A.    Sources of Hazardous Waste

Hazardous waste is from the pretreatment of effluents in order to meet water pollution controls, an example being heavy metal sludges from electroplating, sludges from treating tannery wastes etc.

The degree of hazard posed by hazardous wastes varies;

a.    High risk to human health – this wastes include low flashpoint solvents, highly toxic pesticides or persistent carbonated materials such as PCBs.

b.    Wastes that hazard is less but greater in quantities – this waste includes large of volume mineral wastes as metaliferous slags, mine tailings, phosphogypsum or lime sludges.

B.    Illustrative Examples of Hazardous Wastes

Sector                              Sources                            Hazardous Waste

Commerce and Agriculture    Vehicle servicing                 Waste oils

                                      Airports                            Oils, Hydraulics fluids

                                      Dry cleaning                      Halogenated solvents

                                      Electrical transformers         Polychlorinated Bipheryls (PCBs)

                                      Hospitals                          Pathogenic/infectious waste

                                      Farms, Municipal parks etc.   Unused pesticides, empty containers

Small-scale industry             Metal treating                     Acids, heavy metals

                                        (electroplating, etching       Solvent, acids, silver

                                         Anodizing, and                 Cadmium, mineral acids

                                         Galvanizing)                    Solvents, ink and dyes

                                      Printing                             Solvents, chromium

                                      Leather tanning

Large-scale                       Bauxite processing              Red muds

                                      Oil refining                        Spent catalyst

                                         (petrochemical                Tarry residues, Solvents

                                          Manufacture)

                                      Chemical/pharmaceutical     Mercury

                                      Chlorine production

C.    Treatment Technologies of Hazardous Waste

1.    Physical treatment – these processes include various method of phase separation and solidification; phase separation encompasses lagooning, sludge drying in beds, and prolonged in tank processes.

          Lagooning and sludge drying in beds allow the removal of liquid into waste by decanting, drainage and evaporation, while lagooning and tank storage are used to separate oil and water from mixed waste.

Solidification – these process convert the waste into an insoluble, rock-hard material and they are generally used as pre-treatment prior to landfill disposal. The conversion is achieved by blending the waste with various reactants to produce a cement-like product. Waste should be sealed in bags of polyethylene or casing in concrete.

2.    Chemical treatment – chemical treatment method are used to facilitate the complete breakdown of hazardous waste into non-toxic gases and to modify the chemical properties of the waste (e.g. to reduce water solubility or to neutralize acidity or alkalinity)

3.    Biologic treatment – many hazardous industrial waste are treated by biological methods similar to those used for sewage treatment. The in-plant biological treatment of dilute aqueous effluents is well established, and micro-organisms have been degrade specific toxic chemicals.