of low‐permeability layers because of particle migration and accumulation may have occurred in the experiments. Experiments conducted in 90‐cm‐ and 120‐cm‐diameter tanks showed that the zone of influence of vibrations extended as far as the tank walls. These results suggested that in‐situ implementation of localized vibrations to enhance hydraulic conductivity of clayey soils, and to facilitate controlled mobilization of clay fines and the associated contaminants, may be feasible.

The mineral surface of Wyoming bentonite was modified by replacing inorganic ions by different quaternary ammonium compounds of known molecular structure by an ion‐exchange process. The resulting organically modified bentonites (organobentonites) were evaluated for their suitability as components of compacted earthen liners for waste‐disposal sites. Varying weight percentages of an Ottawa sand, either of five types of organobentonites, untreated bentonite, and water were mixed uniformly and compacted to simulate sand‐and‐bentonite liners for waste‐disposal facilities. The hydraulic conductivities of the compacted soil specimens were measured in triplicate. The hydraulic conductivities of compacted specimens composed of 88% (by mass) Ottawa sand, 8% untreated bentonite, and 4% organobentonite were on the order of 10⁻⁸ cm/s. Benzene sorption from water to Ottawa sand, bentonite, and two of the organobentonites was measured experimentally. Benzene sorption to each organobentonite was significantly greater than benzene sorption to bentonite or Ottawa sand. The sorption and permeability data were input into a one‐dimensional solute transport model to simulate benzene transport through a conventional earthen liner and two liners containing organobentonites. Results of the simulations indicate that earthen liners containing organobentonites significantly retard the transport of benzene relative to a conventional liner.

A stochastic modeling approach for the migration of contaminants in soil and ground‐water is developed to assess the expected magnitude of contamination at receptors located downgradient from a waste pit. The model evaluates uncertainties in contaminant concentrations due to uncertainties in waste composition and hydrogeologic properties of waste sites, and determines the exceedance probabilities of a specified concentration level at receptor points using Monte Carlo (MC), first order (FO), and point‐estimate (PE) methods. A lognormal distribution is assumed for concentration at a receptor point in application of the FO and PE methods to assess exceedance probabilities. A comparison of the three error‐analysis methods is performed to evaluate the efficiency and accuracy of the FO and PE methods relative to the MC method. Results suggest that for nonadsorbing and conservative contaminants, the level of accuracy provided by the FO method is comparable with that of the MC method. For nonconservative contaminants, both FO and PE methods compared less favorably with the MC results.

The mass transfer of gasoline constituents [benzene, toluene, ethylbenzene, xylenes (BTEX), and naphthalene] to air during laboratory‐simulated soil vapor extraction (referred to here as soil venting) was determined. Soil venting was conducted on laboratory‐packed columns brought to residual water then residual gasoline saturations. The columns contained either a sandy, low organic soil (0.10% organic carbon), a sandy organic soil (1.65% organic carbon), or glass beads. Gasoline‐air partitioning at the start of soil venting was experimentally determined to be adequately described by Raoult's law for many gasoline constituents of interest. The application of the local equilibrium assumption used to describe the mass transfer of gasoline constituents from the residual gasoline to air during soil venting was investigated using: (1) Experimental techniques such as reduction of air flow rate; flow‐interruption and discrete sampling of soil following soil venting; and (2) a local‐equilibrium‐based model employing Raoult's law to describe gasoline‐air partitioning. The local equilibrium assumption was shown to be valid for describing the mass transfer of various gasoline constituents until such time when the constituent was nearly depleted from the non‐aqueous‐phase liquid gasoline (NAPL). Mass transfer of constituents became rate‐limiting when the constituent was depleted from the residual gasoline. The effect of soil organic matter content was only slightly apparent at this time, as evidenced by the higher benzene and toluene air concentration in the effluent of columns containing the sandy organic soil.

This paper discusses a number of design modifications and changes in operational conditions that have a major influence on the overall performance of the large Coventry municipal incinerator plant (65 MW), which incorporates a heat recovery system for district heating. Four different secondary air‐injection systems and various primary air‐distribution patterns along the grate were investigated using a computational fluid dynamic model, in an attempt to obtain optimum combustion conditions that would minimize potential emissions of toxic pollutants and reduce maintenance costs at the plant. The modeling work showed that the use of four large high‐speed secondary air jets firing toward a common center, together with optimizing the primary air distribution along the grate, produces substantially longer residence times and improves temperature profile at the exit. The novel feature of the proposed secondary air‐injection system is the formation of a large and significantly strong recirculation zone, located above the burning refuse bed, in the middle of the furnace shaft. This strong recirculation zone improves the overall performance of the incinerator due to intensive mixing of hot gaseous products evolving from the refuse bed with the combustion air supplied as the secondary air, and greatly improves the gas phase combustion and helps to reduce emissions of chlorinated organic compounds. Results obtained clearly demonstrate that the proposed secondary air‐injection system has an excellent potential to satisfy most requirements for emissions levels and gas resident times specified by the European Directives (EEC) for municipal incinerators with a vertical radiation shaft.

Treatment plant hydraulics for environmental engineers include problems dealing with dividing flows such as the design of branch channels, lateral weirs, and diffuser systems. The hydraulic characteristics of these problems can be understood by developing simple models based on solutions related to the flow past a two‐dimensional lateral slot. Existing theoretical solutions to the problem of efflux from a two‐dimensional lateral slot located in the wall of a two‐dimensional closed conduit are based on the free streamline theory. The present study provides experimental data to directly verify the predictions of the theoretical two‐dimensional slot flow model. To this end, the predicted dependence of the slot discharge coefficient on the sum of the pressure and velocity heads of the flow approaching the slot, and the discharge ratio, are compared with the test results.

A new method was developed using a flow‐through cell for the measurement of oxidation‐reduction potential (ORP) for anaerobic microbial cultures. The method involves passing the reactor effluent through a recycle line equipped with an ORP probe. Stable ORP values were obtained for two different anaerobic reactors, operating under steady‐state conditions, for more than 2 months. One of the reactors was strictly methanogenic and the other was sulfate‐reducing. A comparison of ORP measurements by this method with conventional measurement techniques showed this method minimizes uncertainty in the measurement as a result of hydrodynamic limitations, temperature changes, and oxygen contamination of the sample. It prevents fouling of the ORP probe by allowing for easy cleaning of the probe and checking for calibration before use. The method can also be used for both batch reactors and chemostats (CSTR).

The performance of two ceramic filter candles and one silver‐impregnated ceramic filter candle with a layer of activated carbon, randomly selected from those available in the Indian commercial market, were evaluated employing the Escherichia coli and poliovirus challenge tests and a long‐duration filtration test mimicking the home treatment of water. None of the candles appeared reliable as microbiological water purifiers. The study indicated that specifications/standards should require filter candles to retain suspended particles down to a size of 1 μm to ensure at least a bacteriologically safe filtrate. Additional disinfection or boiling must be practiced during suspected outbreaks of waterborne viral diseases.