15.6.3. landfill design - cesnet hazardous waste site eia 200916br292...a constant head triaxial...

Revised Final Environmental Impact Report for the Proposed Regional General and Hazardous Waste

Management Facility in the Eastern Cape

Ch 15 – Preliminary Design July 2010 270

15.6.3. Landfill Design

Site development

The GHWMF will be developed in four phases to provide a 20 year site life, given

current waste stream predictions. Each phase will provide approximately five

years of site life. Table 15.1 below indicates the airspace provided by each phase:

Table 15.1: Airspace provided by phases

Phase Airspace provided [m3]

Phase 1 1’230’000 m3

Phase 2 1’290’000 m3

Phase 3 1’260’000 m3

Phase 4 1’220’000 m3

Total 5’000’000 m3

Following the initial development of Phase 1 and its associated infrastructure, the

development of three subsequent phases with additional drainage systems is

planned; the development of a leachate treatment plant is likely; while the need

for and financial viability of a landfill gas extraction and utilisation or destruction

facility could be assessed.

Given that there are currently unknowns regarding the industries that will

establish in the IDZ as well as start up times, actual waste streams may vary

considerably from current predictions. It is therefore necessary that regular data,

calculation and planning updates be undertaken for further site development.

Site geometry – Phase 1

The site is located in a valley with steep slopes. The base of the valley heads in a

south eastern direction at an average gradient of 1:35. The south facing slope

has an average gradient of 1:30 and the east facing slope has an average

gradient of 1:20. Phase 1 has been shaped to allow upslope runoff to be diverted

around the phase, and to appear as natural as possible.

Basin design – Phase 1

The basin of Phase 1 is formed by two sloping planes that mimic the natural

topography. The depth of the phase is restrained by the shallow calcrete profile

and the slopes of the basin. The depth of the basin at excavation level varies from

2.0m below ground level at its lowest excavation level to 0.17m above ground at

its highest section of fill required to keep the constant slope. The slope of the

sidewalls of the basin is 1:3.




The south facing slope has a gradient of 1:40 and the east facing slope a gradient

of 1:35. The different slopes coincide at the middle of the basin, in which a

leachate collection trench is to be constructed that bisects the basin. The trench is

designed to be 1m below the western edge while the depth of the eastern edge

varies from 2.14m to 0.7m. The slope of the trench side walls is 1:2.

Liner design

The material properties required for a clay liner, in terms of the Minimum

Requirements, are as follows:

Plasticity Index (PI) greater than 10%.

No particles larger than 25mm.

Gravel size fraction must not exceed 10%.

Saturated permeability must be less than 1 x 10-7 cm/sec for H:H landfills.

A constant head triaxial permeability test was carried out on a sample from depth

2.2m in a test pit on site, which is representative of the siltstone material on site.

The permeability stabilised at 1.2 x 10-9cm/s following close to 26 days of testing,

which is almost two orders of magnitude lower than required, necessary to allow

for laboratory testing/ field performance discrepancies.

From the geotechnical testing carried out, the plasticity index of the sample was

27%, the maximum particle size was less than 4.75mm, and the gravel fraction

was less than 1%. The material is therefore suitable for lining material. For

detailed test results, the reader is referred to Jones & Wagener Report number

JW15/08/B494, dated February 2008.

The landfill liner has been designed to meet the Minimum Requirements

specifications, incorporating a double lined system of which the primary liner

comprises a composite geosynthetic and clay liner, and the secondary liner

comprises a conventional clay liner. A detail of the landfill liner is included on

Drawing B494-00-007 of the Design Report. Starting from the waste body, the

liner system is formed from the following in descending order:

Waste body;

Separation geotextile;

300mm layer of 53mm stone;

Protection geotextile (minimum weight 1,5kg/m2);

2mm high density polyethylene (HDPE) geomembrane liner;

4 x 150mm thick layers of selected clay compacted to minimum 98% MOD

AAHSTO at optimum moisture content (omc) to omc + 2%;


150 mm layer of coarse sand (with particles less than 3mm in diameter);




2 x 150mm thick layers of selected clay compacted to minimum 98% MOD

AAHSTO at optimum moisture content (omc) to omc + 2%;and

In situ ripped and re-compacted layer.

