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The mobility of heavy metals in clay amended sewage sludge and municipal solid waste compost

Qiao, Liang (1996) The mobility of heavy metals in clay amended sewage sludge and municipal solid waste compost. PhD thesis, Murdoch University.

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Currently 250,000 dry tonnes of sewage sludge is produced annually in Australia. As the amount of sludge produced is expected to increase greatly, the current sludge treatment, disposal and utilisation practices will have increasing environmental implications. Sewage sludge contains significant organic matter and nutrients that can serve as soil conditioners or fe1tilisers. When sewage sludge is used as a soil conditioner, heavy metal and pathogen contamination can limit its application rates. In a preceding study bauxite refining residue (red mud) amendment was used to control the mobility of heavy metals and pathogen in MSW composting. To meet the requirement of a soil conditioner, sludge composting with an amendment of bauxite refining residue (red mud) was studied in this thesis.

Thorough composting can meet the requirement for controlling pathogens by subjecting the wastes to thermophilic temperatures during the process. Digested sludge composting usually contains a higher moisture content and less available carbon, making it more difficult to reach the thermophilic stage of composting with anaerobically digested sludge alone. Two methods were tried to overcome these problems; reducing moisture content by drying the sludge and supplying available carbon by amending with readily biodegradable bulking agents. It was confirmed that the content of available carbon in digested sewage sludge was a key factor for the optimisation of composting in a forced aeration system.

In order to investigate the long term effect of red mud amendment on the mobility of heavy metals in sludge composting, the speciation of heavy metals was studied. A sequential step extraction was employed to investigate metal speciation into exchangeable. carbonate bound, Mn & Fe oxides bound, organic matter bound and residual phases for the following heavy metals: cadmium, chromium, copper, nickel, lead and zinc. The basis of this investigation is that potential hazards of heavy metals ae dependent on the physio-chemical forms of metals in sludge and soil.

To reduce the concentrations and the mobility of heavy metals in sewage sludge compost, red mud - gypsum was mixed with the sludge prior to the sludge being composted. The red mud compost had the following improvements over conventional sludge compost: lower metal content and mobility; lower moisture content; much higher pH buffer capacity; was subjected to higher temperature and moisture removal; increased nutrient and water retention capacity and a greater stability of the organic fractions.

It was found that heavy metals are present in very stable forms in sludge compost relative to MSW compost. The composting process slightly increased the mobility of Cu and Zn in sludge compost, but significantly decreased the mobility of Pb.

The effect of red mud addition on metal distribution in sludge compost was particularly evident. Twenty per cent red mud addition reduced the amount of heavy metal in the exchangeable fraction by 100% for Ni, 87% for Pb, 81 % for Zn, 60 % for Cu and 15% for Cr. Red mud also efficiently fixed metals, and formed complexes that were irreversibly even under acid digestion. The red mud used in this research contained nearly 27% SiO2 and 5% sodium-aluminium-silicates (DSP), which could bind to the metals. It was confirmed that red mud increased the fraction of metals associated with silicates, which could only be dissolved in hydrofluoric acid. The sludge composting process was improved by red mud addition as well.

The results of the experiment showed that red mud could sufficiently change the speciation of heavy metals in sludge compost through converting the heavy metals from soluble and unstable forms to insoluble and stable forms. Red mud significantly affected the speciation of chromium, lead and zinc, but the effect on speciation of copper was not so clear.

The humic substances and soluble organic matter extracted from sewage sludge compost were also investigated. Composting decreased the amount of soluble organic matter in the sludge compost, but increased levels in MSW compost. However, more than 90% of the heavy metals bound to organic matter were in the insoluble humin fraction in the sludge compost. The rest of the metals, which were associated with soluble organic matter, dominated in the fulvic fraction (> 80%) rather than in the humic fraction of the sludge compost.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Ho, Goen
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