LANDFILL TECHNICAL INFORMATION AND DESIGN
- Site Design
- Landfill Site Layout
- Landfill Layout Plan and Development Plan
- LANDFILL REHABILITATION PLAN AND END USE PLAN
- Surface Drainage Design
- Design for Sub-Surface Drainage
- Monitoring System Design
- Surface Water Sources
- Groundwater Sources
- Erosion Control Design
- Methane Collection System
The site design is a process which follows the site selection and investigation process. Finally, the disposal site design will be based on the outcome of the site investigation as well as the EIA process. One of the most important reasons for the disposal site design is to provide a cost effective, environmentally acceptable waste disposal facility. In case that the best available site, which was identified during the site selection process is not as good from the environmental or geo-hydrological point of view, the responsibility from the design would be to compensate for the shortcoming in the most appropriate way. It is distinguished into two different stages of design, which are conceptual and the technical design. Thus, this section will concentrate on the site layout and the technical design. The scope of work included the following:
i. Liner design and leachate management plan
ii. Sub-surface and surface drainage systems
iii. Capping and re-habitation design
iv. Permanent storm-water diversion and anti-erosion measures
Based on the site investigations and findings the new site was classified as a G: M: B + (South African Standards). The total area is about 100 000 m2. The total available air space is about 50 000 m2 X 3.12 m = 156 000 m3 and based on the calculations it is anticipated that the landfill will be operational for the next 25 years at least.
The recommended designs are as follows:
i. A lining system comprising of an average of 300 mm leachate collection layer, layers of compacted soil of low permeability to prevent leachate from migrating into the ground, a geo-synthetic clay liner (GCL) through which the landfill leachate should seep through and a 150mm base preparation layer
ii. Separation liner between the existing waste body and the new extension comprising of a 300 mm leachate collection layer, geo-synthetic clay liner, and a 200 mm slope preparation layer
iii. Sub-surface drainage network comprising of HO mm diameter perforated (high density polyethylene) HDPE
iv. Leachate collector pipes at 25m centres that drain into 200 mm diameter perforated HDPE pipes then tie to the existing leachate collection system
v. Surface drainage system consisting of earth berms, chutes and toe drains to prevent surface w'ater on top of the landfill from ponding
Landfill Site Layout
Some of the site infrastructure already exists, for example the road access, which is provided via the tarred road which connects with the Bulawayo Road. On the southern side the disposal site will have surface drainage and a storm water diversion drain, which is at the same time the bottom of the slope. Due to this the natural flow of the water will be used to develop the drainage system, whereby possible polluted water will be separated from unpolluted water. Leachate management is required and a leachate detecting system has to be implemented to make sure that leachate is detected as soon as it appears. Professional operation of a waste disposal site also requires a monitoring system for surface and groundwater, which will indicate possible pollution. Therefore, water sampling points and monitoring boreholes will be installed. It is recommended to establish one borehole at the top of the property which is the highest point and another borehole should be installed at the bottom of the future waste disposal facility. The site will be fenced; a gate and gate house will be installed as well. This will avoid unauthorized access to the site. The fencing will be 2m high. The gate will be locked after working hours. During evenings and at night security will be present. Administrative facilities will be implemented. Therefore, an office block will be built, which includes changing rooms for the staff working at the disposal site, toilets, and showers as well as one office for doing all the administrative work which is required at a waste disposal site. Currently there is no municipal piped water and electricity at the site. Either running water connection need to be provided to the area or water tanks for drinking purpose need to be established.
Landfill Layout Plan and Development Plan
The new waste disposal facility will be developed in phases, whereby each phase consists of one cell. Within the first phase of the waste disposal development, phase one will be established. This is the most southern phase, close to the access road and on the bottom of the slope. The first cell which is going to be developed within the first phase will be the most southern cell as well. An entrance road will be developed. The access to the road will be only available via the security gate, since the entire facility will be fenced. A service road inside the premises surrounds the area and 3 m wide fire break will surround the entire landfill site area.
Just after entering the site, the collection vehicle will enter the first cell by passing the office and ablution facility (to be constructed). Thereafter the truck will pass the installed weighbridge and the office where all the information regarding the truck, the amount and type of loaded waste are registered and maintained.
The storm water ablution pond will be opposite the main office. The location of the pond was calculated (details in the facility design) and based on the slope of the area. The soil which will be excavated within the first cell, to provide airspace for disposal, will be heaped close so as to use it for daily cover. This means the first cell will be excavated and prepared for disposal. While the first cell is in operation the second cell will be excavated. The landfill layout plan and development plan will then include the following things.
- • Infrastructure
- • Site access and drainage
- • Excavation and stockpiling of cover
- • Screening berms and screening vegetation.
- • Cell construction sequence
- • Deposition sequence and phases
LANDFILL REHABILITATION PLAN AND END USE PLAN
The intended end-use for the site is currently envisaged as open space. The domed shape should enable runoff to drain freely off the disposal site without penetrating beyond the topsoil.
