LANDFILL RECLAMATION BY EXCAVATION, SCREENING AND SEPARATION
by
Frank C. Ford, P.Eng.
Henderson Paddon & Associates Limited.
OWEN SOUND, Ontario N4K 2K8
INTRODUCTION
Landfill reclamation by excavation, screening and separation of components into "waste overs" and "fines" has been initiated in Ontario.
"Waste overs" comprise the coarse, separated waste components greater than 25mm, which are relandfilled.
The "fines" material comprises the cover material and minor waste components which pass through the 25mm screen. This material is mostly native cover material with some visible small waste components such as metal bottle caps, glass, small bits of plastic, paper, textiles, and minor organics.
This technology is relevant in the following circumstances:
to rehabilitate old polluting sites
to recapture waste space where inefficient/poor operation has maximized cover and minimized waste placement
to extract recyclables and possibly hazardous waste
to create landfill space where none existed previously, possibly without the need for environmental assessments connected with siting new landfills
Landfill reclamation with these methods has been carried out at the following sites:
Lancaster County, Pennsylvania Waste Management Department
- reclaiming the landfill provides supplementary carbonaceous material for feeding an incinerator with a constant waste stream in addition to the waste stream which is collected daily
Collier County, Florida
- relatively small site
Edinburgh, New York
In the last three years, several test screening sites have been carried out as follows:
County of Simcoe - Flos North Landfill Site
- to recapture inefficient landfill placement and rehabilitate landfill overfilling
County of Simcoe - Elmvale Landfill Site
- to recapture inefficient landfill placement and rehabilitate landfill overfilling
County of Simcoe - Essa Landfill Site
- to assist a slope remediation problem, possible bottom liner installation and to recapture lost landfill space due to inefficient past landfilling practices
Township of McDougall Landfill Site
- approximately 5.4 ha landfill screened in connection with a remediation project to stop a contaminant plume and provide appropriate liner systems
The latter site received funding from the Ontario Ministry of Environment and Energy as a prototype project to develop and document the screening technology in Ontario.
METHODOLOGY
The basic separation unit may be an inclined vibrating screen or a large diameter rotating trommel screen. Several proprietary types exist such as the inclined vibrating type, (Read Waste Manager, Power Screen) and the rotating trommel-type (Power Screen, RE-TECH, McCloskey Bros. Manufacturing), to name a few. In the McDougall project, a RE-TECH 723 (7' diameter x 23' long) trommel was purchased by the municipality and other equipment such as stackers (conveyors), dozers, loaders, compactors, excavators and off-road trucks were leased or rented.
The basic method involves pushing waste with a large dozer to a backhoe, which loads the waste from a stationary location into the screener grizzly or charging hopper of the screen. Conveyors take the waste overs and fines away from the screen machine, and a dozer(s) pushes the waste into stockpiles or relandfills same with associated compaction. Dozers, loaders, and/or trucks (off-road or on-road) convey fines to stockpiles or other desired locations on or off-site. Refer to the following Screening Schematic.
The exact size and type of equipment depends upon availability, configuration of the landfill, length of push of waste, number of setups for the screener, size of landfill, and production desired.
The push dozer(s) pushing raw waste to the backhoe is usually a D6 to D8 CAT size machine.
The backhoe, which is used to charge the screen, is approximately 1.2m3 size, and serves a key function as follows:
separates large material for recycling or relandfilling and to prevent undue shock or heavy loads on the screener
works quickly to keep screener at full capacity but not overloaded
may have to distribute materials evenly and at a controlled rate on input grizzly to prevent excess fines entering the overs component
Inclined vibrating screens are shaken with an eccentric, and as the waste moves across the screen, the overs fall off the top of the screen and are removed with a dozer or, alternatively, are conveyed to a waste pile via conveyor. The screened fines fall through the screen and are separated to the other side of the screen with a partition or, alternatively, are conveyed to a stockpile for removal.
Inclined, rotating trommel screens receive waste in the upper end, and continually bounce and rotate the waste and cover material. This action separates the fines from the overs. The fines fall through the screen to an underbelly conveyor where they are conveyed to a stockpile for removal. The waste overs go onto a second conveyor and are conveyed to a separate stockpile for removal.
ISSUES TO BE CONSIDERED BEFORE SCREENING
Several questions and factors should be considered before contemplating screening. These aspects are categorized under the headings Regulations and Work Plan, respectively.
