Journal of Environmental Engineering

The pervasive nature of water and its accessibility to mankind leave it open to a variety of civilization‐induced changes ranging from diversion of water flows to heavy‐metals pollution. The abuses of the last 100 years have led to problems with water quality and quantity. The consequences may not be apparent for some time, and cause and effect may be difficult to identify, due to the large distances involved. Exploratory multidisciplinary research on comparative toxicology, the cycles of energy and nutrients in ecological communities, and the impact of chemicals and other stresses upon structure and species distribution frequency can eventually yield criteria that permit the establishment of no adverse effect tolerance levels for substances nonindigenous to natural systems. The maintenance of resilient ecosystems with a sufficient diversity of organisms is a prerequisite for the well‐being and health of humans.

The U.S. Environmental Protection Agency (EPA) has recently promulgated drinking water regulations known as the surface water treatment rule (SWTR) to control viruses, Giardia lamblia, Legionella, and heterotrophic bacteria. The SWTR specifies overall minimum removal and inactivation efficiencies by filtration and disinfection for Giardia lamblia and viruses. The CT concept is used to predict inactivation efficiencies. CT stands for the product of characteristic exposure time (T) and the characteristic concentration (C) of the disinfectant in the disinfection chamber. This paper presents an analysis of ways to characterize ozone concentration in water disinfection systems for the purpose of calculating ozone inactivation efficiencies. Guidelines to predict the characteristic (or average) ozone concentration are developed for four types of ozone reactors, including rigorously mixed systems, cocurrent and countercurrent contactors, and flow segments (i.e., where no ozone gas is introduced but where residuals in a water phase remain).

The expanded‐bed anaerobic GAC reactor, operating with GAC replacement has been demonstrated to treat toxic wastes effectively. The concentration of adsorbable toxic compounds, the hydraulic detention time within the treatment system, and the GAC replacement rate represent three parameters that are strongly related and effectively control process performance. This study investigates the impact of the aforementioned process variables on treatment efficiency. A steady‐state model that includes representations of the various phenomena that occur within the anaerobic reactors such as biodegradation, inhibition, competitive adsorption, and attached biomass shearing is presented. Using independently determined adsorption parameters, measured shear loss coefficients, and literature values for biodegradation kinetic parameters, the model predictions are found to be in good agreement with the experimental data.

Fluidized bed crystallizers (FBCs), also called pelletizers or pellet reactors, are an attractive alternative to conventional water softening. FBCs produce a coarse, spherically grained, easily dewatered, calcitic sand as the waste product, rather than a gelatinous sludge. Two operating full‐scale municipal FBCs used for softening in southern Florida were sampled and characterized. The study concludes that: (1) Calcium removal is very rapid within the reactor, leaving a large portion of the bed unused at any one time; (2) the calcium removal rate can be described by the Reddy‐Nancollas crystallization model; (3) high supersaturation levels due to poor mixing at the point of chemical mixing promotes microfloc production and solids carryover in the effluent; (4) classification of the pellets within the fluidized bed does not occur; (5) the sampling techniques appear to be adequate for characterizing FBC reactors; and (6) operation of FBCs would be aided by the installation of a pressure gauge on the FBC influent to monitor bed growth.

This paper describes computer modeling experiments for single adsorbate flow through a packed‐bed adsorption reactor to examine the behavior of concentration “wave fronts” for aqueous and solid phases. Conclusions were: (1) The shape of the wave front assumes a steady‐state shape after development; (2) the wave front has constant velocity; (3) the solid‐phase wave front is synoptic with the aqueous‐phase wave front; (4) the wave‐front velocity can be predicted from isotherm data; and (5) the adsorbate‐concentration breakthrough curve can be calculated from wave‐front data. These conclusions relate directly to design and operation. First, the length of the reactor can be calculated by wave‐front velocity, which can be determined from isotherm data. Second, for pilot plant studies the major interest includes wave‐front shape, length, and velocity. Third, in operation, the progress of the wave front should be monitored to predict the breakthrough curve.