Surface Drainage Design
It is proposed that surface water on top of the landfill needs to be diverted down by a system of earth berms, chutes, and channels. As shown in Figure 8.0, the earth berms are oriented in such a way that enhances flow of water. Any excess water from the earth berm will be diverted down by the chutes to the toe drains. The upslope cut-off drains must divert clean storm water around the site into the natural drainage system. The chute systems will have energy dissipaters downstream to protect downstream areas from erosion by reducing the velocity of flow. All drains should be maintained ensuring that they are not blocked by silt or vegetation. The trenches will have to be cleaned regularly because even small deposits of silt reduce the capacity of drainage.
Design for Sub-Surface Drainage
To ensure that no sub-surface leachate accumulate, the leachate should gravitate to leachate collector pipes that drain to a series of leachate drainage networks which lead to the existing sewer system. Based on the design calculations, 110 mm diameter perforated HDPE pipes at 25 mm centers are recommended for sub-surface drainage. The leachate should gravitate into 200 mm diameter perforated HDPE pipes then tie to the existing leachate collection system as shown in the detailed drawings. The new leachate collection system is to connect to the existing sewer system with manholes whose average invert level is 2.9 mm. A provision of inspection manholes has been made and they are well spaced at 75 mm.
Monitoring System Design
The boreholes that will be located in the proximity of the proposed site will be used for monitoring the system design.
Leachate Monitoring and Treatment
One of the most important problems associated with the design, operation, and longterm care of landfills is managing leachate that is formed when w'ater passes through the deposited waste. The leachate generated from municipal solid waste is a mixture of organic and inorganic, and dissolved and colloidal solids. It contains products of decomposition of organic materials and soluble ions which present a potential pollution problem for surface and groundwaters.
Leachate generation rates are primarily dependent on the amount of liquid the waste originally contained, a quantity of precipitation that enters the landfill through the cover or falls directly on the waste. Chemical character will be affected by the biological decomposition of biodegradable organic materials, chemical oxidation processes, and dissolving of organic and inorganic materials in the waste leachate’s chemical composition will change as the landfill goes through the various phases of decomposition similar to the changes in methane production.
Leachate Treatment and Disposal System
Leachate shall be removed from the drainage and collection system when the leachate level in the landfill interferes with landfill operations or when the unit is subject to assessment monitoring. The operator is responsible for the operation of a leachate management system designed to handle all leachate removed from the collection system. The leachate management system shall consist of any combination of storage, treatment, pre-treatment, and disposal options designed and constructed in compliance w'ith EMA requirements.
The leachate management system shall consist of any combination of multiple treatment and storage structures, to allow the management and disposal of leachate during routine maintenance and repairs.
Standards for on-site treatment and pre-treatment:
i. All on-site treatment or pretreatment systems shall be considered part of the facility
ii. The on-site treatment or pretreatment system shall be designed in accordance with the expected characteristics of the leachate. The design may include modifications to the system necessary to accommodate changing leachate characteristics
iii. The on-site treatment or pretreatment system shall be designed to function for the entire design period
iv. All of the facility’s unit operations, tanks, ponds, lagoons, and basins shall be designed and constructed with liners or containment structures to control seepage to groundwater. The ponds, lagoons, and basins shall be inspected prior to use for cracks and settling, and, if leachate is stored in them for more than 60 days, they shall be subject to groundwater monitoring pursuant to this Part
v. All treated effluent discharged to waters of the country shall meet the requirements of national standards for effluent emissions
vi. The treatment system shall be operated by a certified operator
Standards for leachate storage systems:
i. The leachate storage facility must be able to store a minimum of at least five days’ worth of accumulated leachate at the maximum generation rate used in designing the leachate drainage system in accordance. The minimum storage capacity may be built up over time and in stages, so long as the capacity for five consecutive days of accumulated leachate, during extreme precipitation conditions, is available at any time during the design period of the facility
ii. All leachate storage tanks shall be equipped with secondary containment systems equivalent to the protection provided by a clay liner having permeability no greater than 10 7 cm/s
iii. Leachate storage systems shall be fabricated from material compatible with the leachate expected to be generated and resistant to temperature extremes
Standards for Discharge to an Off-site Treatment Works
Leachate may be discharged to an off-site treatment works that meets the following requirements:
i. All discharges of effluent from the treatment works shall meet the EMA expected standards
ii. The treatment system shall be operated by an operator certified under the requirements
iii. No more than 50% of the average daily influent flow can be attributable to leachate from the solid waste disposal facility. Otherwise, the treatment works shall be considered a part of the solid waste disposal facility
iv. The operator is responsible for securing permission from the off-site treatment works for authority to discharge to the treatment works
v. All discharges to a treatment works shall meet the EMA requirements
