Journal of Environmental Engineering

The problem of designing a wastewater storage pond at a land treatment site is similar in many respects to sizing a water supply reservoir subject to variable inflows and outflows. The similarities and differences of the essential features involved in determining the capacity required for a wastewater storage pond and a water supply reservoir are compared and contrasted. In the companion paper, this analogy is exploited by showing how methods developed in applied stochastic reservoir theory can be extended to solve the wastewater storage problem at a land treatment operation.

A new reliability‐based method is presented to size a storage pond at a land treatment site operated for the disposal of municipal wastewater. Reliability is defined as the probability that wastewater levels remain below the pond freeboard zone. Starting with the mass conservation principle and the theorem of total probability, an integral equation representing the cumulative distribution function of wastewater storage levels is developed. This equation is solved with Monte Carlo simulation, a technique often used in stochastic reservoir analysis. Results are presented on dimensionless design charts called equilibrium storage performance functions which show the steady‐state cumulative distribution function of wastewater levels in a storage facility of specified capacity. An example illustrates how the equilibrium storage performance function can be used to design a wastewater storage pond that meets a specified annual reliability.

One of the enironmental concerns associated with on‐site wastewater disposal is nitrate contamination of groundwater. The purpose of this study was to design and monitor an experimental on‐site watewater disposal system with nitrogen‐reducing capability. Incorporated into the system are two fixed‐film reactors: a nitrification/denitrification (N/D) tank, and a polishing filter. Recirculated denitrified effluent sprayed onto the sand filter portion of the N/D tank is nitrified and the nitrified effluent is then mixed with septic tank effluent in the submerged gravel section for denitrification. The remaining portion of the denitrified effluent enters a polishing filter before final discharge into groundwater. With a recirculation ratio of 3.3, the overall reduction in total nitrogen through the N/D tank was approximately 70%. The N/D component of the system also achieved a reduction in BOD, COD, and suspended solids of 87, 80, and 84%, respectively. With the polishing filter, further enhanced removal of these pollutants was achieved, including a 5‐log reduction of fecal coliform. Using the N/D tank as a pretreatment unit for a soil adsorption system may not only reduce the levels of total nitrogen entering the system but may also extend the life of the system.

A strip‐pit pond formed as a result of past coal mining activities in northeastern Ohio is contaminated with alkylbenzenes and chlorobenzene from tanktruck washing operations. Batch aerobic studies are performed on samples from a strip‐pit pond containing about 100 μg/L each of chlorobenzene, toluene, ethylbenzene, xylenes, styrene, and aniline in order to determine the potential for their biotransformation under field conditions. Microorganisms capable of biotransforming the aromatic compounds are present in the strip‐pit pond water. However, under natural conditions, the biotransformation rate is slow as incomplete removal is observed over a 43‐day incubation period. Volatilization rates exceed the natural rates of biotransformation. Addition of acetate, nitrogen, and phosphorus markedly increase the rate of biotransformation of the aromatic compounds. Enhanced biotransformation rates are more than an order of magnitude faster than volatilization rates. Chlorobenzene is mineralized to carbon dioxide. The presence of 0.61 g/L sediment does not significantly affect biotransformation of the aromatic compounds at a 95% level of confidence. The prospect for successfully stimulating and exploiting biotransformation of the contaminants in a bioreclamation scheme appears very good.

The natural cation in Wyoming bentonite is exchanged with Tetramethylammonium $\left({\mathrm{TMA}}^{+}\right)$ ions. The adsorptive properties of TMA‐bentonite increase in direct proportion to the exchange reaction below 60% of the CEC of the clay. The ability of the modified clay to remove benzene decreases at doses higher than 150% of the CEC of the clay. Adsorption isotherm constants on TMA‐bentonite are calculated from equilibrium studies for benzene, toluene, and o‐xylene. The selectivity of the TMA‐tailored bentonite follows the following order: benzene » toluene > o‐xylene. Solute equilibrium concentration is attained within fiveminute contact time. Solution pH plays a secondary role on the adsorptive nature of TMA‐bentonite.

Separation of volatile organic contaminants from water using a microporous hollow‐fiber membrane/oil stripping system is evaluated. Countercurrent flow of water and oil on opposite sides of an air‐filled gas‐permeable membrane is used to concentrate the contaminants in the oil phase. The rate of mass transfer under varying conditions is measured and compared to existing full‐ and laboratory‐scale air‐stripping processes, to membrane‐extraction processes, and to predictions based on known empirical and theoretical correlations. The correlations can be used as the basis for system design and scale‐up. This process incorporates the advantages of other membrane‐stripping processes, such as a high surface area to volume ratio for transfer, and independent control of the separate liquid phases. It also overcomes several disadvantages of extraction and stripping processes. There is no contact between the liquid phases and thus no potential for cross‐contamination or emulsion formation. The membrane separation of phases prevents flow channeling and flooding present in air‐stripping towers. Finally, materials used in this process may be recovered and recycled, preventing further environmental contamination.

