M2M

Proper design of a landfill is very important for the successful operation of the landfill disposal facility. All technological alternatives for construction of landfills must be reviewed prior to their incorporation into the design. The design of the landfill should be capable of accepting the given amount of solid waste material for disposal. To serve as a basis for design, the types and quantities of all refuse expected to be disposed of at the landfill should be determined and analyzed by survey and analysis.

The design of the sanitary landfill should be such that it should not adversely affect the health and safety of nearby inhabitants, and in general should prohibit the following:

S.No	Place	Minimum Siting Distance
1	Habitation	500 meters
2	Water bodies/ Rivers/ Lakes	200 meters
3	Non meandering (canal, drainage etc)	30 meters
4	Highway/ Railway line	300 meters from the center line
5	Public park	No landfill would be permitted within 300 meters of public park
6	Water supply well	No landfill should be constructed within 500 meters any water supply wells
7	Critical habitat area	Not permitted
8	Wetland	Not permitted
9	Ground water table	A landfill would not be constructed in areas where water table is less than 2 meters below ground surface
10	Coastal Regulation Zone	No landfill site would be permitted in this zone
11	Earthquake Zone	500 meters from fault fracture
12	Flood prone Area	Not permitted
13	Airports	No landfill would be constructed within the prescribed limits by regulatory agencies from time to time
14	Unstable zone	Not permitted in unstable areas such as landslide prone areas and fault zones etc.
15	Buffer zone	A landfill should have a buffer zone around it, up to a distance prescribed by regulatory authority
Source: Central Pollution Control Board

The preparation of land for sanitary landfill is carried out by placing 60 cm compacted solid liner, flexible HDPE (high density poly ethylene) geo-membrane liner, geo-textile liner, 30cm drainage material layer, 60 cm protective layer. The provisions for gas collection (through 1.25 cm diameter perforated poly vinyl chloride pipe) and leachate collection (15 cm diameter slotted HDPE pipe) are also made during the preparation of the landfill site. The solid wastes are landfilled by spreading thin layers, compacted to the smallest volume and covering it each day or periodically with some suitable substitute material in a way that minimizes environmental problems. Successive layers are built up until a depth of 10–12 feet (304–365 cm) is achieved. Finally, it is covered with 60 cm of soil layer for final closure.

During the landfill procedure, at least 40% moisture is maintained to achieve maximum microbial degradation. Periodically the leachate collection in the bottom is pumped out to drying beds which are specially prepared for this purpose. Due to scientific landfilling, the maturity is achieved faster and hence gas collection starts even during the landfill procedure. The gas generation and complete extraction can be achieved even after closure of landfill (upto 10 years). This is faster than the ordinary landfill where gas extraction continues even upto 50 years. The figure below shows how, after final soil cover and maturing, the combustible gas can be used for generating power while leachate removal and treatments are carried out simultaneously. Compost retrieval is an optional choice depending on site condition and commercial feasibility.

The volume of waste to be placed in a landfill is computed for the ‘active' period (it ranges from 10-25 years depending upon the availability of land area) of the landfill taking into account the following aspects:

(b) The anticipated increase in the rate of waste generation on the basis of past records or population growth rate.

The required landfill capacity is significantly greater than the waste volume it accommodates. The actual capacity of the landfill depends upon the volume occupied by the liner system and the cover material as well as the compacted density of the waste. In addition to this, the amount of settlement a waste will undergo due to overburden stress and due to biodegradation is also taken into account.

The density of waste varies on account of large variations in waste composition, degree of compaction and state of decomposition. Densities may range as low as 0.40 t/cu.m. to 1.25 t/cu.m. For planning purposes, a density of 0.85 t/cu.m. is usually adopted for biodegradable wastes and typically 1.1 t/cu.m. for inert waste.

Settlement of the waste mass beneath the final cover will inevitably occur as a result of the consolidation of waste within a landfill site. Initial settlement occurs predominantly because of the physical rearrangements of the waste material after it is first placed in the landfill. Later settlement mainly results from biodegradation of the waste, which in turn leads to further physical settlement. Accurate prediction of settlement is difficult because time related settlement data are not readily available.

The total landfill area is approximately 15% more than the area required for landfilling to accommodate all infrastructure and support facilities as well as to allow the formation of a green belt around the landfill. There is no standard method for classifying landfills by their capacity. However the following nomenclature is observed in literature:

Open waste dumps lead to leachate seepage into the ground. Leachate is a liquid generated as a result of percolation of water or other liquid through landfilled waste, and compression of the waste as the weight of overlying materials increases. It is a contaminated liquid, since it contains many dissolved and suspended materials. Good management techniques, which can limit adverse impact of leachate on ground and surface water, are control of leachate production and discharge from a landfill, and collection of the leachate with treatment and disposal. The figure 2 &3 below shows the contamination of ground water due to leachate percolation into the ground.

