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Treating leachate emissions from old
urban landfill sites – a case study
David Rudland
Scheme Manager – Shaw Forest Park Restoration
Swindon Borough Council
Introduction
• Most local authorities have at least one old landfill
to manage
• Leachate is produced at all landfill sites that
contain biodegradable wastes
• Continues to be generated long after the last
deposits of waste
How is leachate generated?
• Leachate is generated principally by rainwater percolating through landfill
• In contact with decomposing solid waste, the percolating water becomes contaminated
• On flowing out of the waste material it is termed leachate.
• Associated degeneration produces typical landfill gases and a complex mix of organic acids, aldehydes, alcohols and simple sugars
The Consequences of leachate
production • May emerge as strong smelling
yellow or orange-coloured cloudy liquid
• Contaminates ground/surface water
• Local vegetation dieback, health risks
• Modern landfills contain barriers to this leachate loss
• Older landfills and those with no membrane between the waste and the underlying geology, leachate may be free to egress
Can’t old landfill sites be left to
fester quietly? • New development in the vicinity often affects local
groundwater levels beyond immediate area of the development
• Rising groundwater may lead to rising leachate levels in turn wetting waste masses that have previously been dry
• This triggers further active decomposition and leachate generation
• Thus stabilised, inactive site can become re-activated -restart significant gas production and significant changes in finished ground levels
Shaw Forest Park – key features 1
• Landfill activity from 1960s to 1980s
• No current licence, urban setting
• All sorts of wastes present
• Part of a series of landfills being landscaped to form public park
• No capping or containment
Shaw Forest Park – key features 2
• 45 ha area public park
• Community forest –
60,000 trees planted
since 1994
• Network of paths
• Volunteer conservation
interest
Environmental controls at Shaw
• None required by licence
• Leachate pumping system via well extraction installed in 2006 in response to breakouts
• Passive gas venting
• In 2006 a mechanical problem lead to escape of leachate to Lydiard Brook
• Prosecution under section 85 Water Resources Act 1991
Leachate arisings
• Extraction wells collect high strength leachate and direct it to sewer
• Leachate breakout from flanks collected in perimeter ditch system, which was incomplete
• Leachate in ditches diluted by precipitation/ surface runoff
• Treatment system required for breakout leachate
Treatment Alternatives
Considered • Mains sewerage solution not
available
• Mechanical solutions – rejected on ground of running costs, carbon emissions, noise and building requirements
• Ongoing tankering – significant impact on environment – unsustainable
• Constructed Wetlands preferred
option
Wetland treatment benefits
• Biodiversity enhancement
• Amenity improvement
• Economy - low energy, maintenance
• Additional water storage capacity
• Estimated tankering 2year on-cost similar to
wetland construction cost
Data Required
• Water balance calculations
– Rainfall input
– Discharges to sewer
– Tankering away volumes
– Uncontrolled escapes
• Assumes 100% capture by wells
or perimeter system
• Leachate quality
Low permeability
clay
Old landfill
Ditch
Treatment requirements
• Wetlands would be secondary system, ie
would not treat high strength leachate
• Concluded there are certain chemicals that
occur in the leachate which we need to
remove: – Suspended Solids
– Nutrients such as Nitrogen, Ammonia and
– Metals such as iron
How to they work?
• Leachate flows over the
surface of the wetland
• Oxygen enters wetland
through diffusion
• Thin layer of water
flowing through
• Clean water flows out
How to they work?
• Sediment and pollutants settle as water flows slowly through system
• Bacteria naturally found in wetlands, living in soil and on plant roots help decompose and consume nutrients
• Harmful pathogens are attenuated
• Nutrients are adsorbed by soil and plants - natural chemical reactions occur to remove them from water.
How can this be made special for
Swindon?
• By creating different areas within the
system to allow a variety of root systems
and various bacteria to enhance the
treatment
• This has the effect of varying the habitats to
attract a variety of species
Key Features of Constructed Wetland
• System designed by WWT Consulting, Slimbridge
• Leachate pumped to a primary treatment pond of open water & marginal vegetation
• Then to 2 parallel systems of 3 beds each
– Wet grassland beds to maximise ammonia, nitrate attenuation
– Reedbed
– Tall emergent mixed community (eg Iris)
– Short emergent community (eg water mint, forget-me-not)
– The latter 3 to reduce suspended solids, COD and bind orthophosphates
• Additional reedbed and tall emergent pond then final polishing pond
• Discharge to stream
Consents
• Flood defence consent – to allow construction of
outfall
• Discharge consent
– BOD = 10mg/l
– Ammoniacal nitrogen = 10mg/l
– List 2 substances = none
– pH
– Suspended solids = 25mg/l
Ongoing maintenance
• Water flow management
• Silt removal from 5 years
• Accumulated plant material clearance – annual
• Algal blooms – either bed draw down or increase
water flows as required
Finished construction
Community involvement
In conclusion……..
www.swindon.gov.uk