Inertisation is thus far the only technology for the conversion of leachate and contaminated water into an inert product, thereby facilitating handling and disposal in MSW landfills.
One of the most complex aspects of the global management of an MSW landfill or treatment plant is leachate. This liquid comes mainly from the degradation of organic matter, added to other liquids present in the waste, rainwater, etc. and constitutes a complex, highly polluted water. It has a dark colour, a strong penetrating smell and, in areas of accumulation and/or stagnation, it has a surface layer of scum of several centimetres.
Leachate is the liquid that filters through solid waste in a landfill extracting solutes and suspended solids. These solutes and suspended solids are determined by a number of factors, including the composition of the solid waste, the way the landfill operates and weather conditions in the location. Leachate composition varies in accordance with landfill age, type of waste and history prior to sampling.
As a result, landfill leachate must be appropriately managed in accordance with physical and chemical characteristics, and composition. The composition of leachate at a landfill depends on the landfill type (hazardous, non-hazardous and inert) and, within the same landfill, on the period during which the landfill has been in operation. Therefore, depending on the age, a distinction can be made between young leachate, found at landfills in operation for 1 or 2 years, and mature leachate, which is found at landfills that have been in operation for over 3 years.
In general, leachate is highly contaminated, mainly due to; high concentrations of organic matter, concentrations of nitrogen, mainly in the form of ammonium; high salt concentrations, mainly chlorides and sulphates. Leachate generally has a low heavy metal content.
Membrane filtration and evaporation are amongst the treatment techniques most commonly applied to this type of wastewater and both are considered secondary treatments. Regardless of the technique applied, two fractions are obtained: a clean fraction (the permeate) and a dirty fraction (the reject or concentrate) which contains all the impurities (salts) of the treated water. This reject or concentrate therefore has a greater environmental impact than the water from which it originates. For this reason it must be treated in order to convert it into a product that can be easily handled or inertised for disposal in an MSW landfill.
The current treatment processes applied to this concentrate are crystallisation, spray drying, and inertisation. Crystallisation and spray drying can minimise this concentrate but the residual solid obtained is generally highly soluble in water, meaning that it cannot be managed in the landfill itself.
Inertisation. This is a physicochemical process that converts an effluent into a solid with structural integrity, preventing the migration of contaminants by reducing their mobility.
Inertisation is a technology widely used in the management of hazardous waste and it consists of two operations: Stabilisation of the waste. This process uses a number of reagents to reduce the hazardous nature of the waste, minimise the speed of migration of contaminants into the environment, and reduce the toxicity of the waste components.
The term sequestration is commonly used as a synonym for stabilisation. Stabilisation is carried out by adding reagents which improve the handling and physical characteristics of the waste, reduce the surface area through which transfer or loss of pollutants might take place, limit the solubility of any contaminant in the waste, and reduce the toxicity of contaminants.
Solidification of the waste is described as a process of reagent dosing for the purpose of solidifying the waste, increasing its resistance and reducing compressibility and permeability. In the broadest sense of the word, inertisation can be defined as the technique through which hazardous waste can be converted into inert waste; where inert waste is understood to be waste that does not undergo significant physical, chemical or biological transformation.
In common practice, the aim is to obtain waste that can be managed as non-hazardous waste.
Types of inertisation
Given the current state of development of this technology, it is possible to speak of two types of leachate inertisation: mass phase inertisation and dispersed phase inertisation.
Mass phase inertisation
This type of inertisation consists of mixing the solid-liquid phases in a unit with suitable reagent/leachate proportions for the formation of the inertised product. The most commonly used equipment includes: drum-type reactors (continuous reactor or discontinuous perfectly mixed reactors) or worm reactors (Figure 2 plug flow reactor).
In a flow reactor, the effectiveness of the contact between the phases (solid-liquid) is limited by the intensity of agitation. For this reason, mass and heat transfer coefficients are in turn limited and decrease considerably (high resistance to the reactions taking place), giving rise to long retention times. To overcome this and other typical disadvantages of this type of solid-liquid contact, high quantities of reagent per unit of waste mass (leachate- liquid) are used.
This result of this is the release of a gas with a high concentration of NH3 accompanied by H2O vapour. This is due to the high ammonium (NH4+) concentration in this type of water (waste) and the displacement of both chemical (increase in pH) and thermal (increase in temperature) balances that takes place, which favours the release of ammonium (NH4+) in the form of gas (NH3). Prior to being released into the atmosphere, this gas is scrubbed to remove all traces of NH3 by means of an absorption- desorption system.
Figure 1 shows the flow diagram of this technologyl model (treatment facility). In this case, reverse osmosis is combined with evaporation to further concentrate the concentrate from the membrane separation process. The resulting concentrate is sent to the inertisation process where, with the appropriate reagents, it is converted into a solid inert product suitable for landfilling.
Dispersed phase inertisation
This type of inertisation consists of spraying of the leachate through a nozzle into a heated chamber to produce small drops and then spraying the reagents into the same chamber. In both cases a mist is formed, one of droplets and the other of particles and these mists are mixed. In this way, a large contact surface area is achieved and heat and mass transfer coefficients are increased, which ensures that the reaction takes place with a high degree of effectiveness. For this reason, reagent requirements are closer to the stoichiometric ratio of the reaction and a smaller quantity of reagent is needed for the reactions to take place.
This technology requires adequate control of both leachate and reagent flows. For this reason, a dosing system is required for the solid part.
Unlike mass phase inertisation, this technology operates at temperatures of between 150 and 200 ºC, meaning that a supply of thermal energy is generally required to heat the air used as a heating and transport medium. Another possibility is the use of the flue gas from biogas gensets, which are generally available at landfills.
This gas, in turn, carries out the function of pneumatic transport and transports the inert product, in the form of dust, from the reaction chamber to the particle separation system, where it can be separated by means of different systems (cyclone-bag filters) and collected for subsequent handling and disposal.
Like the mass phase process, the result of this reaction is the release of a gas containing NH3 accompanied by H2O vapour. But in this case, concentrations are low due to the fact that they are diluted in the heating agent, generally air.
Like the mass phase process, this gas is scrubbed prior to being released into the atmosphere to remove all traces of NH3 and other potential contaminants such as VOCs.
Figure 2 shows the technology model of this type of facility. In this case, reverse osmosis is combined with evaporation to further concentrate the concentrate from the membrane process. The resulting concentrate is sent to the inertisation process where, with the appropriate reagents, it is converted into a solid inert product suitable for landfilling.
Benefits of DPI technology
Another of the aspects that makes dispersed phase inertisation attractive is that the flue gases from biogas combustion engines or the heat of the water used for cooling the engine can be used as a heating medium. Apart from reducing energy costs, this option enables the development of a new (biogas-leachate) model for the management of landfill effluents.
Because of the increase in the contact surface between the phases involved (leachate-solid reagents), this technology enables operating costs to be minimised by reducing the quantity of reagents required compared to mass phase inertisation, thereby making DPI a more attractive technology in terms of costs.