15.6.4. Drainage Systems

Site liquids balance

The liquid balance on a landfill site is dependent on incident precipitation,

evaporation, transpiration, the moisture content of incoming waste streams,

liquid by-products from decomposition processes within the waste body, and on

any other liquids entering or leaving the site.

Typically, little vegetation is present on the waste body itself prior to capping,

liquid by-products from decomposition processes within the waste body are of low

volume, and additional sources of water should not enter the waste body if the

site is properly designed, constructed and operated, so that these factors are

ignored.

The liquid balance is usually used to estimate the size of the contaminated water

and leachate dams required, as well as required take out and treatment rates. A

conceptual site-specific liquids balance has been developed for the Coega regional

hazardous waste management facility, using average rainfall and evaporation

data from the Aloes H:H landfill, on a monthly basis. This was used in the sizing

of the contaminated water dam, as well as in determining the average take out

rate required to keep volumes acceptable. The dam was also sized by estimating

the storage required for runoff from the 1 in 100 year recurrence 24 hour

duration storm event.

Upslope runoff drainage and management

Upslope runoff is diverted around the waste body by a system of storm water

diversion trenches. The system consists of temporary measures and permanent

measures. The temporary trenches serve individual phases and are later covered

by subsequent phases. The permanent measures will service the entire waste

body. The progression of required stormwater drainage measures as the phases

are developed is shown on Drawing B494-00-008 of the Design Report. T

The drains were sized using the rational method. The mean annual precipitation

was estimated to be the same as at the Aloes Waste Facility at 560mm per

annum. The uncontaminated storm water drains into the environment

downstream of the waste facility. A typical detail of the uncontaminated runoff

diversion drains is included on Drawing B494-00-007 of the Design Report.




15.6.5. Contaminated water drainage and management

Contaminated water drainage

The contaminated storm water drainage system also incorporates a similar trench

system to the upslope management system. The drains are smaller as the

catchment area consists of the waste body only. The progression of required

storm water drainage measures as the phases are developed is shown on

Drawing B494-00-008 of the Design Report. A typical detail of the contaminated

water drains is included on Drawing B494-00-007 of the Design Report. The

confluence of the system is in the south eastern corner of the site where the

diversion trenches meet in a drop inlet formed from manhole rings. The drop inlet

leads to a pipe that connects the drainage system to the contaminated storm

water dam south of the site (refer to Figure 15.3).

Storm water dam

The 1 in 100 year 24 hour duration storm event was used to size the storm water

dam, by calculating the storage required for rain falling on the area of various

phases. The most severe case was found to be rain falling on the site once the

fourth phase was being constructed. However, it was assumed that partial

capping would mitigate the amount of storm water being contaminated.

Therefore, the dam was sized for the next severe case of the storm falling over

the site during the construction of the third phase. The required storage volume

was 24 000 m3. The dam has been sized at approximately 32 000m3, to allow for

some contaminated water to be held in storage, and still allow for the

containment of the expected runoff from the 1 in 100 year 24 hour duration

storm event. The contaminated storm water dam has also been sized with an

expected take out of 2 300m3 per month, for dust suppression, to maintain the

levels as low as possible in the dam. The dam will therefore require active

management. A plan of the dam is shown on Drawing B494-00-010 and the

cross-sections shown on Drawing B494-00-011 of the Design Report. A detail of

the storm water dam liner is included on Drawing B494-00-007 of the Design

Report. The liner for the contaminated storm water dam incorporates a single

composite geosynthetic and clay liner as described below starting at the surface

of the dam:

