18 Landfill leachate management

Dr. Yogalakshmi K. N

Objectives:

1.      To understand the process of leachate generation

2.      To know the composition and characteristics of landfill leachate

3.      To know the factors influencing the composition of leachate

4.      To estimate the quantity of leachate generation

5.      To identify the leachate collection system

6.      To study about different treatment options available for leachate

1.0 Introduction

Landfills are engineered structure for the containment of domestic and industrial solid wastes. It is one of the controlled disposal option. Landfills are generally located in the remote places to prevent environmental risk and ensure human health safety. A landfill is either large area of land or excavated ground constructed in a way to receive waste. The waste disposed in the landfills undergoes decomposition over a period of time. The decomposing waste release greenhouse gases as well as toxic liquid known as leachate. These products pose environmental risk and health hazard. Leachate is a watery liquid that oozes out from the waste material during its decomposition. Precipitation that flows through the waste material further increases the quantity of leachate generated from landfills. Leachate is a liquid that contains soluble components of the waste, suspended solids and other degraded products. The characteristic of leachate depends on the composition of waste, presence of biodegradable substances, moisture content, weather conditions, waste holding time and operational procedures. Longer the waste holding time, higher is the concentration of persistent organic pollutants. The stage of biodegradation also influences the composition of leachate. Example, inert waste produces a minimum amount of leachate unlike the biodegradable waste. Likewise, the concentration of pollutant in leachate also varies with the nature of waste. Leachate generally consists of undesirable toxic chemicals such as organic acids, aldehydes and alcohols.

Landfill if not managed properly, leads to many potential hazards. It can seep into the groundwater and contaminate entire aquifer. Once the groundwater is polluted it is very expensive to remediate and restore it. The main goal of managing landfill leachate is to prevent the contamination of groundwater, surface water and soil. Leachate, if not handled properly causes various environmental and health implications. The major problems include water and soil contamination, odour problem, spread of diseases, epidemics, vegetation damage and many more. The aesthetics of the site is also affected by the site. Leachate generation in landfill is influenced by number of factors such as rainfall, landfill cover, waste type, vegetation, climatic conditions, and landfill design. The flow rate of leachate depends on precipitation, surface run-off, evaporation, permeation, groundwater intrusion, type of waste (water content and degree of compaction in the cells), and land filling methods (employing daily cover).

2.0 Composition of leachate

The composition of leachate may vary greatly depending upon the age of the landfill, pH, nature of waste, decomposition rate and weather conditions.

Age of the landfill: The composition of leachate varies greatly with the age of the landfill. Young landfills generate acetogenic leachate while old or matured landfill generates methanogenic leachate. The waste within the landfills undergoes degradation in five different phases. During the initial period when the hydrolysis or aerobic degradation occurs, the leachate generation is minimum and just contains carbonic acid as the major constituent. The pH of leachate during this phase is acidic in nature. The fermentation (stage 2) and acetogenic (stage 3) phase greatly influences the composition of leachate. The major proportion of the complex waste is degraded into simpler substances especially organic acids, ammonia and hydrogen. The pH of the leachate drops below 4, thereby increasing the solubility of metals. Thus, the leachate produced during the acetogenic phase contains a high amount of COD (1, 52,000 mg/L); ammonical nitrogen (922 mg/L) and heavy metals. Higher levels of ammonia are contributed by biodegradation of proteins and other nitrogenous compounds. The BOD/COD ratio is very high for leachate generated during stage 3 indicating high levels of organic materials and its biodegradability. Stage 4 (i.e.) methanogenesis result in leachate of varied composition. The pH of the methanogenic leachate is alkaline in nature showing an average pH of 7.52. The degradation of organic acid to methane and carbon dioxide resulted in pH shift. The solubility of metal ions where restricted due to pH shift. The COD and BOD levels of the methanogenic leachate are comparatively lesser than acetogenic leachate. However, the concentration of ammonical nitrogen remained high in the methanogenic leachate.

Nature of waste: As mentioned earlier, the composition of leachate is influenced by the nature of waste. Municipal solid waste generally generates leachate high in organic matter and ammonia. Hazardous waste landfills generate leachate having a highly variable concentration of different types of compounds viz. salts, metals, halogenated compounds and other organic compounds. Leachate from inert landfills contain minimum amount of organic and inorganic species. Landfills destined for lower levels of biodegradable waste generates leachate of comparatively lesser organic matter.

