sanitary landfills design operation and management

it is notes of SWM as per the syllabus of PTU

1. Solid waste management Sanitary landfills SHUBHAM SHARMA Department of civil engineering BGIET, SANGRUR

2. Sanitary Land Filling or Controlled Tipping Method In this method of refuse disposal, refuse is carried and dumped into the low lying area (are marked as the land fill site) under an engineered operation, designed and operated in an environmentally sound manner, as not to cause any public nuisance or hazards to public health or safety. The EPA defines a landfill as an engineering method of disposing of solid waste on land. As such, landfills are required to protect the environment by spreading waste into thin layers and compacting them into the smallest practical volume. By day’s end, all waste is then covered with earth. Transfer stations have their own regulations. They are required to have their floors clear of all waste by the end of the working day.

3. Design Considerations Geometry of cell Support material Leachate collection Leachate availability Leachate recirculation methods LFG collection Final closure Liners

4. Site selection process The suitability of a landfill site is determined by Its size/area/volume Technical and environmental factors Climate and hydrological conditions. It requires a development of a working plan, description of site location, operation, engineering work and site restoration. People are reluctant to allow construction of new landfill, thus siting approval authority is important.

5. construction A specific method of filling will depend on the characteristics of the site, Such as the amount of available cover material, topography, and the local hydrology. During construction of landfill the following must be determined ➤ Access roads. ➤ Equipment shelters. ➤ Scales if used. ➤ Topsoil stockpiles sites.

6. Leachate collection system Leachate may be defined as the liquid that has percolated through solid waste and has extracted dissolved or suspended materials from it. The rate of seepage of leachate from the bottom of a landfill is estimated by Darcys law. The use of clay has favoured in reducing the leachate percolation

7. Landfill gas In most of the cases as the anaerobic decomposition of the wastes predominates the decomposition process the gases obtain are Carbon dioxide and methane. Carbon dioxide as result of its density will move towards the groundwater which can lower the pH of the groundwater and increases the hardness and mineral content in the ground water. Hence it is important to manage and processing these gases in a safe way.

8. Gas venting system The lateral movement of gases produced in a landfill Can be controlled by installing vents made of materials that are more permeable than surrounding soil. The spacing of vents depends on width of waste cells but usually varies from 18 to 60 m.

9. Outlet for Gas venting system Barrier or well vents also can be used to control the lateral movement of gases. Well vents also can be used to control the lateral movement of gases. The movement of landfill gases through adjacent soil formations can be controlled by constructions of Barriers that are more impermeable than soil E.g.; bentonites, butyl rubber, alites etc;

10. Final cover and post closure The final cover must be 36" thick layer of clay. Boundaries must be well protected. Long term usage of landfill i.e. reusage of landfill. Once the landfill reaches design height, a final cap is placed to minimize infiltration of rainwater. Facilitate long term maintenance of the landfill. The final cover shall have a barrier soil layer. On the top of barrier soil layer, there shall be drainage layer of 15 cm. On the top of the drainage there shall be a vegetative layer of 45 cm to support natural plant growth LANDFILL CAP and to minimize erosion.

11. Final cap Specification A cap consists of from top to bottom Vegetation and supporting soil (6 inches) Filter and drainage layer - protective material (18-36 inches) drainage material (12 inches) A hydraulic barrier- clay layer (24 inches), LDPE barrier Foundation for hydraulic barrier- gravel layer ( 6 inch) sand bedding for LDPE (4 inch) Mulch is a layer of material applied to the surface of soil, for conservation of soil moisture, improving fertility and health of the soil

12. COMPACTION FACTORS Refuse layer thickness is the most important factor. To obtain the greatest density, waste should be spread in layers not more than two ft deep and compacted . The thicker the layer, the less densely a machine can compact it. The number of passes a compaction machine makes over the refuse is another factor that affects density. A pass is defined as a machine traveling over refuse one time in one direction. Whatever the machine, it should make three to four machine passes to achieve best results. More than four passes does not achieve enough additional density to make them economical. Slopes should be kept to a minimum of 4:1 or less . A level surface allows the best compaction.

13. MOISTURE CONTENT Compaction density is considerably affected by moisture content. Water softens and acts as a lubricant for material such as paper and cardboard, and permits tighter consolidation. Field tests show that moisture content varies from 10 to 80 percent, depending on whether the season is wet or dry. The optimum moisture content for maximum compaction is about 50 percent. A minimum amount of moisture can increase refuse compaction density by ten percent. While higher moisture content can provide higher in place densities, it also increases the amount of leachate formation.

