Landfill

A Landfill Site is the ultimate destination for waste, ideally after all forms of Reuse, Recycling, and Recovery have been undertaken and as such it is at the bottom of the Waste Hierarchy.

General Picture of Landfill Compactor compacting MSW Waste

Introduction

The Definition of 'Landfill' is found in the Landfill Directive 1999/31/EC on the landfilling of waste as 'a waste disposal site for the deposit of waste onto or into land (i.e.underground) including:

• internal waste disposal sites (i.e. landfill where a producer of waste is carrying out its own waste disposal at a place of production), and
• a permanent site (i.e. more than one year) which is used for temporary storage of waste'.

The deposit of waste into a Landfill site has evolved in the UK from 'open tips' and uncontrolled tipping in the late 1960's through to the current position comprising highly engineered solutions that contain and stabilize the waste in a 'Sanitary Landfill' - the complexity of the engineering approach and management being based upon the nature of the waste received and the sensitivity of the location of the site, especially in the context of the quality of the underlying groundwater.

Landfill sites have usually been constructed in disused quarries, or parts of quarries. However, the phrase is also used to describe sites built where there had previously been no excavation. Such sites are also known as “land-raise” sites, but in the context of this website, no distinction has been made. To all practical intents and purposes, there is no significant difference between landfill and land-raise.

Whilst Landfill is the ultimate destination for waste, ideally after all forms of Reuse, Recycling, and Recovery have been undertaken, there there will always be a small element of waste that will need to be landfilled in a safe and secure way (under the Landfill Directive no more than 10% of MSW generated by weight is to be landfilled by 2035). In developing countries, with limited infrastructure, the delivery of Sanitary Landfill would represent a huge step forwards in protecting water resources and human health.

The key legal requirements for managing landfill have been implemented in the UK through the Landfill Regulations 2005 which implemented the Landfill Directive and defined 3 types of Landfill:

• Non-Hazardous Landfill (the link goes to a page listing sites in England, Wales and Scotland)
• Hazardous Landfill
• Inert Landfill

and the standards are set out for the key stages of landfill in regards to:

• initial choice of location
• the engineering and containment systems
• the operational management
• the restoration and closure of the landfill
• the monitoring of landfills through each of the above stages

Landfill Overview

The placement of waste in a landfill will slowly degrade over time, and in the case of the presence of Organic Waste within the Residual Waste going to landfill, it will breakdown to produce Landfill Gas and Leachate which have the potential to cause environmental pollution. As a result the selection of the site and the engineering and containment design to protect the environment, and specifically underlying groundwater, is critical to the successful delivery of a modern landfill.

Landfill Cross section - Dilute and Disperse, no Engineering Containment	Cross section of Modern Landfill: source http://www.goic.org.qa/GOICCMS/RECYCLING_FOR_GCC_INDUSTRIES_EN.html
Landfill Cross-Section of Older, Dilute and Disperse Style Landfill, no Engineering Containment	Cross-Section of Modern Engineered Landfill

The 'go-to' guidance for many in the landfill sector in the UK is still the waste management papers produced in the mid and late 1990's:

• Waste Management Paper 26B (WMP26B)
• Waste Management Paper 26A (WMP26A)
• Waste Management Paper 27 (WMP27)

Leachate

Putrescible waste placed in landfill sites undergoes decomposition. This is also described in WMP26B. The process gives rise to a liquid known as leachate. Leachate arises from water entering the waste matter, generally from rainfall on open areas of waste. Leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed. Landfill leachate is strongly polluting in the external environment; one of the principal reasons for constructing landfill sites as containment facilities is to prevent landfill leachate from escaping from the waste mass in an uncontrolled manner. Escape of Leachate into the underlying groundwater is a key factor in assessing the location of a landfill and the engineering containment systems to be design.

Once the design of sites moved from dilute and disperse to containment designs, as shown in the previous cross-sectional diagrams, it became necessary to devise means of avoiding leachate from building up excessively within the waste mass. There are two drivers for this: (i) in extreme circumstances, the volumes could lead to levels over-topping the sides of any containment structures and leaving the site, and (ii) designing the site to maintain levels at a level as low as practically possible would improve the Hydraulic Risk Assessment (HRA) outcome for any given design. Consequently, sites were designed with means to collect and remove leachate.

