4. SOLID WASTE / SOIL MANAGEMENT

Soil contamination take on many forms: municipal land-fills, hazardous waste sites, underground gas / oil storage tanks, pesticides, fertilizers and industrial chemicals are only a few of the many challenges. Substances are often characterized by their potential hazard: flammability (solvents etc.), corrosivity (acids), toxic volatiles (gasoline fumes etc.), radioactivity and reactivity are all concerns for solid waste management. Of particular concern because of their high relative toxicity's are PCBs, pesticides, heavy metals and organic solvents.

i. Domestic garbage

Domestic garbage is generated on a daily basis in varying amounts by different societies.  In developing, lower-income countries there is far more disposal of vegetable-based refuse in landfills and correspondingly, far higher levels of manufactured / synthetic materials like plastic, glass, paper and metals sent to landfills of industrialized nations. Historically, landfills were simply a hole in the earth where refuse was dumped in an uncontrolled manner, although many areas have long since established strict guidelines for solid waste disposal. A simple "hole-in-the-ground" landfill poses serious risks: precipitation moisture permeates through the soil / refuse and leaches soluble contaminants which enter aquifers and other groundwater systems. A more modern process involves continuously (daily / weekly) covering the refuse with soil so that layers of refuse are more isolated and stratified within the site.  The following reading assignment  will present some of the many considerations for landfill operations.

ii. Incineration:

While many solid wastes are discarded in landfills, other solid wastes are eliminated through incineration. Incineration can be conducted to achieve sterility, inactivate toxic compounds, or simply to reduce landfill volume. Commercial incinerators usually have primary and secondary incineration temperatures approaching 800° C and 900° C and have a number of different set-ups. Gaseous discharge from the incineration is reclaimed using scrubbers. Scrubbers may be wet (passing smoke through a liquid to collect materials, or dry, using filtration or dry-chemical inactivation to reduce emissions.

iii. Accumulation of Heavy Metals:

Many toxic chemicals, including heavy metals accumulate in soils and sediments. Heavy metals tend to remain in the upper soil layers and can be metabolized into plant tissues, especially since heavy metal have a high affinity for certain organic materials. Ions of heavy metals can be readily absorbed into organic particles either in soils or in water systems, ultimately becoming deposited as sediments. With their high affinity for organic materials (such as those seen in sewage sludge) there is concern that food crops treated with such natural fertilizing agent s may absorb some of these compounds.

REFERENCE - Sydney Tar Ponds: http://www.tarpondscleanup.ca/ 

iv. Treatment of Contaminated Soils:

When there is an unacceptable accumulation of a given toxic agent in soils, either through accidental "fugitive" release or through gradual accumulation, steps must be taken to control the situation. Containment refers to the idea of limiting a contamination to a specific, defined area, minimizing further spread. This may involve removing and isolating soils for effective treatment, or immobilizing them within the area of heavy contamination. Treatments may include washing the soil with appropriate solutions (i.e. EDTA, a chelator used to remove ions) to reduce a specific contaminant. Surfactants (highly powerful soaps) may be used to emulsify oily contaminants and improve their solubility in water to facilitate improved removal. There are numerous companies specializing in the containment, treatment and handling of toxic chemicals and / or spill sites, offering expertise in treating potentially hazardous situations. One popular current area of contaminated-soil treatment is bioremediation. Bioremediation is essentially uses microorganisms to metabolize toxic compounds into lower-toxicity substances. Bioremediation is conducted either in situ (on site) or in controlled treatment facilities. The concept is simple: there are nearly infinite numbers of microorganisms, each evolving into different sub-species daily, meaning there is essentially an organisms to metabolize most toxic compounds. Bioremediation companies will actively promote genetic selection to obtain microorganisms particularly adept at breaking down key contaminants such as PCBs, long hydrocarbons (fossil fuels), phenols, etc. By aerating a spill site and promoting and maintaining a healthy, active microbial population at the spill site bioremediation can be an especially effective means of cleaning up an area. In simple terms bioremediation is large scale "chemical composting", relying on and hastening natural breakdown of hazardous materials.