14. COVER The right cover material and proper handling techniques help control nuisances as well as health and environmental problems. Cover material: Must be compacted to provide a tight seal. Must be free of organic material and large objects. Must not crack excessively when dry. Cover material is valuable and provides the following benefits: Helps seal in odours and prevents water from entering compacted waste. Prevents the breeding of insects and eliminates a source of food and shelter for rodents and birds. Prevents fires and controls litter. Provides a dense, stable fill that can serve as a good road base. A few landfills have been allowed to permit limited rain water entry to facilitate waste decomposition

15. Determine the Design Designs may vary, but there are three basic methods of building a sanitary landfill: area method Trench method Ramp method

16. AREA METHOD. The area method is best-suited for sites where no natural slopes exist. This method can be adapted, however, to ravines, valleys, quarries, or old surface mines. Disposing of waste in a ravine site requires construction of diversion ditches for runoff water before any waste is received. Here is how the area method works: Waste is pushed into layers, compacted, and adequately covered. During succeeding days, the incoming waste is dumped at the toe of the preceding day’s waste and pushed up the face, compacted, and covered at the end of each working day. A machine, such as a track-type tractor or landfill compactor, spreads and compacts the material. Soil for daily cover must be hauled in from borrow sites using a wheel tractor-scraper or articulated truck.

17. AREA METHOD

18. TRENCH METHOD The trench method is best-suited for flat or gently sloping land where the groundwater table is deep below the surface. The chosen site should have soil that is easy to excavate and suitable for cover. Immediate availability of cover without the need of expensive specialized equipment to haul it long distances can be a major advantage of the trench method. If the landfill is to be brought above ground level, nearby cover material can also be an advantage. The trench does, however, have some disadvantages. If more cover material is excavated than can be used immediately, it will have to be stockpiled and moved again at an additional expense. Drainage, too, can be a problem, but it can be solved by “day lighting” one end of the trench and sloping the trench floor toward that end. Provisions must also be made to allow surface water to run off at the end of the trench. Small trenches usually measure eight to 10 ft deep and are two to three times as wide as the machine excavating them. Larger ones may be 30 to 40 ft deep, 60 to 80 ft wide, and 200 to 300 ft long. These are suitable for sites receiving 300 to 500 tpd. Note that 500 tons is usually the limit to avoid truck traffic congestion.

19. TRENCH METHOD There are three ways to trench: Excavate the entire trench, and windrow the cover material along the sides until it is needed. Excavate only far enough to provide a single day’s working space and dirt cover. This is called the progressive trench method, and it may require handling the cover material only once. Excavate a second trench in segments parallel to the first one, and use the excavated material as cover for the first trench. Take care to leave at least two ft between the two trenches. This method may allow for handling the cover material only once.

20. RAMP METHOD The ramp method is a variation of the area and trenching techniques. Waste is spread and compacted on an existing slope. Cover material is excavated directly in front of the waste. It is then spread over the waste and compacted. The excavated are becomes a part of the cell to be worked the following day. Similar to the progressive trench method, the ramp method is considered ideal by some operators because they do not have to haul in cover material (with its extra cost of expensive handling equipment). Because they may handle the cover only once and do not have to prepare the land in advance, they consider this an excellent way to start a landfill with a minimum of equipment. If more than one lift is required, cover will have to be hauled to the working face at an additional expense. Depth of the water table is another factor, but it is not as critical as with the trench method, which normally requires deeper excavation.

21. RAMP METHOD

22. Operation and closure of sanitary landfill In this method, the refuse is dumped and compacted in layers of about 0.5 m thickness, and after the day's work-when the depth of filling becomes about 1.5 m , it is covered by good earth of about 15 cm thickness . This cover of good earth is called the daily cover. Since the refuse is well compacted with bulldozers, trucks, rollers, etc. and is well covered daily with good-earth, it does not cause any public nuisance, like scattering of wind-blown litter, and evolution of unpleasant odours and foul smells, as may be caused by an ordinary dumping of refuse on land. The filling of refuse is done in sanitary land filling by dividing the entire land-fill area into smaller portions, called cells. These cells are initially filled with daily compacted refuse of about 1.5 m depth, in turn. After filling all the cells with first lift, the second lift is laid in about 1.5 m height and covered with good earth cover of about 0.15 depth, called the intermediate cover. After all the cells have been filled up with of second lift, the third and more lifts can be piled up in about 1.5m depth each, all laid over by the intermediate earth covers, turn by turn. The process will continue till the topmost lift is piled up, over which the final cover of good earth of about 0.6m depth shall be laid, and well compacted. A cap-system may be installed over the top of the final cover. With the passage of time, the filled-up refuse will get stabilized due to the decomposition of organic matter and subsequent conversion into stable compounds . The land filling operation in essentially a biological method of waste treatment, since the waste is stable by aerobic as well as anaerobic bacterial processes.