Leachate removed from the waste mass needs to be disposed of in an appropriate fashion. In general, this requires transmission to an appropriate treatment plant; this might be via a connection to the sewer system for ultimate treatment at an off site sewage treatment plant operated by a utility company, or an on-site private leachate treatment plant. Depending upon the capacity at the receiving utility treatment plant, leachate may require pre-treatment at the landfill site prior to sending to the offsite sewage treatment plant.

In the case of an on-site leachate treatment plant, once appropriate quality standards have been met it may discharge to a receiving watercourse or (less frequently) a soak-away. Such a discharge would be regulated by the EA with an Environmental Permit. Discharge to a sewer, with or without pre-treatment, would require agreement with the relevant utility company as an industrial effluent.

In the event that no on-site treatment is possible, and no local connection to the sewage system is possible, Leachate may also be taken to an off-site facility (e.g utility sewage treatment works) via road-going tanker for treatment and disposal.

Groundwater and Hydrogeology

The underlying geology and hydrogeology of a landfill is especially significant if there is an aquifer that is relied upon for drinking water or other extraction purposes. One definition of aquifer is from Freeze and Cherry (1979): “An aquifer is best described as a saturated permeable geologic unit that can transmit significant quantities of waste under ordinary hydraulic gradients”.

In the England and Wales, the EA categorize aquifers to be consistent with the Water Framework Directive. The categories are Principal Aquifer, Secondary Aquifer and Unproductive Strata. Previously the phrases major aquifer and minor aquifer were utilized. Similar arrangements exist in Scotland and are supervised by SEPA.

The relevance of the hydrogeological setting within which a landfill is located arises from the need to assess the technical precautions and engineering requirements for a landfill to be authorized such that unacceptable environmental impacts are prevented. This assessment is undertaken via a Hydrogeological Risk Assessment (HRA); this is submitted as supporting document to an Environmental Permit application and takes a risk based approach to the location of the landfill and the engineering required. Again, similar arrangements are in place in Scotland, Wales and Northern Ireland.

In the case of Principal Aquifers, which are of the highest quality and use, it is unlikely that any HRA will come to the conclusion, even with extensive engineering, that a landfill will be able to be located above such an aquifer (see Landfill location below).

Landfill Engineering

Landfill engineering is a phrase used to describe the preparation and on-going engineering required to operate a landfill site, and overall design, approach and engineering solution is often referred to as a whole as a containment system. What is required at any particular site in the UK depends upon the proposals made by the permit holder to protect the environment as part of obtaining an Environmental Permit (with the HRA and the containment system and engineering design being a key part of any such submission) and any subsequent modifications agreed between the permit holder and the regulator (EA, NRW or SEPA).

Notwithstanding the risk-based technical design of the landfill engineering, there are minimum requirements imposed by the Landfill Directive and subsequent UK legislation – The Landfill (England and Wales) Regulations 2002 and the Landfill Regulations 2005.

The design will have been produced taking heed of the technical precautions required to satisfy the regulator that the site will not have an unacceptable impact upon the environment and that the approved design will comply with the requirements of the Landfill Directive. These precautions will have been validated against the various risk assessments required to accompany a permit application.

Usually the application will be accompanied by risk assessments of the hydrogeological impact of the site, the potential for landfill gas release and the stability of any engineering designs proposed. There will be additional risk assessments required for operational matters such as dust, noise and amenity impacts; these are largely managed by operational techniques, but occasionally there will be elements of mitigation required that constitute site engineering (e.g. the construction of noise bunds).

Sites which were authorised historically did not require engineering and such sites without an engineered lining system are commonly described as “dilute and disperse”. Notwithstanding the lack of site engineering, some evaluation of their pollution potential was undertaken. This mainly consisted of a simple evaluation of the ability of the surrounding geology to adsorb contamination to determine likely consequential pollution effects. This evaluation considered the ability of the ground to “dilute and disperse” the impact of the site, hence the common name.

Landfill engineering design evolved in the 1980s, such that new dilute and disperse sites became less likely to be granted consent by the regulator. At the time, this was not supported by the implementation of any new specific legislative drivers, the principal legislation remaining the Control of Pollution Act 1974. This required landfills to be licensed by local authorities. In assessing licence applications, local authorities were required to ensure the prevention of pollution to water and danger to public health. This requirement established a rationale for assessment and its implementation evolved over the period from 1976 onwards. With the introduction of subsequent legislation mentioned elsewhere on this page, the regulatory role moved from local authorities to the EA (and in due course the NRW, SEPA and NIEA) and coincided with the Landfill Directive and Groundwater Directive which set minimum standards and requirements.

