Last Topic

1. Last Topic Collected by JHP

2. Physico-chemical treatment of Hazardous waste

3. Physico-chemical treatment • a range of cool processing techniques • aim to reduce the hazardous potential of wastes • may also offer re-use or recycling opportunities • often used in combination to optimise hazardous wastes treatment • Chemical processes use chemical reactions to transform hazardous wastes into less hazardous substances • Physical processes enable different waste components to be separated or isolated, for • re-use or appropriate treatment or disposal

4. Physico-chemical treatments • On-site vs off-site in central treatment facility • Some physical processes on-site eg sedimentation • Treatment may be integrated into manufacturing process • On-site treatment reduces: • volumes needing transport • transport costs

5. Physico-chemical treatment • Off-site treatment allows for dedicated waste handling and treatment systems • Should provide: • Waste receiving station • Storage facilities for wastes awaiting treatment • Treatment areas for number and variety of processes used • Storage and disposal facilities for treatment residues eg reaction products, filter cake and wastewater • Storage for treated wastes to be incinerated, where appropriate • Laboratory services • Trained personnel

6. Treatment residues • All physico-chemical treatment processes generate residues which may: • be hazardous wastes themselves • be more concentrated than original waste • be suitable for recycling • require further treatment • need to be landfilled Sludge from physico-chemical treatment after pressing Source: Safe hazardous waste management systems 2002 ISWA

7. Physical processes • Many different physical treatment processes • Most are simple and low-cost • Choice depends on physical form of waste and its characteristics • Options include: • ·Separation • ·Sedimentation • ·Flotation • ·Drying • ·Evaporation • ·Sludge dewatering • ·Filtration Filter press Source: Safe hazardous waste management systems 2002 ISWA

8. Separation • Examples of separation techniques: • Sieving and screening - for dry materials of different particle size • Distillation - to separate liquids • Use of washing medium - to extract contaminants from soils or soluble components from solid wastes

9. Adsorption

10. Sedimentation • Used to separate particles held in suspension in a liquid which is principally aqueous • Uses gravity • May require mechanical or manual stirring • Suitable for a wide range of hazardous wastes • metals in waste water • neutralised acids and alkalis containing suspended metal hydroxides • metals that have been precipitated • Sludges may need further screening, drying or dewatering • Separated liquid may need further treatment

11. Sedimentation - example Source: Davd S Newby 1991

12. Flotation • Relies on the natural behaviour of particles less dense than water • Is suitable for a range of waste types eg oil/water separation • Efficiency can be improved by blowing air through the liquid • size of air bubbles should be varied according to waste type

13. Drying and evaporation • May be needed after sedimentation • Options include: • Sludge drying beds • Centrifugal separation • Filtering and pressing

14. Drying and evaporation - example am management, Wiley Belt filter - a continuous filtering process widely used for dewatering sludges

15. Chemical processes • change chemical properties of waste • use a chemical to treat a chemical • need details of waste composition and reactivity • need qualified staff to: • assess waste composition • monitor chemical reaction • check reaction results • Options include: • ·Reduction and oxidation • ·Neutralisation • ·Precipitation

16. Reduction and oxidation Some common oxidising and reducing reagents • Oxidising reagents • Sodium or calcium hypochlorite • Hydrogen peroxide • Chlorine • Potassium permanganate • UV • Ozone • Reducing reagents • Ferrous sulphate • Sodium sulphite • Sulphuric acid • Iron • Aluminium • Zinc • Sodium borohydride

17. Oxidation in practice • Needs expert design, careful operation to be safe • Is cost effective • Enables avoidance of harmful side reactions • Commonly used for cyanides • Easiest oxidising reagents: • sodium or calcium hypochlorite

18. Reduction in practice • Commonly used for chromates and chromic acids from chromium plating and tanning industries • Cr VI reduced to Cr III then removed by precipitation • Common reducing reagents: • ferrous sulphate • sodium sulphite/sulphuric acid

19. Chemical Oxidation and reduction: Oxidation reduction methods provide another important chemical treatment alternative for hazardous wastes. One important chemical redox treatment involves the oxidation of cyanide wastes from metal finishing industry, using chlorine in alkali solution. In this reaction CN- is first converted to a less toxic cyanate. Further chlorination oxidises the cyanate to simple carbondioxide and nitrogen gas. important redox treatment process is the reduction of hexavalent chromium Cr(VI) to trivalent chromium Cr(III) in large electroplating operations. Sulphur dioxide is used as the reducing agent and the reactions are as follows.

20. A large variety of oxidisable contaminants in waste water and sludges are oxidised by ozone which can be generated on site by an electrical discharge through dry air or oxygen.

