BRISBANE REGION ENVIRONMENT COUNCIL


Overview of Landfill Wastes
Prepared by BREC volunteer
Dr Abdul Al Chalabi Phd,Msc,Bsc
C/-BREC POB 479 Morningside 4170
Ph 3800 1361 e-mail webink@pasdex.com.au

 For a long time , industrial and domestic wastes have been regarded a cheap land reclamation and landfill material. Amount the many wastes used for these purposes are dredged spoils from harbours and channels, mine tailings, pulverised slags, fly ash from power stations, industrial and household garbage and sewage sludge. These wastes are commonly high in hazardous chemicals and there is now many reports showing that soils derived from these wastes are also enriched with these chemicals. Some of the hazardous chemicals which could be present include:

Toxic Elements: Ozone, White Phosphorous, Halogens( Chlorine, Bromine and Iodine ), heavy metals( Cadmium, Lead, Arsenic and elemental Mercury), cyanide, carbon monoxide, nitrogen oxides, hydrogen halides, asbestos, inorganic phosphorous compounds and inorganic compounds of sulphur.

Organometallic compounds: Organolead compounds, organotin compounds and carbonyls.

Organic compounds: Alkane hydrocarbon, Alkene and alkyne hydrocarbons, Benzene and aromatic hydrocarbons (Toluene, Naphthalene and polycyclic hydrocarbons).

Oxygen containing organic compounds: ethylene and propylene oxides, alcohols, phenols, aldehydes and ketones, carboxylic acids, ethers and esters.

Organonitrogen pesticides: such as carbofuran and herbicidal paraquat.

Organohalide pesticides: such as DDT, Aldrin, dieldrin, endrin, chlordane, heptachlor and isodrin.

Landfill historically has been the most common way of disposing of solid hazardous wastes and some liquids although it is being severely limited in many nations by new regulations and high land costs.

The greatest environmental concern with the landfill of hazardous wastes is the generation of leachate from infiltrating surface water and groundwater with resultant contamination of ground water supplies. Modern hazardous waste landfills provide elaborate systems to contain, collect and control such leachate. Modern hazardous waste landfills typically have dual leachate collection systems. Chemical and biochemical processes have the potential to cause some problems for leachate collection systems. One such problem is the clogging by insoluble manganese and iron hydrated oxides upon exposure to air.

The first step in treating leachate is to characterise it fully, particularly with a thorough chemical analysis and an assessment of its biodegradability. Hazardous wastes leachate can be treated by a variety of chemical processes including acid/base neutralisation, precipitation and oxidisation/reduction. In some cases these treatments must precede biological treatment, for example,

 In the presence of biodegradable wastes, methane and carbon dioxide gases are produced in landfills by anaerobic degradation. Gases may also be produced by chemical processes with improperly pretreated wastes, as would occur in the hydrolysis of calcium carbide to produce acetylene. Odorous and toxic hydrogen sulphide(H2S) may be generated by the chemical reaction of sulfide's with acids or by the chemical reaction of sulfide's with acids or by the biochemical reduction of sulphate by anaerobic bacteria in the presence of biodegradable organic matter. Gases such as these may be toxic, combustible or explosive. Furthermore, gases permeating through landfilled hazardous waste may carry along other waste vapours, such as those of volatile aryl compounds and low molecular weight halogenated hydrocarbons. Of these, the ones of most concern are benzene, di bromo and di chloroethane, carbon tetrachloride, chloroform and vinyl chloride. Therefore, it is important to minimise the production of gases and, if significant amounts of gases are produced, they should be safely vented or treated by activated carbon sorption or flarings.

In-situ immobilisation is used to convert wastes to insoluble forms that will not leach from the disposal site. Heavy metal contaminants including lead, zinc, cadmium and mercury can be immobilised by chemical precipitation as sulfide's by treatment with gaseous H2S or alkaline Na2S solution. Although precipitated metal sulfide's should remain as solids in the anaerobic condition of a landfill, unintentional exposure to air can result in oxidation of the sulfide and remobilisation of the metals as soluble sulfate salts. Oxidation and reduction reactions can be used to immobilise heavy metals in-situ. Oxidation of soluble Fe2+ and Mn2+ to their insoluble hydrous oxides can precipitate these heavy metal ions and coprecipitate other heavy metal ions. However subsurface reducing conditions could later result in reformation of soluble reduced species. Reduction can be used in-situ to convert soluble, toxic chromate to insoluble chromium compounds.

Chelation may convert metal ions to less mobile forms, although with most agents chelation has the opposite effect. A chelating agent called Tetran is supposed to form metal chelates that are strongly bound to clay minerals. The humin fraction of soils humic substances likewise immobilises metal ions.

Evidence for deeper penetration of lead has been encountered only in special soil types where the combined effects of low temperature and the presence of sodium chloride diminish the ability of the alluvial soil to retain the lead in the uppermost layer.

 


To be continued .Part 2 Wastes From Sewage Sludges due to be released by the end of March 1998