Landfill sites that incorporate engineered barriers are known as “containment facilities”. The design of such facilities has more than one form and can incorporate different numbers of layers and different designs. These layers are conventionally known as the “landfill lining system”.

The Landfill Directive sets out minimum requirements for site engineering. In simple terms, all sites require a "geological barrier" and some sites are additionally required to have an "artificial sealing liner". The most common manner in which these requirements are deal with is by having a clay liner of 1m metre thickness of low permeability engineered clay (permeability of less than 1x10-9 metres per second) overlain with 2mm thick welded HDPE (high density polyethylene) lining membrane. Typically, this would be further overlain with a geotextile protection layer and then a drainage layer of aggregate and pipework. A typical installation is shown below.

Typical installed HDPE liner

The Department of the Environment produced a series of Waste Management Papers (WMP) over the period from the late 1970s to 1997. WMP 26B contains a review of the types of designs. The design employed at any particular design would be informed by the outcome of the HRA which would assess the likely impact of a site upon the environment taking account of the design proposed. In general terms, the most sensitive sites would require the greatest degree of engineering.

Landfills are constructed in discrete parts known as “landfill cells”. The sub-division into cells is undertaken so as to minimise the operating area and consequently minimise leachate generation and make the site more manageable in terms of odour and litter control. Sub-division also enables the site engineering to be constructed progressively as the site fills.

Landfill Gas

Another product of waste decomposition is landfill gas. This is a mixture of various components, principally methane and is a Biogas not dissimilar to that generated in an Anaerobic Digestion facility. WMP27B also contains a useful review of the generation and evolution of landfill gas.

Landfill gas needs to be managed to prevent build-up such that it migrates off site through the surrounding soils and rock (where it can accumulate and create explosive conditions) or is released into the atmosphere (as it is a significant potential contribution to greenhouse gases).

Management of landfill gas consists of the prevention of build up by active removal of the gas from the waste mass by extraction (and subsequent destruction by high temperature flare or via the creation of energy for subsequent use on site or for export ot the national grid) and the prevention of migration and escape by the containment barrier/lining system and the restoration/capping systems. WMP27 deals with methods for management of landfill gas.

Landfill gas flare - source Hoffsetter	A typical landfill gas engine
Landfill Gas Flare	Landfill Gas Engine

Landfill Location Policy in the UK

Landfill Directive

The basic criteria for suitable landfill locations are set out in the Landfill Directive, which says in Annex 1:

1.1 The location of a landfill must take into consideration requirements relating to: (a) the distances from the boundary of the site to residential and recreation areas, waterways, water bodies and other agricultural or urban sites;
(b) the existence of groundwater, coastal water or nature protection zones in the area;
(c) the geological and hydrogeological conditions in the area;
(d) the risk of flooding, subsidence, landslides or avalanches on the site;
(e) the protection of the nature or cultural patrimony in the area.

1.2. The landfill can be authorised only if the characteristics of the site with respect to the aforementioned requirements, or the corrective measures to be taken, indicate that the landfill does not pose a serious environmental risk.

UK Implementation of Landfill Directive

The requirements of the Landfill Directive are brought into UK guidance via various pieces of legislation and EA guidance, notably:

• Environmental Permitting (England and Wales) Regulations 2010
• Regulatory Guidance Series, No LFD 1 - Understanding the Landfill Directive
• Environmental Permitting Guidance - The Landfill Directive - For the Environmental Permitting (England and Wales) Regulations 2010
• Groundwater protection: Principles and practice (GP3) as updated by The Environment Agency’s approach to groundwater protection

Source Protection Zones for Groundwater

In essence these policies rely upon a risk assessment of each proposed landfill location, set into the context of the site and whether or not it is within a Source Protection Zone (SPZ) for a protected groundwater abstraction or within a Major Aquifer (as defined by the EA).

