Journal of Environmental Engineering

A eutrophication model package was employed to examine the response of mainstem Chesapeake Bay to nitrogen and phosphorus load reductions. In addition to sensitivity analyses, the bay was examined under limit-of-technology nutrient controls and all-forest conditions. Scenarios were run in a time-variable mode for a minimum of 10 years. Decade-long simulations were required to achieve a near-complete response to load reductions. Analyses indicated that the primary effect of phosphorus controls was the limitation of algal biomass during the spring bloom and in the upper bay during summer. Nitrogen controls limited algal biomass primarily in the lower bay. Nitrogen controls were more effective than phosphorus controls in limiting algal production and in reducing anoxic volume. Limit-of-technology load reductions, under average hydrologic conditions, improved minimum summer-dissolved oxygen by ≈0.5 gm m ⁻³ and reduced anoxic volume by 40%. Anoxia was largely absent under all-forest, average hydrologic conditions but persisted during wet, highly stratified years.

A final-clarifier inlet design for improving the flow conditions and the settling efficiency is described. Numerical simulations and experiments are introduced in order to model and analyze the flow field as well as to evaluate the design criteria. It is demonstrated that inlet structures of primary and secondary clarifiers cannot be optimized by the same procedure. Although primary-clarifier inlet design is mainly concerned with dissipation of kinetic energy, secondary-clarifier inlet design requires the consideration of density effects on the flow field as well as flocculation properties of the activated sludge. The suggested design includes (1) an inlet expanding over the whole width of the tank, positioned at the bottom, and with a relatively low aperture height in order to reduce potential energy flux; (2) a series of angle bars beyond the inlet for dissipation of kinetic energy and decrease of velocity fluctuations; and (3) dimensioning the inlet volume as flocculation chamber. The introduced inlet design must be tested in the future in either full-scale or pilot-scale tanks with activated sludge.

In-situ methanotrophic bioremediation of ground water contaminated with petroleum hydrocarbons and chlorinated aliphatic hydrocarbons (CAHs) often require the addition of O ₂ for compound oxidation. The purpose of this research was to test the feasibility of a down-well gas-transfer apparatus for adding O ₂ directly into recirculating ground water. At times it may be desirable to add other gases as well, such as methane for CAH cometabolism. A high downward velocity of water through the inlet tube of an inverted cone prevented upward movement of injected O ₂ bubbles. The addition of gas-liquid mixing cones within the transfer section of the inverted cone increased the turbulence below the inlet. Complete dissolution of gases is possible. The relationships between O ₂ transfer rate and system variables such as pressure loss, water flow rate, and N ₂ content in the influent water were evaluated. A significant factor affecting O ₂ mass transfer was the total dissolved gas content in the water entering the transfer device. From the results of this study, a design for practical conditions of operation was developed.

Understanding of flow and entrapment of non-aqueous-phase liquids (NAPLs) in aquifers contaminated with organic chemicals is important in the effective design of recovery and remediation schemes. Soil heterogeneities play a significant role in the physical behavior of these chemicals. An experimental facility consisting of a large soil tank (lysimeter) and a dual-gamma spectroscopy system for fluid saturation measurements was developed to simulate and monitor plume migration in water-table aquifers after chemical spills. Experimental techniques and results from a preliminary set of experiments conducted in unsaturated and saturated soils under homogeneous and heterogeneous conditions are presented. The effects of the layered homogeneities were pronounced in modifying the migration pattern and velocity of the plume. Pockets of coarse sand placed across the path of the plume resulted in the soil acting as a light NAPL trap. A fine-sand pocket acted as a barrier. Qualitative and quantitative data generated in the type of experiments presented in this paper can be used to validate multiphase flow models.