200 mm thick layer of <3 mm sand stabilised with 5% cement 1 32.5;

Protection geotextile;

1.5 mm high density polyethylene (HDPE) geomembrane liner;

2 x 150 mm thick layers of selected clay compacted to minimum 98% MOD

AAHSTO at optimum moisture content (omc) to omc + 2%; and


Revised Final Environmental Impact Report for the Proposed Regional General and Hazardous Waste Management Facility in the Eastern Cape


Figure 15.3: Preliminary leachate and storm water dam layout for the GHWMF




It is noted that this liner does not strictly comply with the specifications given in

the Minimum Requirements for Waste Disposal by Landfill third edition draft, but

is based on the successful use of lesser liners at several other hazardous waste

management facilities.

15.6.6. Leachate Drainage

The leachate drainage system is divided into the leachate collection system and

the leakage detection system. The leachate collection system is located above

the primary liner and its functions are to drain leachate out of the landfill before it

penetrates the liner and prevent excessive volumes of leachate being stored in

the waste body. The leakage detection system is constructed between the

primary and secondary clay liners. The purpose of this system is to detect if

leachate has penetrated the primary lining system, to allow measurement of flow

in the leakage detection system, and to prevent a build up of hydrostatic head on

the secondary clay liner. The leachate collection system will comprise a 300mm

thick 53mm stone layer, containing perforated pipes, on the landfill footprint.

Solid pipes will penetrate the side walls, and allow leachate to drain from the

landfill phases into leachate collection manholes and into a solid leachate collector

pipeline.

The leakage detection system will comprise a 150mm thick coarse sand layer,

including smaller perforated pipes. Again, solid pipes will penetrate the side walls,

and allow any leakage to drain from the landfill phases into leakage detection

manholes and into a solid leakage pipeline. Both systems will be extended as

future phases are developed (see proposed layout on Drawing B494-00-009 of

the Design Report). The confluence of the leachate collection and the leakage

detection systems is located in the south eastern corner of the site where the

pipes of each system meet in one large sump which will have one outlet pipe

leading to the leachate dam (refer to Figure 15.4).

Leachate Dam

Given uncertainties in the future waste streams from the Coega IDZ, the sizing of

the leachate dam was not based on the site liquids balance. Instead, this was

sized by assuming that a maximum co-disposal ratio of 1 part liquid to 6 parts

solid by mass was used, and that 20% of the liquids in the waste percolate out as

leachate, from experience on similar sites. This calculation was cross-checked by

assuming that 20% of expected liquid wastes and 10% of all expected sludges

from the Coega IDZ as well as 20% of the existing waste stream to Aloes would

percolate out as leachate. The latter figure was lower than the former, as the

currently predicted co-disposal ratio for the Coega regional and hazardous waste

disposal facility is less than 1 in 6.




The calculation resulted in a smaller storage requirement than for the storm

water dam, in the order of 24’000m3. Further leachate storage dams are planned

for construction if needed after Phase 1; these are shown on the plans

downstream of the current dam.

A plan of the dam is shown on Drawing B494-00-010 and cross-sections shown

on Drawing B494-00-011 of the Design Report. The liner for the leachate storage

dam is designed according to Minimum Requirements, incorporating a double

composite geosynthetic and clay liner as specified for hazardous lagoons. A detail

of the leachate dam liner is included on Drawing B494-00-007 of the Design

Report. The liner system is described below starting at the surface of the dam:

200 mm thick layer of <3 mm sand stabilised with 5% cement 1 32.5;

Protection geotextile (minimum weight 1,5 kg/m2);

2 mm high density polyethylene (HDPE) geomembrane liner;


AAHSTO at optimum moisture content (omc) to omc + 2%;


150 mm layer of coarse sand (with particles less than 3 mm in diameter) for

leakage detection system;

Protection geotextile (minimum weight 1,5 kg/m2);

1.5 mm high density polyethylene (HDPE) geomembrane liner;