The major components of the MSW landfill site (also shown in figure: 4) are listed below:

A Liner System: It is at the base and sides of the landfill, which prevents migration of leachate or gas to the surrounding soil.
A Leachate Collection and Control Facility : It collects and extracts leachate from within and from the base of the landfill and then treats the leachate.
A Gas Collection and Control Facility: It collects and extracts gas from within and from the top of the landfill and then treat it and use it for energy recovery.
A Final Cover System: It is at the top of the landfill, which enhances the surface drainage, prevents infiltration of water and supports surface vegetation.
A Surface Water Drainage System: It collects and removes all surface runoff from the landfill site.
An Environmental Monitoring System : It periodically collects and analyses air, surface water, soil gas and ground water samples around the landfill site.
A Closure and Post-Closure Plan: It lists the steps that must be taken to close and secure a landfill site once the filling operation has been completed and the activities for long term monitoring, operation and maintenance of the completed landfill.

The composition of leachate will determine the effect it will have on the quality of nearby surface water and ground water. Specific contaminants carried in leachate vary, depending on what is in the solid waste, and on the simultaneous physical, chemical, and biological processes occurring within the landfill.

The chemical and biological characteristics of leachate depend on constituents found in the solid waste, the age of the landfill, degree of compaction, and climatological conditions, which includes ambient temperature and rainfall. Young landfills, generally those less than 5 years old, produce leachate with a high organic content made up primarily of fatty acids. Leachate from older landfills may have only 10% organic content, which will be predominantly humic and fulvic acid.

The fundamental approach in controlling leachate is to confine leachate within the limits of the landfill, then collect it and dispose it safely. For successful collection techniques an impermeable soil barrier or artificial liner must be used to confine the leachate within the landfill and to deliver it safely to the disposal site. The most common type of collection system utilizes gravity drainage and consists of a layer of sand or gravel with perforated pipes that carry leachate to the collection point. The location and placement of the pipes are very important for collection of leachate. In small landfills, all manholes and collection pipes can be placed on the edge of excavation where the depth of the waste is thinnest.

Before finalizing the treatment plan for leachate, it is necessary that the quality and quantity of the leachate should be studied. The major factors affecting leachate quality are the composition of the refuse, age of the landfill, depth upto which the refuse is disposed, permeability of the refuse, ambient temperature, availability of moisture from the surrounding environment, available oxygen level from the refuse.

Leachate control within a landfill involves the prevention of migration of leachate from landfill sides and landfill base to the subsoil by a suitable liner system and drainage of leachate collected at the base of a landfill to the sides of the landfill and removal of the leachate from within the landfill.

Liner systems comprise of a combination of leachate drainage and collection layer(s) and barrier layer(s). A competent liner system should have low permeability, should be durable and should be resistant to chemical attack, puncture and rupture. A liner system may comprise of a combination of barrier materials such as natural clays, amended soils and flexible geomembranes.

5.2.1 Single Liner System : This system comprises of a single primary barrier overlain by a leachate collection system with an appropriate separation/protection layer. A system of this type is used for a low vulnerability landfill .

5.2.2 Single Composite Liner System: A composite liner comprises of two barriers, made of different materials, placed in intimate contact with each other to provide a beneficial combined effect of both the barriers. Usually a flexible geomembrane is placed over a clay or amended soil barrier. A leachate collection system is placed over the composite barrier. Single composite liner system is often the minimum specified liner system for non-hazardous wastes such as MSW.

5.2.3 Double Liner System: In a double liner system a single liner system is placed twice, one beneath the other. The top barrier (called the primary barrier) is overlaid by a leachate collection system. Beneath the primary barrier, another leachate collection system (often called the leak detection layer) is placed followed by a second barrier (the secondary barrier). This type of system offers double safety and is often used beneath industrial waste landfills. It allows the monitoring of any seepage which may escape the primary barrier layer.

The minimization and containment of leachate within a landfill ultimately depends on the design of the landfill. Therefore it is important that an impervious cover should be provided to stop leachate percolation into the ground. Figure 7 below shows old landfill with a cover on the waste. Figure 8 shows landfill covering waste with a cut-off wall.

