Journal of Environmental Engineering

A model has been developed to describe dynamic leaching of metal contaminants from solidified wastes using data for calibration that are taken only from batch tests. The model describes the three major factors affecting leaching: (1) acid/base reactions that determine the pH within the waste; (2) pH-dependent reactions that determine whether the contaminants are in mobile or immobile forms; and (3) diffusion that transports mobile contaminants from the waste. Acid/base reactions and mobilization/immobilization reactions of contaminants are described by empirical equations calibrated from batch equilibrium tests. Precipitation or sorption reactions are used to describe partitioning of contaminants among mobile and immobile forms. Fickian diffusion is used to describe transport of soluble compounds. Model predictions for observed diffusivities of copper, chromium, lead, and zinc agreed well with the experimental data obtained from dynamic leaching tests. Little difference was observed in model predictions when contaminant immobilization was assumed to be caused by precipitation or by sorption. Model simulations indicate that the assumption of an infinite bath may not apply to dynamic leach tests when contaminants are strongly immobilized.

Soluble heavy metals present in water could be deleterious to health, and as a result, their discharge into surface waters has been regulated internationally. Many processes for the removal of heavy metals from water and wastewater have been investigated. Coagulation and precipitation are the processes that have been reported to be most effective in the removal of heavy metals. In this study, seawater liquid bittern (LB), as an inexpensive source of magnesium, added to wastewater alkalized with lime or caustic soda is investigated as a possible coagulant. The experiments covered tests on eight metals: arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc. The lime-LB process culminated in high removals (>90%) for cadmium, chromium, lead, mercury, and zinc and reasonably good removals (71, 82, and 75%) for arsenic, copper, and nickel, respectively. These results were superior to those obtained using the caustic-soda–LB process. The concurrent presence of different metals in solution has been shown to have a minor effect on removal efficiencies for most metals. However, in the case of nickel, removal was appreciably increased by 18.5%. Also, higher concentrations of a single metal showed higher removal efficiencies.

Oxygenated compounds such as ethanol and MTBE are added to gasoline in many countries to increase the octane rating and to reduce the carbon monoxide and ozone levels in the air. In Brazil, due to economic reasons, gasoline has had up to 26% ethanol added. The leakage or spillage of such gasolines, referred to as oxygenated gasolines, could induce a cosolvent flushing effect in the contaminated soil as a result of the high solubility of oxygenated compounds in water. This paper focuses on the experimental measurements of the enhanced concentration and the dissolution of 12 organic compounds with increasing percentages of ethanol in the gasoline phase. The validity of Raoult's law and the log-linear cosolvency model to estimate the aqueous concentration of the hydrocarbons from unamended and from oxygenated gasolines is verified. Column tests showed that ethanol is leached out of residual gasoline after a short time, leaving an ethanol-free residual phase.

In 1997, 6.04 × 10⁸ kg of hazardous air pollutants were emitted into the atmosphere from the United States. A bench-scale activated-carbon fiber-cloth adsorption, electrothermal desorption, and condensation system was designed, built, and operated to demonstrate its ability to capture and recover organic hazardous air pollutants from airstreams. The annular-cartridge configuration of the activated-carbon fiber cloth allows low pressure drop and rapid electrothermal regeneration. Adsorption and electrothermal desorption cycling of the system with an airstream containing 1,000 ppmv methyl ethyl ketone (MEK) resulted in closure within 8% by mass. Competitive adsorption between water vapor and MEK reduced MEK adsorption capacity 43% as relative humidity of the gas increased from 4 to 90% at 22°C dry-bulb temperature. Increasing the dry-bulb temperature of the gas stream by 10°C at a constant dew-point temperature resulted in a 37% increase in MEK adsorption compared to the high relative humidity test case, while allowing the water vapor to penetrate the adsorber. The automated system achieves MEK capture efficiency >99.9% by mass while recovering the liquid MEK for reuse.

Vapor phase bioreactors are receiving increasing attention as a cost-effective treatment method for air contaminated with volatile organic compounds (VOCs). In this study, a novel absorption and humidification system was evaluated for its ability to dampen transient VOC loads, and to reduce their detrimental effects on a downstream bioreactor. A model based on the mass transfer characteristics of two target compounds (acetone and toluene) was developed and takes into account a closed water recirculation loop that minimizes fugitive emissions and simultaneously humidifies the inlet gas stream. When water is used as the scrubbing liquid, model and experimental results indicate that the system effectively dampens hydrophilic compounds and segregates them from the hydrophobic compounds in the waste gas stream. The response of a vapor phase bioreactor to the pretreated stream has also been assessed, and results indicate that the load dampening system works effectively for hydrophilic, but not hydrophobic, VOCs. However, when an organic cosolvent is used in conjunction with water, hydrophobic VOCs can also be dampened efficiently.

The measurement of suspended solid concentration profiles in dry weather wastewater of the “Le Marais” combined sewer network, Paris, has shown an increase in concentration in the lower part of the sewer's flow (1–3 g/L). To obtain a precise description of the composition of the water-sediment interface, an observation system was set up, which showed that an immobile layer composed of fibers and organic matter formed at the bottom of trunks over the coarse mineral deposits. The concentration profiles obtained result from the removal of this layer by aspiration. Samples were taken from this organic layer using a specially adapted system. The accumulation rate of the organic layer is 215 g/(m²⋅day). The particles of this organic layer are heavily loaded with pollutants (volatile solids to suspended solids = 66–75%). This layer is located at points where the shear stress is <0.1 N/m². Flush experiments have shown that it can be eroded by small rainfall events. When these results are extrapolated over the whole catchment area by mapping the organic layer, they show that the organic layer is stored in the same quantities as the mass eroded during a rainfall event in the sewer system and it can therefore contribute to pollution in urban storm-water discharge.

