Journal of Environmental Engineering

Poor flow in final settling tanks may be the result of neglecting the effects of density current. This can be countered when inserting guiding flow elements, such as the inlet and the barrier walls in the tank. Based on a simple approach, the optimum height of the inlet aperture increases with both the overflow rate per unit width and the return sludge ratio, but decreases with the relative difference of density. Further, a central barrier wall is introduced to divide the tank into high and low sludge concentration units. More generally, the effect of density can be reduced by correctly positioning the inlet structure. These results are verified by selected prototype observations involving measurements of both velocity and concentration fields. A comparison of original and final designs where effects of density were reduced clearly demonstrates significant improvement in sedimentation and effluent characteristics. The present recommendations may easily be incorporated in an existing tank and, thus, are efficient in improving the final sedimentation.

Subsurface sorption systems (SSS) are zones created within aquifers to enhance the contaminant sorptive capabilities of the porous media. These systems could reduce contaminant mobility by as much as three orders of magnitude, thus delaying contaminant arrival and reducing maximum concentrations downgradient. Reduced contaminant mobility may also allow time for microbial and abiotic degradation reactions to occur. A few investigators explored the use of cationic surfactants in sorption zones. There are limitations to the use of these surfactants, namely reliance upon cation exchange sites, limited contaminant sorption ability, and their biocidal activity. The possible use of nontoxic, water‐insoluble, nonaqueous phase liquids (NAPL) to form SSSs is examined. Experimental results are presented for a bench‐scale system constructed with sand that is partially saturated with a sorbing NAPL (SNAPL). Residual SNAPL saturations, aqueous phase relative permeabilities, and capture efficiency for a hydrophobic organic contaminant (1‐methylnaphthalene) are determined for the sorption zone. An effective SSS is established in the experimental system. In addition to experimental work, this paper introduces the concept of “effective retardation” as a design aid. An application of the concept produces parametric expressions of system performance in terms of residual SNAPL saturation, partition coefficients, and permeabilities.

A design method is presented for a wastewater disposal scheme suitable for coastal communities in both developed and developing countries. The disposal technique exploits the natural flushing mechanisms of estuaries to pump effluent out of the estuary and away from the coast. By placing the outfall within an estuary near the mouth and timing the waste discharge to coincide with the ebbing tide, two advantages ensue: (1) Upstream transport is avoided; and (2) the chance of effluent being re‐entrained at the entrance is minimized. Where there are sensitive environmental targets between the outfall and entrance, Monte Carlo simulation of the transport dispersion processes within the channel is used to predict the distribution of pollutant concentration and, hence, the standard of treatment required. Timing of the discharge cycle is determined by consideration of the dynamics of the ebb and flood flow patterns at the channel entrance. The method is applied to a case study being considered for New South Wales, Australia.

This paper deals with artificial mixing of naturally temperature‐stratified water bodies, such as lakes, reservoirs, and ponds, by selective withdrawal and jet reinjection at a different depth. The effects of jet orientation and jet momentum on mixing processes and efficiency are investigated. A one‐dimensional simulation model, previously verified against experimental data for horizontal jets, is applied to experimental data for vertical jets in this paper. It is found that destratification by a vertical jet is slightly more efficient than by a horizontal jet. Destratification is measured by a stability parameter. More than 80% mixing is achieved when 20% or more of the water volume between withdrawal and reinjection points is recycled. Low‐momentum jets are found more efficient than high‐momentum jets. Efficiency was measured in terms of energy demand relative to change in potential energy. Basins of rectangular or parabolic shape are investigated.