Regulations
The following requirements should be considered before contemplating a screening process. These may include:
regulatory requirements
approvals
permits
mitigation measures
studies
Regulatory Requirements, Approvals, Permits
Depending upon legislative and government policy decisions, the excavation and screening may be considered part of the existing landfill Certificate of Approval or, alternatively, a new process which requires approval, and/or discretionary hearings, public consultation or formal studies and submission for approval.
Submissions for approval may require formal documents, demonstrating the plan of implementation, mitigative measures, or rehabilitation requirements to be carried out in association with the excavation and screening process.
Applications may have to be submitted in regard to provincial or municipal permits for carrying out the work at a specific site.
Specific approvals may be required in regard to rehabilitation measures such as liners, leachate capture, leachate collectors, pumping stations, trucking of leachate, leachate treatment or receipt of leachate at municipal sewage plants.
Mitigation Measures and Studies
It may be necessary to carry out formal studies and submit documentation in regard to such matters as visual screening, control of nuisance factors such as seagulls, odour, dust and noise. The work plan should be implemented to provide maximum mitigative measures to surrounding residents or development.
Studies may be required in regard to formulating a leachate management plan. Leachate wells may be installed to determine existing leachate levels or perched leachate conditions and leachate strengths. Boreholes will reveal soil conditions used as daily cover which may influence moisture content of the waste within the landfill.
Work Plan
Prior to embarking upon an excavation/screening process, several further matters require consideration as follows:
site history
site characteristics
a site development plan
materials handling plan
equipment requirements
a schedule
contingency plan(s)
staff requirements
cost considerations
safety procedures or additional requirements for equipment and on-site requirements
site trailers, water supplies, fire control, odour control, gas detection
Site History
Before screening an existing landfill, the following matters should be considered:
Was the site municipally operated or privately owned and operated?
Is there relative certainty that only municipal solid waste was landfilled at the site?
Was asbestos landfilled at the site and what precautionary measures are required if asbestos in bagged or loose form are encountered?
Are any known hazardous waste likely to be encountered at the site and what contingency plans are available for handling such waste?
Were liquid wastes of any kind landfilled in the site?
What contingency plans are in place if any of the above wastes are encountered?
Was municipal sewage treatment plant sludge landfilled at the site?
What is the leachate level within the landfill site and what plans are in place to pump/treat/dispose of such liquids if encountered?
Are any industrial wastes landfilled at the site which could make screening operations/excavation operations particularly difficult. Are industrial rubbers, belting, webbing (long, stringy materials) or extremely bulky materials likely to be encountered which will slow or make the operation difficult?
Were imported soils used for daily cover material which could affect the screening efficiency or progress of the work?
Site Characteristics
To date, only sandy and sandy silt sites have been screened, with only moderate amounts of perched leachate. Sites which have used fine tills for daily cover can probably be screened, but with less efficiency, and providing extremely wet waste or leachate mounds are not encountered within the sites. Sites which have used clay material for daily cover may not be amenable to screening due to the cohesive nature of the cover material, consequent poor separation of waste from overs, anticipated high moisture content, and possibly high leachate mounds within the site.
Boreholes will provide guidance on soil cover material and waste to cover ratios. If information in regard to the leachate mounding within a site is needed, leachate wells for water level measurement and possibly for leachate characteristic sampling should be installed prior to screening.
Site Development Plan
On a landfill of any size, or if space limitations are a factor, then a site development plan is of utmost importance to incorporate the following:
provide an efficient site operation when excavating and screening the site
to incorporate and consider materials handling problems
plan stockpiles
to best implement any mitigation procedures
to plan and include any remedial measures such as leachate capture or liners
to incorporate and prepare for future or existing landfilling operations
relandfill the waste overs
to incorporate any recycling measures to be implemented during the screening process
Materials Handling
Since screening operations are essentially a large materials handling problem, a plan is required to define within estimating limits what quantities of waste overs and fines are to be produced; are they to be relandfilled or disposed of elsewhere; where are stockpiles to be located; are stockpiles to be covered and are they, temporarily or permanently capped, and what final disposal methods are to be required?
The materials handling plan cannot be overemphasized, since the site can quickly become clogged with stockpiles of materials, perhaps resulting in expensive double handling, moving materials, or remixing of separated materials.
