CIRIA Annual Conference

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