23. initially, the bacterial decomposition occurs under the aerobic conditions, because a certain amount of air is trapped within the landfill. However, the oxygen in the trapped air is soon exhausted within a few days and the long-term decomposition occurs under anaerobic conditions. The entire period of refuse stabilisation can in fact, be divided into five distinct phases: During the first phase of operation, aerobic bacteria and fungi, which are dominant, deplete the available oxygen to effect oxidation of organic matter. As a result of aerobic respiration, the temperature in the fill increases. In the second phase, anaerobic and facultative bacteria develop in decompose the organic matter; and H2 and Co2 gases are thus evolved. through acidogenic activity. In the third phase, methanogenic bacteria develop to cause evolution of methane gas. In the fourth phase of decomposition, the methanogenic activity get stabilized. In the fifth stage, the methanogenic activity subsides, representing depletion of the organic matter; and ultimately, the system returns to aerobic conditions within the landfill.

24. For better biological degradation, the moisture content of the dumped material should be high, say not less than 60% or so, which is sometimes maintained by the aerobic decomposition brought out by fungi, or sometimes by sub-soil water. The refuse, in managed landfills, may usually get stabilised, generally within a period of 2 to 4 months, and settle down by 20-40% of its original height. The filled-up land can, in fact, be used for developing some green land, parks, or other recreational spots. Unequal settlement and odour trouble, may however, be there, and hence, normally, for the first 1-2 years, the land is grassed or planted, fenced, and left out as reserved green land. This can, on a later date, be preferably used for developing some regular playgrounds, or picnic spots. Such sites may also sometimes be used for constructing houses; though they are not generally preferred, because such constructions may prove to be costly due to their deeper foundations for avoiding unequal settlements. Such houses, may further pose problems like those of bad odours and cracks in walls. This method of refuse disposal is very suitable to the heavier type of Indian refuse, and also to the rural communities, hostels, camps, etc. Hence, it is widely adopted in our country. So much so, that about 90% of Indian refuse is disposed of in this manner.

25. Factors to be considered in the design and operation of a sanitary landfill site. Usually, the following factors must be considered for properly designing and managing a sanitary landfill site: Access to the site . The sanitary land fill site must be provided with suitable access roads for easy plying of trucks, carrying the city's solid waste (refuse) to the site. Cell Design and construction . The design for piling the waste at the landfill site will largely depend upon the terrain and position of water-table, etc. of the landfill site, as well as on the type and quantum of daily waste required to be dumped at the filling site. Circumstances under which the different methods of land filling are to be adopted have already been discussed along with the arrangements required to be made for collecting gas and leachate, if required. Depending on the specific requirements, the arrangements may be designed and adopted to suit the specific needs of the given site. Gas Recovery or Prevention of Gas Movement: Arrangements for prevention of gas movement or gas recovery may be suitably designed and made.

26. Cover Material. Approximately 1 m³ of cover earth (good earth) will be required to cover every 4 to 6 m³ of refuse. Clayey soils would be preferable as covers or sealants. Such clayey earth should be available near the sanitary fill site, as otherwise, the same will have to be transported over distances. Bringing of such earth will have to be identified and arranged in advance, for more properly managing a landfill site. Use of onsite earth by excavation, may be maximum to avoid transport of outside earth by preferring the trench system land filling. Equipment Requirements . Heavy earth moving machinery to pile and compact the waste and the good earth will be required at the land fill site, and this requirement may vary with the size of landfills. Fire Prevention. Now potable water outlets, if provided at site to extinguish fires at the landfill sites, must be clearly marked and distinguished. Proper cell separation by earthen embankments will surely help in preventing continuous fire, even if it occurs in one cell. Land Area . Area for filling at the site should be large enough to hold all wastes for a minimum of 1 year but preferably for 5 to 10 years.