Source Protection Zones are defined and determined by the EA as follows:

• SPZ1 - also known as the Inner Zone - is any land within 50 days (groundwater) travel time from a groundwater source, with a minimum radius of 50 metres;
• SPZ2 - also known as the Outer Zone - is any land within 400 days (groundwater) travel time from a groundwater source, with a minimum radius of 250 metres;
• SPZ3 - also known as the Total Catchment - is defined as the area around a source within which all groundwater recharge is presumed to be discharged at the source.

• below the water table in any strata where the groundwater provides an important contribution to river flow, or other sensitive receptors • within SPZ2 or 3 • on or in a principal aquifer

The locations of SPZs are shown on the EA "what's in my backyard" page of their website.

The relevant EA policy is as follows:

• The Environment Agency will normally object to any proposed landfill site in groundwater SPZ1.
• For all other proposed landfill site locations, a risk assessment must be conducted based on the nature and quantity of the wastes and the natural setting and properties of the location.
• Where this risk assessment demonstrates that active long-term site management is essential to prevent long-term groundwater pollution, the Environment Agency will object to sites:

Landfill Management

The management of a Landfill not only encompasses the engineering and and monitoring to ensure permit compliance, but also encompasses the receipt and placement of the waste while the site is open/operational. In general terms, the following day to day activities take place and are further detailed in WMP26B and ISWA's Guidelines:

• waste is received at the site and the vehicle and contents weighed
• all paperwork is checked to ensure that the received materials are acceptable for disposal at the site
• a visual check is made of the waste at the weighbridge to confirm that the material has been correctly described
• the vehicle is dispatched to the tipping area where its content are discharged
• site waste is pushed out into a working area in the operational/active landfill cell and the nature of the waste checked to confirm that it has been correctly consigned and is acceptable for disposal at the site prior to final placement and compaction
• the vehicle leaves the working area and is re-weighed at the weighbridge (empty) and an appropriate receipt given to the driver

The working area utilised on each day is kept to a minimum area and waste is tipped in relatively thin layers. This practice is maintained to minimise the area open to the atmosphere and the potential attraction to avian scavengers. At the end of every day a layer of suitable inert material is spread over that day's input to minimise wind-blown litter and prevent birds and rodents from accessing the tipped waste.

Site operations and waste placement are usually carried out with various items of fixed and mobile plant, such as:

• weighbridge and associated site office
• appropriate mobile plant to push the waste out into layers and provide compaction of the waste (this usually comprises bulldozer and/or wheeled compactor; these can be in multiple if the site has a large daily input)
• wheelwash to clean the wheels of road-going vehicles leaving the site to enter the public highway
• various ancillary equipment to maintain dust suppression, surface water control and to deal with pests and birds/scavengers

Landfill Restoration, Closure and Aftercare

Landfill sites are generally restored (finished) to green space, such as agriculture, forestry or (less often) public open space.

Overview

The final engineering structure is an engineered capping layer on top of the final layer of waste. Sub-soil and then top soil is placed on to the engineered capping layer, often termed the restoration levels or horizon. Landfill sites are usually restored progressively in phases as the waste mass reached final levels. The final restoration is usually defined in the Planning Permission for the site.

The engineered capping can be constructed in several ways and the design selected is evaluated in the same way as the landfill lining system, taking account of the various engineering constraints and impacts to be avoided at each site. The design also undergoes similar risk assessment to the lining system. The principle objectives of the engineering capping layer and restoration soils are:

• To contain landfill gas within the waste
• To repel/shed rainwater and surface water to reduce the production of Leachate
• To provide a barrier between the waste and the roots of vegetation growing in the soils
• To provide an effective growing medium and afteruse for the site

Commonly, the capping layer will consist of a compacted clay layer of 1 metre thickness, placed on the last levels of the waste (which is often a thicker layer of final cover to provide a uniform level for placement of the material). This capping layer can be replaced or supplemented by flexible membranes such as LLDPE (Linear Low Density Polyethylene) or GCL (Geo-synthetic Clay) in varying combinations. Flexible membranes can be laid over-lapped with no fused joints or can be overlapped and welded together similar to engineering lining systems.

Soil thickness will depend upon the proposed afteruse of the site; uses involving deep-rooted plants will require thicker soils to protect the engineered capping layer from root-penetration.

Gas and leachate extraction and monitoring equipment (such as monitoring well and pipework) commonly penetrates the capping and soil layers in order to afford access for maintenance and monitoring. Sometimes this equipment is progressively buried with suitable access points remaining at the surface; this is done principally for aesthetic reasons to improve the appearance of the final restored surface.