In the summer of 1992, a proof-of-concept demonstration of direct electrical heating and air stripping was conducted for enhancing the removal of a volatile organic contaminant, trichloroethylene (TCE), from soil. Six electrodes were buried in shallow boreholes so that a target region 6.1 m in diameter and 3.05 m in height was heated by ohmic dissipation of power-line–frequency electrical currents supplied by a diesel generator. Air stripping of TCE contamination from the same region was accomplished from a single well at the center of the heated volume. The electrical energy used during the demonstration was 3.46 × 10 ¹⁰ J (9,600 kW$\dot{s}$h), and the temperature of the extracted air rose from 16°C to 38°C. An energy balance shows that the input energy is consistent with the temperature rise in the target volume and the amount of water vaporized at the electrodes. Prior to heating, the TCE concentration in the vapor decreased from about 80 parts per million by volume (ppm _V ) to around 60 ppm _V . As soon as electrical heating started, TCE concentrations began to increase. Some concentration data were lost shortly after electrical heating began. After the system was repaired, the TCE concentration fell rapidly from about 140 ppm _V to 5 ppm _V over a period of about 25 days. A simple two-dimensional model for calculation of heating rates is also presented and verified experimentally. Finally some of the operational and safety issues associated with electrical heating are discussed.

This paper describes studies of the effect of firing temperature on the photocatalytic and photoelectrocatalytic degradation of formic acid (HCOOH) using ultraviolet (UV) illuminated TiO ₂ thin films. Other variables, such as the potential applied [versus saturated calomel electrode (SCE)] to the film and the number of layers of film applied to the support were examined to determine optimum conditions for degradation. The transformation of TiO ₂ from anatase to rutile in the thin films was significantly retarded by placing the films on SnO ₂ -covered glass. Results indicate that films fired at 500°C showed the best efficiency for pure photocatalytic degradation, while films fired at 600°C had the best efficiency for the photoelectrocatalytic degradation of HCOOH. The efficiency of these photoelectrocatalytic reactions increased with increasing potential until 2 V (versus SCE) was obtained. A film with one layer (0.27 μm thickness) showed the maximum degradation of HCOOH for photoelectrocatalytic reactions at 2 V (versus SCE), as no further increase in the rate of degradation of HCOOH was obtained by using thicker films.

The simultaneous removal of SO ₂ and NO _x by calcium salts of carboxylic acids was evaluated in a laboratory-scale furnace in atmospheres containing 2,000 ppm SO ₂ , 1,000 ppm NO, 3% O ₂ , and 12% CO ₂ at gas temperatures between 550 and 1,250°C (825 and 1,425 K) with a residence times of 4 s in an isothermal zone. The salts studied included calcium formate (CF, Ca(COOH) ₂ ), calcium acetate (CA, Ca(CH ₂ COOH) ₂ ), calcium propionate CP, (Ca(CH ₂ CH ₂ COOH) ₂ ) and calcium benzoate (CB, Ca(C ₆ H ₄ COOH) ₂ ). Maximum reductions in SO ₂ concentration due to the formation of CaSO ₄ were recorded as follows: 60% for CP, 55% for CF, 35% for CA, and 18% for CB at gas temperatures in the neighborhood of 950°C and at a Ca/S molar ratio near 2.8. With the organic portion of the injected salts acting as secondary fuel for NO _x control, maximum NO _x concentration reductions were 88% for CP, 65% for CB, 22% for CA and less than 10% for CF at gas temperatures near 950°C at bulk equivalence ratios (actual to stoichiometric organic fuel-to-air ratio) that varied between 0.5 for CF and 2.1 for CB. A recently constructed, narrower furnace increased the gas flow velocities and the particle dispersion in the gas. Therein, at residence times as low as 1 s, CP achieved simultaneous SO ₂ and NO _x removal of over 80% at gas temperatures ≥950°C. The addition of an oxidizing zone following the carboxylic acid sorbent injection zone affected the overall SO ₂ -NO _x removal according to the stoichiometry of the sorbent injection zone. When this zone was: (1) fuel-lean, the overall SO ₂ removal improved; (2) stoichiometric, the overall SO ₂ removal was not affected; and (3) significantly fuel-rich, the overall SO ₂ removal worsened. Under most conditions, the overall reduction of NO _x was not affected by the oxidizing zone. At temperatures below 800°C, Ca either was retained as CaCO ₃ or remained bound to the organic compounds in the sorbents. At temperatures between 950 and 1,150°C, increasing amounts of CaO were found. The composition of the calcined sorbent and the availability of Ca was used to interpret the results of a cenosphere sulfation model. The sorbent sulfation kinetics were found to be bounded by those of pure CaO and pure CaCO ₃ .