Transcript Hazardous Wastes Introduction

Chapter 6.2
Physico-chemical treatment
TRP Chapter 6.2 1
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 treatment
facilities
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
Sludge from physicochemical treatment
after pressing
•require further treatment
•need to be landfilled
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
Adsorbents
Application
Activated carbon Solvent recovery
Elimination of odours
Gases purification
Alumina
Drying of gases and
liquids
Bauxite
Treatment of petroleum
fractions
Drying of gases and
liquids
Molecular sieves Selective removal
of contaminants
from hydrocarbons
Silica gel
Drying and purifying gases
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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
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Source: Guyer, Howard H Industrial processes and waste stream management, Wiley
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
Reducing reagents
• Sodium or calcium hypochlorite
•Ferrous sulphate
• Hydrogen peroxide
•Sodium sulphite
• Chlorine
•Sulphuric acid
• Potassium permanganate
•Iron
• UV
•Aluminium
• Ozone
•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
precipitation
then removed by
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
·Coagulation and flocculation
Coagulation and flocculation
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Source: Guyer, Howard H Industrial processes and waste stream management, Wiley
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
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