Because of its high organic matter and dangerous heavy metal concentrations, municipal solid waste leachate, a type of wastewater, can severely harm the environment and contaminate groundwater.
Due to its complicated makeup, this effluent must be carefully treated before being discharged into the environment.
What is the meaning of Landfill Leachate?
Leachate is liquid that enters a landfill from external sources such as rain, surface drainage, subsurface springs and groundwater, as well as the liquid created by garbage putrefaction.
Leachate is produced by all landfills. The way the landfill is designed, as well as the features of the location, determine whether the leachate contaminates groundwater.
The chemical and biological oxygen requirement of landfill leachate is significant.
The content and age of landfill materials, the degrading process, the climate and the hydrological conditions all have an impact on landfill leachate.
Heavy metals, ammonia, phenols, medicines, pesticides and remnants of plastic degradation can all be found in the leachate generated by solid waste landfills.
The majority of these pollutants are in the form of dissolved salts.
The volume of leachate produced by solid waste landfills is never constant and varies considerably from year to year and with rainfall.
This is why a traditional biological procedure will not work; only physical and chemical treatments will be able to handle this sort of effluent.
Factors that influence the composition of landfill leachate
_Waste from landfills: Is the material biodegradable or not? Is it a soluble or insoluble substance? Organic or inorganic materials? Solid or liquid? Toxic or innocuous substances?
_Conditions at the landfill: pH, temperature, rate of decomposition, moisture content, climate and landfill age are all factors to consider.
_Entering water characteristics: pH, temperature and quantity
_Characteristics of the soil under the landfill: Permeability, geologic strata depth and thickness and mineral concentration
The following criteria affect the danger of groundwater pollution by any leachate that escapes collecting systems:
_The water table’s depth. Water will get partially filtered as it percolates downhill through the soil if the water table is low (far below the ground surface).
If the groundwater level is high, Contaminants can reach the groundwater without being filtered by the soil.
_ Concentration of contaminants: Contamination of groundwater is more likely if the leachate has a high concentration of pollutants.
_ Permeability of geological strata: water can infiltrate quickly through very porous rock strata, with little filtration along the way. Water cannot percolate downhill through strata made up of relatively impervious materials like silt and clay.
_ Geologic stratum type: Some earth minerals, like clay, are better at filtering out pollutants than others, not only because they are waterproof, but also because chemicals can bind to the surface of their particles.
_ The contamination’s toxicity: When water filters downhill through a landfill, it picks up dissolved components from the decaying wastes, resulting in leachate.
_ The flow of groundwater in one direction: Groundwater travels through the open areas in soil and rock slowly and constantly.
A plume of pollution will result if a landfill contaminates groundwater.
Wells in the plume will be polluted, while wells not in the plume, even those near the dump, may be unaffected.
Methods of leachate treatment
Biological Treatment Technique
The metabolic activities of microorganisms result in the biological breakdown of pollutants.
Biological approaches are often employed to remove nutrients (e.g., ammonia) and organic compounds due to their cost effectiveness; nevertheless, such techniques may not be able to effectively remove heavy metals and non-biodegradable organics.
Biological procedures are divided into two categories: aerobic biological procedures and anaerobic biological procedures.
Bioreactors have been used to treat wastewater for several years because they are easy to operate, dependable and cost-effective.
However, temperature difficulties and leachate toxicity for microbial populations are the major downsides of bioreactor treatments.
Physical and chemical treatment Techniques
According to the author, adsorption has been widely used to remediate landfill leachate.
The ease of use, simplicity of design, insensitivity to harmful compounds and capacity to remove a range of pollutants are all advantages of this technology.
Pollutants can cling to the surface of the adsorbent in a variety of ways during adsorption.
The adsorbent’s surface has special properties that allow the adsorbate to stick to it.
Adsorption happens under particular conditions, and desorption, a reversible event, is applicable.
Adsorbates can be liberated from the adsorbent’s surface and returned to the liquid during desorption.
