| Abstract | To study the toxicity and removal of toxic organic substances in a model land treatment
process, the experiments involved a lab-scale soil column treatment of naphthalene and phenol
and a pilot-scale land treatment of a petroleum refinery wastewater were conducted.
The lab-scale study was divided into 2 phases, as follows. Phase I was conducted to
investigate the toxic effects of naphthalene and phenol on the soil microbes in utilizing of a
synthetic wastewater (200 mgCOD/L of glucose), and to study the fate of naphthalene and phenol
in the soil column system. Three lab-scale soil columns, filled with a sandy loam soil, were
operated at the liquid loading of 5 cm/wk. It was found that the soil columns could effectively
treat the synthetic wastewater containing either 1-10 mg/L of naphthalene or 10-100 mg/L of
phenol. The added naphthalene, at the concentration of2 mg/L, had some toxic effects on the soil
microbes in utilizing the glucose, but these toxic effects could be reduced by step-increases of
the naphthalene dosages. The added phenol had no toxic effects on the soil microbes and was
utilized by the soil microbes as carbon source. Volatilization and adsorption were found to be the
dominant mechanisms in the removal of naphthalene, while biodegradation was the dominant
mechanism in the removal of phenol. According to the MicrotoxR toxicity tests, naphthalene was
more toxic than phenol. At the same level of toxic effects (EC50 = 13%-26%) on the microbes
in the MicrotoxR reagent, the initial exposure concentration of naphthalene was about 5.6 mg/L,
while the initial exposure concentration of phenol was 100 mg/L. Phase II was conducted to
investigate the performance of the soil columns in treating a synthetic phenol wastewater and its
removal kinetics. Two lab-scale soil columns, filled with the sandy loam soil, were employed in
the experiments. The influent phenol concentrations were varied at 200, 300, 500, and 800 mg/L,
while the liquid loading rates were varied at 5 and 10 cm/wk. Beyond the influent phenol
concentration of about 200 mg/L, the soil column at the liquid loading rate of 5 cm/wk gave
higher removal efficiencies than at the liquid loading rate of 10 cm/wk. The data of the liquid
loading rate of 5 cm/wk gave negative values of the biokinetic constants (Ki> Ks, µm) in the
Haldane equation. At the liquid loading rate of 10 cm/wk, the biokinetic constants (Ki> Ks, µm)
involved in the Haldane equation were evaluated to be 163.93 mg/L, 29.09 mg/L, and 3.52 day1
,
respectively. The influent phenol concentration up to about 200 mg/L exhibited no toxic effects
to the soil microbes in the soil column. The higher the influent phenol concentrations above 200
mg/L, the more the toxic effects on the soil microbes or the reduction of the removal efficiencies
by the soil column.
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The pilot-scale study was conducted to investigate performance of the land treatment
system in treating a petroleum refinery wastewater whose COD, phenol and oil contents were
250, 0.8-3.1 and 1.4-9.5 mg/L, respectively. Three pilot-scale land treatment plots, filled with
the sandy loam soil, were operated at the liquid loading rates of 15, 10, and 5 cm/wk. The
average TCOD removal efficiencies of 61 %, 66% and 72% were, respectively, observed .
Similarly, the average SCOD removal efficiencies of 40%, 49% and 59% and the average
suspended solid removal efficiencies of 81 %, 83% and 85% were obtained, respectively.
All the land treatment plots effectively removed the phenol content to be lower than
detectable level(< 0.5 mg/L); in addition there was disappearance of peaks of unknown organic
compounds when observed by gas chromatography. The oil content was also effectively removed
to be less than 0.1 mg/L. Detoxification of the petroleum refinery wastewater by the land
treatment plots was investigated by means of the MicrotoxR toxicity tests. The very toxic influent,
with the EC50 in the range of 0.4%-9.9%, was detoxified to be the slightly toxic (EC50 > 75%;
90.7%-97.6%) or non toxic (EC50 > 100%) effluent. The highest reliability in detoxification of
the petroleum refinery wastewater was observed when the land treatment plot was operated at
the liquid loading rate of 5 cm/wk. |