Preliminary investigation of nutrient supplementation of, and heavy metal mobilization by, dual (phenol/activated sewage sludge) co-disposal with refuse.

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.