*4.1.4.2.1 First or bottom layer*

This is the liner system. Industrial waste landfills are mostly designed with doubleliner systems especially when they dispose hazardous waste (**Figure 13**), although some are built with a single-liner system (**Figure 14**); these are ideal for disposing C&DD wastes. The liner system acts as a barrier that isolates landfill contents from contact with the environment thereby preventing pollution. Common liner materials include clay, geomembranes, or flexible membrane liners (FML) made from plastic materials such as polyvinyl chloride (PVC) and high-density polyethylene (HDPE),

**Figure 13.**

*Block diagram of a double-liner system mostly used in hazardous waste landfills [33].*

#### **Figure 14.**

*Single-liner system used in C&DD and industrial solid waste landfills [33].*

geotextiles, geosynthetic clay liners (GCL), and geonets [33]. The liner material is applied at the bottom of landfill to prevent seepage of liquids. Sometimes when clay is used as liner, HDPE can be applied on top of the clay as reinforcement [31].

#### *4.1.4.2.2 Second layer*

This is the drainage system. The drainage system controls toxic liquid or leachate produced from decomposing waste materials combined with rainwater runoff or snow in the landfill. The drainage layer helps in draining leachate to avoid contact with the liner system, which may be corroded by the toxicity level of leachate [31]. Leachate should never seep past the liner layer because of contamination of groundwater and soil. In order to reduce the risk of contamination, the landfill is designed with perforated pipes on top of the liners to collect all leachates at the bottom from where it is channeled to leachate treatment plant, where it is treated and then reused [31]. Stormwater and snow which may not have mixed with decomposing matter and had seeped from the top surface into the landfill are also drained away in the drainage layer.

#### *4.1.4.2.3 Third layer*

This is the gas collection system. Just as toxic liquids are produced in landfill, toxic gases (usual biogas) are also released through natural decomposition processes [31]. Methane that is a major constituent of biogas is produced in landfills. Since methane is toxic and known to contribute to global warming, it is prevented from being released into the atmosphere where it causes public health damage, by the landfill design [31]. The design of the landfill is such that gas extraction pipes are fitted into the gas collection system, which entraps methane and transports it to the gas treatment plant for electricity and power generation [31].

#### *4.1.4.2.4 Fourth layer*

This is the topmost and largest layer of the landfill that contains the disposed waste itself. A compressor is used to reduce the size of waste disposed on daily basis to avoid occupying too much space [31]. Also, compact soil is applied on top of the compressed waste to keep away windblown debris, offensive odors produced within the landfill, and pests [31].