Screening Equipment Requirements
The key machine, i.e. the screener, can be rented, leased, or purchased by the owner who is undertaking the work. A cost benefit analysis will indicate which is the least cost or risk.
Screening machines are relatively expensive and, due to the nature of the work, may
be prone to accelerated wear and tear, breakdowns, or malfunctions. Since the technology is relatively new, every generation of screeners employ new modifications and updates. For instance, in the trommel type screens, the desirability of an overload sensor on the underbelly conveyor, which takes away the fines, may be desirable. A failure on the underbelly conveyer will cause fines to build up, stop the screen from rotating and plug the machine, after which a hand dig-out, which is an undesirable, manual and difficult job, is required.
In this regard, workers involved with supervising the operation or, alternatively, the backhoe operator charging the screen, must be alert to stop the loading and let potential plugs clear and to watch the waste and fines conveyors that material flow is not stopped.
Other Machines
Stackers (long conveyors) are usually desirable to get stockpiles away from the screening operation to prevent congestion near the screen and to allow clearing of fines and overs stockpiles. This equipment may be bought or leased, generally not from the screening manufacturer, and requires coordination with the proposed site set up.
Dozers, backhoes, off-road trucks or loaders, must be purchased, leased, or contracted for.
Compactors for compacting stockpiles or relandfilled waste must be obtained.
Contingency plans or allowances are required for machine breakdowns. Wear and tear on bearings, belts, or hydraulic failures can be anticipated. If machinery is contracted, or rented, standby time or down time should be defined in contracts with respect to payment.
Equipment Modifications
Air conditioning or fresh air blowers within an enclosed cab are desirable for equipment operators working on the screening process, for operator comfort and health and safety reasons.
Contingency Plans
It is recommended that a combustible gas indicator (methane meter) be available on-site, and used to check gas levels near the excavated face or other areas in case gas problems are encountered, and to provide documentation in regard to such findings.
Self-contained breathing apparatus should be available on-site in case noxious, extremely odorous, or hazardous materials are encountered. Cover clothing, suits and other safety apparatus should be available on-site in case of encounter with unknown or known hazardous waste material.
Suitable contingency and operating plans and instructions to operators should be provided with firm instructions on procedures in case of encountering unknown orhazardous material, liquid wastes, drums, etc.
Safety procedures should be written and posted, dealing with site operations, chain of command, emergency numbers and what to do in case of dizziness, nausea or other ill feeling. First aid kits must be available.
Schedule
Weather will affect equipment production and overall site efficiency. Heavy rainfall periods are not conducive to efficient screening and may cause shutdown. Screening production in winter conditions is not efficient. Ontario shutdown time is expected from December 15th to March 15th.
Production Rates
If typical 10 to 12-hour construction days (1 shift) are contemplated, actual screening time may average 80%, with the other time being devoted to maintenance, breakdowns, breaks, moving screen location or other site considerations.
Experience indicates that production rates, on average, will be 80m3 to 200m3/hour,of in-place waste and cover, depending upon the site, equipment haul distances and procedures. The higher production figure might be expected in sites which have waste to cover ratios of approximately 1:4, with the lower production rates where waste to cover ratios are approximately 1:1. The above figures also apply to sites covered with sand, silty sand, or sandy silt material, as opposed to silt or clayey silt, or extremely wet sites.
Experience to date in Ontario indicates that ratios of waste to cover vary from 1:1 to as low as 1:4. The very low waste to cover ratios are indicative of inefficient operations of small municipal sites where daily cover and interim cover were used excessively, or the rate of waste input at the site did not allow efficient use of cover material.
Odour Control
At most sites, no odour control equipment or chemicals have been used in the screening process. At the McDougall Township site, periodic fogging of the working face, where waste is being excavated or where the overs are stockpiled, is carried out with spray fogging nozzles utilizing the chemical "Ecosorb". This chemical reacts with hydrogen sulphide and mercaptans to destroy the odour-producing compounds and has worked relatively effectively. Other odour control chemicals are available.
Odours are most problematic when screening relatively fresh garbage or relatively wet waste. Still humid days exacerbate the odour problems. Fresh air blowers and air conditioning in equipment cabs maximize operator comfort in this regard.