Decomposition rate and weather conditions: Once, water seeps through the waste, anoxic condition is created by the decomposition by bacteria and fungi as they consume all the available oxygen. The waste which undergoes decomposition, cause increase in the temperature and fall in the pH. It results in dissolution of metal ions in the leachate which are otherwise insoluble in neutral pH. The waste undergoing decomposition processes release water adding volume to the leachate.

Leachate is blackish, orangish or yellowish liquid that emits strong odour. The strong odour can be attributed to the presence of hydrogen-, nitrogen- and sulfur-rich organic compounds. Table 1 summarizes the composition of acidogenic and acetogenic leachate. In addition to the above mentioned characteristics, leachate also contains more than 200 types of xenobiotic organic compounds. Major elements, trace metals, and microbiological substances are present both in dissolved and suspended form. The pollutants of leachate can be broadly classified into four groups.

• Dissolved organic matter including alcohols, acids, aldehydes, short chain sugars; volatile fatty acids; fluvic and humus like materials.
• Inorganic macro components including common cations and anions. Heavy metals.
• Xenobiotic organic compounds.

3.0 Toxicity of leachate

Bioassay studies prove leachate to be highly toxic liquid. Ammonia, chloride, heavy metals, acidity and alkalinity are few of the toxic pollutants present in leachate. Leachate generated from municipal solid waste possesses mutagenic and carcinogenic substances.

Table 1 Composition of acetogenic and methanogenic leachate from large landfill sites with high waste input rate and relatively dry environments (mg/L)

4.0 Estimation of leachate generation

The factors used for estimation of leachate generation include.

• Amount of rainfall
• Amount and rate of rainwater Infiltration through the landfill Potential of waste to absorb water
• Amount of liquid waste disposed in the landfill Weight of the waste after the absorption of water Leakage via seepage or discharge

The amount of leachate generated for municipal solid waste is estimated as per the formula mentioned below

Lo= [ER+ LIW+IRA] – [LTP + aW+ DL]

Lo= [Free leachate retained at the site (leachate production minus leachate leaving the site)

ER = Effective rainfall (or actual on an active surface area); this may need to be modified to account for run- off, especially after capping

LIW= liquid industrial waste (including any surplus water from sludges)

IRA = infilteration through restored and capped area

LTP = discharge of leachate off- site

A = unit absorptive capacity of wastes

W = weight of absorptive waste

DL = Designed seepage (if appropriate)

5.0 Leachate Management

A lined leachate evaporation ponds are constructed for managing landfill leachate. The leachate is collected in the ponds through extraction of leachate from the waste containing landfill cell by using a leachate collection system. The leachate cells are usually lined using synthetic material or by using clay liner. The liner system prevents leachate from seeping the soils underneath.

The leachate pond commonly uses floating aerators which helps in treating it and reducing the odours by preventing it from becoming anaerobic. The pond also induces evaporation and reduces the leachate volume. The Sludge formed is removed from time to time and either deposited within the landfill cell or other safe location.

6.0 Components of leachate collection systems

The components leachate collection system includes pumps, manholes, discharge lines and liquid level monitors. However, liners, filters, pumps and sumps are the four main components governing the overall efficiency of the system.

Liners: Liners are used as both collection device and means for separating leachate within the landfill. Liners can be either natural or synthetic. It is used to protect the soil and groundwater underneath. The liners should essentially be strong and impermeable in nature. Moreover, an effective liner should flexible, possess high tensile strength, tolerant to temperature fluctuation and should be black in colour (to resist UV light). The liners should be easy to install, economical and resistance to scratch, and chemical degradation by leachate. The common liners used in leachate management are geomembranes, geosynthetic clay liners, geotextiles, geogrids, geonets, and geocomposites. Each liner comes with specific features and uses. The geomembranes are utilized as a barrier between mobile polluting materials released from wastes and the groundwater. They are generally used in the closing of landfills to provide a low-permeability cover barrier which restricts the incursion of rain water. Geosynthetic clay liners (GCLs) are prepared by spreading sodium bentonite (having low permeability) uniformly between woven and non-woven geotextiles. Geotextiles are used as partition between two different types of soils to check the pollution of the lower layer by the upper stratum. It protects synthetic layers against perforation from underlying and overlaying rocks. Geogrids are kind of structural synthetic materials utilized in slope surface stability for creating stable cover soils over synthetic liners. Geonets are also a kind of are synthetic materials used instead of sand and gravel. Geocomposites are a combination of synthetic materials that are used individually. Geocomposites provide filter and drainage medium.

