Chapter 6.2 Physico-chemical treatment

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Chapter 6.2 Physico-chemical treatment
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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

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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

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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

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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
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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
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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

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Adsorption
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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

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Sedimentation - example
Source Davd S Newby 1991
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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

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Drying and evaporation

• May be needed after sedimentation
• Options include
• Sludge drying beds
• Centrifugal separation
• Filtering and pressing

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

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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

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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

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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

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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

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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

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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

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Combined physical chemical processes
Two common examples Solvent extraction Coagulat
ion and flocculation
Coagulation and flocculation
Source Guyer, Howard H Industrial processes and
waste stream management, Wiley
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Physico-chemical treatment
Source David S Newby
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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

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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