A design aid is presented to size detention basins and outlet facilities for removal of particulate pollutants from storm runoff. The single‐design‐event approach is employed. The basins that are sized using the proposed method detain the runoff for a specified period of time to achieve the desired level of treatment. At the same time, the basins empty within a specified time period before the next design runoff event occurs. The method is based on the reservoir‐routing technique, which couples the hydrologic storage equation with an expression describing the hydraulics of the outlet structure. Orifice‐ and weir‐type outlet structures are considered. The equations are written in dimensionless form and solved using a finite‐difference computer model. The numerical results obtained in terms of the governing dimensionless parameters represent predetermined solutions to the reservoir‐routing problem. The results are generalized based on the concept of hydrologic similarity, and presented in chart form. As illustrated through a sample application, these pond charts are useful in determining the stage‐storage relationship for the basin, the size of the outlet structure, and the required storage volume.

A dynamic model that describes gas transfer in the oxygen‐activated sludge process was used to compare four gas phase control strategies under dry and wet weather flow conditions. Typical influent flows and oxygen uptake rates for the treatment plant at Hot Springs, Arkansas, were used for simulation purposes, The conventional control scheme was shown to be sensitive to sudden changes in the reactor liquid level during wet weather flows, and it was unable to prevent dissolved oxygen (DO) depletion when operating with relatively low levels of oxygen in the exit gas. The vacuum exhaust control (VEC) strategy, which utilizes an exhaust apparatus to control the effluent mixed liquor DO, was found to provide more stable oxygen feed and effluent DO during wet weather flows. Two other control strategies, which use measurements of the effluent mixed liquor DO to control the oxygen feed, were found to have results similar to the VEC strategy, but they generally had larger pressure variations. For both dry and wet weather flows, the alternative control strategies required approximately 10% less oxygen feed than the conventional control scheme.

Annual mass load estimators are evaluated for application to a range of tributary types including highly responsive systems. Evaluations are performed by conducting retrospective studies with comprehensive sets of field data for total phosphorus concentration and discharge. Four estimators from three broad categories are selected for study based on their performance in previous research. Extimators are evaluated using Monte Carlo sampling studies in which load estimates made from random subsamples are compared to true loads calculated from a complete loading record. The performance of the estimators differs strongly among the tributary test cases. Hydrograph characteristics and/or concentration/discharge relationships appear to play a strong role in the precision of all of the estimators and in the bias of averaging and regression estimators. Beale's ratio estimator is the only estimator to provide unbiased estimates for both stable and responsive systems, although stratification is necessary under event sampling. Consistent accuracy of the ratio estimator is valuable because it will allow the design of future sampling programs and accurate estimation of confidence intervals from collected sample data.

A mathematical mode is presented to determine the optimal operation of consecutive landfills while incorporating recycling programs. The model is capable of handling currently available landfills as well as numerous proposed future landfills. Model sensitivity analysis is performed by varying the capacities of present and future landfills, in addition to conventional objective function sensitivity analysis and right‐hand‐side sensitivity. In addition, the model incorporates the possible utilization of recycling programs. The model is driven by overall long‐term costs of operation. The optimization model indicates landfill use characteristics throughout the planning horizon, including the times at which landfills are closed and opened and the amount of waste that is diverted to recycling. Model solutions also indicates that recycling options may be implemented even though recycling costs exceed the cost of the current disposal option, in order to extend the lifespan of less expensive landfills and to defer more expensive future landfills.

Chlorination of water containing organic compounds results in generation of undesired chlorinated hydrocarbons. Enhanced removal of precursors present in raw water is one of the viable options for effectively reducing the formation potential of undesired reaction products. Total organic carbon (TOC) and turbidity removal by Fe (III) coagulation under low pH, and coagulation under high pH with Fe (III) combined with lime softening were investigated for a lake water. The results of jar tests showed that low pH coagulation removed 42% TOC and 96% turbidity at an optimum pH of 6.3 when 20 mg/L Fe (III) was used as a coagulant. High pH coagulation with lime softening removed about 81% TOC and 97% turbidity at pH 10.3 when 9.5 mg/L Fe (III) was used as a primary coagulant.

Removal efficiency from column settling tests for type II or flocculent sedimentation for a given time is evaluated using mathematical relationships. Two types of equations are in use and both give the same numerical results. However, there has been no proof of the equivalence of these mathematical relationships. Proof is presented that establishes this equivalence.