Leachate drainage system: The leachate drainage system is used to collect and transport the leachate which is collected inside the liner. The pipes of the drainage system are located at the bottom of the cell. The pipeline network can be flexible or rigid but the joints connecting the pipes should be flexible to withhold enormous amount of weight and pressure. The collection pipe network drains, collects, and transports leachate throughout the drainage layer to a collection sump where it is separated for the treatment or disposal. The pipe network also helps in draining the waste within the drainage system to reduce the mounding of leachate. These pipes are fabricated with cuts in such a way that they are inclined to 120 degrees and prevent the entry of solid particles.

Filters: The filter layer is used on top of the drainage layer in leachate collection system. Granular and geotextile are two types of filters used in practices. Granular filters come with one or more soil layers which have coarse gradation in the direction of the leakage than the soil to be protected. When liquid enters the landfill cell, it progresses down the filter, passes through the pipeline and rests in the sump. For efficient operation, the number, location, and size of the sumps are very important. While designing sumps, it is important to consider the amount of leachate and liquid expected. The size of sump is larger in the areas falling under higher than average rainfall. The capacity of pump is also an important criterion in sump planning. Larger the pump capacity smaller is the sump size. It is vital for the capacity of the sump to be able to accumulate the expected liquid between the pumping cycles. Leachate is conveyed by the collection pipes through the gravity to one or more sumps depending on the extent of the area drained. Leachate collected in the sump is disposed by pumping either to a vehicle or to the holding facility for vehicle pickup or to an on-site treatment facility.

Membrane and collection for treatment: Some landfills are equipped with some forms of membrane separating the waste from the surrounding ground. In such sites the leachate collection pipe network is laid on the membrane to put across the leachate to a collection or treatment site. The membranes are porous to some extent because, over time a low volume of leachate crosses the membrane. The design of landfill membranes is at low volumes so that they never have a quantifiable adverse impact on the quality of the receiving groundwater.

Reinjection into landfill: In leachate management, leachate is collected and re-injected into the waste mass for recirculation. These processes greatly increase the decomposition and gas production. It positively impacts the conversion of leachate volume into landfill gas and thus reduces the overall volume of leachate for disposal.

7.0 Treatment methods

The on-site treatment is usually practiced in landfill management. In on-site leachate treatment, the leachate is pumped from the sump into the treatment container. The leachate is then sometimes mixed with some chemical reagents to adjust the pH, coagulate and settle solids. It also reduces the concentration of hazardous substance. However, the treatment methods for landfill leachate are ideally classified as biological, physicochemical and combination of biological and physicochemical methods.

7.1 Biological methods: The main biological processes include activated sludge, the rotating biological contractors, sequencing batch reactor (SBR), reed beds, biological aerated filters and lagoons. Others are up- flow anaerobic sludge blanket (UASB), moving bed biofilm reactor (MBBR) and the membrane bioreactors (MBR) and anaerobic filters. In activated sludge process, oxygen is provided in the aeration tank as the leachate flows all along the system. Microorganisms thrive in the tank forming biological flocs. The microorganisms which are in suspension consume the organic matter in the leachate and transform it into new microbial biomass, carbon dioxide and water. Microbial activity reduces the organic content of the leachate. At the clarifier or settling tank, the sludge formed settles at the floor of the tank while the supernatant is runoff as effluent. Part of the sludge is returned to the aeration tank to re-seed incoming leachate.

Rotating biological contractor: The rotating biological contractor, known as a bio-rotor uses attached growth system. It consists of circular plastic discs mounted on a shaft which is partly submerged in a tank holding the leached water. As the shaft rotate, the microorganisms adhered to the disc, assimilate and treat organic matter from the leached water as they pass over the surface of the disc. Aerobic conditions are maintained while the disc rotates out of the leachate. The disc provides the contact between biomass and the leachate, mixes the mixed liquor and aerates the leachate. However, performance of rotating biological contractor is lower than activated sludge technique.

Sequence batch reactors (SBR): In SBRs, microorganisms are in suspension. However, the main difference is that the aeration and sludge settlement take place in the same tank in a batch mode based on operation cycle.

Reed beds: Reed beds come with gentle sloping beds lined with impervious barrier and fixed with emergent hydrophytes such as reeds (Phragmites), bulrush (Scirpus), or cattails (Typha). Reed beds sometimes have an inlet zone of crushed stone in order to distribute wastewater uniformly over the bed with an outlet zone of compacted stone to collect and discharge the effluent. Leachate enters the inlet and moves slowly through the bed flowing in the horizontal path. As the leachate flows, oxygen circulates into the beds. Then an aerobic bacterium surrounding the rhizomes of reeds consumes it to oxidize organic substance in leachate as it moves on through the bed. The gravel /soil in which the reeds are planted acts as a filter medium.

