Biosolids-amended soils: Part I. Effect of biosolids application on soil quality and ecotoxicity.

Investigations of potential risk from biosolids generally indicate that land application does not threaten human or ecosystem health, but questions continue to arise concerning the environmental effects of this practice. This research project was initiated to evaluate ecotoxicity resulting from the amendment of soils with biosolids from municipal wastewater treatment plants. Toxicity was evaluated using standard tests, including earthworm mortality, growth, and reproduction; seedling germination and root elongation; microbial respiration; and nematode mortality and reproduction. Nineteen municipal wastewater treatment plants were identified to participate in an initial screening of toxicity, and five were chosen for a more detailed evaluation. In addition, two soils with historically high applications of high-metal biosolids were evaluated. Contaminants examined were zinc, copper, nickel, chromium, arsenic, cadmium, lead, and coplanar polychlorinated biphenyls (PCBs). Single applications had no effect on soil metal concentrations. Coplanar PCBs were not detectable in any of the soils or biosolids. All target organisms were sensitive to reference toxicants. Limited toxicity was observed in a small number of the amended soils, but no patterns emerged. Approximately one-half of the negative effects of biosolids on bioindicators could be attributed to routine properties, such as slight depression of pH and/or elevated salinity. None of the accumulated metal concentrations were excessive, and most would not be considered elevated. These observations suggest that current regulations for application of biosolids to soils are providing adequate ecosystem protection.

Kinetics of anaerobic degradation of glycol-based type I aircraft deicing fluids.

The kinetics of anaerobic degradation of glycol-based Type I aircraft deicing fluids (ADFs) were characterized using suspended-growth fill-and-draw reactors. Both Type I ADFs tested showed near-complete anaerobic degradability. First-order degradation rate constants of 3.5 d(-1) for the propylene glycol-based Type I ADF and 5.2 d(-1) for the ethylene glycol-based Type I ADF were obtained through continuous-culture means under mesophilic conditions (35 degrees C). Fill-and-draw operation at lower temperatures affected anaerobic degradability only minimally down to 25 degrees C but substantially below 25 degrees C. High Type I ADF feed concentrations substantially affected degradability. Batch testing of fill-and-draw reactors resulted in first-order degradation rate constants of 1.9 d(-1) for propylene glycol-based Type I ADF and 3.5 d(-1) for ethylene glycol-based Type I ADF.

Best management practices for airport deicing stormwater.

With the advent of new regulations concerning aircraft deicing and management of spent aircraft deicing fluids (ADFs), many airports now face the dual challenges of simultaneously maintaining public safety and protecting the environment. This paper provides a theoretical assessment of the potential environmental impact of stormwater runoff and offers detailed current information on alternative deicing fluid application methods and materials, collection and treatment practices.

Characterization of compost biofiltration media.

The effect of compost quality on performance of biofiltration media was investigated. Three types of compost media were characterized in terms of their key properties, including pH, total organic carbon (TOC) and nitrogen, moisture content, oxygen uptake rates and heterotrophic and fungal plate counts. Laboratory-scale biofilter columns were each filled with a different compost medium mixed with buffer and a bulking agent. The columns were operated with a volatile organic compound (VOC) mixture consisting of ethyl alcohol, butyraldehyde, ethyl acetate, and 1,1-diethoxybutane and subjected to various operating conditions. After 13 months, the columns were sampled and post-characterization of the media was performed. A mixture of biosolids/horse manure compost, which had a high initial microbial activity, had the shortest acclimation period. Best overall performance, however, was observed with yard waste compost, which was the most stable medium. Media hydrophobicity was a key factor in long-term performance. Significant changes in compost structure, moisture content, pH, volatile solids, oxygen uptake rates, and microbial population densities were observed over the course of the study. Analyses of organic nitrogen content showed that significant amounts of nitrogen were not depleted from the media.

Alcohol and acid formation during the anaerobic decomposition of propylene glycol under methanogenic conditions.

Intermediates formed during the anaerobic decomposition of propylene glycol under methanogenic conditions were studied using a serum bottle technique. The pathway is similar to the anaerobic decomposition of ethylene glycol as previously reported. For both compounds, the decomposition is believed to proceed via an initial disproportionation of the glycol to form equal molar amounts of the volatile fatty acid and normal alcohol of the same chain length. In the case of ethylene glycol, disproportionation results in the formation of acetate and ethanol, while disproportionation of propylene glycol produces propionate and n-propanol. Following disproportionation, the alcohols produced from glycol fermentation are oxidized to their corresponding volatile fatty acid with the reduction of protons to form hydrogen. Ethanol and propionate oxidation to acetate proceeds via a well-established syntrophic pathway that is favorable only under low hydrogen partial pressures. Subsequent degradation of acetate proceeds via acetoclastic methanogenesis with the production of carbon dioxide and methane. Despite the production of hydrogen in the initial steps of glycol degradation, both compounds are completely degradable under the methanogenic conditions tested in this study.

Influence of mass transfer limitations on determination of the half saturation constant for hydrogen uptake in a mixed-culture CH(4)-producing enrichment.

There is strong evidence in the literature supporting the existence of significant mass transfer limitations on the kinetics of exogenous H(2) consumption by methanogens. The half saturation constant for H (2) uptake by a mixed-culture, CH(4) producing enrichment was measured using an experimental protocol that avoided internal mass transfer limitations. The value obtained was two orders of magnitude smaller than any other previously reported. A mathematical model for acetogenic syntrophic associations was developed to check the capacity of H(2) as electron transporter between syntrophic partners. It was found that H(2) diffusion could account for the rate of transport of electrons between the syntrophic microorganisms and that formate is not a necessary intermediate. The possibility that formate may be an intermediate in this system was not ruled out. A Monod-type kinetic equation was modified to include the observed H(2) threshold effect. This modified equation was used to predict the CH(4)-production rate in a batch-fed digester. The results show that the external and internal H(2) pools are kinetically coupled.

Comparison of diffusion and reaction rates in anaerobic microbial aggregates.

The ability of hydrogen diffusion to account for the rates of methane production in microbial aggregates was studied in a defined coculture consisting of a sulfate reducer grown as a syntrophic hydrogen producer in the absence of sulfate and a methanogen. The hydrogen uptake kinetics of the methanogen were determined using the infinite dilution technique. The maximum hydrogen uptake velocity was 7.1 nmol/min/µg protein and the half saturation constant for hydrogen uptake was 386 nmol/liter. A threshold of 28 nmol/liter below which no further hydrogen consumption occurred was observed. The reconstituted co-culture was shown to produce methane at rates similar to mixed culture enrichments grown on lactate. The diffusion model demonstrated that for the particular system studied, the rates of hydrogen diffusion could account for the overall rate of methane production.