Journal of Environmental Engineering

A steady‐state approach for designing biological wastewater treatment systems is developed for inhibitory substrates using the Haldane rate expression. Steady‐state solutions are derived for a continuous‐flow completely mixed reactor with recycle of the biomass. It is determined that two steady‐state operating regions exist at any one mean cell residence time; an unstable region, where the substrate concentration increases and the biomass concentration decreases as the mean cell residence time increases, and a stable region, where the substrate concentration decreases and the biomass concentration increases with mean cell residence time. A critical mean cell residence time, ${\theta }_c^{*}\text{,}$ is established as a design parameter, for inhibitory substrates, to maintain a biological reactor in the stable operating region. A survey of published data for phenol exhibits an excellent logprobability relationship for ${\theta }_c^{*}\text{.}$ For design purposes, the hydraulic residence time, ${\theta }_h\text{,}$ is set equal to ${\theta }_c^{*}\text{,}$ This restricts the mean cell residence time, ${\theta }_c\text{,}$ to values higher than ${\theta }_h\text{,}$ i.e., ${\theta }_c\ge {\theta }_h\text{,}$ and maintains a reactor in the stable operating region at all times (based on steady‐state analysis). For phenol, a very biodegradable inhibitory substrate, the data indicate a log‐mean ${\theta }_c^{*}$ of 0.29 days, ${\theta }_c^{*}\text{.}$ However, values of ${\theta }_c^{*}$ guaranteeing a less‐than‐1% chance that ${\theta }_c^{*}$ will be exceeded are in excess of one day.

Mesophilic biomethanation of distillery spentwash has been investigated at a pilot scale with a diphasic anaerobic process. An operational strategy to suppress sulfide inhibition through pH control in the methane phase has also been validated. The acid phase has been operated on lime‐neutralized spentwash at an organic loading rate of $30\mathrm{kg}\mathrm{COD}/m^3/\mathrm{day}\text{.}$ Methane phase operation without solids recycling enabled 65% COD reduction and $0.3m^3$ of biogas production with 60% methane per kilogram COD utilized at an organic loading rate of $3.25\mathrm{kg}\mathrm{COD}/m^3/\mathrm{day}\text{.}$ The contact methane phase at $5\mathrm{kg}\mathrm{COD}/m^3/\mathrm{day}$ and 27 day mean cell‐residence time achieved 70% COD reduction and $0.3m^3$ gas production with 70% methane per kilogram of COD utilized. An energy audit revealed over 13 times the energy production per unit energy consumed in the process. A mathematical model for the acid phase predicted volatile acid concentrations within 15% of experimental results. The methane phase model incorporated inhibition and was independent of growth or nongrowth associated phenomena. The model predicted effluent substrate concentrations and gas yields within 10% and 15% of experimental values in the methane phase, respectively.

Discrete molecular size distributions of dissolved organic matter (DOM) in natural waters and wastewaters are usually determined using an array of ultrafiltration membranes in stirred cells. However, many researchers neglect membrane rejection, resulting in an underestimation of DOM in low molecular weight size classes. In this paper, we develop a simple model, based on a permeation coefficient, that can be used to improve the accuracy of size distributions. Since permeation coefficients are a function of membrane material and water composition, these coefficients must be determined for every water sample. The utility of the model is demonstrated by comparing unadjusted and adjusted size distributions of dissolved organic carbon (DOC) in ground waters and municipal wastewaters. For example, when membrane rejection is neglected, 41% of the DOC in Biscayne Aquifer ground water has an apparent molecular weight greater than 5,000 amu. Size distributions determined using the permeation coefficient model, however, indicate only 16% of the DOC is greater than 5,000 amu.

A linear programming (LP) based simulation model is used to examine the effects of variable meteorology on optimal controls for acid‐rain abatement in eastern North America. The LP model minimizes the total cost of pollution control corresponding to 235 controllable point sources subject to satisfying the maximum acid deposition limits at 20 receptor locations. The meteorlogic inputs are transformed into transfer coefficients that relate emissions from sources to depositions at receptors. The meteorologic inputs vary from season to season and year to year, reflecting meteorologic uncertainty. The resulting optimal control strategies for five years of meteorologic data are used in a simulation model to develop a curve of control costs versus the cumulative violation of a critical deposition limit at the selected receptor locations in order to study the trade‐off between emission reduction cost and “damage” reduction.

Serum bottle (SB) activated sludge toxicity test procedure is compared with the OECD procedure. The purpose of this study is to compare and contrast the two testing procedures so that one can complement the other. If the toxicity measurements obtained from the two testing procedures are comparable, users can select either procedure based on time constraint and equipment availability. Seventeen organic toxicants are selected for the comparison. The results show: (1) For chemicals with low volatility, the OECD procedure and SB procedure give comparable results for obtaining the IC50 of organic toxicants; and (2) for volatile chemicals, the SB procedure can be reliably used whereas the OECD procedure cannot because aeration in an open vessel strips out the chemical of interest and the culture is subjected to a concentration decrease with time. Four phenolic compounds are used for studying the effect on the IC50 results due to the origin of the activated sludge. Lab‐activated sludge grown on a complex, defined organic medium and sewage activated sludge gives comparable IC50 results.