Biologically aerated filter (BAF): A BAF is a kind of treatment tank with submerged aerated fixed film biological filtration systems which provide a surface for the biomass and hold back suspended solids. It acts as a biological contactor and a filter which eliminates the need for a separate sedimentation step.

7.2 Physiochemical treatment of leachate: Few of the popularly used physiochemical methods of leachate treatment is elaborated below

Adsorption: Adsorption which is a process of accumulating substances in solution on a suitable interface involves transfer of constituent in the liquid phase to the solid phase. Mostly used adsorbents include synthetic polymeric, activated carbon and silica-based adsorbents. Activated carbon is used most commonly in advanced wastewater treatment applications. PACT (Powdered Activated Carbon Treatment) is also used sometimes as an adsorbant. The process involves the continuous addition of PAC to the activated sludge bioreactor to adsorb toxic contaminants. The adsorbed sludge is later incinerated to destroy the organics. Pre-treatment is a prerequisite for adsorption process. Suspended solids must be removed before subjecting leachate to carbon adsorption process, otherwise grease and oils may accumulate on the first few inches of surface and hinder the process. This treatment process has various advantages as it does not require precipitation and sedimentation.

Membrane technology: Membranes are used as a final or polishing step in leachate treatment. Microfiltration and ultrafiltration have been found to be effective in the removal of large organics from aqueous leachate streams. Best application of these membrane technologies is carried out at sites where leachate contains only one primary contaminant. As membranes having greater productivity and chemical resistance are developed, microfiltration and ultrafiltration have become more viable treatment alternatives. Reverse osmosis has not been widely applied to the full-scale treatment of waste leachate because of the delicate nature of reverse osmosis membranes and the strength and complexity of leachate. RO has just been used as a polishing step subsequent to other more conventional processes. Reverse osmosis can remove dissolved inorganics (metals, metal-cyanide complexes, and other ionic species) and high-molecular-weight organics (e.g., pesticides) from leachate. Leachate is pre-treated before subjecting it to reverse osmosis process.

Chemical treatment: Under the umbrella of chemical treatments comes those techniques in which we add certain chemicals to detoxify and remove harmful constituents present in leachate. Chemical methods are fast and do not cause damage to the set up. Some important methods of chemical treatment are given below.

Coagulation and precipitation: This process involves addition of certain chemicals to leachate which react with impurities and render them harmless, which are finally removed by sedimentation. This treatment method is useful on leachate with very high molecular weight organic material such as fulvic and humic acids.

Chemical oxidation: Chemical oxidation is another method of waste treatment where oxidative degradation or transformation of wastes is carried out. Usually wastewaters that are resistant to biodegradation or create toxicity in biological reactors are given chemical oxidation treatment. A variety of chemical oxidants used for leachate treatment include hydrogen peroxide, ozone, chlorine. A point to remember while selecting an oxidation agent is to check oxidation ability of the oxidant. More the oxidation ability better will be the oxidation process undertaken. This method is a preferred technique for the treatment of low BOD5/COD (i.e. stabilized) landfill leachates. Oxidants, such as chlorine, potassium permanganate, ozone, and calcium hydrochloride are popular.

Ammonia Stripping: In ammonia stripping large quantities of air is passed over exposed surface of the leachate, which causes the partial pressure of the ammonia gas within the water to convert the ammonia in the liquid to the gas phase. Ammonia stripping can be used by water falling through a flow of air in the form of bubbles. Stripping towers have been found to be more effective since there is a better contact between the gas and liquid phases when dispersion of liquid takes place in the form of fine droplets.

Summary

To summarize, in this module we have studied about

•      Landfill leachate

•      Composition of landfill leachate

•      Factors influencing landfill leachate composition

•      Estimation of landfill leachate generation

•      Landfill leachate collection and treatment

Reference

• Paul T William (2005) Waste Treatment and Disposal, John Wiley and sons, Second Edition.
• Wiszniowski, J., Robert, D., Surmacz-Gorska, J., Miksch, K., & Weber, J. V. (2006). Landfill leachate treatment methods: A review. Environmental Chemistry Letters, 4(1), 51-61.
• Tchobanoglous, G. (1993). Integrated solid waste management engineering principles and management issues (No. 628 T3).
• Madu, J. I. (2008). New leachate treatment methods. Department of Chemical Engineering Lund University, Sweden.
• Renou, S., Givaudan, J. G., Poulain, S., Dirassouyan, F., & Moulin, P. (2008). Landfill leachate treatment: review and opportunity. Journal of hazardous materials, 150(3), 468-493.