Preliminary investigation of nutrient supplementation of, and heavy metal mobilization by, dual (phenol/activated sewage sludge) co-disposal with refuse.
Loading...
Date
1999
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Investigation of landfill co-disposal technology, with emphasis on nutrient
supplementation and heavy metal mobilization, was made. For the purpose of this
study, co-disposal is defined as the combined disposal of wastewaters and/or sludges
with refuse. It is, currently, the most cost-effective method of waste treatment and
disposal. To assess whether refuse could be characterized as nutrient limited and to
determine the effects of nutrient additions on the refuse solid-state methanogenic
fermentation, nutrient supplementations were made to refuse (control), co-disposal
(activated sewage sludge with refuse) and dual co-disposal (activated sewage sludge
plus phenol with refuse) microcosms. The results showed that the domestic refuse used
was not nutrient limited. For the controls, previously reported solid-state fermentation
patterns resulted. Self-generating redox gradients were established with concomitant
reductions in leachate Chemical Oxygen Demand and initiation of sulphate reduction.
Thus, hydrogen sulphide and methane were both evolved. In contrast, nutrient
supplementation, particularly with macronutrients and macronutrients plus trace
elements, effected fermentation imbalances such that protracted low pH values and
high volatile fatty acid concentrations were apparent. Redox gradient generation was
slowed which militated against sulphate reduction and the onset of methanogenesis. In
the absence of nutrient supplementation, low residual phenol concentrations
characterized the dual co-disposal microcosms whereas elevated concentrations
persisted in the equivalent nutrient supplemented microcosms.
To investigate the implications of heavy metal retention / mobility during landfill
co-disposal operation, microcosms were packed with "young" synthetic refuse and/or
activated sewage sludge at packing ratios of 4.1:1 (1) or 4.1:2 (2). The sludge was
"spiked" with each of four heavy metals, Cr(3+), Cu(2+), Ni(2+) and Zn(2+), to a concentration of
100 mg ⌠(1) (refuse/sludge ratio 1) or 200 mg ⌠(1) (ratio 2) while the control received the
same concentrations of metals dissolved in distilled water.
The heavy metal concentrations were increased progressively to 800 mg ⌠(1) (ratio 1)
and 1 600 mg ⌠(1) (ratio 2). For all the microcosms, including an unperturbated control,
unbalanced fermentations (acidogenesis > acidotrophy) resulted as evidenced by the
low pH values. Thus, heavy metal toxicity was not the sole cause. The leached metal
concentrations were in a consistent order with high Zn and Ni concentrations detected
compared with immobilized Cr and Cu. After 15 weeks of operation with the higher
applied loading, despite extensive retention, increases in Cr, Ni and Zn were detected
in the microcosm leachates. Due to the elevated redox potentials, precipitation of the
metals as insoluble sulphides was not operable. After 28 weeks of operation,
microcosm depth samples (15, 25 and 40 cm) were collected and analysed for
immobilized metals. Chromium was characterized by maximum retention at a depth of
15 cm. In contrast, nickel concentrations were comparable throughout the
refuse/sludge profile while no specific adsorption patterns emerged for Cu and Zn. The
implications of these findings in relation to co-disposal landfill site operation are
discussed.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1999.
Keywords
Refuse and refuse disposal., Sewage disposal., Sewage sludge., Sanitary microbiology., Sewage--Microbiology., Theses--Microbiology.