27. Land filling Method: Selection of method will vary with the terrain and available cover. Litter control: Moveable fences may be used at the unloading areas: crews may be deployed to pick up litter atleast once per month, or as required. Unloading Area: Unloading area should be kept small, generally under 30 m. Drainage Arrangements: To divert the surface, run off (from rains, etc.) to avoid its percolation through the filled-up site, drainage ditches shall be installed, and final top earth cover shall be smoothly finished with 1 to 2% slope to avoid ponding of water. xii) Protection of underground water: Underground springs, if any, may be suitably diverted. Sealant barrier for leachate control maybe installed. Well for gas and groundwater monitoring may also be installed.

28. (xiii) Communications Facilities: Telephones may be installed at the site ensure proper communication in case of emergencies, like fires, etc. (xiv) Operational Records: Proper personnel may be positioned to receive the city's refuse and solid waste. Tonnages, transactions, and billing may be installed, if any disposal fee is to be charged. (xv) Employees Facilities. Restrooms and drinking water should be provided. (xvi) Days and Hours of Operation. Usual practice is to adopt 5 to 6 days a week, with 8 to 10 h/day. Night shifts are also provided in big cities, since carriage of refuse is usually done during night hours. (xvii) Equipment Maintenance. A covered shed or workshop at the site may be installed for field maintenance of equipment.

29. Advantages This method is most simple and economical. No costly plant or equipment is required in this method, as is required in other methods of incineration or pulverization. Separation of different kinds of refuse, as required in incineration method, is also not required in this method. There are no residues or byproducts left out/evolved in this method, and hence no further disposal is required; this being a complete method in itself. Low lying water-logged areas and odd quarry pits can be easily reclaimed and put to better use. The mosquito-breeding places are also, thus, eliminated.

30. Disadvantages: Low lying depressions or dumping sites may not always be available ;or even if they are available today, they may ultimately become scare or unavailable in future, since the production of solid waste is a continuous process. There is a continuous evolution of foul gases near the fill site, especially during the times the refuse is being dumped there. These gases may often be explosive in nature and are produced by the decomposing or evaporating organic matter. These gases, known as landfill gases, become a serious environmental problem at sanitary landfill sites. These gases is need be estimated, properly disposed off. Since the dumped garbage may contain harmful and sometimes carcinogenic non-bio-degradable substances, such as plastics, unused medicines, paints, insecticides, sanitary napkins, etc., they may start troubling on a later date, particularly during rainy season, when excess water seeping through the area, may come out of the dump, as a coloured liquid, called leachate . This highly poisonous and polluted leachate , containing organic compounds like chlorinated hydrocarbons, benzene, toluene, xylene, etc.; is likely to seep to the underground water-table, to contaminate the ground water, leading to diseases, like cholera, typhoid, polio, etc. In order to avoid such harmful effects, the leachates may have to be scientifically assessed, collected, and disposal of.

31. Controlling the movement of leachate in properly managed sanitary landfill sites Under normal conditions in hazardous landfills, leachate is found at the bottom of the landfills. From there, its downward movement occurs through the underlying strata, although some lateral movement may also occur, depending upon the characteristics of the surrounding soil. The rate of downward seepage from the bottom of the landfill can be estimated by Darcy's law , by assuming that the material below the landfill to the top of the water-table is saturated, and that a small layer of leachate exists at the bottom of the fill. Under these conditions, the discharge rate of leachate per unit area is equal to the value of K (coefficient of permeability) expressed in m/day . This value will indicate the maximum amount of seepage that would be expected, and this value can be used for designing a suitable system for controlling or collecting the leachate. As leachate percolates through the underlying soil strata of the landfill, many of the chemical and biological constituents originally contained in it, will be removed by the filtering and adsorptive action of the soil strata. The degree of removal of pollutants from the leachate will usually depend on the characteristics of the soil strata, especially its clay content.

32. However, because of the inherent potential risk involved in allowing leachate to percolate to the groundwater (even with the removal of some of its organics and chemicals), it is always better to either eliminate the production of leachate, or to collect and treat it separately, as stated earlier. The use of clay liners or synthetic liners like geotextiles** has been the favoured method for reducing or eliminating the percolation of leachate. Simultaneous use of clay lining, and synthetic membrane liner may be required for all hazardous landfill sites, where hospital wastes are dumped frequently, and where chances of production of leachates are high. The use of synthetic liners may prove to be costly and require care as not to get damaged during the filling of refuse at the site over the liner, although such a lining layer proves to be quite effective in preventing the downward movement of the leachate to the groundwater. The leachate can also be collected over this layer through open jointed pipes , etc. For it to be an effective barrier, the thickness of clay liner with permeability not more than 1 x 10^-7 cm/s, usually provided at the bottom of the fill, should be at least about 0.9 m or so. The synthetic liner with permeability not exceeding 1 x 10^-12 cm/s may be acceptable for the upper liner.

