Chapter 6.3
Biological treatment
TRP Chapter 6.3 1
Application of biological treatment
Biological treatment of organic waste:
• optimises a natural process
• uses ubiquitous micro-organisms (eg bacteria,
fungi)
• requires control of temperature and nutrient
balance
• may be aerobic or anaerobic
For hazardous waste treatment, used for:
• low contaminant-concentration organic wastes
eg sludges
TRP Chapter 6.3 2
Factors influencing biological
treatment
Suitability of the waste:
•composition
•physical form
•pH
• Biological treatment is only suitable for
organic wastes with relatively low toxicity
• It is not 100% efficient in destroying organic
material
TRP Chapter 6.3 3
Process conditions
Biological treatment processes require control of:
• temperature
• moisture
• pH
• level of aeration
• inhibitors such as metals
• nutrients
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Advantages of biological waste
treatment
In the right conditions - ie temperature, humidity
and pH - biological treatment is:
• Effective
• Tolerant to changes in waste composition these may result in a short period of inactivity,
but do not halt the process
TRP Chapter 6.3 5
Scope of treatment application
• Wastewater treatment is the most widely used
application of biological treatment eg for
industries such as paper manufacture and
recycling, food processing, tanneries and the
pharmaceutical industry, and for landfill leachate
Also:
• In-situ bio-remediation of contaminated soil
• Slurry-phase treatment
• Land treatment
• Co-composting
TRP Chapter 6.3 6
On-site vs off-site treatment
• For hazardous wastes, most processes
suitable for on-site treatment
• For wastewater, central treatment - but
requires pre-segregation of toxic
effluents
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Typical wastewater treatment
process
Pre-treatment
Treatment
Post-treatment
eg physical/chemical
treatment, solids
separation
Biological treatment
eg filtration,
adsorption
eg aerobic/ anaerobic
suspended/attached
batch/continuous
BIOREACTOR
Discharge of
clean effluent
TRP Chapter 6.3 8
Wastewater treatment - liquid
phase
• Staged process
• Must meet discharge limits
• Data collection needed to prove compliance
• Variations in composition must be expected
• Use fixed or aqueous medium
• May need complementary/finishing
treatments eg lagoons, activated carbon
TRP Chapter 6.3 9
Wastewater treatment - example 1
Bacterial filter beds
• Fixed media
• Stable process
• Good for high charges of effluent
• Suitable for effluents with high organic
loadings eg from canning industry, sugar
industry
TRP Chapter 6.3 10
Wastewater treatment - example 2
Activated sludge
Widely used eg refineries, canneries, pharmaceuticals
Suitable for wastewater, aqueous hazardous wastes
with < 1% suspended solids
• Bacteria are aggregated in floating flocs
• Recirculation ensures constant bacterial charge
• Flexible but more sensitive process than bacterial
filter beds
• Bioreactor needs efficient mixing and high level of
dissolved oxygen
• Cleaner effluent than other biological processes
• Needs a lot of energy
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Wastewater treatment - example 3
Lagoons
• Shallow impermeable water basins used for
degradation of batches of effluent
• Depends on action from bacteria, algae or aquatic
vegetation
• Low cost process
• Suitable for wastewater with low organic contents
• Not suitable for wastes with mainly chemical
components
• Often used as pretreatment stage
TRP Chapter 6.3 12
Slurry-phase treatment
Used for solid waste, sludge or contaminated soil
Needs mixing to:
•homogenise slurry
•break down solid particles
•oxygenate
•increase contact with microorganisms
Degrades waste at a faster rate, needs less land,
than solid-phase treatment
Potential for use with additional waste streams eg
wastes from wood preserving, petroleum refinery
wastes
TRP Chapter 6.3 13
Land treatment - solid phase
• involves applying wastes uniformly to prepared land
at controlled rates
• aimed at degradation of organic constituents
• removes other waste constituents eg suspended
solids, heavy metals
• purely a treatment process - land NOT suitable for
cultivation
Suitable for:
• oily sludges and waste oils
• organic sludges and liquids
• widely used in the USA by petroleum industry
• also possible to treat wastes from wood treatment eg
preservatives such as creosote, pentachlorophenol
TRP Chapter 6.3 14
Co-composting - solid phase
Mixing of hazardous wastes with biodegradable solids
to act as:
• bulking agent - to create void spaces for passage of air
• thermal source - by biological decomposition
Usually need two different materials as:
• good bulking agent eg wood chips are poor thermal source
• good thermal source eg dry molasses are poor bulking agent
Process takes place:
• in windrows, turned to ensure adequate aeration
• in static piles where air is forced or sucked through material
• in vessel which offers greater process control/ VOC containment
Used for:
• soils contaminated with coal tar
• for TNT-contaminated sediments and soils
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In-situ bio-remediation
• Enables treatment without excavation and
removal of contaminated material
• An aerobic process
• Enhances natural biodegradation
• Influenced by hydrogeological factors
• Can reduce contamination to acceptable
levels in relatively short time eg 1-2 years
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Soil heaping - solid phase
• Combines land treatment and windrow
composting
• Effective for treating large volumes of
contaminated soil and other wastes with
low concentration of organics
• Valuable where available land area is
restricted
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Emerging applications for
biological treatment
•For many waste types, biological treatment
is still in early stage of development
•New applications are being tested and
developed eg
• for treating additional hazardous waste
streams
• for integrating biological processes with
physical-chemical treatment
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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 6.3 Summary
Biological treatment of hazardous waste
• optimises a natural process
• is suitable for low concentration organic
wastes eg sludges
• requires good control of process conditions
• is relatively low cost, effective and tolerant to
changes in waste
• is most widely used for wastewater treatment
• may be on-site or off-site
• new applications being developed
TRP Chapter 6.3 20