Landfill sites where all tipping has ceased and the all infrastructure for post-closure management has been installed are deemed "closed" following agreement with the EA.

The landfill operator is required to continue to maintain monitoring and management of the site post-closure until it can be shown to the satisfaction of the EA that the active management of the site by the operator is no longer required. Assessment of such a state is measured against criteria identified in WMP26A and subsequent EA guidance. It is generally accepted for non-hazardous landfill sites that this is unlikely to be able to shown for at least 60 years post-closure, probably somewhat longer. This period is known as the aftercare period.

There is published guidance on landfill permit surrender available from the EA.

Settlement and Surcharge

As the waste mass decomposes, the waste infill material settles under its own weight to occupy less volume. This process is known as settlement.

Settlement in landfilled waste occurs due to loading and other processes including chemical and microbial actions. These processes are time-dependent, and are controlled by factors such as leachate composition, pH, temperature and moisture. As a result, settlement in landfilled waste, especially long-term settlement, is complex and difficult to predict in both magnitude and timing. Notwithstanding that, reasonably accurate estimates are commonly used.

Landfill sites operate under the terms of a Planning Permission. Amongst the matters controlled by the permission will be the final levels to which the site must be restored. As the waste ages and decomposes the final level will settle to a lower level. In order to ensure that the agreed final levels are achieved after the majority of the decomposition and related settlement have finished, it is common to agree that the final levels can be exceeded initially - known as surcharging.

The degree of surcharging permitted is agreed as a Landfill Surcharge Allowance. This figure is usually quoted as a percentage which relates to the depth of the site at each point. The percentage agreed between the planning authority and the developer will vary according to the nature of the waste accepted, the depth of fill and the morphology of the landfill void.

It is conventional to have two sets of contours agreed with the planning authority - pre-settlement contours and post-settlement contours. The former represent the level to which the site can be surcharged, the latter represent the levels to which the site will settle. The post-settlement contours are sometimes also known as final levels.

Landfill Monitoring

Landfill sites are monitored to ensure that all activities are being undertaken in compliance with permit and other regulatory requirements. Furthermore, monitoring is undertaken to ensure that the site is performing as designed (such that if it is not performing remedial action can be considered) and other day to day impacts are managed to ensure that they are not unacceptable nor cause nuisance.

General Monitoring Requirements

Various aspects of the landfill are monitored, such as:

• Quantity and type of waste received including any assessments of suitability for acceptance at the site
• Daily activity and waste placement log
• Leachate volume and quality within the waste mass
• Leachate volume and quality extracted from the site and disposed
• Landfill gas quantity and quality within the waste mass
• Presence of landfill gas outside the site perimeter
• Emissions of landfill gas from the surface of the site
• Groundwater quality outside the perimeter of the site
• Surface water quality in nearby water bodies
• Noise emissions associated with the site
• Dust emissions associated with the site
• Odour emissions associated with the site
• Pest activity (e.g. birds, rodents) in, on and around the site
• Complaint records held at the site

Reasons for Leachate, groundwater and surface water monitoring

The specific reasons for leachate, groundwater and surface water monitoring at landfills are:

• to demonstrate that the landfill is performing as designed;
• to provide reassurance that leachate controls are preventing pollution of the environment (by reference to a pre-established baseline);
• to meet the control and monitoring requirements of legislation and in particular Regulations 14 and 15, as well as Schedule III, of the Landfill Regulations, this includes the requirement for control monitoring;
• to demonstrate compliance with the Groundwater Control and Trigger level requirements of Schedule 3 of the Landfill Regulations;
• to indicate whether further investigation is required and, where the risks are unacceptable, the need for measures to prevent, reduce or remove pollution by leachate;
• to identify when a site no longer presents a significant risk of pollution or harm to human health (to enable an application for a certificate of completion to be made, and thereby formally end the licensing or permitting process and the legal duty to monitor).

Dip Tape	Monitoring well head	Well sampling
Measurement Equipment	Well head for groundwater	Sampling from leachate well
Monitoring Multiple Gas Extraction wells	Monitoring single gas well head	Gas well extraction system
Monitoring Multiple Gas Extraction wells	Monitoring single gas well head	Gas well extraction system

There is considerable published guidance available on how, when and why to undertake this monitoring.