21. Ozonolysis: Ozone is a very powerful oxidising agent. Although this process has not been demonstrated in any full-scale facility, its application to TCDD and PCBs is quite promising. With respect to TCDD it was demonstrated that if the dioxins were suspended as an aerosol combined with CCl4, 97% degradation of TCDD was possible. Ozone in conjunction with UV radiation has been shown effective for the destruction of polychlorinated phenols and pesticides. In both the cases the key requirements were to concentrate the TCDD in a medium where they were susceptible to attack and provide a free radical for reaction with dioxin molecule.

22. Neutralisation • A batch process • Used for wide variety of acidic and alkaline wastes • Acid wastes are neutralised by alkalis, and vice versa • Used to treat liquid wastes, sludges and gases • Reactions must be laboratory tested to control pH, identify complementary reagents • Neutralised liquid usually sent for sedimentation

23. Precipitation • Causes soluble substances to become less soluble/insoluble • Often used in combination with other treatment processes eg reduction, neutralisation • Effective treatment for wastewater containing toxic metals which arise in metal-plating and finishing industry, and mining • Calcium hydroxide (lime) most widely used reagent

24. Chemical precipitation: This technique can be applied to almost any liquid waste stream containing a precipitable hazardous constituent. By properly adjusting pH, the solubility of toxic metals can be decreased, leading to the formation of a precipitate that can be removed by settling and filtration. Quite often lime [Ca(OH)2] or caustic soda is used for precipitation of the metal ions as metal hydroxides. For example the following reaction suggests the use of lime to precipitate the metal as hydroxide.

25. Chemical ppt method Chromium is precipitated as hydroxide. Sodium carbonate also has been used to precipitate metals as hydroxides (Fe(OH)3•XH2O), carbonates (CdCO3), basic carbonate salts (2PbCO3•Pb(OH)2). Carbonate ion hydrolyses in water to give hydroxide ion

26. Chemical ppt methods Even lower concentrations of metals in the effluent can be removed by precipitating them as sulphides. Ferrous sulphide can be used as a safe source of sulphide ion to produce sulphide precipitates with other metals that are less soluble than ferrous sulphide. Reducing agents such as sodium borohydride can be used to precipitate the metal ions from solution in the elemental form.

27. Chemical Methods Neutralisation Waste acid with an alkali e.g. sulfuric acid with sodium carbonate: H2SO4 + CO3 2-→ SO4 2- + CO2 + H2O Oxidation Using common oxidising substances such as hydrogen peroxide or calcium hypochlorite e.g. cyanide waste with calcium hypochlorite: CN- + OCl-→ OCN- + Cl-OCN- + H3O+ → CO2 + NH3 Reduction Used to convert inorganic substances to a less mobile and toxic form e.g. reducing Cr(VI) to Cr(III) by the use of ferrous sulphate: 14H+ + Cr2O7 2-+ 6Fe2+ → 6Fe3+ + 2Cr3+ + 7H2O Hydrolysis Decomposition of hazardous organic substances e.g. decomposing certain organophosphorus pesticides with sodium hydroxide. Precipitation Particularly useful for converting hazardous heavy metals to a less mobile, insoluble form prior to disposal to a landfill e.g. precipitation of cadmium as its hydroxide by the use of sodium hydroxide: Cd2+(aq) + 2OH- → Cd(OH)2(s)

28. Hydrolysis: Hydrolysis treatment can be given to those hazardous waste constituents which are very reactive with water. Examples of those substances are halides, carbide, hydride, alkoxide, and active metal.

29. Ion exchange: Ion exchange is judged to have some potential for the application of interest in situations where it is necessary to remove dissolved inorganic species. However other competing processes - precipitation, flocculation and sedimentation - are broadly applicable to mixed waste streams containing suspended solids and a spectrum of organic and inorganic species. These competing processes also usually are more economical. The use of ion exchange is therefore limited to situations where polishing step was required to remove an inorganic constituent that could not be reduced to satisfactory levels by preceding treatment processes. One example for this is the use of anion exchanges for the removal of anionic nickel cyanide complex and chromate ions from waste solutions. Ion -exchange resins have also been used in the removal of radionuclides from radioactive wastes

30. Wet air oxidation: It is the aqueous phase oxidation of dissolved or suspended organic substances at elevated temperatures (150-325oC) and pressures (2000 kPa to 20,000 kPa) water. Which makes up the bulk of the aqueous phase, serves to catalyse the oxidation reactions so they proceed at relatively low temperature, and at the same time serves to moderate the oxidation rates removing excess heat by evaporation. It also acts as excellent heat transfer medium, which enables the wet air oxidation process to be thermally self-sustaining with relatively low organic feed concentrations. The high pressures allow high concentration of oxygen to be dissolved in water and the high temperature assist the reaction to occur. In wet air oxidation, the waste is pumped into the system with high-pressure pump and mixed with air from an air compressor. The waste is passed through a heat exchanger and then to a reactor where atmospheric oxygen reacts with the organic matter waste, sometimes in the presence of catalysts. The oxidation is accomplished by a temperature increase. The gas and liquid phases are separated. System pressure is controlled to maintain the reaction temperature. The process can be used for the removal of cyanide from electroplating waste solutions.