vi. Pumps, meters, valves, and monitoring stations that control and monitor the flow of leachate from the unit and which are under the control of the operator shall be considered part of the facility and shall be accessible to the operator at all timesvii. Leachate shall be allowed to flow into the sewerage system at all times (subject to the town’s decision or strategy); however, if access to the treatment works is restricted or anticipated to be restricted for longer than five days, an alternative leachate management system shall be constructed in accordance with world standards
viii. Where leachate is not directly discharged into a sewage system, the operator shall provide storage capacity sufficient to transfer all leachate to an off-site treatment works. The storage system shall meet the national requirements
Representative samples of leachate shall be collected from each unit and tested quarterly. The frequency of testing may be changed to once per year for any monitored constituent, if it is not detected in the leachate for four consecutive quarters. However, if such a constituent is detected in the leachate, testing frequency shall return to a quarterly schedule and the constituent added to the groundwater monitoring program. In such a case, the testing frequency shall remain on a quarterly schedule until such time as the monitored constituent has remained undetected for four additional quarters. Leachate and discharges of leachate from units shall be monitored for constituents determined by the characteristics of the waste to be disposed of in the unit. They shall include, at a minimum:
iii. Any other constituents listed in the operator’s discharge permit
iv. All of the indicator constituents
v. The operator shall also monitor the leachate head within each unit
Time of Operation of the Leachate Management System
The operator shall collect and dispose of leachate for a minimum period of 5 years after closure until treatment is no longer necessary. Treatment is no longer necessary if the leachate constituents do not exceed the wastewater effluent standards (EMA standards).
If the results of testing of leachate samples in accordance with subsection above show that the leachate exceeds the limits for low risk waste as defined shall:
i. Notify the Agency in writing of this finding within 10 days following the finding
ii. Verify the exceedance by taking additional samples within 45 days after the initial observation
iii. Report the results of the verification sampling to the Agency within 60 days after the initial observation
iv. Determine the source of the exceedance, which may include, but not be limited to, the waste itself, natural phenomena, sampling or analysis errors, or an off-site source, within 90 days after the initial observation; and notify the Agency in writing of a confirmed exceedance and provide the rationale used in such a determination within ten days after the determination
If, as a result of further testing of the leachate and the background groundwater and analysis, it is determined that the facility leachate exceeds the set limits for low-risk waste, the facility shall no longer be subject to the low-risk waste landfill but hazardous and be subject to the requirements for hazardous waste landfills.
Surface Water Sources
According to the hydrogeological report, in the area of the chosen site, there is no significant surface water occurrence within a radius of O.l km of the proposed waste disposal facility. Although surface flow does occasionally occur along the non-perennial stream to the southeast of the study area, it is not utilized as a sustainable water source. This stream may, however, act as a transport path for liquid contaminants originating at the proposed facility. The site exhibits a high risk that liquid pollutants from the facility may reach important surface water sources. The site exhibits a slight risk that liquids moving laterally through the soil horizons may eventually emerge at the non-perennial stream to the southeast of the facility.
No boreholes were found to occur in the vicinity of the proposed site; however, boreholes will be dug around the landfill to monitor any groundwater contamination. In light of this, it is inferred that the groundwater exhibits a gradient roughly parallel to the regional topography. The site exhibits a high risk that liquids moving laterally through the soil horizons may reach groundwater sources.
Erosion Control Design
There are two types of erosion which need to be avoided: wind and water. In order to provide protection from water the outer slope of the waste disposal site will be provided with storm water channels to avoid possible contamination of storm water with the waste body. Therefore, the water will be navigated around the facility. The surface between the drains should be planted with indigenous vegetation to avoid further erosion via wind. Around the entire perimeter fence a 5 m width should be cleared of vegetation. This is necessary to provide a firebreak. The land operation should strictly practice daily covering of the waste.
Methane Collection System
Various processes take place within the buried waste resulting in generation of various end products. Bacteria in the landfill break down the trash in the absence of oxygen (anaerobic) because the landfill is airtight. A byproduct of this anaerobic breakdown is landfill gas, which contains approximately 50% methane and 50% carbon dioxide with small amounts of nitrogen and oxygen. The composition varies from one disposal site to another and is influenced by waste types. This presents a hazard because the methane can explode and/or burn. So, the landfill gas must be removed via a collection system. To do this, a series of pipes are embedded within the landfill to collect the gas.
The landfill gas represents a usable energy source. The methane can be extracted from the gas and used as fuel. In some developed countries, companies collect the landfill gas, extract the methane, and sell it to chemical companies to power their boilers. The extraction system is a split system, meaning that methane gas can go to the boilers and/or the methane flares that burn the gas. The reason for the split system is that in case of a landfill producing quantities of gases which exceed industrial demand, the gas can be burned into carbon dioxide, water, and other trace gases which are less potent than methane. Alternatively, the gas can be compressed into liquid and sold.
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