In this research, three anaerobic process configurations—namely, two-phase dual sludge (TPDS), two-stage single sludge (TSSS), and single-stage—were evaluated for effluent COD concentration. The same temperature, SRT, and glucose substrate were used in all experiments. In every case, TPDS and TSSS configurations significantly outperformed the single stage. All experiments were carried out at a temperature of 35°C, and all reactors were operated as daily fill-and-draw with HRT = SRT. The following ranges for each design parameter were studied: pH (4.5, 5.5, and 6.5); first stage HRT (3, 8, and 24 h); and floc load (3, 9, and 27 gCOD/gVSS). The overall HRT/SRT of all systems was 30 days, and a pH-Stat system was used to control the pH in the acidification reactors at the desired value. Statistical evaluation of the results indicated that a floc load of 3 in the first reactor of TPDS yielded the lowest effluent COD concentrations under the studied range of parameters, while for the TSSS reactor configuration the staging of the system itself was the controlling phenomena responsible for reduced effluent COD.

Settling characteristics of mixed liquor from seven advanced wastewater treatment (AWT) and two conventional wastewater treatment facilities in the Tampa Bay Metropolitan Area were evaluated. An algorithm was developed that relates zone settling velocity (V_s) of the mixed liquor to initial mixed-liquor suspended solids (MLSS) concentration and unstirred sludge volume index (uSVI) of the mixed liquor. Two-tailed, paired comparison, Student's t-tests indicated there was no significant difference between the sludge volume index (SVI) as measured in a 1 L unstirred graduated cylinder and that measured in a stirred, 28 L Plexiglas column. A secondary clarifier operating diagram is presented that can be used by engineers and operators in the design and operation of secondary clarifiers. Bye and Dold's model could be used to predict the SVIs of the mixed liquors evaluated in this study.

A steady-state laboratory-scale sequencing batch reactor process for biological phosphorus removal (BPR) was developed, and the influence of wastewater biodegradability on BPR was studied in batch tests. Biodegradability was expressed in this work as the readily biodegradable fraction of wastewater COD (rbCOD) present in the mixed liquor after the anaerobic stage of the anaerobic/oxic cycle of the BPR process. The rbCOD fraction was changed by varying the organic composition of synthetic wastewater (different carbohydrates were used: saccharose, cellobiose, starch, and cellulose) or varying the anaerobic retention time (1.25, 4, 9, and 24 h) when only one kind of low biodegradable synthetic wastewater (starch composed) was used. A clear positive trend was observed between rbCOD and anaerobic P release, but such a clear relationship was not observed with BOD₅ measurements. Soluble carbohydrates allowed a BPR mechanism, but particulate carbohydrates seemed to cause nonbiological P removal. An increase in anaerobic retention time improved rbCOD concentrations up to 50%, approximately, and P removal, but excessive retention times, >9 h, should be used to reach good BPR results.

A nitrification process under low dissolved oxygen (DO) concentration is proposed in a completely stirred biofilm reactor. The reactor was fed with a synthetic wastewater containing 250 mg NH₄–N/L. A stable nitrite accumulation in the effluent was obtained during >110 days' operation; NO₂–N:(NO₂–N + NO₃–N) in the effluent reached >90% under 0.5 mg DO/L. Ammonium was completely converted and NH₄–N in the outlet was as low as 5 mg/L. A transient increase of the DO concentration in the reactor induced a complete conversion of ammonia and nitrite to nitrate after only 2 days. A return to a low DO concentration again induced nitrite accumulation. These results show that the nitrite oxidizers were always present in the reactor but were outcompeted at low DO concentration, due to their lower affinity for oxygen, compared with ammonia oxidizers. Nitrite accumulation could also be favored by free nitrous acid accumulation inside the biofilm.

The efficiency of a sequencing batch reactor in denitrification of drinking water with relatively high nitrate concentrations (40–250 mg/L as N) was evaluated. Ethanol at a COD/N of 2.00 was found sufficient to reduce nitrate concentrations to acceptable levels (<10 mg/L as N). Within the first 6 min of reaction, nitrite accumulation in the range of 0.03–3.5 mg/L as N was observed increasing with the increase of initial nitrate concentrations. In the first hour, nitrate removal was significantly high in the range of 85.7–91.5%. Anoxic reaction times of 3, 5, and 7 h were required for nitrate concentrations of 40–160, 200, and 250 mg/L (as N) to achieve acceptable levels of nitrate and nitrite. Alkalinity of the denitrified water increased on average by 3.53 mg as CaCO₃ for each milligram of nitrate reduced and pH increased from 7.3 to the range of 8 to 9. Idle times between the operation cycles, in the range of 1–14 h, had an insignificant effect on denitrification. Residual COD concentrations in the range of 5–15 mg/L and sulfide concentrations (at initial nitrate concentrations ≥120 mg/L as N) in the range of 0.2–0.4 mg/L were recorded in the finished water. Elevated concentrations of COD in general are not advisable in drinking water, and specifically in this case, it could result in toxic sulfide formation in the treated water. There is a need to further study the optimization of the use of ethanol and polishing of the treated water. A sequencing batch reactor has the potential of being used as an alternative configuration for biological denitrification of drinking water.