Based on a comprehensive analysis of four groups of chemical and biological reactions, a general pseudo‐steady‐state mathematical model of the anaerobic‐digeston buffer system was developed. According to the characteristics of the wastewater being treated and the biological efficiencies of the reactors, this model was able to predict the pH, the concentration of bicarbonate in the effluent, and the percentages of methane, carbon dioxide, and hydrogen sulfide in the biogas from several laboratory‐scale reactors of different configurations treating different types of influent. The model showed that the composition of the wastewater, such as the concentration of volatile fatty acids (VFA), sulfate, nitrogenous compounds, and chemical oxygen demand (COD), has a profound effect on the chemical demands for pH control in anaerobic digestion. By means of simulation, some operational factors, such as the flow pattern in the reactors, recirculation and the effectiveness of different chemicals for pH adjustment are also discussed.

The ammonia oxidation rate of Nitrosomonas spp. is tested at various nitrite concentrations from 14 to 296 mg‐N/L in fresh water at 28.4 °C. The nitrite oxidation rate of Nitrobacter spp. is tested at various nitrate concentrations of 11–250 mg N/L. The oxidation rates of both bacteria are found to be linearly correlated to free‐energy changes $\left(\delta G\right)$ with coefficients of determination of 0.841 and 0.934. It is shown that the Monod kinetic model with end‐product inhibition can be more simply and precisely modeled by a proposed thermodynamic model. At the same ratio of ammonia/nitrite and nitrite/nitrate over the range of 0.001–0.1, the ammonia conversion rate is about double that of the nitrite conversion rate. Management strategies can be devised to maximize the nitrite removal rate. Further, by knowing the thermodynamic model's parameters, the reactor's performance can be optimized during the peak loading period while minimizing nitrite pulses in the effluent.

Spatial distribution of water discharge can be obtained through a manifold consisting of main pipes and perforated laterals. This study formulates the problem of discharge distribution among lateral pipes and from laterals through orifices uniformly spaced along them. A nonlinear difference equation for flow in the pipes is solved numerically to determine the driving head as a function of the number of ports and a geometrical ratio. A dimensionless solution for simple manifolds is developed to obtain a direct method of evaluating discharges through all the orifices. The approach presented in this paper makes it possible to formulate the problem of restricting the maximum outlet‐discharge variation through a dividing‐flow manifold. A graphical solution to this problem is obtained that permits one to indicate the lateral‐to‐manifold diameter ratio for limiting this variation to a prescribed value. The results of this paper offer a means for comparing the uniformity of distribution expected from various combinations of the number of orifices and laterals and their sizes for a given manifold.

A novel approach for including uncertainty in the water‐quality‐management process is presented. The approach is based on the use of multiple scenarios that reflect possible combinations of hydrologic, meteorologic, and pollutant loading design conditions. Each scenario, which represents possible “future management design conditions,” defines a set of transfer coefficients that describes the pollutant transport process under these design conditions. A general waste‐load‐allocation model is developed for which all sets of transfer coefficients are simultaneously incorporated in the water‐quality constraint set. Several optimization formulations of this general model are examined to evaluate efficient management solutions that account for a range of possible future conditions. The approach is applied to the management of BOD from 10 dischargers on the Willamette River in Oregon. The results indicate that model formulations based on minimizing a measure of total deviations from a target DO level identify efficient solutions for the waste‐load allocation.

An algebraic expression in diraensionless form, relating effluent substrate concentration to retention time for plug‐flow reactors is developed. Monod kinetics is used, and longitudinal concentration gradients are considered. An expression for mean cell residence time is also derived. The model is illustrated by an example using the data of a field study. Results of the developed model are compared with those of constant biomass assumption and differences in biomass and effluent substrate concentrations found more meaningful for values of $(1+r){\theta }_x/\theta <5\text{.}$ A graphical procedure enabling a simplified methodology for process design is presented. The method can be used in the design of batch or plug‐flow reactors.