Experimental data show that the breakthrough curves observed in filtration in the case of both surface‐force‐dominated deposition and flocculation‐dominated deposition display a general similarity. Some theoretical observations on these extreme cases of deposition behavior are presented. The analyses presented focus on the prediction of the breakthrough curves that could be expected in each case. Solutions for breakthrough curves are given, and their relationship to the deposition mechanisms and the corresponding boundary conditions is examined. The models provide a general approach for surface‐force‐dominated deposition, contact‐flocculation filtration, direct filtration, and related phenomena. The parameters included in the formulation represent both the gross accumulation (deposition) mechanisms and the mechanisms or forces that oppose deposition. The models are applicable to various types of waters, flocculants, filter beds, and soils.

A new unit operation called a sludge freezing bed is proposed for dewatering sludges produced by treatment facilities in cold regions. This unit operation uses natural freeze‐thaw to condition the sludge for dewatering. It can dewater all types of aqueous sludges up to a depth of 2.0 m. Basic construction details are identified, and procedures for operating the bed are discussed. Equations for predicting the design depth are presented along with an example of how they can be used. Convection was found to be the controlling heat transfer mechanism during freezing. Solar radiation, ambient air temperature, and the thermal conductivity of the settled sludge layer over the frozen sludge are important parameters controlling the thawing rate. Data from various sludge freezing operations indicate that the design equations are valid.

Impoundments or natural lakes having short hydraulic detention times may become eutrophic through anthropogenic nutrient loading episodes. Subsequent lake recovery may be influenced by nutrient transport from the lake bottom to the water column. Order‐of‐magnitude analysis is used to derive simple mathematical models of eutrophic lake recovery. Total phosphorus concentration (P) is used as an indicator of lake water quality. Sedimentation of particulate P from the water column and cycling from lake sediments control the rate of change of lake water P in response to a step change in external P loading. The rate of P desorption from the solid phase is more important than film transport rates across the lake bottom/water column boundary. Comparisons between exact and approximate longterm predictions of lake water total phosphorus concentration generally differ by less than 0.2%. The approximate models are convenient because they are more compact in form and appear to behave similarly to an exact analytical solution to the lake model.

Hydrologic considerations, such as the water storage capacity of peat, are critical in properly designing peatlands for wastewater treatment. Water storage capacity is estimated by measuring the specific yield of peat in a north Michigan cedar swamp. Field estimates of specific yield are performed by measuring the response of water table to rainfall. Specific yield, taken as the ratio of rainfall to water table rise, is found to decrease exponentially with depth, ranging between 0.4–0.7 for the surface layers and between 0.05–0.15 for peat layers at 0.5 m depth. A model predicting the water table response to rainfall is developed and successfully tested. Applications of the model in designing wetland wastewater treatment systems include computation of maximum wastewater quantity that can be sprinkled on a wetland before flooding and overland flow. Such information is critical in systems where optimum treatment is to be achieved when wastewater is absorbed into the peat matrix.

In situ microcosms are often effective systems for performing trophic studies in aquatic environments. In this paper, the behavior of replicate control microcosms in three experiments is compared to each other and to the behavior of the reservoir in which the experiments were conducted. The results indicate that the microcosms accurately and consistently simulated the vertical profiles of temperature in the reservoir. The microcosms accurately simulated the reservoir concentrations of the nutrients nitrogen and phosphorus, chlorophyll a, and light penetration in two experiments, but not in the third experiment. In all three experiments, the microcosms were unable to simulate accurately the reservoir concentrations of dissolved oxygen. Finally, the replicate microcosms did perform as replicates.

Improperly functioning soil absorption systems are often cited as sources of groundwater contamination. Clogging of the infiltration surfaces and uneven distribution of wastewater along the length of the trench are believed to be a primary reason for poor treatment. A new soil absorption system design, the capillary seepage trench (CST), is similar to conventional systems except that an impermeable liner is placed at the bottom of the trench. The liner prevents direct percolation and causes the wastewater to move upwardly and horizontally by capillary action before percolating downwards. Preliminary experiments comparing the CST to conventional soil absorption system designs were conducted. Removal of organic matter in the CST was found to be significantly better than in the conventional system. Possible reasons for superior treatment of the CST compared with the conventional system include: increased residence time in the aerobic soil zone and a more even distribution of wastewater along the trench.