31. Other chemical processes • Practical options can include: • Hydrolysis eg for some pesticides • Electrolysis eg for silver recovery from photographic wastewaters • Dechlorinationeg for solvents • Chlorolysiseg for residues from chlorinated hydrocarbon manufacture

33. Hazardous waste compatibility

34. Combined physical & chemical processes • Two common examples: • ·Solvent extraction • ·Coagulation and flocculation Coagulation and flocculation

35. Physico-chemical treatment Source: David S Newby

36. Thermal treatment

37. Thermal treatment = destruction of hazardous waste by thermal decomposition • Thermal treatment methods include: • incineration - complete combustion using excess oxygen • gasification - incomplete combustion in the partial absence of oxygen • pyrolysis - thermal decomposition in the total absence of oxygen

38. Application of thermal treatment • Suitable for organic wastes • Thermal treatment processes: • require high capital investment • are highly regulated • need skilled personnel • require high operating and safety standards • have medium to high operating costs

39. Good practice in hazardous waste combustion • 3 Ts: • Time • Temperature • Turbulence • Flue gas cleaning systems

40. Examples of Calorific Value Mixed waste from plant cleaning operations 10,000 - 30,000 kj/kg Wastewater 5,000 kj/kg (0 - 10,000kj/kg depending on organic content) Industrial sludge 1,000 - 10,000 kj/kg Paints and varnishes >20,000 kj/kg Chlorinated hydrocarbons 5,000 - 20,000 kj/kg For comparison, MSW = ~10,000kj/kg Source: Indaver

41. Combustion • Requires: • addition of excess air • mechanical mixing of waste • even distribution and aeration of waste • Behaviour of waste during combustion varies according to its heat value and its form • Some low CV wastes burn easily = straw • Some low CV wastes are difficult to burn = wet sludges • Some high CV wastes burn easily = tank bottoms • Some high CV wastes are difficult to burn = contaminated soils, certain plastics • Certain wastes change their physical characteristics during combustion

42. Combustion techniques Bed plate furnaces: use gravity to mix waste - used for homogeneous and wet wastes such as sludge cake Fluidised bed furnaces: waste is introduced into a bed of sand which is kept in suspension - used for wastes of similar size and density Incineration grates: wastes fed onto the grate are turned or moved to ensure aeration of the waste mass via holes in the grate - used for solid wastes eg municipal wastes, not liquids or sludges Rotary kilns: wastes are placed in slowly rotating furnace - suitable for solids, sludges and liquids

43. Operation of the furnace Must be consistent Needs: • good understanding of waste characteristics • technical skills • control of waste feed • mixing of wastes • temperature to be kept at required level despite variations in waste • excess air • flue gas control • regular maintenance Source: David C Wilson

44. Energy recovery • Waste combustion produces heat • but combustion of low CV wastes may not be self-supporting • Energy recovery is via production of steam to generate electricity • Only steam production: 80% efficiency is typical • Steam can be used for in-house demands • Steam can be delivered to adjacent users eg other industrial plants • Electricity can be generated: 25% efficiency typical • Opportunities to sell heat are improved where facilities are in industrial areas • Sale of surplus energy improves plant economics

45. Waste-to-energy Incinerator

46. By-products of incineration • May be: • solid • liquid • gaseous • Comprise: • recovered materials such as metals, HCl • flue gases • slag and ash • products of the flue gas treatment, also called air pollution control (APC) residues • wastewater

47. Flue gases • Quantity and type of pollutants in emissions depend on: • pollutants in waste • technology • efficiency of operation • Average 6 - 7 Nm3 of flue gas per kg waste Specific collection/treatment for: Dust - staged filters Chlorine - neutralised by scrubbing with lime Sulphur - washing stage Dioxins - combustion control, activated carbon Source: David C Wilson

48. Dioxins • Family of around 200 chlorinated organic compounds, a few of which are highly toxic • Widespread in the environment • Present in waste going to incineration • Can be re-formed in cooling stages post-combustion • 3Ts help destroy dioxins in waste, reduce reformation • Use of activated carbon to filter from flue gases • Emissions limits extremely low

49. Example of flue gas cleaning technology Source: Indaver

50. Fuel blending Buildings and civil constructions Main process NORCEM Reception/storage of waste in drums and bulk Shredding and separation of solids and liquids Treatment of solids Storage ‘Hot-Mix’ Treatment of liquids Washing and recovery of metal Export Reception of liquid waste in bulk Sale Tankfarm Control and safety systems Source: Ian Miller