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Chapter 6.2 Physico-chemical treatment

Chapter 6.2 Physico-chemical treatment . 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

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Chapter 6.2 Physico-chemical treatment

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  1. Chapter 6.2 Physico-chemical treatment TRP Chapter 6.2 1

  2. 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 TRP Chapter 6.2 2

  3. Physico-chemical treatmentfacilities • 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 TRP Chapter 6.2 3

  4. Physico-chemical treatment in central treatment facility • 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 TRP Chapter 6.2 4

  5. 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 TRP Chapter 6.2 5

  6. 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 TRP Chapter 6.2 6

  7. 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 TRP Chapter 6.2 7

  8. Adsorption TRP Chapter 6.2 8

  9. 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 TRP Chapter 6.2 9

  10. Sedimentation - example Source: Davd S Newby 1991 TRP Chapter 6.2 10

  11. 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 TRP Chapter 6.2 11

  12. Drying and evaporation • May be needed after sedimentation • Options include: • Sludge drying beds • Centrifugal separation • Filtering and pressing TRP Chapter 6.2 12

  13. Drying and evaporation - example Belt filter - a continuous filtering process widely used for dewatering sludges TRP Chapter 6.2 13 Source: Guyer, Howard H Industrial processes and waste stream management, Wiley

  14. 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 TRP Chapter 6.2 14

  15. 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 TRP Chapter 6.2 15

  16. 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 TRP Chapter 6.2 16

  17. 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 TRP Chapter 6.2 17

  18. 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 TRP Chapter 6.2 18

  19. 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 TRP Chapter 6.2 19

  20. Other chemical processes • Practical options can include: • Hydrolysis eg for some pesticides • Electrolysis eg for silver recovery from photographic wastewaters • Dechlorination eg for solvents • Chlorolysis eg for residues from chlorinated hydrocarbon manufacture TRP Chapter 6.2 20

  21. Combined physical & chemical processes • Two common examples: • ·Solvent extraction • ·Coagulation and flocculation Coagulation and flocculation TRP Chapter 6.2 21 Source: Guyer, Howard H Industrial processes and waste stream management, Wiley

  22. Physico-chemical treatment TRP Chapter 6.2 22 Source: David S Newby

  23. Key considerations • Waste reduction and avoidance by generators should always be a priority • Role of on-site vs off-site technologies • Need to consider residues from treatment processes and their disposal • Transitional technologies may be used until final high-quality installations are available TRP Chapter 6.2 23

  24. Chapter 6.2 Summary • Physical and chemical treatment includes a range of cool processing techniques • Often used in combination • Suitable for a wide range of waste types • May enable re-use or recycling • Treatment can be on-site or off-site • Processes inevitably generate residues TRP Chapter 6.2 24

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