A general Lagrangian jet model formulation is presented for an inclined buoyant jet in a current, with a three‐dimensional trajectory. The shear‐induced entrainment is computed as a function of the local densimetric Froude number and jet orientation, while the forced entrainment is taken as the ambient flow intercepted by the “windward” side of the buoyant jet. Model predictions are compared with both trajectory and tracer concentration data for a wide range of discharge and ambient flow conditions: horizontal buoyant jets in a coflow; vertical jets, oblique jets, and dense plumes in a cross flow; buoyant jets in a stagnant uniform or stratified fluid; and a horizontal buoyant jet in a perpendicular cross flow. The model is also consistent with the concept of asymptotic flow regimes, and reproduces the correct behavior in both the near and far field of a vertical momentum of buoyancy‐dominated jet in a cross flow. The connection of the model with traditional integral jet models is also discussed.

A sampling and analysis procedure for the tracer measurement of gas transfer is presented. Dissolved propane was used as a gas‐transfer tracer. The propane samples were analyzed by a headspace gas chromatographic technique that was tested as an alternative to the purge‐and‐trap technique previously used for tracer measurements. Headspace analysis is simple, fast, precise, and does not require the extensive apparatus required by the purge‐and‐trap technique, so that any gas‐chromatography laboratory can perform it. Headspace analysis was found to give excellent results with low uncertainty and a low limit of detection. Gas‐transfer measurements were performed on flow across a large‐scale laboratory weir. A detailed uncertainty analysis was conducted for the transfer measurements. The results are applicable to other tracer measurements of gas transfer, and the head‐space technique may be applied, with modifications, for the laboratory analysis of any volatile compound.

Concentration profiles of combined residual chlorine (CRC) are measured in a small stream just below a municipal wastewater outfall. Loss of CRC is found to be due primarily to volatilization. Vertical and longitudinal concentration profiles are successfully modeled with a 2‐dimensional diffusive transport model, not incorporating a stagnant film. To calibrate this model, a Henry's Law constant for CRC is determined in the laboratory. The model proves superior in the case studied to the classical two‐film model for the prediction of depth‐integrated combined residual chlorine. The diffusive transport model is nearly identical to the two‐film model under conditions of imposed gas‐phase limitation. However, the two‐film theory predicts liquid‐phase limitation for the data in this study, and as a result, significant differences exist between the two sets of model predictions. Field data clearly indicates gas‐phase limitation, which was predicted by the 2‐dimensional diffusive transport model.

A comparison is conducted between Monte Carlo simulation and first‐order uncertainty analysis within the framework of an exposure assessment procedure. The modeled scenario consists of contaminated soil located directly above an aquifer and upgradient of a water supply well. For the exposure assessment procedure, the exposure point concentration is predicted by performing Monte Carlo simulation using a one‐dimensional vertical, soil‐water solute‐transport model coupled with a two‐dimensional, horizontal ground‐water fate and transport model. The result is expressed as a probability distribution of the contaminant concentration at the point of human exposure (water supply well). To complete the exposure assessment procedure, the concentration corresponding to a prespecified risk level (usually 5%) is evaluated against the appropriate and/or relevant health‐based standards for the site. First‐order uncertainty analysis is another probabilistic method that under certain circumstances may be applied in place of Monte Carlo simulation. For the hypothetical site modeled in this research, first‐order uncertainty analysis was found to be a useful tool for examining the relative importance of the various site parameters, although the method was not found to be a substitute for Monte Carlo simulation in the context of this particular exposure assessment procedure.

The effect of correlation among the input parameters and variables on the output uncertainty of the Streeter‐Phelps water‐quality model is examined. Three uncertainty analysis techniques are used: sensitivity analysis, first‐order error analysis, and Monte Carlo simulation. A modified version of the Streeter‐Phelps model that includes nitrification, net algal oxygen production, and sediment oxygen demand is used. Analyses are performed for a wide variety of simulated stream‐flow conditions. Results show that the standard deviation of the predicted dissolved oxygen deficit (DOD) with correlated inputs potentially can be 20‐40% larger than with independent inputs. Under conditions of moderate to high velocity, the reaeration and bio‐oxidation coefficients are the dominant contributors to DOD uncertainty, while net oxygen production from algal activity and sediment oxygen demand are the major factors at low velocity. The largest effect of input correlation on DOD occurs in the vicinity of the sag point. Uncertainty results from first‐order analysis differ by at most 10% from those of a Monte Carlo simulation for both correlated and independent inputs.

This paper presents a heuristic approach for the design of sewer networks that can handle the introduction of lift stations and the use of standard diameters. This heuristic provides good and logical (rather than optimal) designs of sewer networks. The necessary and sufficient conditions to determine the position of a lift station are presented in a theorem. Based on this theoretical result, a heuristic design algorithm, using either the Manning or the modified Hazen‐Williams hydraulic equation, is developed. The proposed heuristic design algorithm takes into account all the structural and hydraulic requirements. The approach is validated and illustrated by designing several hypothetical and two real large sewer networks on a microcomputer. The CPU time for designing sewer networks using the proposed heuristic is less than 10% of the time required for the optimization procedure. Such application indicates the effectiveness of the heuristic in rapidly providing “good” sewer network designs that are comparable to the optimal ones.

Nomographs and tables are used to facilitate the application of Manning's equation for different flow conditions and pipe materials. Different computer programs are also used to avoid the laborious calculations required to find the optimum and economic design. There is no exact algebraic solution for the calculation of the depth of flow in a partially filled pipe when its diameter, slope, and flow rate are known. Therefore, iterative methods are applied to calculate the depth of flow. In this paper, a simple algebraic equation is proposed that will facilitate the calculation of the water surface angle and thus the depth of flow when the diameter, slope of the energy grade line, and the flow rate are known in partially filled pipes. The equation is valid for water surface angles less than or equal to 265°. The application of the proposed equation is illustrated by calculating the flow area, depth of flow, and velocity in a gravity-sewer example problem.