Landfill leachate is routinely treated using biological methods.
A biological approach, on the other hand, is insufficient to remove the majority of refractory pollutants from landfill leachate.
As a result, researchers have advocated integrating biological and physical/chemical techniques in landfill leachate treatment to increase biodegradability ratios and biological performance.
Reverse Osmosis treatment Technique
Because of the high rejection rate of organic and inorganic pollutants, a well-constructed membrane, either as a primary step in a landfill leachate treatment chain or as a single post-treatment step, is an indispensable way of landfill leachate treatment.
Reverse osmosis with high rejection may retain dissolved particles and metals throughout a wide range, and the elimination rates can occasionally exceed 99 percent.
The usage of reverse osmosis membrane as a key stage in a landfill leachate treatment chain or as a single post-treatment step has shown to be an indispensable way of landfill leachate treatment due to the development of high rejection rates and appropriately built membrane modules.
Although reverse osmosis technology has superior removal for both dissolved organic and inorganic substances, the high level of suspended solids, colloids, dissolved organic matters, metal oxides, bacteria and their metabolites in landfill leachate can invariably lead to membrane fouling after a certain period of operation.
Membrane fouling can result in a decrease in permeate flux as well as an increase in Transmembrane pressure.
When salt concentrations on the reverse osmosis feed side surpass their solubility (scaling), salt precipitation on the reverse osmosis membrane surface occurs, resulting in a decrease in permeate flux, an increase in trans-membrane pressure and the need for periodic chemical membrane cleaning.
Coagulation and Flocculation
Stabilized stage and old age leachate are treated using the coagulation-flocculation process.
The primary goal of this method is to remove organic molecules from the leachate.
Sludge is formed during the coagulation process, depending on the properties of the leachate and the pollutant removal effectiveness.
The charge neutralization of negatively charged colloids by cationic hydrolysis products is followed by the inclusion of contaminants in an amorphous hydroxide precipitate by flocculation in this process.
The different coagulation procedures are as follows:
_ Coagulation with calcium hydroxide and alum can remove up to 69 percent COD and 54 percent turbidity from the leachate. Nowadays, coagulants such as ferric chloride and ferric sulfate are also used.
_ Polyaluminum chloride (PACl) offers a higher coagulant efficiency and a cheaper cost than standard coagulants. It has a good structure and a higher charge density, requiring less dose and producing less sludge. Color, COD, and ammonia may all be removed from water and wastewater with PACl.
_ The coagulant is generated by electrolytic oxidation of an appropriate anode material that leads, pH appropriate, to an insoluble metal hydroxide that is capable of removing a wide variety of pollutants.
It neutralizes the electrostatic charges on suspended solids and oil droplets to facilitate agglomeration or coagulation and separation from the aqueous phase.
PAC is a class of extremely efficient coagulants used in water treatment that have largely replaced conventional aluminum coagulants due to their low dose, high efficiency, low cost and ease of use.
Another benefit of this procedure is that the leachate does not need to be pre-treated.
Reference
1- Fast, Leachate Management, [online] Available at: https://www.osmeo.com/leachatemanagement/?gclid=EAIaIQobChMIq92JqPPr9wIV8YBQBh3guw89EAAYASAAEgIAG_D_BwE
2- Landfill Leachate, [online] Available at: http://cwmi.css.cornell.edu/TrashGoesToSchool/Landfill.html
3- Mojiri, A., Zhou, J. L., Ratnaweera, H., Ohashi, A., Ozaki, N., Kindaichi, T., & Asakura, H. (2021). Treatment of landfill leachate with different techniques: an overview. Water Reuse, 11(1), 66-96. [online] Available at: https://iwaponline.com/jwrd/article/11/1/66/78524/Treatment-of-landfill-leachate-with-different
4- Leachate Treatment Technologies, 5 Nov, [online] Available at: https://www.ripublication.com/ijeem_spl/ijeemv4n5_05.pdf