Table: 5.2Types of Cover Systems
Type	Characteristics
Cover system A	It is the most impervious system and is mainly used for hazardous landfills. It is also recommended for MSW landfills for better gas recovery. (Refer figure:9)
Cover system B	It is less impervious than cover system A and is mainly recommended for MSW landfills. (Refer figure:10)
Cover system C	This cover system doesn't have low permeability and is suitable for waste that has very low potential for contamination. (Refer figure:11)

Fig: 9 Cover System A

Fig: 10 Cover System B

Fig: 11 Cover System C

Gas generated from some landfills is negligible, but landfills are expected to generate a significant quantity of gas. The quality of gas depends mainly on the type of solid waste. As with leachate, the quality and quantity of landfill gas both vary with time. Landfill gases, specifically methane gas, are natural by products of anaerobic microbial activity in the landfill. The quality of gas varies with time. The quantity of gas generated depends on waste volume, waste composition and on the time since the waste is deposited in the landfill. Gas production from the landfill can be increased by adding nutrients, such as sewage sludge or agricultural waste and by removing bulky metallic goods.

It is important that arrangements should be made for proper venting or extracting the gas from the landfill. It is necessary that the gas should be treated after extracting. Venting of gas from the landfill depends upon the following reasons:

Gas pressure : The pressure generated by landfill gases should be estimated. The pressure generated should be low so that the landfill cover is not ruptured.

Stress on vegetation : Landfill gases that diffuse upward through the landfill cover may have an adverse effect on the vegetation growing above the cover. This stress will cause vegetation to deteriorate which in turn will lead to increased erosion of the final cover.

Toxicity of gases : It is very important to study the toxicity of landfill gases. The diffusion rate, concentration of gas and its toxicity determines whether such release will violate air quality regulations.

Location of landfill : Gases diffusing through a landfill cover may pose a health risk to the resident population in the immediate vicinity of the landfill, therefore proper monitoring of the area should be done. If a landfill is located near to or within a community then the gas collection and disposal techniques should minimize the nuisance of odors and explosive potentials of gas.

Each design for gas disposal requires different design decisions relative to acceptable landfill wastes, leachate control and treatment, liner design, and gas production expectations.

A typical landfill gas has 50% of CH4 concentration. The landfill gas can be utilized directly for heating as medium heating value gas (raw gas), high heating value gas (filtered gas) and also for power generation in internal combustion engines, and in gas and steam turbines. However, the most economic option is the direct use for process heating and boiler fuel. In the power generation projects, the cost of engines or turbines is more than 60% of the total plant cost. The only way to make the power generation viable is to force the utilities to purchase power from LFG at higher cost. The figure 12 below shows typical gas collection system from a landfill.

Control of storm water runoff at a landfill disposal facility is necessary to minimize the potential of environmental damage to ground and surface waters. Direct surface water contamination can result when solid waste and other dissolved or suspended contaminants are picked up and carried by storm water runoff that comes into contact with the working face of the landfill. Uncontrolled surface water runoff can also increase leachate production, thereby increasing the potential for groundwater contamination. The resulting unwanted gas generation may also increase the potential for explosions.

A general design shown in figure 13 consists of 12 inches or more of top soil, followed by a 12 inch drainage layer of sand, and a hydraulic barrier of 24 inches of compacted clay over a gas collection system. Drain tile or perforated pipe can be installed in the drainage layer to facilitate drainage; however, they must be designed to prevent crushing. There must be 12 to 24 inches of compacted soil between the gas collection system and the compacted waste. If a sufficient quantity of good quality clay is not available, a 40 ml thick flexible membrane liner (20 ml minimum) is recommended. In some cases both a compacted clay layer and a flexible membrane liner are required. Grass or other native vegetation with finely branched root systems that will stabilize the soil without penetrating the hydraulic barrier must be planted. Trees must not be allowed to grow in the cover unless necessary for the planned final use of the landfill, and then must be enclosed in planters. Generally, a much thicker soil layer is required for planted trees.

In planning a sanitary landfill, consideration should be given to support facilities that are based primarily on the size of the operation and the climate. Support facilities must be as per specific landfill. These facilities generally include the following:

A landfill design life will comprise of an ‘active' period and a ‘closure and post closure' period. The ‘active' period may typically range from 10 to25 years depending on the availability of land area. The ‘closure and post-closure' period for which a landfill will be monitored and maintained will be 25 years after the ‘active period' is completed.

Before closing a landfill, the final cover must be installed as approved by the regulatory agency. Closure activities must begin no later than 30 days after the last waste is received unless it is reasonable to expect additional waste. However, even if additional waste is expected, the landfill must be closed if no waste is received for one year (longer periods must be approved by the regulatory agency).