L2: Flashcards
landfill in the beginning
- was an open dump
- leachate leaked into the surrounding geology following rainfall
why are landfill processes influenced by waste composition?
- leachate generation
- flushing of contaminants
- degradation
- gas generation
- settlement
the emission of landfill gas (CH4 and CO2) as waste degrades
xxx
what was the loscoe explosion?
the loscoe was the site of a landfill gas migration explosion on 24th March 1986. Although there were no fatalities, one house was completely destroyed by the blasts and its three occupants injured
engineered features deteriorate over time
- drains clog
- linear degrades
- cap settles and cracks
- waste- degrades and settles
What are some examples of geotechnical failure modes?
increased filtration
leackage of leachate
gas escape
what is a sustainable landfill?
- the contents of the landfill must be managed so that outputs are released to the environment in a controlled and acceptable way
- the residues left in the site do not pose an unacceptable risk to the environment and the need for after care and monitoring should not be passes on to the next generation
- future use of groundwater and other resoures should not be compromised
What is landfill completion?
The landfill has reached a stable state, in hydraulic equilibrium with the surrounding environment with contaminant release at a rate that will not damage the receiving environment
degradation, settlement and gas generation have substantially stopped
leachate is non- or marginally polluting i.e. mobile recalcitrant contaminants have been flushed out
liabilities can be transferred from the operator to society
timescale of centuries with current methods of landfill operation
Sustainable landfill?
engineered feastures address the first point during the managed life of the landfill
waste degradation/ flushing is unlikely to be complete when engineered features fail
need to ensure that engineered feastures ‘fail safe’, so that residual contaminant load is not released into the environment unsustainably- release of remaining contaminants is controlled by the ‘failed’ engineered feastures of the boundary conditions they impose.
what are the landfill processes influencing the residual containment systems?
- Degradation settlement gassing
- degradation flushing
- leachate quality when linear fails
- wells and drains
what is degredation, gassing and settlement in lf?
who knows
what is the role of liquid in landfill?
- water content
- encourages microbial degradation: >35% water content needed for methanogenesis - water flow
- transports seed bacteria and nutrients for AD
- flushes out non-degradable contaminants - both processes essential for ‘completion’.
what is darcy’s law?
an equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on the results of experiments on the flow of water through beds of sand, forming the basis of hydrogeology, a branch of earth sciences.
why is permeability important?
It governs water (leachate) flow and hence is a controlling factor in degradation and flushing
If waste is processed to reduce its particle size, will the permeability be reduced so much that no significant flow occurs and degradation is slowed?
- large scale investigation using the pitsea compression cell
- found that average hydraulic conductivity reduces with average applied stress (vertical)
- waste density increases with effective stress
Pearmeability vs vertical effective stress
- single correlation between vertical permeability and vertical effective stress in first loading
- differences in permeability resulting from
what are the limitations of a continuum flow model?
> not always valid in conventional sized systems
Channel flow: completion challenges
organic matter degradation: ?
Flushing of leachate pollutants- have to conform to drinking water standards
describe the tail end of LFG curve years after closure?
LFG generation drops to 0.5 to 2m3/t.a
remaining gas potential 75m3/t (out of ~250 m3/t)
describe flushing to completion
Leachate dilution at full scale landfill with high water inputs
- ~exponential decline
- ~3-5m3 water tonne waste required (~7 bed volumes)
- finally gets to the point where it isn’t considered harmful
Describe dual porosity (DP) model: saturated flushing
A) conceptual model
- diffusion between fissure and matrix
B) a representation in DP-Pulse: Slab geometry
- advection flow from cell to cell in fissure
describe landfill today: the UK picture
three categories
- standard rate
- lower rate
- exempt
- all declining
what is still being landfilled?
- food
- paper and card
- note there has been a significant reduction in the quantity of garn waste sent to landfill, however a sig rise in the quantity of food waste
- MSW only ~25% of waste to landfill, but is a major controlling influence
- 50% reduction in landfilled waste
- Higher proportion of food waste in 2011, though total mass landfilled is less
Describe the EU landfill directive
aims to reduce fugitive ghg emissions
biodegradable waste going to landfill to be reduced to3 35% to 1995 amounts by 2016 (2020 in the UK)
requires pretreatment of biodegradable wastes prior to landfill
describe MBT preparation of waste
- current options for treatment of wastes prior to disposal are mostly either thermal and mechanical/ MBT
MBT is a generic term- sorting and screening to remove recyclables; shredding; plus aerobic and/or anaerobic degradation
MBT residues are likely still to need landfilling
MBT wastes still bioactive to some extent, with very different particle properties from raw MSW and implications for their behaviour in landfills
why will there be a diminished future role for landfill
- a sink for hazardous substances in producrs that would otherwise accumulate in the environment
- strategic storage of materials for which there are currently no (economic, or resource efficient) recycling options
- storage to cover failure of other systems and following extreme events (e.g. foot and mouth, flooding)
Challenges (1)
- for practice, an acceptance of the spatial variability of waste hydraulic conductivity in landfills (especially with stress/depth), and of anisotrophy and gassing, is essential to understand patterns of leachate head and for effective leachate management
- we need a better understanding of the mechanism of gas generation and its effect on hydraulic conductivity
challenges (2)
- we need better characterization of waste structure and how it affects flow through the landfill; particularly degradation (undegraded pockets) and flushing of recalcitrant contaminants
- there will be a future role for landfill in the disposal of residual and emergency wastes
- we need to be able to manage the legacy of old landfills to completion/ a stable non-polluting state