Quality of Fines
The quality of fines and the ultimate disposition of such material must be addressed. At the time of writing, Ontario regulators have adopted the conservative position that existing legislation (Regulation 347 under the EPA Act) states that material not defined as inert fill (earth or rock fill or waste of a similar nature that contains no putrescible materials or soluble or decomposable chemical substances) is defined as waste. However, a special Policy Committee is considering the matter to determine whether the material can be defined and used under some other category. For instance, guidelines exist for compost; excess soil guidelines (MOEE Proposed Policy for Management of Excess Soil, Rock and Like Materials, August 1992); and Schedule 4 - Leachate Extraction Tests Under Regulation 347 to define material as hazardous, designated substances or non-registerable waste; or dredge material.
Aside from Schedule 4 defining a waste, other guidelines pertain to potential off-site usage of the fines at locations other than landfill sites.
Even if fines are designated under new regulations for use only on landfill sites, considerable flexibility will still exist for the following alternatives:
use as daily cover for future landfilling on-site or at other landfill sites
stockpiles on-site, with or without clay caps and vegetated
In County or Regional jurisdictions, where some sites are short of daily cover materials, these may provide ideal outlets for excess screened fines, and would still comply with the use of fines material defined under regulations as "waste". It is suggested that a leach test (Schedule 4, Regulation 347) be conducted to determine if any compounds, which may leach from the fines piles with time, are problematic for the site. Clay capping and vegetation may reduce the infiltration rates and leachability to the extent that environmental compliance is not a problem. As a generalization, attenuation type sites, which are not presently a problem, will not likely be a problem with stockpiled fines adjacent to the "landfill footprint" (area where waste is licensed to be landfilled), providing stockpiles are not too close to buffer zones or other properties. However, if the purpose of excavation and screening of the site is for rehabilitation purposes, caused by migrating contaminant plumes off-site, then the location of fines stockpiles may be more of an issue and must be carefully considered.
Appendix "A" contains data in regard to quality of the screened fines from various sites to give the reader some appreciation of the contaminants within the materials.
Aside from reuse on landfill sites, the fines material has the most likelihood to be reused in "urban industrial fills" where water and sewage system servicing is available, fills are limited, and the material is covered with other clean material. Use as road fills may also be permissible in certain circumstances.
Other Properties of Screened Fines
Grain size analyses of screened fines closely parallel the native material used as cover material, as expected. However, two precautionary aspects are noted:
Fines should be used/stored above water table elevation.
Due to the organic component of the material, higher moisture contents may be present, and it may be found that the material, especially at higher moisture contents may become more slick, less workable, and less compactable than the native soil material.
Ignition tests, as expected, indicate higher loss of organic material than native soil material and higher axial strain for a given compressive stress, which may accelerate or exaggerate settlement of the material over time. To minimize this aspect, if settlement under a liner, for instance, were of concern, the use of fines as fills should be limited to approximately 1m, or, as a minimum, to provide uniform depth of fill to prevent differential settlements.
Experience with fines at the McDougall site resulted in removal of some of the fines and subsequent replacement with native soil material, which would "bridge" soft spots. This action was required to prevent "cracking" due to flexing of the fines material under a proposed clay liner. This seemed to indicate some "sponginess" in the fines material when applied too deep, resulting in compression on compaction and some rebound after the compactor passed.
Use of the fines material as topsoil, particularly if used with fertilizer applications or if mixed with sewage sludge, grows medium to excellent vegetation cover. However, use of the material in this way may depend upon regulatory agencies, the proposed locations, and after uses. Obviously, use of the material on future playgrounds would be unsuitable if the fines contain pieces of glass, nails, bottle caps or other foreign material.
Costs
Costs of excavation and screening are anticipated to be $4.50 to $7.00/m3 of in-place waste material, including removal of existing final cap, and relandfilling and compacting old waste, recapping, topsoiling and vegetating the waste mound. Typical equipment rates are assumed as follows:
D8 dozer - $90/hr
Backhoe - $100/hr
D6 dozer - $60/hr
D4 dozer - $50/hr
Screener - $100/hr
Stackers - $30/hr
Supervisory and labouring staff - $150/hr
Obviously, costs will depend upon the production rates achieved which, in turn, are dependent upon site variables and production efficiency. Costs generally anticipate one screener. Some economies of scale could be achieved if two or more screeners were working on the same site. The above costs do not consider remediation initiatives such as liners, purge wells, or other leachate management requirements.