33. Another important method to control the production of leachate is to provide an impervious clay layer over the top of the fill, which should be appropriately sloped (1 to 2 percent) and provided with adequate surface drainage. This will prevent the infiltration of surface water, which is the major contributor to the total volume of the leachate.

34. Gas production and its control at properly managed sanitary land fill sites. The rate of decomposition of solid waste in landfills, as measured by gas production, reaches a peak within the first 2 years, and then tapers off, continuing in many cases, for periods as large as up to 25 years or more. The total volume of gases released during anaerobic decomposition can be estimated in a number of ways. If all the organic constituents in the refuse (with the exception of plastics, rubber and leather) are represented with a generalised formula of the form Ca, Hb, Oc, Nd, then the total volume of gas can be estimated by using the equation given below, with the assumption of complete conversion to carbon dioxide and methane:

35. Under ideal conditions, the gases generated from a landfill should either be vented out to the atmosphere; or in larger landfills, it may be collected and supplied to houses for warming or cooking, or used for production of energy. In most cases, over 90% of the gas volume produced from the decomposition of solid wastes, consists of methane and carbon dioxide. Although most of the methane escapes to the atmosphere, both methane and carbon dioxide have been found in large concentrations, of say up to 40%, at lateral distances of up to 120 m from the edges of the landfills. Hence, if the gases are allowed to be released to the atmosphere in an uncontrolled manner (as happens in most of the cases in India), then methane may accumulate below buildings, or in enclosed spaces, or close to the sanitary land fill site (upto about 200 m), since the sp. gravity of methane gas is less than that of air. This methane collection may prove to be dangerous, as it may develop an explosive mixture with air. even when methane is present in 5-15% by volume. On the other hand, carbon dioxide gas tends to move towards the bottom of the landfill, since carbon dioxide is about 1.5 times as dense as air, and is 28times as dense as methane. As a result, the concentration of CO, in the lower portions of the landfill mazy remain high for years.

36. Ultimately, because of its density, Co2, will also move downward through the underlying formation, until it reaches the ground water. Because C02 is readily soluble in water, it usually lower the PH of the ground water, in turn may increase the hardness and mineral content ground water, through the solubilization of calcium and magnesium carbonates. The movement of gases in landfills, should therefore, be controlled with properly designed engineering methods such as by constructing vents and barriers, and by gas recovery, as discussed below . Control of Gas Movement with Vents and Barriers . The lateral movement of gases produced in a landfill can be controlled by installing gas vents, made of materials that are more permeable than the surrounding soil. Typically, as shown in Fig, gas vents are constructed of gravel. The gravel layer of a cell vent is laid directly above the daily cell cover. The spacings of cell vents depends on the width of the waste cells, but usually varies from 18 to 60 m. The thickness of the gravel layer should be such that it will remain continuous, even though there may be differential settling. A depth of about 0.3 m to 0.45 m is usually recommended.

37. In the trench vent , a trench is dug as deep as the solid wastes and filled with gravel. A well vent , consisting of a perforated pipe surrounded with gravel, may also be installed through the waste fill, to control the lateral movement of gases. Such pipe vents will collect and convey the gases to riser pipes for venting. If the gases are to be collected for recovery for its heating value, then the riser pipe may be connected to a main for pumping, to the processing plant. Control of downward movement of gases can be accomplished by installing perforated pipes in a gravel layer at the bottom of the landfill. If the gases cannot be vented laterally, it may be necessary to install gas wells and to vent the pumped gas to the atmosphere. The movement of landfill gases through adjacent soil formations (neighbours lands) can be controlled by erecting vertical well barriers of materials that are more impermeable than the soil. Some of the landfill sealants that are available for this use. Out of these available sealants, the compacted clay is most commonly used.

38. In large sized landfills, the gases evolved due to decomposition of refuse may be collected through installing gas recovery wells. The recovered gas may be used for generation of electric power or may be supplied to nearby houses for domestic use. although gas recovery systems have been installed in some advance countries in some large municipal landfills, yet the economics of such operations are not well defined. The cost of cleaning of the generated gas and that of the processing equipment may limit the recovery of landfill gases, especially from small landfills. control of Gas Movement by Gas Recovery

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