AAHSTO at optimum moisture content (omc) to omc + 2%; and


Leachate Treatment

It is impractical to contain the leachate that could be generated from the Site

over a 20 year site life. The design has therefore included a leachate dam based

on the first three years’ expected waste stream, taking average rainfall and

evaporation into account. The intention is that monitoring of leachate quality and

quantity will take place during the first year of operation, treatability tests, design

and pilot treatment studies will take place in the second year, and the leachate

treatment plant will be constructed and commissioned by the middle of the third

year of operation. It would be an advantage to design a modular system that can

easily be expanded as the need arises, given current uncertainties in the

expected waste stream composition. The leachate dam would then continue to

provide sufficient buffer capacity to accommodate seasonal peaks in order to

match the rate of treatment.



Figure 15.4: Preliminary leachate collection and drainage plan for the GHWMF



Chapter 15 – Preliminary Design July 2010 278

Future leachate dams could be constructed on site, but it would be preferable to

refrain from storing significant volumes of leachate on site, as this would increase

environmental risk, contribute to site odour and have significant associated costs.

It is anticipated that the leachate quality will be reasonably similar to that

generated at Aloes, with a high Total Dissolved Solids (TDS) count of 40 000 to

50 000 mg/l. The TDS will predominantly be made up of Chloride and Sodium.

The Fishwater Flats Sewage Works will not accept such highly saline waste and

therefore direct discharge to sewer will not be an option. In addition, as no sewer

connection is available close to the proposed Regional WMF it would mean

transporting the leachate to the closest sewer connection which could be 20km

away. The recommended options for the GHWMF would be to treat the leachate

to a standard that will allow discharge directly into the environment or for re-use

on site for dust control, for irrigation of vegetated areas and for fire-water. The

quality of water to be discharged should comply with the quality objectives set by

DWEA for the catchment and should ensure that the downstream water remain fit

for the purposes it is used for. This aspect should be agreed with DWEA.

Likely steps in a treatment process could include the following (Ardeer, personal

communication with T. Hopkins of J&W, April 2004):

Pre-treatment (pH adjustment and removal of suspended solids);

Ultra filtration;

High pressure reverse osmosis step; and

Low pressure reverse osmosis step.

Should the leachate at the Regional WMF contain a higher organic content, an

additional step to address this component, e.g. biological treatment, would be

needed. The anticipated treatment technologies listed above are well established

technologies that are currently used for the treatment of waste water and which

will ensure that the required discharge standard is achieved. An accurate

prediction of leachate generation rates is not possible at this stage, as it will

depend on many factors, including the stage of landfill development, cell

geometry, co-disposal ratio, operational controls and waste characteristics.

Ideally, the cell should be operated for a period before determining the size and

design of the treatment facility required.

15.6.7. Landfill Gas Management Systems

LFG can be managed by passive or active venting from a landfill, depending on

the quantities generated. The design has included for the installation of

horizontal LFG collection wells, to be installed during the operation at

approximately 10m vertical and 30m horizontal spacing, see proposed layout and



Chapter 15 – Preliminary Design July 2010 279

typical horizontal well detail on Drawing B494-00-014 of the Design Report (refer

to Figure 15.5 overleaf). These wells are relatively uncomplicated to install, and

are to be installed by the operator on an ongoing basis. Gas collection in the wells

is to be monitored, and if necessary and/or financially viable, the installation of

an active extraction system and gas flare or utilisation system is to be

considered.

Given that a bed of waste of 10m thick is required prior to the installation of

these wells, it is likely to be in the region of 2 years before the wells can be

installed, and probably 4 years before suction can commence. If installed, the

gas management system will need to be carefully managed, as per the

instructions of the design engineers.



Figure 15.5: Conceptual gas extraction layout for the GHWMF

15.6.3. landfill design - cesnet hazardous waste site eia 200916br292...a constant head triaxial...

Documents